Biomedical Engineering Reference
In-Depth Information
valves (each valving three diferent luidic channels) closed all three channels, routing the luid
through a fourth channel (
Figure 3.58a
). Diferent pressure settings allow for selecting difer-
ent valves (
Figure 3.58b-d
). he terms “ternary valve” and “quaternary valve” are somewhat
misleading—each valve is inherently binary (their operation is thresholded), but they function
as if they had three states (ternary) or four states (quaternary) when they are part of an ensemble
of valves.
3.8.4 Micropumps
In Section 3.6.2, we have already introduced the basic practical concerns about how to drive low
into microchannels (using syringe pumps, gravitational low, etc.), and we have covered ways to
drive low using electrical ields (i.e., electro-osmotic low in Section 3.3.2 and electrowetting in
Section 3.3.4). In this section, we will focus on nonelectrical strategies that have been designed
speciically for pumping luids on the microscale.
3.8.4.1 Micropumps Driven by Surface Tension
Small luid volumes in contact with microstructured surfaces move spontaneously as a result of
an interplay between the liquid's surface tension and both the surface's chemical composition
and topography—always in the direction that minimizes the free energies between the vapor,
luid, and solid interfaces. his behavior is most strikingly revealed in the spontaneous wetting
of small channels or capillaries and has also been exploited in valve and pump luids. Early in
2002, Emmanuel Delamarche's group at IBM Zurich (Switzerland) reported a
capillarity pump
that moves luid from one area of the device onto another (
Figure 3.59
). Eventually, the capil-
larity pump drains the liquid out of the service port until it is pinned at the capillary retention
valve (“CRV” in
Figure 3.59c
); a second solution can then be dispensed into the drained service
port, a process that can can be repeated as many times as desired (16 sequential steps were dem-
onstrated), as long as the channels of the capillarity pump are not entirely illed. Flow rates of
220 nL/s and average low speeds of 55 mm/s were observed (the device occupies 100 × 100 μm
2
).
Note that Delamarche's pumping scheme is inherently limited to small footprints (which
limits the designs to which it can be applied) and by the fact that the low must be powered at
all times by the surface tension of the meniscus at the end of the microchannel (so there is a
point in which it runs “out of steam”). A variation on this concept, presented in 2002 by David
Beebe's group from the University of Wisconsin at Madison, does not sufer from these limita-
tions because it uses the surface tension of droplets placed at inlets/outlets of microchannels to
drive the low (
Figure 3.60
). he low rates are dictated by the curvature of the droplets, which
in turn are controlled by the amount of luid dispensed. his droplet-based strategy allows for
a
b
c
d
e
f
CP
Closed
section
etc.
Cover
CRV
Open
channel
Service
port
FIGURE 3.59
Capillarity. pump.. (From. David. Juncker,. Heinz. Schmid,. Ute. Drechsler,. Heiko. Wolf,.
Marc. Wolf,. Bruno. Michel,. Nico. de. Rooij,. and. Emmanuel. Delamarche,. “Autonomous. microlu-
idic.capillary.system,”.
Anal. Chem.
.74,.6139,.2002..Reprinted.with.permission.of.the.American.
Chemical.Society..Figure.contributed.by.Emmanuel.Delamarche.)
