Page 269 - Mechanics of Microelectromechanical Systems

256 Chapter 4
10 OTHER FORMS OF TRANSDUCTION

Actuation and sensing can be produced by other means, as well. Figure
4.60 for instance, shows an elastic, membrane-like layer that covers an
enclosure. If heating is provided to an element in the enclosure, the gas
trapped inside the enclosure will expand and the resulting pressure will
deform the membrane outward.

Figure 4.60 Actuation by gas expansion and elastic membrane: (a) Initial undeformed state;
(b) High-pressure deformed state

In the case of a general gas transformation, the pressure after the temperature
increase has been applied can be calculated as:

Another equation can be written relating the pressure in the final state with
the deformation, and therefore the new volume and the solution to these
two equations will characterize the transduction problem.
The membrane can also deform by introducing fluids under pressure in
the provided enclosure, such that the case is with hydraulic or pneumatic
transduction.
Hydrogels, which can undergo large volume changes (swelling or
shrinking) under diverse stimuli such as variations in the water pH, solute
concentration, electric field, light or temperature, are also utilized in MEMS
transduction, as microcomponents that actuate flow-control components in
microfluidics – see, for instance, Liu, Yu and Beebe [13] . Hydrogel-based
transduction needs no external power and is capable of producing relatively
large amounts of displacement and force in actuation, and to be very
sensitive to small environmental changes. Reducing the scale of hydrogel
MEMS components improves the time response of swelling-unswelling.
Electroactive polymers (EAP), also called artificial muscles, are actually
electrostrictive materials which can reversibly change shape through electric
field exposure, and are consequently used as transduction materials,
especially in multimorph configurations.