Physicists suggest a path to faster and more flexible robots

In an article dated May 15, published in the magazine Review of physical sheetsVirginia Tech physicists have discovered a microscopic phenomenon that could significantly improve the performance of soft devices such as dexterous flexible robots or microscopic capsules for drug delivery.

The paper, written by doctoral student Chinmay Katke, associate professor K. 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. On the one hand, this opens up the possibility for hydrogels to replace the rubber-based materials used to make flexible robots, allowing these manufactured materials to move with speed and dexterity close to human hands.

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

Like our own bodies, hydrogels are mostly water and are everywhere around us, like food 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 environments.

What did Virginia Tech scientists do?

Living organisms use osmosis for activities such as breaking seeds, dispersing fruits in plants, or absorbing water in the intestines. We usually think of osmosis as the flow of water moving across a membrane that larger molecules like polymers cannot move through. Such membranes are called semi-permeable and were thought to be necessary for the initiation of osmosis.

Previously, Korewaar and Kaplan conducted experiments using a thin layer of hydrogel film consisting of polyacrylic acid. They observed that even though 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 inside the hydrogel and shrinks again.

Katke, Korewaar, 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 if the released ions are unevenly distributed within the hydrogel. They called it “diffusion-phoretic swelling of hydrogels”. In addition, this newly discovered mechanism allows hydrogels to swell much faster than previously possible.

Why is this change important?

Kaplan explained that currently, soft dexterous robots are made of rubber that “does the job, but their shape changes hydraulically or pneumatically. This is undesirable because it is difficult to imprint a network of tubes to feed air or liquid into these robots. .”

Imagine, Kaplan said, how many different things you can do with your hands and how fast you can do them thanks to your neural network and the movement of ions under your skin. Because rubber and hydraulics are not as versatile as your biological tissues, which is hydrogel, today’s soft robots can only perform a limited number of movements.”

How can it improve our lives?

Katke explained that the process they investigated allows hydrogels to change shape and then return to their original shape “so much faster” 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 take hours to change shape, Katke said. Using a new method of diffusion phoresis, centimeter-sized soft robots can transform in just a few seconds, which is subject to further study.

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.