Hydrogels can generate voltages when touched, but scientists did not clearly understand how. A team of researchers at UBC devised a unique experiment, published in Science, that 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.
“How hydrogel sensors work is they produce voltages and currents in reaction to stimuli, such as pressure or touch — what we are calling a piezoionic effect. But we didn’t know exactly how these voltages are produced,” said the study’s lead author Yuta Dobashi, who started the work as part of his master’s in biomedical engineering at UBC.
Working under the supervision of UBC researcher Dr. John Madden, 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. 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 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 added.
Another application is a soft hydrogel sensor worn on the skin that can monitor a patient’s vital signs while being totally unobtrusive and generating its own power.
“We can imagine a future where jelly-like ‘iontronics’ are used for body implants. 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 continues to grow. “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.”
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