As an analog to microelectronics,  the intrinsic advantage of microfluidics, including minimal sample volume, quick execution and potential of scale-up, attracts vast research interests from scientists and engineers. To name a few of the applications, physicists design more efficient micro-mixers, engineerings is looking for easy mobilization methods for fluid in nano liter scale; chemists try to mimic separation system in chip formate and make reactors in side the micro-sized chamber; biologists are trying to leverage the micro system to accelerate the understanding the beauty of the nature. I, as an engineering, keep striving to design best micro platforms for bio-analysis.

    The core advantage of miniaturization for microelectronics is to integrate more and more transistors into a single die, yielding extreme computation power as feature size goes down as small as 32 nanometers (will down to 22 or even 10 nm in the future, according to Intel. Inc).  The underlying difference between microelectronic and microfluidic is obvious: physics involved works differently when scaled down to sub microns. The electrons keep the same behavior at nano scale until meets the limit of material itself. But fluids has more and more surface force coming to play and behaves differently at micro size. It was affected by the scale laws more quickly. It brings several opportunities and challenges for us:

1. Capillary force will be a core consideration when a microfluidic system is designed.
2. Electrostatic force can be used to manipulate fluids.
3. Diffusion dominates transport process. (Turbulent mixing becomes challenging).
4. Surface science dominates: surface charge, surface tension, surface roughness, surface hydrophobicity.

    Due to minimal scale to maintain beneficial physics and chip-to-world interface, it is difficult to integrate hundreds of micrfluidic components onto a small chip and work just as electronics. However, it opens a door to a different direction: Chip in a lab. Perfusion and detection equipments are outside of chip but the system has a core function that's hard to realize in macrosystem.

    For example, if the sample volume involved is smaller, with same amount of molecules, the concentration of sample is higher and a number of physical rules tell us it is beneficial, especially for some biological applications where the sample is always a treasure. The clinical sample is hard to get and always in minimum amount; the product of reaction needs extreme high detection sensitivity.

    Another example is temperature control and perfusion control. In some of biological studies, changing buffer conditions and temperatures is a way to reveal complex characteristic of a reaction or a molecule. The small buffer volume render this easier and more precise.

Resources:

• A comprehensive review by Dr. T.M. Squires and Dr. S.R. Quake was published at July 2005 at Reviews of Modern Physics,vol, 77, page 977-1026.  Link.
• In some cases, inertial effects become significant and can be taken advantage of. A review by Dr. Calos can by found in Lab Chip, 2009, vol, 9, page 3038 - 3048. Link.
• A lot of  applications utilizing the different physics in microfluidics can be found in a great journal: Lab on a chip.

    We can look at microfluidic in a different prospective. Nanotechnology and its applications are developing in a way that can revolutionarily changes solutions to scientific and social problems.  Nano-materials such as monoliths, nanotubes, nanobelts and quantum dots are capable to provide unique advantages in various applications. Bio-analysis benefits from nano-technology without exception but it requires a connecting bridge between nano-scale materials and macro-scale manipulations. Microfluidic system is such a bridge, a tool to use nanotechnology. A nice example is monoliths which is employed as stationary phase in microfluidic high performance liquid chromatography (HPLC). My college Dr. Liu has a paper on this subject, published on Analytic Chemistry :Polymer microchips integrating solid-phase extraction and high-performance liquid chromatography using reversed-phase polymethacrylate monoliths