4D-Printed Shape Memory Polymer Composites: Innovations in Smart Structures, Processing Strategies, and Application Prospects

Over the past decade, four-dimensional (4D) printing has emerged as a prominent fabrication technique, garnering significant attention. Unlike conventional manufacturing methods, 4D printing enables the transition of materials from static to dynamic configurations. The essence of this technology lies in the integration of smart materials with 3D printing techniques, enabling printed structures to undergo modification of properties under external stimuli. This paper elaborates on 4D printing technologies, including fused deposition modeling (FDM), selective laser sintering (SLS), direct ink writing (DIW), and vat photopolymerization (VP). Furthermore, it provides a comprehensive analysis of diverse shape memory polymers categorized by their actuation mechanisms and examines both current and prospective applications of 4D printing in aerospace, biomedicine, robotics, and electronic devices. These advancements underscore their transformative potential in redefining conventional manufacturing and provide novel solutions to complex engineering challenges. In addition, this review addresses the prevailing challenges in 4D printing technology, including material performance optimization, high-precision/high-efficiency printing, and the need for novel design methodologies. These discussions aim to accelerate the advancement of this burgeoning field.