Extremely Stretchable Strain Sensors Based on Conductive Self‐Healing Dynamic Cross‐Links Hydrogels for Human‐Motion Detection

摘要

Extremely stretchable self-healing strain sensors based on conductive hydrogels are successfully fabricated. The strain sensor can achieve autonomic self-heal electrically and mechanically under ambient conditions, and can sustain extreme elastic strain (1000%) with high gauge factor of 1.51. Furthermore, the strain sensors have good response, signal stability, and repeatability under various human motion detections. Stretchable, wearable, flexible, and human friendly soft electronic devices are of significance to meet the escalating requirements of increasing complexity and multifunctionality of modern electronics.1-6 Strain sensors can generate repeatable electrical changes upon mechanical deformations. They have found particular interest and broad applications in robotics, sports, health monitor, and therapeutics, etc. To date, several representative strain sensors using carbon nanotubes,7-9 metal/semiconductor,10-12 graphene,13-15 conductive polymer,16, 17 and microfluidic18, 19 as conductive materials combining with elastomeric substrates have been successfully fabricated. However, most of these devices can only be stretched to a very limited extent (usually less than 200%). Lewis and co-workers20 have developed a capacitive soft strain sensor using an ionically conductive fluid and silicone elastomer as the conductor and dielectric/encapsulant respectively, which can be stretched up to 700%, but the gauge factor is small (0.348 ± 0.11). We can define the gauge factor as (ΔR/R0)/ε, where ΔR/R0 is relative resistance change, R0 is the resistance at 0% strain, R is the resistance under stretch, and ε is the applied strain.21 In addition, introducing self-healing properties to these soft electronic devices that can repeatably recover mechanical and electrical performance under room temperature, even at the same damaged location or under extremely stretchable situation, is of high importance to avoid the degradation of the device performance during the deformation. Nowadays, self-healing materials have attracted increasing attention, especially in soft electronics field. Haick and co-workers22 have reported a self-healing flexible sensing platform by dispersing metal particles in polyurethane diol as self-healing electrode. Bao and co-workers23 have demonstrated a self-healing electronic sensor skin based on nanostructured μNi particles-supramolecular organic composite. Park and co-workers24 have developed self-healing conductive hydrogel by polymerizing pyrrole in agarose solution. However, none of these self-healing electronic devices can be stretched over 100%. Recently, there are intense research on highly stretchable hydrogels, which are mainly focusing on ionic conductors due to their excellent transparency and small resistance variation under high stretching states.25-29 In particular, conductive hydrogels are promising materials for the fabrication of ionic skin, bioelectrodes, and biosensors because many hydrogels with high water concentration have biocompatibility properties.24, 30-32 Therefore, it is of great interest to fabricate highly stretchable self-healing strain sensor by combining the advantages of both biocompatible hydrogels and electronic conductors for applications in robotics, human motion detection, entertainment, medical monitoring, and treatment etc. Herein, we introduce a new type of extremely stretchable self-healing piezoresistive strain sensor using different electronic conductors comprised of single wall carbon nanotube (SWCNT), graphene, and silver nanowire in self-healing hydrogel (SWCNT, graphene, and silver nanowire/hydrogel) as the conductive sensing channel built on a commercial transparent elastic substrate. The conductive hydrogel exhibits a fast self-healing capability which can restore 98 ± 0.8% of its initial conductivity within 3.2 s healing time. Moreover, no external stimuli (such as heat, pH, light, or catalyst) are required. The fast self-healing process of the SWCNT/hydrogel en