Microfluidics - Introduction to BioMEMS - page 114

wetting (“ electrowetting ,” see Section 3.3.4) for shuttling individual droplets across a lat sub-
strate or in a chamber. he newly demonstrated ability of microluidic devices to produce a
stream of droplets of water in a carrier solution of oil has revolutionized many assays (“ droplet
microluidics ,” see Section 3.7).
3.2.11 The Surface-to-Volume Problem
he most serious problem with microluidic systems is just that they are so small. In shrinking
a conventional chemical measurement system, the surface-to-volume ratio increases. his ratio
is a serious problem in chemical detection if there is any tendency for an important species to
adsorb to the channel walls. In such a circumstance, all pipes between points in the microlu-
idic device become chromatographic columns; in the worst of cases, the analyte of interest may
never reach the point of detection if it adsorbs to the walls and desorbs slowly (or not at all). he
smaller the channel, the worse the problem.
he science of this problem is both fascinating and frustrating. here is not just one event that
causes loss of analytes from solution, but several. First, there is a close approach by the analyte
to the surface by difusion. If the electrostatics are favorable, the analyte near the surface may
become adsorbed loosely to the surface. If it stays in that state long enough, large analytes may
make a transition to a diferent (denatured) coniguration that sticks very well to the surface.
Each process is vulnerable to being blocked.
Much efort has gone into the creation of nonfouling surfaces for such systems but, regrettably,
no perfect material has been found. Some surfaces work very well to prevent adsorption of some
compounds. he best of the nonfouling coatings has been polyethylene glycol (PEG or PEO), which
has been applied in many diferent forms. PEG coatings drastically reduce adsorption by most
proteins, but they are not particularly stable in the presence of biological samples, and have a few
annoying traits, such as triggering platelet activation. he next-best solution has been used for half
a century or more, which is adsorbing an “inert” protein to the surface before use by the interesting
samples. he most commonly used proteins are albumin (from serum) and casein (from milk); they
are both inexpensive and have been shown to stick well to both hydrophobic and some hydrophilic
surfaces, and to greatly reduce further adsorption by other proteins. he most common method of
using them is to lood the device with a concentrated solution of the protein and to keep the chan-
nels illed long enough (minutes to hours, unfortunately), for the protein to adsorb and then par-
tially denature on the surface to make the coating stick better. It is even also sometimes necessary
to “bake” on the coatings by subsequent drying at elevated temperatures before use.
An alternate way to prevent adsorption by analytes of interest is to mix your sample with an
antifouling compound that does not stick permanently, but simply outcompetes the analytes for
the surface sites. Such a “dynamic coating method” can be used with proteins, synthetic anti-
fouling polymers like Pluronic (see Section 2.6.1.4) and even conventional surfactants. Simply
adding a high concentration of albumin to a sample can oten solve the adsorption problem, but
only if adding albumin to the sample does not interfere with the operation of the microluidic
device (e.g., albumin could have a confounding/deleterious efect on the analytical measure-
ment of interest or on the cells cultured in the device, if any).
3.3 Fluids in Electrical Fields
Here, we shall see what happens when an electric ield is introduced in a microchannel, or more
generally, in a luid. As it turns out, electric ields can be used to move luids around (i.e., used as
pumps or valves) and to selectively separate certain chemical species from others—not without
diiculty. In this section, we review briely the three most important phenomena that arise when
luids are subjected to electric ields: electrophoresis , electro-osmosis , and dielectrophoresis .
Examples of devices that exploit these phenomena will be reviewed in Chapter 4.