Materials scientists would kill to be able to produce such incredible material as the biological muscle, which can retract on command, stretch of about 70% without damage, and heal his own cuts and tears. Now researchers say they are closer to a synthetic material that can do all these things, but not as well as natural muscle. The advance could one day be useful in robotics and prosthetic.
The concept of an artificial muscle back decades. Researchers have proposed many different starting materials, from carbon nanotubes thick carbon tubes called to ceramics to metal alloys. In 00, scientists have shown that certain rubbery polymers called elastomers can be stretched reversibly to a maximum of three times their length by applying a voltage across them. Like almost all synthetic materials, however, these elastomers needed someone to repair them if they are damaged. Working separately, other scientists used elastomers such as basic self-healing polymers, materials that can repair tears, seal holes and even joining the cut edges. However, most of them were quite small and lacked elasticity, making them poor artificial muscles. And nobody has produced an artificial muscle that can be repaired.
Until now, that is. Materials chemist Zhenan Bao of Stanford University in Palo Alto, California, and colleagues reveal today Nature Chemistry a group of elastomers called Fe-Hpdca PDMS. The material comprises long chain and randomly entangled polymer containing silicon, oxygen, nitrogen and carbon atoms in admixture with an iron salt. The chemical bonds of forms of iron with oxygen and nitrogen atoms in the polymer, linking the polymer chains to both themselves and to each other, such as strings attached with elastic bands at the crossing points. These crosslinks do not prevent the polymer chains to move altogether, so that the material can stretch. But do not stop crosslinks the chains to slide completely freely. For the form change material, the crosslinks must be stretched, distorted, and sometimes broken and rearranged. When the material is made of stretch, the cross-links return to their original shape, giving both the material strength and elasticity.
Next, there is self-healing. If a hole is drilled in the material, iron atoms on a side of the hole are attracted by oxygen atoms and nitrogen on the other hand, reforming atomic bonds and the closing of the orifice in the 72 hours. Even when the researchers cut the polymers into two separate pieces, the ends of the cutting edges almost perfectly joined if they have been contacted, recovering almost all of their strength and 0% of their scalability, even at temperatures as lower than -20 ° C.
When the researchers applied an electric field across the polymer (similar to how muscle tissue is activated), the length of the material rapidly increased approximately 2% . When the field was switched off, the material is returned to its original size
A notable weakness of the matter is that the size change after the electric field is applied is still low. Even if the material can normally be stretched up to 45 times its original length and still return to its original shape, size change when the field was turned on was much smaller than the real muscle (which can shorten up 40%). This would mean that the robotic legs could not bend nearly as well as natural.
"In our case, the goal was not to make the best artificial muscle, but rather to develop new design rules for stretchable materials and self-healing materials," says Bao. "Artificial Muscle is a potential application for our material." The Bao team is currently planning further work on increasing the effects of electric fields.
"It is very interesting and extremely elegant work, "says Marek Urban polymer chemist at Clemson University in South Carolina. He said the polymer could possibly be used to make synthetic muscles needed to move artificial limbs or to replace missing for disabled people or to allow robots to move things as a human can. He also said the material could have other applications. Materials that expand and contract in response to an electric field are often used as pressure or strain sensors, sometimes self-correction. Self-healing may be useful when the sensors should be placed in extreme conditions such as in space, where repair is difficult or impossible. "If a material is to be placed in an environment where there is a risk of damage [and] as self-repairing material, which is a huge advantage," says Urban.
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