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The recent popularity of soft robots for marine applications has established a need for the reliable fabrication of actuators that enable locomotion, articulation, and grasping in aquatic environments. These actuators should also reduce the negative impact on sensitive ecosystems by using biodegradable materials such as organic hydrogels. Freeform Reversible Embedding of Suspended Hydrogels (FRESH) printing can be used for additive manufacturing of small-scale biologically derived, marine-sourced hydraulic actuators by printing thin-wall structures out of sustainably sourced calcium-alginate hydrogels. However, controlling larger alginate robots with complex geometries and multiple actuation mechanisms remains challenging due to the reduced strength of such soft structures. For tethered hydrogel hydraulic robots, a direct interface with fluid lines is necessary for actuation, but the drag forces associated with tethered lines can quickly overcome the actuation force of distal and extremity structures. To overcome this challenge, in this study, we identify printing parameters and interface geometries to allow the working fluid to be channeled to distal components of FRESH-printed alginate robots and demonstrate a proof-of-concept biodegradable robotic arm for small object manipulation and grasping in marine environments. 

Abstract Despite the impressive performance of recent marine robots, many of their components are non‐biodegradable or even toxic and may negatively impact sensitive ecosystems. To overcome these limitations, biologically‐sourced hydrogels are a candidate material for marine robotics. Recent advances in embedded 3D printing have expanded the design freedom of hydrogel additive manufacturing. However, 3D printing small‐scale hydrogel‐based actuators remains challenging. In this study, Free form reversible embedding of suspended hydrogels (FRESH) printing is applied to fabricate small‐scale biologically‐derived, marine‐sourced hydraulic actuators by printing thin‐wall structures that are water‐tight and pressurizable. Calcium‐alginate hydrogels are used, a sustainable biomaterial sourced from brown seaweed. This process allows actuators to have complex shapes and internal cavities that are difficult to achieve with traditional fabrication techniques. Furthermore, it demonstrates that fabricated components are biodegradable, safely edible, and digestible by marine organisms. Finally, a reversible chelation‐crosslinking mechanism is implemented to dynamically modify alginate actuators' structural stiffness and morphology. This study expands the possible design space for biodegradable marine robots by improving the manufacturability of complex soft devices using biologically‐sourced materials.