Biodegradable All-Leaf E-Skin for Enhanced Gesture Recognition and Human Motion Monitoring

Electronic skins (e-skins) are flexible, stretchable films that mimic some of the functionalities of human skin, including the ability to sense pressure, temperature, humidity, and motion. They have a wide range of potential applications, from wearable technology and soft robotics to healthcare and prosthetics. While e-skins need to be flexible and stretchable, finding materials that can withstand repeated stretching and flexing without degradation over time is challenging. Moreover, for continuous monitoring, e-skins must operate efficiently with minimal power consumption and developing low-power sensors that can be embedded in e-skins is a significant challenge. Furthermore, producing e-skins on a large scale at a reasonable cost while maintaining high quality and performance is difficult. The manufacturing process must be compatible with the delicate materials and sensors involved. Additionally, integrating e-skins into wearable devices in a way that is comfortable for the user remains also a challenge. The ideal e-skin must be breathable, lightweight, and have minimal impact on the user’s daily activities. Addressing these challenges is essential for their successful integration into practical applications. To this account, a new study published in Chemical Engineering Journal by Dr. Mohammad Zarei, and Professor Seung Goo Lee from the Department of Chemistry at University of Ulsan in South Korea alongside Dr. Jung Hoon Kim and Dr. Joong Tark Han from the Nano Hybrid Research Center at Korea Electrotechnology Research Institute, the researchers fabricated and evaluated the performance of a biodegradable, breathable, flexible, and electrically modulated all-leaf capacitive electronic skin (e-skin) for gesture recognition and human motion monitoring. Their experimental approach encompassed material synthesis, structural and electrical characterization, sensitivity testing, and application demonstration. The team initiated their study by synthesizing oxidized single-walled carbon nanotubes (Ox-SWCNTs) and incorporated them into silver nanowire (AgNW) networks. The utilization of Ox-SWCNTs and AgNWs as the conductive materials, combined with the structural integrity of leaf skeletons as the substrate, marks a significant departure from traditional e-skin materials. This novel approach of employing leaf skeletons enhances the biodegradability and environmental sustainability of the e-skin and also leverages the natural microstructural properties of leaves to improve the sensor’s performance while incorporating Ox-SWCNTs, which are known for their ultra-high thermal conductivity, into the AgNW network will significantly enhance the electrical and thermal properties of the electrodes, thereby improving the e-skin’s sensitivity and stability.

After fabricating the electrodes, the researchers performed a series of characterizations. Scanning Electron Microscopy (SEM) analyses revealed the hierarchical microstructure of the leaf skeletons, including the presence of non-smooth microdomes and protuberances that contributed to the e-skin’s sensitivity. They showed that incorporation of Ox-SWCNTs into the AgNW network to significantly improve thermal stability and electrical conductivity, which was crucial for the AgNW-based electrodes’ performance. They demonstrated the pivotal role of Ox-SWCNTs in tuning the electrical properties of the AgNW network using electrical modulation experiments. Additionally, by adjusting the concentration of Ox-SWCNTs, they were able to optimize sheet resistance and electrical conductivity, and achieved the optimum balance at 4 wt% concentration. This approach allowed for the modulation of sheet resistance and electrical conductivity, thereby enhancing the e-skin’s ability to accurately detect and monitor a wide range of pressures and motions. Indeed, the e-skin demonstrated high sensitivity and excellent linearity across a broad range of pressures, making it suitable for both subtle physiological monitoring and dynamic motion detection.

The core of their experimentation focused on testing the e-skin’s sensitivity and performance in detecting pressure variations and monitoring human motions. According to the authors, the e-skin exhibited high sensitivity (0.86 ± 0.16 kPa⁻¹) across a wide sensing range (0.01–97 kPa), with excellent linearity for low- and high-pressure regimes. This high sensitivity allowed the e-skin to accurately monitor subtle human physiological signals such as exhalation and vocal cord vibrations, as well as detect and monitor human motion with high precision. Moreover, the team conducted several demonstrations to showcase the practical applications of their e-skin. These included attaching the e-skin to various body parts (e.g., joints like elbows and knees, and the throat) to monitor different types of human movements and physiological signals. The e-skin successfully detected joint movements, including flexion and extension, and was able to monitor vocal cord vibrations indicative of speech patterns. Additionally, they tested the e-skin’s performance in a humid environment and its breathability and showed its robustness and comfort in wearable applications. It is noteworthy to mention, the authors’ work highlights the potential of using plant-based materials and nanotechnology in creating advanced wearable electronics. In 2024, drawing insights from this study, the authors published an advanced article in the Chemical Engineering Journal focusing on the development of a biodegradable all-leaf piezoresistive e-skin with enhanced sensitivity and improved breathability. This innovative e-skin, designed for tactile sensing and physiological monitoring, is disposable, biodegradable, and exceptionally breathable, making it well-suited for analyzing subtle human movements and comprehensive physiological signals. Its remarkable breathability, ultra-low limit of detection (LOD ~0.27 Pa), and enhanced sensitivity (19.75 ± 1.50 kPa⁻¹, <3 kPa) are facilitated by its high electrical conductivity, structural porosity, and effective interlayer contacts. The piezoresistive e-skin developed, characterized by the highly porous structure of leaf skeletons, exhibits exceptional breathability and permeability to both vapor and water. This attribute holds significant importance in ensuring comfort when worn. Enhanced sensitivity is achieved through optimized stress concentration, increased contact area, and improved stress distribution across interlocking vein arrays within multiple layers. These features empower the piezoresistive e-skins to effectively monitor a wide range of stimuli, spanning from low to high pressure levels. In conclusion, the application potential of the developed e-skins by Dr. Zarei, Professor Lee, Dr. Kim, and Dr. Han is vast, ranging from healthcare and rehabilitation to robotics and human-machine interfaces.