High Robust And Conductive Cellulose Hydrogel For Flexible Strain Sensors

Conductive hydrogels are three-dimensional cross-linked polymer networks with high water content and conductive properties.Conductive hydrogels have emerged as promising candidates for various applications including flexible strain sensing and electronic skin owing to their biocompatibility,adjustable mechanical flexibility,good conductivity,and multiple stimuli-responsive properties.However,traditional sensors do not have the characteristics of mechanical flexibility,bendability,and good elasticity,which can no longer meet the needs of people.To meet the demand of developing renewable resources and solve the shortcomings of hydrogels,this paper combined the natural polymer material cellulose with conductive ions or conductive materials to construct a series of cellulose conductive hydrogels.Then the preparation process and formation mechanism were clarified and different designs of cellulose hydrogels with the combination of conductive ions or conductive materials were also presented,focusing on applications under special conditions.Current challenges and future research perspectives of cellulose conductive hydrogels are proposed,which aims to bring new insights to the development of flexible electronics.The main content of the paper was divided into the following three parts.（1）Cellulose was dissolved in the alkali/urea solution system,and 1,4-butanediol diglycidyl ether（BDDGE）was selected as a long-chain chemical cross-linking agent to prepare chemically crosslinked cellulose hydrogels.BDDGE was injected dropwise into the stirred cellulose solution.The introduction of the long-chain chemical cross-linkers significantly provided a large hidden length,thereby improving the resilience and tensile strength of cellulose hydrogel.The incorporation of Li Cl/Glycerol imparted reinforced polymer architecture and stable conductive path to the cellulose hydrogel along with strong hydration effect.Li OH hydrates in solvent is easily attracted to the hydroxyl groups of cellulose chains to form new hydrogen bonds at low temperature via destroying the original intramolecular and inter-molecular hydrogen bonds among cellulose chain.Urea hydrates as shell rapidly surround the formed hydrogen bond network through dynamic self-assembly via van der Waals force to obtain the worm-like inclusion complex（IC）with sheath like structure,thus improving the mechanical properties of cellulose hydrogel.At the same time,the conductive Li⁺ions contained in the cellulose solvent are well retained after double-crosslinking,and the introduction of conductive ions Li⁺and Cl^-in the coagulation bath Li Cl/Glycerol further enhances the excellent conductivity of cellulose hydrogel materials,which greatly improve the sensitivity of the cellulose hydrogel.In addition,conductive ions Li⁺can combine with water to form strong hydration effect,thereby improving the water retention and frost resistance of the material.In addition,Glycerol further enhanced the antifreeze and water retention capacity of the hydrogel by inhibiting the formation of ice crystals inside the hydrogel.The strain sensing experiment showed that cellulose lonic conductive hydrogel demonstrated the high strain sensitivity and ultrafast response,which provides novel ideas and designs for the construction of multifaceted sensors for human motions.（2）Cellulose and AgNO₃ were selected as materials to fabricate antibacterial and sensing cellulose hydrogels.By taking advantage of the original alkaline environment and the hydroxyl group on the cellulose chains,Ag NO₃ was reduced to silver nano cubes in situ and distributed evenly in the hydrogel,greatly improving the agglomeration of traditional nano fillers.At the same time,with the help of chemical and physical double crosslinking,a multi-level pore network structure was prepared,which is not only to enhance the mechanical properties of hydrogel,but also help the energy dissipation of hydrogels when forced.Dense micropores or mesopores maintain the elasticity of hydrogel under external forces,endowing it excellent tensile strength and elasticity.The uniform distribution of nano silver in the hydrogel endows it with excellent conductivity and antibacterial properties.The antibacterial experiment showed that CAH had excellent antibacterial activity against Escherichia coli and Staphylococcus aureus（≈99.99%）.The sensing experiment showed that CAH could monitor the large movement of the human body in real time,and monitor the subtle actions,such as speech.Interestingly,CAH displayed the good capacitive touch sensing ability.（3）Cellulose and MWCNT-OH were selected as materials to fabricate conductive cellulose hydrogels by low-temperature dissolution technology.MWCNT-OH combined with hydroxyl groups on cellulose molecules to form hydrogen bonds,greatly improving its dispersion and stability in cellulose solution.The effect of MWCNT-OH content on the mechanical properties and electrical conductivity of the cellulose conductive hydrogels was investigated.Thermal stability of pure CH and CH/MWCNT-OH composite hydrogels by thermogravimetric analysis.The strain sensing experiment showed that CH/MWCNT-OH not only displayed the resistance response to the bending of finger and wrist,but also could effectively detect the swallowing movement.