Microactuators are required to drive the resonant sensors, above, to oscillate at their resonant frequency. They are also required to produce the mechanical output required of particular microsystems: this may be moving micromirrors to scan laser beams, or switch them from one fibre to another; to drive cutting tools for microsurgical applications; to drive micropumps and valves for microanalysis or microfluidic systems; or these may even be microelectrode devices to stimulate nervous tissue in neural prosthesis applications.
Within the following section a variety of methods for achieving microactuation are briefly outlined: electrostatic, magnetic, piezoelectric, hydraulic, and thermal. Of these, piezoelectric and hydraulic methods currently look most promising, although the others have their place. Electrostatic actuation runs a close third, and is possibly the most common and well developed method, but it does suffer a little from wear and sticking problems. Magnetic actuators usually require relatively high currents (and high power), and on the microscopic scale, electrostatic actuation methods usually offer better output per unit volume (the limit is somewhere in the region of going from 1cm cubed devices to a few mm cubed - depending on the application). Thermal actuators also require relatively large amounts of electrical energy, and the heat generated also has to be dissipated.
When dealing with very smooth surfaces, typical of micromachined devices, sticking or cold welding of one part to another can be a problem. These effects can increase friction to such a degree that all the output power of the device is required just to overcome it, and they can prevent some devices from operating at all. Careful design and selection of materials can be used to overcome these problems; but they still cause trouble with many micromotor designs. Another point to be aware of is that when removing micromachined devices from wet etch baths, the surface tension in the liquid can be strong enough to stick parts together.
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