Biomedical Engineering Reference
In-Depth Information
D , typically less than 10
nm for aqueous buffers. As the fl uid moves in the Debye layer, it carries the bulk liq-
uid in the channel. As a result, the velocity profi le is essentially fl at across the channel
[165]. This type of fl ow is ideal for separations based on the charge-to-size ratio of the
molecules in biological samples, since broadening of the separated bands of differing
species occurs only by diffusion, not as a result of the differences in fl ow velocity across
the channel. Advantages of electroosmotic fl ow are that the blunt velocity profi le avoids
many of the diffusion non-uniformities that occur with pressure driven fl ow. However,
electroosmotic fl ow has its own weaknesses: concerns on sample dispersion in the form
of band broadening for the pumping, sensitivity to impurities that adsorb on the wall of
the channel, ohmic generation of heat in the fl uid, and the need for high voltages (order
of kilovolts). The variability of surface properties can also affect the fl ow. Proteins, for
example, can adsorb to the walls, substantially change the surface charge characteristics
and, thereby, change the fl uid velocity. This can result in unpredictable long-term time
dependencies in the fl uid fl ow. Thus, electroosmotic fl ow is usually not well suited for
transporting multicomponent solutions when separation is not desired.
Valves are often classifi ed by whether they work by themselves (passive or check
valves) or if they need an external energy to work (active valves). Ideal valves are
hoped to have characteristics of zero leakage, zero energy consumption, zero dead vol-
ume, and low cost. Figure 11.33a (see Plate 13 for color version) shows an example of
a valve that exploits the electrometric character of polydimethylsiloxane (PDMS) [166].
This passive check valve is a fl uidic rectifi er: pressure in one side of the device pre-
vents fl uid from fl owing through the device; pressure in the other side opens a fl ap and
allows the fl uid to fl ow. An active valve requires an actuator that mechanically moves
a part to open or close the fl ow passage. The actuation principles are various including
pneumatic (compressed air), thermopneumatic (heated fl uids), piezoelectric (materials
expansion when voltage applies), electrostatic (electric attraction), shape “memory”
alloy (shape changes reversibly vs temperature), and electromagnetic actuation.
The importance of mixing in a chemical/biological microsystem is obvious, particu-
larly in a microreactor. Passive mixing in a microsystem solely depends on diffusion.
In terms of Eq. (18), the diffusion time can be reduced by 100 times if the diffusion
distance, x , is reduced by ten times. We can design a narrow or high aspect ratio chan-
nel to reach a high effi cient mixing design. A high aspect ratio, narrow channel would
result in high fl uidic resistance and an expensive manufacturing process. An alternative
strategy is to design standard size microchannels followed by splitting each channel
into an array of smaller channels, and then merging them again. In such a design, the
fl uidic resistance increase can be eliminated. The Peclet number in Eq. (21) is a good
indicator for diffusion effi ciency. The higher the Peclet number, the harder the diffu-
sion mixing. Figure 11.33b (see Plate 13 for color version) shows a mixer designed
by David Beebe's group at the University of Wisconsin (formerly at the University of
Illinois) and illustrates the types of new devices that must be developed to perform
familiar functions when turbulence is no longer available as an aid [167]. This 3D-ser-
pentine channel acts as a passive mixer for laminarly fl owing fl uids based on a type of
chaotic fl ow known as chaotic advection. Chaotic advection appears in certain steady
interactions, called a Debye screening layer, has a thickness,
λ
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