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In a May 15 paper published in the journal Physical examination lettersVirginia Tech physicists have uncovered a microscopic phenomenon that could greatly improve the performance of soft devices, such as flexible flexible robots or microscopic capsules for drug delivery.

The paper, written by PhD student Chinmay Katke, assistant professor C. Nadir Kaplan, and co-author Peter A. Korevaar of Radboud University in the Netherlands, proposes a new physical mechanism that can accelerate the expansion and contraction of hydrogels. On the one hand, this opens up the possibility that hydrogels could replace the rubber-based materials used to make flexible robots, allowing these manufactured materials to move with speed and dexterity close to that of human hands.

Soft robots are already used in manufacturing, where a hand-like device is programmed to grab an item from a conveyor belt—think a hot dog or a bar of soap—and place it in a container to be packaged. But the ones used now rely on hydraulics or pneumatics to change the shape of the “arm” to lift the object.

Like our own body, hydrogels contain mostly water and are all around us, for example nutritional jelly and shaving gel. Research by Katke, Korevaar and Kaplan appears to have found a method that allows hydrogels to swell and shrink much faster, which would improve their flexibility and ability to function in a variety of settings.

What did Virginia Tech scientists do?

Living organisms use osmosis for such activities as bursting seeds, dispersing fruits in plants, or absorbing water in the intestines. We usually think of osmosis as a flow of water moving through a membrane, with larger molecules such as polymers unable to pass through it. Such membranes are called semipermeable membranes and are believed to be necessary for osmosis to occur.

Previously, Korevaar and Kaplan had done experiments using a thin layer of hydrogel film composed of polyacrylic acid. They observed that although the hydrogel film allows water and ions to pass through it and is not selective, the hydrogel quickly swells due to osmosis when ions are released inside the hydrogel and shrinks again.

Katke, Korewaar, and Kaplan developed a new theory to explain the above observation. This theory says that microscopic interactions between ions and polyacrylic acid can cause the hydrogel to swell when the released ions inside the hydrogel are unevenly distributed. They called this “diffusion-phoretic swelling of the hydrogels”. Furthermore, this newly discovered mechanism allows the hydrogels to swell much faster than previously possible.

Why is this change important?

Kaplan explained: Currently, soft flexible robots are made with rubber that “does the work, but their shapes change hydraulically or pneumatically. This is not desirable because it is difficult to print a network of tubes in these robots to deliver air or liquid into them.”

Imagine, Kaplan said, how many different things you can do with your hand and how quickly you can do them thanks to your neural network and the movement of ions under your skin. Because rubber and hydraulics are not as flexible as your biological tissues, which are hydrogel, the most advanced soft robots can only perform a limited number of movements.”

How can this improve our lives?

Katke explained that the process they studied allows the hydrogels to change shape, then return to their original shape “significantly faster that way” in soft robots that are larger than ever before.

Currently, only microscopic-sized hydrogel robots can respond to a chemical signal fast enough to be useful, and larger ones require hours to change shape, Katke said. By using the new diffusiophoresis method, centimeter-sized soft robots may be able to transform in just a few seconds, subject to further research.

Larger agile soft robots that could respond quickly could improve assistive devices in healthcare, pick-and-place functions in manufacturing, search and rescue operations, cosmetics used in skin care, and contact lenses.