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

An encapsulated sandwich-structured flexible strain sensor with high sensitivity and simple repairability was prepared using a Sn–Bi alloy film as the sensitive layer. Conductive composite materials were prepared by combining graphene with Ecoflex, to completely encapsulate the sensitive layer. The resultant flexible strain sensor demonstrated a wide sensing range (50%), high sensitivity coefficient of 16323 (0 < ε < 3.84%) and 2125 (3.84% < ε < 50%), rapid response time (approximately 46 ms), and high durability (∼4800 stretch-release cycles before needing repair). The high sensitivity was attributed to the cracks generated in the Sn–Bi alloy film during the stretching process, and the EG (Ecoflex/graphene) layer maintained the conductive pathways under large strains, greatly expanding the sensing range. Furthermore, the EG layers on the outside surfaces provided robust protection for the Sn–Bi alloy film, endowing the sensor with water, dust, and friction resistance, which is needed in various daily life scenarios. After 6000 cycles of stretching and releasing, the accumulation of cracks in the Sn–Bi alloy film resulted in significant residual resistance. With the melting point of the Sn–Bi alloy film as low as 47 °C, a simple thermal pressing treatment was used to rapidly and efficiently restore the damaged Sn–Bi alloy film to its initial state. This work presents an effective approach for achieving both high sensitivity and a wide sensing range in strain sensors. The encapsulated sandwich structure design endows the sensor with repair capabilities and resistance to external environmental interference. The sensor demonstrates significant potentiality for applications in health monitoring, electronic skin, and wearable devices.