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New self-healing liquid metal actuators could transform robotics and wearables
(Nanowerk Spotlight) A material that stretches, bends, and heals itself after being damaged could change the way we think about robotics, wearable devices, and even artificial muscles. Recent research published in Advanced Functional Materials ("Core–Shell Nanostructured Assemblies Enable Ultrarobust, Notch-Resistant and Self-Healing Materials") has moved this vision closer to reality with a new type of composite material that combines flexibility, strength, and the ability to self-repair—all without sacrificing performance. This breakthrough opens the door to technologies that can endure heavy use and continue functioning without the need for constant repairs or replacements.
Imagine a robotic arm that repairs itself after being torn or an artificial muscle that continues to function after sustaining damage. This new material – developed through the incorporation of liquid metal nanostructures into a polyurethane matrix – could make such scenarios possible. The combination of adaptive liquid metals and a flexible polymer creates a system capable of withstanding mechanical stress while retaining its ability to heal, an essential feature for dynamic environments.
At the heart of this development are tiny droplets of gallium-based liquid metal, known for their ability to stretch and conduct heat without breaking down. These particles, when encapsulated in an organic polyphenol shell, form a "core-shell" structure that interacts seamlessly with the surrounding polyurethane. This design addresses a common issue in flexible materials: the trade-off between strength and adaptability. By using this nanostructured assembly, the material retains high tensile strength even under extreme stretching, while also remaining flexible and resistant to damage.
Materials design of the ultrarobust, notch-resistant, and self-healing actuators
Materials design of the ultrarobust, notch-resistant, and self-healing actuators. a) Schematic illustration of the mechanism of encapsulating liquid metal nanoassemblies (LMNs) under ultrasonication. b) Schematic illustration of core–shell structured deformable LMNs. c) Schematics and images of the nanostructure of the sample during stretching and recovery. d) Various properties of the composites. (Image: Reproduced with permission by Wiley-VCH Verlag) (click on image to enlarge)
Soft actuators – devices that convert energy into mechanical motion – have typically faced limitations when it comes to durability. Those made from rigid materials are strong but too stiff to perform complex movements, while soft materials, though flexible, tend to degrade quickly under stress. This new material solves that issue by offering a balance between strength and flexibility, ideal for actuators that need to operate under challenging conditions, such as in robotics, where materials undergo repetitive motions and may come into contact with sharp objects.
One of the key innovations of this research is its focus on "notch resistance" – the ability of a material to resist damage from small cuts or tears. In typical soft materials, even minor imperfections can grow into major cracks, leading to mechanical failure.
However, in this new composite, the liquid metal nanoparticles deform along with the surrounding polyurethane, redistributing stress and preventing cracks from spreading. This resistance to fracture significantly extends the lifespan of the material, making it suitable for applications where durability is essential. Testing showed that the material exhibited a fracture energy of 58.8 kJ/m2, a major improvement over traditional soft materials, meaning it can absorb more energy without breaking.
Another standout feature is the material’s ability to heal itself autonomously at room temperature. Unlike previous self-healing materials that required external heat or light to trigger repair, this composite fixes itself naturally over time. When the material is damaged, hydrogen bonds within the supramolecular interface break and reform, allowing the damaged area to knit itself back together.
In tests, the material recovered 92.5% of its original tensile strength and 96% of its stretchability after self-healing, an impressive feat that reduces the need for external intervention. This self-healing process works without the need for added heat, making it ideal for real-world applications where repairs need to happen on the fly, without special equipment.
The photothermal properties of the liquid metal nanoparticles give the material another unique ability: actuation through light. When exposed to near-infrared (NIR) light, the nanoparticles absorb the energy and convert it into heat, which causes the material to expand or contract in a controlled way. This feature could enable the design of devices that move and change shape in response to light, useful in applications such as soft robotics or artificial muscles, where precise, rapid movements are essential.
For example, the material can be shaped into a spiral that unravels when exposed to NIR light or formed into a gripping tool that opens and closes on command. The photothermal responsiveness also helps the material recover quickly from deformation, ensuring it can perform repetitive tasks without losing its shape or function.
The material’s thermal stability further enhances its versatility. In testing, the composite showed less than 2% mass loss at temperatures up to 275 °C, meaning it can maintain its integrity in high-temperature environments. This makes it suitable for applications in fields like aerospace or industrial robotics, where materials are often exposed to extreme heat.
But perhaps the most exciting aspect of this research is its potential to revolutionize entire industries. In robotics, where machines need to operate continuously in tough conditions, the ability to repair damage without human intervention could reduce downtime and maintenance costs.
Wearable technology, which must withstand constant movement and contact with the body, could benefit from materials that adapt to stress and last longer without replacement. Even biomedical devices could become more reliable, with materials that heal themselves while retaining the flexibility to move with the body.
The real impact of this material lies in its combination of properties – flexibility, durability, self-repair, and responsiveness to light – none of which have been easy to achieve in the same system. The liquid metal nano-assemblies are key to this success, offering not just mechanical strength but also the ability to adapt to and recover from damage. By using a core-shell structure and a polyurethane matrix, the researchers have created a material that can perform complex functions while withstanding the wear and tear that comes with long-term use.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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