The development of soft robots, inspired by biological systems, offers safer human-machine interaction and adaptable movements compared to traditional rigid robots. Various actuation methods exist, including pressure, thermal, magnetic, light, combustion, and phase transition. However, each has limitations: pneumatic/hydraulic systems lack portability; combustion-driven pumps are difficult to control; thermal activation is slow; magnetic and light-responsive systems require bulky equipment. No single method effectively balances powerful actuation, controllability, speed, reliability, and portability for untethered applications. Biological systems, like spiders, utilize built-in hydraulic systems for agile movement, driven by the heart's pumping action and self-healing hemolymph. This paper explores the creation of soft electronic pumps mimicking this biological function, aiming for a fully soft, powerful, built-in, versatile, long-lasting, quiet, and self-healing power source for untethered soft robots.

Existing research on soft robot actuation highlights the limitations of current technologies. Pneumatic and hydraulic actuators, while common, suffer from portability issues due to external pumps and compressors. Combustion-driven systems offer high speed and pressure but lack control and reusability. Thermal actuation, though offering high force, is slow due to poor thermal control. Magnetic and light-activated systems are suited for micro/nano robots but necessitate complex external equipment. Stretchable electrohydrodynamic pumps offer improved portability but need enhanced flow rate and force. The self-healing aspect, drawing inspiration from biological systems, is a relatively new area of research in soft robotics, aiming to improve the longevity and robustness of these systems in unpredictable environments.

The researchers designed and fabricated fully soft electronic pumps, inspired by a spider's heart. These pumps consist of soft ring grounding electrodes with holes, soft ring positive electrodes with needles, insulated supports, and an insulated elastomer shell (silicone or PDMS). Fabrication involved silicone casting and coating techniques. The pumps operate via electron and ion migration under an applied electric field: electrons separate from liquid molecules near positive electrodes, leaving positive ions that move towards the grounding electrode, dragging the liquid along. This creates a jet flow. The process is reversible for bidirectional pumping. A self-healing liquid (dibutyl sebacate-tung oil solution) was developed. Tung oil's properties enable it to solidify upon air exposure, thus sealing any damage to the pump. The pumps' pumpability was customized by altering applied voltage, electrode configuration (needle and hole diameter, inter-electrode gap), and liquid type. Series and parallel integration of electrode pairs affected pressure and flow rate. A lightweight, miniature high-voltage power converter (HVPC), powered by a battery or wireless system, was created to actuate the pumps. The pumps were integrated into a robotic fish and a robotic vehicle to demonstrate untethered motion. The performance of the soft electronic pumps was compared with commercially available pumps and compressors. Theoretical modeling and numerical simulation using COMSOL Multiphysics were employed to understand the electric potential, charge density, and velocity distributions within the pumps. Material properties (density, viscosity, permittivity, conductivity, ionic mobility) of the functional liquid were experimentally determined.

The researchers successfully designed and fabricated fully soft electronic pumps that utilize electron and ion migration to pump liquid. The pumps are lightweight, portable, and capable of powerful and controllable actuation. The key parameters impacting performance include the applied voltage, electrode configuration, and the type of liquid used. Bidirectional pumping was demonstrated. The self-healing liquid effectively repaired punctures in the system, restoring functionality within a reasonable timeframe (6h at 35°C, 1 day at 24°C). The customizable appearances of the pumps allow for integration into various soft robots. Performance comparisons with commercially available pumps demonstrated the superior specific pressure and flow rate of the soft pumps. A miniature HVPC enabled untethered operation of a robotic fish and vehicle, showcasing the pumps' potential as universal power sources for soft robotics. Theoretical modeling and numerical simulations validated the operational principle of the pumps.

The findings demonstrate the feasibility of fully soft, self-healing, and customizable electronic pumps for powering untethered soft robots. The superior performance characteristics of the developed pumps over existing technologies suggest a significant advancement in soft robotics. The ability to customize both the appearance and pumpability offers versatility for integrating into various robotic systems. The self-healing capability significantly improves the reliability and longevity of the robotic systems, addressing a critical challenge in the field. The successful integration into a robotic fish and vehicle provides strong evidence for the pumps’ broad applicability. The combination of powerful actuation, high speed, portability, and self-healing makes these pumps a promising universal power source for untethered soft robots.

This study presents a significant advancement in soft robotics by introducing bio-inspired, fully soft electronic pumps with self-healing capabilities. These pumps offer superior performance in terms of actuation power, speed, portability, and reliability compared to existing technologies. The customizable nature of the pumps, along with the development of a miniature HVPC, enables the integration into diverse robotic systems. Future research could focus on exploring different self-healing mechanisms and liquid compositions, further miniaturizing the pumps and HVPC, and integrating more complex control systems for more sophisticated robotic applications.

While the self-healing functionality is effective, the healing time is dependent on temperature and air exposure. The current study focused on specific liquid compositions; further research is needed to explore a broader range of liquids for diverse applications. The high voltage required for operation may pose safety considerations. Long-term stability of the self-healing liquid needs further evaluation. Although the frequency limit of the pumps has been tested to 10Hz, improving its frequency limit is another area that needs more research.