A review on 4D printing: Material structures, stimuli and additive manufacturing techniques

Advances in 3D printable smart materials are leading to the development of four-dimensional printing (4DP). Assembling stimulus-responsive materials and combining their functionalities into a single printed construct is an efficient way to improve the shape recovery capability of the smart structure. Three types of multi-material structures are reported in the literature: uniform distribution, gradient distribution, and special patterns. Smart materials have the unique property of changing their shape and properties when activated by external stimuli such as heat, water, or pressure. This article provides an overview of current smart material structures, types of stimuli, and three-dimensional printing (3DP) technologies used to manufacture 4D smart structures.