The next generation of prosthetic devices may incorporate materials that are smarter and more life-like than plastic and metal. An ionic skin might even be the key to letting users sense pressure.
Soft and flexible like natural skin, engineer John Madden is working on a biocompatible hydrogel that can send information about pressure in a way that could potentially communicate with the human nervous system.
Most pressure sensors depend on the movement of electrons to transform mechanical forces into electrical signals. While this has many possible applications, most biological systems use the movement of ions to send information about pressure from the skin to the brain.
“When we connect our sensor to a nerve, it produces a signal in the nerve. The nerve, in turn, activates muscle contraction,” said Madden, professor of electrical and computer engineering at the University of British Columbia, in a press release.
“You can imagine a prosthetic arm covered in an ionic skin. The skin senses an object through touch or pressure, conveys that information through the nerves to the brain, and the brain then activates the motors required to lift or hold the object. With further development of the sensor skin and interfaces with nerves, this bionic interface is conceivable.”
While ionic skins were previously known to generate voltages when touched, Madden’s team figured out how it works by loading a jelly-like hydrogel with salts. Their study was published in Science.
When dissolved, the salts provided positive and negative ions of different sizes. By applying strong magnetic fields, the team was able to track how the ions moved when the sensor was pressed.
They found that squeezing the hydrogel made the ions move, but at different speeds. The positive ions tend to be smaller, so they can move more quickly through the gel. The uneven distribution of these charged ions creates an electric field, which is how the hydrogel senses pressure. The system is completely self-powered.
Testing an ionic skin, Madden’s team also demonstrated that they could stimulate muscle excitation in response to touch, suggesting that it could be possible to create bionic sensory interfaces. Ionic hydrogels might also be useful for other applications, like a pressure-sensing artificial knee cartilage that could be programmed to release drugs after a joint replacement.
Being able to mimic the way that biological systems sense pressure with a soft and biocompatible material brings us one step closer to making devices that can communicate directly with living cells.