Research On Preparation And Performance Regulation Of Multifunctional Flexible Strain Sensors

In recent years,flexible and wearable sensors have demonstrated important applications in various fields,such as health monitoring,human-machine interaction,E-skin,and so on.They have attracted extensive attention from many researchers at home and abroad.Flexible and wearable sensors based on different sensitive materials and sensing mechanisms have been proposed in the past years and important research progress has been achieved.Compared with the traditional strain sensors based on rigid materials,flexible strain sensors have the advantages of simple structure,high sensitivity,wide detection range,and good adhesion to human skin.They can be used to monitor weak signals such as pulse,respiration,speech,and the human motion signals such as joint bending,and so on.However,it is still difficult for most flexible strain sensors to simultaneously possess the characteristics of low-cost fabrication,high sensitivity,multifunctional integration,ultrastretchability,and good biocompatibility,which will limit their practical applications.How to solve these challenging problems has very important practical significance for the development of flexible strain sensors.Therefore,the laser-induced graphene-based flexible strain sensors are investigated in this thesis to realize the low-cost fabrication,high sensitivity,and multifunctional integration applications.Based on this research,to further achieve ultrastretchability and good biocompatibility,flexible strain sensors based on the conductive hydrogels with a hybrid double network structure and dual physically cross-linked double network structure are also studied in this thesis.These researches will provide an important research foundation and explorable directions for the realization of flexible strain sensors with low cost,high sensitivity,ultrastretchability,good biocompatibility,and multi-functional integration.The main researches are summarized as follows:(1)Based on the literature research on the status at home and abroad,the important significance is pointed out for the selection and design of flexible substrates and sensitive materials to the performance regulation of flexible strain sensors.The preliminary research conception by combining conductive networks of nanomaterials and hydrogels is proposed to construct the flexible strain sensor with high sensitivity,super-extension,and good biocompatibility.Based on this,the overall research framework is proposed for this thesis.(2)Aiming at the problem of the high cost for the fabrication of flexible strain sensors with high sensitivity,the laser-induced graphene patterning method is proposed to improve the sensitivity of the strain sensor based on the advantage of laser direct writing technology.Firstly,since the polyimide film has no stretchable performance,the method for transferring the laser-induced graphene from polyimide to PDMS substrate is studied to improve the stretchable property of the sensor.Secondly,the effect of the patterning structure on the strain sensing performance based on laser-induced graphene is studied,and the sensing mechanism is analyzed.Finally,to verify the high sensitivity of the strain sensor,it is used to monitor the signals of human pulse,respiration,joint bending,and so on.(3)Aiming at the multi-functional integrated application of flexible strain sensors,a bilayer asymmetrical pattern of laser-induced graphene is proposed to realize the integrated application of strain sensing and early warning.First,the effects of laser power and writing direction on the electrothermal properties of laser-induced graphene are studied.Then,the effect of different laser repeated writing on the electrothermal properties is studied and the repeated writing method is proposed to optimize the preparation of laser-induced graphene and its electrothermal properties.Finally,combined with the mechanical and thermal properties of laser-induced graphene,a bilayer asymmetric integration structure of laser-induced graphene is proposed to realize the multifunctional integrated application of sensing and early alarm.(4)Aiming at the problem of the limited tensile properties of most of the current flexible strain sensors and the mechanical mismatch with human skin,a highly transparent ultrastretchable strain sensor based on the conductive hydrogel with a hybrid double network structure is prepared.Firstly,based on the double network structure,the conductive hydrogel is constructed based on ionically crosslinked carrageenan and covalently cross-linked polyacrylic acid.The influence of the ionically crosslinked network on the mechanical and electrical properties is analyzed.Secondly,to further improve the conductivity property of the conductive hydrogel,it is proposed to introduce the ionic liquid into the network structure.The post-drying treatment is proposed to further improve the mechanical properties and anti-freezing properties.Therefore,a high-transparency conductive hydrogel is prepared with excellent fracture strain and low Young’s modulus,as well as good electrical conductivity and anti-freezing properties.Finally,as a strain sensor,the conductive hydrogel is used to detect various human motion signals.(5)Aiming at the problem that the current conductive hydrogels are difficult to meet the sensing performance,mechanical performance,and environmental stability at the same time,based on a dual physically cross-linked double network,a nanocomposite organohydrogel is prepared.First,the introduction of carbon nanotubes into the network structure is studied to construct nanocomposite hydrogels to improve the mechanical properties and electrical properties.Secondly,using the solvent displacement method,the introduction of glycerol solvent into the hydrogel structure is studied to construct nanocomposite organohydrogels to improve environmental stability.Besides,Na Cl is introduced to the solvent to solve the problem of reduced electrical conductivity.Finally,the strain sensing performance of the nanocomposite organohydrogel is studied and its mechanism is analyzed.(6)Based on the above research,the main research results are further summarized to provide some important ideas for the follow-up research on the preparation of multifunctional flexible strain sensors.At the same time,by combining the conductive porous graphene network and hydrogel,future research prospect is proposed to construct multifunctional flexible strain sensors with high sensitivity,ultra-extensibility,and good biocompatibility.