4D printing is based on 3D printing technology with an added dimension, where the printed structure evolves as a function of time. Materials that are stimuli-responsive are used for 4D printing such as shape memory alloys and polymers etc. Once these materials are 3D printed, they can morph into complex geometries on being exposed to external stimuli such as heat, temperature, pH etc. Among various stimuli responsive materials, gels are of great interest to soft robotics community because of their toughness and flexibility. Designing of complex 4D printed gel objects that are preprogrammed to morph into otherwise unattainable geometries opens the door for numerous applications in every field of science and technology. This paper presents a comprehensive review of current trends, prospects and challenges in 4D printing of gel materials with a focus towards their applications in soft robotics and bionic devices.
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