4D printing: Technological developments in robotics applications

The idea of four-dimensional (4D) printing is the formation of intricate stimuli-responsive 3D architectures that transform into different forms and shapes upon exposure to environmental stimuli. 4D printing (4DP) of smart/intelligent materials is a promising and novel approach to generate intricate structures for biomedical, food, electronics, textile, and agricultural fields. Nowadays, soft robotics is a growing research field focusing on developing micro/nanoscale 4D-printed robots using intelligent materials. Herein, recent advancements in 4DP of soft robotics, actuators, and grippers are summarized. This review also highlights some recent developments in novel robotics technologies and materials including multi-material printing, electro-, and magneto-active soft materials (MASMs), and metamaterials. It also sheds light on different modeling mechanisms including numerical models and machine learning (ML) models for fabricating highly precise and efficient micro/macro-scaled robots. The applications of shape-memory polymers (SMPs), hydrogels, and liquid crystal elastomers (LCEs)-based 4D-printed soft and intelligent robots in different engineering fields are highlighted. Lastly, this review incorporates current challenges which are hindering the actual utilization of 4D-printed soft robotics and their possible remedies.