Research On Flexible Sensing Properties Of Silk Nanofibers Baced Multifunctional Hydrogels

In recent years,due to the advantages of wearability,stretchability,remote operation and real-time feedback,wearable flexible sensors have received more and more attention and been widely used in various fields,such as electronic skin,soft robots and human health detection.However,most of the current flexible sensors rely on stiff conductive materials（graphene,carbon nanotubes and metal nanowires,etc.）to achieve mechanical signal conversion.These stiff conductive materials lack stretchability and recoverability,which greatly limits the sensitivity and accuracy of flexible sensors.Because of its inherent conductivity,good flexibility and biocompatibility,conductive hydrogel-based flexible sensor could be regarded as an ideal choice for the flexible sensors in the future.However,in practical application,hydrogels couldn’t meet various complex conditions of use,which caused hydrogel-based sensors still facing huge challenges.Many current hydrogel materials had many shortcomings in practical application,such as poor mechanical property,greatly affected by environmental conditions,poor mechanical cyclic stability and single function,which caused their application in wearable devices to be greatly restricted.For example,the weak tensile strength of hydrogel would limit its practical application range.The environmental stability of hydrogel was poor and it was prone to fail under harsh environmental conditions,which caused hydrogel sensors unable to be used under low or hot environmental conditions.Besides,the cyclic stability of hydrogel needed to be improved and the hydrogel materials had single function and lacked antibacterial and adhesive properties,etc,which would affect their actual use.Therefore,this article used polyvinyl alcohol（PVA）as the matrix to study the preparation of multifunctional PVA hydrogel and its application in flexible sensors.In this paper,we used one-dimensional（1D）silk nanofibers（SNFs）and two-dimensional（2D）graphitic carbon nitride（g-C₃N₄）nanosheets to enhance the mechanical property of PVA organohydrogel.The addition of only 0.1%silk nanofibers and little g-C₃N₄ nanosheets could greatly improve the tensile strength（about 3.2 times）and toughness（about 7.7 times）of the organohydrogel.By using a binary solvent system composed of water and ethylene glycol,the organohydrogel had anti-freezing function.Even at-18°C,it could maintain the flexibility and conductivity.Due to the addition of Al³⁺,conductive polyvinyl alcohol-silk nanofiber-graphitic carbon nitride nanosheet（PVA-SNF-CN）organohydrogel had a significant antibacterial effect on both E.coli and S.aureus.The sandwich-like flexible strain sensor based on PVA-SNF-CN organohydrogel had a large linear sensing range（0%–100%）,fast response（276 ms）and excellent fatigue resistance（1000 cycles）.It could not only successfully detect human motions（wrist bending and knee bending）and facial expressions（smiling and frowning）,but also still maintain the accuracy of the output signal for up to 20 days of use.Many hydrogel flexible sensors lack adhesive property and need to be adhered to surface of the skin with tape,which would not only bring poor experience to users,but also affect the accuracy of detective datas.In the third chapter,we prepared multifunctional adhesive PVA organohydrogel.Inspired by the adhesive mechanism of mussel,we deposited tannic acid（TA）on the surface of silk nanofibers,so that the surface of silk nanofibers contained a large number of catechol functional groups.The addition of modified silk nanofibers could not only improve the tensile strength and tensile strain of the organohydrogel,but also endue the organohydrogel good adhesion.In addition,adding appropriate borax solution caused the organohydrogel possessing self-healing ability.The polyvinyl alcohol-silk nanofibers-tannic acid（PVA-SNF@TA）organohydrogel combined high strength,anti-freezing,self-healing,adhesion,transparency（91%）and UV-filtering.The flexible strain sensor based on PVA-SNF@TA organohydrogel not only had good sensing ability and anti-fatigue property,but also had excellent sensitivity,which could accurately detect different human motions.The flexible pressure sensor based on cylindrical PVA-SNF@TA organohydrogel could detect different motion states of the human body（walking and running）.During repetitive motion,the organohydrogel pressure sensor still maintained excellent sensing stability.In this paper,PVA-SNF-CN organohydrogel and PVA-SNF@TA organohydrogel were prepared by using nanocomposite method.PVA-SNF-CN organohydrogel had high strength,large deformation,conductivity,anti-freezing,moisturizing and antibacterial property.PVA-SNF@TA organohydrogel integrated high strength,conductivity,anti-freezing,adhesion,self-healing,transparency and UV-filtering.The multifunctional PVA-SNF-CN organohydrogel and PVA-SNF@TA organohydrogel greatly expanded the practical application of hydrogel in the field of flexible sensors.In addition,the preparation strategy of the multifunctional conductive organohydrogel in this paper will provide new insights for constructing multifunctional conductive organohydrogel.