Preparation And Performance Of Antifreezing Hydrogel Electronic Skin Based On Nanocellulose/polypyrrole

With the advent of the intelligent era,wearable electronic devices have attracted more and more attention.As a flexible electronic device capable of transmitting physiological signals,electronic skin has gradually become a research hotspot.Electronic skin has a form and function similar to human skin,capable of sensing force,temperature or airflow,etc.As an environmentally friendly and renewable natural material,nanocellulose（TOCN）has incomparable advantages over traditional polymers,and plays an important role in alleviating the energy crisis and environmental pollution.Polypyrrole（PPy）,as a non-toxic and highly efficient conductive polymer,is a good choice for the preparation of electronic devices.Therefore,we used nanocellulose and polypyrrole as the main raw materials to conduct a series of explorations on electronic skin.First,by designing the microstructure to endow the TOCN/PPy electronic skin with excellent pressure-sensing properties.And by utilizing the feature that temperature and light can affect the conductivity of polypyrrole,a multifunctional electronic skin that can detect temperature and light is obtained.Then,in order to improve the problem of easy freezing of hydrogels at low temperature,inorganic salt calcium chloride（Ca Cl₂）was introduced into TOCN/PPy hydrogels by solvent replacement method,and TOCN/PPy/Ca Cl₂strain sensor with excellent low temperature tolerance was prepared.The sensor can detect stress changes at low temperatures.Finally,in order to improve the hydrogel’s easy water loss and drying and further enhance the low temperature tolerance of the hydrogel,the TOCN/PPy hydrogel was treated with glycerol（Gly）to obtain a strain sensor with resistance to drying and low temperature.The specific work content is as follows:（1）2,2,6,6-tetramethylpiperidine-1-oxyl radical（TEMPO）was used to oxidize cellulose to obtain nanocellulose with high aspect ratio and nanometer size.Three-dimensional network TOCN hydrogels with abundant pores were prepared by cross-linking nanocellulose with ferric ions.Taking advantage of the volatile nature of pyrrole molecules,polypyrrole was introduced into the TOCN three-dimensional framework to obtain TOCN/PPy hydrogels.PPy grew along the nanofibers to form a complete conductive path,laying the foundation for electrical signal conduction.The nylon gauze pattern was printed on the TOCN/PPy hydrogel film dried at low temperature by screen printing,and the hydrogel film was assembled to prepare a piezoresistive electronic skin with excellent performance.This TOCN/PPy electronic skin exhibits a sensitivity of 3.13 k Pa^-1,an ultrafast response time（<10 ms）,and excellent cycling stability（9000 cycles）.This electronic skin can detect basic physiological activities of the human body,such as chewing,finger bending and wrist flexion.In addition,this electronic skin can also be used to detect temperature and light,with ultra-high sensitivity to temperature（1.215%/℃）,excellent linear correlation（R²=0.989）and wide detection range（5～110℃）.It also has obvious response to illumination（0.093%/W）and better linearity（R²=0.944）.（2）In order to make the application range of hydrogels more extensive,especially to enable them to be used in low temperature environment,the work in this chapter studies to improve the low temperature tolerance of hydrogels.First,the TOCN/PPy hydrogel was prepared according to the method in the previous chapter,and then calcium chloride was added to the inside of the hydrogel by a simple and efficient solution replacement method to endow the hydrogel with low temperature tolerance.The TOCN/PPy/Ca Cl₂hydrogel prepared by this method still has excellent electrical conductivity,good mechanical properties and relatively excellent strain sensing performance at low temperature of-35°C（strain sensitivity GF=2.02,response time 100 ms）.It can also monitor joint bending,chewing,swallowing and other behaviors.（3）On the basis of the previous work,the low temperature resistance of the hydrogel was further improved and the antidrying ability was given to the hydrogel.First,TOCN/PPy hydrogel was prepared according to the same method as the previous two chapters,and then a glycerol/water binary system solution was prepared.Finally,the solvent replacement method was used to exchange the glycerol and the water inside the hydrogel to obtain TOCN/PPy/Gly hydrogel.The TOCN/PPy/Gly hydrogel can further expand the use temperature of the hydrogel,and can still maintain a certain shape and weight of the hydrogel in the environment of100°C.The hydrogel still has electrical conductivity and sensing ability（strain sensitivity GF=2.41,response time 100 ms）in the cold environment of-45°C.