The chemistry behind 4D printing

One of the latest hot topics in materials science is four-dimensional printing, where the fourth dimension refers to time. A component with a 4D effect can change its shape over time upon an external stimulus. The combination of additive manufacturing techniques with knowledge based on shape -memory mate- rials allows exploring different approaches for the fabrication of 4D devices. However, the chemistry responsible for the morphing effect of polymeric materials is often not explicit, despite the increasing number of published papers on this topic. For specific applications, only the chemical modification of the polymeric materials allows obtaining the required features and characteristics for 4D printing. This paper aims to provide a state of the art of the polymeric materials and available printing techniques used in 4D fabrication. The chemical concepts responsible for shape transformations, as well as the chemical reactions conducted for base material modification, are discussed. (C) 2020 Elsevier Ltd. All rights reserved.