PEDOT:PSS-based Conducting Polymer Hydrogel As Strain Sensor Materials And Their Application In Wearable Electronics

As the largest organ of the human body,the skin can directly contact the external environment,feel various external stimuli and respond to stress.It has the advantages of flexibility,lightness,softness,stretchability,high sensitivity and high environmental adaptability.Due to its special anisotropic microstructure,the skin has outstanding mechanical properties（the modulus of human skin is about 10-1000 k Pa,the water content is above 70%,and the stretchability is 140-180%）.However,it is still a huge challenge to make wearable electronic devices or robots realize the sensing function like human skin and provide real-time feedback.As a key component in wearable electronic devices,strain sensors have been widely used in sports monitoring and health monitoring,but traditional strain sensors are usually composed of metals or semiconductors,and the stretchability is generally less than 5%,which leads to their mechanical properties.It does mismatch the human body,has obvious interface problems,and has disadvantages such as stiffness,bulkiness,poor touch,poor sensory function,and poor environmental adaptability.Therefore,it is urgent to develop sensing materials based on soft materials similar to human body or biological tissue.In order to achieve long-term stable interface monitoring of the human body,the strain sensing material needs to meet the following basic requirements:firstly,the softness and hardness of the material can match the human tissue,and it can achieve seamless and stable attachment when applied on the surface of the human body;second,it has both excellent mechanical properties and strain sensitivity;third,it has good biocompatibility and can be compatible with human tissues.Conducting polymer hydrogel combines the electrical conductivity of conducting polymers with the mechanical properties of softness and stretchability of hydrogels.It has high electrical conductivity and can achieve ion-electronic dual conduction,excellent biocompatibility,and long-term stability.It is expected to greatly improve the performance matching between sensing materials and human tissues,so as to achieve better integration of human-machine interface.In order to develop high-performance PEDOT-based strain sensing materials,poly（3,4-ethylenedioxythiophene）:polystyrene sulfonate（PEDOT:PSS）hydrogel was used as the conductive phase in this paper,and it was combined with the mechanical support network of highly flexible viscoelastic polymers,a PEDOT:PSS-PVA hydrogel sensing material with high-efficiency self-healing properties and PEDOT:PSS/PAM-SA with ideal self-adhesive properties were designed and prepared respectively.Hydrogel interface sensing material,fatigue-resistant PEDOT:PSS-PVA hydrogel strain sensing material with highly ordered anisotropic microstructure.Systematically explored their morphology,structure,mechanics,sensing and other properties,as well as the influence of hydrogel component concentration,cross-linking method,microstructure,configuration and structure on the sensing performance of strain sensors,revealing the material structure-performance correlation rules;materials with excellent performance ratios were screened out.Then it is assembled into a wearable electronic skin for human motion detection and monitoring of subtle physiological signals,and a wireless transmitter is constructed by combining the sensing function of the strain sensor with the traditional Morse Code coding rules;further in order to demonstrate more complex sensing functions,five hydrogel strain sensors were assembled into a multi-channel wearable electronic skin for precise gesture recognition;finally,the wearable electronic skin was integrated with a robot to realize practical applications such as motion monitoring of robotic fish.The specific research content and results are as follows:1、A self-healing S/PEDOT:PSS-PVA hydrogel is designed by chemical cross-linking,and a wearable strain sensor that can be used repeatedly for a long time was obtained,realizing the long-term stable use of the sensor.Using PEDOT:PSS as the conductive network,polyvinyl alcohol（PVA）as the mechanical support network,through the covalent cross-linking of hydroxyl groups between borax and polyhydroxy polymer PVA,due to this special reversible covalent bond,the hydrogel bonds between them can be continuously broken-repaired,resulting in a hydrogel material with self-healing properties.The system characterized and tested its mechanical properties,self-healing properties,strain-sensing properties,and cycle stability,and explored the relationship between material structure and properties,and then used it as a sensing layer to assemble strain-sensing devices and use them for human motion monitoring.The results show that PEDOT:PSS-PVA hydrogel has excellent self-healing properties,and the healing efficiency（HE）can reach 83.5%,which greatly prolongs the service life of strain sensing devices.Meanwhile,the PEDOT:PSS-PVA hydrogel exhibits excellent mechanical properties with high stretchability（300%）and low modulus（24.69 k Pa）.As a strain sensing material,it has high sensitivity（GF=3.93）and linearity（R²=0.98）.Using it as a wearable electronic device can be used to monitor human body movement,not only to monitor tiny physiological movements,but also to monitor the movement of human joints.2.Design PEDOT:PSS/PAM-SA hydrogel with interpenetrating double network structure through chemical cross-linking,obtain a high-performance self-adhesive interface sensing material,and realize the strain sensing performance of conducting polymer hydrogel improvement.PEDOT:PSS as the conductive phase and PAM/SA as the mechanical phase were chemically cross-linked to obtain PEDOT:PSS/PAM-SA hydrogel with an interpenetrating network.The system characterized and tested its surface morphology,rheological properties,adhesion properties,mechanical properties,strain sensing properties and cycle stability,and explored the relationship between the structure and properties of the material.Subsequently,it was used as a sensing layer to assemble a wearable strain sensing device,and the application of human motion detection,wireless information transmission and gesture recognition was successfully realized.The results show that the hydrogel precursor solution possesses the general characteristics of pseudoplastic fluids in aqueous polymer solutions,namely shear thinning and shear yielding.PEDOT:PSS/PAM-SA hydrogel exhibits certain adhesiveness,good conformal attachment ability with human tissue,and also exhibits low modulus（18.3-9.3 k Pa）and high stretchability（680%）,so that the flexible strain sensor has an ultra-wide detection range（0-500%）.Meanwhile,the PEDOT:PSS/PAM-SA hydrogel has an ultrahigh sensitivity～11.35（better than most conducting polymer hydrogels,GF<10）,and an ultralow hysteresis～1.52%.In addition,the hydrogel exhibits excellent durability and strain cycle stability,exhibiting extraordinary mechanical and sensing stability after cyclic stretching for 1000 cycles at 100%strain.Finally,it was assembled into a wearable skin electronic patch.Thanks to its ultra-high sensitivity and ultra-wide detection range,it can be used to detect subtle physiological signals such as swallowing and frowning,as well as large-deformed human motions such as fingers and knees;through wireless resistance acquisition modules,and with traditional morse.The combination of password coding rules realizes real-time and accurate information transmission;the recognition of gestures is realized by integrating five strain sensors to form a multi-channel electronic skin.3.The PEDOT:PSS-PVA hydrogel with highly ordered anisotropic microstructure was prepared by directional freezing-assisted salting-out design,and the high fatigue threshold of the conducting polymer hydrogel was realized,and the possible wearable electronic skin integrated with underwater robot for long-term stable real-time monitoring of strain sensors.The freeze-redispersed PEDOT:PSS nanofibers were fully mixed with the viscoelastic polymer PVA,and an ordered micro-nano structure was formed inside the hydrogel material through the one-way ice-template freeze-casting technique,and then soaked in sodium citrate solution creates a unique highly ordered anisotropic conducting polymer hydrogel network.The system characterizes its microscopic morphology,mechanical properties,anti-fatigue properties,sensing properties,long-term stability,etc.,and studies its application performance as wearable electronic skin and robot skin.The results show that,thanks to its special ordered anisotropic microstructure design,PEDOT:PSS-PVA hydrogel has an ultra-high fatigue threshold(301 J m^-2),mechanical and sensing cycle stability（10000 cycles）.Based on the strain sensing performance of PEDOT:PSS-PVA hydrogel,it exhibits a wide detection range（0-300%）,excellent sensitivity（3.93）,outstanding linearity（0.95）and low hysteresis（4%）.By using PEDOT:PSS-PVA hydrogel as the sensing layer and VHB elastomer on both sides as the encapsulation layer,a wearable electronic skin was assembled,and human motion detection under small strain,medium strain and large strain was successfully realized.A multi-channel sensing array assembled using five sensors enables real-time recognition of gestures.In addition,by integrating the electronic skin with the underwater robot,real-time long-term robot motion monitoring and remote control are realized.In summary,this paper designed and prepared three kinds of PEDOT:PSS conducting polymer hydrogel strain sensing materials with different functions,and realized the preparation and application of hydrogel materials through phase engineering and structural engineering.Various strain sensing performance indicators have been improved,and the relationship between the internal structure of the material and the material performance has been revealed.At the same time,the exploration of various application scenarios such as conducting polymer hydrogel as wearable electronic devices and robot electronic skin has been realized,which provides material and technical reference for the next generation of conducting polymer hydrogel wearable electronic devices.