Publication

Abstract : The technology of additive manufacturing has been a breakthrough in the field of biomedical engineering, and 4D printing is a paradigm shift because of the introduction of time in the constructs manufactured. The gap in the literature that this review fills is the absence of a synthetically-based, mechanistically-understood synthesis of smart materials, printing strategies, stimuli-sensitive processes, and biomedical implementation in 4D printing. In contrast to traditional 3D-printed non-biological constructs that are stationary, 4D-printed ones have dynamic shape-morphing properties in response to external stimuli, leading to never-before-seen functionality in tissue engineering, drug delivery, and implantable devices. This is a methodical review of the intrinsic relationship between the structure of materials, compatibility with printing technology, and stimulus-response behaviour of shape memory polymers, alloys, hydrogels, ceramics, and liquid crystal elastomers. We offer quantitative relative schemes of printing methods (FDM, DIW, SLA, DLP, inkjet, SLS/SLM) and of the types of stimuli (thermal, photo, hydro, pH, electromagnetic, biological) that elucidate the deformation mechanisms and the response-time behavior and stability of cycling. Most importantly, we discuss multi-stimuli synergistic systems that are involved in complex physiological adaptation and the AI/ML-based inverse design and predictive modeling that is required to be translated into clinical contexts. The biocompatibility testing formats and the mechanical characterisation test procedures are thoroughly assessed to provide regulatory compliance. This combined analysis allows us to name the fundamental issues as interfacial bonding in multi-material printing, having tight control over deformation, scalability issues, and regulatory ambiguities, and it suggests strategic remedies. The review gives a systematic decision-making guide to the researchers and clinicians to select materials, optimize processes, and design applications, which will eventually expedite the clinical translation of 4D printing technology in various fields of biomedicine.