A Bio-inspired Robotic Fish Fin with Mechanosensation Using Conductive Liquid-Metal-Infused Soft Actuators

Fish fins not only function as the propeller during swimming but also play an essential role in perceiving the underwater environment. Previous anatomical studies show that the nerves embedded in the fin ray muscles and the membrane. These fin nerves function as the sensory feedback for the swimming of live fishes. Inspired by this feature, we designed and fabricated a multi-material biomimetic fin prototype with sensory capability. Eight typical spiral eGain soft sensors were embedded in the robotic fish fin ray base to mimic the biological fin nerves. This bio-inspired lever-like design could not only respond to robotic fish fin's lateral swing motion but also enhance the fin ray's sensitivity to the external force. The soft actuators with mechanosensation allow the fin to have undulation/folding motions and the capacity of detecting flow disturbance (sensitivity: 0.03m/s water flow). We examined the mechanosensation capability of the prototype in the uniform flow with different velocities and the tilted oncoming flow jet. Through assembling the biomimetic fins to an undulatory robotic fish, we found that the soft fin could sense both the undulatory body motions and the external disturbance (including the strength and direction of the stimuli). This work provides a new approach for robotic underwater sensing over a range of flow speeds and vortex jets and may help to enrich our understanding of the sensory mechanism of live fishes.