Construction Of Organic Nanocomposite Hydrogels And Their Applications In Strain Sensors

Flexible sensors are one of the core components of wearable electronic devices,which can transform mechanical changes such as stretching,compression,bending and torsion into electrical signal changes such as resistance,capacitance and current.They are widely used in electronic skin,human-computer interaction,soft robot and other fields.The key to determine the sensing performance of a flexible sensor lies in the material properties of the sensor.Flexible sensors are increasingly made of conductive hydrogels due to their characteristic such as excellent biocompatibility,conductivity,compliance and stretchability.But traditional hydrogels usually have poor mechanical properties and lack of versatility,which limits their application in flexible sensors.It is a significant task to design conductive hydrogel materials with good performance.Therefore,this paper uses carboxylate modified cellulose nanofibril（c-CNF）as a functional filler to fabricate a hydrogel which have conductivity,self-healing,adhesion,high stretchability,frozen,and magnetic response.Systematic research is carried out around hydrogel structure adjustment,mechanical properties,and sensor performance of hydrogel strain sensors.The main research contents include:（1）Self-repairing flexible strain sensors are fabricated with poly（AA-co-SMA）/c-CNF/Fe³⁺nanocomposite hydrogels（AA:acrylic acid,SMA:stearyl methacrylate,c-CNF:carboxylate modified cellulose nanofibril）.Because the nanocomposite hydrogels are fully supramolecularly cross-linked by hydrophobic association and ionic interaction,so the flexible strain sensors can repair themselves upon damage and recover their sensing ability.The introduction of c-CNF significantly improved the resilience of nanocomposite hydrogels.After ten load-unloading cycles,the residual strain of hydrogels without c-CNF is 63%.The residual strain of the hydrogel introduced with c-CNF is 26%,and the residual strain is reduced by 37%,which makes the flexible strain sensor in this work have essentially reversible sensing ability.In addition,such sensors also have integrated large sensing range（0～800%）,low response time（0.25 s）,high sensitivity（GF=4.4）and excellent durability（～1000 cycles）for guaranteeing their applications in whole-body monitoring.（2）Covalently cross-linked polyacrylamide（PAM）network reinforced by carboxylate cellulose nano fibril（c-CNF）is designed as the recoverable framework.Ternary-semi-interpenetrating network is constructed by simultaneously introducing polyvinyl alcohol（PVA）and polyvinylpyrrolidone（PVP）.The ternary-semi-interpenetrating ionic（TSII）hydrogels were prepared.Because of the ternary-semi-interpenetrating structure,the resulting hydrogels PAM/c-CNF/（PVA-PVP）exhibit improved stretchability and outstanding self-recoverable properties.Flexible strain sensors with a sensing range of 0～750%,high sensitivity（GF=5.51）,low response time（～140 ms）and excellent stability（>1000 cycles）have been fabricated utilizing the ternary-semi-interpenetrating ionic hydrogels.Wireless strain sensors have also been demonstrated by integrating the sensors into a bluetooth signal transmission and reception system.Moreover,ternary-semi-interpenetrating organohydrogels are further developed by introducing a binary solvent of glycerol/water.The organohydrogel have reversible adhesion and exhibits inherent adhesiveness even at-20℃ and can be applied in sensitive capacitive pressure sensors（GF=0.97 kPa-1）.Finally,various strains and pressures in daily life are monitored by the flexible sensors.（3）P（AM-co-NIPAM）/PVA/Fe₃O₄@C-CNF magnetic nanocomposite hydrogel is prepared by acrylamide（AM）and N-isopropyl acrylamide（PNIPAM）crosslinked by N,N’-methylene bisacrylamide（MBAA）,polyvinyl alcohol（PVA）and c-CNF-dispersed Fe₃O₄ magnetic nanoparticles.Among them,the chemically crosslinked AM-co-NIPAM network is used as a rigid skeleton of hydrogel and provides heat sensitivity,and PVA is used as a linear polymer to penetrate hydrogel network reinforcing stretchability,and c-CNF-dispersed Fe₃O₄ magnetic nanoparticles increase hydrogel mechanical properties and provides magnetic properties for hydrogels.Magnetic nanocomposite hydrogels have conductivity,magnetic sensitivity,temperature sensitivity,and controlled adhesion.Based on this hydrogel,a flexible sensor of the magnetic field,temperature,and strain multi-response is built.The flexible sensor has a large sensor range（0～350%）,high sensitivity（GF=4.31）,low response time（～140 ms）,excellent self-recovery,temperature sensitivity,and controlled adhesion properties.Moreover,the flexible sensor has two sensing modes of contact sensing and non-contact sensing,which can monitor people’s walking/run in the case of contact;monitor people’s riding in the case of non-contact.