A Foldable and Agile Soft Electromagnetic Robot for Multimodal Navigation in Confined and Unstructured Environments

Abstract: Multimodal locomotion is crucial for an animal's adaptability in unstructured wild environments. Similarly, in the human gastrointestinal tract, characterized by viscoelastic mucus, complex rugae, and narrow sphincters like the cardia, multimodal locomotion is also essential for a small-scale soft robot to conduct tasks. Here, we introduce a small-scale compact, foldable, and robust soft electromagnetic robot (M-SEMR) with more than nine locomotion modes designed for such a scenario. Featuring a six-spoke elastomer body embedded with liquid metal channels and driven by Laplace forces under a static magnetic field, the M-SEMR is capable of rapid transitions (< 0.35 s) among different locomotion modes. It achieves exceptional agility, including high-speed rolling (818 mm/s, 26 BL/s), omnidirectional crawling, jumping, and swimming. Notably, the robot can fold to reduce its volume by 79%, enabling it to traverse confined spaces. We further validate its navigation capabilities on complex terrains, including discrete obstacles, viscoelastic gelatin surfaces, viscous fluids, and simulated biological tissues. This system offers a versatile strategy for developing high-mobility soft robots for future biomedical applications.

• The paper presents a foldable soft electromagnetic robot capable of over 9 locomotion modes, including rapid rolling (up to 818 mm/s) and adaptable terrain navigation.
• The design integrates modular silicone elastomer actuators with liquid metal channels, enabling fast posture transitions (<0.35 s) and precision control via electrical signals.
• Demonstrated robustness across varied substrates and fluids indicates promising biomedical applications, such as minimally invasive procedures in the GI tract.

Foldable Soft Electromagnetic Robots for Agile Multimodal Navigation in Complex Environments

Overview and Motivation

This work presents a miniaturized, fully soft electromagnetic robot (M-SEMR) that achieves high-speed, multimodal locomotion and compact foldability, targeting navigation in highly constrained and unstructured environments such as the human gastrointestinal (GI) tract. The robot integrates a six-spoke elastomeric structure with liquid metal channels, actuated by Laplace forces in a static magnetic field. The M-SEMR demonstrates >9 distinct locomotion modes, rapid posture transitions, and robust adaptation to diverse terrains and fluids. The primary motivation is to address unmet challenges in soft robotics—particularly, combining versatility, speed, and environmental adaptability at small scales for scenarios like minimally invasive biomedical operations.

Design Principles and Actuation

The M-SEMR features a modular design: six soft electromagnetic actuator modules are assembled via mortise and tenon joints to minimize weight (robot mass: 3.84 g). Each module comprises a silicone elastomer shell containing a micro-patterned channel filled with conductive Galinstan liquid metal, providing soft, electrically responsive actuation. The robot is driven by precisely controlled currents (up to 2 A) through individual modules in a uniform magnetic field (MRI-compatible), leveraging Laplace-force-based deformations for fast, programmable shape changes.

Critically, the actuators support both bending and twisting deformations. Sequential square-wave current actuation enables rolling, crawling, walking, and rotational maneuvers. Structural compliance, validated by both dynamic simulations and experiments, multiplies the work output relative to rigid designs and underpins the agility of the robot.

Locomotion Modalities and Performance

The M-SEMR achieves multimodal locomotion, partitioned across two principal postures (standing, lying):

• Rolling: Fastest reported rolling velocity among small-scale soft robots, peaking at 818 mm/s (26 BL/s), with robust performance (>500 mm/s) across diverse substrates (metal, polymer, sandpaper, paper).
• Walking: Three distinct walking gaits, tunable by selectively activating top or bottom modules, achieving up to 24.26 mm/s and precise directionality modulation.
• Crawling: Six-directional crawling enabled by dual-module actuation strategies (O-, M-, P-actuation), with in-place rotation, fault tolerance, and complex trajectory execution (e.g., tracing “ZJU” patterns).
• Jumping and Steering: Manifest as intermittent jumps during rapid rolling and on-demand in-place turns via differential actuation.
• Swimming: Underwater rolling and V-actuation, verified at frequencies from 5 Hz to 100 Hz, with an average underwater rolling speed of 51.7 mm/s.

Notably, posture transitions are achieved in <0.35 s (lying–standing/standing–lying), enabling rapid switching across all locomotion modes with a single system—without external reconfigurable actuators. The robot can reduce its volume by 79% via folding, facilitating passage through strict bottlenecks (e.g., a simulated gastric cardia).

Environmental Robustness

Experiments demonstrate traversal over stepped obstacles (up to 0.37× body height), periodic or sinusoidal surfaces, viscoelastic gelatin (mimicking tissue/mucus), viscous non-Newtonian fluids (yogurt solution), and direct media transitions from aquatic to terrestrial environments. Locomotion is preserved even if some modules fail, attesting to intrinsic fault tolerance.

In a representative scenario, a folded robot is deployed through a narrow tube (diameter 14.5 mm), unfolds automatically in water upon dissolution of a PVA encapsulation, and resumes locomotion. Integrated drug release was mediated by a chamber with platinum electrodes, using controlled electrolysis for precise dosing and location-targeted delivery in a simulated gastric environment.

Comparative Analysis

Relative to prior art, the M-SEMR establishes several strong claims:

• Fastest rolling velocity for soft robots at the described size scale.
• Unprecedented multimodality: >9 discrete locomotive modes, including omnidirectional crawling and amphibious operations, all controlled via electrical signals without mechanical reconfiguration.
• Superior adaptability: Demonstrated versatility in terrain, compressive resilience, and post-damage functional recovery.
• Biomedical relevance: The volume reduction during folding (down to 21% of original) and safe operation in MRI-strength magnetic fields align with requirements for in vivo biomedical devices.

These performance metrics are supported by a combination of experimental measurements, dynamic modeling, and systematic comparative analysis to existing state-of-the-art soft robots.

Limitations and Future Research Directions

The present implementation is limited by fabrication constraints; hand-assembly and molding constrain further miniaturization and integration. The prototype is tethered; embedding onboard power/control (potentially with micro-batteries and electronics) remains a challenge for untethered deployment. Further optimization of actuation frequency and dynamic behaviors (especially in viscous/heterogeneous environments) is open for systematic study, potentially via multilayered modeling and AI-based control.

Future developments could include:

• Scaling down via high-precision multimaterial 3D printing to reach truly submillimeter insertable robots.
• Fully wireless operation using micro-electronic integration and wireless energy transfer.
• Advanced closed-loop control exploiting real-time feedback for autonomous navigation in biological tissue.
• Integration with tissue/interventional imaging and multi-agent cooperation for distributed tasks inside the body.

The implications for soft robotic design are significant: this modular, Laplace-force-based scheme could generalize to other use cases requiring extreme adaptability, fault tolerance, and safe operation in both medical and search-and-rescue contexts.

Conclusion

This work demonstrates a compact, foldable, and agile soft electromagnetic robot capable of high-speed, multimodal locomotion across diverse and challenging environments. The integration of modular soft electromagnetic actuation, rapid posture switching, superior environmental adaptability, and a demonstrated pathway toward biomedical applications marks a substantial advance in soft robotics. While fabrication and system integration challenges remain, the M-SEMR architecture offers a promising foundation for future minimally invasive, adaptive robotic systems designed for complex, confined, or dynamic environments.