Biomedical Engineering Reference
In-Depth Information
microdroplet platform by air. Although the concept is simple, the technical challenge is to pre-
vent wetting of the walls (otherwise the droplets would continuously lose material to the walls),
which is achieved by a proprietary hydrophobic coating (“Fluorosurf ”); droplet speeds of 13
mm/s were demonstrated without breaking the integrity of the droplets. To prevent cross-con-
tamination, a lushing plug can be pushed down the channel. his platform could be ideally
suited for cell encapsulation applications because the extracellular medium would be able to
rapidly equilibrate its gas concentrations and thus might not sufer the hypoxic conditions that
can be encountered in oil carrier microdroplets.
3.8 Active Flow Control
Here we review the miniaturization of various active elements or “actuators”—such as valves,
pumps, and others—some of which are inspired in design by their macro-world counterparts,
whereas others are based on unique micron-scale phenomena that do not work when scaled up
to larger scales. he microvalve and micropump world-expert Nam-Trung Nguyen has under-
taken an extensive categorization of all the existing designs based on their operating physical
and chemical principles, and ranked them based on their ranges of operation of pressure and
response times (
Figure 3.33
). For example, pneumatic pumps are capable of producing large
pressures but the pneumatic pressure is typically delivered through compliant tubing, which
introduces capacitance in the lines (so pumping and valving on the order of only several hertz
can be achieved in practice). Pumps based on electrostatic actuators, on the other hand, can
be extremely fast but cannot deliver large pressures. In practice, some clever designs push the
pressures and response times by one order of magnitude, but the graph reveals very valuable
rule-of-thumb trends. hese values can be used as a starting point for the BioMEMS designer
who is considering incorporating microvalves or micropumps, since some operating principles
may be fundamentally incompatible with the pressures and response times required by a given
application.
Pressure
10
2
10
3
10
4
1
10
kPa
Electromagnetic
Disk type piezoelectric
Electrostatic
Electrochemical
Chemical
Pneumatic
Thermopneumatic
Shape-memory alloy
Thermomechanic
Response time
10
-4
10
-3
10
-2
10
-1
Second
1
Piezoelectric
Electromagnetic
Thermopneumatic
Thermomechanic
Pneumatic
Shape-memory alloy
Electrochemical
Chemical
Electrostatic
FIGURE 3.33
Pressure. and. response. times. typical. of. actuators. used. in. microvalves. and. micro-
pumps..(Figure.contributed.by.Nam-Trung.Nguyen.)
