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An SU-8 polymeric actuator with an embedded silicon skeleton has recently been reported to have good actuation performance. As a composite, it has room for performance improvements by design. Yet, it has not been optimized for the best performance. In this paper, we perform a parametric study, based on analytical and finite-element models, to find the optimum design. Properties and actuation capabilities are related to design parameters of the composite. In a design case to maximize work density, the parametric study found an optimum design consisting of 70% SU-8 and 30% Si. As compared with pure SU-8, the optimum composite design has a Young's modulus that is 2.2 times higher, a thermal stress that is 2.5 times higher, a work density that is 2.6 times higher, a coefficient of thermal expansion that is 5% higher, and a shorter thermal-response time. The optimum design can deliver a maximum strain of 2.77% and a maximum stress of 196 MPa at the maximum operating temperature of 200°C. Moreover, it has a rather high work density of 2.71 MJ/m3, which is comparable with other thermal phase-change actuators. In conclusion, this polymeric composite actuator is very powerful for microactuation.