Scientists have worked out how the piezoionic effect works in hydrogels, an advance that could further the application of ionic skins to prosthetic arms or robot hands.
Ionic skins are designed to mimic the sensing capabilities of natural skin and are made of flexible, biocompatible hydrogels that use ions to carry an electrical charge. In contrast to smart skins made of plastics and metals, the hydrogels have the softness of natural skin and are more comfortable to wear.
These hydrogels can generate voltages when touched - the piezoionic effect - but scientists did not clearly understand how until a team at the University of British Columbia (UBC) devised a unique experiment that is published in Science.
Working under the supervision of UBC researcher Dr. John Madden, the study’s lead author Yuta Dobashi devised hydrogel sensors containing salts with positive and negative ions of different sizes. He and collaborators in UBC’s physics and chemistry departments applied magnetic fields to track precisely how the ions moved when pressure was applied to the sensor.
“When pressure is applied to the gel, that pressure spreads out the ions in the liquid at different speeds, creating an electrical signal,” Dobashi said in a statement. “Positive ions, which tend to be smaller, move faster than larger, negative ions. This results in an uneven ion distribution which creates an electric field, which is what makes a piezoionic sensor work.”
The researchers said this new knowledge confirms that hydrogels work in a similar way to how humans detect pressure, which is also through moving ions in response to pressure, inspiring potential new applications for ionic skins.
“The obvious application is creating sensors that interact directly with cells and the nervous system, since the voltages, currents and response times are like those across cell membranes,” said Dr. Madden, an electrical and computer engineering professor in UBC’s faculty of applied science. “When we connect our sensor to a nerve, it produces a signal in the nerve. The nerve, in turn, activates muscle contraction.”
He continued: “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.”
According to UBC, another application is a soft hydrogel sensor worn on the skin that can monitor a patient’s vital signs while being unobtrusive and generating its own power.
“We can imagine a future where jelly-like ‘iontronics’ are used for body implants,” said Dobashi. “Artificial joints can be implanted, without fear of rejection inside the human body. Ionic devices can be used as part of artificial knee cartilage, adding a smart sensing element. A piezoionic gel implant might release drugs based on how much pressure it senses, for example.”
Dr. Madden added that the market for smart skins is estimated at $4.5bn in 2019 and is growing. “Smart skins can be integrated into clothing or placed directly on the skin, and ionic skins are one of the technologies that can further that growth.”
The research included contributions from UBC chemistry PhD graduate Yael Petel and Carl Michal, UBC professor of physics, who used the interaction between strong magnetic fields and the nuclear spins of ions to track ion movements within the hydrogels. Cédric Plesse, Giao Nguyen and Frédéric Vidal at CY Cergy Paris University in France helped develop a new theory on how the charge and voltage are generated in the hydrogels.