Molecules in biological fluids such as serum and urine can reveal a lot of information about a person’s health.  A pregnancy test kit is a good example of a diagnostic tool that analyzes a biological sample to produce a quick and fairly reliable result.  For many other medical conditions, however, no inexpensive diagnostic kit exists.  A major reason for this lack of easy-to-use diagnostic tools is the fact that biological fluids are complex mixtures in which molecules of interest are often present at very low concentrations.  Proteins, lipids, and sugars cannot be amplified in vitro, so in order to detect these molecules, one must usually separate them from background molecules and then concentrate them.  Microfluidic devices are being developed to accomplish these tasks, and this technology could lead to valuable new diagnostic devices.

Seeking to separate biomolecules, researchers have created nano-channels capable of physically filtering molecules one by one.  These channels are small enough to permit only one protein to pass at a time, and the channel diameter can be adjusted according to the size of the desired protein.  To prevent these channels from becoming clogged, researchers in the lab of Jongyoon Han, PhD, have designed a separating device that consists of massively parallel sets of nano-channels running perpendicular to larger micro-channels.  Fluid flows through the micro-channels, and an electric field is applied parallel to the nano-channels.  A molecule’s path through this grid will depend on its size and on its electrical properties.  Thus, one input stream of molecules can be separated into many different output streams. 

A combination of micro-channels and nano-channels can also be used to concentrate biomolecules.  Connecting two micro-channels via a nano-channel creates an ion depletion zone around the mouth of the nano-channel.  Most biomolecules are slightly charged and do not move into the ion-depleted area, piling up at the edge of this area instead.  Concentrating molecules in this manner could potentially increase the sensitivity of immunoassays by a factor of more than one hundred, even in the presence of background molecules.  This technology promises to significantly lower the detection threshold for many molecules present in biological fluids.

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