Tunable Microfluidic Devices for Hydrodynamic Fractionation of Cells and Beads: A Review
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paper_title: Microfluidics for cell separation

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The need for efficient cell separation, an essential preparatory step in many biological and medical assays, has led to the recent development of numerous microscale separation techniques. This review describes the current state-of-the-art in microfluidics-based cell separation techniques. Microfluidics-based sorting offers numerous advantages, including reducing sample volumes, faster sample processing, high sensitivity and spatial resolution, low device cost, and increased portability. The techniques presented are broadly classified as being active or passive depending on the operating principles. The various separation principles are explained in detail along with popular examples demonstrating their application toward cell separation. Common separation metrics, including separation markers, resolution, efficiency, and throughput, of these techniques are discussed. Developing efficient microscale separation methods that offering greater control over cell population distribution will be important in realizing true point-of-care (POC) lab-on-a-chip (LOC) systems.
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paper_title: Chip integrated strategies for acoustic separation and manipulation of cells and particles

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Acoustic standing wave technology combined with microtechnology opens up new areas for the development of advanced particle and cell separating microfluidic systems. This tutorial review outlines the fundamental work performed on continuous flow acoustic standing wave separation of particles in macro scale systems. The transition to the microchip format is further surveyed, where both fabrication and design issues are discussed. The acoustic technology offers attractive features, such as reasonable throughput and ability to separate particles in a size domain of about tenths of micrometers to tens of micrometers. Examples of different particle separation modes enabled in microfluidic chips, utilizing standing wave technology, are described along a discussion of several potential applications in life science research and in the medical clinic. Chip integrated acoustic standing wave separation technology is still in its infancy and it can be anticipated that new laboratory standards very well may emerge from the current research.
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paper_title: Cell manipulation in microfluidics

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Recent advances in the lab-on-a-chip field in association with nano/microfluidics have been made for new applications and functionalities to the fields of molecular biology, genetic analysis and proteomics, enabling the expansion of the cell biology field. Specifically, microfluidics has provided promising tools for enhancing cell biological research, since it has the ability to precisely control the cellular environment, to easily mimic heterogeneous cellular environment by multiplexing, and to analyze sub-cellular information by high-contents screening assays at the single-cell level. Various cell manipulation techniques in microfluidics have been developed in accordance with specific objectives and applications. In this review, we examine the latest achievements of cell manipulation techniques in microfluidics by categorizing externally applied forces for manipulation: (i) optical, (ii) magnetic, (iii) electrical, (iv) mechanical and (v) other manipulations. We furthermore focus on history where the manipulation techniques originate and also discuss future perspectives with key examples where available.
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paper_title: Microfluidics technology for manipulation and analysis of biological cells

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Analysis of the profiles and dynamics of molecular components and sub-cellular structures in living cells using microfluidic devices has become a major branch of bioanalytical chemistry during the past decades. Microfluidic systems have shown unique advantages in performing analytical functions such as controlled transportation, immobilization, and manipulation of biological molecules and cells, as well as separation, mixing, a nd dilution of chemical reagents, which enables the analysis of intracellular parameters and detection of cell metabolites, even on a single-cell leve l. This article provides an in-depth review on the applications of microfluidic devices for cell-based assays in recent years (2002–2005). Various cell manipulation methods for microfluidic applications, based on magnetic, optical, mechanical, and electrical principles, are described with selected examples of microfluidic devices for cell-based analysis. Microfluidic devices for cell treatment, including cell lysis, cell culture, and cell electroporation, are surveyed and their unique features are introduced. Special attention is devoted to a number of microfluidic devices for cell-based assays, including micro cytometer, microfluidic chemical cytometry, biochemical sensing chip, and whole cell sensing chip. © 2005 Elsevier B.V. All rights reserved.
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paper_title: Consideration of Nonuniformity in Elongation of Microstructures in a Mechanically Tunable Microfluidic Device for Size-Based Isolation of Microparticles

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This paper presents an investigation of the nonlinearity behavior in deformation that appears during the linear stretching of the elastomeric microstructures in a pillar-based microfilter. Determining the impact of the nonuniformity in strain on the geometry and performance of such a planar device under in-plane stretch is the motivation for this study. A semiempirical model is used to explain the physical strain–stress behavior from the root to the tip of the micropillars in the linear arrays in the device. For microfabrication of the device, the main substrate is elastomeric polyurethane methacrylate, which is utilized in an ultraviolet-molding method. Optical imaging and scanning electron microscopy were used to evaluate the deformation of the microstructures under different loading conditions. It was demonstrated that by applying mechanical strains of $\Delta L/L_{o})$ on the elastomeric device using a modified syringe pump, the spacing of the pillars is increased effectively to about three times the size of the initial setting of 5.5 $\mu $ m, which corresponds to a strain of above 180% in the absence of nonuniformity effects. This simple yet interesting behavior can be exploited to rapidly adjust a microfluidic device for application to the separation of microbeads or blood cells, which would normally require the geometrical redesign and fabrication of a new device. [2014-0213]
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paper_title: Inertial microfluidics for continuous particle filtration and extraction

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In this paper, we describe a simple passive microfluidic device with rectangular microchannel geometry for continuous particle filtration. The design takes advantage of preferential migration of particles in rectangular microchannels based on shear-induced inertial lift forces. These dominant inertial forces cause particles to move laterally and occupy equilibrium positions along the longer vertical microchannel walls. Using this principle, we demonstrate extraction of 590 nm particles from a mixture of 1.9 μm and 590 nm particles in a straight microfluidic channel with rectangular cross-section. Based on the theoretical analysis and experimental data, we describe conditions required for predicting the onset of particle equilibration in square and rectangular microchannels. The microfluidic channel design has a simple planar structure and can be easily integrated with on-chip microfluidic components for filtration and extraction of wide range of particle sizes. The ability to continuously and differentially equilibrate particles of different size without external forces in microchannels is expected to have numerous applications in filtration, cytometry, and bioseparations.
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paper_title: A technique of optimization of microfiltration using a tunable platform

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The optimum efficiency of size-based filtration in microfluidic devices is highly dependent on characteristics of design, deformability of microparticles/cells, and fluid flow. The effects of filter pores and flow rate, which are the two major determining and related factors of characterization in the separation of particles and cells are investigated in this work. An elastomeric microfluidic device consisting of parallel arrays of pillars with mechanically tunable spacings is employed as an adjustable microfiltration platform. The tunable filtration system is used for finding the best conditions of separation of solid microbeads or deformable blood cells in a crossflow pillar-based method. It is demonstrated that increasing flow rate in the range of 1.0–80.0 µl min−1 has an adverse effect on the device performance in terms of decreased separation efficiency of deformable blood cells. However, by tuning the gap size in the range of 2.5–7.5 µm, the selectivity of the separation is controlled from about 5.0 to 95.0% for white blood cells (WBCs) and 40.0 to 95.0% for red blood cells (RBCs). Finally, the best range of trapping and passing efficiencies of ~70–80.0% simultaneously for WBCs and RBCs in whole blood sample is achieved at optimum gap size of ~3.5–4.0 µm.
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paper_title: Optical Manipulation of Objects and Biological Cells in Microfluidic Devices

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In this paper, we review optical techniques used for micro-manipulation of small particles and cells in microfluidic devices. These techniques are based on the object's interaction with focused laser light (consequential forces of scattering and gradient). Inorganic objects including polystyrene spheres and organic objects including biological cells were manipulated and switched in and between fluidic channels using these forces that can typically be generated by vertical cavity surface emitting laser (VCSEL) arrays, with only a few mW optical powers. T-, Y-, and multi-layered X fluidic channel devices were fabricated by polydimethylsiloxane (PDMS) elastomer molding of channel structures over photolithographically defined patterns using a thick negative photoresist. We have also shown that this optical manipulation technique can be extended to smaller multiple objects by using an optically trapped particle as a handle, or an “optical handle”. Ultimately, optical manipulation of small particles and biological cells could have applications in biomedical devices for drug discovery, cytometry and cell biology research.
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paper_title: Cell manipulation with magnetic particles toward microfluidic cytometry

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Magnetic particles have become a promising tool for nearly all major lab-on-a-chip (LOC) applications, from sample capturing, purification, enrichment, transport to detection. For biological applications, the use of magnetic particles is especially well established for immunomagnetic separation. There is a great amount of interest in the automation of cell sorting and counting with magnetic particles in LOC platforms. So far, despite great efforts, only few fully functional LOC devices have been described and further integration is necessary. In this review, we will describe the physics of magnetic cell sorting and counting in LOC formats with a special focus on recent progress in the field.
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paper_title: Tunable liquid-filled microlens array integrated with microfluidic network

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An elastomer-based tunable liquid-filled microlens array integrated on top of a microfluidic network is fabricated using soft lithographic techniques. The simultaneous control of the focal length of all the microlenses composing the elastomeric array is accomplished by pneumatically regulating the pressure of the microfluidic network. A focal length tuning range of hundreds of microns to several millimeters is achieved. Such an array can be used potentially in dynamic imaging systems and adaptive optics.
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paper_title: A tunable microfluidic-based filter modulated by pneumatic pressure for separation of blood cells

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This article presents a new microfluidic-based filter for the separation of microbeads or blood cells with a high filtration rate. The device was composed of a circular micropump for automatic liquid transport, and a normally closed valve located at the filter zone for separation of beads or cells. The filtration mechanism was based on the tunable deformation of polydimethylsiloxane (PDMS) membranes that defined the gap between a floating block structure and the substrate which determined the maximum diameter of the beads/cells that can pass through the filter. Another unique feature of this filter is an unclogging mechanism using a suction force, resulting in a back flow to remove any trapped beads/cells in the filter zone when the PDMS membrane was restored to its initial state. The separation performance of the proposed device was first experimentally evaluated by using microbeads. The results showed that this device was capable of providing size-tunable filtration with a high recovery efficiency (95.25–96.21%) for microbeads with sizes smaller than the defined gap in the filter zone. Furthermore, the proposed device was also capable of performing separation of blood cells and blood plasma from human whole blood. Experimental results showed that an optimum filtration rate of 21.40 and 3.00 μl/min correspond to high recovery efficiencies of 86.69 and 80.66%, respectively, for red blood cells (RBCs) and blood plasma. The separation method developed in this work could be used for various point-of-care diagnostic applications involving separation of plasma and blood cells.
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paper_title: A pillar-based microfilter for isolation of white blood cells on elastomeric substrate

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Our goal is to design, fabricate, and characterize a pillar-based microfluidic device for size-based separation of human blood cells on an elastomeric substrate with application in the low-cost rapid prototyping of lab-chip devices. The single inlet single outlet device is using parallel U-shape arrays of pillars with cutoff size of 5.5 μm for trapping white blood cells (WBCs) in a pillar chamber with internal dead-volume of less than 1.0 μl. The microstructures are designed to limit the elastomeric deformation against fluid pressures. Numerical analysis showed that at maximum pressure loss of 15 kPa which is lower than the device conformal bonding strength, the pillar elastomeric deformation is less than 5% for flow rates of up to 1.0 ml min−1. Molding technique was employed for device prototyping using polyurethane methacrylate (PUMA) resin and polydimethylsiloxane (PDMS) mold. Characterization of the dual-layer device with beads and blood samples is performed. Tests with blood injection showed that ∼18%–25% of WBCs are trapped and ∼84%–89% of red blood cells (RBCs) are passed at flow rates of 15–50 μl min−1 with a slight decrease of WBCs trap and improve of the RBCs pass at higher flow rates. Similar results were obtained by separation of mixed microspheres of different size injected at flow rates of up to 400 μl min−1. Tests with blood samples stained by fluorescent gel demonstrated that the WBCs are accumulated in the arrays of pillars that later end up to blockage of the device. Filtration results of using elastomeric substrate present a good consistency with the trend of separation efficiencies of the similar silicon-based filters.
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paper_title: Review of cell and particle trapping in microfluidic systems

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The ability to obtain ideal conditions for well-defined chemical microenvironments and controlled temporal chemical and/or thermal variations holds promise of high-resolution cell response studies, cell-cell interactions or e.g. proliferation conditions for stem cells. It is a major motivation for the rapid increase of lab-on-a-chip based cell biology research. In view of this, new chip-integrated technologies are at an increasing rate being presented to the research community as potential tools to offer spatial control and manipulation of cells in microfluidic systems. This is becoming a key area of interest in the emerging lab-on-a-chip based cell biology research field. This review focuses on the different technical approaches presented to enable trapping of particles and cells in microfluidic system.
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paper_title: Optical trapping, manipulation, and sorting of cells and colloids in microfluidic systems with diode laser bars

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We demonstrate a new technique for trapping, sorting, and manipulating cells and micrometer-sized particles within microfluidic systems, using a diode laser bar. This approach overcomes the scaling limitations of conventional scanned laser traps, while avoiding the computational and optical complexity inherent to holographic optical trapping schemes. The diode laser bar enables us to control a large trapping zone, 1 μm by 100 μm, without the necessity of scanning or altering the phase of the beam.
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paper_title: Deformability considerations in filtration of biological cells

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Biological cells are highly sensitive to variation in local pressure because cellular membranes are not rigid. Unlike microbeads, cells deform under pressure or even lyse. In isolating or enriching cells by mechanical filtration, pressure-induced lysis is exacerbated when high local fluidic velocity is present or when a filter reaches its intended capacity. Microfabrication offers new possibilities to design fluidic environments to reduce cellular stress during the filtration process. We describe the underlying biophysics of cellular stress and general solutions to scale up filtration processes for biological cells.
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paper_title: Manipulation and sorting of magnetic particles by a magnetic force microscope on a microfluidic magnetic trap platform

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We have integrated a microfluidic magnetic trap platform with an external magnetic force microscope (MFM) cantilever. The MFM cantilever tip serves as a magnetorobotic arm that provides a translatable local magnetic field gradient to capture and move magnetic particles with nanometer precision. The MFM electronics have been programmed to sort an initially random distribution of particles by moving them within an array of magnetic trapping elements. We measured the maximum velocity at which the particles can be translated to be 2.2mm∕s±0.1mm∕s, which can potentially permit a sorting rate of approximately 5500particles∕min. We determined a magnetic force of 35.3±2.0pN acting on a 1μm diameter particle by measuring the hydrodynamic drag force necessary to free the particle. Release of the particles from the MFM tip is made possible by a nitride membrane that separates the arm and magnetic trap elements from the particle solution. This platform has potential applications for magnetic-based sorting, manipulati...
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paper_title: Cell Separation by Non-Inertial Force Fields in Microfluidic Systems.

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Cell and microparticle separation in microfluidic systems has recently gained significant attention in sample preparations for biological and chemical studies. Microfluidic separation is typically achieved by applying differential forces on the target particles to guide them into different paths. This paper reviews basic concepts and novel designs of such microfluidic separators with emphasis on the use of non-inertial force fields, including dielectrophoretic force, optical gradient force, magnetic force, and acoustic primary radiation force. Comparisons of separation performances with discussions on physiological effects and instrumentation issues toward point-of-care devices are provided as references for choosing appropriate separation methods for various applications.
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paper_title: A microfabricated deformability-based flow cytometer with application to malaria

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Malaria resulting from Plasmodium falciparum infection is a major cause of human suffering and mortality. Red blood cell (RBC) deformability plays a major role in the pathogenesis of malaria. Here we introduce an automated microfabricated “deformability cytometer” that measures dynamic mechanical responses of 103 to 104 individual RBCs in a cell population. Fluorescence measurements of each RBC are simultaneously acquired, resulting in a population-based correlation between biochemical properties, such as cell surface markers, and dynamic mechanical deformability. This device is especially applicable to heterogeneous cell populations. We demonstrate its ability to mechanically characterize a small number of P. falciparum-infected (ring stage) RBCs in a large population of uninfected RBCs. Furthermore, we are able to infer quantitative mechanical properties of individual RBCs from the observed dynamic behavior through a dissipative particle dynamics (DPD) model. These methods collectively provide a systematic approach to characterize the biomechanical properties of cells in a high-throughput manner.
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paper_title: Cell research with physically modified microfluidic channels: A review

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An overview of the use of physically modified microfluidic channels towards cell research is presented. The physical modification can be realized either by combining embedded physical micro/nanostructures or a topographically patterned substrate at the micro- or nanoscale inside a channel. After a brief description of the background and the importance of the physically modified microfluidic system, various fabrication methods are described based on the materials and geometries of physical structures and channels. Of many operational principles for microfluidics (electrical, magnetic, optical, mechanical, and so on), this review primarily focuses on mechanical operation principles aided by structural modification of the channels. The mechanical forces are classified into (i) hydrodynamic, (ii) gravitational, (iii) capillary, (iv) wetting, and (v) adhesion forces. Throughout this review, we will specify examples where necessary and provide trends and future directions in the field.
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paper_title: Elastomer based tunable optofluidic devices

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The synergetic integration of photonics and microfluidics has enabled a wide range of optofluidic devices that can be tuned based on various physical mechanisms. One such tuning mechanism can be realized based on the elasticity of polydimethylsiloxane (PDMS). The mechanical tuning of these optofluidic devices was achieved by modifying the geometry of the device upon applying internal or external forces. External or internal forces can deform the elastomeric components that in turn can alter the optical properties of the device or directly induce flow. In this review, we discuss recent progress in tunable optofluidic devices, where tunability is enabled by the elasticity of the construction material. Different subtypes of such tuning methods will be summarized, namely tuning based on bulk or membrane deformations, and pneumatic actuation.
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paper_title: Mechanically tunable optofluidic distributed feedback dye laser

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We demonstrated a continuously tunable optofluidic distributed feedback (DFB) dye laser on a monolithic poly(dimethylsiloxane) (PDMS) chip. We obtained /spl sim/60 nm tuning range by mechanically varying the grating period. Single-mode operation was maintained with <0.1 nm linewidth.
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paper_title: A Mechanically Tunable Microfluidic Cell-Trapping Device.

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Abstract Controlled manipulation, such as isolation, positioning, and trapping of cells, is important in basic biological research and clinical diagnostics. Micro/nanotechnologies have been enabling more effective and efficient cell trapping than possible with conventional platforms. Currently available micro/nanoscale methods for cell trapping, however, still lack flexibility in precisely controlling the number of trapped cells. We exploited the large compliance of elastomers to create an array of cell-trapping microstructures, whose dimensions can be mechanically modulated by inducing uniformly distributed strain via application of external force on the chip. The device consists of two elastomer polydimethylsiloxane (PDMS) sheets, one of which bears dam-like, cup-shaped geometries to physically capture cells. The mechanical modulation is used to tune the characteristics of cell trapping to capture a predetermined number of cells, from single cells to multiple cells. Thus, enhanced utility and flexibility for practical applications can be attained, as demonstrated by tunable trapping of MCF-7 cells, a human breast cancer cell line.
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paper_title: Tuneable separation in elastomeric microfluidics devices.

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We describe how the elastomeric properties of PDMS (polydimethylsiloxane) can be utilised to achieve tuneable particle separation in Deterministic Lateral Displacement devices via strain controlled alteration of inter-obstacle distances, a development that opens up new avenues toward more effective separation of particles in microfluidics devices.
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paper_title: Electrokinetic motion of particles and cells in microchannels

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This paper provides an overview of the electrokinetic phenomena associated with particles and cells in microchannel systems. The most important phenomena covered include electrophoresis, dielectrophoresis, and induced-charge electrokinetics. The latest development of these electrokinetic techniques for particle or cell manipulations in microfluidic systems is reviewed, in terms of the basic theories, mathematical models, numerical and experimental methods, and the key results/findings from the published literatures in the most recent decades. Some of the limitations associated with the negative field effects are discussed and the perspectives for the future investigations are summarized.
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paper_title: Deformability-based cell classification and enrichment using inertial microfluidics

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The ability to detect and isolate rare target cells from heterogeneous samples is in high demand in cell biology research, immunology, tissue engineering and medicine. Techniques allowing label-free cell enrichment or detection are especially important to reduce the complexity and costs towards clinical applications. Single-cell deformability has recently been recognized as a unique label-free biomarker for cell phenotype with implications for assessment of cancer invasiveness. Using a unique combination of fluid dynamic effects in a microfluidic system, we demonstrate high-throughput continuous label-free cell classification and enrichment based on cell size and deformability. The system takes advantage of a balance between deformability-induced and inertial lift forces as cells travel in a microchannel flow. Particles and droplets with varied elasticity and viscosity were found to have separate lateral dynamic equilibrium positions due to this balance of forces. We applied this system to successfully classify various cell types using cell size and deformability as distinguishing markers. Furthermore, using differences in dynamic equilibrium positions, we adapted the system to conduct passive, label-free and continuous cell enrichment based on these markers, enabling off-chip sample collection without significant gene expression changes. The presented method has practical potential for high-throughput deformability measurements and cost-effective cell separation to obtain viable target cells of interest in cancer research, immunology, and regenerative medicine.
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paper_title: Inertial microfluidics for continuous particle separation in spiral microchannels.

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In this work we report on a simple inertial microfluidic device that achieves continuous multi-particle separation using the principle of Dean-coupled inertial migration in spiral microchannels. The dominant inertial forces coupled with the Dean rotational force due to the curvilinear microchannel geometry cause particles to occupy a single equilibrium position near the inner microchannel wall. The position at which particles equilibrate is dependent on the ratio of the inertial lift to Dean drag forces. Using this concept, we demonstrate, for the first time, a spiral lab-on-a-chip (LOC) for size-dependent focusing of particles at distinct equilibrium positions across the microchannel cross-section from a multi-particle mixture. The individual particle streams can be collected with an appropriately designed outlet system. To demonstrate this principle, a 5-loop Archimedean spiral microchannel with a fixed width of 500 microm and a height of 130 microm was used to simultaneously and continuously separate 10 microm, 15 microm, and 20 microm polystyrene particles. The device exhibited 90% separation efficiency. The versatility of the device was demonstrated by separating neuroblastoma and glioma cells with 80% efficiency and high relative viability (>90%). The achieved throughput of approximately 1 million cells/min is substantially higher than the sorting rates reported by other microscale sorting methods and is comparable to the rates obtained with commercial macroscale flow cytometry techniques. The simple planar structure and high throughput offered by this passive microfluidic approach make it attractive for LOC devices in biomedical and environmental applications.
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paper_title: Cellular enrichment through microfluidic fractionation based on cell biomechanical properties

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The biomechanical properties of populations of diseased cells are shown to have differences from healthy populations of cells, yet the overlap of these biomechanical properties can limit their use in disease cell enrichment and detection. We report a new microfluidic cell enrichment technology that continuously fractionates cells through differences in biomechanical properties, resulting in highly pure cellular subpopulations. Cell fractionation is achieved in a microfluidic channel with an array of diagonal ridges that are designed to segregate biomechanically distinct cells to different locations in the channel. Due to the imposition of elastic and viscous forces during cellular compression, which are a function of cell biomechanical properties including size and viscoelasticity, larger, stiffer and less viscos cells migrate parallel to the diagonal ridges and exhibit positive lateral displacement. On the other hand, smaller, softer and more viscous cells migrate perpendicular to the diagonal ridges due to circulatory flow induced by the ridges and result in negative lateral displacement. Multiple outlets are then utilized to collect cells with finer gradation of differences in cell biomechanical properties. The result is that cell fractionation dramatically improves cell separation efficiency compared to binary outputs and enables the measurement of subtle biomechanical differences within a single cell type. As a proof-of-concept demonstration, we mix two different leukemia cell lines (K562 and HL60) and utilize cell fractionation to achieve over 45-fold enhancement of cell populations, with high purity cellular enrichment (90% to 99%) of each cell line. In addition, we demonstrate cell fractionation of a single cell type (K562 cells) into subpopulations and characterize the variations of biomechanical properties of the separated cells with atomic force microscopy. These results will be beneficial to obtaining label-free separation of cellular mixtures, or to better investigate the origins of biomechanical differences in a single cell type.
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paper_title: A tunable micro filter modulated by pneumatic pressure for cell separation

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Abstract This study reports a new microfluidic-based filter for size-tunable separation of microbeads or cells. The filtration separation mechanism is based on the pneumatically tunable deformation of poly-dimethylsiloxane (PDMS) membranes, which block the fluid channel with a varied degree. This defines the dimensions of the open area of the fluid channel and thus determines the maximum diameter the microbeads or cells which can pass through. The proposed device incorporates pneumatic micropumps for automatic liquid handling. Another unique feature of this filter is an unclogging mechanism using a back-flush operating mode, by which a reverse-directional flow is utilized to flush the clogged filter zone. The separation performance of the proposed device has been experimentally evaluated. Results show that this developed device is able to provide precise size-dependent filtration, with a high passage efficiency (82–89%) for microbeads with sizes smaller than the defined void space in the filter zone. Also, the proposed separation mechanism is also capable of providing a reasonable filtration rate (14.9–3.3 μl/min). Furthermore, the separation of chondrocytes from a 30 μl suspension of enzymatically digested tissue is successfully demonstrated, showing an excellent cell passage efficiency of 93% and a cell viability of 96%. The proposed device is therefore capable of performing cell separation in situations where either the harvested specimen is limited or the sample cell content is sparse. It also paves a new route to delicately separate or to isolate cells in a simple and controllable manner.
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paper_title: Microfluidics for flow cytometric analysis of cells and particles.

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This review describes recent developments in microfabricated flow cytometers and related microfluidic devices that can detect, analyze, and sort cells or particles. The high-speed analytical capabilities of flow cytometry depend on the cooperative use of microfluidics, optics and electronics. Along with the improvement of other components, replacement of conventional glass capillary-based fluidics with microfluidic sample handling systems operating in microfabricated structures enables volume- and power-efficient, inexpensive and flexible analysis of particulate samples. In this review, we present various efforts that take advantage of novel microscale flow phenomena and microfabrication techniques to build microfluidic cell analysis systems.
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paper_title: Particle focusing in microfluidic devices

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Focusing particles (both biological and synthetic) into a tight stream is usually a necessary step prior to counting, detecting, and sorting them. The various particle focusing approaches in microfluidic devices may be conveniently classified as sheath flow focusing and sheathless focusing. Sheath flow focusers use one or more sheath fluids to pinch the particle suspension and thus focus the suspended particles. Sheathless focusers typically rely on a force to manipulate particles laterally to their equilibrium positions. This force can be either externally applied or internally induced by channel topology. Therefore, the sheathless particle focusing methods may be further classified as active or passive by the nature of the forces involved. The aim of this article is to introduce and discuss the recent developments in both sheath flow and sheathless particle focusing approaches in microfluidic devices.
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paper_title: Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering

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Polydimethylsiloxane (PDMS) elastomers are extensively used for soft lithographic replication of microstructures in microfluidic and micro-engineering applications. Elastomeric microstructures are commonly required to fulfil an explicit mechanical role and accordingly their mechanical properties can critically affect device performance. The mechanical properties of elastomers are known to vary with both curing and operational temperatures. However, even for the elastomer most commonly employed in microfluidic applications, Sylgard 184, only a very limited range of data exists regarding the variation in mechanical properties of bulk PDMS with curing temperature. We report an investigation of the variation in the mechanical properties of bulk Sylgard 184 with curing temperature, over the range 25 ◦ C to 200 ◦ C. PDMS samples for tensile and compressive testing were fabricated according to ASTM standards. Data obtained indicates variation in mechanical properties due to curing temperature for Young’s modulus of 1.32‐2.97 MPa, ultimate tensile strength of 3.51‐7.65 MPa, compressive modulus of 117.8‐186.9 MPa and ultimate compressive strength of 28.4‐51.7 GPa in a range up to 40% strain and hardness of 44‐54 ShA.
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paper_title: Hydrodynamic filtration for on-chip particle concentration and classification utilizing microfluidics

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We propose here a new method for continuous concentration and classification of particles in microfluidic devices, named hydrodynamic filtration. When a particle is flowing in a microchannel, the center position of the particle cannot be present in a certain distance from sidewalls, which is equal to the particle radius. The proposed method utilizes this fact, and is performed using a microchannel having multiple side branch channels. By withdrawing a small amount of liquid repeatedly from the main stream through the side channels, particles are concentrated and aligned onto the sidewalls. Then the concentrated and aligned particles can be collected according to size through other side channels (selection channels) in the downstream of the microchannel. Therefore, continuous introduction of a particle suspension into the microchannel enables both particle concentration and classification at the same time. In this method, the flow profile inside a precisely fabricated microchannel determines the size limit of the filtered substances. So the filtration can be performed even when the channel widths are much larger than the particle size, without the problem of channel clogging. In this study, concentrations of polymer microspheres with diameters of 1–3 µm were increased 20–50-fold, and they were collected independently according to size. In addition, selective enrichment of leukocytes from blood was successfully performed.
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paper_title: Free Flow Acoustophoresis: Microfluidic-Based Mode of Particle and Cell Separation

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A novel method, free flow acoustophoresis (FFA), capable of continuous separation of mixed particle suspensions into multiple outlet fractions is presented. Acoustic forces are utilized to separate particles based on their size and density. The method is shown to be suitable for both biological and nonbiological suspended particles. The microfluidic separation chips were fabricated using conventional microfabrication methods. Particle separation was accomplished by combining laminar flow with the axial acoustic primary radiation force in an ultrasonic standing wave field. Dissimilar suspended particles flowing through the 350-microm-wide channel were thereby laterally translated to different regions of the laminar flow profile, which was split into multiple outlets for continuous fraction collection. Using four outlets, a mixture of 2-, 5-, 8-, and 10-microm polystyrene particles was separated with between 62 and 94% of each particle size ending up in separate fractions. Using three outlets and three particle sizes (3, 7, and 10 microm) the corresponding results ranged between 76 and 96%. It was also proven possible to separate normally acoustically inseparable particle types by manipulating the density of the suspending medium with cesium chloride. The medium manipulation, in combination with FFA, was further used to enable the fractionation of red cells, platelets, and leukocytes. The results show that free flow acoustophoresis can be used to perform complex separation tasks, thereby offering an alternative to expensive and time-consuming methods currently in use.
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paper_title: Tuneable hydrophoretic separation using elastic deformation of poly(dimethylsiloxane)

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This paper demonstrates a method for tuning elastomeric microchannels for hydrophoretic separation made in poly(dimethylsiloxane) (PDMS). Uniform compressive strain is imposed on the elastomeric microchannel between two acrylic substrates by fastening the bolts. The elastomeric microchannel can change its cross-section during compression, simultaneously tuning the criterion for hydrophoretic ordering. The change of the channel cross-section under compression is studied using a confocal microscope and finite element method (FEM). By pressing the channel for hydrophoretic separation, we achieved tuning of the separation criterion from 7 to 2.5 µm in particle diameter.
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paper_title: Hydrodynamic mechanisms of cell and particle trapping in microfluidics

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Focusing and sorting cells and particles utilizing microfluidic phenomena have been flourishing areas of development in recent years. These processes are largely beneficial in biomedical applications and fundamental studies of cell biology as they provide cost-effective and point-of-care miniaturized diagnostic devices and rare cell enrichment techniques. Due to inherent problems of isolation methods based on the biomarkers and antigens, separation approaches exploiting physical characteristics of cells of interest, such as size, deformability, and electric and magnetic properties, have gained currency in many medical assays. Here, we present an overview of the cell/particle sorting techniques by harnessing intrinsic hydrodynamic effects in microchannels. Our emphasis is on the underlying fluid dynamical mechanisms causing cross stream migration of objects in shear and vortical flows. We also highlight the advantages and drawbacks of each method in terms of throughput, separation efficiency, and cell viability. Finally, we discuss the future research areas for extending the scope of hydrodynamic mechanisms and exploring new physical directions for microfluidic applications.
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paper_title: A particle trapping chip using the wide and uniform slit formed by a deformable membrane and air bubble plugs

paper_content:

We present a high-efficient particle trapping chip, where a wide and uniform slit is formed by a deformable membrane barrier with air bubble plugs. The air bubble plugs remained in the extended microchannel during sample filling process, block the particle passage at the both side ends of the membrane, thus all particles flow through the uniform slit gap. Therefore, high-efficient particle trapping without particle loss can be achieved. In the experimental study using 10.3μm-diameter polystyrene beads, the membrane barrier with the air bubble plugs successfully trapped the injected beads with the trapping efficiency of 100% at the flow rate of 10μl/min, while the barrier without the air bubble plugs showed low efficiency of 20%. The present simple and effective particle trapping device is applicable for the high-efficient bioparticle isolation and recovery in the micro total analysis system.
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paper_title: Tunable Open-Channel Microfluidics on Soft Poly(dimethylsiloxane) (PDMS) Substrates with Sinusoidal Grooves

paper_content:

On soft poly(dimethylsiloxane) (PDMS) substrates with 1D sinusoidal wrinkle patterns, we study the anisotropic wetting behavior and fluidic transport as a function of surface energy and groove geometry. On grooved substrates with a contact angle greater than 90°, liquids form dropletlike morphology, and its contact angle in the direction perpendicular to the grooves is larger than that parallel to the grooves. This wetting anisotropy, for a fixed Young’s contact angle, is found to increase when the grooves become deeper. On substrates with a contact angle smaller than 90° and deep grooves (aspect ratio ≥0.3), liquids form filament-like morphology. When the groove depth is further increased by compressing the PDMS film beyond a threshold value, which depends on the surface wettability, fluid starts imbibing the grooves spontaneously. The dynamics of the liquid imbibition of grooves is studied, and a square-law dependence between the length of the liquid filament and time is found, which obeys Washburn’s la...
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paper_title: Clog-free cell filtration using resettable cell traps

paper_content:

The separation of cells by filtration through microstructured constrictions is limited by clogging and adsorption, which reduce selectivity and prevent the extraction of separated cells. To address this key challenge, we developed a mechanism for simply and reliably adjusting the cross-section of a microfluidic channel to selectively capture cells based on a combination of size and deformability. After a brief holding period, trapped cells can then be released back into flow, and if necessary, extracted for subsequent analysis. Periodically clearing filter constrictions of separated cells greatly improves selectivity and throughput, and minimizes adsorption of cells to the filter microstructure. This mechanism is capable of discriminating cell-sized polystyrene microspheres with <1 μm resolution. Rare cancer cells doped into leukocytes can be enriched ~1800× with ~90% yield despite a significant overlap in size between these cell types. An important characteristic of this process is that contaminant leukocytes are captured by non-specific adsorption and not mechanical constraint, enabling repeated filtration to improve performance. The throughput of this mechanism is 900 000 cells per hour for 32 multiplexed microchannels, or ~1 200 000 cells cm−2 h−1 on a per area basis, which exceeds existing micropore filtration mechanisms by a factor of 20.
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paper_title: Squeeze-chip: a finger-controlled microfluidic flow network device and its application to biochemical assays

paper_content:

We designed and fabricated a novel microfluidic device that can be operated through simple finger squeezing. On-chip microfluidic flow control is enabled through an optimized network of check-valves and squeeze-pumps. The sophisticated flow system can be easily constructed by combining a few key elements. We implemented this device to perform quantitative biochemical assays with no requirement for precision instruments.
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paper_title: Microfluidic applications of magnetic particles for biological analysis and catalysis.

paper_content:

Keywords: Spin-Valve Sensors ; Cell Tracking Velocimetry ; On-A-Chip ; Polymerase-Chain-Reaction ; Total Analysis Systems ; Iron-Oxide Nanoparticles ; Field-Flow Fractionation ; Cross-Coupling Reactions ; Circulating Tumor-Cells ; Mode Magnetophoretic Microseparator Reference LMIS2-ARTICLE-2010-004doi:10.1021/cr9001929View record in Web of Science Record created on 2010-01-20, modified on 2016-08-08
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paper_title: Mechanically switchable wetting on wrinkled elastomers with dual-scale roughness

paper_content:

We report the fabrication of a new superhydrophobic surface with dual-scale roughness by coating silica nanoparticles on a poly(dimethylsiloxane) (PDMS) elastomer bilayer film with micro-scaled ripples. The wetting behavior of the surface can be reversibly tuned by applying a mechanical strain, which induces the change in micro-scale roughness determined by the ripples. The dual-scale roughness promotes the wetting transition of the final dual-structure surface from Wenzel region into the Cassie region, thus, reducing the sliding angle at least three times in comparison to that from the surfaces with single-scale roughness (either from the nanoparticle film or the wrinkled PDMS film). In addition, three-times and fast-response tunability of the sliding angle by applying mechanical strain on this dual-roughness surface is demonstrated.
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paper_title: Microfluidic immunoaffinity separations for bioanalysis.

paper_content:

Microfluidic devices often rely on antibody-antigen interactions as a means of separating analytes of interest from sample matrices. Immunoassays and immunoaffinity separations performed in miniaturized formats offer selective target isolation with minimal reagent consumption and reduced analysis times. The introduction of biological fluids and other complicated matrices often requires sample pretreatment or system modifications for compatibility with small-scale devices. Miniaturization of external equipment facilitates the potential for portable use such as in patient point-of-care settings. Microfluidic immunoaffinity systems including capillary and chip platforms have been assembled from basic instrument components for fluid control, sample introduction, and detection. The current review focuses on the use of immunoaffinity separations in microfluidic devices with an emphasis on pump-based flow and biological sample analysis.
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paper_title: A Mechanically Tunable Microfluidic Cell-Trapping Device.

paper_content:

Abstract Controlled manipulation, such as isolation, positioning, and trapping of cells, is important in basic biological research and clinical diagnostics. Micro/nanotechnologies have been enabling more effective and efficient cell trapping than possible with conventional platforms. Currently available micro/nanoscale methods for cell trapping, however, still lack flexibility in precisely controlling the number of trapped cells. We exploited the large compliance of elastomers to create an array of cell-trapping microstructures, whose dimensions can be mechanically modulated by inducing uniformly distributed strain via application of external force on the chip. The device consists of two elastomer polydimethylsiloxane (PDMS) sheets, one of which bears dam-like, cup-shaped geometries to physically capture cells. The mechanical modulation is used to tune the characteristics of cell trapping to capture a predetermined number of cells, from single cells to multiple cells. Thus, enhanced utility and flexibility for practical applications can be attained, as demonstrated by tunable trapping of MCF-7 cells, a human breast cancer cell line.
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paper_title: What is the Young's Modulus of Silicon?

paper_content:

The Young's modulus (E) of a material is a key parameter for mechanical engineering design. Silicon, the most common single material used in microelectromechanical systems (MEMS), is an anisotropic crystalline material whose material properties depend on orientation relative to the crystal lattice. This fact means that the correct value of E for analyzing two different designs in silicon may differ by up to 45%. However, perhaps, because of the perceived complexity of the subject, many researchers oversimplify silicon elastic behavior and use inaccurate values for design and analysis. This paper presents the best known elasticity data for silicon, both in depth and in a summary form, so that it may be readily accessible to MEMS designers.
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paper_title: Consideration of Nonuniformity in Elongation of Microstructures in a Mechanically Tunable Microfluidic Device for Size-Based Isolation of Microparticles

paper_content:

This paper presents an investigation of the nonlinearity behavior in deformation that appears during the linear stretching of the elastomeric microstructures in a pillar-based microfilter. Determining the impact of the nonuniformity in strain on the geometry and performance of such a planar device under in-plane stretch is the motivation for this study. A semiempirical model is used to explain the physical strain–stress behavior from the root to the tip of the micropillars in the linear arrays in the device. For microfabrication of the device, the main substrate is elastomeric polyurethane methacrylate, which is utilized in an ultraviolet-molding method. Optical imaging and scanning electron microscopy were used to evaluate the deformation of the microstructures under different loading conditions. It was demonstrated that by applying mechanical strains of $\Delta L/L_{o})$ on the elastomeric device using a modified syringe pump, the spacing of the pillars is increased effectively to about three times the size of the initial setting of 5.5 $\mu $ m, which corresponds to a strain of above 180% in the absence of nonuniformity effects. This simple yet interesting behavior can be exploited to rapidly adjust a microfluidic device for application to the separation of microbeads or blood cells, which would normally require the geometrical redesign and fabrication of a new device. [2014-0213]
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paper_title: A pillar-based microfilter for isolation of white blood cells on elastomeric substrate

paper_content:

Our goal is to design, fabricate, and characterize a pillar-based microfluidic device for size-based separation of human blood cells on an elastomeric substrate with application in the low-cost rapid prototyping of lab-chip devices. The single inlet single outlet device is using parallel U-shape arrays of pillars with cutoff size of 5.5 μm for trapping white blood cells (WBCs) in a pillar chamber with internal dead-volume of less than 1.0 μl. The microstructures are designed to limit the elastomeric deformation against fluid pressures. Numerical analysis showed that at maximum pressure loss of 15 kPa which is lower than the device conformal bonding strength, the pillar elastomeric deformation is less than 5% for flow rates of up to 1.0 ml min−1. Molding technique was employed for device prototyping using polyurethane methacrylate (PUMA) resin and polydimethylsiloxane (PDMS) mold. Characterization of the dual-layer device with beads and blood samples is performed. Tests with blood injection showed that ∼18%–25% of WBCs are trapped and ∼84%–89% of red blood cells (RBCs) are passed at flow rates of 15–50 μl min−1 with a slight decrease of WBCs trap and improve of the RBCs pass at higher flow rates. Similar results were obtained by separation of mixed microspheres of different size injected at flow rates of up to 400 μl min−1. Tests with blood samples stained by fluorescent gel demonstrated that the WBCs are accumulated in the arrays of pillars that later end up to blockage of the device. Filtration results of using elastomeric substrate present a good consistency with the trend of separation efficiencies of the similar silicon-based filters.
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paper_title: A new UV-curing elastomeric substrate for rapid prototyping of microfluidic devices

paper_content:

Rapid prototyping in the design cycle of new microfluidic devices is very important for shortening time-to-market. Researchers are facing the challenge to explore new and suitable substrates with simple and efficient microfabrication techniques. In this paper, we introduce and characterize a UV-curing elastomeric polyurethane methacrylate (PUMA) for rapid prototyping of microfluidic devices. The swelling and solubility of PUMA in different chemicals is determined. Time-dependent measurements of water contact angle show that the native PUMA is hydrophilic without surface treatment. The current monitoring method is used for measurement of the electroosmotic flow mobility in the microchannels made from PUMA. The optical, physical, thermal and mechanical properties of PUMA are evaluated. The UV-lithography and molding process is used for making micropillars and deep channel microfluidic structures integrated to the supporting base layer. Spin coating is characterized for producing different layer thicknesses of PUMA resin. A device is fabricated and tested for examining the strength of different bonding techniques such as conformal, corona treating and semi-curing of two PUMA layers in microfluidic application and the results show that the bonding strengths are comparable to that of PDMS. We also report fabrication and testing of a three-layer multi inlet/outlet microfluidic device including a very effective fluidic interconnect for application demonstration of PUMA as a promising new substrate. A simple micro-device is developed and employed for observing the pressure deflection of membrane made from PUMA as a very effective elastomeric valve in microfluidic devices. (Some figures may appear in colour only in the online journal)
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paper_title: Construction of Stess-Strain Curves for Brittle Materials by Indentation in a Wide Temperature Range

paper_content:

A test method procedure for constructing stress-strain curves by indentation of brittle and low plastic materials under temperature ranging from 20 to 900°C was developed recently by Yu. Milman, B. Galanov et al. According to this test method procedure stress-strain curves σ - Є for Si, Ge, SiC, TiB2 and WC/Co hard alloy were constructed in the above temperature region and mechanical parameters such as elastic point, σe, yield stress, σs, etc. were extracted by using the measurement results obtained by a set of trihedral pyramid indenters with different angles at the tip, γ1, ranging from 45 to 85°C.
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paper_title: Poisson's ratio and modern materials

paper_content:

Poisson's ratio describes the resistance of a material to distort under mechanical load rather than to alter in volume. On the bicentenary of the publication of Poisson's Traite de Mecanique, the continuing relevance of Poisson's ratio in the understanding of modern materials is reviewed.
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paper_title: Rapid prototyping polymers for microfluidic devices and high pressure injections.

paper_content:

Multiple methods of fabrication exist for microfluidic devices, with different advantages depending on the end goal of industrial mass production or rapid prototyping for the research laboratory. Polydimethylsiloxane (PDMS) has been the mainstay for rapid prototyping in the academic microfluidics community, because of its low cost, robustness and straightforward fabrication, which are particularly advantageous in the exploratory stages of research. However, despite its many advantages and its broad use in academic laboratories, its low elastic modulus becomes a significant issue for high pressure operation as it leads to a large alteration of channel geometry. Among other consequences, such deformation makes it difficult to accurately predict the flow rates in complex microfluidic networks, change flow speed quickly for applications in stop-flow lithography, or to have predictable inertial focusing positions for cytometry applications where an accurate alignment of the optical system is critical. Recently, other polymers have been identified as complementary to PDMS, with similar fabrication procedures being characteristic of rapid prototyping but with higher rigidity and better resistance to solvents; Thermoset Polyester (TPE), Polyurethane Methacrylate (PUMA) and Norland Adhesive 81 (NOA81). In this review, we assess these different polymer alternatives to PDMS for rapid prototyping, especially in view of high pressure injections with the specific example of inertial flow conditions. These materials are compared to PDMS, for which magnitudes of deformation and dynamic characteristics are also characterized. We provide a complete and systematic analysis of these materials with side-by-side experiments conducted in our lab that also evaluate other properties, such as biocompatibility, solvent compatibility, and ease of fabrication. We emphasize that these polymer alternatives, TPE, PUMA and NOA, have some considerable strengths for rapid prototyping when bond strength, predictable operation at high pressure, or transitioning to commercialization are considered important for the application.
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paper_title: Stress-Strain Response of PMMA as a Function of Strain-Rate and Temperature

paper_content:

The strain rate response of PMMA was investigated under uniaxial compression at room temperature at strain-rates ranging from 0.0001/sec to about 4300/sec. In addition, the temperature response of PMMA was investigated at strain-rates of 1/sec and 0.001/sec at temperatures ranging from 0°C to 115°C (below Tg). High rate experiments at room temperature (greater than 1/sec rates) were conducted using a split-Hopkinson Pressure bar (SHPB) with pulse-shaping. This is necessary to induce a compressive loading on the specimen at a constant strain rate to achieve dynamic stress equilibrium. Results conducted at room temperature show that PMMA is strain rate sensitive from quasi-static to dynamic loading. Additionally, the stress-strain response exhibits a decrease in the flow stress with an increase in temperature. These experimental data are being used to develop constitutive behavior models of PMMA.
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paper_title: Mechanical properties of silicones for MEMS

paper_content:

This paper focuses on the mechanical properties of polydimethylsiloxane (PDMS) relevant for microelectromechanical system (MEMS) applications. In view of the limited amount of published data, we analyzed the two products most commonly used in MEMS, namely RTV 615 from Bayer Silicones and Sylgard 184 from Dow Corning. With regard to mechanical properties, we focused on the dependence of the elastic modulus on the thinner concentration, temperature and strain rate. In addition, creep and thermal aging were analyzed. We conclude that the isotropic and constant elastic modulus has strong dependence on the hardening conditions. At high hardening temperatures and long hardening time, RTV 615 displays an elastic modulus of 1.91 MPa and Sylgard 184 of 2.60 MPa in a range up to 40% strain.
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paper_title: Low cost prototyping of microfluidic structure

paper_content:

An elastomeric microfluidic structure was developed by polymerization of a liquid photopolymer using UV lithography process. Integration of different components of a microfluidic device including microchannel, micromixer, and microchamber, on one layer and by using one photomask is practiced. More complicated geometries and 2-layer chip was examined in this research and gives promising results. This process enables well control over the thicknesses of the layers, and dimensions of the components considering. The proposed simple process benefits from low cost in development time and expenses of the materials.
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paper_title: Design of sealed cavity microstructures formed by silicon wafer bonding

paper_content:

Three fabrication issues related to the design and fabrication of micromechanical devices using sealed cavities within bonded silicon wafers are discussed. The first concerns the resultant residual gas pressure within a sealed cavity between two bonded wafers after bonding and a high-temperature anneal. The second concerns the prediction of plastic deformation in capping layers of single-crystal silicon over sealed cavities. Exposure of sealed cavity structures to a high-temperature environment causes the trapped residual gas to expand, which can result in the plastic deformation of the capping layer. A model for analytically predicting the occurrence of plastic deformatio in these silicon capping layers has been developed. The third fabrication issue concerns the prediction of the resultant height of plastically deformed capping layers of silicon after cooling. A model which gives a lower and an upper bound on the height, based on an analytical spherical shell membrane stress equation, has been developed. >
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paper_title: Exploiting the Oxygen Inhibitory Effect on UV Curing in Microfabrication: A Modified Lithography Technique

paper_content:

Rapid prototyping (RP) of microfluidic channels in liquid photopolymers using standard lithography (SL) involves multiple deposition steps and curing by ultraviolet (UV) light for the construction of a microstructure layer. In this work, the conflicting effect of oxygen diffusion and UV curing of liquid polyurethane methacrylate (PUMA) is investigated in microfabrication and utilized to reduce the deposition steps and to obtain a monolithic product. The conventional fabrication process is altered to control for the best use of the oxygen presence in polymerization. A novel and modified lithography technique is introduced in which a single step of PUMA coating and two steps of UV exposure are used to create a microchannel. The first exposure is maskless and incorporates oxygen diffusion into PUMA for inhibition of the polymerization of a thin layer from the top surface while the UV rays penetrate the photopolymer. The second exposure is for transferring the patterns of the microfluidic channels from the contact photomask onto the uncured material. The UV curing of PUMA as the main substrate in the presence of oxygen is characterized analytically and experimentally. A few typical elastomeric microstructures are manufactured. It is demonstrated that the obtained heights of the fabricated structures in PUMA are associated with the oxygen concentration and the UV dose. The proposed technique is promising for the RP of molds and microfluidic channels in terms of shorter processing time, fewer fabrication steps and creation of microstructure layers with higher integrity.
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paper_title: A new USP Class VI-compliant substrate for manufacturing disposable microfluidic devices.

paper_content:

As microfluidic systems transition from research tools to disposable clinical-diagnostic devices, new substrate materials are needed to meet both the regulatory requirement as well as the economics of disposable devices. This paper introduces a UV-curable polyurethane-methacrylate (PUMA) substrate that has been qualified for medical use and meets all of the challenges of manufacturing microfluidic devices. PUMA is optically transparent, biocompatible, and exhibits high electroosmotic mobility without surface modification. We report two production processes that are compatible with the existing methods of rapid prototyping and present characterizations of the resultant PUMA microfluidic devices.
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paper_title: What is the Young's Modulus of Silicon?

paper_content:

The Young's modulus (E) of a material is a key parameter for mechanical engineering design. Silicon, the most common single material used in microelectromechanical systems (MEMS), is an anisotropic crystalline material whose material properties depend on orientation relative to the crystal lattice. This fact means that the correct value of E for analyzing two different designs in silicon may differ by up to 45%. However, perhaps, because of the perceived complexity of the subject, many researchers oversimplify silicon elastic behavior and use inaccurate values for design and analysis. This paper presents the best known elasticity data for silicon, both in depth and in a summary form, so that it may be readily accessible to MEMS designers.
---
paper_title: Consideration of Nonuniformity in Elongation of Microstructures in a Mechanically Tunable Microfluidic Device for Size-Based Isolation of Microparticles

paper_content:

This paper presents an investigation of the nonlinearity behavior in deformation that appears during the linear stretching of the elastomeric microstructures in a pillar-based microfilter. Determining the impact of the nonuniformity in strain on the geometry and performance of such a planar device under in-plane stretch is the motivation for this study. A semiempirical model is used to explain the physical strain–stress behavior from the root to the tip of the micropillars in the linear arrays in the device. For microfabrication of the device, the main substrate is elastomeric polyurethane methacrylate, which is utilized in an ultraviolet-molding method. Optical imaging and scanning electron microscopy were used to evaluate the deformation of the microstructures under different loading conditions. It was demonstrated that by applying mechanical strains of $\Delta L/L_{o})$ on the elastomeric device using a modified syringe pump, the spacing of the pillars is increased effectively to about three times the size of the initial setting of 5.5 $\mu $ m, which corresponds to a strain of above 180% in the absence of nonuniformity effects. This simple yet interesting behavior can be exploited to rapidly adjust a microfluidic device for application to the separation of microbeads or blood cells, which would normally require the geometrical redesign and fabrication of a new device. [2014-0213]
---
paper_title: A tunable microfluidic-based filter modulated by pneumatic pressure for separation of blood cells

paper_content:

This article presents a new microfluidic-based filter for the separation of microbeads or blood cells with a high filtration rate. The device was composed of a circular micropump for automatic liquid transport, and a normally closed valve located at the filter zone for separation of beads or cells. The filtration mechanism was based on the tunable deformation of polydimethylsiloxane (PDMS) membranes that defined the gap between a floating block structure and the substrate which determined the maximum diameter of the beads/cells that can pass through the filter. Another unique feature of this filter is an unclogging mechanism using a suction force, resulting in a back flow to remove any trapped beads/cells in the filter zone when the PDMS membrane was restored to its initial state. The separation performance of the proposed device was first experimentally evaluated by using microbeads. The results showed that this device was capable of providing size-tunable filtration with a high recovery efficiency (95.25–96.21%) for microbeads with sizes smaller than the defined gap in the filter zone. Furthermore, the proposed device was also capable of performing separation of blood cells and blood plasma from human whole blood. Experimental results showed that an optimum filtration rate of 21.40 and 3.00 μl/min correspond to high recovery efficiencies of 86.69 and 80.66%, respectively, for red blood cells (RBCs) and blood plasma. The separation method developed in this work could be used for various point-of-care diagnostic applications involving separation of plasma and blood cells.
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paper_title: Tuneable separation in elastomeric microfluidics devices.

paper_content:

We describe how the elastomeric properties of PDMS (polydimethylsiloxane) can be utilised to achieve tuneable particle separation in Deterministic Lateral Displacement devices via strain controlled alteration of inter-obstacle distances, a development that opens up new avenues toward more effective separation of particles in microfluidics devices.
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paper_title: Development and multiplexed control of latching pneumatic valves using microfluidic logical structures.

paper_content:

Novel latching microfluidic valve structures are developed, characterized, and controlled independently using an on-chip pneumatic demultiplexer. These structures are based on pneumatic monolithic membrane valves and depend upon their normally-closed nature. Latching valves consisting of both three- and four-valve circuits are demonstrated. Vacuum or pressure pulses as short as 120 ms are adequate to hold these latching valves open or closed for several minutes. In addition, an on-chip demultiplexer is demonstrated that requires only n pneumatic inputs to control 2(n-1) independent latching valves. These structures can reduce the size, power consumption, and cost of microfluidic analysis devices by decreasing the number of off-chip controllers. Since these valve assemblies can form the standard logic gates familiar in electronic circuit design, they should be useful in developing complex pneumatic circuits.
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paper_title: A tunable micro filter modulated by pneumatic pressure for cell separation

paper_content:

Abstract This study reports a new microfluidic-based filter for size-tunable separation of microbeads or cells. The filtration separation mechanism is based on the pneumatically tunable deformation of poly-dimethylsiloxane (PDMS) membranes, which block the fluid channel with a varied degree. This defines the dimensions of the open area of the fluid channel and thus determines the maximum diameter the microbeads or cells which can pass through. The proposed device incorporates pneumatic micropumps for automatic liquid handling. Another unique feature of this filter is an unclogging mechanism using a back-flush operating mode, by which a reverse-directional flow is utilized to flush the clogged filter zone. The separation performance of the proposed device has been experimentally evaluated. Results show that this developed device is able to provide precise size-dependent filtration, with a high passage efficiency (82–89%) for microbeads with sizes smaller than the defined void space in the filter zone. Also, the proposed separation mechanism is also capable of providing a reasonable filtration rate (14.9–3.3 μl/min). Furthermore, the separation of chondrocytes from a 30 μl suspension of enzymatically digested tissue is successfully demonstrated, showing an excellent cell passage efficiency of 93% and a cell viability of 96%. The proposed device is therefore capable of performing cell separation in situations where either the harvested specimen is limited or the sample cell content is sparse. It also paves a new route to delicately separate or to isolate cells in a simple and controllable manner.
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paper_title: Dynamic trapping and high-throughput patterning of cells using pneumatic microstructures in an integrated microfluidic device.

paper_content:

Microfluidic trapping methods create significant opportunities to establish highly controlled cell positioning and arrangement for the microscale study of numerous cellular physiological and pathological activities. However, a simple, straightforward, dynamic, and high-throughput method for cell trapping is not yet well established. In the present paper, we report a direct active trapping method using an integrated microfluidic device with pneumatic microstructures (PμSs) for both operationally and quantitatively dynamic localization of cells, as well as for high-throughput cell patterning. We designed and fabricated U-shape PμS arrays to replace the conventional fixed microstructures for reversible trapping. Multidimensional dynamics and spatial consistency of the PμSs were optically characterized and quantitatively demonstrated. Furthermore, we performed a systematic trapping investigation of the PμSs actuated at a pressure range of 0 psi to 20 psi using three types of popularly applied mammalian cells, namely, human lung adenocarcinoma A549 cells, human hepatocellular liver carcinoma HepG2 cells, and human breast adenocarcinoma MCF-7 cells. The cells were quantitatively trapped and controlled by the U-shape PμSs in a programmatic and parallel manner, and could be opportunely released. The trapped cells with high viability were hydrodynamically protected by the real-time actuation of specifically designed umbrella-like PμSs. We demonstrate that PμSs can be applied as an active microfluidic component for large-scale cell patterning and manipulation, which could be useful in many cell-based tissue organization, immunosensor, and high-throughput imaging and screening.
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paper_title: Tuneable hydrophoretic separation using elastic deformation of poly(dimethylsiloxane)

paper_content:

This paper demonstrates a method for tuning elastomeric microchannels for hydrophoretic separation made in poly(dimethylsiloxane) (PDMS). Uniform compressive strain is imposed on the elastomeric microchannel between two acrylic substrates by fastening the bolts. The elastomeric microchannel can change its cross-section during compression, simultaneously tuning the criterion for hydrophoretic ordering. The change of the channel cross-section under compression is studied using a confocal microscope and finite element method (FEM). By pressing the channel for hydrophoretic separation, we achieved tuning of the separation criterion from 7 to 2.5 µm in particle diameter.
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paper_title: A particle trapping chip using the wide and uniform slit formed by a deformable membrane and air bubble plugs

paper_content:

We present a high-efficient particle trapping chip, where a wide and uniform slit is formed by a deformable membrane barrier with air bubble plugs. The air bubble plugs remained in the extended microchannel during sample filling process, block the particle passage at the both side ends of the membrane, thus all particles flow through the uniform slit gap. Therefore, high-efficient particle trapping without particle loss can be achieved. In the experimental study using 10.3μm-diameter polystyrene beads, the membrane barrier with the air bubble plugs successfully trapped the injected beads with the trapping efficiency of 100% at the flow rate of 10μl/min, while the barrier without the air bubble plugs showed low efficiency of 20%. The present simple and effective particle trapping device is applicable for the high-efficient bioparticle isolation and recovery in the micro total analysis system.
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paper_title: A Dynamic Microarray with Pneumatic Valves for Selective Trapping and Releasing of Microbeads

paper_content:

This paper describes a dynamic microarray device with pneumatic valves for trapping and releasing microbeads selectively. We fabricated thin membranes of polydimethylsiloxane (PDMS) inside our conventional dynamic microfluidic device. The membrane works as a pneumatically driven valve that changes the fluidic resistance, which ultimately determines the modes of the device: trapping, passing, or releasing. Using this device, we successfully controlled these three modes by using 100 ?m polystyrene microbeads. Moreover, we succeeded in arraying two different types of microbeads alternately in a single device by selectively activating and deactivating the pneumatic valves.
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paper_title: Consideration of Nonuniformity in Elongation of Microstructures in a Mechanically Tunable Microfluidic Device for Size-Based Isolation of Microparticles

paper_content:

This paper presents an investigation of the nonlinearity behavior in deformation that appears during the linear stretching of the elastomeric microstructures in a pillar-based microfilter. Determining the impact of the nonuniformity in strain on the geometry and performance of such a planar device under in-plane stretch is the motivation for this study. A semiempirical model is used to explain the physical strain–stress behavior from the root to the tip of the micropillars in the linear arrays in the device. For microfabrication of the device, the main substrate is elastomeric polyurethane methacrylate, which is utilized in an ultraviolet-molding method. Optical imaging and scanning electron microscopy were used to evaluate the deformation of the microstructures under different loading conditions. It was demonstrated that by applying mechanical strains of $\Delta L/L_{o})$ on the elastomeric device using a modified syringe pump, the spacing of the pillars is increased effectively to about three times the size of the initial setting of 5.5 $\mu $ m, which corresponds to a strain of above 180% in the absence of nonuniformity effects. This simple yet interesting behavior can be exploited to rapidly adjust a microfluidic device for application to the separation of microbeads or blood cells, which would normally require the geometrical redesign and fabrication of a new device. [2014-0213]
---
paper_title: Tuneable separation in elastomeric microfluidics devices.

paper_content:

We describe how the elastomeric properties of PDMS (polydimethylsiloxane) can be utilised to achieve tuneable particle separation in Deterministic Lateral Displacement devices via strain controlled alteration of inter-obstacle distances, a development that opens up new avenues toward more effective separation of particles in microfluidics devices.
---
paper_title: A microfluidic-based hydrodynamic trap: design and implementation.

paper_content:

We report an integrated microfluidic device for fine-scale manipulation and confinement of micro- and nanoscale particles in free-solution. Using this device, single particles are trapped in a stagnation point flow at the junction of two intersecting microchannels. The hydrodynamic trap is based on active flow control at a fluid stagnation point using an integrated on-chip valve in a monolithic PDMS-based microfluidic device. In this work, we characterize device design parameters enabling precise control of stagnation point position for efficient trap performance. The microfluidic-based hydrodynamic trap facilitates particle trapping using the sole action of fluid flow and provides a viable alternative to existing confinement and manipulation techniques based on electric, optical, magnetic or acoustic force fields. Overall, the hydrodynamic trap enables non-contact confinement of fluorescent and non-fluorescent particles for extended times and provides a new platform for fundamental studies in biology, biotechnology and materials science.
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paper_title: Hydrodynamic trap-and-release of single particles using dual-function elastomeric valves: design, fabrication, and characterization

paper_content:

This paper introduces a simple method for trapping and releasing single particles, such as microbeads and living cells, using dual-function elastomeric valves. Our key technique is the utilization of the elastomeric valve as a dual-function removable trap instead of a fixed trap and a separate component for releasing trapped particles, thereby enabling a simple yet effective trap-and-release of particles. We designed, fabricated, and characterized a microfluidic-based device for trapping and releasing single beads by controlling elastomeric valves driven by pneumatic pressure and a fluid flow action. The fluid flow is controlled to ensure that beads flowing in a main stream enter into a branch channel. A bead is trapped by deflected elastomeric valves positioned at the entrance of a branch channel. The trapped bead is easily released by removing the applied pressure. The trapping and releasing of single beads of 21 μm in diameter were successfully performed under an optimized pressure and flow rate ratio. Moreover, we confirmed that continuous trapping and releasing of single beads by repeatedly switching elastomeric valves enables the collection of a controllable number of beads. Our simple method can be integrated into microfluidic systems that require single or multiple particle arrays for quantitative and high-throughput assays in applications within the fields of biology and chemistry.
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paper_title: Hydrodynamic mechanisms of cell and particle trapping in microfluidics

paper_content:

Focusing and sorting cells and particles utilizing microfluidic phenomena have been flourishing areas of development in recent years. These processes are largely beneficial in biomedical applications and fundamental studies of cell biology as they provide cost-effective and point-of-care miniaturized diagnostic devices and rare cell enrichment techniques. Due to inherent problems of isolation methods based on the biomarkers and antigens, separation approaches exploiting physical characteristics of cells of interest, such as size, deformability, and electric and magnetic properties, have gained currency in many medical assays. Here, we present an overview of the cell/particle sorting techniques by harnessing intrinsic hydrodynamic effects in microchannels. Our emphasis is on the underlying fluid dynamical mechanisms causing cross stream migration of objects in shear and vortical flows. We also highlight the advantages and drawbacks of each method in terms of throughput, separation efficiency, and cell viability. Finally, we discuss the future research areas for extending the scope of hydrodynamic mechanisms and exploring new physical directions for microfluidic applications.
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paper_title: Consideration of Nonuniformity in Elongation of Microstructures in a Mechanically Tunable Microfluidic Device for Size-Based Isolation of Microparticles

paper_content:

This paper presents an investigation of the nonlinearity behavior in deformation that appears during the linear stretching of the elastomeric microstructures in a pillar-based microfilter. Determining the impact of the nonuniformity in strain on the geometry and performance of such a planar device under in-plane stretch is the motivation for this study. A semiempirical model is used to explain the physical strain–stress behavior from the root to the tip of the micropillars in the linear arrays in the device. For microfabrication of the device, the main substrate is elastomeric polyurethane methacrylate, which is utilized in an ultraviolet-molding method. Optical imaging and scanning electron microscopy were used to evaluate the deformation of the microstructures under different loading conditions. It was demonstrated that by applying mechanical strains of $\Delta L/L_{o})$ on the elastomeric device using a modified syringe pump, the spacing of the pillars is increased effectively to about three times the size of the initial setting of 5.5 $\mu $ m, which corresponds to a strain of above 180% in the absence of nonuniformity effects. This simple yet interesting behavior can be exploited to rapidly adjust a microfluidic device for application to the separation of microbeads or blood cells, which would normally require the geometrical redesign and fabrication of a new device. [2014-0213]
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paper_title: A technique of optimization of microfiltration using a tunable platform

paper_content:

The optimum efficiency of size-based filtration in microfluidic devices is highly dependent on characteristics of design, deformability of microparticles/cells, and fluid flow. The effects of filter pores and flow rate, which are the two major determining and related factors of characterization in the separation of particles and cells are investigated in this work. An elastomeric microfluidic device consisting of parallel arrays of pillars with mechanically tunable spacings is employed as an adjustable microfiltration platform. The tunable filtration system is used for finding the best conditions of separation of solid microbeads or deformable blood cells in a crossflow pillar-based method. It is demonstrated that increasing flow rate in the range of 1.0–80.0 µl min−1 has an adverse effect on the device performance in terms of decreased separation efficiency of deformable blood cells. However, by tuning the gap size in the range of 2.5–7.5 µm, the selectivity of the separation is controlled from about 5.0 to 95.0% for white blood cells (WBCs) and 40.0 to 95.0% for red blood cells (RBCs). Finally, the best range of trapping and passing efficiencies of ~70–80.0% simultaneously for WBCs and RBCs in whole blood sample is achieved at optimum gap size of ~3.5–4.0 µm.
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paper_title: A pillar-based microfilter for isolation of white blood cells on elastomeric substrate

paper_content:

Our goal is to design, fabricate, and characterize a pillar-based microfluidic device for size-based separation of human blood cells on an elastomeric substrate with application in the low-cost rapid prototyping of lab-chip devices. The single inlet single outlet device is using parallel U-shape arrays of pillars with cutoff size of 5.5 μm for trapping white blood cells (WBCs) in a pillar chamber with internal dead-volume of less than 1.0 μl. The microstructures are designed to limit the elastomeric deformation against fluid pressures. Numerical analysis showed that at maximum pressure loss of 15 kPa which is lower than the device conformal bonding strength, the pillar elastomeric deformation is less than 5% for flow rates of up to 1.0 ml min−1. Molding technique was employed for device prototyping using polyurethane methacrylate (PUMA) resin and polydimethylsiloxane (PDMS) mold. Characterization of the dual-layer device with beads and blood samples is performed. Tests with blood injection showed that ∼18%–25% of WBCs are trapped and ∼84%–89% of red blood cells (RBCs) are passed at flow rates of 15–50 μl min−1 with a slight decrease of WBCs trap and improve of the RBCs pass at higher flow rates. Similar results were obtained by separation of mixed microspheres of different size injected at flow rates of up to 400 μl min−1. Tests with blood samples stained by fluorescent gel demonstrated that the WBCs are accumulated in the arrays of pillars that later end up to blockage of the device. Filtration results of using elastomeric substrate present a good consistency with the trend of separation efficiencies of the similar silicon-based filters.
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paper_title: A Mechanically Tunable Microfluidic Cell-Trapping Device.

paper_content:

Abstract Controlled manipulation, such as isolation, positioning, and trapping of cells, is important in basic biological research and clinical diagnostics. Micro/nanotechnologies have been enabling more effective and efficient cell trapping than possible with conventional platforms. Currently available micro/nanoscale methods for cell trapping, however, still lack flexibility in precisely controlling the number of trapped cells. We exploited the large compliance of elastomers to create an array of cell-trapping microstructures, whose dimensions can be mechanically modulated by inducing uniformly distributed strain via application of external force on the chip. The device consists of two elastomer polydimethylsiloxane (PDMS) sheets, one of which bears dam-like, cup-shaped geometries to physically capture cells. The mechanical modulation is used to tune the characteristics of cell trapping to capture a predetermined number of cells, from single cells to multiple cells. Thus, enhanced utility and flexibility for practical applications can be attained, as demonstrated by tunable trapping of MCF-7 cells, a human breast cancer cell line.
---
paper_title: Tuneable separation in elastomeric microfluidics devices.

paper_content:

We describe how the elastomeric properties of PDMS (polydimethylsiloxane) can be utilised to achieve tuneable particle separation in Deterministic Lateral Displacement devices via strain controlled alteration of inter-obstacle distances, a development that opens up new avenues toward more effective separation of particles in microfluidics devices.
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paper_title: Continuous Particle Separation Through Deterministic Lateral Displacement

paper_content:

We report on a microfluidic particle-separation device that makes use of the asymmetric bifurcation of laminar flow around obstacles. A particle chooses its path deterministically on the basis of its size. All particles of a given size follow equivalent migration paths, leading to high resolution. The microspheres of 0.8, 0.9, and 1.0 micrometers that were used to characterize the device were sorted in 40seconds with a resolution of ∼10nanometers, which was better than the time and resolution of conventional flow techniques. Bacterial artificial chromosomes could be separated in 10 minutes with a resolution of ∼12%.
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paper_title: Tuneable hydrophoretic separation using elastic deformation of poly(dimethylsiloxane)

paper_content:

This paper demonstrates a method for tuning elastomeric microchannels for hydrophoretic separation made in poly(dimethylsiloxane) (PDMS). Uniform compressive strain is imposed on the elastomeric microchannel between two acrylic substrates by fastening the bolts. The elastomeric microchannel can change its cross-section during compression, simultaneously tuning the criterion for hydrophoretic ordering. The change of the channel cross-section under compression is studied using a confocal microscope and finite element method (FEM). By pressing the channel for hydrophoretic separation, we achieved tuning of the separation criterion from 7 to 2.5 µm in particle diameter.
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paper_title: Single-Cell Enzyme Concentrations, Kinetics, and Inhibition Analysis Using High-Density Hydrodynamic Cell Isolation Arrays

paper_content:

High-quality single-cell data are required for a quantitative systems biology description of cellular function. However, data of this type are difficult and time-consuming to collect using traditional techniques. We present a robust and simple microfluidic method for trapping single cells in large arrays to address this problem. Ordered single-cell isolation arrays allow for high-density microscopic analysis with simplified image processing. Moreover, for fluorescent assays, on-chip sample preparation (e.g., fluorescent labeling, washing) can be performed, as opposed to manual intensive operations of incubation, centrifugation, and resuspension in previous techniquessaving time and reagents. This technology was applied to determine novel single-cell enzyme kinetics for three different cell types (HeLa, 293T, Jurkat). A kinetic model of this process predicted this varied response was due to variation in the concentration of carboxylesterase between cell types. Nordihydroguaiaretic acid (NDGA) was also char...
---
paper_title: A Mechanically Tunable Microfluidic Cell-Trapping Device.

paper_content:

Abstract Controlled manipulation, such as isolation, positioning, and trapping of cells, is important in basic biological research and clinical diagnostics. Micro/nanotechnologies have been enabling more effective and efficient cell trapping than possible with conventional platforms. Currently available micro/nanoscale methods for cell trapping, however, still lack flexibility in precisely controlling the number of trapped cells. We exploited the large compliance of elastomers to create an array of cell-trapping microstructures, whose dimensions can be mechanically modulated by inducing uniformly distributed strain via application of external force on the chip. The device consists of two elastomer polydimethylsiloxane (PDMS) sheets, one of which bears dam-like, cup-shaped geometries to physically capture cells. The mechanical modulation is used to tune the characteristics of cell trapping to capture a predetermined number of cells, from single cells to multiple cells. Thus, enhanced utility and flexibility for practical applications can be attained, as demonstrated by tunable trapping of MCF-7 cells, a human breast cancer cell line.
---
paper_title: Continuous blood cell separation by hydrophoretic filtration.

paper_content:

We propose a new hydrophoretic method for continuous blood cell separation using a microfluidic device composed of slanted obstacles and filtration obstacles. The slanted obstacles have a larger height and gap than the particles in order to focus them to a sidewall by hydrophoresis. In the successive structure, the height and gap of the filtration obstacles with a filtration pore are set between the diameters of small and large particles, which defines the critical separation diameter. Accordingly, the particles smaller than the criterion freely pass through the gap and keep their focused position. In contrast, the particles larger than the criterion collide against the filtration obstacle and move into the filtration pore. The microfluidic device was characterized with polystyrene beads with a minimum diameter difference of 7.3%. We completely separated polystyrene microbeads of 9 and 12 microm diameter with a separation resolution of approximately 6.2. This resolution is increased by 6.4-fold compared with our previous separation method based on hydrophoresis (S. Choi and J.-K. Park, Lab Chip, 2007, 7, 890, ref. 1). In the isolation of white blood cells (WBCs) from red blood cells (RBCs), the microfluidic device isolated WBCs with 210-fold enrichment within a short filtration time of approximately 0.3 s. These results show that the device can be useful for the binary separation of a wide range of biological particles by size. The hydrophoretic filtration as a sample preparation unit offers potential for a power-free cell sorter to be integrated into disposable lab-on-a-chip devices.
---
paper_title: Tuneable hydrophoretic separation using elastic deformation of poly(dimethylsiloxane)

paper_content:

This paper demonstrates a method for tuning elastomeric microchannels for hydrophoretic separation made in poly(dimethylsiloxane) (PDMS). Uniform compressive strain is imposed on the elastomeric microchannel between two acrylic substrates by fastening the bolts. The elastomeric microchannel can change its cross-section during compression, simultaneously tuning the criterion for hydrophoretic ordering. The change of the channel cross-section under compression is studied using a confocal microscope and finite element method (FEM). By pressing the channel for hydrophoretic separation, we achieved tuning of the separation criterion from 7 to 2.5 µm in particle diameter.
---
paper_title: Cell handling using microstructured membranes.

paper_content:

Gentle and precise handling of cell suspensions is essential for scientific research and clinical diagnostic applications. Although different techniques for cell analysis at the micro-scale have been proposed, many still require that preliminary sample preparation steps be performed off the chip. Here we present a microstructured membrane as a new microfluidic design concept, enabling the implementation of common sample preparation procedures for suspensions of eukaryotic cells in lab-on-a-chip devices. We demonstrate the novel capabilities for sample preparation procedures by the implementation of metered sampling of nanoliter volumes of whole blood, concentration increase up to three orders of magnitude of sparse cell suspension, and circumferentially uniform, sequential exposure of cells to reagents. We implemented these functions by using microstructured membranes that are pneumatically actuated and allowed to reversibly decouple the flow of fluids and the displacement of eukaryotic cells in suspensions. Furthermore, by integrating multiple structures on the same membrane, complex sequential procedures are possible using a limited number of control steps.
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paper_title: A microfluidic-based hydrodynamic trap: design and implementation.

paper_content:

We report an integrated microfluidic device for fine-scale manipulation and confinement of micro- and nanoscale particles in free-solution. Using this device, single particles are trapped in a stagnation point flow at the junction of two intersecting microchannels. The hydrodynamic trap is based on active flow control at a fluid stagnation point using an integrated on-chip valve in a monolithic PDMS-based microfluidic device. In this work, we characterize device design parameters enabling precise control of stagnation point position for efficient trap performance. The microfluidic-based hydrodynamic trap facilitates particle trapping using the sole action of fluid flow and provides a viable alternative to existing confinement and manipulation techniques based on electric, optical, magnetic or acoustic force fields. Overall, the hydrodynamic trap enables non-contact confinement of fluorescent and non-fluorescent particles for extended times and provides a new platform for fundamental studies in biology, biotechnology and materials science.
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paper_title: Hydrodynamic trap-and-release of single particles using dual-function elastomeric valves: design, fabrication, and characterization

paper_content:

This paper introduces a simple method for trapping and releasing single particles, such as microbeads and living cells, using dual-function elastomeric valves. Our key technique is the utilization of the elastomeric valve as a dual-function removable trap instead of a fixed trap and a separate component for releasing trapped particles, thereby enabling a simple yet effective trap-and-release of particles. We designed, fabricated, and characterized a microfluidic-based device for trapping and releasing single beads by controlling elastomeric valves driven by pneumatic pressure and a fluid flow action. The fluid flow is controlled to ensure that beads flowing in a main stream enter into a branch channel. A bead is trapped by deflected elastomeric valves positioned at the entrance of a branch channel. The trapped bead is easily released by removing the applied pressure. The trapping and releasing of single beads of 21 μm in diameter were successfully performed under an optimized pressure and flow rate ratio. Moreover, we confirmed that continuous trapping and releasing of single beads by repeatedly switching elastomeric valves enables the collection of a controllable number of beads. Our simple method can be integrated into microfluidic systems that require single or multiple particle arrays for quantitative and high-throughput assays in applications within the fields of biology and chemistry.
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paper_title: Chromatographic behaviour of single cells in a microchannel with dynamic geometry.

paper_content:

We present the design of a microchannel with dynamic geometry that imparts different flow rates to different cells based on their physical properties. This dynamic microchannel is formed between a textured surface and a flexible membrane. As cells flow through the microchannel, the height of the channel oscillates causing periodic entrapment of the larger cells, and as a result, attenuating their velocity relative to the bulk liquid. The smaller cells are not slowed by the moving microstructure, and move synchronously with the bulk liquid. The ability of the dynamic microchannel to selectively attenuate the flow rate of eukaryotic cells is similar to a size-exclusion chromatography column, but with the opposite behavior. The speed of smaller substances is attenuated relative to the larger substances in traditional size-exclusion chromatography columns, whereas the speed of the larger substances that is attenuated in the dynamic microchannel. We verified this property by tracking the flow of single cells through the dynamic microchannel. L1210 mouse lymphoma cells (MLCs), peripheral blood mononuclear cells (PBMCs), and red blood cells (RBCs) were used as model cells. We showed that the flow rate of MLC is slowed by more than 50% compared to PBMCs and RBCs. We characterized the operation of the microchannel by measuring the velocity of each of the three cell types as a function of the pressures used to oscillate the membrane position, as well as the duty cycle of the oscillation.
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paper_title: A tunable micro filter modulated by pneumatic pressure for cell separation

paper_content:

Abstract This study reports a new microfluidic-based filter for size-tunable separation of microbeads or cells. The filtration separation mechanism is based on the pneumatically tunable deformation of poly-dimethylsiloxane (PDMS) membranes, which block the fluid channel with a varied degree. This defines the dimensions of the open area of the fluid channel and thus determines the maximum diameter the microbeads or cells which can pass through. The proposed device incorporates pneumatic micropumps for automatic liquid handling. Another unique feature of this filter is an unclogging mechanism using a back-flush operating mode, by which a reverse-directional flow is utilized to flush the clogged filter zone. The separation performance of the proposed device has been experimentally evaluated. Results show that this developed device is able to provide precise size-dependent filtration, with a high passage efficiency (82–89%) for microbeads with sizes smaller than the defined void space in the filter zone. Also, the proposed separation mechanism is also capable of providing a reasonable filtration rate (14.9–3.3 μl/min). Furthermore, the separation of chondrocytes from a 30 μl suspension of enzymatically digested tissue is successfully demonstrated, showing an excellent cell passage efficiency of 93% and a cell viability of 96%. The proposed device is therefore capable of performing cell separation in situations where either the harvested specimen is limited or the sample cell content is sparse. It also paves a new route to delicately separate or to isolate cells in a simple and controllable manner.
---
paper_title: A particle trapping chip using the wide and uniform slit formed by a deformable membrane and air bubble plugs

paper_content:

We present a high-efficient particle trapping chip, where a wide and uniform slit is formed by a deformable membrane barrier with air bubble plugs. The air bubble plugs remained in the extended microchannel during sample filling process, block the particle passage at the both side ends of the membrane, thus all particles flow through the uniform slit gap. Therefore, high-efficient particle trapping without particle loss can be achieved. In the experimental study using 10.3μm-diameter polystyrene beads, the membrane barrier with the air bubble plugs successfully trapped the injected beads with the trapping efficiency of 100% at the flow rate of 10μl/min, while the barrier without the air bubble plugs showed low efficiency of 20%. The present simple and effective particle trapping device is applicable for the high-efficient bioparticle isolation and recovery in the micro total analysis system.
---
paper_title: Clog-free cell filtration using resettable cell traps

paper_content:

The separation of cells by filtration through microstructured constrictions is limited by clogging and adsorption, which reduce selectivity and prevent the extraction of separated cells. To address this key challenge, we developed a mechanism for simply and reliably adjusting the cross-section of a microfluidic channel to selectively capture cells based on a combination of size and deformability. After a brief holding period, trapped cells can then be released back into flow, and if necessary, extracted for subsequent analysis. Periodically clearing filter constrictions of separated cells greatly improves selectivity and throughput, and minimizes adsorption of cells to the filter microstructure. This mechanism is capable of discriminating cell-sized polystyrene microspheres with <1 μm resolution. Rare cancer cells doped into leukocytes can be enriched ~1800× with ~90% yield despite a significant overlap in size between these cell types. An important characteristic of this process is that contaminant leukocytes are captured by non-specific adsorption and not mechanical constraint, enabling repeated filtration to improve performance. The throughput of this mechanism is 900 000 cells per hour for 32 multiplexed microchannels, or ~1 200 000 cells cm−2 h−1 on a per area basis, which exceeds existing micropore filtration mechanisms by a factor of 20.
---
paper_title: A tunable microfluidic-based filter modulated by pneumatic pressure for separation of blood cells

paper_content:

This article presents a new microfluidic-based filter for the separation of microbeads or blood cells with a high filtration rate. The device was composed of a circular micropump for automatic liquid transport, and a normally closed valve located at the filter zone for separation of beads or cells. The filtration mechanism was based on the tunable deformation of polydimethylsiloxane (PDMS) membranes that defined the gap between a floating block structure and the substrate which determined the maximum diameter of the beads/cells that can pass through the filter. Another unique feature of this filter is an unclogging mechanism using a suction force, resulting in a back flow to remove any trapped beads/cells in the filter zone when the PDMS membrane was restored to its initial state. The separation performance of the proposed device was first experimentally evaluated by using microbeads. The results showed that this device was capable of providing size-tunable filtration with a high recovery efficiency (95.25–96.21%) for microbeads with sizes smaller than the defined gap in the filter zone. Furthermore, the proposed device was also capable of performing separation of blood cells and blood plasma from human whole blood. Experimental results showed that an optimum filtration rate of 21.40 and 3.00 μl/min correspond to high recovery efficiencies of 86.69 and 80.66%, respectively, for red blood cells (RBCs) and blood plasma. The separation method developed in this work could be used for various point-of-care diagnostic applications involving separation of plasma and blood cells.
---
paper_title: Cell handling using microstructured membranes.

paper_content:

Gentle and precise handling of cell suspensions is essential for scientific research and clinical diagnostic applications. Although different techniques for cell analysis at the micro-scale have been proposed, many still require that preliminary sample preparation steps be performed off the chip. Here we present a microstructured membrane as a new microfluidic design concept, enabling the implementation of common sample preparation procedures for suspensions of eukaryotic cells in lab-on-a-chip devices. We demonstrate the novel capabilities for sample preparation procedures by the implementation of metered sampling of nanoliter volumes of whole blood, concentration increase up to three orders of magnitude of sparse cell suspension, and circumferentially uniform, sequential exposure of cells to reagents. We implemented these functions by using microstructured membranes that are pneumatically actuated and allowed to reversibly decouple the flow of fluids and the displacement of eukaryotic cells in suspensions. Furthermore, by integrating multiple structures on the same membrane, complex sequential procedures are possible using a limited number of control steps.
---
paper_title: A tunable microfluidic-based filter modulated by pneumatic pressure for separation of blood cells

paper_content:

This article presents a new microfluidic-based filter for the separation of microbeads or blood cells with a high filtration rate. The device was composed of a circular micropump for automatic liquid transport, and a normally closed valve located at the filter zone for separation of beads or cells. The filtration mechanism was based on the tunable deformation of polydimethylsiloxane (PDMS) membranes that defined the gap between a floating block structure and the substrate which determined the maximum diameter of the beads/cells that can pass through the filter. Another unique feature of this filter is an unclogging mechanism using a suction force, resulting in a back flow to remove any trapped beads/cells in the filter zone when the PDMS membrane was restored to its initial state. The separation performance of the proposed device was first experimentally evaluated by using microbeads. The results showed that this device was capable of providing size-tunable filtration with a high recovery efficiency (95.25–96.21%) for microbeads with sizes smaller than the defined gap in the filter zone. Furthermore, the proposed device was also capable of performing separation of blood cells and blood plasma from human whole blood. Experimental results showed that an optimum filtration rate of 21.40 and 3.00 μl/min correspond to high recovery efficiencies of 86.69 and 80.66%, respectively, for red blood cells (RBCs) and blood plasma. The separation method developed in this work could be used for various point-of-care diagnostic applications involving separation of plasma and blood cells.
---
paper_title: A Mechanically Tunable Microfluidic Cell-Trapping Device.

paper_content:

Abstract Controlled manipulation, such as isolation, positioning, and trapping of cells, is important in basic biological research and clinical diagnostics. Micro/nanotechnologies have been enabling more effective and efficient cell trapping than possible with conventional platforms. Currently available micro/nanoscale methods for cell trapping, however, still lack flexibility in precisely controlling the number of trapped cells. We exploited the large compliance of elastomers to create an array of cell-trapping microstructures, whose dimensions can be mechanically modulated by inducing uniformly distributed strain via application of external force on the chip. The device consists of two elastomer polydimethylsiloxane (PDMS) sheets, one of which bears dam-like, cup-shaped geometries to physically capture cells. The mechanical modulation is used to tune the characteristics of cell trapping to capture a predetermined number of cells, from single cells to multiple cells. Thus, enhanced utility and flexibility for practical applications can be attained, as demonstrated by tunable trapping of MCF-7 cells, a human breast cancer cell line.
---
paper_title: A tunable micro filter modulated by pneumatic pressure for cell separation

paper_content:

Abstract This study reports a new microfluidic-based filter for size-tunable separation of microbeads or cells. The filtration separation mechanism is based on the pneumatically tunable deformation of poly-dimethylsiloxane (PDMS) membranes, which block the fluid channel with a varied degree. This defines the dimensions of the open area of the fluid channel and thus determines the maximum diameter the microbeads or cells which can pass through. The proposed device incorporates pneumatic micropumps for automatic liquid handling. Another unique feature of this filter is an unclogging mechanism using a back-flush operating mode, by which a reverse-directional flow is utilized to flush the clogged filter zone. The separation performance of the proposed device has been experimentally evaluated. Results show that this developed device is able to provide precise size-dependent filtration, with a high passage efficiency (82–89%) for microbeads with sizes smaller than the defined void space in the filter zone. Also, the proposed separation mechanism is also capable of providing a reasonable filtration rate (14.9–3.3 μl/min). Furthermore, the separation of chondrocytes from a 30 μl suspension of enzymatically digested tissue is successfully demonstrated, showing an excellent cell passage efficiency of 93% and a cell viability of 96%. The proposed device is therefore capable of performing cell separation in situations where either the harvested specimen is limited or the sample cell content is sparse. It also paves a new route to delicately separate or to isolate cells in a simple and controllable manner.
---
paper_title: Dynamic trapping and high-throughput patterning of cells using pneumatic microstructures in an integrated microfluidic device.

paper_content:

Microfluidic trapping methods create significant opportunities to establish highly controlled cell positioning and arrangement for the microscale study of numerous cellular physiological and pathological activities. However, a simple, straightforward, dynamic, and high-throughput method for cell trapping is not yet well established. In the present paper, we report a direct active trapping method using an integrated microfluidic device with pneumatic microstructures (PμSs) for both operationally and quantitatively dynamic localization of cells, as well as for high-throughput cell patterning. We designed and fabricated U-shape PμS arrays to replace the conventional fixed microstructures for reversible trapping. Multidimensional dynamics and spatial consistency of the PμSs were optically characterized and quantitatively demonstrated. Furthermore, we performed a systematic trapping investigation of the PμSs actuated at a pressure range of 0 psi to 20 psi using three types of popularly applied mammalian cells, namely, human lung adenocarcinoma A549 cells, human hepatocellular liver carcinoma HepG2 cells, and human breast adenocarcinoma MCF-7 cells. The cells were quantitatively trapped and controlled by the U-shape PμSs in a programmatic and parallel manner, and could be opportunely released. The trapped cells with high viability were hydrodynamically protected by the real-time actuation of specifically designed umbrella-like PμSs. We demonstrate that PμSs can be applied as an active microfluidic component for large-scale cell patterning and manipulation, which could be useful in many cell-based tissue organization, immunosensor, and high-throughput imaging and screening.
---
paper_title: A particle trapping chip using the wide and uniform slit formed by a deformable membrane and air bubble plugs

paper_content:

We present a high-efficient particle trapping chip, where a wide and uniform slit is formed by a deformable membrane barrier with air bubble plugs. The air bubble plugs remained in the extended microchannel during sample filling process, block the particle passage at the both side ends of the membrane, thus all particles flow through the uniform slit gap. Therefore, high-efficient particle trapping without particle loss can be achieved. In the experimental study using 10.3μm-diameter polystyrene beads, the membrane barrier with the air bubble plugs successfully trapped the injected beads with the trapping efficiency of 100% at the flow rate of 10μl/min, while the barrier without the air bubble plugs showed low efficiency of 20%. The present simple and effective particle trapping device is applicable for the high-efficient bioparticle isolation and recovery in the micro total analysis system.
---
paper_title: Clog-free cell filtration using resettable cell traps

paper_content:

The separation of cells by filtration through microstructured constrictions is limited by clogging and adsorption, which reduce selectivity and prevent the extraction of separated cells. To address this key challenge, we developed a mechanism for simply and reliably adjusting the cross-section of a microfluidic channel to selectively capture cells based on a combination of size and deformability. After a brief holding period, trapped cells can then be released back into flow, and if necessary, extracted for subsequent analysis. Periodically clearing filter constrictions of separated cells greatly improves selectivity and throughput, and minimizes adsorption of cells to the filter microstructure. This mechanism is capable of discriminating cell-sized polystyrene microspheres with <1 μm resolution. Rare cancer cells doped into leukocytes can be enriched ~1800× with ~90% yield despite a significant overlap in size between these cell types. An important characteristic of this process is that contaminant leukocytes are captured by non-specific adsorption and not mechanical constraint, enabling repeated filtration to improve performance. The throughput of this mechanism is 900 000 cells per hour for 32 multiplexed microchannels, or ~1 200 000 cells cm−2 h−1 on a per area basis, which exceeds existing micropore filtration mechanisms by a factor of 20.
---
paper_title: Continuous blood cell separation by hydrophoretic filtration.

paper_content:

We propose a new hydrophoretic method for continuous blood cell separation using a microfluidic device composed of slanted obstacles and filtration obstacles. The slanted obstacles have a larger height and gap than the particles in order to focus them to a sidewall by hydrophoresis. In the successive structure, the height and gap of the filtration obstacles with a filtration pore are set between the diameters of small and large particles, which defines the critical separation diameter. Accordingly, the particles smaller than the criterion freely pass through the gap and keep their focused position. In contrast, the particles larger than the criterion collide against the filtration obstacle and move into the filtration pore. The microfluidic device was characterized with polystyrene beads with a minimum diameter difference of 7.3%. We completely separated polystyrene microbeads of 9 and 12 microm diameter with a separation resolution of approximately 6.2. This resolution is increased by 6.4-fold compared with our previous separation method based on hydrophoresis (S. Choi and J.-K. Park, Lab Chip, 2007, 7, 890, ref. 1). In the isolation of white blood cells (WBCs) from red blood cells (RBCs), the microfluidic device isolated WBCs with 210-fold enrichment within a short filtration time of approximately 0.3 s. These results show that the device can be useful for the binary separation of a wide range of biological particles by size. The hydrophoretic filtration as a sample preparation unit offers potential for a power-free cell sorter to be integrated into disposable lab-on-a-chip devices.
---
paper_title: A pillar-based microfilter for isolation of white blood cells on elastomeric substrate

paper_content:

Our goal is to design, fabricate, and characterize a pillar-based microfluidic device for size-based separation of human blood cells on an elastomeric substrate with application in the low-cost rapid prototyping of lab-chip devices. The single inlet single outlet device is using parallel U-shape arrays of pillars with cutoff size of 5.5 μm for trapping white blood cells (WBCs) in a pillar chamber with internal dead-volume of less than 1.0 μl. The microstructures are designed to limit the elastomeric deformation against fluid pressures. Numerical analysis showed that at maximum pressure loss of 15 kPa which is lower than the device conformal bonding strength, the pillar elastomeric deformation is less than 5% for flow rates of up to 1.0 ml min−1. Molding technique was employed for device prototyping using polyurethane methacrylate (PUMA) resin and polydimethylsiloxane (PDMS) mold. Characterization of the dual-layer device with beads and blood samples is performed. Tests with blood injection showed that ∼18%–25% of WBCs are trapped and ∼84%–89% of red blood cells (RBCs) are passed at flow rates of 15–50 μl min−1 with a slight decrease of WBCs trap and improve of the RBCs pass at higher flow rates. Similar results were obtained by separation of mixed microspheres of different size injected at flow rates of up to 400 μl min−1. Tests with blood samples stained by fluorescent gel demonstrated that the WBCs are accumulated in the arrays of pillars that later end up to blockage of the device. Filtration results of using elastomeric substrate present a good consistency with the trend of separation efficiencies of the similar silicon-based filters.
---
paper_title: Single-Cell Enzyme Concentrations, Kinetics, and Inhibition Analysis Using High-Density Hydrodynamic Cell Isolation Arrays

paper_content:

High-quality single-cell data are required for a quantitative systems biology description of cellular function. However, data of this type are difficult and time-consuming to collect using traditional techniques. We present a robust and simple microfluidic method for trapping single cells in large arrays to address this problem. Ordered single-cell isolation arrays allow for high-density microscopic analysis with simplified image processing. Moreover, for fluorescent assays, on-chip sample preparation (e.g., fluorescent labeling, washing) can be performed, as opposed to manual intensive operations of incubation, centrifugation, and resuspension in previous techniquessaving time and reagents. This technology was applied to determine novel single-cell enzyme kinetics for three different cell types (HeLa, 293T, Jurkat). A kinetic model of this process predicted this varied response was due to variation in the concentration of carboxylesterase between cell types. Nordihydroguaiaretic acid (NDGA) was also char...
---
paper_title: Continuous Particle Separation Through Deterministic Lateral Displacement

paper_content:

We report on a microfluidic particle-separation device that makes use of the asymmetric bifurcation of laminar flow around obstacles. A particle chooses its path deterministically on the basis of its size. All particles of a given size follow equivalent migration paths, leading to high resolution. The microspheres of 0.8, 0.9, and 1.0 micrometers that were used to characterize the device were sorted in 40seconds with a resolution of ∼10nanometers, which was better than the time and resolution of conventional flow techniques. Bacterial artificial chromosomes could be separated in 10 minutes with a resolution of ∼12%.
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Title: Tunable Microfluidic Devices for Hydrodynamic Fractionation of Cells and Beads: A Review

Section 1: Introduction
Description 1: Introduce the significance of effective fractionation of beads and cells in microfluidic devices and outline the paper's focus on tunable microfluidic devices using mechanical actuators for hydrodynamic-based fractionation.

Section 2: Mechanical Tunability
Description 2: Discuss the concept of mechanical tunability in microfluidic devices, including the effect of forced deformation on microstructures and how it can alter fluid flow parameters and interaction with microbeads or cells.

Section 3: Elastomeric Substrate
Description 3: Review the materials used in fabrication of microfluidic devices, focusing on elastomeric substrates like PDMS and their importance for achieving substantial deformation under small forces.

Section 4: Microstructure Design
Description 4: Explore the design of microstructures for size-based hydrodynamic manipulation of beads and cells, detailing methods like compressing or stretching to achieve mechanical tuning.

Section 5: Device Tuning Methods
Description 5: Describe different models introduced for manipulating microbeads and cells via mechanical tuning, including bulk deformation and local or membrane-based deformation.

Section 6: Deformation of Microstructure Layer
Description 6: Examine how physical manipulation of cells and beads is implemented through bulk deformation by altering the overall dimensions of the microfluidic device.

Section 7: Separation by Arrays of Micropillars
Description 7: Detail the use of elastomeric microfluidic devices with pillar-based microstructures for separation of microbeads and cells, including specific examples and their tunable characteristics.

Section 8: Cup-Shaped Elements for Trapping
Description 8: Discuss tunable microfluidic devices with cup-shaped elements for cell trapping, including methods for adjusting the trapping capacity by stretching the device.

Section 9: Tuning of Hydrophoretic Effect
Description 9: Review the concept and methods of tunable hydrophoretic continuous separation by elastomeric deformation of microchannel cross-sections.

Section 10: Elastomeric Membrane Deformation
Description 10: Analyze methods of local deformation of thin membranes under pneumatic pressures for mechanical tuning in microfluidic devices, focusing on techniques like blockage of microchannel cross-sections and dynamic cup-shaped elements.

Section 11: Conclusions
Description 11: Summarize the advancements in mechanically tunable microfluidic devices for size-based fractionation of cells and beads, discussing their potential applications, limitations, and future development opportunities.