Hydraulic hydrogel actuators and robots optically and sonically camouflaged in water

Abstract

Sea animals such as leptocephali develop tissues and organs composed of active transparent hydrogels to achieve agile motions and natural camouflage in water. Hydrogel-based actuators that can imitate the capabilities of leptocephali will enable new applications in diverse fields. However, existing hydrogel actuators, mostly osmotic-driven, are intrinsically low-speed and/or low-force; and their camouflage capabilities have not been explored. Here we show that hydraulic actuations of hydrogels with designed structures and properties can give soft actuators and robots that are high-speed, high-force, and optically and sonically camouflaged in water. The hydrogel actuators and robots can maintain their robustness and functionality over multiple cycles of actuations, owing to the anti-fatigue property of the hydrogel under moderate stresses. We further demonstrate that the agile and transparent hydrogel actuators and robots perform extraordinary functions including swimming, kicking rubber-balls and even catching a live fish in water.

Figure 1. Design and fabrication of leptocephalus-inspired hydrogel actuators.

(a) Leptocephalus in oceanic environment (the image is credited to HRF U/W Production). (b) Schematic illustration of leptocephalus-inspired hydrogel actuators capable of high-speed, high-force actuation with optical and sonic transparency in water. (c) Schematic illustration of osmotic-driven actuation of hydrogels. (d) Schematic illustration of hydraulic-driven actuation of hydrogels. (e) Schematic illustration of the fabrication of complex and robust hydrogel structures for hydraulic hydrogel actuators.

Figure 2. Hydraulic actuation of hydrogel in water.

(a) Actuation time is defined as a ratio of actuation volume to supply rate. Actuation time versus supply rate from the pump for the unit-segment hydraulic hydrogel actuator in both experiments and finite element simulations. (b) Actuation force versus applied pressure for the unit-segment hydraulic hydrogel actuator in both experiments and finite element simulations. (c) Fast bending actuation of the hydraulic hydrogel actuator with actuation frequency around 1 Hz. The finite element simulation also captures the fully actuated state of the actuator and its maximum principal strain (middle inset image). (d) Slow swelling-driven osmotic actuation of a bulk PAAm-alginate hydrogel assembled with a Sylgard 184 layer. (e) Comparison of the actuation forces and frequencies between the hydraulic hydrogel actuators in this study and typical osmotic hydrogel actuators and muscle-powered bioactuators. Note that the hydrogel actuators in a–d are dyed with colour for better visual representation. Scale bars, 1 cm (c,d).

Figure 3. Characterization of the hydraulic hydrogel actuators under cyclic actuations.

(a) Pressure-volume hysteresis curve and the resultant energy analysis for the unit-segment hydrogel actuator. (b) Pressure-volume hysteresis curve and the resultant energy analysis for the bending hydrogel actuator. (c) Pressure-volume hysteresis curves for the bending hydrogel actuator in the 1st, 10th, 100th and 1,000th cycles of actuations with the actuation frequency of 0.5 Hz. (d) The number of cycles to failure for the PAAm-alginate hydrogel versus the applied nominal stress. The cyclic fatigue tests are performed using stress-controlled cyclic tension of the hydrogel samples at a frequency of 1 Hz. The dotted line indicates the maximum stress level in the fully actuated hydrogel actuator in Fig. 2a. Note that hydrogel actuators in a,b are dyed with colour for better visual representation.

Figure 4. Optically camouflaged hydrogel actuators and robots in water.

(a) Transmittance in visible light range for the PAAm-alginate hydrogel, Ecoflex, Elastosil and Sylgard 184. (b,c) Images of the hydraulic fish-like actuators made of Ecoflex in mono-colour (b) and multi-colour (c) backgrounds. (d) Image of the fully actuated bending hydraulic hydrogel actuator in multi-colour background. (e,f) Images of the hydraulic fish-like actuators made of PAAm-alginate hydrogel in mono-colour (d) and multi-colour (e) backgrounds. Dotted lines are introduced in d–f to indicate the boundaries of transparent hydrogel structures in water. Scale bars, 1 cm (b–f).

Figure 5. Sonically camouflaged hydrogel actuators and robots in water.

(a–c) Speed of sound measurements for pure water, PAAm-alginate hydrogel, Ecoflex, Elastosil and Sylgard 184. The curves indicate the ultrasound signals travel through the samples from the transducer to the hydrophone with the source frequency of 40 kHz (a), 200 kHz (b) and 1 MHz (c). The t=0 corresponded to the time at which the ultrasound signal was sent from the transducer and the signal amplitudes were measured by the hydrophone upon the arrival of the transmitted ultrasound signals through the samples. The ultrasound signals sent at each frequency had the same amplitude while the attenuation varied among sample materials due to different acoustic impedance and viscous effect of each material. (d,e) Ultrasound image of the hydraulic fish-like actuators made of Ecoflex (d) and PAAm-alginate hydrogel (e) in the water tank. Dotted lines are introduced in e to indicate the boundaries of transparent hydrogel structures in water.

Figure 6. Various applications of naturally camouflaged hydrogel actuators and robots.

(a) Forward fish-like swimming of a hydrogel robotic fish in water. The hydrogel fish can keep the camouflaged state when swimming over rainbow-coloured background owing to its optical transparency. (b) A transparent hydrogel actuator kicks a rubber-ball in water. The high-speed and high-force hydraulic actuation enables effective ball-kicking motion. (c) A transparent hydrogel gripper catches, lifts and releases a live ryukin goldfish. The agile actuation and optical transparency of the hydrogel gripper allow its successful capture of the goldfish. The gripper holds and then releases the captured goldfish without harm owing to the gripper's softness. Dotted lines are introduced in a–c to indicate the boundaries of transparent hydrogel structures in water. Scale bars, 1 cm (a–c).