Polymer electrode suitable for e-skin applications

South Korean materials scientists, led by the Daegu Gyeongbuk Institute of Science & Technology (DGIST), have improved electrical conductivity in a polymer electrode for e-skin applications. The researchers have published their findings in the journal Biosensors and Bioelectronics.

Scientists around the world are working to develop electronic skins that attach to the body and monitor vital signs. These e-skins need to be comfortable, breathable and flexible for everyday use. Gold is typically used to fabricate the electrodes that conduct electric signals in these applications, but gold is expensive, involves a complicated manufacturing process and must be sterilised for use on the human body.

Among promising alternative materials for electrodes is the polymer PEDOT:PSS, as it is biocompatible with human skin, flexible, relatively cheap and can be easily manufactured and made into an electrode. Unfortunately, it doesn’t conduct electricity as well as gold — and while scientists have found ways to improve its conductivity, these methods involve toxic products, like acids, which can leave residues and are therefore not ideal for e-skin applications. Now DGIST researchers and their colleagues have found a non-toxic method that significantly improves PEDOT:PSS performance.

“We developed a hydrothermal treatment, involving humidity and heat, that enhanced the conductivity of PEDOT:PSS films by a factor of 250,” said materials scientist Sungwon Lee, who led the study. Specifically, the researchers found that applying 80% humidity and more than 60°C heat to a PEDOT:PSS thin film led to structural changes within the material that enhanced its ability to conduct electricity.

PEDOT:PSS is made up of water-insoluble, conductive PEDOT molecules and water-soluble, insulating PSS molecules. Adding humidity to a thin film of PEDOT:PSS separated the two types of molecules with a screen of water, while adding heat expanded the PEDOT chains, increasing the material’s overall crystallinity. These structural changes improved the material’s conductivity from 0.495 to 125.367 Siemens per centimetre (S/cm).

The scientists then made electrodes from the enhanced PEDOT:PSS material and found it stably conducted electricity when exposed to air, heat, bending and stretching. They also found that the electrodes worked well when sprayed onto e-skin devices used for monitoring joint movements, skin temperature and the heart’s electrical activity.

The new approach is simple and cheap, but further enhancements are needed for PEDOT:PSS to become a viable alternative to gold electrodes. Indeed, treating the polymer with acids can improve its electrical conductivity all the way up to 2244 S/cm.

“Our results are, nonetheless, noteworthy, with our novel hydrothermal treatment showing significant potential for use in biomedical applications,” Lee said.

Image caption: The scientists found their hydrothermally treated polymer electrodes functioned well in e-skin devices.

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