Monday, November 19, 2012

The Wonderful Adventure of a Small Little Tiny Drop

What is microfluidics?
As explained in my first post, nanotechnology is all about reducing size and discovering new phenomena. Scaling down the volume inside a channel filled with water is no exception to this rule. Microfluidics is all about fluidic structures with a volume of about 1 microliter (as a comparison: one drop of water has a volume of approximately 60 µl). One main characteristics of microfluidics is that inside the channels, there is a so called laminar flow. This means the flow is not turbulent like we usually see a water stream. If you turn on your faucet and water flows out, you’ll see a lot of turbulences, this doesn’t happen with microfluidics. It’s incredible hart to mix two fluids in a microfluidic channel, as can be seen in the image below. Although different colored food-dye runs from separate channels into one big channel, it does not mix, it rather flows side by side. Mixing only occurs after a while due to diffusion. This main phenomenon of microfluidics can be tricky to handle, but also super useful.

Turbulent (left) vs. laminar flow (right)
(Image by: J. Schulze and D.B. Weibel)

How do you design a microfluidics device?
The most common material used for microfluidics is a polymer called polydimethylsiloxane (short PDMS). It is a viscous solution and will harden upon placing into an oven. Lets discover how you can create a simple channel (0.05 mm wide, that's 50 µm) with an in- and outlet. First, prepare a master mold of your structure to pour the PDMS into. This is done by spin coating a thin layer of a photoresist (which is a lightactive solution which will harden upon illumination) onto a wafer. Next, a photolithographic mask of your desired channel structure is illuminated onto the photoresist. This will harden the desired structure. The rest is washed away and one is left with a negative of your channel. Finally, PDMS is poured onto the master, cured in the oven and simply peeled of your master. Punch two holes into, one for your in- and one for your outlet. Because the channel is open on one side, the PDMS is bond to e.g. a glass substrate and you're done It's that simple!

Taking it to the next level
Because it can be that simple to fabricate a microfluidics structure, several people are using microfluidics to handle small sample volumes, reduce costs and be able to fast fabricate these devices. Several applications do exist on the market or are promising candidates for market ready device. Most of them are in the field of biosensing. As explained before one can detect a disease in a smaller volume more easily and most importantly less costly. However, microfluidics devices do not only have one straight channel and that's it. Rather it can get really complex. You can integrate all sorts of features like pumps and mixers. Hundreds of in-/outlets, valves and channels can be integrated into one microfluidic chip.

Valve in action, closed position (left) and
open position (right)

What is paper microfluidics?
One really awesome application platform is the so-called paper microfluidics. These devices are simply made from paper, no PDMS, no glass etc. On a paper the outer lines of your channels are printed with a hydrophobic solution, i.e. liquids will avoid these areas and therefore stay within your desired channel configuration. One of the first of these structures is shown in the image below and used for glucose testing for people having diabetes. These devices are easy to use, disposable and very cheap. More and more applications are coming out of these paper based microfluidics, like body-on-a-chip, lab-on-a-chip and organ-on-a-chip devices (which I will cover in more detail in one of my next posts).

Paper microfluidics to test the glucose level in urine
(Image by: Whitesides, Harvard)

Microfluidics meets art!
Now sit back and enjoy these beautiful microfluidic art pieces. Doesn't they look just amazing? Science can be so great.

Images taken from (in order top to bottom): 
Albert Folch, Michael Roukes, Stephen Quake, Albert Folch, Ann-Lauriene Haag

Sunday, November 11, 2012

Extreme Science

Why do we care about ultra, micro, high and fast?
Science is trying to push human knowledge a bit further every day. In science you can find people working with extreme conditions to push these limits and gain an even deeper knowledge about fundamental science. To be able to know what happens in fundamental processes, you have to push these limits. Here, I want to walk you through some of the main extreme sides of research in physics and try to explain why we care about them.

As this blog focuses mostly on nanotechnology, let’s start with that extreme. Trying to work on smaller and smaller length scales opens up a whole new dimension and possible discoveries. Why? If you want to study or manipulate single atoms or molecules, you have to work in the nano scale, which is a hundred thousand times smaller than the width of a hair.

One of the four huge detectors at CERN. In the centre
is the actual beamline (image: CERN). 
One of the most famous example of high-energy physics is the large hadron collider (LHC) established in 2008 at Cern in Geneva, Switzerland. This massive huge ring underneath the surface is accelerating protons, the positive charged particles in an atom, to energies up to 7 Teraelectronvolt. The energy is so high that two protons are accelerated in the ring to almost near the speed of light, to be precise it's 99.9999991% of the speed of light. The protons travel in opposite directions in this ring and at some point they collide. Detectors like the one in the left image are recording these collisions. Why? The purpose of the LHC is to discover new particles. Once the two protons collide, they will break into smaller particles  One specific particle, the Higgs-Boson is of special interest, as this is supposed to be the particle giving mass to all other particles.

Many experiments on single atoms are performed at low temperatures. Temperature is a measure of how fast atoms move. If the temperature of e.g. water is above 100°C it will boil, the movement of water molecules is so strong that it will become a gas. If the temperature is below 0°C, the atoms do not move that much anymore and ice is formed. The absolute zero for the temperature is -273.15°C (or 0 Kelvin, the temperature unit system used most commonly in science). At this temperature, atoms do not move any more. However, it is really difficult to reach this zero value. Most low-temperature experiments usually run at the milli Kelvin scale, which is 1000 times colder than 1 Kelvin (-272.15°C), so already pretty cold. Why? Because the atoms almost do not move anymore, you can measure them very precisely. It's like taking a picture of moving object, it's so much easier to take a sharp image of still objects. You can study quantum mechanical effects e.g. to better understand how to improve quantum computers. 

Typical UHV set-up
Ultra High Vacuum (UHV)
In my last blog, I talked about cleanrooms. An UHV system is like an even better cleanroom on smaller scale. However, you can not enter a UHV system, as there is almost no air in there, only the samples are inside. For a vacuum in the range of 10^-11 mBar, one atom has to travel for about 7000 km to collide with another particle. In practice, one atom is hitting the wall of the chamber much earlier than any other particle. It’s amazing how few atoms are in the whole system. It is also amazing to see how these UHV systems look like. Usually, you'll find a UHV system covered with a huge amount of aluminium foil and cables hanging everywhere. ;-) Why? In a UHV system, one can study atomic structures without ANY other unwanted particles. As explained before, it's like the ultimate cleanroom. 

If you want to know more about these individual techniques, subscribe to my blog. See you soon.