Fast-moving soft electronic fish

Abstract

Soft robots driven by stimuli-responsive materials have unique advantages over conventional rigid robots, especially in their high adaptability for field exploration and seamless interaction with humans. The grand challenge lies in achieving self-powered soft robots with high mobility, environmental tolerance, and long endurance. We are able to advance a soft electronic fish with a fully integrated onboard system for power and remote control. Without any motor, the fish is driven solely by a soft electroactive structure made of dielectric elastomer and ionically conductive hydrogel. The electronic fish can swim at a speed of 6.4 cm/s (0.69 body length per second), which is much faster than previously reported untethered soft robotic fish driven by soft responsive materials. The fish shows consistent performance in a wide temperature range and permits stealth sailing due to its nearly transparent nature. Furthermore, the fish is robust, as it uses the surrounding water as the electric ground and can operate for 3 hours with one single charge. The design principle can be potentially extended to a variety of flexible devices and soft robots.

Keywords: aquatic robot; dielectric elastomer; hydrogel; soft robot.

Fig. 1. Fabrication of electro-ionic fish.

(A) Muscle laminate fabrication: A thin hydrogel film electrode was sandwiched between two biaxially prestretched (3 × 3) DE membranes (VHB membrane; initial thickness, 1.5 mm); the assembly was then fixed within ABS frames. (B) Fin fabrication: Two silicone films (thickness, 0.5 mm) and two rigid “L”-shaped acrylic frames (thickness, 1 mm) were glued together to form two pectoral fins, which were placed between two ABS frames. (C) The fin and muscle laminates were stacked together. The white and brown dashed lines indicate the locations of encapsulated hydrogel and feed line by DE membranes. (D) Liquid silicone precursor was poured into the mold (ABS frames) to fabricate the soft body. (E) The soft body bends after demolding. (F) Installation of the silicone tail and electromagnets.

Fig. 2. Operation mechanism of electro-ionic fish.

Front view of the actuating mechanisms. (A) In water, the soft body (silicone body) and the muscle laminates (two DE membranes and one hydrogel film) are deformed by the shrinking of the prestretched DE membranes with a bending curvature. (B) When a high voltage (HV) is applied to the muscle laminates, the electric field drives the ions in both the surrounding water and the hydrogel. Positive and negative charges accumulate on both sides of the DE membranes, inducing Maxwell stress and relaxing the DE membranes. The bending of the electro-ionic fish decreases. The surrounding water functions as the electric ground. (C) Front view (FEA) of the robotic fish in the rest state with a large bending angle θ₁. (D) Front view (FEA) for the actuated state of the robotic fish with a small bending angle θ₂. (E) Tilted view of FEA simulation for the rest state of the robotic fish. (F) Tilted view of FEA simulation for the actuated state of the robotic fish. Red dashed curves indicate the variation of bending. (G) Snapshot (similar tilted view) of a swimming manta ray. The body and fins of the manta ray buckle down with a large bending angle and (H) a small bending angle.

Fig. 3. Live snapshots of the swimming fish with wired power.

t = 0 is defined as the beginning of the cycle with the fin in the actuated state, and T represents the time required for one full flapping cycle. (A) Bending variations of the soft body and fins (front views). (B) Forward motion of the fish and undulatory motion of the fins (side views; the white dashed lines highlight the fin edges). (C) The top views correspond to (B).

Fig. 4. Quantitative performance evaluation via wired power.

(A) Voltage signal from the signal generator is amplified through a high-voltage amplifier and fed to the soft robotic fish. (B) The speed of robotic fish demonstrates a double peak distribution and reaches a maximum of 13.5 cm/s.

Fig. 5. Performance of the untethered fish.

(A) Tilted view of the fish showing the onboard system for power and remote control. (B) Live snapshots of the swimming of the robotic fish under remote control (voltage of 8 kV and 5 Hz).

Fig. 6. The thermal tolerance and visual disguise of the fish.

The soft robotic fish can swim in a wide range of water temperatures from (A) 0.4°C to (B) 74.2°C. The stealth sailing of the fish with (C) wired power and (D) onboard power.