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

This paper presents a compact, foldable soft electromagnetic robot (M-SEMR) capable of rapid transitions among over nine locomotion modes, including high-speed rolling and jumping, enabling agile navigation through confined and unstructured environments such as the human gastrointestinal tract.

The Gist

Imagine a tiny, super-flexible robot that can squeeze through the tightest spaces, roll like a wheel, crawl like a bug, jump like a flea, and even swim like a fish—all without changing its shape or swapping parts. That is exactly what a team of engineers at Zhejiang University has created. They call it the M-SEMR (Multimodal Soft Electromagnetic Robot).

Here is a simple breakdown of how it works, what it can do, and why it matters, using some everyday analogies.

1. The "Origami" Body

Think of the robot as a six-spoked wheel made of soft rubber, but instead of being solid, it's hollow and filled with liquid metal (like a super-conductive mercury).

• The Fold: The coolest part? It's like a piece of origami. When it needs to get through a tiny door (like the entrance to your stomach), it can fold itself up into a tight cylinder, shrinking its size by nearly 80%.
• The Unfold: Once it's inside, it pops back open like a spring-loaded toy, ready to work.

2. The "Magic" Engine

This robot doesn't have batteries or motors inside it. Instead, it's powered by magnetic fields (like the giant magnets in an MRI machine).

• How it moves: Imagine the robot is a soft toy with liquid metal wires inside. When you zap it with electricity while it's sitting in a magnetic field, the liquid metal pushes against the magnet. This creates a "Laplace force" that bends the robot's soft arms.
• The Result: By turning these "zaps" on and off in a specific rhythm, the robot can bend, twist, and roll. It's like conducting an orchestra where the conductor is a magnetic field, and the robot is the instrument playing different tunes (modes of movement).

3. The "Swiss Army Knife" of Movement

Most robots are good at one thing: a car rolls, a fish swims, a spider crawls. This robot is a multitasking master. It has nine different ways to move, switching between them in less than half a second (faster than you can blink).

• Rolling: It can spin like a wheel at incredible speeds (up to 818 mm/s). That's like a race car zooming across a table.
• Walking/Crawling: It can stand up and walk like a bug, or lie down and crawl. It can even walk backward or turn in place.
• Jumping: It can launch itself over small obstacles.
• Swimming: It can dive into water and swim using a "V-shape" motion, similar to how a jellyfish propels itself.

4. Why Do We Need This? (The "Stomach" Scenario)

The main reason the scientists built this is for medicine, specifically for navigating the human body.

• The Problem: Your stomach and intestines are messy, sticky, and full of narrow twists and turns. Old robots get stuck, can't turn, or are too big to fit through the "cardia" (the tight valve at the top of the stomach).
• The Solution: This robot is small, soft, and foldable.
  1. Delivery: You swallow it (or insert it) in its folded state. It slips through the tight valve.
  2. Deployment: Once inside the stomach, it unfolds.
  3. Navigation: It can roll over the bumpy walls of the stomach, swim through the mucus, and even climb over obstacles.
  4. The Mission: It can go to a specific spot and release medicine (like a tiny, smart pill) exactly where it's needed, then swim away.

5. Tough as a Rubber Band

The robot is incredibly durable.

• Crush Test: If you squish it flat with a heavy weight, it bounces back to its original shape and keeps working.
• Broken Parts: If one of its six "legs" (modules) breaks, the other five can still carry the robot. It's like a car that can still drive even if one tire is flat.

The Big Picture

Think of this robot as the ultimate rescue worker for the inside of your body. Instead of invasive surgery, a doctor could send this tiny, agile explorer into your digestive tract to check for problems, deliver drugs to a tumor, or clear a blockage. It combines the speed of a race car, the flexibility of a gymnast, and the resilience of a rubber band, all controlled by a magnetic field.

While it currently needs a wire for power (like a toy on a string), the future goal is to make it fully wireless and even smaller, turning it into a true "micro-surgeon" for the human body.

Technical Summary

1. Problem Statement

Current small-scale soft robots face significant challenges when navigating complex, unstructured environments, particularly within the human gastrointestinal (GI) tract. Key limitations include:

• Environmental Constraints: The GI tract features viscoelastic mucus, complex rugae (folds), and narrow sphincters (e.g., the cardia) that restrict mobility.
• Lack of Multimodal Adaptability: Existing robots often excel in a single mode (e.g., swimming, rolling, or jumping) but fail to transition effectively between modes or adapt to abrupt terrain changes.
• Structural Rigidity and Size: Many devices cannot fold to reduce volume for passage through narrow openings, nor can they recover from physical damage or physical constraints.
• Control Limitations: Existing solutions often suffer from slow response times, bulky tethering systems, or safety concerns regarding high voltages.

2. Methodology

The authors developed a Multimodal Soft Electromagnetic Robot (M-SEMR) designed to overcome these limitations through a synergy of material science, structural design, and control strategies.

• Design and Fabrication:

  • Structure: The robot consists of six identical modular units arranged in a six-spoke elastomer configuration.
  • Materials: Each module is a soft elastomeric shell (a composite of Ecoflex-30 and PDMS) embedded with liquid metal (Galinstan) channels acting as soft conductive coils.
  • Assembly: Modules are connected via mortise and tenon joints to minimize mass. The total weight is 3.84 g.
  • Foldability: The robot can be folded into a compact cylinder, reducing its volume by 79% (occupying only 21% of its original volume) to pass through narrow constraints like the gastric cardia.

• Actuation Mechanism:

  • Principle: The robot is driven by Laplace forces generated when current passes through the liquid metal channels within a static global magnetic field (e.g., an MRI environment).
  • Control: A custom PWM controller allows independent actuation of each module with square-wave currents. This enables precise control over bending, twisting, compression, and extension.
  • Postures: The robot operates in two primary postures: Standing (for rolling and walking) and Lying (for crawling and V/U-actuation).

• Control Strategies:

  • Posture Switching: The robot can transition between standing and lying postures in under 0.35 seconds using specific current sequences.
  • Locomotion Modes: By varying the activation sequence and polarization of the modules, the robot achieves nine distinct locomotion modes:
    1. Rolling
    2. Walking (3 variants: Walk-1, Walk-2, Walk-3)
    3. Crawling (3 variants: O-, M-, and P-actuation based on xylene isomer positions)
    4. Jumping/Steering/Backward motion
    5. Swimming (V-actuation)

3. Key Contributions

• Highest Multimodal Diversity: The M-SEMR possesses the most abundant locomotion modes (nine) among reported small-scale soft robots, all achieved through a single structural design without extra reconfigurable actuators.
• Record-Breaking Speed: It achieves a maximum rolling speed of 818 mm/s (26 body lengths per second), the fastest reported for small-scale soft robots.
• Rapid Posture Transition: The ability to switch between standing and lying postures in <0.35 s allows for seamless transitions between high-speed rolling and low-profile crawling.
• Extreme Foldability: The design allows the robot to reduce its height from 27 mm to 16 mm (and volume by 79%), enabling navigation through the cardia and other narrow anatomical structures.
• Robustness and Fault Tolerance: The robot can continue locomoting even if only one module is functional and can recover fully after being compressed to 20% of its height.

4. Key Results

• Locomotion Performance:
  • Rolling: Achieved 818 mm/s on flat surfaces. On complex substrates (sandpaper, paper, silicone), speeds exceeded 500 mm/s.
  • Crawling: In the lying posture, the robot demonstrated omnidirectional crawling with speeds up to 10.51 mm/s (P-actuation) and the ability to trace complex trajectories (e.g., "ZJU").
  • Underwater: Successfully deployed from a dissolvable PVA film, unfolded autonomously, and switched between rolling (5 Hz) and V-actuation swimming (50 Hz).
• Terrain Adaptability:
  • Successfully traversed discrete obstacles, viscoelastic gelatin surfaces (226.2 mm/s), and viscous non-Newtonian fluids (yogurt solution, 800 mPa·s).
  • Navigated a 3D-printed stomach model with gastric rugae topography at speeds comparable to flat terrain.
• Biomedical Application Simulation:
  • Demonstrated a controllable drug release mechanism integrated into the robot. Using electrolysis to generate gas pressure, the robot released liquid drugs at specific target locations within a simulated gastric environment.
• Navigation in Constrained Spaces:
  • In a demonstration, the robot rolled toward an obstacle, switched to a lying posture to pass under a 26 mm barrier (where its standing height was 27 mm), crawled through, and switched back to a standing posture to resume rolling.

5. Significance

This work presents a paradigm shift in soft robotics for biomedical applications. The M-SEMR addresses the critical need for versatility in unstructured environments. By combining high-speed rolling with low-profile crawling and rapid posture switching, it offers a viable solution for:

• Targeted Drug Delivery: Navigating the complex GI tract to deliver medication to specific sites.
• Minimally Invasive Surgery: Performing tasks in confined spaces where rigid or single-mode robots fail.
• Search and Rescue: Operating in disaster zones with debris and uneven terrain.

The study highlights that structural compliance and magnetic actuation can overcome the trade-offs between speed, adaptability, and size, paving the way for the next generation of autonomous, in vivo medical robots. Future work aims to miniaturize the system further and develop untethered power and control systems.