Design,Preparation And Performance Studies Of Functional Conductive Double Network Hydrogel Based Flexible Sensor

Health is the most precious wealth of mankind,and it is also the eternal theme of humans.The report of the 19th National Congress of the Communist Party of China put forward the great strategy of"Healthy China 2030",aiming at providing all-round health services for the people.Flexible sensor present great application potential in the field of human health monitoring due to its excellent performance and high adaptation.Hydrogel,as a typical soft-wet material,is an ideal candidate material for preparing flexible sensors because of its high elongation,low modulus and unique ion conduction channel.However,there are still some challenges to using it as a flexible sensor,such as weak mechanical property,poor self-recovery performance,low temperature freezing and high temperature easy to lose water resulting in loss of toughness,and dependence on solid-state power supply,etc.This thesis aimed to prepare conductive double network hydrogel based flexible sensor with high adaptability to the environment by molecular and structure design,and explore its application in human motion monitoring.(1)Design and performances of DNA/PHEAA hydrogel with high mechanical and fast recovery performanceSalmon sperm DNA,N-hydroxyethyl acrylamide(HEAA),initiator and sodium bromide were mixed in bottle and stirred to form a clear solution at 95°C.Then,cooled to room temperature,the DNA was induced by sodium bromide to self-assemble to form the DNA rigid first network.After that,the hydrogel was transferred to ultraviolet lamp,and the PHEAA flexible second network was formed by photopolymerization.Finally,the fully physically crosslinked DNA/PHEAA double network hydrogel was synthesized.The obatained hydrogel showed excellent mechanical properties(tensile stress/fracture strain:0.89 MPa/2140.79%),rapid self-recovery performance(resting time at room temperature for 30s,peak stress/energy dissipation recovery efficiency:99.72%/95.38%)and good biocompatibility(survival rate of MCF-7 cells>98%).The flexible sensor based on the hydrogel could stably monitor human movements such as limb bending,swallowing and blinking.Meanwhile,the monitoring signals could be synchronized in real time through the Bluetooth of the mobile phone.In this work,by combining full physical crosslinking strategy and double network toughening method,the hydrogel has high mechanical properties and fast self-recovery performance.Thus the flexible sensor based on the double network hydrogel shows high adaptability to complex load environments.The results fully demonstrated the unique advantages of multi-hydrogen bonding molecules(DNA,HEAA)and fully physically crosslinked double network strategy in improving the mechanical properties and self-recovery performance of hydrogel flexible sensors.(2)Preparation and performances of Gelatin/PHEAA/Gly/Li Cl hydrogel with anti-freezing and anti-drying propertiesBased on previous studies,we replaced the expensive DNA with cheap and easily available gelatin to construct the rigid first network,and introduced water/glycerin(Gly)binary solvent and lithium chloride to prepare Gelatin/PHEAA/Gly/Li Cl hydrogel with anti-freeze and moisturizing properties by one-pot heating-cooling-ultraviolet polymerization method.The obtained hydrogel had excellent mechanical properties(tensile stress/strain:2.14 MPa/1637.49%),rapid self-recovery properties(resting time at room temperature for 10 min,recovery rate of peak stress/energy dissipation:91.17%/79.21%),anti-freezing and anti-drying properties(the hydrogel still maintained flexibility at-40°C,and the quality of hydrogel lost only 5%after 135 h in dry environment).The hydrogel was used to prepare a flexible strain sensor,which could monitor human movements and physiological behaviors at-20°C.This work successfully realized the compatibility of high mechanical strength,rapid self-recovery properties,anti-freezing and anti-drying capabilities of hydrogel,which enabled the hydrogel-based flexible strain sensor applicable in low temperature/dry harsh environment,and effectively broadened the application range of hydrogel-based flexible strain sensor.(3)Construction and performances of self-powered flexible sensing system based on PNH hydrogel with high ionic conductivity and high toughnessThe above reserches effectively solved the problems of weak mechanical properties,poor self-recovery and lack of anti-freezing and anti-drying abilities of hydrogel-based flexible sensors.In this chapter,we hoped to build a self-powered flexible sensing system to relieve the dependence of hydrogel flexible sensor on solid state power supply.According to the fully physically crosslinked double network strategy and salting-out effect,we successfully prepared polyvinyl alcohol/sodium chloride/poly(N-hydroxyethyl acrylamide)(PNH)hydrogel with high ionic conductivity(1.13 S/m)and high toughness(14.66 MJ/m~3)by using one-pot freeze-thaw-ultraviolet polymerization method.A flexible capacitor was assembled by using the PNH hydrogel as a solid electrolyte and carboxylic acid functionalized multi-walled carbon nanotubes/indium tin oxide/polyethylene terephthalate film as a flexible electrode.The area capacitance of the capacitor could reach 2.9 m F/cm~2,and the retention rate of the capacitor was more than 90%after 2000 charge-discharge cycles,and the capacitance did not change obviously after arbitrary bending.Furthermore,the flexible sensor based on PNH hydrogel and flexible capacitor were assembled into a self-powered flexible sensing system.The system could monitor the bending and frowning of human limbs.In this work,a flexible capacitor with similar modulus to the hydrogel flexible sensor was constructed,which realized the centralized flexibility of sensor element and energy supply element.This behavior helpes the hydrogel flexible sensor to get rid of its dependence on the solid-state power supply,and provides a new idea for the integration and application of the hydrogel flexible sensor.