Shape memory alloys (SMA) form a group of metals which have the interesting property that, when deformed while below a martensite finish temperature and then heated to above an austenite temperature, the alloy returns to its shape existing before the deformation. Actuating devices made with SMA technology for large scale applications are well known. Typical actuators employ a SMA member that is deformed in some manner and a return bias spring mechanically connected in some manner to the SMA device. When such a SMA device is heated, thermally or by other means, the actuator moves to perform some work function. The bias spring returns the actuator to its original position after cooling. Hashimoto et al discuss one example of this type of actuator with reference to FIG. 4a of their paper "Application of Shape Memory Alloy to Robotic Actuators", Journal of Robotic Systems, Vol. 2(1), March 1985.
A second type of actuator, called a differential type, uses two SMA devices connected mechanically in series. Heating of one device shrinks the apparatus in one direction while heating of the second device shrinks the apparatus in the other direction. Actuators of the bias and differential types have been applied to such macro applications as controlling movement of robotic joints.
Homna et al have attempted to apply SMA technology to microactuator technology by using thin (0.2 mm diameter) SMA wire and by heating the wire with pulsed electrical current. Their efforts are discussed in two papers, "Digital Actuator Applied Shape Memory Effect," Transactions of the Japan Society of Mechanical Engineers, Vol. 49, pp. 2163-2169, 1984, and "Tactile Detection Performed by SME Actuator", Bulletin of the Japan Society of Precision Engineering, Vol. 18, September 1984, pp. 274-276,. These microactuators use bias springs to return the SMA portion of an actuator to its deformed state after cooling. The use of such bias springs in microtechnology is undesirable because of the physical size requirements imposed, among other things, by the biasing device, and because of slow time constants associated with cooling.