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This paper investigates the use of shape-memory polymer thin films in microelectromechanical systems (MEMS). shape-memory polymers possess the capacity to recover large-strain deformations by the application of heat and are candidates for small-scale transduction. The key advantages of shape-memory polymers are their low material/fabrication cost coupled with their simplicity of integration/operation. In the present study, shape-memory polymers are spin coated onto a standard Si wafer and polymerized by thermal annealing. The thermomechanics of strain storage and recovery in the polymer films are studied using instrumented microindentation. The sharp microindents demonstrate full recovery at all load levels, establishing the feasibility of microscale actuation. The microindentation response of the polymer film is shown to depend on temperature and the cooling cycle during indentation. In turn, the subsequent recovery behavior of an indent depends on the thermal history during indentation. Indents performed at higher temperatures are larger in size, but have smaller stored strain energy compared to indents performed at low temperature. The larger stored strain energy in low temperature indents results in lower shape recovery temperatures. The effects of indentation temperature and load are systematically investigated to provide a framework for the use of shape-memory polymers in microsystems. Application of shape-memory polymers is demonstrated through the development of an active microfluidic reservoir. The reservoir was created by indentation at the end of a microfluidic channel and was activated by local heating. The collapse of the filled reservoir caused the motion of fluid down the microfluidic channel.
Date of Publication: June 2004