Flexible bioelectrodes: the future of healthcare wearables?

Researchers from the Tokyo University of Technology have developed a novel bioelectrode material for wearable devices by combining single-wall carbon nanotubes and poly(styrene-b-butadiene-b-styrene) nanosheets. The material is stretchable, permeable to humidity and conforms closely to the skin, making it suitable for prolonged use. Its development addresses the limitations of current bioelectrode materials, to provide more comfortable and effective wearables for healthcare and fitness applications.

The use of wearable electronics that continuously monitor biosignals is becoming increasingly ubiquitous. With this rapid growth, there is rising demand for high-quality bioelectrodes capable of accurately recording biosignals over extended periods. However, many of the materials currently used for bioelectrodes, such as metals, conductive polymers and hydrogels, have limitations. They often lack the flexibility to stretch the skin without breaking and have low humidity permeability, leading to sweat build-up and discomfort.

To address these limitations, the researchers — led by Assistant Professor Tatsuhiro Horii and Associate Professor Toshinori Fujie — have developed a bioelectrode material that is composed of layers of conductive fibrous networks consisting of single-wall carbon nanotubes (SWCNTs) on a stretchable poly(styrene-b-butadiene-b-styrene) nanosheet. The nanosheet conforms to the skin, enabling precise biosignal measurements, while the carbon nanotube fibres maintain the material’s stretchability and humidity permeability.

The researchers applied SWCNTs as aqueous dispersions onto the nanosheets, creating multiply layers reaching a thickness of 431 nm. Each coating of SWCNTs increased the density and thickness of the fibres, modifying the bioelectrode’s characteristics. While adding more SWCNT layers increased nanosheet stiffness, the bioelectrode material maintained its flexibility. Pristine SBS nanosheets and those with one or three layers of SWCNTs (SWCNT 3rd-SBS) were stretched elastically by 380% of their original length before permanent deformation.

Another requirement for bioelectrodes is high water vapour permeability to prevent sweat build-up. Adding SWCNTs is beneficial as its fibrous network structure improves breathability compared to continuous films.

The bioelectrode material is also designed to be resilient, for extended use. To test the material’s durability, the researchers immersed the bioelectrodes in artificial sweat and subjected them to repeated bending, measuring the change in resistance. The researchers found that the resistance increased by only 1.1 times in sweat and by 1.3 times over 300 cycles of bending. The SWCNT 3rd-SBS nanosheets also showed little to no detachment after being rubbed 10 times, indicating their suitability for prolonged use.

To assess the material’s real-world performance, the researchers compared an SBS nanosheet with three layers of SWCNT to commercially available bioelectrode materials such as Ag/AgCl gel electrodes. The bioelectrodes were attached to the forearm and surface electromyography measurements were taken during gripping motions. The performance of the SWCNT-SBS nanosheet was comparable to that of commercial Ag/AgCl gel electrodes, achieving similar signal-to-noise ratios of 24.6 dB and 33.3 dB respectively.

“We obtained skin-conformable bioelectrodes with high water vapour permeabilities, which showed comparable performance in sEMG measurements to those of conventional electrodes,” Fujie said.

The research findings were published in the journal NPG Asia Materials.

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