Research On Flexible Strain Sensors Based On Liquid-impregnated Elastic Fabric

The emergence and popularization of new applications such as human motion monitoring,medical health monitoring,human-machine interface,and smart clothing have driven the development of the wearable sensing devices.The flexible strain sensors are one of the important components of the wearable sensing devices.The sensing principle of the flexible strain sensors is that the sensors are deformed due to external force,which changes the electrical characteristics of their conductive materials.The commonly used conductive materials include solid-state and liquid-type materials.Due to its excellent fluidity and ductility,the liquid conductive materials can overcome the shortcomings of solid conductive materials that are prone to failure when highly stretched,and have attracted more and more attention from researchers.However,for most flexible strain sensors based on liquid conductive materials,the curing of the elastomer should be experienced twice to complete the liquid channel fabrication and sensor packaging,which is inefficient.Furthermore,flexible strain sensors based on liquid conductive materials rarely exhibit both tensile and compressive strain sensing capabilities,which shrinks their application potentials.Therefore,this paper carried out the innovative research by simplifying the manufacturing process of flexible strain sensors based on liquid conductive materials,using porous elastic fabrics to carry sensing liquids with different viscosities,and further compounding with silicone elastomers,successfully preparing the LIEF flexible strain sensors.The sensor exhibits good stretchability,linearity,durability and low hysteresis,and more importantly,it shows the ability to sense tensile strain and compressive stress at the same time.Specific work and research results are as follows:(1)We innovatively impregnated the porous elastic fabric directly with three kinds of liquid conductive materials with different viscosities,such as water,ethylene glycol and hydrogel,and then embed them in the silicone elastomer respectively,thus preparing the LIEF flexible strain sensor,making the production simple,convenient,and no special steps for making channels.The research results show that:the lower the viscosity of the liquid conductive material,the smaller the hysteresis of the sensor.The LIEF flexible strain sensor was able to sense tensile strain and compressive stress at the same time.Simultaneously,high stretch strain over 100%and pressures up to 612 kPa can be subjected.A minimum tensile strain of 0.5%and a minimum applied pressure of 5 kPa can be detected.In particular,our device exhibited the excellent long-term durability after completing>6000 stretching-releasing cycles(100%strain)and>10000 compression-expansion cycles(50 kPa).(2)We systematically studied the effect of the amount of impregnated liquid,ambient temperature,the shape of the elastomer and the length of the liquid channel on the sensor performance.The test results show that:the different amount of impregnated liquid has no major impact on sensing performance.When the ambient temperature increased from room temperature(25?)to 100?,the thermal sensitivity of the water-based,ethylene glycol-based,and hydrogel-based sensors was calculated to be 0.0076,0.006,and 0.0029 ?-1,which is in the same order of magnitude as most reported temperature sensors.The sensitivity of a fibrous sensor is slightly lower than that of a rectangular sensor,but their hysteresis is similar.And the longer the length of the liquid channel,the higher sensitivity to tensile strain and the smaller hysteresis of the LIEF strain sensor.(3)Benefits from the good sensing performance demonstrated by the LIEF flexible strain sensor,we have successfully applied it to the field of human motion monitoring,realizing the monitoring of human joint flexion from finger to knee,as well as the monitoring of human muscle undulations,coughing and swallowing,playing volleyball,etc.We also made the sensor into a smart insole,realizing real-time monitoring of walking,running and jumping.