Multi-material Hydrogel Additive Manufacturing for Soft Robots

 Naturally occurring and synthetic hydrogels have a wide range of attractive properties including biodegradability and stimuli-responsiveness and have been used to build soft and biohybrid machines. Recent advances in hydrogel 3D printing have placed hydrogel additive manufacturing among the most promising fabrication modes to create hydrogel-based robots with diverse actuation modality. Freeform Reversible Embedding of Suspended Hydrogels (FRESH) is a state-of-the-art hydrogel 3D printing technique and has been implemented to build cell-powered actuators. However, FRESH printed actuators that do not rely on labor-intensive cell culture have not been reported. Compared with many traditional building materials for soft robots with toxicity and/or limited biodegradability, many bioink materials for FRESH, such as sodium alginate, show great potentials toward soft machines that are environmentally friendly and sustainable. With the ultimate goal of creating powerful, autonomous soft machines that can safely co-exist with living organisms, my dissertation aims to contribute to this goal by developing a multi-material hydrogel additive manufacturing toolbox for hydraulically driven soft robots with enhanced structural rigidity, exceptional biodegradability, environmental compatibility, and sustainability. I first present strategies to fabricate a candidate long-fiber material with tunable mechanical and geometric properties that can be embedded in hydrogels for structural reinforcement: Electrochemically Aligned Collagen (ELAC). Then I develop an open-source, multi-material fabrication platform that integrates long-fiber embedding and FRESH, termed LFE-FRESH, which achieves structural reinforcement of FRESH printed hydrogels with high compatibility to different fiber and ink materials and versatility in embedding patterns. Next, I modify the FRESH technique to create bending actuators consisting of thin-wall, pressurizable, and watertight hydrogel chambers, addressing a major challenge in 3D printing hydraulically-driven hydrogel actuators. Finally, I establish a fabrication technique and post-fabrication processing methods to achieve 3D printed, biodegradable, sustainable hydrogel actuators with shape morphing capability. These millimeter-scale actuators have millinewton-scale force output and can handle objects with a variety of surface texture, size, geometry, and weights. Additionally, they exhibit a unique combination of robustness to extended cyclic loading, exceptional biodegradability, and complex actuation geometry and are safely edible and digestible to Aplysia californica. A reversible chelation-crosslinking-based method is proposed to modify and reconfigure their actuation geometry to achieve additional functionality. Overall, this dissertation represents a promising next step toward multi-functional soft robots that can be safely deployed in the natural environment and co-exist with living organisms.