4D Printing of Shape Memory Polymers, Blends, and Composites and Their Advanced Applications: A Comprehensive Literature Review

4D printing is defined as the additive manufacturing process of smart (stimuli-responsive) materials. Shape memory materials are sensitive to specific stimuli such as heat, electricity, magnetic fields, etc., which can change their form or properties. This characteristic gives the material a dynamic behavior over time (the fourth dimension). The application of the 4D printing technique is currently being explored in various fields, including soft robotics, electrical devices, deployable structures, medical implants, and medicine delivery systems. This article first examines the fundamentals of 3D printing techniques, their advantages, and limitations. Then, the shape memory materials are categorized and reviewed according to their type (shape memory polymers, shape memory composites, polymer blends, etc.) and stimulus responsiveness. Finally, different properties of shape memory materials like shape memory effect, thermomechanical properties, and their compatibility with different types of additive manufacturing processes are discussed.