Microscale machines are able to perform a number of tasks like micromanipulation, drug-delivery, and noninvasive surgery. In particular, microscale polymer machines that can perform intelligent work for manipulation or transport, adaptive locomotion, or sensing are in-demand. To achieve this goal, shape-morphing smart polymers like hydrogels, liquid crystalline polymers, and other smart polymers are of great interest. Structures fabricated by these materials undergo mechanical motion under stimulation such as temperature, pH, light, and so on. The use of these materials renders microscale machines that undergo complex stimuli-responsive transformation such as from planar to 3D by combining spatial design like introducing in-plane or out-plane differences. During the past decade, many techniques have been developed or adopted for fabricating structures with smart polymers including microfabrication methods and the well-known milestone of 4D printing, starting in 2013. In this review, the existing or potential active smart polymers that could be used to fabricate active microscale machines to accomplish complex tasks are summarized.
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