Physicists propose a path to faster, more flexible robots

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Virginia Tech physicist C. Nadir Kaplan (left) and doctoral student Chinmay Katke (right) discovered a microscopic phenomenon that could significantly improve the performance of soft devices, such as dexterous flexible robots or microscopic drug delivery capsules. Credit: Spencer Coppage for Virginia Tech.

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Virginia Tech physicist C. Nadir Kaplan (left) and doctoral student Chinmay Katke (right) discovered a microscopic phenomenon that could significantly improve the performance of soft devices, such as dexterous flexible robots or microscopic drug delivery capsules. Credit: Spencer Coppage for Virginia Tech.

In a May 15 article published in the magazine Physical Assessment LettersVirginia Tech physicists revealed a microscopic phenomenon that could significantly improve the performance of soft devices, such as dexterous flexible robots or microscopic drug delivery capsules.

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 could accelerate the expansion and contraction of hydrogels. First, this opens the possibility for hydrogels to replace rubber-based materials used to make flexible robots – perhaps allowing these fabricated materials to move with speed and dexterity close to that of human hands.

Soft robots are already being used in manufacturing, programming a hand-like device to grab something from a conveyor belt – think a hot dog or bar of soap – and place it in a container for packaging. But those in use now rely on hydraulics or pneumatics to change the shape of the “hand” to pick up the item.

Just like our own bodies, hydrogels usually contain water and are present all around us, for example nutritional jelly and shaving gel. Katke, Korevaar and Kaplan’s research appears to have found a method that allows hydrogels to swell and contract much faster, which would improve their flexibility and ability to function in different environments.

What did the scientists at Virginia Tech do?

Living organisms use osmosis for activities such as bursting seeds, spreading fruit in plants or absorbing water in the intestines. Normally we think of osmosis as a flow of water moving through a membrane, which larger molecules such as polymers cannot pass through. Such membranes are called semi-permeable membranes and were thought to be necessary to initiate osmosis.

Previously, Korevaar and Kaplan had done experiments with a thin layer of hydrogel film consisting of polyacrylic acid. They had observed that although the hydrogel film is permeable to both water and ions and is not selective, the hydrogel quickly swells due to osmosis when ions are released into the hydrogel and shrinks again.

Katke, Korevaar and Kaplan developed a new theory to explain the above observation. This theory states that microscopic interactions between ions and polyacrylic acid can cause the hydrogel to swell when the released ions in the hydrogel are unevenly dispersed. They called this ‘diffusio-phoretic swelling of the hydrogels’. Furthermore, this newly discovered mechanism allows hydrogels to swell much faster than previously possible.

Why is that change important?

Kaplan explained: Soft, agile robots are currently made of rubber, which “does the work, but changes their shapes hydraulically or pneumatically. This is not desirable because it is difficult to print a network of tubes into these robots to make them work.” introduce air or liquid.

Imagine, said Kaplan, 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 the rubber and hydraulics are not as versatile as your biological tissues, which is a hydrogel, state-of-the-art soft robots can only perform a limited range of movements.”

How could this improve our lives?

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

Currently, only microscopic-sized hydrogel robots can respond quickly enough to a chemical signal to be useful, and larger ones take hours to change shape, Katke said. Using the new difusiophoresis method, soft robots as large as a centimeter can potentially transform in just a few seconds, which is the subject of further research.

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

More information:
Chinmay Katke et al., Diffusiophoretic rapid swelling of chemically responsive hydrogels, Physical Assessment Letters (2024). DOI: 10.1103/PhysRevLett.132.208201

Magazine information:
Physical Assessment Letters

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