Driving force
The motivation behind BioMEMS technology is introduced briefly in the "Micro-PCR" section of this site. In summary, researchers are working to create portable, handheld devices that can outperform the functions of a full room-sized laboratory. This allows for implementation of on-site biological assays that are faster, easier and cheaper. Such technology is vital in the fight against biological warfare and natural infectious agents. These systems will provide the ability to diagnose diseases within developing nations with little access to electricity and medical help. Applications like this are the driving force for many of the researchers in this field.
Actuation alternative
These devices operate around the ability to precisely control the motion of biological fluids and particles so as to replicate laboratory procedures at a length scale equivalent to (and often smaller than) the thickness of human hair. When designing and operating mechanical devices at such a length scale, simplicity is of paramount importance. In an effort to reduce the number of mechanical components used in these transport processes, some researchers choose to take advantage of the interactions between electric fields and the biological media. It is possible to generate electrical forces to pump fluids while transporting and shifting particles.
Three "categories" of electrokinetics
This area of microfluidics is often categorized into three depending on what you are moving and how you are moving it. Electroosmosis, for example, is the flow of an electrolyte through a stationary insulating channel under the influence of an externally applied electric field. Excess counterions near the solid-liquid interface are drawn to one end of the electric field. As they move, they drag the bulk fluid with them due to the viscous forces. Thus, you can bulk fluid motion without a pressure gradient. A similar form of transport occurs when an insulating particle is suspended within the medium. Electrostatic charge naturally develops on the surface of the particle when in contact with the medium. Under the same electric field, the particle will then be drawn toward one end of the field depending on its electrical properties. This comes from the net charge of the particle within the electric field and it is referred to as electrophoresis. It is a similar effect when compared to electroosmosis, except the surface is free to move. The third category has many forms, but all are referred to as dielectrophoresis. One example of this type of transport is a dielectric particle suspended in a dielectric media. When an electric field is applied, a dipole is induced. In a spatially non-uniform electric field, there exists an unbalanced Coulombic force that can be used to "push" the particle in a transverse direction.
How it all comes together
Take into consideration a bodily fluid like blood. It contains red blood cells, white blood cells, platelets and plasma. From an engineering perspective , you have three types of biological particles (some smaller than others) and a fluid that is mostly water. When a biological assay (test) is going to be performed on a sample of blood, many steps are performed using a series of devices that measure, separate, rinse, mix, dispense, lyse and incubate the sample. These are only a handful of the actual steps required of a full laboratory. In order to be able to replicate these steps at the length scale of a handheld device, we need to be able to manipulate the fluids and cells without our hands or various devices found on a lab bench.
A quick example
Separation is a good example that is commonly researched in microfluidics. A centrifuge is used in a room-sized laboratory to separate materials within a sample. In a microfluidic network, one could move particles by means of electrophoresis. The particles could be hydrodynamically focused with two external flow streams so they are in a straight line. This will make it easier to sort and count them. The particles will flow along a single streamline unless otherwise acted upon due to the nature of Stokes flow. We could then implement dielectrophoresis to selectively push the particles based on their electrical properties or size. Two branches at the end of the channel would then collect the two particle-laden streamlines. Based on the concentration of particles, one could make an assessment of the sample. This is referred to as flow cytometry and it is all done in the palm of your hand.
Related Texts
Electrokinetics in Microfluidics
by Dongqing Li
The objective of this book is to provide a fundamental understanding of the interfacial electrokinetic phenomena in several key microfluidic processes, and to show how these phenomena can be utilized to control the microfluidic processes. For this purpose, this book emphasizes the theoretical modeling and the numerical simulation of these electrokinetic phenomena in microfluidics. However, experimental studies of the electrokinetic microfluidic processes are also highlighted in sufficient detail. This is the first book which systematically reviews electrokinetic microfluidics processes for lab-on-a chip applications.
Electromechanics of Particles
by Thomas B. Jones
This is a straightforward book covering various forms of field-particle interaction. It covers different models for predicting motions such as dielectrophoresis and electrorotation. Its purpose is to provide a general framework of common electromechanics of particles. This is a very concise book that explains details of the causes of dielectrophoresis very well. It is heavily referenced in literature.
Dielectrophoresis: The behavior of neutral matter in nonuniform electric fields
by Herbert A. Pohl
This text is commonly regarded as the bible of dielectrophoresis. You will rarely read a published article on dielectrophoresis that does not reference this book. As an incredibly thorough review of the field, it is a must read for anyone studying it. Pohl covers both macroscopic and molecular points of view. Although mathematically intense, the details are well-explained.
