Reinforced Gels and Elastomers for Biomedical and Soft Robotics Applications

Natural tissues possess a so-called J-shaped strain-stiffening behavior, being soft and compliant at small strains followed by a rapid stiffening at higher strains to prevent tissue damage. In addition, they have significant anisotropy and local variations of mechanical properties that are difficult to reproduce with homogeneous materials. Inspired by many biological organisms, biomimetic and soft robotics research has been focusing on composites of soft components being reinforced by structured stiff components. Especially, structures from mature textile technology and recently emerging additive manufacturing techniques are effective in achieving complex biomimetic structures in a deterministic manner. This review focuses on composites of soft elastomers and gels reinforced by woven or knitted fibers and 3D printed structures, often with conductive or magnetic functionalities for active actuation. Discussions include describing the property requirements from biomedical and soft robotics applications with examples of specific cases, followed by a summary of a few technical solutions that have been suggested. For soft matrix materials, we focus on silicone elastomers because they are the most widely used for biomedical and soft robotics applications. Among reinforcements, knitted/woven fabrics, kirigami/origami structures, and 3D printed metamaterial reinforcements are the topics of interest because their local and global structure can be designed to govern properties of the composites and direct complex motions of soft robots in a deterministic manner. Then, application examples in the fields of biomimetic materials, soft robotics, and tissue engineering are reviewed. Finally, we emphasize the importance of advancing numerical modeling and simulation tools to predict the mechanical properties and actuation behavior of the designed composite materials.