Preparation And Performances Of Cellulose-Based Antifreezing Conductive Hydrogels

A sensor is a detection device that can sense physical or chemical changes in the external world and convert the detected signals into electrical or other required forms of information output through certain regulations.At present,flexible wearable devices with the advantages of wearability,miniaturization,and flexibility are attracting much attention,and they have broad application prospects in the fields of medical health,sports monitoring,human-computer interaction,electronic skin,intelligent robotics,and energy storage.As a core component of wearable devices,flexible sensors can detect and record physical signals such as temperature and pressure,electrophysiological signals,chemical substances,and other subtle changes due to their high flexibility,stretchability,and excellent electrical conductivity,which are of significance to the increasing requirements in the fields of healthcare and sports monitoring.Hydrogels are considered to be one of the most promising materials for the development of next-generation bioelectronic devices due to their similarity to biological tissues.Currently,cellulose-based conductive hydrogels are drawing extensive interest in the field of flexible sensors for human motion monitoring due to their superior electrical conductivity,ductility,and biocompatibility.As the most abundant natural polymeric polysaccharide on earth,cellulose is green,widely sourced,and recycled sustainably.However,cellulose-based hydrogels generally have weak mechanical properties.Taking advantage of the natural crystalline structure and high elastic modulus of cellulose nanofibers（CNF）itself,the enhancement of the mechanical properties of cellulose hydrogels can be achieved by nanocomposites.In addition,traditional cellulose-based conductive hydrogels cannot be applied to winter sports body monitoring.When the conductive hydrogel with high water content is exposed to low temperatures,it would freeze,resulting in the loss of mechanical flexibility and electrical conductivity,which limits the application of cellulose-based conductive hydrogels in the field of flexible sensors.In this dissertation,polyacrylamide（PAAm）,CNF,and gelatin（Gel）were used as the main materials,and Fe Cl₃,amphoteric betaine,and glycerol（Gly）were used as the main antifreeze agents to prepare nanocomposite hydrogels by low-temperature gelation and UV polymerization,and further improve the comprehensive performance of the composite hydrogels by solution replacement.The hydrogels were used to assemble flexible sensors and explore their sensing performance,and the potential of the flexible sensors for human motion monitoring was also evaluated.The main research contents and findings are as follows:（1）Ionic antifreezing conductive hydrogels were prepared by introducing Fe³⁺based on CNF and PAAm.The hydrogel exhibited excellent electrical conductivity up to 10.78 S/m.In addition,the hydrogel was able to maintain mechanical flexibility at a low temperature of-30°C,indicating its superior antifreezing properties.The ionic coordination bonds between Fe³⁺and PAAm and CNF during solution substitution effectively improve the mechanical properties of the ionic hydrogels with a compressive stress of 1.93 MPa at 90%strain.In addition,the non-covalent interaction between hydrogen bonds and ionic coordination bonds can be used to dissipate energy,giving the ionic hydrogels good elasticity and cyclic properties.Using ionic hydrogels assembled into flexible strain sensors under 60 compression cycles,signal stability and sensitivity up to about 6.42 can be achieved,which can be used as flexible sensors to monitor human motion in low-temperature environments.（2）Tough,freeze-resistant,and highly conductive zwitterion hydrogels were prepared by constructing a double-network hydrogel structure and solution replacement method.The double network structure of the zwitterion hydrogel consists of a brittle Gel physical network and a tough PAAm chemical network.The zwitterion betaine can hinder the formation of hydrogen bonds between water molecules,allowing the ampholytic hydrogels to remain mechanically flexible at low temperatures of-40°C.The addition of（NH₄）₂SO₄ not only improves the electrical conductivity of the hydrogel up to～1.5 S/m,but also produces a salting effect with Gel,which greatly improves the mechanical properties of the hydrogel,with stresses up to～5.72 MPa at a compressive strain of 92%.Zwitterion hydrogel flexible sensors exhibit good performance,such as high sensitivity,exhibiting up to 3.73 at a strain of 70%sensitivity,and the ability to achieve 100 signal-stabilized compression cycles,the sensor enables real-time monitoring of movements such as human joint flexion and frowning.（3）An organic hydrogel with multiple physical cross-linking and chemical cross-linking,including salt precipitation effect,ionic interaction,and hydrogen bonding,was prepared by introducing Na₃Cit/Gly/H₂O ternary solvent into the PAAm/Gel/CNF double network hydrogel using the solvent replacement method,and based on the above interaction forces,the organic hydrogel has good mechanical properties,and the stress is about 1.55MPa under the condition of about 90%compressive strain.In addition,the partial replacement of water molecules in the hydrogel by Na⁺,Cit^3-and Gly molecules gives the organic hydrogel excellent frost resistance,capable of withstanding weather extremes as low as-50°C,while maintaining the good electrical conductivity of about 0.24 S/m at low temperatures.Organic hydrogel flexible sensors are capable of real-time monitoring of human motion,including repetitive Bending large movements and frowning subtle movements.