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Schematic illustration of 4D printing of size-controllable microrollers and microscrews using external stimuli. (a) Schematic illustration of the components of stimuli-responsive material-based photoresists and printing process using a direct laser writing (two-photon polymerization) system. (b) Schematic representation of the two-way size-controllable microrollers/microscrews mechanism when induced by multiple stimuli, such as temperature, pH, and Ca2+ ions.
