on April 28, 2022
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Published on November 16, 2022 Updated on August 31, 2023

Under ionic skin

University of British Columbia (UBC) - © Kai Jacobson Photography
University of British Columbia (UBC) - © Kai Jacobson Photography - University of British Columbia (UBC) - © Kai Jacobson Photography

Cédric Plesse, Giao Nguyen and Frédéric Vidal of the Laboratory of Physicochemistry of Polymers and Interfaces (LPPI) of CY Cergy Paris University participated, within the framework of a study carried out by Canadian researchers, in the development of a new theory on the how charge and tension are generated in hydrogels. Explanations on the applications, particularly in medicine, of the results published in "Science" on April 28, 2022.

In the quest to build smart skin that mimics the sensing abilities of the skin's natural receptors, ionic gels show significant benefits. These flexible and biocompatible hydrogels use ions to transport electrical charges. Unlike smart skins made of rigid plastic or metal sensors, ionic skins have mechanical properties similar to those of human skin, making it possible to offer a more natural sensation to prosthetic arms or a robot hand that would be covered and making them comfortable to wear in smart clothes.

These hydrogels can indeed generate a voltage and an electric current in response to stimulation, such as a touch or finger pressure, a phenomenon called the piezoionic effect (from the Greek "piezo" to press). However, the mechanisms involved in converting a mechanical stimulation into an electrical signal were not fully understood.
These hydrogels contain an electrolyte, i.e. positive and negative ions of different sizes dissolved in a solvent such as water. By studying how each of these ions moved under the action of mechanical pressure, it was possible to determine that these ions did not move at the same speed, leading to their non-homogeneous distribution in the gel and to the creation of an electric field.
This new knowledge confirms that hydrogels function in a manner similar to natural skin mechanoreceptors, also based on the movement of ions in response to pressure, inspiring new potential applications for ionic skins.

For John Madden, professor of electrical and computer engineering at the University of British Columbia (UBC) School of Applied Science, a future application will be to create sensors that interact directly with cells and the nervous system by imagining a prosthetic arm covered with an ionic skin that detects an object by touch or pressure and transmits this information through nerves to the brain, activating the motors necessary to lift or hold the object.

For Yuta Dobashi, a doctoral student at the University of Toronto, another application for these flexible, stretchable and transparent hydrogel sensors is to use them directly on the skin in order to follow, for example, the physiological signals of a patient while remaining totally discreet and generating its own energy. Implantable artificial joints could also be envisaged, without fear of rejection by the human body, for example as artificial knee cartilage, adding an intelligent sensing element.

The results of this study, published in Science on April 28, 2022, also includes contributions from Yael Petel, who holds a doctorate in chemistry at UBC, and Carl Michal, professor of physics at UBC, who used the interaction between strong magnetic fields and ion nuclear spins to track ion motions in hydrogels.

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