Muscle-inspired magnetic actuators that push, pull, crawl, and grasp

This paper presents muscle-inspired magnetic actuators (MMAs) fabricated via laser powder bed fusion of a TPU/Nd2Fe14B composite, which enable precise control over stiffness and magnetization to achieve versatile, fatigue-resistant soft robotic functions including lifting, crawling, and grasping.

The Gist

Imagine you could build a robot out of a single, smart piece of plastic that acts just like a human muscle. It could squeeze, pull, crawl like a worm, or grab a strawberry without squishing it. That is exactly what this research team has done.

Here is the story of their invention, broken down into simple concepts:

1. The "Magic Dough" (The Material)

Think of the material they used as a special kind of playdough.

• The Soft Part: They started with a flexible plastic called TPU (the kind used in shoe soles or phone cases).
• The Magnetic Part: They mixed in tiny, super-strong magnets (Nd-Fe-B) that are like invisible iron filings.
• The Result: A composite material that is soft and stretchy but also reacts strongly to magnets.

2. The "Magic Oven" (The 3D Printer)

Usually, 3D printers just melt plastic to make shapes. This team used a high-tech laser printer (called LPBF) that acts like a smart oven.

• The Secret Sauce: They didn't just print the shape; they changed the "heat" of the laser as they printed.
• The Analogy: Imagine baking a cake where the bottom layer is hard as a rock, but the top layer is soft as a cloud. By adjusting the laser, they could make parts of the robot stiff (to hold weight) and other parts soft (to bend easily), all in one single piece of plastic. No glue, no screws, no separate parts.

3. The Two "Super Powers" (The Actuators)

Using this magic dough and oven, they built two types of "muscles":

A. The Squeezing Muscle (The Elongated Actuator)

• What it does: It looks like a zig-zag accordion. When a magnetic field hits it, it shrinks (contracts) just like a bicep muscle when you lift a weight.
• The Feat: This tiny piece of plastic (weighing less than a paperclip) can lift a 50-gram weight (like a small apple). That is like a human lifting a car! It can do this over and over again without getting tired.
• The Crawler: They attached little "feet" with rough textures to this muscle. When the muscle shrinks and stretches, the feet grip the ground in one direction and slide in the other, allowing the robot to crawl across surfaces like an inchworm.

B. The Grabbing Hand (The Expandable Actuator)

• What it does: This one looks like a flower or a starfish. When the magnet turns on, it closes its petals. When the magnet turns off, it opens.
• The Feat: It can gently grab delicate things like wild berries without crushing them, or grab hard things like 3D-printed blocks. It can even reach inside a tube, expand to press against the walls, and hold itself there (anchoring) while hanging a weight.

4. Why This is a Big Deal

• No Wires, No Batteries: These robots don't need wires or batteries inside them. They are controlled entirely by a magnet held outside the robot. It's like using a remote control, but the "signal" goes through walls and water.
• One Material, Many Jobs: Usually, to make a robot that is both strong and flexible, you need to glue different materials together. This team made it all from one single material, printed in one go.
• Tiny and Tough: They managed to print hinges that are thinner than a human hair (0.5 mm) that can bend thousands of times without breaking.

The Future: What's Next?

Right now, the robot moves based on a pre-set plan (it knows how to move, but not what it's touching). The researchers want to add "senses" (like touch sensors) so the robot can feel how hard it's squeezing. This would allow it to pick up a fragile egg without breaking it, or help doctors perform delicate surgery inside the human body without making big cuts.

In short: They turned a simple mix of plastic and magnets into a versatile, wireless, muscle-like robot that can lift heavy loads, crawl, and grab delicate objects, all controlled by a magnet from the outside.

Technical Summary

1. Problem Statement

Soft robotics aims to replicate the adaptive motion and versatility of biological systems, yet current magnetic soft actuators face several critical limitations:

• Limited Motion Modes: Most existing magnetic actuators rely on bending or torsion, struggling to achieve significant axial contraction (linear shortening) similar to natural muscle fibers.
• Fabrication Constraints: Traditional casting or lamination methods often result in weak interfacial bonding, leading to delamination and functional fatigue during cyclic operation. They also struggle to produce sub-millimeter flexural features necessary for distributed compliance.
• Lack of Integration: There is a lack of a single, monolithic material platform that can simultaneously perform linear contraction, directional locomotion, and adaptive gripping without complex multi-material assembly or external power sources.
• Stiffness vs. Performance Trade-off: Achieving high load-bearing capacity while maintaining large strain and tunable stiffness remains a central challenge in magnetoactive composites.

2. Methodology

The authors developed a Muscle-Inspired Magnetic Actuator (MMA) system using a single-material approach via Laser Powder Bed Fusion (LPBF).

• Material System: A composite of Thermoplastic Polyurethane (TPU) as a flexible binder and Nd₂Fe₁₄B (MQP-S) spherical microparticles as a hard-magnetic filler (50 wt%).
• Fabrication Process:
  • The composite powder was printed using an LPBF system (Sinterit Lisa Pro).
  • Key Design Variable: The laser-energy scale (ranging from 1.0 to 3.0) was tuned to control interlayer densification and porosity. This allowed for the precise programming of mechanical stiffness and magnetic remanence within a single print job.
• Actuator Architectures: Two distinct geometries were designed from the same composite material:
  1. Elongated Actuator: Mimics linear muscle fibers using a zig-zag hinge structure with 0.5 mm-thick flexural hinges. It was magnetized in a contracted state to induce axial shortening under a uniform field.
  2. Expandable Actuator: Features a radially symmetric, four-lobed design with inner/outer hinges. It was magnetized to enable reversible opening and closing (grasping) or radial expansion for anchoring.
• Integration: The elongated actuator was integrated with non-magnetic, 3D-printed feet featuring asymmetric micro-textures to create anisotropic friction, enabling directional crawling.

3. Key Contributions

• Process-Property Linkage: Demonstrated that LPBF energy scaling acts as a continuous design parameter to tune tensile strength (0.28 to 0.99 MPa) while preserving high elasticity (30–45% strain at break).
• Sub-Millimeter Hinge Fabrication: Successfully printed 0.5 mm-thick flexural hinges within a magnetoactive composite, a scale previously difficult to achieve with powder bed fusion, enabling large, reversible deformations without mechanical failure.
• Multimodal Functionality: Achieved three distinct actuation modes (pushing, pulling, crawling, grasping, and anchoring) within a single, untethered material system without onboard electronics or wiring.
• High Load-Bearing Capacity: The system demonstrated the ability to lift loads significantly exceeding its own weight (up to 32x) and sustain performance over repeated cycles.

4. Key Results

A. Material Characterization

• Increasing the laser-energy scale from 1.0 to 3.0 increased tensile strength from 0.28 MPa to 0.99 MPa.
• Strain at break remained stable between 30% and 45%, confirming that elasticity was preserved despite increased stiffness.
• Magnetic remanence increased from 47 mT to 76 mT with higher energy scales, while coercivity remained constant (~885 mT).

B. Elongated Actuator Performance

• Contraction: Under a 500 mT field, the actuator achieved linear contraction.
• Load Lifting: It successfully lifted 50 g (approx. 23–32 times its own self-weight, depending on the print density).
• Durability: The actuator maintained >60% of its displacement amplitude over 50 cycles under a 100 g load, with no observed cracking or delamination.
• Locomotion: Integrated into a crawling robot, it achieved 90% success on SiC abrasive paper and 100% success on laboratory tissue by leveraging anisotropic friction feet.

C. Expandable Actuator Performance

• Grasping: Successfully grasped, lifted, and released 13 diverse objects (including soft wild berries and rigid 3D-printed geometries) with a 100% success rate under a 300 mT field.
• Anchoring: The actuator could expand radially to grip the inner wall of a tube, supporting a suspended 50 g load without detachment, demonstrating potential for confined space applications.

5. Significance and Outlook

This work establishes a material-centric pathway for creating adaptive, untethered soft machines. By combining LPBF, tunable stiffness, and programmable magnetization, the authors have overcome the limitations of traditional casting methods.

• Biomedical Potential: The use of biocompatible TPU and the ability to operate in sealed or fluidic environments (via remote magnetic control) opens doors for minimally invasive surgical tools, such as soft tissue manipulation and anchoring in confined anatomical spaces.
• Future Directions: The authors propose moving from open-loop (predefined geometry) to closed-loop control by embedding soft sensors (strain/pressure) to enable force-controlled grasping. Future work will also explore spatially graded LPBF to co-print compliant and rigid zones and further miniaturization for operation with compact permanent magnets.

In summary, this paper presents a breakthrough in soft robotics by demonstrating that a single, additively manufactured magnetic composite can perform complex, load-bearing tasks (pushing, pulling, crawling, grasping) with high fatigue resistance and remote controllability.