Preparation Of Nanocellulose/Polyacrylic Acid Conductive Hydrogels And Their Sensing Applications

Flexible,stretchable and wearable humanized soft electronic devices play an important role in meeting the complexity and multifunctionality of modern electronic devices due to their comprehensive characteristics of wearability,comfort,remote operation and timely feedback.As an important field of wearable devices,wearable strain sensors for transducing mechanical deformation into electrical signals can be directly installed on human skin for human activity monitoring and health management.To meet the requirements of complex human motion and safety for direct skin contact,a stretchable,recoverable,and biocompatible substrate materials is highly desirable,thus the green,environmentally friendly and sustainable natural biomass material nanocellulose has attracted wide attention.Due to its biocompatibility,excellent mechanical properties and modifiability,it provides the possibility for green wearable strain sensors.Electroconductive hydrogel（ECHs）is a kind of"soft material"with high ionic conductivity similar to biological tissue.It has the characteristics of high flexibility and excellent stretchability,so it can be the ideal choice of green wearable strain sensor conductor materials together with nanocellulose.This paper intends to prepare TEMPO-oxidized cellulose nanofibers（TOCNs）by TEMPO（2,2,6,6 tetramethylpiperidine-1-oxy）oxidation method.Because TOCNs is uniform and stable,it has the ability to assist in dispersion,which can assist graphene（GN）to disperse in water to form the TOCNs-GN complex.The TOCNs-GN complex is introduced into polyacrylic acid（PAA）hydrogel to construct a nano-reinforced conductive network.At the same time,it improves the mechanical strength of PAA hydrogel and plays a double reinforcing role.In the case of the optimal ratio of TOCNs,the influence of the reasonable amount of GN on the mechanical strength,electrical conductivity,self-healing and other properties of PAA hydrogels is explored,and assemble it into a green wearable strain sensor to monitor human motion.In order to connect the sensor directly to the skin and make the application of the epidermal sensor a reality,polydopamine（PDA）oxidized by synthetic lithium magnesium silicate nano-clay（Laponite）is added to give it self-adhesion.At the same time,acrylamide（AM）monomer is introduced to prepare double-network polyacrylic acid-polyacrylamide（PAAAM）hydrogel to enhance its self-recovery and anti-fatigue performance.The effects of different content of Laponite on the performance of hydrogel strain sensor is investigated.To ensure normal use in extreme environments,organic glycerol（Gl）is introduced to prepare a hydrogel with anti-freezing,heat resistance and long-term stability by solvent replacement method,which provides more possibilities for the multi-functional development of green wearable strain sensors.Details as follows:（1）The stretchable,self-healing and high-strength hydrogel strain sensor was successfully prepared by simple in situ free radical polymerization and metal-ligand interaction.The sensor was formed by physical and chemical double crosslinking of PAA chain to produce TOCNs-GN/PAA composite hydrogel with excellent mechanical properties.TOCNs are prepared by TEMPO oxidation method using green,environmentally friendly,renewable and degradable bleached wood pulp fibers as raw materials.The presence of nanocellulose makes GN show good dispersibility in the hydrogel solution.When the content of GN is 0.7 wt%of the mass of AA monomer,the comprehensive performance of hydrogel is the best.At the same time,due to the increase of the internal hydrogen bond of the hydrogel,the crosslinking density is increased,maintaining its mechanical stability.When the compressive strain is 60%,the mechanical strength is up to 2.54MPa.The elongation at break is 850%,the electrical conductivity is 2.75 S m^-1 and the self-healing efficiency is up to 96%.It further shows that the stretchable,self-healing and high-performance hydrogel strain sensor made of this composite material can monitor and detect various human movements and other mechanical stimuli.It has a wide range of potential applications in artificial intelligence,human health monitoring,electronic skin and biosensors.（2）PAAAM hydrogels are prepared by adding AM to construct a double-layer network skeleton and introducing PDA oxidized by nano-clay Laponite to give the hydrogel strain sensor adhesion and increase its mechanical properties.When Laponite accounted for 10 wt%of the total AA and AM monomer,TOCNs-GN/PAAAM-LP composite hydrogel strain sensor showed excellent performance.The compressive strength can reach 3.34 MPa at 55%strain,and has excellent self-recovery performance（60 min recovery efficiency is 95.7%）.The structure of the double network and the addition of PDA have also improved the tensile toughness of the hydrogel and increased the tensile strength to 120 k Pa.At the same time,it has excellent fatigue resistance.After 10th cycles,the energy dissipation of the hydrogel tends to be stable.The adhesion mainly comes from the catechol group provided by the PDA.For different materials,the adhesion strength of the obtained glass is up to 23.8 k Pa,and it can still be maintained at 19 k Pa through multiple adhesion cycles to the steel material.Finally,through the cell compatibility test,the TOCNs-GN/PAAAM-LP hydrogel strain sensor can be directly used in human skin for human motion monitoring.（3）The Gl-water binary solvent is introduced on the basis of the above to prepare a strain sensor that is resistant to anti-freezing,heat-resistant and long-term stability in extreme environments.Gl-water system forms a strong hydrogen bond,which suppresses the formation of ice at low temperatures and prevents water evaporation at high temperatures.This study records the weight of TOCNs-GN/PAAAM-LP-Gl hydrogels with different contents of Gl at different times.The results show that when the volume ratio of Gl is 50%of the Gl-water solution,the anti-freeze and heat resistance effect is the best in extreme environments,and the hydrogel remains in its original state.Due to the viscosity of Gl itself,the adhesion strength of the hydrogel can be increased.Moreover,Gl-water introduces non-covalent bond interactions in the polymer,effectively improving the mechanical properties.In the experiment,the organic gel treated with Gl-water binary solvent and the untreated hydrogel are compared in various properties.It is found that the elongation at break increases to 973.5%,the tensile strength is about 110.3 k Pa,and the compressive strength is 2.64 MPa at 55%strain.The strain sensor based on TOCNs-GN/PAAAM-LP-Gl hydrogel can achieve a high GF value of about 15 and still exhibits sensor stability and excellent mechanical durability after multiple cycles of stretching.It has excellent strain sensitivity and high sensitivity under mechanical deformation such as bending,stretching and pressing.Strain sensors with good biocompatibility can be further used for real-time monitoring of large-scale or tiny movements of the human body,including finger expressions,arm joint movements,writing or smiling,frowning,swallowing and other micro-expression monitoring.Therefore,TOCNs-GN/PAAAM-LP-Gl hydrogel strain sensor opens up a new way for multifunctional antifreeze and heat-resistant hydrogel,and opens up many future research directions and application fields.