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Sensing and actuation are intricately connected in soft robotics, where contact may change actuator mechanics and robot behavior. To improve soft robotic control and performance, proprioception and contact sensors are needed to report robot state without altering actuation mechanics or introducing bulky, rigid components. For bioinspired McKibben-style fluidic actuators, prior work in sensing has focused on sensing the strain of the actuator by embedding sensors in the actuator bladder during fabrication, or by adhering sensors to the actuator surface after fabrication. However, material property mismatches between sensors and actuators can impede actuator performance, and many soft sensors available for use with fluidic actuators rely on costly or labor-intensive fabrication methods. Here, we demonstrate a low-cost and easy-to manufacture-tubular liquid metal strain sensor for use with soft actuators that can be used to detect actuator strain and contact between the actuator and external objects. The sensor is flexible, can be fabricated with commercial-off-the-shelf components, and can be easily integrated with existing soft actuators to supplement sensing, regardless of actuator shape or size. Furthermore, the soft tubular strain sensor exhibits low hysteresis and high sensitivity. The approach presented in this work provides a low-cost, soft sensing solution for broad application in soft robotics. 

Supplying continuous power is a major challenge in the creation and deployment of sensors and small robots for marine applications. Glucose-based enzymatic fuel cells (EFCs) are a possible solution for sustainably powering such devices when mounted on or implanted in living organisms. The two main barriers to developing implantable EFCs for marine organisms are their power output and in vivo feasibility. Ideally, an in vivo EFC should be minimally invasive, remain mechanically secure, and output relatively consistent power over a predefined lifespan, ranging from weeks to months. The shape and chemistry of EFC electrodes can each contribute to or detract from the overall power production potential of the cells. This paper assesses the feasibility of EFCs using the marine sea slug, Aplysia californica’s, hemolymph as an analyte and presents methods to enhance the power produced by EFCs by altering their chemistry and form factor. We found that perfluorodecalin-soaked cathodes and spirally-rolled cells demonstrated increased power output compared to their respective control specimens. Cells tested in Aplysia saline mirrored the power output trends of cells tested in hemolymph but with higher power output. This work suggests the feasibility of creating implantable EFCs for marine sea slugs that could one day serve as sustainable biohybrid robotic platforms. 

Stimuli-responsive hydrogels are candidate building blocks for soft robotic applications due to many of their unique properties, including tunable mechanical properties and biocompatibility. Over the past decade, there has been significant progress in developing soft and biohybrid actuators using naturally occurring and synthetic hydrogels to address the increasing demands for machines capable of interacting with fragile biological systems. Recent advancements in three-dimensional (3D) printing technology, either as a standalone manufacturing process or integrated with traditional fabrication techniques, have enabled the development of hydrogel-based actuators with on-demand geometry and actuation modalities. This mini-review surveys existing research efforts to inspire the development of novel fabrication techniques using hydrogel building blocks and identify potential future directions. In this article, existing 3D fabrication techniques for hydrogel actuators are first examined. Next, existing actuation mechanisms, including pneumatic, hydraulic, ionic, dehydration-rehydration, and cell-powered actuation, are reviewed with their benefits and limitations discussed. Subsequently, the applications of hydrogel-based actuators, including compliant handling of fragile items, micro-swimmers, wearable devices, and origami structures, are described. Finally, challenges in fabricating functional actuators using existing techniques are discussed.