4D printing materials for vat photopolymerization

Vat photopolymerization is a widely adopted additive manufacturing process category owing to its high accuracy and accessibility. These technologies utilize a light source to cure the surface of a vat filled with liquid photopolymer resin layer by layer and recent developments in resin composition have led to developments in 4D printing. 4D printing is an extension of 3D printing in which the object transforms itself into a predefined structure upon the application of external stimuli such as heat, humidity, or light amongst others. This review paper presents a comprehensive review on the materials used to manufacture stimuli-responsive/shape morphing structures via vat photopolymerization technologies (stereolithography (SLA), micro-SLA (PµSL), digital light processing (DLP), and volumetric printing). Overall, the literature review has shown that acrylate-based resins are the most widely used for 4D printing and DLP is the most widely adopted technology owing to the possibility of implementing grayscale lithography. The goal of this review is to emphasize the need for new materials and faster development to enable further applications and wider adoption of this technology in industry and not only research.