Opportunity
Epidermal electronic devices have great promise in wearable human-machine interfaces and intelligent healthcare sensing, yet their widespread adoption is hindered by several critical challenges. Conventional epidermal electronics often suffer from complex and costly manufacturing processes, limited adhesion duration on the skin, and poor adaptability to different skin surfaces. Once adhesion is lost, the entire device must be replaced, causing inconvenience, increased cost, and reduced patient compliance. Similarly, emerging "Silly Putty-like" hydrogels, while offering deformability, still rely on expensive conductive materials such as silver nanowires, liquid metals, or MXene, and involve complicated fabrication steps that limit shape customization and reusability. Moreover, increasing the conductive filler content to improve electrical performance often degrades mechanical properties. There is an urgent need for a low-cost, easily customizable, fully reusable, and highly conformable epidermal substrate that combines good electrical conductivity, mechanical robustness, adhesion, and self-healing capability for practical wearable sensing applications.
Technology
The present invention introduces an ultra-deformable, bio-adhesive, self-healing, and electromechanically durable wearable electronic slime (termed "E-slime") for in-situ formation of epidermal electronics. The substrate material is composed of biocompatible polyvinyl alcohol (PVA) as the matrix, glycerol as a plasticizer, tannic acid as a physical property-enhancing agent, and a cost-effective conductive mixture of graphite and carbon black. A polymerization agent such as borax pentahydrate is used to crosslink the network. The fabrication process is remarkably simple: PVA, glycerol, and tannic acid are dissolved in deionized water under heating (80°C for 2 hours), then graphite and carbon black are added and mixed (60°C for 10 minutes), followed by dropwise addition of borax solution to form the final hydrogel. This method uses only readily available, inexpensive raw materials and requires no complex equipment. The E-slime exhibits a hierarchical three-dimensional network with dynamic borate ester bonds and hydrogen bonds, enabling extreme stretchability (up to 2600% strain), high sensitivity (gauge factor of 2.95), a wide sensing range (up to 400% strain), and a low detection limit (~1% strain). The island-bridge structure formed by graphite flakes and carbon black granules provides efficient conductive pathways. The material also demonstrates strong skin adhesion (up to 5.84 kPa) due to catechol groups from tannic acid, rapid self-healing within ~1 second even after repeated cuts, and excellent reusability for up to 100 cycles with minimal performance degradation. It can be easily reshaped, peeled off, and reapplied to different body locations, adapting intimately to wrinkled or curved skin surfaces.>
Advantages
- Extremely simple and low-cost fabrication using PVA, glycerol, tannic acid, graphite, and carbon black (total material cost ~0.089 CNY/g)
- High electromechanical performance: gauge factor 2.95, conductivity 0.33 S/m, sensing range up to 400% strain, detection limit ~1% strain
- Outstanding mechanical properties: stretchability up to 2600%, elastic modulus ~5.3 kPa, adhesive strength up to 5.84 kPa
- Rapid self-healing ability: recovers conductivity in ~1 second, maximum healing distance ~5 mm, stable over 100 cutting-healing cycles
- Full reusability: can be peeled off, reshaped, and reapplied for up to 100 cycles with stable electrical and mechanical performance
- Strong conformability and adhesion to human skin, even on wrinkled or irregular surfaces, without additional adhesives or fixation
- Biocompatible with high cell viability (98.72%), suitable for long-term epidermal contact
- Versatile sensing capability: detects large joint motions and subtle physiological signals (pulse, throat movements)
Applications
- Wearable strain sensors for human motion detection: finger, wrist, elbow, knee, ankle, and hand gesture recognition
- Facial expression monitoring: frown, smile, mouth movements
- Physiological signal monitoring: pulse detection, swallowing, coughing
- Human-machine interfaces: handwriting recognition (English and Chinese characters), mouse click detection
- Personal health tracking: electrocardiogram (ECG), blood glucose level monitoring, wound healing status
- Emergency circuit repair and flexible electronic interconnections
- Smartphone interaction and soft robotics
- Next-generation green electronics and intelligent medical sensing devices
