Page 148 - An Introduction to Microelectromechanical Systems Engineering

Actuators and Actuated Microsystems 127

development activities appears to be. An application for micropumps is likely to
be in the automated handling of fluids for chemical analysis and drug delivery
systems.
Stand-alone micropump units face significant competition from traditional
solenoid or stepper-motor-actuated pumps. For instance, The Lee Company of
Westbrook, Connecticut, manufactures a family of pumps measuring approxi-
mately 51 mm × 12.7 mm × 19 mm (2 in × 0.5 in × 0.75 in) and weighing, fully
packaged, a mere 50g (1.8 oz). They can dispense up to 6 ml/min with a power
consumption of 2W from a 12-V dc supply. But micromachined pumps can have a
significant advantage if they can be readily integrated along with other fluid-
handling components, such as valves, into one completely automated miniature
system. The following demonstration from the Fraunhofer Institute for Solid State
Technology of Munich, Germany [48], illustrates one successful effort at making a
bidirectional micropump with reasonable flow rates.
The basic structure of the micropump is rather simple, consisting of a stack of
four wafers (see Figure 4.37). The bottom two wafers define two check valves at the
inlet and outlet. The top two wafers form the electrostatic actuation unit. The appli-
cation of a voltage between the top two wafers actuates the pump diaphragm, thus
expanding the volume of the pump inner chamber. This draws liquid through the
inlet check valve to fill the additional chamber volume. When the applied ac voltage
goes through its null point, the diaphragm relaxes and pushes the drawn liquid out
through the outlet check valve. Each of the check valves comprises a flap that can
move only in a single direction: The flap of the inlet check valve moves only as
liquid enters to fill the pump inner chamber; the opposite is true for the outlet check
valve.
The novelty of the design is in its ability to pump fluid either in a forward or
reverse direction—hence its bidirectionality. At first glance, it appears that such a

V Fixed electrode

Electrostatic unit

Pump diaphragm actuation

value

Check unit
Chamber

Insulator
Check-valve flap
Inlet Outlet Silicon

Figure 4.37 Illustration of a cutout of a silicon micropump from the Fraunhofer Institute for Solid
3
State Technology of Munich, Germany [48]. The overall device measures7×7×2mm . The
electrostatic actuation of a thin diaphragm modulates the volume inside a chamber. An increase in
volume draws liquid through the inlet check valve. Relaxation of the diaphragm expels the liquid
through the outlet check valve.