Inspecting safety-critical built environments like power plants and bridges requires accessing difficult-to-reach areas, often involving narrow gaps (near-millimeter and sub-millimeter high) and diverse terrains (solid, liquid, aerial). Existing robots, while adept at navigating macro-scale obstacles, struggle with these micro-scale challenges and domain transitions. Humanoid, wheeled/tracked, and quadrupedal robots excel in larger spaces but lack the agility for confined areas. Smaller robots, such as worm-like and insect-like designs, usually possess a single locomotion mode, limiting their versatility. The need for robots capable of multimodal locomotion in narrow spaces across multiple domains remains a significant challenge. Narrow access points (millimeter scale) are ubiquitous in built environments: under doors, within duct systems, and inside complex machinery. Millimeter-scale thickness and multimodal locomotion are crucial for effective navigation. Soft robots, actuated by pneumatics, dielectric elastomers, shape memory alloys, or magnetic fields, have been developed, primarily for medical applications. However, magnetic actuation is impractical in built environments due to interference from ferromagnetic materials. Dielectric elastomer actuators (DEAs) offer high-power density, flexibility, and robustness, making them a suitable choice for building thin, multi-degree-of-freedom (DoF) robots. The ability to climb inclined or vertical surfaces further necessitates reversible attachment mechanisms, with various approaches like bio-inspired dry adhesion, vacuums, and electroadhesion being explored. This research proposes a novel approach to develop TS-Robots for multimodal locomotion in solid and liquid environments, including domain transitions. The design comprises a Thin Soft Dielectric Elastomer Actuator (TS-DEA) and Electrostatic Adhesive Pads (EA-Pads), configurable in symmetric and asymmetric arrangements. The capabilities of the TS-Robots are further enhanced by the ability to join multiple robots for complex navigation and manipulation within confined spaces.

Numerous studies have explored the development of robots for locomotion in various environments. Humanoid robots like HRP-5P demonstrate advanced capabilities, while wheeled/tracked and quadrupedal robots offer versatile mobility across different terrains. However, accessing small, confined spaces presents a unique challenge. Worm-like and insect-like robots have been proposed, but often these are limited to single locomotion modes. Existing soft robotic solutions often employ pneumatic actuation, dielectric elastomers, shape memory alloys, or external magnetic fields. Many small-scale systems, especially those for medical applications, rely on external magnetic fields for actuation. While effective in controlled environments, this method is not practical in human-made environments due to the large dimensions and potential interference from common materials. DEAs present a viable alternative, offering high power density, flexibility, and robustness for small-scale robots. Several designs of miniature DEA-driven soft robots capable of single or multiple locomotion modes within single domains have been developed. However, the challenge remains to develop robots capable of multimodal locomotion across multiple domains (solid, liquid, air) within highly confined spaces.

The TS-Robots are designed with two subsystems: a TS-DEA for generating movement and force, and EA-Pads for surface adhesion. The TS-DEA employs a dual-actuation sandwich structure with two actuation layers driving a compressible tensioning mechanism. This design allows for both in-plane linear compression/extension and out-of-plane bending motion. The Poisson's ratio of the tensioning mechanism is tunable, enabling control over the anisotropy of the motion along the X and Y axes. Analytical models are developed to describe the linear and bending motion of the TS-DEA, considering the forces generated by the actuation layers and the tensioning mechanism. EA-Pads utilize electrostatic adhesion technology to generate attraction forces with substrate surfaces. Three types of robots were fabricated for evaluation: * **Type-A TS-Robot:** Zero Poisson's ratio TS-DEA (ν = 0) with two EA-Pads, performing extension/contraction and bending in one direction. * **Type-B TS-Robot:** Negative Poisson's ratio TS-DEA (ν = -1) with four EA-Pads, enabling extension/contraction and steering in two directions. * **Type-C TS-Robot:** Uses silicone-based elastomers for high-frequency actuation. Two variants (Type-C-I and Type-C-II) were tested, differing in tensioning mechanism stiffness and number of actuation layers. The TS-DEAs were characterized for static and dynamic displacement, blocking force, resonant frequency, output power, and energy conversion efficiency. Ageing tests were conducted to assess performance changes over time. Locomotion tests included crawling, climbing, steering, swimming, and landing. Gait and control strategies were detailed. The robots were tested on various materials (PET, wood, paper, PVC) to determine the impact of substrate material on locomotion speed. Multiple joined TS-Robots were tested for surface transitions and complex manipulations. A twin system of Type-A robots with a passive hinge joint, and a serial kinematic system (SK-TS-Robot) combining Type-A and Type-B robots with an active hinge joint were evaluated. Collaboration with other robots was explored using a Flying-TS-Robot (Type-A TS-Robot + drone) for cross-domain locomotion (terrestrial and aerial). The materials used for fabrication included VHB 4910 and silicone-based elastomers (Ecoflex 00-30 and Sylgard 184). Electrodes were made of multi-walled carbon nanotubes, and the tensioning mechanisms were made of PETG. EA-Pads were fabricated using a silver ink printed onto a polyimide film.

The TS-Robots demonstrated remarkable capabilities: * **Multimodal Locomotion:** Successfully performed crawling, climbing, steering, swimming, and landing across solid and liquid domains. * **High Performance:** Achieved a maximum speed of 43.1 mm/s (1.16 body length/s and 13.1 body thickness/s) with the Type-C-II TS-Robot, and a maximum crawling speed of 2.3 mm/s (3.5% body length/s) and 1.7 mm/s (2.8% body length/s) on horizontal and vertical surfaces respectively, for the Type-A TS-Robot. * **Tunable Actuator:** The resonant frequency of the TS-DEAs was easily tuned by adjusting the stiffness of the tensioning mechanism, offering a less time-consuming alternative to chemical methods. * **Cross-Domain Locomotion:** Demonstrated transitions between solid and liquid domains, seamlessly switching between crawling and swimming gaits. * **Collaborative Locomotion:** Multiple joined TS-Robots successfully performed surface transitions and complex manipulations, such as reconnecting a damaged circuit. * **High Payload Capacity:** The robots exhibited a high payload-to-weight ratio, allowing them to carry additional robots for extended capabilities. * **Cross-Domain Collaboration:** A hybrid Flying-TS-Robot (TS-Robot + drone) demonstrated successful cross-domain (terrestrial-aerial) locomotion for object delivery. Specific speeds and forces were measured for different robots and gaits under various conditions, detailed in the paper. The impact of different substrate materials on locomotion speed was also quantified.

The results address the challenge of designing thin soft robots for multimodal locomotion in narrow, complex environments. The TS-Robots successfully demonstrated the ability to navigate confined spaces and transition between different domains, overcoming limitations of existing robots. The high output force and speed, coupled with the adaptability of the gaits, highlight the significant potential of this technology. The ability to tune the resonant frequency mechanically simplifies the actuator design and fabrication process. The collaborative capabilities of multiple robots greatly enhance the operational range and complexity of tasks that can be undertaken. The collaboration with other robot types, as showcased with the Flying-TS-Robot, demonstrates a path towards even more versatile and adaptable robotic systems. These findings are significant for applications in various fields, including inspection, maintenance, and exploration of hazardous or difficult-to-access environments.

This paper presents a novel design of ultra-thin soft robots capable of multimodal locomotion across multiple domains in highly confined spaces. The robots demonstrate exceptional performance, adaptability, and collaborative capabilities. Future research could focus on enhancing the autonomy of the robots, developing more sophisticated control algorithms, exploring new materials for improved performance, and expanding the range of applications in diverse fields such as infrastructure inspection, search and rescue operations, and minimally invasive surgery.

The current design of the EA-Pads limits the maximum actuation frequency due to the charging and discharging time. The performance of the electrostatic adhesion is also dependent on the surface properties of the substrate. While the robots demonstrated impressive capabilities, further research is needed to optimize the design and materials for improved robustness and longer operational lifespan. The complexity of the control system for multiple robots may need to be improved for more complex tasks.