By Lesley Evans Ogden

Scientists are on a quest to create materials that mimic the sensory abilities of human skin for uses like the surfaces of robots and prosthetics. Canadian and French scientists recently made progress in this direction by designing a skin-like salt-infused gel that uses ionic currents to facilitate sensing, transmission and processing of tactile information.

In seeking methods for developing synthetic skin, one pathway has been to use electronics — conventional sensors, transistors and piezoelectric materials, the latter referring to electricity generated by touch. Another approach, explains Yuta Dobashi, lead author of the new research published in Science, uses gelatinous material that functions as an ionic, rather than electric, sensor.

Ionic sensors exploit the flow of positively and negatively charged ions. Dobashi, now pursuing a doctorate in medical sciences at the University of Toronto, embarked on this work while a Master’s student with electrical engineer John Madden at the University of British Columbia. Madden’s lab had for many years been working on artificial muscle. “These are materials that you can charge and discharge electrochemically,” says Madden, as ions are inserted or removed. As ions are inserted, the material swells. As ions are removed, it contracts. Madden’s team realized that the reverse should also be true; pressing or pulling these materials should generate a voltage.

With that premise, the hydrogel material they developed is comprised of polyacrylamide plus salts like sodium chloride. It looks and feels “much like Jello,” says Dobashi. “When you press on it, as if you were squeezing a dishwashing sponge, it would squeeze the electrolyte.” The team applied nuclear magnetic resonance to track ion movement when the sensor was pressed gently with a finger. Pressing lightly on the hydrogel generates low (millivolt) levels of voltage but a relatively high charge.

This property is valuable because currents transmitted by our nervous system tend to operate at low voltages. “Most of our electronics are at higher voltages than we would normally withstand,” says Madden, adding that “the body doesn’t tend to tolerate them, either mechanically or electrically.”

Their series of hydrogel tests included connection to nerves supplying the hind leg of a rat. This experiment, says Madden, is similar to the now-famous 1780s investigation by Lucia Galeazzi Galvani and Luigi Galvani testing nerve response in a dead frog hooked up to a battery. In the present day experiment, after delivering pressure to the hydrogel, the rat’s hind leg visibly moved, indicating successful signal transmission.

Signalling via thin hydrogels is something the team calls a piezoionic effect. It’s a technology they think could provide a better interface with the nervous system or brain. “That’s the exciting part,” says Dobashi, noting it opens up possibilities for bionics – materials that can connect or extend directly to the human body without an electrical interface.

“It’s a creative way to generate [a] spike train nerve-like signal by controlling the anion and cation mobilities,” says chemical engineer Zhenan Bao at Stanford University, who was not involved in the study but has worked extensively on flexible electronic materials inspired by human skin. Bao notes that while the type of signal these new hydrogels can generate is analogous to how human mechanoreceptors encode information about external forces, “On the other hand, the frequency of spike train usually corresponds to the level of force.” She suspects that further tuning of hydrogels with electronics will be needed to truly mimic those generated by skin mechanoreceptors.

Acknowledging that potential medical applications of this new material are still “very far out,” Madden imagines ionic skin feeding directly into the nervous system, whether from a prosthetic limb, or as replacement skin for burn victims. Those kinds of real world applications are still “sort of a dream at the moment” says Madden, noting one advantage of the material is that it doesn’t require any external electrical power.

“You don’t need a battery,” says Madden. “You don’t necessarily need any wires,” though for transmission of signals over longer distances, he acknowledges, “you might need to have a wire to carry that current, because ions aren’t as conductive as electrons.”

“The future of iontronics (as we like to call devices we create out of hydrogels) looks promising,” says neurosurgeon Victor Yang, Dobashi’s doctoral supervisor at the University of Toronto. “Smart materials that can simultaneously provide therapeutic and diagnostic capabilities may open a new paradigm of personalized medicine, and a piezoionic gel may be a key discovery to enable it.”

Madden notes that one major advantage of using ionic skin on bionic devices — those that connect biological to mechanical body parts — is that it could provide a mechanism for signal transmission across the skin without having to break through it, thus eliminating an interface with a high risk of infection.