Revolutionizing manufacturing: A review of 4D printing materials, stimuli, and cutting-edge applications

The concept of four-dimensional (4D) printing emerged from the addition of the time dimension into the more established additive manufacturing (AM) methods, namely 3D printing technologies. Incorporating the element of time into 4D printing allows the resulting structures to change and adapt over time in response to external or internal stimuli such as light, heat, magnetism, electricity, solvent, pH, etc. Thus, only smart materials such as shape memory polymers, alloys, hydrogels, ceramics, and composites that possess the ability to respond to a stimulus can be used. In comparison to 3D printing, the change in structure and/or property allows for broader applications of 4D printed materials in the aerospace, biomedical, soft robotics, engineering, and fashion in-dustries, and it has the potential to completely revolutionize the manufacturing industry. Herein, we have reviewed in detail the current developments in 4D printing, the available and new smart (shape-changing) materials, and how they respond to internal or external stimuli, following a short description of the definition, concept, and background of 4D printing. The cutting-edge current and potential applications of the 4D printed structures in the field of aerospace, biomedical, soft robotics, engineering, and fashion and apparel are reviewed in detail. Lastly, the review addresses future prospects including the potential future research areas, and market trends, as well as the challenges that need to be overcome for its widespread adoption. Although 4D printing technology is still in its very early phases of development, the possibilities are unlimited, and much like 3D printing, the future seems exciting. This document provides a cutting-edge review of this fascinating new field of 4D printing technology.