Self-protection soft fluidic robots with rapid large-area self-healing capabilities

Soft fluidic robots are widely used due to their large deformation, simple fabrication, and low cost, yet development is limited by bulky tethered fluidic power sources, limited performance of embedded miniature rigid pumps, vulnerability to damage, and the lack of intelligent self-protection. Alternative untethered actuation strategies (phase change, peroxide decomposition, combustion) remove the tether but typically reduce speed, force, or controllability. Electrohydrodynamic (EHD) pumping offers portability and controllability, but existing EHD pumps embedded in robots often yield slow response, small forces, limited stroke, or constrained designs. In parallel, current self-healing efforts in soft robotics primarily repair elastomers and are typically slow and limited to small-area damages where fracture surfaces contact. Prior liquid-based healing could cure upon air exposure but suffered from poor adhesion, low stretchability (~20%), and long cure times (>6 h). Therefore, the authors aim to develop an integrated design for soft fluidic robots that achieves high-speed untethered actuation and intelligent self-protection, including self-sensing, autonomous damage judgment, active self-heating, and rapid large-area self-healing.

The paper situates its contribution within soft robotics powered by EHD pumps and self-healing materials. Prior EHD solutions include electro-conjugate fluid (ECF) pumps, stretchable pumps, and soft electronic pumps, which offer portability and controllability but, when embedded, often exhibit slow actuation (seconds-scale), small forces, and limited stroke. Untethered strategies like gas–liquid phase change, H2O2 decomposition, and combustion have removed tethers but introduce slow dynamics, reduced output, or poor control. Self-healing research in soft materials has focused on intrinsically healable elastomers and soft electronics, enabling recovery after small-area damages but typically requiring long times and/or heat, with difficulty in bridging gaps where fracture surfaces do not contact. Some devices offer sensing, damage detection, or heating to assist healing, and liquid metal composites can autonomously regain conductivity, but a unified soft system integrating self-sensing, autonomous damage judgment, and self-heating to realize rapid large-area healing had not been demonstrated. The authors also reference prior soft fluidic systems and actuators, indicating gaps in high-speed untethered performance and integrated intelligence.

Design strategy: Self-protection soft fluidic robots integrate four core components: (i) soft EHD pumps, (ii) soft actuators, (iii) liquid-metal E-skins for sensing/heating, and (iv) healing electrofluids. The design workflow: design actuators and E-skins; design soft EHD pumps; optimize via finite element analysis (FEA); fabricate and integrate into robots. Motions (bending, twisting, contracting) are generated by pumping fluid between chambers to deform actuators. Wireless control uses an ESP8266 WiFi module and a smartphone app (Gizwits IoT platform). Soft EHD pumps: Working principle is EHD flow in dielectric liquids driven by inhomogeneous electric fields. Authors propose a conical-array–porous-plate electrode pair to enhance field strength and flow versus cylindrical or planar electrodes. Multi-material FDM 3D printing (Raise3D E2, dual-nozzle) fabricates soft electrodes and supports using conductive TPU (Eel, 90A, NinjaTek) for electrodes and non-conductive TPU (60A, NinjaTek) for structure. Individual components are printed and assembled. Measured electrode resistances: conical array ~4–10 kΩ; porous plate ~0.3–0.9 kΩ. Pumps are deformable and remain functional unless shorted or dielectric breakdown occurs. System dielectric breakdown voltage reported ~17 kV. Multi-electrode-pair integration increases flow and pressure. Actuators and E-skins: Actuators are mold-cast Ecoflex 00-20 or 00-30 (1:1 parts A/B; vacuum degas ~3 min; room-temp cure ~6 h). E-skin is a liquid metal (Ga-In-Sn alloy; 62% Ga, 25% In, 13% Sn; melting point 5 °C) patterned as conductive traces either coated on actuator surfaces or embedded and sealed with Ecoflex 00-10. E-skin provides two switchable functions: sensing (resistive strain) and Joule heating (cannot be simultaneous; switched by control electronics). Healing electrofluids: Motivated by observed mutual solubility of methyltracetoxysilane (C6H12O6Si) and dibutyltindilaurate (C32H64O4Sn) with certain electrofluids, the healing electrofluid is prepared by dissolving methyltracetoxysilane and dibutyltindilaurate in dibutyl sebacate functional liquid at a 3:10 mass ratio using magnetic stirring. Measured properties: viscosity ~1.116×10^-2 Pa·s (rotational viscometer), conductivity ~1.99×10^8 S/m (conductivity meter), relative permittivity ~4.2815 at 1 GHz (coaxial reflection). Upon air exposure, the electrofluid cures into a self-healed film with high stretchability and strong adhesion to silicone rubbers. Temperature-dependent curing time is characterized using a temperature-controlled chamber. At room conditions, time to failure of the electrofluid is ~10 days. Electrofluids swell silicone rubbers, stabilizing at ~5% swelling; time to 5%: linalyl acetate ~2 h; dibutyl sebacate ~20 days; Fluorinert FC-40 ~50 days. Robot fabrication: Actuators and soft EHD pump are bonded (704 white adhesive) to form two connected chambers; after 3 h room-temp cure, pump surfaces are sealed with Ecoflex 00-20. Chambers are filled with ~20 mL healing electrofluid via syringe and ports sealed with adhesive. Actuation principle and control: A two-chamber system with dual EHD electrode pairs moves fluid back and forth to produce bidirectional bending or other motions. Wireless control via ESP8266 and smartphone app; bench tests use LabVIEW, NI USB-6341 DAQ, Trek 20/20C high-voltage amplifier, and a camera for motion capture. Finite element analysis: Abaqus with Yeoh hyperelastic model fitted from ASTM D638 tensile tests. For Ecoflex 00-30: C10=0.0122 MPa, C20=-0.0018 MPa, C30=0.0005 MPa; for Ecoflex 00-20: C10=0.0093 MPa, C20=-0.0017 MPa, C30=0.0004 MPa. Pressure loads equivalent to pump output are applied to inner actuator surfaces. Dynamic testing and control: Contracting robot tested 0.5 Hz–1 kHz at 14 kV; an elastic band provides restoring force. A PID closed-loop control system (Arduino UNO, camera-based stroke feedback, high-voltage converters) demonstrates position control with fast stabilization across setpoints. Self-sensing and self-judgment: E-skin resistance varies with deformation; consistent waveforms at fixed frequency indicate reliable sensing. From datasets of healthy vs damaged robots, a self-judgment model flags damage when resistance amplitude falls below 60% of the normal amplitude at the same actuation frequency. Self-heating for rapid healing: For damage events, control switches E-skin from sensing to heating (~4 W input). Temperature rises to ~160 °C in ~3 min and stabilizes there; based on curing curve, an additional ~1 min at 160 °C yields rapid curing, so a 4-min heating cycle is used. After cooling to room temperature, E-skin reverts to sensing. Power and integration: Component power consumptions: electronics <1 W; data transmission <1 W; soft EHD pump max ~6 W; E-skin sensor <1 W; E-skin heater ~4 W. A 7.4 V, 350 mAh battery supports 7–10 min of actuation; a full self-protection cycle may consume a full battery. Application demonstrations: - Load enhancement via added electrode pairs (actuation modules) to increase force. - An untethered soft gripper (two bending actuators, onboard microchip, battery) remotely controlled via smartphone to catch a 65 cm/s falling ping-pong ball. - A three-actuator mechanical sieve performing sequential tilting to separate beads in ~28 s.

- High-speed untethered actuation using soft EHD pumps with conical-array–porous-plate electrodes fabricated by multi-material 3D printing. Embedded response time <0.25 s, over 120× faster than stretchable pumps, >40× faster than ECF pumps, and >4× faster than soft electronic pumps (Table 1). - Motion performance: • Bending robot: bidirectional bending up to ~85° at 14 kV, 2 Hz; simulation agrees with experiments. • Twisting robot: ~20° max twisting at 14 kV, 2 Hz. • Contracting robot: ~14 mm stroke at 1 Hz under 20 g load; large stroke relative to soft robot benchmarks. Load–stroke decreases with load; output force >0.8 N (Table 1). • Frequency response: noticeable deformation up to ~20 Hz; diminished stroke by 50 Hz; at 1 kHz pump produces high-frequency vibration with no apparent stroke. PID control achieves rapid stabilization across positions. - Healing electrofluid forms self-healed films with excellent stretchability (>1200%) and strong adhesion to soft silicones (Ecoflex 00-20, Silicone 5A, Elastosil M4601). Even at substrate stretch up to 700%, films remain bonded and functional. Mechanical properties of self-healed Silicone 5A samples approach pristine behavior. - Large-area self-healing (fracture surfaces not in contact) achieved for gaps less than ~5 cm; larger gaps risk hole formation and failure. Temperature strongly accelerates curing: ~83 s at 150 °C and ~10 s at 250 °C; at ~160 °C, healing time ~75 s. - E-skin enables integrated intelligence: reliable self-sensing (stable resistance waveforms at fixed frequency), autonomous self-judgment (damage threshold: 60% of normal resistance amplitude), and self-heating (~4 W to ~160 °C in ~3 min) to realize rapid large-area self-healing in an autonomous cycle (~4 min heating). - Function enhancement via combining electrodes/actuators: • Load capacity increased from 50 g (10 mm stroke) to 330 g (9 mm stroke) by adding electrode pairs. • Untethered soft gripper caught a fast falling object (65 cm/s). • Mechanical sieve completed bead separation in ~28 s versus 418 s reported previously, demonstrating fast, programmable sequential control.

The work addresses two key barriers in soft fluidic robots—limited, bulky fluidic power sources and susceptibility to damage—by integrating high-performance soft EHD pumps, a rapid-curing healing electrofluid, and a multifunctional E-skin. The conical-array–porous-plate electrode design and multi-material 3D printing deliver a compact, deformable pump with substantially faster system response than prior embedded EHD pumps, enabling high-speed untethered actuation across bending, twisting, and contracting modes. The healing electrofluid uniquely enables large-area healing where fracture surfaces do not contact, overcoming limitations of solid self-healing elastomers and earlier liquid healing approaches; its excellent stretchability and adhesion restore functionality close to pristine levels. The E-skin closes the loop by providing self-sensing for deformation monitoring, an empirical self-judgment model to detect damage (≤60% resistance amplitude threshold), and on-demand thermal actuation to accelerate curing, producing an autonomous self-protection cycle that mimics biological responses. Demonstrations (enhanced load capacity through electrode pairing, fast untethered grasping, rapid programmable sieving) illustrate scalability and application versatility. Collectively, the results advance the concept of physical intelligence in soft robots, coupling fast actuation with embedded perception and self-repair mechanisms.

This study introduces a unified design strategy for self-protection soft fluidic robots that combines soft, high-performance EHD pumps, multifunctional liquid-metal E-skins, and a rapid, strongly adhesive healing electrofluid. The robots achieve fast, untethered actuation with large deformation and demonstrate autonomous self-sensing, self-judgment, self-heating, and rapid large-area self-healing, significantly expanding reliability and functionality. Applications showcase enhanced load handling via electrode/actuator combinations, high-speed grasping, and rapid programmable material handling. Future research directions include improving electrofluid formulations (e.g., reducing swelling and extending lifetime), increasing pump efficiency and breakdown margins, optimizing actuator architectures for higher frequency performance, integrating closed-loop control that leverages onboard E-skin sensing, and extending self-protection strategies to more complex multi-DOF systems and wearable interfaces.

- Large-area healing is effective when the distance between fracture surfaces is less than ~5 cm; larger gaps tend to form holes in the cured film, leading to failure. - The E-skin cannot perform sensing and heating simultaneously; it must switch modes and cool to room temperature before returning to sensing. - Healing electrofluid has a finite usable lifetime (~10 days at room temperature) and can swell silicone substrates (~5% at stabilization), with swelling kinetics dependent on the electrofluid. - Thermal self-healing requires elevated temperatures (up to ~160 °C) and a ~4-min heating cycle, imposing energy demands (~4 W) and potential thermal constraints for certain applications. - Power is limited in fully untethered operation; a 7.4 V, 350 mAh battery supports 7–10 min of actuation and roughly one full self-protection cycle. - Pumps may fail under dielectric breakdown (~17 kV threshold), and deformation-induced shorting must be avoided. - Frequency response degrades at higher frequencies (reduced stroke by 50 Hz, negligible stroke at 1 kHz), limiting dynamic range for large-amplitude motions.