Recent advances in polymer 4D printing: 3D printing techniques, smart material design, and healthcare applications

Fourth-dimensional (4D) printing has progressed tremendously since its first conceptualization in 2013. 4D printing is an emerging branch of three-dimensional (3D) printing that allows printed parts to change their shapes and properties as a function of time under external stimuli. It has revolutionized the fabrication of smart polymer composites with customized geometry and programmed dynamic functions for expanding engineering and healthcare applications. This review provides a comprehensive overview of recent advances in the 4D printing of polymer composites, emphasizing three pivotal areas: 3D printing methodologies, smart material design, and their healthcare applications. We start with 3D printing techniques, encompassing traditional methods, multimaterial printing approaches, and other emerging technologies for functional polymer systems. We discuss the molecular engineering of shape-shifting smart polymers, including shape memory polymers, liquid crystal elastomers, magnetoactive soft materials, and hydrogel composites. The structural design strategies and modeling-guided design of smart materials are also covered. We summarize the emerging healthcare applications of 4D-printed polymer composites in medical devices, soft robotics, wearables, drug delivery, and tissue repair/regeneration. Finally, challenges, opportunities, and future directions are highlighted in material design and printing techniques for 4D printing to advance next-generation healthcare solutions.