Preparation And Performance Of Nanocellulose/Ti₃C₂T_X Multifunctional Electronic Skin

As flexible electronic skins show more and more extensive application value in the fields of human health monitoring,artificial intelligence and human-computer interaction.However,how to endow electronic devices more additional functions to realize electronic skins with complex functions even beyond human skin is a challenge that need to be solved urgently now.In this work,2,2,6,6-tetramethylpiperidine-1-oxygen radical（TEMPO）oxidized cellulose nanofibers（TOCN）and two-dimensional layered active MXene（Ti₃C₂T_X）prepare by minimum strength layering method as basic materials for the preparation of multifunctional flexible electronic skins.Among them,nanocellulose has the advantages of high aspect ratio,good water dispersibility,and good biocompatibility,but its poor electrical conductivity makes it difficult to be used as electronic skin alone.MXene materials have excellent electrical conductivity,biocompatibility,and good antibacterial properties.TOCN and Ti₃C₂T_Xcan be composited so that the functions of the two can complement each other to obtain a multifunctional composite material,which is then used to prepare a multifunctional flexible electronic skin.The main contents of the paper are as follows:（1）The oxidized nanocellulose was prepared by the TEMPO oxidation method.The prepared TOCN had a large aspect ratio and good water dispersibility,and could be mixed with the MXene nanosheet dispersion evenly,the TOCN/Ti₃C₂T_Xcomposite hydrogel film was prepared by vacuum-assisted suction filtration,and the shell-like layered structure of the composite film showed good mechanical properties with a tensile strength of 124.6 MPa and a fracture energy of 21.6 MJ/m³.At the same time,the hydrogel film also maintained good flexibility.In addition,the composite material also showed good biocompatibility,antibacterial performance（antibacterial rate>97%）and electromagnetic shielding performance（38 d B）,which greatly expanded the application range of electronic skin.By replicating the hemispherical microstructure on the surface of the composite membrane for the fabrication of flexible electronic skin,the resulting sensing device exhibited a high sensitivity of11.6 k Pa^-1,a response and recovery time as low as 10 ms,a detection limit as low as0.2 Pa,and a long-term stability（>11000 cycles）,could also be used to monitor human physiological activities such as finger bending,chewing,pulse,etc.The prepared multifunctional electronic skin can not only meet the needs of different functional sensors such as non-toxic,antibacterial,and electromagnetic shielding,but also show great application prospects in the fields of flexible wearable devices,health monitoring,and multifunctional sensing.（2）Cellulose-based aerogels have the advantages of light weight and good compressive strength,and have great application prospects in the field of electronic skin.However,the TOCN/Ti₃C₂T_Xcomposite aerogel prepared by general freeze-drying are difficult to be used in the practical application of electronic skin due to their internal disordered and discontinuous structure with poor compressive elasticity.Therefore,we used the bidirectional freeze-drying method to prepare composite aerogels with high compressive elasticity.Due to the orderly stacked wavy layered structure inside,the composite aerogels had good compressive elasticity.However,due to the strong water absorption of nanocellulose-based materials,the internal structure was damaged and the compression elasticity was lost after water absorption.Therefore,we deposited hydrophobic Methyltrimethoxysilane（MTMS）on the surface of the composite aerogel by thermal chemical vapor deposition,making it hydrophobic so that the compressive elasticity was preserved.The final fabricated electronic skin exhibited the advantages of light weight（6 mg/cm³）,high compressive elasticity（compressive strength retention>80%after 300 compression cycles）,and excellent electromagnetic shielding performance（51 d B）.The fabricated electronic skin can also be used for humidity and pressure sensing,and the fabricated humidity sensor exhibited a high humidity sensitivity of 1.12 RH^-1and a fast response time of 15 s.The prepared pressure sensor exhibited a high sensitivity of 4.05 k Pa^-1,a low detection limit of 1 Pa and a response time of 50 ms,and can also be used to monitor physiological activities such as human finger bending,vocal cord vibration,foot bending,and chewing.This makes the prepared TOCN/Ti₃C₂T_Xcomposite aerogel not only suitable for practical long-term use,but also has great application value in the fields of portable wearable electronic devices,multifunctional sensors,and human-computer interaction.（3）In order to realize the electronic skin with self-healing and self-adhesive ability like human skin,we added polyvinyl alcohol（PVA）and used borax as a cross-linking agent on the basis of TOCN and MXene materials.The preparation of TOCN/Ti₃C₂T_X/PVA composite hydrogel not only solves the disadvantage of TOCN/Ti₃C₂T_Xcomposite hydrogel being poor and easy to be damaged,but also the final TOCN/Ti₃C₂T_X/PVA composite hydrogel exhibited good mechanical properties（tensile strength up to 81 k Pa）.At the same time,the composite hydrogel also has good self-healing properties and self-adhesion properties.The self-healing efficiency of 36 hours after damage was as high as 90.2%.It showed good adhesion strength for various materials,and the maximum adhesion strength can be up to 12 k Pa.The prepared electronic skin can also have good mechanical properties,self-healing and self-adhesion properties.In addition,the electronic skin prepared by TOCN/Ti₃C₂T_X/PVA composite hydrogel also exhibited good stress and strain sensing.The sensitivity to pressure was up to 1.18 k Pa^-1and the sensitivity to strain was up to12.5.At the same time,the sensor also exhibits a detection limit as low as 5.2 Pa and a response time of 100 ms.It can also be used for monitoring human physiological activities such as vibration,wrist flexion and swallowing.The prepared multifunctional composite hydrogel can not only meet the needs of daily use as a sensor,but also can be used in other occasions with different functional requirements,showing great potential in multifunctional sensors,health monitoring and human-computer interaction.