Study On The Construction And Performance Of Flexible Strain Sensor Based On Fiber Assemblies Structure

With the development of wearable electronic devices,flexible strain sensors that fully interact with the human body have attracted more attentions.Textiles commonly used in daily life which exhibited flexibility,double curvature effect,light weight,breathability,durability and other advantages with their unique fiber assemblies structure(yarns,fabrics),are an ideal structural platforms for strain sensor.However because of the intrinsic insulation,viscoelasticity of the fibers and the complexity of the structure of the fiber assemblies,.it is necessary to study electrical conductivity and sensing mechanism of fiber assemblies,the relationship between fiber assemblies structure and sensing performances.Herein starting from the realization of the electrical conductivity of the cotton fibers and using the technical for large-scale production of textiles,the fiber assemblies structure at all levels from yarns to fabrics were fully constructed.The conductive of the fiber assemblies,the relationship between the structure of the fiber assemblies and the sensing performances,and the sensing performances under static and dynamic conditions were systematically studied.The contents included:1.The fabrication of cellulose/graphene composite and study on its electrical conductive and interface properties:Firstly the polyvinylpyrrolidone(PVP)was used as the graphene dispersant,by using UV-vis spectrum,Raman spectrum and TEM analysis it was found that when the PVP content was 0.2wt%,the ultrasonic time was 4-8 hours,a stable graphene solution with concentration of 0.76mg/m L can be formed.Based on this,cellulose/graphene composites were prepared by simple solution casting and hot pressing processes.Then using SEM,XRD FITR analysis it was found that due to cross-linking effect of PVP,the strong interactions between graphene and cellulose had been formed.Finally,the percolation theory was proposed to study the conductive mechanism of the composite materials and it was found that the surface conductivity reached 0.40S/cm when the amount of graphene content reached the percolation threshold(2.1wt%).2.Fabrication of conductive yarns and its conductive mechanism and electromechanical properties:Firstly the sizing process was proposed to prepare conductive cotton yarns with the similar process of dipping and hot-pressing and feasible for large-scale production.The sizing process was optimized by orthogonal design to produce the conductive cotton yarns with conductivity of 0.45S/cm(14s).Then probability theory was proposed to study the conductive mechanism of cotton yarns,it was found that its conductivity was not only related to the conductivity of the fiber itself,but also had a close relationship with the connection probability of the fibers in the yarn.Finaly the electromechanical properties of cotton yarns was studied by tensile experiments,it was found that due to the discontinuous distribution of fibers in the yarns and the viscoelastic characteristics of the fibers itself,the conductive yarns exhibited small strain range(<4.1%),long response time(300 ms)and poor repeatability(>17.8%),which indicated that the staple yarns structure were not suitable for strain sensing and must be further processed into fabrics structure.3.The conductive/sensing mechanism and sensing performance of fabrics structure:Firstly the cotton yarns were processed into woven fabrics(plain and twill webbing)and knitted fabrics(1?1rib),respectively.By using Peirce's classic geometric structure model it was found that the main factor affecting the equivalent resistance of fabrics was the contact resistance at the point of the yarns interwoven,the contact resistance and yarn interwoven angle conformed to the fitting equation_(8)=2.780⁽(82).The change of resistance caused by the change of interwoven angle during stretching was the mechanism of strain sensing of fabrics structure.For the static sensing performance of the fabrics sensor,the woven fabrics structure exihited the small strain and high sensitivity(GF>10),while knitted sructure exhibited large strain and low sensitivity(GF<1).For the dynamic sensing performances on three dynamic strain inputs:transition,slope and sine,the frequency amplitude,phase frequency,and damping coefficient of knitted fabrics structure in low frequency mode were better than woven structure,while woven structure exhibited shorter response time(184 ms)under high frequency conditions.The results demonstrated that woven fabrics structure was suitable for sensing high-frequency strain signals with small strain and high sensitivity,while knitted structure was suitable for sensing low-frequency strain signals with large strain and low sensitivity.4.The sensing perfoirmance on practical application:The strain sensor of woven fabrics structure was constructed to monitor breathing and knitted structure was to recognize body movements,the results were basically in line with the theoretical analysis conclusion.In summary,this work systematically discussed the construction methods of strain sensors based on fiber assemblies from the three structural levels of fibers,yarns,and fabrics,and deeply studied of their conductive mechanism and sensing performance.Not only did it take into account the feasibility of large-scale production,but also established the relationship between the structure of the fiber assemblies and their sensing performances theoretically.Therefore,the conclusions of this work can provide new ideas for the application of strain sensors based on fiber assemblies structure and the development of wearable"electronic textiles"in the future.