Applications of Micro/Nanoparticles in Microfluidic Sensors: A Review
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paper_title: Dispersion of carbon black in a continuous phase: Electrical, rheological, and morphological studies

paper_content:

This work investigates the dispersion of carbon black (CB) aggregates into various polymeric matrices to increase electric conductivity. The effect of matrix viscosity on CB morphology and, consequently, on the blend conductivity was thoroughly addressed. The electric conductivity increases from 10–9 to 10–4 when less than 3% CB aggregates were dispersed into the PDMS liquid of various viscosities. The CB threshold loading was found to increase from 1% to 3% as the viscosity rose from 10 cp to 60 000 cp. This finding shows that an ideal loading with CB aggregates is far below that (generally 15%) of a typical pelletized CB loading. Moreover, the microscope and RV tests reveal that CB aggregates diffuse and form an agglomerate-network when the conductivity threshold is reached in a low-viscosity matrix. However, a CB aggregate-network was observed when the threshold value was attained in a high-viscosity matrix. These two mechanisms can be distinguished at approximately 1000 cp. Finally, experimental observation shows that the increase of viscosity during curing does not influence the conductivity of the composite while the CB aggregates dispersed in a thermoset matrix. The minimum viscosity during curing, however, was found to be critical to CB dispersion morphology and, consequently, to ultimate electric conductivity.
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paper_title: High-throughput particle manipulation by hydrodynamic, electrokinetic, and dielectrophoretic effects in an integrated microfluidic chip

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Integrating different steps on a chip for cell manipulations and sample preparation is of foremost importance to fully take advantage of microfluidic possibilities, and therefore make tests faster, cheaper and more accurate. We demonstrated particle manipulation in an integrated microfluidic device by applying hydrodynamic, electroosmotic (EO), electrophoretic (EP), and dielectrophoretic (DEP) forces. The process involves generation of fluid flow by pressure difference, particle trapping by DEP force, and particle redirect by EO and EP forces. Both DC and AC signals were applied, taking advantages of DC EP, EO and AC DEP for on-chip particle manipulation. Since different types of particles respond differently to these signals, variations of DC and AC signals are capable to handle complex and highly variable colloidal and biological samples. The proposed technique can operate in a high-throughput manner with thirteen independent channels in radial directions for enrichment and separation in microfluidic chip. We evaluated our approach by collecting Polystyrene particles, yeast cells, and E. coli bacteria, which respond differently to electric field gradient. Live and dead yeast cells were separated successfully, validating the capability of our device to separate highly similar cells. Our results showed that this technique could achieve fast pre-concentration of colloidal particles and cells and separation of cells depending on their vitality. Hydrodynamic, DC electrophoretic and DC electroosmotic forces were used together instead of syringe pump to achieve sufficient fluid flow and particle mobility for particle trapping and sorting. By eliminating bulky mechanical pumps, this new technique has wide applications for in situ detection and analysis.
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paper_title: An integrated microfluidic system for reaction, high-sensitivity detection, and sorting of fluorescent cells and particles.

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Presented is a novel approach for an integrated micro total analysis system (microTAS) based on a microfluidic on-chip device that supports ultrasensitive confocal detection of fluorescent cells and particles and subsequently allows for their precise sorting in the fluid phase with respect to spectroscopic properties, such as brightness and color. The hybrid silicone elastomer/glass chip first comprises a branched channel system to initiate fluid mixing and to hydrodynamically focus the sample solution down to a thin flow layer, matching the size of the confocal detection volume placed at that position and, thus, providing a high detection efficiency. In the subsequent on-chip module, the dispersed cells or particles can be sorted into two different output channels. The sorting process is realized by a perpendicular deflection stream that can be switched electrokinetically. The performance of the automated sorting routine is demonstrated by precise partition of a mixture of differently colored fluorescent beads. Moreover, the specifically branched channel geometry allows for direct implementation of reaction steps prior to detection and sorting, which is demonstrated by inducing a selective recognition reaction between the fluorescent protein R-phycoerythrin and a mixture of live bacterial cells exhibiting or lacking the respective surface antigens.
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paper_title: Microfluidic devices fabricated in Poly(dimethylsiloxane) for biological studies

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This review describes microfluidic systems in poly(dimethylsiloxane) (PDMS) for biological studies. Properties of PDMS that make it a suitable platform for miniaturized biological studies, techniques for fabricating PDMS microstructures, and methods for controlling fluid flow in microchannels are discussed. Biological procedures that have been miniaturized into PDMS-based microdevices include immunoassays, separation of proteins and DNA, sorting and manipulation of cells, studies of cells in microchannels exposed to laminar flows of fluids, and large-scale, combinatorial screening. The review emphasizes the advantages of miniaturization for biological analysis, such as efficiency of the device and special insights into cell biology.
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paper_title: Logic control of microfluidics with smart colloid.

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We report the successful realization of a microfluidic chip with switching and corresponding inverting functionalities. The chips are identical logic control components incorporating a type of smart colloid, giant electrorheological fluid (GERF), which possesses reversible characteristics via a liquid-solid phase transition under external electric field. Two pairs of electrodes embedded on the sides of two microfluidic channels serve as signal input and output, respectively. One, located in the GERF micro-channel is used to control the flow status of GERF, while another one in the ither micro-fluidic channel is used to detect the signal generated with a passing-by droplet (defined as a signal droplet). Switching of the GERF from the suspended state (off-state) to the flowing state (on-state) or vice versa in the micro-channel is controlled by the appearance of signal droplets whenever they pass through the detection electrode. The output on-off signals can be easily demonstrated, clearly matching with GERF flow status. Our results show that such a logic switch is also a logic IF gate, while its inverter functions as a NOT gate.
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paper_title: Polydimethylsiloxane-based conducting composites and their applications in microfluidic chip fabrication.

paper_content:

This paper reviews the design and fabrication of polydimethylsiloxane (PDMS)-based conducting composites and their applications in microfluidic chip fabrication. Owing to their good electrical conductivity and rubberlike elastic characteristics, these composites can be used variously in soft-touch electronic packaging, planar and three-dimensional electronic circuits, and in-chip electrodes. Several microfluidic components fabricated with PDMS-based composites have been introduced, including a microfluidic mixer, a microheater, a micropump, a microdroplet controller, as well as an all-in-one microfluidic chip.
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paper_title: Nanocomposite Carbon‐PDMS Material for Chip‐Based Electrochemical Detection

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This paper presents an alternative approach to create low-cost and patternable carbon electrodes suitable for microfluidic devices. The fabrication and the electrochemical performances of electrodes made of Polydimethylsiloxane doped with commercially available carbon black (C-PDMS) are described. Conductivity and electrochemical measurements performed on various carbon to PDMS ratios showed that electrodes with suitable electrochemical properties were obtained with a ratio of 25 %.
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paper_title: Characterizing and Patterning of PDMS-Based Conducting Composites**

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In recent years, there has been considerable progress on fabricating microfluidic devices with multiple functionalities, with the goal of attaining lab-on-a-chip [1–3] integration. These efforts have benefited from the development of microfabrication technologies such as soft lithography. [4] In this context the material polydimethylsiloxane (PDMS) has played an important role, not only serving as the stamp for pattern transfer, but also as an unique material in chip fabrication owing to its properties such as transparency, biocompatibility, and good flexibility. [5] Because such microfluidic devices may be constructed using simple manufacturing techniques such as micromolding, they are generally inexpensive to produce. By employing PDMS, micropumps, valves, mixer/reactors, and other components have been integrated into all-in-one chips with complex functionalities, used in chemical reactions, bio-analysis, drug discovery, etc. [2] However, PDMS is a non-conducting polymer, on which patterning metallic structures during the fabrication of microdevices is challenging due to the weak adhesion between the metal and PDMS. Hence, the integration of conducting structures into bulk PDMS has been a critical issue, especially for those applications such as electrokinetic micropumps, microsensors, microheaters, electro-rheological (ER) actuators, etc., [6–8] which require electrodes for control and signal detection. Patterning metallic structures is popular in microelectronics, but the metals cannot adhere to PDMS strongly due to the low surface energy of PDMS. Lee et al. reported the transfer and subsequent embedding of thin films of gold patterns into PDMS via chemical adhesion mediated by a silane coupling agent. [9] Lim et al. [10] developed a method of transferring and stacking metal layers onto a PDMS substrate by using serial and selective etching techniques. However, the incompatibility between PDMS and the metal usually caused failures in the fabrication process, especially in the bonding of thin layers. To minimize the difference in material properties, other conductive materials were considered. Gawron et al. reported the embedding of thin carbon fibers into PDMS-based microchips for capillary electrophoresis detection. [11] Carbon
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paper_title: A Simple Water-Based Synthesis of Au Nanoparticle/PDMS Composites for Water Purification and Targeted Drug Release

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AuNP/PDMS nanocomposites have been synthesized in the form of gels, foams, and films with distinctive structure and morphology. A simple in situ process in aqueous medium for the formation of such composite materials is described. The nanoparticles are held firmly within the PDMS while still being chemically accessible to substances soluble in PDMS. We demonstrate the utility of this property for water purification applications such as removing aromatic solvents and sulfur-containing contaminants from water. The contaminants can be freed from the composite with a simple thermal treatment, allowing the material to be reused. We also demonstrate chemically selective uptake and release of a fluorescent dye by the nanocomposite as a drug delivery model system.
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paper_title: Electrical conductivity and Young"s modulus of flexible nanocomposites made by metal ion implantation of Polydimethylsiloxane: the relationship between nanostructure and macroscopic properties

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The mechanical and electrical properties of nanocomposites created by gold and titanium implantation into Polydimethysiloxane (PDMS) are reported for doses from 1015 at/cm2 to 5x1016 at/cm2, and for ion energies of 2.5 keV, 5 keV and 10 keV. TEM cross-section micrographs allowed detailed microstructural analysis of the implanted layers. Gold ions penetrate up to 30 nm and form crystalline nanoparticles whose size increases with ion dose and energy. Titanium forms a nearly homogeneous amorphous composite with the PDMS up to 18 nm thick. Using TEM micrographs, the metal volume fraction of the composite was accurately determined, allowing both electrical conductivity and the Young’s modulus to be plotted vs. the volume fraction, enabling quantitative use of percolation theory for nanocomposites less than 30 nm in thickness. This allows linking the composite’s Young’s modulus and conductivity directly to the implantation parameters and volume fraction. Electrical and mechanical properties were measured on the same nanocomposite samples, and different percolation thresholds and exponents were found, showing that while percolation explains very well both conduction and stiffness of the composite, the interaction between metal nanoparticles occurs differently for determining mechanical and electrical properties.
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paper_title: Polydimethylsiloxane-based conducting composites and their applications in microfluidic chip fabrication.

paper_content:

This paper reviews the design and fabrication of polydimethylsiloxane (PDMS)-based conducting composites and their applications in microfluidic chip fabrication. Owing to their good electrical conductivity and rubberlike elastic characteristics, these composites can be used variously in soft-touch electronic packaging, planar and three-dimensional electronic circuits, and in-chip electrodes. Several microfluidic components fabricated with PDMS-based composites have been introduced, including a microfluidic mixer, a microheater, a micropump, a microdroplet controller, as well as an all-in-one microfluidic chip.
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paper_title: Microdroplet-based universal logic gates by electrorheological fluid

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We demonstrate a uniquely designed microfluid logic gate with universal functionality, which is capable of conducting all 16 logic operations in one chip, with different input voltage combinations. A kind of smart colloid, giant electrorheological (GER) fluid, functions as the translation media among fluidic, electronic and mechanic information, providing us with the capability of performing large integrations either on-chip or off-chip, while the on-chip hybrid circuit is formed by the interconnection of the electric components and fluidic channels, where the individual microdroplets travelling in a channel represents a bit. The universal logic gate reveals the possibilities of achieving a large-scale microfluidic processor with more complexity for on-chip processing for biological, chemical as well as computational experiments.
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paper_title: Electrorheological-fluid-based microvalves

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We present the successful design and fabrication of push-and-pull microvalves that use a giant electrorheological (GER) fluid. Our multilayer microvalves, including the GER fluid control channel, the electrode, the flow channel, and the flexible membrane, are fabricated with polydimethylsioxane-based materials by soft lithography techniques. The GER effect is able to provide high-pressure changes in GER control channel so as to fully close and open an associated flow channel. The fast response time of the GER fluid and the push-and-pull valve design adopted assure fast switching time of the valve less than 10ms and sound reliability. This GER-fluid-based microvalve has other advantages of easy fabrication and biocompatibility and is suitable for most microfluidic applications.
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paper_title: Universal logic gates via liquid-electronic hybrid divider

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We demonstrated two-input microdroplet-based universal logic gates using a liquid-electronic hybrid divider. All 16 Boolean logic functions have been realized by manipulating the applied voltages. The novel platform consists of a microfluidic chip with integrated microdroplet detectors and external electronic components. The microdroplet detectors act as the communication media for fluidic and electronic information exchange. The presence or absence of microdroplets at the detector translates into the binary signal 1 or 0. The embedded micro-mechanical pneumatically actuated valve (PAV), fabricated using the well-developed multilayer soft lithography technique, offers biocompatibility, flexibility and accuracy for the on-chip realization of different logic functions. The microfluidic chip can be scaled up to construct large-scale microfluidic logic computation. On the other hand, the microfluidic chip with a specific logic function can be applied to droplet-based chemical reactions for on-demand bio or chemical analysis. Our experimental results have presented an autonomously driven, precision-controlled microfluidic chip for chemical reactions based on the IF logic function.
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paper_title: Polydimethylsiloxane-integratable micropressure sensor for microfluidic chips.

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A novel microfluidic pressure sensor which can be fully integrated into polydimethylsiloxane (PDMS) is reported. The sensor produces electrical signals directly. We integrated PDMS-based conductive composites into a 30 μm thick membrane and bonded it to the microchannel side wall. The response time of the sensor is approximately 100 ms and can work within a pressure range as wide as 0–100 kPa. The resolution of this micropressure sensor is generally 0.1 kPa but can be increased to 0.01 kPa at high pressures as a result of the quadratic relationship between resistance and pressure. The PDMS-based nature of the sensor ensures its perfect bonding with PDMS chips, and the standard photolithographic process of the sensor allows one-time fabrication of three dimensional structures or even microsensor arrays. The theoretical calculations are in good agreement with experimental observations.
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paper_title: Electrorheological fluid-actuated microfluidic pump

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The authors report the design and implementation of an electrorheological (ER) fluid-actuated microfluidic pump, with programmable digital control. Our microfluidic pump has a multilayered structure fabricated on polydimethylsiloxane by soft-lithographic technique. The ER microfluidic pump exhibits good performance at high pumping frequencies and uniform liquid flow characteristics. It can be easily integrated with other microfluidic components. The programmable control also gives the device flexibility in its operations.
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paper_title: Active microfluidic mixer chip

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We report the design and fabrication of a chaotic mixer based on the electrorheological (ER) fluid-controlled valves. The flow in the main channel is perturbed by liquid flow in orthogonal side channels, driven by hydrodynamic pulsating pumps. Each pulsating pump consists of a chamber with diaphragm plus two out-of-phase ER valves operating in a push-pull mode. All the valves, pumps, and mixing channels are integrated in one polydimethylsioxane chip. Mixing characteristics in the main channel are controlled by the strength and frequency of external electric fields applied on the ER fluid.
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paper_title: Electrorheological fluids: smart soft matter and characteristics

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An electrorheological fluid, a special type of suspension with controllable fluidity by an electric field, generally contains semiconducting or polarizable materials as electro-responsive parts. These materials align in the direction of the applied electric field to generate a solid-like phase in the suspension. These electro-responsive smart materials, including dielectric inorganics, semiconducting polymers and their hybrids, and polymer/inorganic composites, are reviewed in terms of their mechanism, rheological analysis and dielectric characteristics.
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paper_title: The giant electrorheological effect in suspensions of nanoparticles

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Electrorheology (ER) denotes the control of a material's flow properties (rheology) through an electric field1,2,3,4,5,6,7,8,9,10. We have fabricated electrorheological suspensions of coated nanoparticles that show electrically controllable liquid–solid transitions. The solid state can reach a yield strength of 130 kPa, breaking the theoretical upper bound on conventional ER static yield stress that is derived on the general assumption that the dielectric and conductive responses of the component materials are linear. In this giant electrorheological (GER) effect, the static yield stress displays near-linear dependence on the electric field, in contrast to the quadratic variation usually observed11,12,13,14,15,16. Our GER suspensions show low current density over a wide temperature range of 10–120 °C, with a reversible response time of <10 ms. Finite-element simulations, based on the model of saturation surface polarization in the contact regions of neighbouring particles, yield predictions in excellent agreement with experiment.
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paper_title: Review Article—Dielectrophoresis: Status of the theory, technology, and applications

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A review is presented of the present status of the theory, the developed technology and the current applications of dielectrophoresis (DEP). Over the past 10 years around 2000 publications have addressed these three aspects, and current trends suggest that the theory and technology have matured sufficiently for most effort to now be directed towards applying DEP to unmet needs in such areas as biosensors, cell therapeutics, drug discovery, medical diagnostics, microfluidics, nanoassembly, and particle filtration. The dipole approximation to describe the DEP force acting on a particle subjected to a nonuniform electric field has evolved to include multipole contributions, the perturbing effects arising from interactions with other cells and boundary surfaces, and the influence of electrical double-layer polarizations that must be considered for nanoparticles. Theoretical modelling of the electric field gradients generated by different electrode designs has also reached an advanced state. Advances in the technology include the development of sophisticated electrode designs, along with the introduction of new materials (e.g., silicone polymers, dry film resist) and methods for fabricating the electrodes and microfluidics of DEP devices (photo and electron beam lithography, laser ablation, thin film techniques, CMOS technology). Around three-quarters of the 300 or so scientific publications now being published each year on DEP are directed towards practical applications, and this is matched with an increasing number of patent applications. A summary of the US patents granted since January 2005 is given, along with an outline of the small number of perceived industrial applications (e.g., mineral separation, micropolishing, manipulation and dispensing of fluid droplets, manipulation and assembly of micro components). The technology has also advanced sufficiently for DEP to be used as a tool to manipulate nanoparticles (e.g., carbon nanotubes, nano wires, gold and metal oxide nanoparticles) for the fabrication of devices and sensors. Most efforts are now being directed towards biomedical applications, such as the spatial manipulation and selective separation/enrichment of target cells or bacteria, high-throughput molecular screening, biosensors, immunoassays, and the artificial engineering of three-dimensional cell constructs. DEP is able to manipulate and sort cells without the need for biochemical labels or other bioengineered tags, and without contact to any surfaces. This opens up potentially important applications of DEP as a tool to address an unmet need in stem cell research and therapy.
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paper_title: A novel method to construct 3D electrodes at the sidewall of microfluidic channel

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We report a simple, low-cost and novel method for constructing three-dimensional (3D) microelectrodes in microfluidic system by utilizing low melting point metal alloy. Three-dimensional electrodes have unique properties in application of cell lysis, electro-osmosis, electroporation and dielectrophoresis. The fabrication process involves conventional photolithography and sputtering techniques to fabricate planar electrodes, positioning bismuth (Bi) alloy microspheres at the sidewall of PDMS channel, plasma bonding and low temperature annealing to improve electrical connection between metal microspheres and planar electrodes. Compared to other fabrication methods for 3D electrodes, the presented one does not require rigorous experimental conditions, cumbersome processes and expensive equipments. Numerical analysis on electric field distribution with different electrode configurations was presented to verify the unique field distribution of arc-shaped electrodes. The application of 3D electrode configuration with high-conductive alloy microspheres was confirmed by particle manipulation based on dielectrophoresis. The proposed technique offers alternatives to construct 3D electrodes from 2D electrodes. More importantly, the simplicity of the fabrication process provides easy ways to fabricate electrodes fast with arc-shaped geometry at the sidewall of microchannel.
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paper_title: Continuous manipulation and separation of particles using combined obstacle- and curvature-induced direct current dielectrophoresis

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This paper presents a novel dielectrophoresis-based microfluidic device incorporating round hurdles within an S-shaped microchannel for continuous manipulation and separation of microparticles. Local nonuniform electric fields are generated due to the combined effects of obstacle and curvature, which in turn induce negative dielectrophoresis forces exerting on the particle that transport throughout the microchannel electrokinetically. Experiments were conducted to demonstrate the controlled trajectories of fix-sized (i.e. 10 or 15x μm) polystyrene particles, and size-dependent separation of 10 and 15 μm particles by adjusting the applied voltages at the inlet and outlets. Numerical simulations were also performed to predict the particle trajectories, which showed reasonable agreement with experimentally observed results. Compared to other microchannel designs that make use of either obstacle or curvature individually for inhomogeneous electric fields, the developed microchannel offers advantages such as improved controllability of particle motion, lower requirement of applied voltage, reduced fouling, and particle adhesion, etc.
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paper_title: Continuous particle focusing in a waved microchannel using negative dc dielectrophoresis

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We present a waved microchannel for continuous focusing of microparticles and cells using negative direct current (dc) dielectrophoresis. The waved channel is composed of consecutive s-shaped curved channels in series to generate an electric field gradient required for the dielectrophoretic effect. When particles move electrokinetically through the channel, the experienced negative dielectrophoretic forces alternate directions within two adjacent semicircular microchannels, leading to a focused continuous-flow stream along the channel centerline. Both the experimentally observed and numerically simulated results of the focusing performance are reported, which coincide acceptably in proportion to the specified dimensions (i.e. inlet and outlet of the waved channel). How the applied electric field, particle size and medium concentration affect the performance was studied by focusing polystyrene microparticles of varying sizes. As an application in the field of biology, the focusing of yeast cells in the waved mcirochannel was tested. This waved microchannel shows a great potential for microflow cytometry applications and is expected to be widely used before different processing steps in lab-on-a-chip devices with integrated functions.
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paper_title: Polydimethylsiloxane-based conducting composites and their applications in microfluidic chip fabrication.

paper_content:

This paper reviews the design and fabrication of polydimethylsiloxane (PDMS)-based conducting composites and their applications in microfluidic chip fabrication. Owing to their good electrical conductivity and rubberlike elastic characteristics, these composites can be used variously in soft-touch electronic packaging, planar and three-dimensional electronic circuits, and in-chip electrodes. Several microfluidic components fabricated with PDMS-based composites have been introduced, including a microfluidic mixer, a microheater, a micropump, a microdroplet controller, as well as an all-in-one microfluidic chip.
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paper_title: Valveless impedance micropump with integrated magnetic diaphragm.

paper_content:

This study presents a planar valveless impedance-based micropump for biomedical applications comprising a lower glass substrate patterned with a copper micro-coil, a microchannel, an upper glass cover plate, and a PDMS diaphragm with an electroplated magnet on its upper surface. When a current is passed through the micro-coil, an electromagnetic force is established between the coil and the magnet. The resulting deflection of the PDMS diaphragm creates an acoustic impedance mismatch within the microchannel, which in turn produces a net flow. The performance of the micropump is characterized experimentally. The experimental results show that a maximum diaphragm deflection of 30 microm is obtained when the micro-coil is supplied with an input current of 0.5 A. The corresponding flow rate is found to be 1.5 microl/sec when the PDMS membrane is driven by an actuating frequency of 240 Hz.
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paper_title: Magnetically Actuated Patterns for Bioinspired Reversible Adhesion (Dry and Wet)

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DOI: 10.1002/adma.201303087 Over the last decade the unique “strong but reversible” characteristics of gecko’s and tree frog’s adhesive pads have been intensively investigated both in the natural systems and artificial mimics.[1,2] Whereas adhesive strength in the natural systems has been matched and even surpassed with synthetic micro and nanostructured surfaces,[3,4] strategies to effectively switch between adhesive and non adhesive states, drastically or gradually, are still scarce and limited either in performance or by the complexity of the preparation method.[5–9] Reversibility in patterned adhesives relies on a significant surface pattern reorganisation via application of an external stimulus. In artificial systems this has been achieved by temperature changes using patterns of responsive polymer materials (shape memory polymers[5] and liquid crystalline polymers[6]) or by mechanical stretching of wrinkled patterns supported by elastomeric films.[7–9] Switching adhesion with temperature based methods is slow and cannot be tuned. Adhesion changes by mechanical forces only work on stretchable films and the principle is not applicable when these are supported by rigid solids.[7–9] Methods for reversible and tunable adhesion controlled by noncontact external stimuli (temperature, light, etc.) remain a scientific and technical challenge. Polymer-based magnetically actuated microcomponents have been reported to undergo predesigned, complex twoand threedimensional motions upon application of magnetic fields.[10–12] Elastomeric materials filled with magnetic nanoparticles shaped in different microgeometries by soft moulding methods have been reported, including pillar patterns.[11,13–15] These structures have been applied in microfluidics,[15] for inducing localized traction forces to cells[11,13] and to generate anisotropic motion of microsized objects.[10] However, all these examples are based on either small movements or movement of isolated components. None of the reported cases allow homogeneous, robust and strong magnetic-driven movement of microcomponents over large areas, which is a prerequisite for efficient switching of adhesion on structured surfaces. Here we report on a facile strategy to obtain magnetically actuated arrays of micropillars able to undergo reversible, homogeneous, drastic and tunable geometrical changes upon application of a magnetic field with variable strength. We demonstrate, for the first time, a magnetically tunable adhesive that works under dry and wet conditions. Arrays of magnetic micropillars with 47 μm height and 18 μm diameter were obtained by soft moulding[16–18] using PDMS precursors containing NdFeB microparticles (see Figure 1 and Experimental Section for details). A drop of magnetic PDMS precursor was cast on the mould and a suitable magnetic field gradient was applied by placing the mould on top of a permanent magnet. In this way, the microparticles accumulated inside the pillars, where the magnetic field gradient was stronger (Figure 1, step 1). This step was crucial for a homogeneous and strong magnetic response of the pillars across the pattern. The residual magnetic PDMS layer on the mould was scraped off and a previously cured PDMS thin film was pressed against the mould (Figure 1, step 2). A non magnetic backing layer is important in order to avoid interfering magnetic interaction with the pillars. The sandwich PDMS mould/PDMS-NdFeB/PDMS film (Figure 1) was cured in an oven and then placed in a strong homogeneous magnet for magnetizing the embedded NdFeB particles (Figure 1, step 3). Control patterns were also prepared where the magnetization step was omitted. After demoulding, homogeneous arrays of magnetic micropillars supported by a non magnetic backing layer were obtained (Supporting Information (SI) Figure SI-1). The microparticles were homogeneously distributed along the pillars, as observed by optical microscopy (Figure 1). Alternatively the sandwich PDMS mould/PDMS-NdFeB/PDMS film was magnetized before curing and then placed on a magnet in order to attract the highly magnetized microparticles in the fluid PDMS to the top of the pillars (Figure SI-2). However, in this case irreversible stick of neighbouring pillars was typically observed after demoulding. This was a consequence of the high concentration of magnetized particles at the top of the pillars, which makes neighbouring pillars to behave like two interacting micromagnets (Figure SI-3). For this reason this method was rejected in the following studies. In order to test the magnetic response of the micropillars, a cylindrical NdFeB permanent magnet mounted on a micromanipulator was approached to the sample (Figure SI-4) and the response of the micropillars was followed by optical microscopy. Micropillars bended and rotated about their own axes when the magnet approached the sample and was moved around it (Figure 2A, SI-movie 1). The movement was homogeneous across the pattern (SI-movie 2). At stronger field gradients, a more pronounced bending and contact between pillars (Figure 2B1, SI-movie 3) or contact between the upper part of the pillar and the backing layer of the array (Figure 2B2, SI-movie 4 and 5) was observed depending on the bending direction of the pillars. When the magnet was removed, the pillars returned to
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paper_title: Application of magnetorheological elastomer to vibration absorber

paper_content:

Abstract Traditional dynamic vibration absorber (DVA) is widely used in industries as a vibration absorption equipment. However, it is only effective at narrow working frequency range. This shortcoming has limited its stability and application. This paper develops an adaptive tuned vibration absorber (ATVA) based on unique characteristics of magnetorheological elastomers (MREs), whose modulus can be controlled by an applied magnetic field. This ATVA works in shear mode and consists of dynamic mass, static mass and smart spring elements with MREs. Based on the double pole model of MR effects, the shift-frequency capability of the ATVA has been theoretically and experimentally evaluated. The experimental results demonstrated that the natural frequency of the ATVA can be tuned from 27.5 Hz to 40 Hz. To study its vibration absorption capacity, a beam structure with two ends supported has been employed. To analyze the vibration absorption capacity, a dynamic model of coupling beam and absorber has been established. Both the calculation and experimental results show that the absorption capacity of the developed ATVA is better than the traditional TVA and can achieve as high as 25 dB which was justified by the experiment.
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paper_title: Magnetic field sensitive functional elastomers with tuneable elastic modulus

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The main purpose of the present work was to establish the effect of external magnetic field on the elastic modulus. We have prepared poly(dimethyl siloxane) networks loaded with randomly distributed carbonyl iron particles. It was found, that the elastic modulus of magnetoelasts could be increased by uniform magnetic field. In order to enhance the magnetic reinforcement effect, we have prepared anisotropic samples under uniform magnetic field. This procedure results in formation of chain-like structures from the carbonyl iron particles aligned parallel to the field direction. The effect of particle concentration, the intensity of uniform magnetic field as well as the spatial distribution of particles on the magnetic field induced excess modulus were studied. It was established that the uniaxial field structured composites exhibit larger excess modulus compared to the random particle dispersions. The most significant effect was found if the applied field is parallel to the particle alignment and to the mechanical stress. A phenomenological approach was proposed to describe the dependence of elastic modulus on the magnetic induction. The magnetic field sensitive soft materials with tuneable elastic properties may find usage in elastomer bearings and vibration absorber.
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paper_title: Shear properties of a magnetorheological elastomer

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This paper presents an experiment testing the damped free vibration of a system composed of a magnetorheological elastomer and a mass. The goal of this experiment was to obtain the dependence of the natural frequency and the damping ratio of the structure on the applied magnetic field. The shear properties, including the shear storage modulus and the damping factor, were therefore determined. The experimental results revealed that the shear storage modulus could reach a value of 60% of the zero-field modulus and was dominated by the magnetic field, but the change in the damping factor could be neglected. Furthermore, when the field was moderate and saturation did not occur, the shear storage modulus increased proportionally with the applied field. This interesting phenomenon was analysed, and it is suggested that the subquadratic field dependence, which arises from the saturation of the magnetization near the poles of closely spaced pairs of spheres, must be taken into consideration.
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paper_title: Study of fluid damping effects on resonant frequency of an electromagnetically actuated valveless micropump.

paper_content:

As fluid flow effects on the actuation and dynamic response of a vibrating membrane are crucial to micropump design in drug delivery, this paper presents both a mathematical and finite-element analysis (FEA) validation of a solution to fluid damping of a valveless micropump model. To further understand the behavior of the micropump, effects of geometrical dimensions and properties of fluid on the resonant frequency are analyzed to optimize the design of the proposed micropump. The analytical and numerical solutions show that the resonant frequency decreases with the slenderness ratio of the diffuser and increases with the opening angle, high aspect ratio, and thickness ratio between the membrane and the fluid chamber depth. A specific valveless micropump model with a 6-mm diameter and 65-μm thickness polydimethylsiloxane (PDMS) composite elastic membrane was studied and analyzed when subjected to different fluids conditions. The resonant frequency of a clamped circular membrane is found to be 138.11 Hz, neglecting the fluid. For a gas fluid load, the frequency is attenuated by slightly shifting to 104.76 Hz and it is significantly reduced to 5.53 Hz when the liquid fluid is loaded. Resonant frequency remarkably shifts the flow rate of the pump; hence, frequency-dependent characteristics of both single-chamber and dual-chamber configuration micropumps were investigated. It was observed that, although the fluid capacity is doubled for the latter, the maximum flow rate was found to be around 27.73 μl/min under 0.4-A input current with an excitation frequency of 3 Hz. This is less than twice the flow rate of a single chamber of 19.61 μl/min tested under the same current but with an excitation frequency of 4.36 Hz. The proposed double-chamber model analytical solution combined with the optimization of the nozzle/diffuser design and assuming the effects of damping proved to be an effective tool in predicting micropump performance and flow rate delivery.
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paper_title: Design parameters for magneto-elastic soft actuators

paper_content:

Novel soft actuators can be designed from ferrogels by combining the elastic behavior of a polymer matrix with the magnetic properties of a magnetic filler. A thorough understanding of the mechanical behavior of ferrogel actuation is essential for optimizing actuator performance. For actuation by linear magnetostriction, the influence of geometrical parameters on the onset and magnitude of hysteretic loss, the range for the continuous deformation ratio, the rate of change of the deformation ratio with respect to the field strength, and the saturation elongation were modeled. These results demonstrate that geometrical design parameters such as specimen length, aspect ratio, and distance from the magnetic field source can be used to tune the performance of ferrogels.
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paper_title: Analysis and fabrication of patterned magnetorheological elastomers

paper_content:

This paper presents analysis, fabrication and characterization of patterned magnetorheological (MR) elastomers. By taking into account the local magnetic field in MREs and particle interaction magnetic energy, the magnetic-field-dependent mechanical properties of MREs with lattice and BCC structures were theoretically analyzed and numerically simulated. Soft magnetic particles were assembled in a polydimethylsiloxane (PDMS) matrix to fabricate new MR elastomers with uniform lattice and BCC structures, which were observed by a microscope. The field-dependent moduli of the new MR elastomers were characterized by using a parallel-plate MR rheometer. The experimental results agreed well with numerical simulations.
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paper_title: An integrated micro-chip for rapid detection of magnetic particles

paper_content:

This paper proposes an integrated micro-chip for the manipulation and detection of magnetic particles (MPs). A conducting ring structure is used to manipulate MPs toward giant magnetoresistance (GMR) sensing elements for rapid detection. The GMR sensor is fabricated in a horseshoe shape in order to detect the majority of MPs that are trapped around the conducting structure. The GMR sensing elements are connected in a Wheatstone bridge circuit topology for optimum noise suppression. Full fabrication details of the micro-chip, characterization of the GMR sensors, and experimental results with MPs are presented in this paper. Experimental results showed that the micro-chip can detect MPs from low concentration samples after they were guided toward the GMR sensors by applying current to the conducting ring structure.
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paper_title: Enhanced separation of magnetic and diamagnetic particles in a dilute ferrofluid

paper_content:

Traditional magnetic field-induced particle separations take place in water-based diamagnetic solutions, where magnetic particles are captured while diamagnetic particles flow through without being affected by the magnetic field. We demonstrate that replacing the diamagnetic aqueous medium with a dilute ferrofluid can significantly increase the throughput of magnetic and diamagnetic particle separation. This enhancement is attributed to the simultaneous positive and negative magnetophoresis of magnetic and diamagnetic particles, respectively, in a ferrofluid. The particle transport behaviors in both ferrofluid- and water-based separations are predicted using an analytical model.
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paper_title: Biosensing utilizing the motion of magnetic microparticles in a microfluidic system

paper_content:

Abstract The study for the design of a compact and inexpensive biosensing device, which can be operated either by primary care personnel or by patients as opposed to skilled operators, is presented. The main parts of the proposed device are a microfluidic channel, permanent magnets and functionalized magnetic microparticles. The innovative aspect of the proposed biosensing method is that it utilizes the volumetric increase of magnetic microparticles when analyte binds to their surface. Their velocity decreases drastically when they are accelerated by an externally applied magnetic force within a microfluidic channel. This effect is utilized to detect the presence of analyte e.g. microbes. Analytical calculations showed that a decrease in velocity of approximately 23% can be achieved due to the volumetric change of a magnetic microparticle of 1 μ m diameter when HIV virions of approximately 0 , 135 μ m are bound to its surface and by keeping its magnetic properties the same. Preliminary experiments were carried out utilizing superparamagnetic microparticles coated with streptavidin and polystyrene microparticles coated with biotin.
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paper_title: Cell manipulation with magnetic particles toward microfluidic cytometry

paper_content:

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: Microfluidic applications of functionalized magnetic particles for environmental analysis: focus on waterborne pathogen detection

paper_content:

The continuous surveillance of drinking water is extremely important to provide early warning of contamination and to ensure continuous supplies of healthy drinking water. Isolation and detection of a particular type of pathogen present at low concentration in a large volume of water, concentrating the analyte in a small detection volume, and removing detection inhibiting factors from the concentrated sample, present the three most important challenges for water quality monitoring laboratories. Combining advanced biological detection methods (e.g., nucleic acid-based or immunology-based protocols) with microfluidics and immunomagnetic separation techniques that exploit functionalized magnetic particles has tremendous potential for realization of an integrated system for pathogen detection, in particular, of waterborne pathogens. Taking advantage of the unique properties of magnetic particles, faster, more sensitive, and more economical diagnostic assays can be developed that can assist in the battle against microbial pathogenesis. In this review, we highlight current technologies and methods used for realization of magnetic particle-based microfluidic integrated waterborne pathogen isolation and detection systems, which have the potential to comply in future with regulatory water quality monitoring requirements.
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paper_title: Manipulation and sorting of magnetic particles by a magnetic force microscope on a microfluidic magnetic trap platform

paper_content:

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: Magnetophoresis of Nanoparticles

paper_content:

Iron oxide cores of 35 nm are coated with gold nanoparticles so that individual particle motion can be tracked in real time through the plasmonic response using dark field optical microscopy. Although Brownian and viscous drag forces are pronounced for nanoparticles, we show that magnetic manipulation is possible using large magnetic field gradients. The trajectories are analyzed to separate contributions from the different types of forces. With field gradients up to 3000 T/m, forces as small as 1.5 fN are detected.
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paper_title: Manipulation of magnetic particles by patterned arrays of magnetic spin-valve traps

paper_content:

Abstract A novel platform for microfluidic manipulation of magnetic particles is discussed. The particles are confined by an array of magnetic spin valves with bistable ferromagnetic “ON” and antiferromagnetic “OFF” net magnetization states. The switchable fringing fields near the spin-valve traps can be used to selectively confine or release particles for transport or sorting. Spin-valve traps may be potentially used as magnetic molecular tweezers or adapted to a low-power magnetic random access memory (MRAM) switching architecture for massively parallel particle sorting applications.
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paper_title: A model for predicting magnetic particle capture in a microfluidic bioseparator

paper_content:

A model is presented for predicting the capture of magnetic micro/nano-particles in a bioseparation microsystem. This bioseparator consists of an array of conductive elements embedded beneath a rectangular microfluidic channel. The magnetic particles are introduced into the microchannel in solution, and are attracted and held by the magnetic force produced by the energized elements. Analytical expressions are obtained for the dominant magnetic and fluidic forces on the particles as they move through the microchannel. These expressions are included in the equations of motion, which are solved numerically to predict particle trajectories and capture time. This model is well-suited for parametric analysis of particle capture taking into account variations in particle size, material properties, applied current, microchannel dimensions, fluid properties, and flow velocity.
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paper_title: Combined microfluidic-micromagnetic separation of living cells in continuous flow.

paper_content:

This paper describes a miniaturized, integrated, microfluidic device that can pull molecules and living cells bound to magnetic particles from one laminar flow path to an- other by applying a local magnetic field gradient, and thus se- lectively remove them from flowing biological fluids without any wash steps. To accomplish this, a microfabricated high- gradient magnetic field concentrator (HGMC) was integrated at one side of a microfluidic channel with two inlets and outlets. When magnetic micro- or nano-particles were intro- duced into one flow path, they remained limited to that flow stream. In contrast, when the HGMC was magnetized, the magnetic beads were efficiently pulled from the initial flow path into the collection stream, thereby cleansing the original fluid. Using this microdevice, living E. coli bacteria bound to magnetic nanoparticles were efficiently removed from flow- ing solutions containing densities of red blood cells similar to that found in blood. Because this microdevice allows large numbers of beads and cells to be sorted simultaneously, has no capacity limit, and does not lose separation efficiency as particles are removed, it may be especially useful for separations from blood or other clinical samples. This on- chip HGMC-microfluidic separator technology may poten- tially allow cell separations to be carried out in the field outside of hospitals and clinical laboratories.
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paper_title: Rare cell separation and analysis by magnetic sorting.

paper_content:

The separation and or isolation of rare cells using magnetic forces are commonly used and growing in use ranging from simple sample prep for further studies to a FDA approved, clinical diagnostic test. This growth is the result of both the demand to obtain homogeneous rare cells for molecular analysis and the dramatic increases in the power of permanent magnets that even allow the separation of some unlabeled cells based on intrinsic magnetic moments, such as malaria parasite-infected red blood cells.
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paper_title: On-chip bio-analyte detection utilizing the velocity of magnetic microparticles in a fluid

paper_content:

A biosensing principle utilizing the motion of suspended magnetic microparticles in a microfluidic system is presented. The system utilizes the innovative concept of the velocity dependence of magnetic microparticles (MPs) due to their volumetric change when analyte is attached to their surface via antibody–antigen binding. When the magnetic microparticles are attracted by a magnetic field within a microfluidic channel their velocity depends on the presence of analyte. Specifically, their velocity decreases drastically when the magnetic microparticles are covered by (nonmagnetic) analyte (LMPs) due to the increased drag force in the opposite direction to that of the magnetic force. Experiments were carried out as a proof of concept. A promising 52% decrease in the velocity of the LMPs in comparison to that of the MPs was measured when both of them were accelerated inside a microfluidic channel using an external permanent magnet. The presented biosensing methodology offers a compact and integrated solution...
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Title: Applications of Micro/Nanoparticles in Microfluidic Sensors: A Review

Section 1: Introduction
Description 1: Introduce the concept of lab-on-a-chip (LOC) systems and discuss the role of microfluidics and nanofluidics, highlighting the importance of micro/nanoparticles.

Section 2: Design and Fabrication of PDMS-Based Conducting Composites
Description 2: Discuss the integration of conducting particles into PDMS to create conducting composites and their applications in microfluidic devices.

Section 3: GERF Application in Microfluidics
Description 3: Explain the use of Giant Electrorheological Fluid (GERF) in microfluidic applications such as actuators, valves, and logic control.

Section 4: Particles' Electric Force in Microfluidic Channels
Description 4: Describe the electric forces acting on particles in microfluidic channels, including electrophoretic and dielectrophoretic forces, and their applications.

Section 5: Design and Fabrication of Magnetic PDMS Composites
Description 5: Detail the fabrication of magnetic PDMS composites by incorporating magnetic nanoparticles and their applications in microfluidic devices.

Section 6: Particles' Magnetic Force in Microfluidic Channels
Description 6: Outline the manipulation of magnetic particles in microfluidic channels using external magnetic fields and their applications.

Section 7: Conclusions
Description 7: Summarize the key points discussed in the paper regarding the use of particles in microfluidic systems, their behaviors, and potential applications.