Moisture Powers Leaping Plastic Pieces

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A newly invented plasticlike material curls up and moves around on its own whenever it's exposed to moisture. Such self-moving materials could power low-energy sensors in the future.
Courtesy of Dr. Ning Zhang

By Francie Diep, TechNewsDaily Staff Writer

Chemist Mingming Ma was working to make a plastic-like material for electrodes when he noticed his plastic was doing something strange. When he put a piece of it in his hand, it would curl up on itself and creep along his palm.

"I put it in my own hand and I found the polymer curved and it's, like, traveling in my hand," Ma told TechNewsDaily. "So I wanted to find out what's the reason, what's the mechanism of this movement?"

Ma soon learned that the moisture from his skin drove the material, a film made of two different kinds of polymers, or chemicals made of repeating units of the same molecule. Now, after further work with his colleagues in chemical engineer Robert Langer's laboratory at MIT, Ma has made small pieces of polymer material that continually curl, creep and leap when placed on a surface that's moister than the air. The researchers have also hooked the polymer pieces up to system that harvests tiny amounts of electricity from the polymer's movement.

Ray Baughman, a materials scientist at the University of Texas in Dallas who was not involved in Ma and Langer's work, called the polymer pieces "tiny robots" and noted they look alive as they move around on their own. "This is a very highly creative work by an MIT team that's known for its creativity," he said.

In the future, ingenious man-made materials like Ma and his colleagues' invention could power simple sensors that don't require much electricity, researchers say. Moisture-driven patches could also power small devices embedded in clothes, Ma thought. He had one interesting idea: Gym clothes with embedded heart rate monitors, powered by people's evaporating sweat.

Before gym-goers can put their sweat to good use, however, there's still plenty left for Ma left to do to improve his self-moving polymer. One major thing he'll be focusing on is making sure the polymer makes enough electricity to be useful.

Firm but Yielding

Ma's group isn't the first to discover that some materials change shape and move when exposed to moisture. There's even a fish-shaped kids' toy that will wiggle in the hand after absorbing water molecules from the skin. Ma and his colleagues worked to make a material that makes more dramatic movements, including leaps in the air, so it will generate more electricity.

The researchers combined two different kinds of polymers to make a material that has just the right balance of softness and stiffness for bending and jumping. The resulting material has a microscopic structure reminiscent of the layer of dermis just underneath the top layer of people's skin, which also combines thicker, rigid fibers with thinner, more flexible fibers, Ma said.

More Power Needed

To convert the polymer's mechanical energy — the energy of the its creeping and jumping — into electricity, Ma and his team added a layer of commercially available piezoelectric film to their self-moving polymer. Piezoelectric materials produce electricity when they're bent or otherwise stressed.

The conversion rate Ma got was low, however. Less than 0.01 percent of the polymer's mechanical energy turns into electricity. A 2-inch-long piece makes just 5.6 nanowatts, or 0.0000000056 watts, of power. That's close to enough for ultra-low-power sensors, such as some sensors used to detect temperature or humidity that use less than one microwatt (1,000 nanowatts) of power, Ma said.

For other applications, however, he'll need to raise the electricity production. Even a pacemaker needs about 10 microwatts of power.

Ma envisioned it would take five to 10 years of continued research before tiny, self-propelled generators like his would start appearing in devices outside of laboratory work. But he's already thinking about what to do next. He's considering custom-making a piezoelectric material to get better conversion rates. He may also try to make larger versions of the self-moving film, "maybe the size of a desk or maybe even bigger," he said. Such larger pieces should be able produce more electricity. Ma and his colleagues published their work today (Jan. 10) in the journal Science.

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