Researchers have created a cutting-edge bioelectrode material for wearable devices by combining single-wall carbon nanotubes and poly(styrene-b-butadiene-b-styrene) nanosheets. This new material is flexible, moisture-permeable, and closely adheres to the skin, making it perfect for long-term use. This breakthrough addresses key limitations of current bioelectrode materials, promising more comfortable and efficient wearables for healthcare and fitness applications.
Wearable electronics that monitor biosignals continuously have revolutionized the healthcare and fitness sectors. These devices are increasingly popular and are expected to reach a market value of around USD 572.06 billion by 2033. With this rapid expansion, there is a growing need for high-quality bioelectrodes that can accurately capture biosignals over extended periods. However, many existing bioelectrode materials, such as metals, conductive polymers, and hydrogels, have limitations. They often lack the flexibility to stretch with the skin without damage and have poor moisture permeability, leading to sweat accumulation and discomfort.
To overcome these challenges, a team of researchers led by Assistant Professor Tatsuhiro Horii and Associate Professor Toshinori Fujie from the Tokyo Institute of Technology developed a bioelectrode material that is stretchable, moisture-permeable, and closely conforms to the skin. This innovative material comprises layers of conductive fibrous networks made of single-wall carbon nanotubes (SWCNTs) on a flexible poly(styrene-b-butadiene-b-styrene) (SBS) nanosheet. The nanosheet fits snugly to the skin, enabling precise biosignal measurements, while the carbon nanotube fibers maintain the material’s flexibility and moisture permeability.
“To allow for natural skin deformation without restricting movement, we need self-supporting electrodes that are stretchable, moisture-permeable, and conform to skin bumps,” explained Horii.
The researchers applied SWCNTs as aqueous dispersions onto SBS nanosheets, forming multiple layers with a thickness of just 431 nm. Each layer of SWCNTs enhanced the density and thickness of the fibers, altering the bioelectrode’s properties. While increasing the number of SWCNT layers made the nanosheet stiffer (initially 48.5 MPa elastic modulus to 60.8 MPa for one layer and 104.2 MPa for five layers), the bioelectrode retained remarkable flexibility. Pristine SBS nanosheets and those with one or three layers of SWCNTs (SWCNT 3rd-SBS) could stretch elastically by 380% of their original length before permanent deformation. This flexibility exceeds that of metal electrodes like gold, with Young’s moduli in the several-hundred-GPa range and able to stretch less than 30% of their original length before breaking.
Another critical requirement for bioelectrodes is high moisture vapor permeability to prevent sweat buildup during physical activity. Adding SWCNTs is advantageous as their fibrous network structure enhances breathability compared to continuous films. In water vapor transmission rate (WVTR) experiments, the SWCNT 3rd-SBS showed a WVTR of 28,316 g m-2 (2 h)-1, double that of normal skin.
The bioelectrode material is also highly durable for extended use. To assess its durability, the researchers immersed the bioelectrodes in artificial sweat and subjected them to repeated bending, measuring changes in resistance. In these tests, the resistance increased minimally, by only 1.1 times in sweat and 1.3 times over 300 bending cycles. Additionally, the SWCNT 3rd-SBS nanosheets showed little or no detachment after rubbing ten times, indicating their suitability for prolonged use.
In a real-world performance evaluation, the researchers compared an SBS nanosheet with three layers of SWCNT to commercially available bioelectrode materials like Ag/AgCl gel electrodes. The bioelectrodes were attached to the forearm, and surface electromyography (sEMG) measurements were taken during gripping movements. The performance of the SWCNT-SBS nanosheet was on par with that of commercial Ag/AgCl gel electrodes, achieving similar signal-to-noise ratios of 24.6 dB and 33.3 dB, respectively.
“We have developed skin-conforming bioelectrodes with high moisture vapor permeability, demonstrating comparable performance in sEMG measurements to traditional electrodes,” Fujie concluded, underscoring the material’s promising potential for healthcare wearables.
