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In the last decade, 3D printing has attracted significant attention and has resulted in benefits to many research areas. Advances in 3D printing with smart materials at the microscale, such as hydrogels and liquid crystalline polymers, have enabled 4D printing and various applications in microrobots, micro-actuators, and tissue engineering. However, the material absorption of the laser power and the aberrations of the laser light spot will introduce a decay in the polymerization degree along the height direction, and the solution to this problem has not been reported yet. In this paper, a compensation strategy for the laser power is proposed to achieve homogeneous and high aspect ratio hydrogel structures at the microscale along the out-of-plane direction. Linear approximations for the power decay curve are adopted for height steps, discretizing the final high aspect ratio structures. The strategy is achieved experimentally with hydrogel structures fabricated by two-photon polymerization. Moreover, characterizations have been conducted to verify the homogeneity of the printed microstructures. Finally, the saturation of material property is investigated by an indirect 3D deformation method. The proposed strategy is proved to be effective and can be explored for other hydrogel materials showing significant deformation. Furthermore, the strategy for out-of-plane variations provides a critical technique to achieve 4D-printed homogeneous shape-adaptive hydrogels for further applications. 

Abstract 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.