Are bioelectrodes the future of healthcare wearables?

This material was stretchable, permeable to humidity, and closely conformed to the skin, making it suitable for prolonged use. This development addressed significant limitations of current bioelectrode materials, offering more comfortable and effective wearables for healthcare and fitness applications.

Creating a stretchable, humidity-permeable electrode material

The integration of wearable electronics for continuous biosignal monitoring has significantly impacted the healthcare and fitness industries. These devices have become increasingly prevalent, with a market valuation expected to reach around $572.06 billion by 2033. This rapid expansion has heightened the demand for high-quality bioelectrodes that can accurately record biosignals over long periods. However, current materials for bioelectrodes, including metals, conductive polymers, and hydrogels, face challenges. They often lack the necessary flexibility to move with the skin without breaking and have low humidity permeability, leading to sweat accumulation and discomfort.

To overcome these challenges, a research team led by Assistant Professor Tatsuhiro Horii and Associate Professor Toshinori Fujie from the Tokyo Institute of Technology (Tokyo Tech) developed a new bioelectrode material. Published in the journal NPG Asia Materials on 20th June 2024, their study introduced a stretchable, humidity-permeable material that closely conforms to the skin. This novel material comprises layers of conductive fibrous networks made of single-wall carbon nanotubes (SWCNTs) on a stretchable poly(styrene-b-butadiene-b-styrene) (SBS) nanosheet. The nanosheet tightly adheres to the skin for precise biosignal measurement, while the carbon nanotube fibres ensure the material's stretchability and humidity permeability.

"Self-supporting electrodes that are stretchable, permeable to humidity, and conformable to skin surface bumps are required to allow for the natural deformation of skin without restricting body movements," says Horii.

The researchers applied SWCNTs as aqueous dispersions onto SBS nanosheets, creating multiple layers with a thickness of just 431 nm. Each SWCNT coating increased the fibre density and thickness, altering the bioelectrode's properties. Although adding more SWCNT layers made the nanosheet stiffer (with the elastic modulus rising from an initial 48.5 MPa to 60.8 MPa for one layer and 104.2 MPa for five layers), the bioelectrode remained highly flexible. Pristine SBS nanosheets and those with one or three SWCNT layers (SWCNT 3rd-SBS) could stretch elastically by 380% of their original length before permanent deformation. This flexibility greatly exceeded that of metal electrodes like gold, which have Young's moduli in the several-hundred-GPa range and can stretch less than 30% before breaking.

Another important requirement for bioelectrodes is high water vapour permeability to prevent sweat buildup during exercise. The fibrous network structure of SWCNTs improved breathability compared to continuous films. In water vapour transmission rate (WVTR) tests, the SWCNT 3rd-SBS exhibited a WVTR of 28,316 g m-2 (2 h)-1, which is double that of normal skin.

The bioelectrode material also demonstrated high resilience for extended use. To test durability, the researchers immersed the bioelectrodes in artificial sweat and subjected them to repeated bending, measuring resistance changes. The resistance increased negligibly, by only 1.1 times in sweat and by 1.3 times over 300 bending cycles. Additionally, the SWCNT 3rd-SBS nanosheets showed minimal detachment after being rubbed ten times, indicating their suitability for prolonged use.

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

Concludes Fujie: "We obtained skin-conformable bioelectrodes with high water vapor permeabilities, which showed comparable performance in sEMG measurements to those of conventional electrodes," enthusiastic about the material's promising capabilities for healthcare wearables.