Programming the time into 3D printing: current advances and future directions in 4D printing

3D printing technology has revolutionized various fields since it was first developed in the 1980s. In 2013, time was introduced to the spatial dimensions of the 3D printing as a new dimension leading to 4D printing. This emerging technology integrates stimuli-responsive materials with 3D printing technologies and opened up new possibilities for challenging problems by allowing the fabrication of complex structures that can undergo programmed temporal changes in response to external stimuli. Despite extensive research on advanced materials and printing techniques, the programming pathways of time into the structures and materials are still in the early stages. In this review, we comprehensively reviewed the potential programming routes of time utilized in 4D printing. These programming routes were identified and classified into three main approaches based on the timing of the programming during the 4D printing processes. These categories are designated as pre-, peri- and post-printing approaches. Then, these main categories were further expanded based on the methods employed during 4D printing to achieve temporal changes. We have also classified the computational tools used to design, program, and fabricate 4D printed structures, specifically focusing on materials modeling and structural design approaches. Finally, we have discussed the current challenges and roadblocks that need to be overcome within 4D printing frameworks to make 4D printing a highly accessible technology.