Preparation Of Nanocellulose Reinforced Multifunctional Hydrogel Sensors And Their Applications In Human Motion Monitoring

Hydrogel has been widely applied in energy storage,sensing,actuators,biomedicine and human-machine interfaces.With the development of science and technology and social progress,hydrogel,as one part of the flexible wearable electronics,has shown great potential in human health monitoring and some other fields.However,traditional ionic conductive hydrogels are mainly prepared by impregnating the prefabricated hydrogel in a high concentration electrolyte solution,which is a time-consuming process and also causes the waste of electrolyte solution.Furthermore,most of the conventional ionic conductive hydrogels are always lack of frost resistance.Water molecules in the hydrogel networks will form ice crystals in extreme conditions,which limits the applications of ionic conductive hydrogels.Therefore,this paper mainly focuses on preparing ionic conductive hydrogel strain sensors with high strength,flexibility,stretchability,self-adhesion and frost resistance for human motion monitoring through a simple and economic feasible method.In order to solve the problems existing in the traditional ionic conductive hydrogels,the main research contents of this paper are as follows:（1）A simple one-pot freezing-thawing processing under low electrolyte concentration was proposed to prepare cellulose nano fibril（CNF）reinforced flexible,stretchable hydrogel sensor with high strength and good ionic conductivity for human motion monitoring.The semi-crystalline hydroxy-rich polymer PVA was selected as the hydrogel substrate to form the hydrogel skeleton.Specifically,the TEMPO-oxidized CNF with high strength,large aspect ratio,large specific surface area and rich in hydroxyl and carboxyl functional groups was used as a strengthening agent to improve the mechanical properties and ionic conductivity of the hydrogel.Furthermore,Zn SO₄ was also introduced into the hydrogel precursor solutions before the one-pot freezing-thawing proceeding.In addition to enhance the ionic conductivity of the hydrogel,Zn²⁺could form coordination bonds with PVA and CNF.More importantly,SO₄^2-facilitated the formation of the PVA microcrystalline domain through the Hofmeister Effect,which further enhanced the mechanical properties of the hydrogel.The ionic conduction of the hydrogel was 0.32 S·m^-1when the concentration of Zn SO₄ was 0.07 M,and it reached up to 1.50 S·m^-1when the concentration increased to0.28 M.The P₂₀C_0.4Z_0.07H achieved a tensile stress of 0.79 MPa and the strain of 242%.Furthermore,the hydrogel sensor exhibited a linear gauge factor up to 1.70 in a strain range from 0 to 200%.The hydrogel sensor with good ionic conductivity,high strength and excellent sensing performance was successfully prepared by the simple and economic feasible freezing-thawing under low electrolyte concentration for human motion monitoring.（2）The simple one-pot freezing-thawing processing under low electrolyte concentration was proposed to fabricate the polyaniline modified cellulose nanofibers（PANI@CNF）reinforced hydrogel sensor with high strength,flexibility and anti-freezing performance for human motion monitoring.The glycerol was introduced into the hydrogel system as the anti-freezing additive to endow the hydrogel with anti-freeze performance.For the sake of alleviating the side effect on the conductivity of the hydrogel due to the introducing of low dielectric constant,high viscosity glycerol,we take the TEMPO oxidized CNF as biomass template.And then,the protonated aniline monomer adsorbed on the surface of CNF through the electrostatic interaction was oxidized under the presence of oxidizing agent ammonium persulfate（APS）to form the polyaniline modified cellulose nanofibers（PANI@CNF）,which solved the problem of the easy flocculation of pure polyaniline in hydrophilic hydrogel network.The PANI@CNF in the hydrogel system not only enhanced he conductivity of the hydrogel,but also the mechanical performances of the hydrogel.The fracture strength of PVA-0.6%PANI@CNF-Zn SO₄ hydrogel reached 1033 KPa,the corresponding tensile strain was606%,and the ionic conductivity at room temperature and-18℃was 0.081 and 0.0730S·m^-1respectively.Comparing to the blank group,the ionic conductivity increased by 88.4%and 118.4%,respectively,which significantly enhanced the ionic conductivity of the hydrogel and showed excellent anti-freezing performance.（3）The simple one-pot freezing-thawing processing under low electrolyte concentration was chosen to prepare the multiple physical interactions reinforced hydrogel sensor with high strength,flexibility and anti-freezing performance for human motion monitoring,for which the tannin（TA）was used as physical crosslinking point to regulate the physical and chemical properties of the hydrogel.Except for increasing the content of the conductive nano filler in hydrogel system or introducing of electron-conducting nano materials,we introduced a versatile small molecule crosslinking agent to enhance the crosslinking density of hydrogel to supply more conductive path for the ion transport to enhance the ionic conductivity and mechanical properties of the of the anti-freezing hydrogel.TA was selected as physical crosslinking point for which can form hydrogen bond cross-linking with PVA and CNF,and form coordination crosslinking interaction with metal ion.Thus,the mechanical and physicochemical properties of the hydrogel were regulated on-demand.Detailly,we chose glycerin and water as anti-freezing system,PVA as the water hydrogel substrate,CNF as strengthening agent,TA,was used as physical crosslinking point.Take advantage of synergistic effect of the hydrogen bond interaction and coordination interaction to regulate the physicochemical property of the hydrogel.In this hydrogel system,increasing TA content can significantly enhance the mechanical properties of the gel,which is ascribed to the strong hydrogen bond interaction between TA and PVA,CNF and coordination cross-linking with electrolyte cation.Compared with the hydrogels crosslinked with different electrolytes,Fe₂（SO₄）₃ can form strong coordination crosslinking with TA,thus endows the gel with uniform porous honeycomb network structure,the best mechanical properties,good ionic conductivity and solvent loss resistance.The tensile fracture strength and fracture strain of PVA₁₀-CNF_0.4-TA₈-Fe₂（SO₄）₃ hydrogel were 980 KPa and 570%,and the ionic conductivity of the hydrogel at room temperature and-18℃was 0.220 S·m^-1and0.183 S·m^-1,respectively,which testified the excellent frost resistance of the hydrogel.Furthermore,the gauge factor of the hydrogel sensor reached up to 1.75.（4）Combining the freezing-thawing processing with the free radical polymerization,a self-adhesive,anti-freezing and transparent polyvinyl alcohol-polyacrylic acid（PVA-PAA）double network hydrogel reinforced with tannic acid modified cellulose nanocrystals（TA@CNC）was designed.Based on the chemical adhesion mechanism of catechol-based chemical groups,TA with catechol-based groups was coated on the surface of cellulose nanocrystals,and it was introduced into the anti-freezing double-network hydrogel of PVA-PAA,endowing the hydrogel with adhesiveness and enhancing the mechanical properties of the hydrogel.Particularly,the first covalent-bonded PAA network was prepared through heat induced free radical polymerization.And then,the second network cross-linked by PVA microcrystalline formed through the simple one-pot freezing-thawing method.It is important to note that,due to the existence of special glycerol-water anti-freezing system and TA in the hydrogel network,more micron particles exhibited on the surface of the hydrogel with the increase of mass ratio of TA to CNC.The emergence of these micron particles is beneficial to increase the contact area between hydrogel and matrix,thus exhibited excellent adhesion property.TA makes a great contribution to the self-adhesive properties of hydrogels.However,high TA content may affect the mechanical properties of hydrogels to a certain extent due to its free radical scavenging ability and the strong interaction between the components in the glycerol-water system.The fracture strength of PVA-TA@CNC_3-1-PAA-Zn SO₄ was 102.8KPa and the tensile strain was 506%.Most importantly,it exhibited the best ionic conductivity of 0.140 S·m^-1.and the best adhesive strength of 8.56 KPa.The hydrogel was assembled into strain sensor for human motion monitoring（such as the joint movement of finger,wrist,elbow knee and the neck,etc.）,and the hydrogel sensor presented excellent sensing performance,such as stability,repeatability and reliability.