4D printing of gels and soft materials

Four-dimensional (4D) printing of soft functional materials such as polymeric hydrogels, elastomers, and macromolecular systems has grown significant interest in the field of robotics and bionics due to their sophisticated morphing behavior such as deformation, shrinkage, swelling, and morphology. Furthermore, the mechanical and biocompatibility characteristics of these soft materials exhibit closer resemblance to those of biological structures, thus making them potential candidates in prototyping bioengineering tools and systems. In contrast to rigid 4D-printed systems composed of stiff polymers, soft material-based structures consisting of ductile polymers show added advantages such as sustainability in wet environments, rigidity, material permeability, and biological compatibility. Hydrogels are extensively studied and emphasized when it comes to soft materials; however, this chapter will cover a variety of different soft material options. In this chapter, the focus has been given to a wide variety of soft materials which include smart hydrogels, composite hydrogels, electroactive polymers, elastomers, and nanocomposites. The advantageous and demanding aspects of 4D printing with soft materials along with an overview of the 4D printing mechanism, 3D fabrication methods, and functions of different types of these soft materials have been discussed. Their prospects and challenges toward applications in 4D-printed soft robotics and bionic systems will also be addressed.