Review on recent advances in 4D printing of shape memory polymers

Four-dimensional (4D) printing is gaining interest in the past few years. The prototyping freedom of 3D printing coupled with the abilities of smart materials and their properties gave rise to a new field of active structures. 4D printed structures are responsive to outside stimuli, thus allowing them to change some of their characteristics. This behaviour presents a base for revolutionary advancements in the field of robotics, biomedicine, and industry. This review presents an overview of all steps involved in the 4D printing of shape memory polymers rooted in recent applications. Furthermore, this paper aims to provide researchers with a basis for design when choosing materials and printing methods that suit their application needs. The shape memory effect of 4D-printed designs, along with 4D printing techniques, and shape programming of the materials are discussed. Recent advances in 4D printing of shape memory polymers with their applications are the focus of this review, whether single or multi material, with the programming of such materials for specific actuation. In addition, this paper discusses multiple methods that can be used to control the shape memory effect of 4D-printed designs, which range from the designs of the structure, the materials used, the methods and orientations of printing, activation criteria, and others. Moreover, this paper explores the range of complexity in 4D printing, which allows for control of the actuation in terms of time, force, speed, and stages, opening a wide range of applications that could benefit from this technology