Minimachines that move without batteries

Chemistry powers tiny tools that have big potential

In a laboratory at Johns Hopkins University in Baltimore, Md., scientists have created machines that can open and close, and can grab other objects. That may not seem like news, except for two things.

One, these machines don’t have batteries or any other source of power. Two, the machines are smaller than mosquitoes.

David Gracias led the recent study on how to make these tiny machines. He is an engineer at Johns Hopkins, and in his laboratory, miniaturization is the name of the game. Scientists who work on miniaturization build small and useful machines. Gracias’ new invention may be useful in medicine. It may be used to remove tiny bits of tissue, a procedure called a biopsy. Biopsies are often used to diagnose disease. The machines may also be able to deliver small amounts of drugs to disease sites in the body.

The tools begin in an open position (far left) and then snap shut when triggered with enzymes (left to right).
The tools begin in an open position (far left) and then snap shut when triggered with enzymes (left to right). D. Gracias et al/JACS 2010
Gracias’ tiny tools have five extensions that look like fingers on a hand or arms on a starfish. When the device is exposed to a certain chemical or combination of chemicals, the fingers snap shut — and the device looks more like a closed fist.

The tiny tools use biology, rather than batteries, to open and close. Gracias is inspired by the natural world, where he sees many minimachines at work — like the snapping “jaws” of a Venus flytrap or the slow wave of sunflowers following the sun.

“In nature, and in us, these [machines] respond to chemistry,” Gracias told Science News.

His machine may be inspired by nature, but you’re not going to find it growing in a field. Gracias and his team worked hard to find just the right combination of materials to make the tiny tool work.

They started with a base layer of silicon — the same stuff used to make computer chips and some kinds of glass. (Silicon is called a metalloid, which means it shares some properties with metals.) To the silicon they added three thin layers of metals: chromium, nickel and gold. The scientists built hinges so the fingers could open and close.

Finally, they coated the layers with polymers. Polymers are big molecules made of many atoms (as many as millions). These atoms are joined together in repeating patterns into a long chain, like beads on a necklace. Some polymers are strong and don’t break easily, which is why they’re used to make materials like plastics.

Gracias didn’t use plastic. He used special polymers, called biopolymers, made from molecules that occur naturally in the body. These polymers fall apart when they mix with certain enzymes. Enzymes are proteins made by living cells to help with chemical reactions.

Gracias tested two polymers. One comes from collagen, which helps keep the human body together. It’s found in the skin and ligaments, among other places. The other polymer comes from cellulose, which is found in the cell walls of plant cells. Both of these natural polymers break down around certain enzymes.

When the machines find certain enzymes, the polymers fall apart and, because of the way it’s built, the machine snaps shut.

To test their machine, the scientists used resin to build a model of internal organs. Inside the model was a piece of tissue. Using a magnet, the scientists steered the machine through the model, to the tissue. An enzyme added to the mix activated the machine, and it clamped down — around the piece of tissue. The scientists then steered the machine out of the model. Success! The practice run was similar to a biopsy.

The first trial is promising — but it is still a first step. Scientists have a lot of work to do before these mini-movers can roam and help heal the human body. But one day, perhaps we’ll fight diseases like cancer with mini-machines.

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News Explores since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

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