DOI | Paper

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.

Figure: A) Biodegradable, edible, morphing hydraulic actuators are fabricated using a biologically-sourced ink using FRESH printing. B,C) Individual actuators have been characterized using motion tracking, force, and pressure measurements. Scale bar: 5 mm. D) This approach can be used to fabricate small-scale soft robotic systems capable of delicate object handling, such as manipulating the sea grape shown here using an alginate-based FRESH-printed robot gripper. Scale bar: 5 mm. E) Our biologically-sourced hydraulic actuators are biodegradable and safely edible by marine organisms and can be safely digested (see inset). F) Comparison of the weight lifting ratio and degradation time between FRESH-printed grippers in this study and representative soft robotic grippers, grouped and color-coded based on actuation mechanisms (stimuli-responsive polymers,[6, 30] hydraulic actuation,[19, 31-33] shape memory alloys and polymers,[4, 5] electro-responsive polymers,[34, 35] and cell-powered actuation[36]). Weight lifting ratios are displayed as reported in the original publications, and the degradation time is estimated based on the building materials of soft grippers.