Materials scientists would kill to be able to produce a material as amazing as biological muscle, which can retract on command, stretch by about 70% without damage, and heal its own nicks and tears. Now, researchers say they’re getting closer with a synthetic material that can do all these things, though not as well as natural muscle. The advance could one day be useful in robotics and prosthetics.
The concept of an artificial muscle dates back decades. Researchers have proposed numerous different starting materials, from atom-thick tubes of carbon called nanotubes to ceramics to metal alloys. In 2000, scientists showed that some rubberlike polymers called elastomers could be reversibly stretched to up to three times their length by applying a voltage across them. Like almost all synthetic materials, however, these elastomers needed someone to fix them if they were damaged. Working separately, other scientists have used elastomers as the basis for self-healing polymers—materials that can repair tears, seal holes, and even join cut edges. However, most of these have been quite weak and lacked elasticity, making them poor artificial muscles. And nobody has produced an artificial muscle that can repair itself.
Until now, that is. Materials chemist Zhenan Bao of Stanford University in Palo Alto, California, and colleagues unveil today in Nature Chemistry a group of elastomers called Fe-Hpdca-PDMS. The material comprises long, randomly entangled polymer chains containing silicon, oxygen, nitrogen, and carbon atoms mixed with an iron salt. The iron forms chemical bonds with the oxygen and the nitrogen atoms in the polymer, joining the polymer chains both to themselves and to each other, like strings joined with elastic bands at the crossing points. These crosslinks do not prevent the polymer chains from moving altogether, so the material can stretch. But the crosslinks do stop the chains from sliding completely freely. For the material to change shape, the crosslinks have to be stretched, distorted, and sometimes broken and rearranged. When the material is done stretching, the crosslinks return to their original shape, giving the material both strength and elasticity.
Then there’s the self-healing. If you poke a hole in the material, iron atoms on one side of the hole are attracted to oxygen and nitrogen atoms on the other, reforming atomic bonds and closing up the hole within 72 hours. Even when the researchers cut the polymers into two separate pieces, the cut edges ends rejoined almost perfectly if they were placed in contact, recovering almost all of their strength and 90% of their stretchability, even at temperatures as low as -20°C.
When the researchers applied an electric field across the polymers (similar to how muscle tissue is activated), the material’s length increased rapidly by about 2%. When the field was turned off, the material returned to its original size.
One notable weakness of the material is that the change in size after the electric field was applied is still small: Even though the material can normally be stretched up to 45 times its original length and still return to its original shape, its change in size when the field was turned on was much smaller than that of real muscle (which can shorten by up to 40%). This would mean that those robotic legs couldn't bend nearly as well as natural ones.
“In our case, the goal was not to make the best artificial muscle, but rather to develop new materials design rules for stretchable and self-healing materials,” Bao explains. “Artificial muscle is one potential application for our materials.” Bao’s team is now planning further work on increasing the effects of electric fields.
“It's very interesting and extremely elegant work,” says polymer chemist Marek Urban of Clemson University in South Carolina. He says the polymer could eventually be used to make the synthetic muscles needed to move artificial limbs, either to replace missing ones for disabled humans or to allow robots to move things like a human can. He also says the material might have other applications. Materials that expand and contract in response to an electric field are often used as pressure or strain sensors, sometimes self-correcting ones. Self-healing could be useful when sensors have to be placed in extreme conditions such as in space, where repair is sometimes difficult or impossible. "If a material has to be placed in an environment where there's a potential for damage [and] that material self-repairs, that's a huge advantage," Urban says.