Multi-dimensional additive manufacturing for self-driven soft actuators: a materials perspective

Recent technological advances in additive manufacturing (AM) have focused on incorporating stimuli-responsive “smart” materials to extensively broaden the functionalities of printed objects beyond their shapes and structures. In this context, advancements toward soft actuators that can convert external ambient stimuli into actuation, i.e., self-driven soft actuators, have attracted tremendous interest, as they can promote sustainable energy autonomy for compact integrated systems requiring dynamic stimuli-responses such as soft robotics, wearable healthcare, and environmental monitoring. However, to practically implement such functionalities through AM, the printing process should be capable of accommodating stimuli-responsive adaptations of the physical/chemical properties of materials; this requires the use of specific processing schemes, depending on the targeted stimulus-response pairs. Therefore, it is crucial to create innovations in materials, geometries, and fabrication processes that suit various stimulus-response and application fields. To help guide such research efforts, this article provides a comprehensive overview of technological frameworks for materials, chemical/physical architectures, and corresponding process design strategies for self-driven actuators through AM. Firstly, we classify the material prerequisites for reliable and facile emulation of various stimuli-responses and the compatible printing techniques for them. Secondly, recent progress in various three-dimensional(3D)-printed untethered stimuli-responsive actuators has been categorized and summarized, depending on the targeted stimuli and energy sources. Thirdly, as a rapidly growing strategy for increasing the stability and precise control of various functional actuations, AM of functionally graded materials and multi-materials is overviewed. Finally, we propose an outlook for future research directions to resolve the remaining challenges of achieving scalable, reproducible, and multifunctional systems. Future directions are proposed to address limitations and unlock the full potential of these materials for autonomous systems.