Despite their remarkable flexibility, today’s soft artificial muscles struggle to deliver meaningful force. This limits their use in real-world machines, even though they can bend, stretch, and twist in ways that rigid motors cannot.
They hold promise for robots operating in tight spaces, wearable systems that move naturally with the body, and surgical tools gentle enough for living tissue – but their molecular softness still keeps them weak.
One way engineers capture this bottleneck is through work density – the amount of mechanical energy an actuator can deliver per unit volume, expressed in kilojoules per cubic meter. In practical terms, it reflects how much pushing, pulling, or lifting a material can do for its size. Soft polymers stretch easily but generate little force, while stiffer ones produce force yet barely move.
Surpassing key barrier in artificial muscle performance
For years, every major artificial muscle technology has run into the same wall: it can either stretch a lot or produce strong force, but not both. Dielectric elastomers stretch widely but need high voltages and remain weak. Carbon nanotube yarns are light yet lift little. Liquid crystal elastomers move elegantly but deliver modest energy. Even natural muscle combines only moderate strain and low work density.
Now, researchers at the Ulsan National Institute of Science and Technology in South Korea have broken this pattern. Their dual cross-linked magnetic polymer actuator achieves a work density of 1,150 kJ m⁻³ and an actuation strain of 86.4%, setting a new performance benchmark for soft artificial muscles, Nanowerk writes.
This new material combines extreme flexibility with remarkable strength. It can stretch to more than twelve times its original length before breaking and shift from a soft, rubber-like state to a rigid, plastic-like one – changing stiffness by over a thousandfold. The breakthrough comes from a smart molecular design that lets the same polymer behave as both rubber and plastic. Inside the material, two types of links hold the network together.
New polymer actuator stores and releases energy on demand
What makes this material especially powerful is its ability to work in two different modes. When heated above about 99 °F, the polymer softens, allowing a magnetic field to stretch, bend, or twist it. Turning off the field lets it spring back. Cooling below about 80 °F then locks the new shape in place and stores energy. Reheating releases that stored energy, causing the actuator to contract and perform useful work.
Through testing, the team found an optimal formula using about 0.39 ounces of magnetic particles and a tiny amount of cross-linker. At this ratio, the actuator reached 86.4% strain and a record work density of roughly 33,000 ft-lb per cubic foot. It recovered nearly 100% of its shape, kept over 87% of its performance after 300 cycles, and could still hold more than 4,000 times its own weight. Its power output reached about 0.54 horsepower per ton, making it competitive with today’s best artificial muscles.
Robotic tests showed that the material can work in real machines. In its soft state, the actuator magnetically gripped objects. A focused laser then heated it, causing it to contract and lift weights between about 2.7 and 4.1 ounces, with shape recovery between 39% and 52%. When compared with other technologies, this actuator operates in a performance range no one had reached before – hydrogels can stretch 40% to 65% but stay below about 11,800 ft-lb per cubic foot.
Source: Interesting Engineering
China’s hair-thin fiber chips bring computer-level processing into washable fabric
New magnetic polymer enables stronger and more flexible artificial muscles in soft robotics