Design And Research Of A Highly Sensitive And Stretchable Flexible Strain Sensor

Flexible strain sensors have wide application prospects in the fields of mobile healthcare,human/machine interaction,and intelligent robots.Carbon nanotubes(CNTs)with high aspect ratio,good electrical conductivity,and excellent mechanical properties,have become one of the most attractive nanomaterials for flexible strain sensors.However,flexible strain sensors based on carbon nanotubes exhibit low sensitivity.There are still facing some urgent challenges,it is difficult to solve the contradiction between high sensitivity and large stretchability and low-cost fabrication of strain sensors.In this thesis,a high-performance flexible strain sensor based on double layer conductive films with microstructures was presented.Two different types of coordinated conductive structures,including multi-walled carbon nanotubes(MWCNTs)/Ecoflex conductive composites film and freestanding MWCNTs extending from the composites film surface,were fabricated via dispersing MWCNTs on the semi-cured Ecoflex surface to achieve high sensitivity and high stretchability at the same time.Furthermore,the working mechanism,electrical properties and application scenarios of the flexible strain sensor were researched in detail.The main research work of the thesis is summarized as follow:(1)The fabrication of MWCNTs-MWCNTs/Ecoflex flexible strain sensor with microstructures.The characterization of the fabricated MWCNTs-MWCNTs/Ecoflex composite film with microstructures,revealing the working mechanism of the flexible strain sensor: under small strain,the interlaced MWCNTs in the freestanding state extending from the MWCNTs/Ecoflex composite film surface are disconnected from each other,it is attribute to the increase resistance of the strain sensor.Under large strain,the entangled MWCNTs embedded in the elastomer are gradually separated to provide the increase resistance of the strain sensor.In addition,compared with flat structure,the stretching strain is primarily concentrated in the valley regions,which leads to the MWCNTs located in the valley regions be disconnected preferentially compared to other areas and further improves the sensitivity of the strain sensor.(2)The electrical performances of MWCNTs-MWCNTs/Ecoflex flexible strain sensor with microstructures are researched to verify the correctness of the working mechanism of the flexible strain sensor.After exploring the influences of different MWCNTs deposition density and Ecoflex substrate curing degree,MWCNTs-MWCNTs/Ecoflex flexible strain sensors with microstructures shows excellent comprehensive electrical properties.The strain sensor has high sensitivity(the gauge factor(GF)is 13.01 at 0-50% strain and 29.64 at 50%-200%strain).It exhibits the strain sensor obey the Ohm's law well under different strain and maintain approximately the same value at different frequencies(0.012-0.104HZ).Its response time is about 200 ms.After 9000 loading-unloading cycles test,it can still maintain good stability.The strain sensor twisting itself up to 180° maintains a linear resistance response.(3)The MWCNTs-MWCNTs/Ecoflex flexible strain sensor with microstructures is attached to the surface of human skin to research applications as wearable devices for human motion detection and health monitoring.The strain sensor can effectively detect the motions of human joints such as finger bending,wrist bending and elbow bending.It can also detect small strains such as human muscles movement,vocalization,touch vibration and wrist radial artery pulse.It demonstrates the flexible strain sensor fabricated by this method has potential applications in human motion detection and mobile healthcare,human/machine interaction,electronic skin,intelligent robots,and so on.In summary,this thesis proposes a flexible strain sensor based on double layer film with microstructures fabricated by depositing MWCNTs on the surface of semi-cured Ecoflex substrate.The fabrication process of proposed strain sensor is simple and low-cost.The sensor shows excellent performance in sensitivity,stretchability,linearity,stability,hysteresis and response speed.It also realizes the function of body motion detection,vocalization,touch and pulse monitoring tests when attached to the surface of human skin,and demonstrates attractive prospects as wearable devices.