Study On The Relationship Between Structures And Performances Of Fabric-based Piezoresistive Sensing Materials

Fabric materials are considered as an ideal substrate for the preparation of wearable flexible sensing materials due to their excellent three-dimensional flexibility,comfortability,and air with moisture permeability.Wearable piezoresistive sensors based on fabric materials can noninvasively obtain weak pressure signals generated by the environment or human body,showing a great potential for the application in medical rehabilitation,sports fitness,artificial intelligence and so on,and gradually becoming the current research hotspot.The fundamental mechanism of fabric-based piezoresistive sensors is that the strain causes reversible changes in the conductive percolation network formed by the fiber itself or between them under cyclic pressure,leading to changes in the resistance of fabric materials.The pressure sensing is realized by establishing the relationship between resistances and external pressure.Therefore,the electrical properties and compression deformation behavior of fabric materials greatly affect the sensing performances of fabric-based pressure sensors such as the sensitivity and working pressure range.Although some literatures about fabric-based pressure sensors have been reported,the optimization of sensing performances based on the structure control of fabric materials remains to be investigated deeply and systematically.This work attempts to systematically study the influences on fabric-based piezoresistive sensing materials and device performance from three aspects:regulating electrical parameters,designing fiber materials as well as fabric structures,and constructing fabric materials with a multi-level structure.Firstly,a polypyrrole conductive fabric was prepared by in-situ polymerization with a flexible cotton fabric as the substrate,and the dependance of surface resistance about conductive cotton fabric on the pyrrole mass percentage was explored.Combined with the experimental results about the surface morphologies,polypyrrole loading rate and surface resistance of the fabric,it was found that the surface resistance reached the smallest（14.7 kΩ）and the resistance uniformity was great（the standard deviation was about 1.28）when the mass percentage was 100%（pyrrole:cotton fabric=1:1）.Through the characterization of stress-strain curves,compressive force-resistance model,in-situ compression electron microscope and force distribution simulation,the law of pressure change and the form of internal fiber movements is clarified under pressure.Combined with the equivalent resistance diagram and relative current change diagram,the relationship between the stress and resistance is obtained.Eventually,the cotton fabric-based pressure sensor obtained a high sensitivity up to 0.131 k Pa^-1in a working range of 0-10 k Pa.Besides,it had a good cycle stability,a fast response time（the trigger time and recovery time are 10 ms and 35 ms,respectively）and an excellent pressure recognition ability.In order to further study the influence of fabric structures and fiber materials on the compressive properties and sensing performance of pressure sensors,the fabrics with different structures and raw materials were obtained via a facile weaving technique and the corresponding conductive fabrics were prepared by in-situ polymerization.Firstly,the surface structure of used fabrics and the polypyrrole distribution after conductive treatment were observed by scanning electron microscope and optical microscope.Combined with the characterization of loading rate and surface resistances,the results showed that the surface resistance of aramid conductive fabric with a plain structure was the lowest（4.7Ω·cm）.According to the Van Wyk compression theory and the compression properties about 9 kinds of samples,it is shown that the deformation ability of aramid conductive fabric with a plain structure exhibits the worst and the compressive strain is only 15.3%at 15 k Pa.Finally,the assembled pressure sensor based on the aramid fabric with a plain structure showed a high sensitivity of 5.094 k Pa^-1at a pressure of 25 k Pa and the working range could reach up to 75k Pa.At the same time,it also had a good cycle stability,a fast response time（the trigger time and recovery time are 100 ms and 250 ms,respectively）and a great ability for pressure recognition.Finally,considering the contents of the above two chapters,a multi-level fabric-based pressure sensor was designed by spraying PVA-co-PE nanofibers on the surface of cotton fabrics and then conductive treatment.The effects of nanofiber spraying methods and times on the compressive properties and sensing performances of composite conductive materials were farther studied.Firstly,the PVA-co-PE nanofibers suspension were prepared via a high-speed shearing technique.Then,the two sides of cotton fabrics were sprayed with nanofibers suspension at different times.After drying,the required fabric-based pressure sensor with a multi-level structure was prepared via in-situ pyrrole polymerization.Subsequently,loading rate of polypyrrole,surface morphologies and surface resistances of composite conductive materials were characterized.The results showed that surface resistances of the side sprayed with PVA-co-PE nanofibers suspension were lower（0.85 kΩ）compared with the side without nanofibers suspension.Finally,a multi-level structured fabric-based pressure sensor with a high sensitivity(26.107 k Pa^-1)in the working range of0-10 k Pa was prepared by this method.The maximum detection pressure can reach 180 k Pa and it also has a good cycle stability,a fast response time（the trigger time and recovery time are 5 ms and 20 ms respectively）and an excellent pressure recognition ability.