From 3D to 4D printing: A review

3D printing, or additive manufacturing, allows for static, three-dimensional components to be produced through a point-by-point material consolidation method. 4D printing is an emerging technique in the field of AM which allows for static structures to become dynamic with the use of smart materials and the later exposure to changing stimuli such as temperature, light, and electricity. The fourth dimension in 4D printing refers to the transformation over time experienced by the component. The concept of self-assembly and programmable materials was initially introduced in 2012 by scientist Skylar Tibbits from the Massachusetts Institute of Technology (MIT) in order to fulfil applications where a dynamic function was needed. Over the past ten years, the number of publications on the field has grown exponentially and gained increased popularity around researchers and engineers of various disciplines. 4D printing structures are mainly produced using fused deposition modelling (FDM) and stereolithography (SLA) processes, however, different techniques are implemented depending on the class of materials being used and the complexity of the shape being produced. As 4D printing began to allow for active structures to autonomously transform over time without adding time, cost or extra components to the previous static components, several potential applications were raised. Among these applications are smart biomedical devices, soft robotics, smart packaging solutions, self-healing devices, among others. This paper will review the status of the 4D printing technology by sorting relevant publications based on the different techniques utilised by researchers to achieve autonomous transformation of components post-printing.