Wearable technology is the cream of the medical innovations and improvement of health outcomes. Wearable devices are smart devices that can be worn on the skin, close to the skin or embedded on a user’s body to collect health data and monitor the user’s health conditions, among other potential applications. Such applications require the use of highly durable and stretchable electrodes for high performance and adequate reliability. Extensive research has been conducted to fabricate high-performance intrinsically stretchable electrodes, including sophisticated detecting sensors and high-density integrated circuits. Unfortunately, most of the available electrical component materials, particularly conductors, are either organic or inorganic. And such conductors fail or undergo permanent deformation when mechanically stretched, thus limiting their application in wearables.
Different methods have been proposed to improve the outcome of wearable devices. Some employ in- and out-of-plane architectural designs, while others use intrinsically stretchable nanocomposites like metallic nanomaterials that create mechanically percolated networks facilitating the formation of the conduction pathway. Despite the good progress in the research and fabrication of high-performance intrinsically stretchable conductors, obtaining improved electrical conductivity and high mechanical integrity simultaneously has remained a great challenge due to trade-off effects. Additionally, the lack of adequate knowledge on the effects of external stimuli like wear on the properties of the stretchable electrodes is a great hindrance to their commercialization and practical applications.
On this account, a team of researchers comprising of Ph.D. students: Seongsik Jeong, Seojun Heo, Minseong Kang and led by Professor Hae-Jin Kim from the Gyeongsang National University in South Korea explored the mechanical durability enhancement of gold nanosheet (AuNS) stretchable electrodes and their application in wearable bio-signal sensors for humans. The aim was to improve the understanding of the mechanical resilience of these electrodes and how they respond to external stimulations for increased practical applications. Their work is currently published in the research journal, Materials and Design.
Briefly, the intrinsically stretchable electrodes used in the study were fabricated on mechanically percolated two-dimensional (2D) AuNSs via optimized hot-pressing treatment conditions. The hot-pressing conditions involved different contact pressures, temperatures and repetitive sliding cycles. The mechanical behaviors and electrical conductivity of the resulting electrodes were assessed and compared under different fabrication conditions. Finally, the feasibility of the fabricated electrodes was demonstrated in bio-signal detection sensors to acquire the human electromyography (EMG) and electrocardiography (ECG) spectrum signals.
The authors found out that the fabricated AuNS electrodes demonstrated high durability, high stretchability and improved electrical conductivity due to the formation of the mechanically percolated network at the AuNS interfaces that increased the contact area and bonding strength between interfaces. The electrodes fabricated under a temperature of 250 °C and contact pressure of 10kPa exhibited significantly higher durability against repetitive sliding than those fabricated under other conditions. Moreover, the electrodes showed very low electrical conductivity variations, below 5 Ω/sq), under 1000 repetitive stretch-release cycles and up to 50% increase in the mechanical strain. Furthermore, the AuNS electrodes formed conformal contact with human skin to capture dynamic bio-signals under different circumstances.
In summary, a study of the intrinsically stretchable AuNS electrodes fabricated by optimized hot-pressing treatment was reported. Hot-pressing fabrication method facilitated firm contact between the gold nanosheets, ensuring that they remained intact when mechanically strained or stretched. The resulting electrodes exhibited enhanced mechanical durability and electrical conductivity. Their successful verification in ECG and EMG bio-detecting sensors applications to capture dynamic bio-signals demonstrated their superior performance than commercially available wearable devices. In a statement to Advances in Engineering, Professor Hae-Jin Kim noted that the high-performance AuNS electrodes prepared by hot-pressing treatments would enable fabrication of future stretchable electronics for soft-robot applications.
Jeong, S., Heo, S., Kang, M., & Kim, H. (2020). Mechanical durability enhancement of gold-nanosheet stretchable electrodes for wearable human bio-signal detection. Materials & Design, 196, 109178.