Study And Application Of Flexible Transparent Humidity Sensor Based On Nano-cellulose Composite Film

Humidity is one of the essential indexes of air,which is important for industrial and agricultural production even our daily life,the measurement of human body humidity has great implications for assessing human health.However,the manufacturing process of humidity sensors often involves complex preparation,expensive instruments and even toxic chemicals.Nanocelluloses are the most important derivatives of cellulose.Because of their degradability,biocompatibility,high specific surface area and strong hydrophilicity,nanocelluloses are considered as potential materials for the preparation of flexible humidity sensors.In order to solve the above problem,this paper has developed a flexible and transparent humidity sensor based on nanocellulose composite film,which is used to monitor human respiratory humidity and skin humidity after packaging.Firstly,in this paper,a composite enzyme solution composed of cellulase and xylanase（enzyme activity ratio 9:1）with a total enzyme activity of 10 u/m L was used to hydrolyze eucalyptus pulp at 50°C for 12 h to obtain rod-shaped CNC.TiO₂/CNC composite film was prepared by ultrasonic composite and suction filtration method.Observed by scanning electron microscope（SEM）,CNC fibers are entangled with each other to form a dense spatial network structure.Therefore,the TiO₂/CNC composite film is prepared with CNC as a biological scaffold,and the TiO₂particles are uniformly distributed.The composite film has high transparency（above 84.5%）,good thermal stability（thermal decomposition temperature of 346°C）,and certain mechanical properties（tensile stress of 44.66 MPa,strain of 57.82%）.The sensor achieves excellent humidity response(_RH/₀=450.9),fast response（response time is 22 s,recovery time is 13 s）,good linearity(R_11-54%²=0.991,R_54-95%²=0.968),folding（20 times）durability（sensitivity deviation<1%）and good long-term stability（more than 40days,sensitivity deviation<2%）.Materials Studio software was used to simulate the force between TiO₂and CNC,and the moisture sensitivity mechanism was further elaborated.Secondly,CNC were added as a reinforcing agent to polyvinyl alcohol（PVA）solution,prepare CNC/PVA hydrogel film by freeze-thaw method,and soak the hydrogel film in KCl solution to obtain CNC/PVA-KCl hydrogel composite film.The composite film has high transparency（above 86.8%）,good mechanical properties（tensile stress of 80.37 MPa,strain of 57.46%）.The sensor achieves excellent humidity sensitivity:high humidity response（R_a/R_g=37.67）,fast response（response time of 12 seconds,recovery time of 16 seconds）,good linearity（R²=0.9807）;folding（50 times）durability（sensitivity deviation<2%）,good long-term stability（over 40 days,sensitivity deviation<2%）,showing potential applications in real-time monitoring.Finally,the CNC/TiO₂and CNC/PVA-KCl humidity sensor were packaged and applied to respiratory humidity monitoring and human skin humidity monitoring.In breathing monitoring,we can distinguish the changes in frequency and depth between normal breathing,fast breathing,and slow breathing from the number of peaks in the curve.Each exhalation and inhalation corresponds to a peak and quickly returns to the starting point,which means that the sensor can respond quickly to breathing and reach a steady state,which is important for accurate detection of breathing depth.Secondly,in the human skin humidity monitoring,the humidity sensor prepared can measure the humidity of different parts of the human body before and after movement in real time,and also can monitor the humidity changes after the application of moisturizer.In human skin humidity monitoring,TiO2/CNC and CNC/PVA-KCl humidity sensors are very close to the data independently collected by commercial skin moisture detectors（deviation<3%）.Through the skin humidity sensor design and development of material price accounting,a single sensor material price within 10yuan,compared with the market of the same type of humidity sensor module,has the same function at the same time,but also achieve low price.Therefore,the humidity sensor prepared in this study has practical application in human health.