Highly Sensitive, Repairable, and Flexible Strain\nSensors with a Wide Sensing Range Based on an EG/Sn–Bi/EG-Encapsulated\nSandwich Structure

An\nencapsulated sandwich-structured flexible strain sensor with\nhigh sensitivity and simple repairability was prepared using a Sn–Bi\nalloy film as the sensitive layer. Conductive composite materials\nwere prepared by combining graphene with Ecoflex, to completely encapsulate\nthe sensitive layer. The resultant flexible strain sensor demonstrated\na wide sensing range (50%), high sensitivity coefficient of 16323\n(0 < ε < 3.84%) and 2125 (3.84% < ε < 50%),\nrapid response time (approximately 46 ms), and high durability (∼4800\nstretch-release cycles before needing repair). The high sensitivity\nwas attributed to the cracks generated in the Sn–Bi alloy film\nduring the stretching process, and the EG (Ecoflex/graphene) layer\nmaintained the conductive pathways under large strains, greatly expanding\nthe sensing range. Furthermore, the EG layers on the outside surfaces\nprovided robust protection for the Sn–Bi alloy film, endowing\nthe sensor with water, dust, and friction resistance, which is needed\nin various daily life scenarios. After 6000 cycles of stretching and\nreleasing, the accumulation of cracks in the Sn–Bi alloy film\nresulted in significant residual resistance. With the melting point\nof the Sn–Bi alloy film as low as 47 °C, a simple thermal\npressing treatment was used to rapidly and efficiently restore the\ndamaged Sn–Bi alloy film to its initial state. This work presents\nan effective approach for achieving both high sensitivity and a wide\nsensing range in strain sensors. The encapsulated sandwich structure\ndesign endows the sensor with repair capabilities and resistance to\nexternal environmental interference. The sensor demonstrates significant\npotentiality for applications in health monitoring, electronic skin,\nand wearable devices.