Polymers can be used to constitute "artificial muscles" that actuate under an electric stimulus. These polymers include dielectric elastomers and thermally expandable polymers. They are insulating and relatively compliant. Their electric activation is enabled with integration of electrodes, heat conductors or heaters. However, the electrodes or heaters are stiff and inevitably restrain actuation of the polymers. Confinement effects on the polymers need to be clarified before the polymers are effectively exploited as actuation materials. The present theoretical and numerical study suggests that the constrained thermal expansion delivers more powerful actuation than the constrained dielectric elastomer does. Understanding of the confinement effects motivates development of various layout designs of the embedded electrode, heaters or skeletons for the compliant polymers. However, fabrication of the micro-actuators using a thermally expandable polymer is more successful than that using a dielectric elastomer. Based on the present research, a new class of polymer thermal micro-actuators with embedded heat conductors (or skeletons, in the other words) is developed. This actuator design features an adequate actuation strain, a large actuation stress, high work energy density and improved heat transfer. It outperforms many other thermal actuator designs based on either pure polymers or silicon. Various layout designs of heaters and skeletons are developed for thermally expandable polymers. These include meandering skeletons of symmetric, asymmetric and V- shapes. Polymer actuators with the different skeleton layout could deliver varying characteristics of motion and force. The generated motion can be rectilinear, curvilinear, in-plane or out-of-plane. The design embodiments confirm that the confined thermally expandable polymers are effective for actuation.