By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Apr 22 2024A recent publication in the journal Communications Materials delves into the advancements in high-performance electrodes for wearable skin-contacting devices. The article examines ways to enhance functionality through structural modifications, the use of flexible and soft conductive materials, and the design of hybrid structures.
a Skin device’s requirements (mechanical properties, electrical properties, and biocompatibility) and associated characteristics. b The occurrence of cracks with low stretchability and high stretchability. c Comparison of electrode-to-skin separation due to differences in adhesion. d Differences in void occurrence according to conformability. e Scanning electron microscope image of platinum nanowires (PtNWs) (Scale bar: 400 nm) (top) and Impedance of Au, Pt/Au, PtNW/Au electrodes with respect to frequency of applied signal (bottom). f Electromyography recordings for three subjects measured with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) gel electrode (red) and conventional electrode (gray). e Reproduced with permission. Copyright 2014 Springer Nature. f Reproduced with permission. Copyright 2023 John Wiley and Sons. Image Credit: https://www.nature.com/articles/s43246-024-00490-8
Background
Wearable electronics have become crucial healthcare devices due to their non-invasive and real-time monitoring and diagnosis capabilities. Skin-interfaced wearable electronics can be categorized as sensors/biomarkers and delivery vehicles, both of which rely on electrodes. Thus, high-performance electrodes are necessary to ensure the effectiveness of wearable skin devices.
The electrodes in wearable devices interact with the skin to capture electrical signals or provide simulation, which can be haptic feedback, electrical muscle stimulation, or neural simulation. Thus, optimization of electrodes requires careful consideration of skin properties such as elasticity (skin can stretch up to around 50 %, moduli varying between a few kPa to tens of MPa), sweating, electrical resistance (1000 to 100,000 Ohms), microstructures (like fingerprints and wrinkles) and variations with race, age, health conditions, and external damage.
This review highlights essential mechanical properties of the electrodes required for wearable skin devices, such as strength (mainly elasticity), electrical performance, and biocompatibility for skin-electrode interface. Additionally, different electrode structures are proposed, which exhibit high flexibility, adhesion, and conformity to the continuously changing shape of the skin.
Mechanical Requirements
The mechanical properties of electrodes are crucial for ensuring effective interaction with the skin, despite disturbances from body movements and sweat. These properties are fundamental in the design of wearable electronics. Firstly, an electrode must be sufficiently stretchable to accommodate the continuously changing shape of the skin caused by the wearer’s movements. Materials with poor stretchability, such as metals, can fracture or crack, hindering consistent signal acquisition or electrical stimulation.
Adhesion is another important property of electrodes, essential for patient comfort and stable monitoring. Electrodes should remain securely fixed to the skin, resisting displacement from the skin’s flexibility, movements, or external forces. Moreover, excellent conformability with minimal voids at the skin-electrode interface is vital for efficient device functioning. Poor conformability can damage the skin or apply excessive pressure.
The electrical properties of electrodes, such as impedance and signal-to-noise ratio, also determine their performance in electrical sensing and stimulation. High contact impedance at the skin-electrode interface deteriorates signal quality. Therefore, materials like soft conductive polymer hydrogels and nanowires, which exhibit conformal contact with the skin, are preferred as they minimize impedance and enhance the signal-to-noise ratio.
Additional mechanical properties crucial for effective wearable devices include biocompatibility, breathability, transparency, size, and density. Noble metals like gold and platinum, as well as 3D-printed carbon nanotubes, are favored for their safe tissue interaction and immune response compatibility. Employing these materials in nano-mesh form also enhances device breathability. Furthermore, using transparent polymers, which can be designed in various shapes and sizes, can significantly improve user experience.
Structural Approaches
Successful integration of wearable devices with our bodies largely hinges on the devices' ability to conform to the dynamic stretching of skin. This can be achieved through the use of soft materials or by structurally modifying non-stretchable materials to absorb stresses and strains. The latter option often involves conventional solid-state metals, which hold a higher potential for conformal integration into wearable electronics.
In their study, the researchers proposed several different functional structures that enable the electrodes to conform to the skin contours. For example, mesh and wavy architectures not only provide high flexibility but also enhance wearability over extended periods. Other effective designs include staggered nanosheets, 3D patterned interwoven patterns, coils, island bridges, and hollow structures. Each of these structures necessitates specific fabrication methods and utilizes a diverse range of materials.
Materials that are flexible (able to bend without breaking) and soft (easily moldable) facilitate the seamless integration of irregular electronic systems with skin. Low-dimensional conductive materials like graphene, carbon nanotubes, MXenes, and silver nanowires are particularly advantageous for such applications. The degree of flexibility and softness provided by these materials is largely dependent on the specific fabrication processes used.
Hybrid formation of materials is another approach suggested in this review for developing efficient electrodes. Hybrid materials are a strategic solution to the tradeoff encountered in conventional electrode materials, according to which increased softness generally leads to reduced electrical conductivity and vice versa. The composites of hard inorganic materials and soft elastomers can help realize unobtrusive and long-term functional wearables.
Future Prospects
Wearable skin device technology extends beyond basic sensing and stimulation functions, holding vast potential for a variety of applications thanks to advancements in electrode materials and structures. Recent innovations are paving the way for multifunctional devices that can perform both sensing and stimulation tasks with high efficiency.
This article highlights the integration of artificial intelligence as a promising direction for smart devices. The application of machine learning and deep learning techniques enables these devices to process information through artificial neural networks, which mimic the functionality of human neurons. This technological integration facilitates a unified platform for comprehensive diagnostics, treatment, and alert systems within a single device.
Journal Reference
Lim, K., Seo, H., Won Gi Chung, Song, H., Oh, M., Seoung Young Ryu, Kim, Y., & Park, J.-U. (2024). Material and structural considerations for high-performance electrodes for wearable skin devices. Communications Materials, 5(1). https://doi.org/10.1038/s43246-024-00490-8, https://www.nature.com/articles/s43246-024-00490-8
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