power of minus 9: 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).