Fabrication And Application Performance Of Flexible Sensor Based On Silver Nanowires

Flexible sensors are widely used in human physiological index detection,health monitoring,clinical diagnosis,virtual electronics,flexible touch screen,flexible electronic skin,and even industrial robots because of their good flexibility,extensibility,low detection limit and stability.However,it was found that the flexible sensors had the problems of low sensitivity,narrow detection range and single response in the process of practical application,which seriously limited its application range.In view of the problems of low sensitivity,narrow detection range and single response of flexible sensors,silver nanowires（Ag NWs）with high aspect ratio were prepared by solvothermal process.The silver nanowire/polyurethane（Ag NWs/TPU）active material was fabricated by compounding the as-prepared Ag NWs with high aspect ratio and TPU with good elasticity and extensibility,and the Ag NWs/TPU flexible strain sensor was fabricated.The flexible strain sensor had high sensitivity and detection range simultaneously.In order to further improve the sensitivity of Ag NWs-based flexible sensor and more effectively identify the movement of different parts of the human body,silver nanowires/graphene/polyurethane（Ag NWs/r GO/TPU）composite conductive materials were prepared by graphene oxide（GO）being introduced on the basis of high aspect ratio Ag NWs and compounding the silver nanowires/graphene conductive materials with thermoplastic polyurethane nanofibers（TPUNFs）film,the sensitivity of the Ag NWs-based flexible sensor was improved,and the application range of flexible strain sensor was expanded.In order to solve the problem of single response of flexible strain sensor,the Ag NWs/CB-OH composite conductive hydrogel multifunctional flexible sensor was fabricated by using Ag NWs/CB-OH as conductive materials,and compounding with three dimensional network structure hydrogel to imitate the structure of neurons,which could provide high sensitivity,strain/pressure responsiveness and wider detection range for Ag NWs-based flexible sensors.Thus,the versatility and wearability of the flexible sensors were realized.The specific research contents and results were as follows:（1）Preparation and conductivity of silver nanowires with high aspect ratioIn view of the low aspect ratio of Ag NWs,in this part,solvothermal method was used to prepare the Ag NWs with high aspect ratio,the growth mechanism of Ag NWs with high aspect ratio was explored,and the conductivity of Ag NWs with high aspect ratio was study.The results showed that the successful fabrication of Ag NWs with high aspect ratio may be benefit from the combined results of PVP,ethylene glycol and copper bromide.The macromolecular chain of PVP and Cu²⁺were beneficial to improving the length of Ag NWs（the average length of the obtained Ag NWs was about 158.3μm）.The weak reducibility of ethylene glycol,the combination of Br^-and Ag⁺,and the directional adsorption of PVP and Br^-were beneficial to reducing the diameter of Ag NWs（the average diameter of the obtained Ag NWs was about 77.6 nm）.Thus,the aspect ratio of the obtained Ag NWs could reach 2000.Then the as-prepared Ag NWs with high aspect ratio were used to fabricate Ag NWs conductive film,the conductive film was transparent,and it had excellent conductivity.（2）Fabrication and application performance of Ag NWs/TPU flexible strain sensorIn order to fabricate flexible sensor with high sensitivity and wide detection range simultaneously,Ag NWs/TPU composite conductive films were prepared with the obtained Ag NWs with high aspect ratio and TPU solution with good elasticity and extensibility by mixing,dispersing and casting film.On this basis,the Ag NWs/TPU flexible strain sensor with high sensitivity and wide detection range was fabricated by3M VHB adhesive packaging.The results showed that the high aspect ratio silver nanowires formed a vertical and horizontal conductive network structure in the polyurethane substrate,which enabled the flexible sensor to have a low initial resistance,which was beneficial to improving the sensitivity of the flexible sensor.The maximum GF of Ag NWs/TPU flexible strain sensor could reach 329.43.The good elasticity and extensibility of TPU could provide wide detection range for the Ag NWs/TPU flexible strain sensor（the detection range of the flexible sensor could reach 250%）.The amide groups of PVP on the surface of Ag NWs could form hydrogen bonding with the carbamate groups of TPU,which could improve the interfacial force and the structural stability between Ag NWs and TPU substrate in composite conductive film.Thus,the Ag NWs/TPU flexible strain sensor had good cyclic stability.It could be used to detect human movement such as finger,wrist,elbow and knee bending,as well as the vibration of expression and small sound,however,it had little difference in the relative resistance change patterns for motion detection of different parts of the human body.（3）Preparation and the application performance of graphene-dopped enhanced silver nanowire based flexible strain sensorIn order to further improve the sensitivity of Ag NWs-based flexible sensor and more effectively identify the movement of different parts of the human body,the as-prepared Ag NWs with high aspect ratio were mixed and dispersed with GO,and GO was reduced to prepare Ag NWs/r GO dispersion in this part.The Ag NWs/r GO active materials were drop-coated on the surface of TPUNFs to fabricate Ag NWs/r GO/TPUNFs composite conductive film.On this basis,the Ag NWs/r GO/TPUNFs flexible strain sensor was fabricated by 3M VHB adhesive packaging.The results showed that through the combination of one-dimensional Ag NWs and two-dimensional GO conductive components,the conductivity of Ag NWs was improved significantly,the conductive path of conductive materials was expanded,and the initial resistance of Ag NWs-based flexible strain sensor was reduced.The design of Ag NWs and GO composite conductive film increased the sensitivity of Ag NWs-based flexible strain sensor,the maximum GF of Ag NWs/r GO/TPUNFs flexible strain sensor could reach 2513.23,it was obviously better than Ag NWs/TPU flexible strain sensor.The high extensibility of TPUNFs film could endow Ag NWs/r GO/TPUNFs flexible strain sensor wide detection range（about187%）.The uneven interface structure and its physical force between Ag NWs/r GO conductive film and TPUNFs film（including mechanical bite force and physical force）could endow the Ag NWs/r GO/TPUNFs flexible strain sensor good cyclic stability.The flexible strain sensor could be used to detect human motion such as finger,wrist and knee bending,as well as small expression.Compared with Ag NWs/TPU flexible strain sensor,the relative resistance change patterns of Ag NWs/r GO/TPUNFs flexible strain sensor to different parts of human body showed great differences,it showed its great application potential in motion recognition of different parts of the human body.（4）Preparation and application performance of neuron-inspired Ag NWs-based multifunctional flexible strain/pressure sensorIn order to solve the problem of single strain response of flexible sensor,inspired by neuron structure,the soma was imitated by CB-OH conductive materials,the neuritis was imitated by Ag NWs,and they were compounded with three dimensional network structure hydrogel to fabricate Ag NWs/CB-OH composite conductive hydrogel multifunctional flexible sensor with strain/pressure response and wide detection range simultaneously.The results showed that the Ag NWs/CB-OH conductive active materials of neuron-inspired structure could endow the flexible sensor strain/pressure response and high sensitivity,and the maximum GF of the flexible sensor could reach 68.64.The sensitivity was 0.229 k Pa^-1under the pressure of less than 0.375 MPa,the sensitivity was 0.0312 k Pa^-1under the pressure of0.375-8.053 MPa range,and the sensitivity was 0.0023 k Pa^-1at the pressure higher than 8.053 MPa.The high extensibility of hydrogels and the imitation neuron structure of Ag NWs/CB-OH conductive active materials could endow the flexible sensor wide strain detection range（the strain detection range of the flexible sensor could reach 355%）.The carbonyl group,amino group and hydroxyl group of hydrogel could form hydrogen bonding with the polar groups of Ag NWs/CB-OH,which could improve the interfacial force and the stability of interfacial structural between the active materials and hydrogel in composite conductive hydrogel and endow the flexible sensor good cyclic stability.In addition,Ag NWs/CB-OH composite conductive hydrogel multifunctional flexible sensor also had good flexibility,fast response/recovery,stability and reproducibility.The Ag NWs/CB-OH composite conductive hydrogel multifunctional flexible sensor could detect human activities such as finger bending,wrist bending,elbow bending,knee bending,opening mouth,fistting,speaking,walking and jumping,which indicating that the composite conductive hydrogel multifunctional flexible sensor would be an ideal material for real-time monitoring of human motion.In summary,in view of the problems of low sensitivity,narrow detection range and single strain response of flexible sensors,Ag NWs with high aspect ratio were prepared by solvothermal method,the growth mechanism of Ag NWs with high aspect ratio was explored.The Ag NWs/TPU composite conductive active material and its flexible strain sensor were prepared by compounding the as-prepared Ag NWs with high aspect ratio and TPU with good elasticity and extensibility,the Ag NWs-based flexible strain sensor had good sensitivity and wide detection range,and it could be used in the detection of human physiological.In order to improve the sensitivity of Ag NWs-based flexible strain sensor and more effectively identify the movement of different parts of the human body,on the basis of high aspect ratio Ag NWs,GO was introduced to prepare the Ag NWs/GO disperse liquid and compounded with TPUNFs film to prepare Ag NWs/r GO/TPUNFs composite conductive material by dip coating method,the sensitivity of Ag NWs-based flexible strain sensor was improve and the application scene of flexible strain sensor was expanded,the flexible strain sensor could be used in motion recognition of different parts of the human body.In order to solve the problem of single strain response of flexible sensor,the Ag NWs/CB-OH composite conductive hydrogel multifunctional flexible strain/pressure sensor was fabricated with high aspect ratio Ag NWs and CB-OH as conductive materials and hydrogel of three-dimensional network structure by imitating the structure of neuron,the multifunctional flexible sensor could be applied for real-time monitoring of human motion.