Artificial muscles can be made with cables on pulleys, driven by heat, or twisted together from carbon nanotubes.
But it's been hard to duplicate the way muscle cells contract and relax at the cellular level, driven by little more than the impulses from the brain.
That is starting to change. A recent study by a team headed by Nicolas Giuseppone at France's Université de Strasbourg shows it's possible to make molecules that respond to changes in the acidity of a solution, known as pH.
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By stringing the molecules together, they got a chain that contracted when the pH was higher and expanded when the pH was low. The total change in length was about one nanometer, or a billionth of a meter.
That's enough for a virus to notice, but not enough to be useful for people.
So the team strung lots of the molecules together in a kind of chain. The total amount of expansion was 10 micrometers. Although still small, it's the approximate size of spider silk fibers -- just big enough to see with a good magnifying glass.
"We show it's possible to make [the molecules] work together to cross into the macroscopic scale," Giuseppone told Discovery News.
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The way it works is with a chemical called rotaxane. Rotaxane is a molecule that has a dumbbell shape, surrounded by a ring called a macrocycle.
The macrocycle has an affinity for negative charges, because the molecule itself is rich with positive ones. When there are more positive charges -- hydrogen ions -- in the surrounding environment, it will move to one side of the dumbbell. When there are fewer, it moves to the other side.
When two rotaxane molecules are placed alongside each other, they function like the extension on a seat belt or handbag strap. Each of these pairs is connected by another molecule, called a ligand, combined with certain metal ions, making a "daisy chain." A higher pH means the molecules sit closer together, while a lower one means they stretch further apart.
"[The experiment] is coming one step closer to the dream of materials that employ the concerted motions of countless molecular machines to deliver work against a load on the macroscopic scale, as do the muscles in our own bodies," said Fraser Stoddart, a professor of chemistry at Northwestern University, in an email. Stoddart was not involved in the research.
"The work sounds like real significant progress from molecules that change their dimensions to large aggregates that do so," wrote Miklos Ketesz, a professor of Chemistry at Georgetown University and head of the Kertesz lab, via email.
It will still take some time before anyone can build anything like an artificial muscle, he added, but the work is proof of principle.
Giuseppone said the idea came in part from the research that Stoddart had done in the field, as well as some earlier work by Jean-Pierre Sauvage of the Université Louis Pasteur in Strasbourg, France. Their experiments got him thinking about the mechanics of the links between rotaxane molecules.
The next challenge, he said, is to make the aggregate big enough. "You need to take several of the chains and bind them into a fiber," he said.
The work was published online in the journal Angewandte Chemie International Edition on Oct. 18.