Design And Response Characteristics Of Force-Sensitive Sensor Based On AC Impedance Effect Of Conductive Hydrogel

Recently,flexible conductive polymer composites based force sensors have attracted much attention for its excellent processability,structural controllability,convenient signal processing,rich material selection and so on.Among them,conductive hydrogels with excellent elasticity,adhesion and self-healing have unique advantages in the field of flexible force sensors.However,the unique structure and intrinsic ionic conductivity of hydrogels have led to some limitations in the application of traditional DC resistance test and analysis methods.Meanwhile,the unstable response signal,low sensitivity and easy dehydration of flexible conductive hydrogel force sensor still needs further investigation.This study focuses on the unique composition and conductivity mechanism of conductive hydrogels,and the exploration of the AC impedance effect and force sensitive response under AC excitation,expecting to obtain stable electrical signal output.As a result,we proposed the strategies of introducing hydrogel particles,microstructured electronic conductive coatings and microstructured hydrogel surface to enhance the comprehensive response performance of sensors from the point of structure design.Besides,the relationship between microstructure and force sensing performance was also established,aiming to provide some theoretical guidance for the design and development of new-generation and high-performance hydrogel force sensors.The main research results are listed as follows:1 Polyacrylamide/Gelatin（PAM/Gelation,PAMG）double network ionic conductive hydrogel with high tensile strength（>2196%）and good transparency（>95%）was prepared by two step method.The electrical behavior of PAMG hydrogel during the DC resistance test indicates that the aggregation of conductive ions at the interface between the hydrogel and the electrode was the main reason for signal mutation and baseline drift in DC mode.The electrical behavior of the sensor under AC excitation was further studied,and the electrochemical impedance spectroscopy（EIS）under different strain shows that the AC impedance of the sensor possesses obvious strain response characteristics,and the ions transport impedance in the hydrogel and the interface electric double layer capacitance of the hydrogel/electrode interface are the key factors affecting the AC impedance response of the sensor.The prepared PAMG double network ionic conductive hydrogel exhibited high sensitivity,fast response/recovery capability（200 ms/200 ms）and stable AC impedance response in a wide strain range（5-600%strain）under 1000 Hz AC excitation,showing good detection ability for different human motion and biological signals,which means that the study of strain impedance response under AC excitation mode is an effective way to study the performance of conductive hydrogel force sensor.2 Inspired by the sheath-core structure of axons,a sheath-core hydrogel strain sensor was prepared innovatively by using the Cellulose Hydrogel Particles（CHPs）as the response medium.The results show that the microparticle strategy can introduce a new ion transport interface in the hydrogel matrix,which is beneficial to improve the responsivity of the ion transfer impedance through the reconstructed interfaces between hydrogel particles,improving the sensitivity of the sensor.In addition,increasing the particle size of hydrogel is conducive to boost the interfacial reconstruction effect,making the sensor exhibit higher strain sensitivity.Nevertheless,increasing the ion concentration can enhance the ability of ion to cross the interface,attenuating the interface reconstruction effect and leading to a low sensitivity.As a result,the prepared CHPs（20-80 mesh,0.05 M ion concentration）strain sensor shows a relative impedance change（ΔZ/Z₀）and sensitivity（GF）of 441.5 and 630.1 respectively.The response process of the sensor was further analyzed by electrochemical impedance analysis method,and the transmission impedance is dominant in the low strain region,while the contribution of double-electric-layer capacitance gradually increases in the high strain region,especially in the strain region greater than 350%,which plays a key role in improving the sensitivity of the sensor.Finally,stable and high-precision monitoring of human and physiological signals can be successfully achieved by the CHPs sensors using the AC excitation provided by triboelectric nanogenerator,providing a reference for developing the next-generation self-powered hydrogel force sensing system.3 PAMSA/CNT（PSC）composite conductive hydrogel with ionic conductive pathway and electronic conductive network was prepared by constructing Island-Bridge microstructured CNT electronic conductive coating on the surface of PAM/SA（PS）hydrogel.The results show that the highly sensitive microcrack structure derived from stressed Island-Bridge CNT electronic conductive coating and the stable Island-Bridge structure can significantly improve the sensitivity of the sensor in a wide strain range.In addition,reducing the concentration of conductive ions can also enhance the strain effect of transmission impedance,and the sensitivity of the sensor is further improved.As a result,ΔZ/Z₀ of the prepared PSC composite conductive hydrogel reached 305.35 at 600%strain,and the sensitivity also increased tenfold compared with the pure hydrogel,where a GF of 76.54 is achieved.Besides,the sensor also displays a fast response/recovery speed（110 ms/110 ms）.All these stated above enable the sensor to be appliable for real-time movement detection of different human body parts.Importantly,the sensor also possesses excellent cycle stability and temperature tolerance due to the treatment in high concentration of glycerol solution4 A PAM/SA（PS）ionic conductive hydrogel with microcolumn structured surface was prepared by the template method and used as the response medium to assemble a sandwich structure hydrogel pressure sensor.According to the analysis of the equivalent circuit and in situ structure change characterization,the obvious change of hydrogel/electrode interface electric double layer capacitance and the inductance capacitance between the electrodes caused by the deformation microstructure under stress can effectively enhance the capacitive response performance of the sensor.By investigating the relationship between the response performance and the spatial distribution,as well as the height of microstructure,it is found that the sensitivity of the sensor rises significantly with decreased dimensional ratio or increased microstructure height.As a result,the sensitivity of the microstructured PS ionic conductive hydrogel sensor（1.5 mm in microcolumn diameter,and 1.5 mm in height）was improved by 39.7 times under 100 k Pa stress compared with the planar one,and excellent stability and fatigue resistance（19.8 k Pa,5000 cycle）in wide stress range（0.05-1100 k Pa）were also achieved.Besides,the prepared sensor can be used as a reliable device to monitor various loads such as static force,impact force,joint movement and gait,and shows high environmental tolerance and anti-overload ability.