4D printing for biomedical and robotic applications: Materials, mechanisms, stimuli, and emerging trends – A comprehensive review

Four-dimensional (4D) printing represents a paradigm shift in additive manufacturing by which three-dimensionally printed objects are able to perform temporally changing shape, property, or function upon stimulation by environmental signals. In this review, the state-of-the-art of 4D printing technology, especially as applied to biomedical and robotics fields, has been extensively explored. We establish the basis of time-dependent transformation and the behavior of smart materials, then categorize different 4D printing technologies including fused deposition modeling, stereolithography, direct ink writing, PolyJet, and selective laser sintering. The working principles of various stimuli thermal, moisture, light, magnetic, electric, and pH/chemical are discussed with respect to their transformation mechanisms and suitability for different application areas. The available smart materials shape memory polymers, shape memory alloys, hydrogels, liquid crystal elastomers, and advanced composites are thoroughly reviewed. We critically evaluate the performance of 4D printing in biomedical applications including tissue engineering scaffolds, drug delivery systems, stents, implants, and bio-actuators, as well as robotic applications including soft robots, self-folding robots, grippers, actuators, and bio-inspired systems. Existing challenges material limitations, multi-material printing difficulties, scalability issues, precision control requirements, and biocompatibility concerns are identified. Future trends including AI-integrated 4D printing, nano-4D printing, personalized medicine, and intelligent robotic systems are proposed. Through quantitative comparisons and critical analysis, this review provides researchers and clinicians with a systematic framework for material selection, process optimization, and application design to accelerate the clinical and commercial translation of 4D printing technology.