4D printing of polylactic acid (PLA)/thermoplastic polyurethane (TPU) shape memory polymer - a review

Recently, there has been rising interest in 4D printing (4DP) technology. It is a new technology that emerged from 3D printing technology but can produce a dynamic product that can change its shape and properties when subjected to external stimuli. 4DP is an additive manufacturing process that uses materials receptive to stimuli, such as shape memory polymer (SMP), that can alter its shape once exposed to applied stimuli like heat, water, light, electricity, pressure, etc. Two frequently used SMPs are polylactic acid (PLA) and thermoplastic polyurethane (TPU), owing to their excellent shape memory properties, biodegradability, and biocompatibility. The shape memory performance of the 4D printed SMP is quantified by shape recovery ratio (Rr) and shape fixity ratio (Rf), which is highly dependent on the printing parameters. Some of the important printing parameters that influence shape memory performance are printing speed, raster angle, layer thickness, and nozzle temperature. Due to 4DP's ability to produce a dynamic product, it is widely used to revolutionize several fields such as biomedical, textile, aerospace, soft robotic, and electronic fields. This review paper discusses and provides a better understanding of the 4DP technology, the shape recovery mechanism, the effect of 4DP parameters on shape recovery performance, applications of 4DP technology, challenges faced, and future perspectives regarding the 4DP technology.