The Basic Microfluidics Concepts

A microfluidics device can be identified by the fact that it has one or more channels with at least one dimension less than 1 mm. Common fluids used in microfluidics devices include whole blood samples, bacterial cell suspensions, protein or antibody solutions and various buffers. Microfluidic devices can be used to obtain a variety of interesting measurements including molecular diffusion coefficients fluid viscosity, pH, chemical binding coefficients and enzyme reaction kinetics. Other applications for microfluidic devices include capillary electrophoresis, isoelectric focusing, immunoassays, flow cytometry, sample injection of proteins for analysis via mass spectrometry, PCR amplification, DNA analysis, cell manipulation, cell separation, cell patterning and chemical gradient formation. Many of these applications have utility for clinical diagnostics.

The use of microfluidic devices to conduct biomedical research and create clinically useful technologies has a number of significant advantages. First, because the volume of fluids within these channels is very small, usually several nanoliters, the amount of reagents and analytes used is quite small. This is especially significant for expensive reagents. The fabrications techniques used to construct microfluidic devices, discussed in more depth later, are relatively inexpensive and are very amenable both to highly elaborate, multiplexed devices and also to mass production. In a manner similar to that for microelectronics, microfluidic technologies enable the fabrication of highly integrated devices for performing several different functions on the same substrate chip. One of the long term goals in the field of microfluidics is to create integrated, portable clinical diagnostic devices for home and bedside use, thereby eliminating time consuming laboratory analysis procedures.

Understanding Microfluidics

Microfluidics is the science of designing, manufacturing, and formulating devices and processes that deal with volumes of fluid on the order of nanoliters or picoliters (symbolized pl and representing units of 10 -12 liter). The devices themselves have dimensions ranging from millimeters (mm) down to micrometers (?m), where 1 ?m = 0.001 mm.
Microfluidics hardware requires construction and design that differs from macroscale hardware. It is not generally possible to scale conventional devices down and then expect them to work in microfluidics applications. When the dimensions of a device or system reach a certain size as the scale becomes smaller, the particles of fluid, or particles suspended in the fluid, become comparable in size with the apparatus itself. This dramatically alters system behavior. Capillary action changes the way in which fluids pass through microscale-diameter tubes, as compared with macroscale channels. In addition, there are unknown factors involved, especially concerning microscale heat transfer and mass transfer, the nature of which only further research can reveal.
The volumes involved in microfluidics can be understood by visualizing the size of a one-liter container, and then imagining cubical fractions of this container. A liter is slightly more than one U.S. fluid quart. A cube measuring 100 mm (a little less than four inches) on an edge has a volume of one liter. Imagine a tiny cube whose height, width, and depth are 1/1000 (0.001) of this size, or 0.1 mm. This is the size of a small grain of table sugar; it would take a strong magnifying glass to resolve it into a recognizable cube. That cube would occupy 1 nl. A volume of 1 pl is represented by a cube whose height, width, and depth are 1/10 (0.1) that of a 1-nl cube. It would take a powerful microscope to resolve that.
Microfluidic systems have diverse and widespread potential applications. Some examples of systems and processes that might employ this technology include inkjet printers, blood-cell-separation equipment, biochemical assays, chemical synthesis, genetic analysis, drug screening, electrochromatography, surface micromachining, laser ablation, and mechanical micromilling. Not surprisingly, the medical industry has shown keen interest in microfluidics technology.