Preparation And Study Of Sensing Behaviors Of Flexible Bio-Based Multifunctional Sensing Materials

With the development of science and technology,mankind is stepping into the era of intelligence.Flexible electronic devices based on artificial intelligence technology have shown extensive application potentials in emerging fields such as intelligent wearable devices,electronic skins and human machine interfaces.Benefiting from good flexibility,ductility and excellent mechanical properties,flexible sensing materials can withstand a large degree of bending,twisting and stretching.Meanwhile,the flexible sensing materials can convert physiological signals and external stimuli such as mechanical deformations,humidity changes,temperature changes into different detectable electrical signals.These merits enable the flexible sensing materials to be utilized in many fields such as electronic skin,human motion monitoring,health monitoring and soft robots.Currently,flexible sensing materials endow with conductive and sensing property are mainly by introducing conductive fillers into a flexible polymer matrix and constructing conductive pathways within it.However,the widespread use of flexible sensing materials synthesized from petroleum will produce a large amount of non-degradable electronic waste.This inevitably causes environmental pollution and wastes of resources,which does not meet the current requirements of green and sustainable development of society,thus severely limiting its application in flexible electronic devices and other fields.Therefore,using bio-based polymer materials with wide sources,diverse chemical components,excellent biocompatibility and biodegradability is of great significance for the development of flexible sensing materials.In this thesis,bio-based materials are selected to prepare flexible sensing materials with multifunctional sensing ability,which can be used as strain sensors,temperature sensors,humidity sensors for human motion detection and health monitoring.The specific research contents and conclusions are discussed as follows:（1）Highly transparent and recyclable flexible sensing material for multifunctional response to strain,temperature and humidity:Highly transparent and recyclable flexible sensing material composed of poly（vinyl alcohol）（PVA）,citric acid（CA）and Ag nanoparticles（Ag NPs）with multiple sensing capabilities in detecting strain,temperature and humidity are prepared by a simple in-situ reduction and solvent casting technology.The as-prepared PVA/CA/Ag NPs sensing material displays not only excellent stretchability（elongation at break up to 596%）but also outstanding transparency（the transmittance>89%）.As flexible strain sensing material,it offers an excellent linearity of the output signals（correlation coefficient:0.998）,wide strain detection range（1-200%）,and fast response time（90 ms）.Moreover,the sensing material can also detect subtle temperature changes because of its high-precision thermo-sensation（0.1℃）and excellent temperature sensitivity（temperature coefficient of resistance is-0.076/℃ in the range of 30-40℃）.In addition,the constructed sensing material has excellent humidity sensing performance thanks to the good hygroscopicity of PVA and CA,which can provide real-time moisture information of human skin.More importantly,the designed sensing material possesses excellent recyclability.Through simple dissolution-recasting process,flexible multifunctional sensing material with excellent comprehensive performance can be obtained again,and can be repeated many times,effectively reducing the generation of electronic waste.（2）Preparation and strain sensing performances of ultrastretchable,antifreezing and anisotropic conductive hydrogel:Inspired by the anisotropic structure of muscle,the flexible material polyacrylamide（PAM）and the relatively rigid biological polymer sodium alginate（SA）are selected to prepare anisotropic PAM/SA-Ca²⁺hydrogel through pre-stretching and Ca²⁺cross-linking strategy.The generated hydrogel exhibits remarkable mechanical properties,including excellent toughness,extremely high stretchability,and comparable modulus to that of human tissues.In addition,due to the anisotropy in the ion transport channel,the hydrogel possesses anisotropic conductivity.In particular,the hydrogel-based sensing material which perpendicular to the orientation direction has excellent strain sensing capability,such as ultra-low strain detection limit（0.1%）,fast response time（123 ms）,wide strain working range（0.1%-1585%）,good durability,excellent stability and repeatability.Ca²⁺cross-linking strategy not only fixes the oriented structure of the hydrogel network,but also endows the hydrogel with superior anti-freezing property and prolongs its working temperature.These advantages enable anisotropic hydrogel to show great application potentials in wearable strain sensors to detect and differentiate different human movements such as facial expression,finger bending,wrist bending,voice recognition,as well as to realize information encryption and wireless transmission.（3）Preparation and study on strain/pressure sensing and triboelectric performances of antifreezing,self-healing and stretchable conductive hydrogel:The poly（acrylamide）/poly（vinyl alcohol）double network hydrogel obtained by thermal initiation polymerization and then immersed in lithium chloride/tannic acid（Li Cl/TA）mixture solution to prepare antifreezing,self-healing,stretchable conductive hydrogel for strain detection,pressure detection and triboelectric nanogenerator.The obtained hydrogel exhibits extraordinary mechanical and sensing performances,such as ultra-low strain detection limit（0.1%）,rapid response time（100 ms）,wide strain sensing range（0.1%-1097%）and pressure sensing range（0-90 k Pa）as well as excellent self-healing property（the self-healing efficiency calculated by strain recovery value is80.3%,and the self-healing efficiency calculated by stress recovery value is 82.9%）.The hydrogel also can be assembled into single electrode triboelectric nanogenerator,which shows excellent triboelectric output performance.Additionally,the strong bonding between Li⁺and H₂O molecules resulted in the generation of hydrated ions,reducing the freezing point of water.This endows the hydrogel with excellent anti-freezing performance and extends its working temperature.With these benefits,hydrogel possesses a wide range of applications in wearable strain and pressure sensors,enabling to detect and differentiate different human movements such as blinking,finger bending,wrist bending and pulse.