Multimodal locomotion ultra-thin soft robots for exploration of narrow spaces

Robots operating in built environments face diverse obstacles such as doors, stairs, narrow gaps, and mixed domains (solid, liquid, aerial), which hinder access to targets. While macro-scale humanoid, wheeled/track, and quadruped robots can handle large obstacles, accessing small, confined spaces requires millimetre-scale platforms. Existing small soft robots (worm- or insect-like) typically offer a single locomotion mode tailored to specific obstacles, limiting their applicability in complex real-world environments that demand multimodal locomotion and cross-domain transitions. Narrow accesses at millimetre scales are prevalent in civil and industrial environments (e.g., under doors, ducts, machine gaps like turbine engines), necessitating thin robots (millimetre/sub-millimetre thickness) with multiple gaits. Soft robots actuated by pneumatics, dielectric elastomers, shape memory alloys, thermal polymers, or magnetic fields have shown promise; many miniature systems target biomedical applications with magnetic actuation and some multimodal capabilities. However, applying external magnetic control across large, ferromagnetically cluttered built environments is challenging due to scale and material interference. This work proposes ultra-thin DEA-driven robots with multimodal locomotion across solid and liquid domains, overcoming access constraints in built environments and enabling transitions between domains and surfaces.

Prior work includes macro-scale robots capable of navigating complex terrains but unsuitable for near-millimetre gaps. At small scales, soft robots have used various actuation modalities: magnetic actuation for untethered control and multimodality, pneumatics for compliant locomotion, thermal and SMA-based approaches, and electrostatic/electroadhesion for adhesion and mobility. DEA-driven robots have demonstrated high power density and robustness with single or limited multimodal locomotion in single domains (jumping, crawling) suitable for environmental monitoring. Attachment methods for vertical locomotion include dry adhesion, vacuum, and electroadhesion; electroadhesion is particularly suitable for lightweight, thin robots. Magnetic control, despite high versatility in confined biomedical contexts, is less feasible in large built environments due to scale and ferromagnetic interference. This work builds on DEA actuation and electroadhesion to realize ultra-thin, multimodal robots capable of cross-domain locomotion and transitions between surfaces.

System architecture: TS-Robots comprise (i) a Thin Soft Dielectric Elastomer Actuator (TS-DEA) producing in-plane extension/contraction and out-of-plane bending, and (ii) electrostatic adhesive pads (EA-Pads) providing reversible anchoring on diverse substrates. The TS-DEA employs a dual-actuation sandwich structure: two actuation layers (elastomer membranes with compliant electrodes) bonded via adhesive to a central compressible tensioning mechanism. Synchronized actuation of both layers yields linear extension/contraction; differential actuation yields bending. The tensioning mechanism uses re-entrant architectures with tunable Poisson’s ratio (negative to zero) by adjusting the diagonal strut angle, enabling anisotropic motion along X and Y. Analytical modeling relates DEA stress, voltage-induced forces, and tensioning mechanism restoring forces for linear and bending motions, allowing prediction of displacement and bending moment as functions of input voltage and geometry. Mechanical tuning of the tensioning stiffness adjusts the resonant frequency without changing elastomer chemistry. Electroadhesion: EA-Pads, fabricated via printed silver electrodes on thin polyimide, provide surface attachment; polarization/depolarization time (~100–300 ms) constrains terrestrial gait frequency. Robot variants: - Type-A TS-Robot: TS-DEA with zero Poisson’s ratio (ν=0) and two EA-Pads; one-dimensional extension/contraction and bending. - Type-B TS-Robot: TS-DEA with negative Poisson’s ratio (ν=−1) and four EA-Pads; bidirectional steering (X and Y). - Type-C TS-Robots (silicone-based for high-frequency actuation): Type-C-I (two actuation layers) and Type-C-II (four layers); L-Type-C (low-voltage, pure Ecoflex). Control and gaits: For crawling, Type-A uses a sinusoidal voltage for the DEA and square-wave voltages for front/rear EA-Pads in a two-step cycle to alternately anchor and advance pads; reversing phase enables backward motion. For swimming, bending actuation drives an undulatory gait using a rear EA-Pad as a tail to generate thrust. Wiring methods (two- and three-pole) were developed for safe aquatic operation. Collaborative assemblies: Two Type-A robots coupled via a passive hinge (twin system) enable transitions between horizontal and vertical surfaces using three gaits (dual crawling/climbing, push-assisted flip, pull-assisted flip). A Serial Kinematic TS-Robot (SK-TS-Robot) connects Type-A and Type-B via an active hinge (biasing bending DEA), enabling flat and L-shaped configurations and four gaits (linear crawl, front flip-up, back flip-up, steering). Hybrid system: A Flying-TS-Robot combines a Type-A TS-Robot with an off-the-shelf drone using an additional EA-Pad for attachment and a front EA-Pad on a thin PETG beam for payload pickup/release. Materials and fabrication: Type-A/B use VHB 4910 elastomer; Type-C use silicone elastomers (Ecoflex 00-30 and Sylgard 184 at 40:1 cross-linker in a 1:1 mix; or pure Ecoflex 00-30 for low-voltage L-Type-C), spin-coated to target thickness. Compliant electrodes are multi-walled carbon nanotube layers. Adhesives: ultra-thin VHB (3M 9460) for VHB-based DEAs, Bond Flex for silicone-based. Tensioning mechanisms are laser-cut PETG (typically 1 mm; 0.25 mm for curved Type-A). EA-Pads: printed silver electrodes (six layers) on 25 µm polyimide with controlled droplet spacing and thermal post-processing, total thickness ~28 µm. Electronics: Signal generator and high-voltage amplifiers for silicone-based designs; Arduino-based control for EA-Pads; thin lead wires connect DEAs and pads. Characterization included static/dynamic displacement, blocking force, resonant frequency, output power and efficiency calculations, and ageing over 15 days.

- Ultra-thin, multimodal robots: TS-Robots with thickness as low as 1.7 mm (Type-A/B) and 0.8 mm (curved Type-A; Type-C) access 2–3 mm gaps and transition across surfaces and domains. - High force-to-weight: Type-A DEA produced up to 0.57 N (≈48× its own weight), enabling carrying other robots (e.g., a 36 g drone) and payloads up to 3 g (L-Type-C; 2.5× self-weight). - Terrestrial locomotion: Type-A achieved maximum crawling speeds of 2.3 mm/s (3.5% BL/s) horizontally and 1.7 mm/s (2.8% BL/s) vertically on PET at 4 Hz; speeds vary with substrate (PET, wood, paper, PVC). Type-B (steerable) reached 0.57 mm/s (X) and 0.51 mm/s (Y) at 4 Hz on PET. - Aquatic locomotion: Modified Type-A swam at ~45.5 mm/s (0.7 BL/s) in silicone oil (viscosity 5) at 12 Hz, nearly 20× faster than terrestrial crawling; undulatory gait observed via high-speed imaging. - High-frequency silicone DEAs: Type-C-I resonant frequency 51 Hz (0.22 mm dynamic displacement at 21.7 V/µm); Type-C-II resonant 67 Hz (0.13 mm), with reduced displacement due to interlayer friction. Type-C-II robot achieved 43.1 mm/s (1.16 BL/s; 13.1 BT/s) at resonance. A low-voltage L-Type-C (220 V) reached 12.4 mm/s (0.33 BL/s; 3.75 BT/s) at 86 Hz. - Mechanical tuning: Resonant frequency is readily tuned by changing tensioning stiffness without altering elastomer chemistry. - Ageing: VHB-based Type-A showed gradual displacement reduction over ~10 days before stabilizing; silicone-based Type-C-I stabilized after ~5 days. - Real-world demonstration: Curved Type-A (0.8 mm thick) navigated a 1.2 mm air gap between rotor and stator of a Rolls-Royce AE2100 generator mock-up for inspection feasibility. - Multi-robot collaboration: Twin Type-A system transitioned from horizontal to vertical surfaces using coordinated gaits. SK-TS-Robot (Type-A + Type-B with active hinge) completed a two-floor mission through 3 mm gaps and wall transitions in nine steps, reconnected a damaged circuit, and transmitted SOS Morse code via controlled EA-Pad contact; speeds ranged 0.34–1.18 mm/s depending on gait and direction. - Cross-domain hybrid system: Flying-TS-Robot (TS-Robot + drone) crawled 200 mm through a 50 mm-high tunnel in 223 s while carrying the drone, flew over a trench in 2 s, then the TS-Robot crawled through a 3 mm gap to deliver and release a letter. Payload affects speed; optimal frequency (3 Hz) selected for 1 mm/s while carrying the drone. - Stiffness and performance: Reported linear and bending stiffness of 311.5 N/m and 4.5 N/m; maximum payload/self-weight ratio up to 29:1.

The work targets two main challenges: achieving multimodal locomotion across multiple domains in near-millimetre spaces (Ch1) and coordinating multiple robots to transition between surfaces and handle complex tasks (Ch2). The proposed dual-layer TS-DEA with a tunable re-entrant tensioning mechanism provides both linear and bending gaits, enabling crawling, climbing, steering, swimming, and landing within 2–3 mm gaps and across solid–liquid boundaries. By tuning the Poisson’s ratio, anisotropic motions allow steering and enhanced maneuverability (Type-B). Mechanical stiffness tuning of the tensioning mechanism offers a practical method to set resonant frequencies, avoiding time-consuming chemical modifications. Electroadhesion affords reversible attachment on various substrates, enabling vertical locomotion and transitions. The robots demonstrated high force-to-weight and stiffness sufficient for collaborative tasks—twin and serial kinematic assemblies transition between horizontal and vertical surfaces, traverse narrow gaps, and manipulate objects/circuits (e.g., reconnecting a damaged circuit and transmitting Morse code). Cross-domain collaboration with a drone extends operational envelopes to include aerial transitions while maintaining access to confined spaces. These capabilities position TS-Robots as strong candidates for inspections and operations in industrial and civil infrastructures, security monitoring, and hazardous environment observation.

This paper presents ultra-thin, DEA-driven TS-Robots capable of multimodal locomotion across solid and liquid environments and transitions between surfaces, enabled by a dual-actuation sandwich DEA with tunable Poisson’s ratio tensioning and electroadhesive anchoring. The robots achieve high force-to-weight ratios, tunable dynamics via mechanical stiffness adjustment, and practical gaits for crawling, climbing, steering, swimming, and collaborative manipulation. Demonstrations include navigation through 2–3 mm gaps, inspection within a 1.2 mm generator air gap, two-floor traversal with circuit repair, and a hybrid crawling–flying delivery task with a drone. Future improvements identified by the authors include reducing damping by integrating independent actuation layers into multi-layer structures to increase dynamic displacement, adding more layers to raise resonant frequency and speed, and advancing towards power autonomy using low-voltage designs. The approach opens avenues for robust, deployable thin soft robots for inspection, maintenance, and operations in complex built environments.

- Electroadhesion dynamics: EA-Pad polarization/depolarization time (100–300 ms) limits terrestrial gait frequency and thus crawling speed. - Material ageing: VHB-based actuators exhibit viscoelastic creep and displacement reduction over the first ~10 days; silicone-based actuators stabilize after ~5 days, indicating performance drift over time. - Dynamic damping: In multi-layer silicone DEAs (Type-C-II), friction between adjacent actuation layers reduces dynamic displacement; integration into a single multilayer structure is proposed to mitigate damping. - Substrate dependence: Locomotion speed varies with surface material (PET, wood, paper, PVC) due to differing adhesion/friction characteristics. - Payload effects: Increased payload reduces crawling speed due to higher frictional forces at EA-Pads. - Power and control: Experiments used external HV amplifiers and tethers; although a low-voltage variant was demonstrated, fully autonomous onboard power/control was not shown.