Stretchable electrode recreates sensations on skin

Led by the University of California San Diego (UCSD), the team’s device is said to reproduce a more varied and realistic range of touch sensations. Their findings are detailed in Science Robotics.

The device consists of a soft, stretchable electrode attached to a silicone patch and can be worn like a sticker on the fingertip or forearm. The electrode is connected to an external power source via wires. By sending a mild electrical current through the skin, the device can produce sensations of pressure or vibration depending on the signal’s frequency.

In a statement, Rachel Blau, a nano engineering postdoctoral researcher at the UC San Diego Jacobs School of Engineering, said: “Our goal is to create a wearable system that can deliver a wide gamut of touch sensations using electrical signals - without causing pain for the wearer.”

According to the UCSD, existing technologies that recreate a sense of touch through electrical stimulation often induce pain due to the use of rigid metal electrodes that do not conform well to the skin. The air gaps between these electrodes and the skin can result in painful electrical currents.

To address these issues, Blau and a team of researchers led by Darren Lipomi, a professor in the Aiiso Yufeng Li Family Department of Chemical and Nano Engineering at UC San Diego, developed a soft, stretchy electrode that conforms to the skin.

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The electrode is made of a new polymer material constructed from the building blocks of two existing polymers: a conductive, rigid polymer called PEDOT:PSS, and a soft, stretchy polymer called PPEGMEA. “By optimising the ratio of these [polymer building blocks], we molecularly engineered a material that is both conductive and stretchable,” said Blau.

The polymer electrode is laser-cut into a spring-shaped, concentric design and attached to a silicone substrate.

“This design enhances the electrode’s stretchability and ensures that the electrical current targets a specific location on the skin, thus providing localised stimulation to prevent any pain,” said Abdulhameed Abdal, a Ph.D. student in the Department of Mechanical and Aerospace Engineering at UC San Diego and the study’s other co-first author.

In tests, the electrode device was worn on the forearm by 10 participants. In collaboration with behavioural scientists and psychologists at the University of Amsterdam, the researchers first identified the lowest level of electrical current detectable. They then adjusted the frequency of the electrical stimulation, allowing participants to experience sensations categorised as either pressure or vibration.

“We found that by increasing the frequency, participants felt more vibration rather than pressure,” said Abdal. “This is interesting because biophysically, it was never known exactly how current is perceived by the skin.”

The new insights could pave the way for the development of advanced haptic devices for applications such as virtual reality, medical prosthetics and wearable technology.