Developments in 4D-printing: a review on current smart materials, technologies, and applications

Recent advances in additive manufacturing (AM), commonly known as three-dimensional (3D)-printing, have allowed researchers to create complex shapes previously impossible using traditional fabrication methods. A research branch that originated from 3D-printing called four-dimensional (4D)-printing involves printing with smart materials that can respond to external stimuli. 4D-printing permits the creation of on-demand dynamically controllable shapes by integrating the dimension of time. Recent achievements in synthetic smart materials, novel printers, deformation mechanism, and mathematical modeling have greatly expanded the feasibility of 4D-printing. In this paper, progress in the 4D-printing field is reviewed with a focus on its practical applications. We discuss smart materials developed using 4D-printing with explanations of their morphing mechanisms. Additionally, case studies are presented on self-constructing structures, medical devices, and soft robotics. We conclude with challenges and future opportunities in the field of 4D-printing.