Preparation Of Gelatin-based Hydrogel Film Sensor With Application In Humidity Sensing

In recent years,the field of flexible electronics has made many academic achievements in full flourish.Hydrogel sensors have attracted much attention in the field of wearable electronic devices by large of scientific researchers in due to its inherent flexibility,viscosity and the controllability of elastic modulus.Compared with traditional sensors,hydrogel sensors can overcome their shortcomings such as low stretchability,time-consuming fabrication processes,and uncomfortable wearing sense.However,most of them reported are only limited to the detection of strain with stretching or compressing,especially it cannot provide feedback of temperature or humidity changes on skin surface,which limits its application in the field of flexible electronic skin.Thus it is necessary to construct a hydrogel strain sensor which is both sensitive to humidity and temperature in the field of flexible wearable electronic devices.In this paper,a hydrogel film with the thickness of only 0.1 mm was firstly prepared by mixing gelatin,chitosan,glycerin and sodium chloride in one-pot method.On the one hand,the design of the film structure endows the hydrogel with high transparency（the transmittance up to 98%）.On the other hand,it has a dense network structure and exhibits excellent mechanical properties（the maximum breaking strength is 3.1 MPa,the maximum elongation at break is 361%,and the maximum toughness is 2.15 MJ/m³）.The dynamic physical interactions（including electrostatic interactions,hydrogen bonding）between the molecular networks make the film have anti-fatigue and good energy dissipation properties.In addition,a large number of hydrophilic functional groups（-OH,-COOH,-NH₂）in the system make it have good water retention（the water loss rate in 30 days is51%）.The binary solvent composed of glycerol and water ensures the migration of ions at low temperature.Based on these excellent properties,the second part of the work further explores the humidity and temperature sensing properties of the hydrogel film.Under changes of different humidity,the humidity sensitivity coefficient calculated by linear fitting is 2.31.The hydrogel film can not only detects continuously changing humidity,but also repeatedly detects 30 times humidity cycles of 40%-85%RH.At the same time,the reasons for humidity and temperature sensitivity are analyzed from the perspective of molecular and network structure.As a humidity and temperature sensor,it has a wide detection range（20%-90%RH,-30°C-50°C）which can stably monitor the humidity and temperature changes on the skin surface in real time.Due to its high sensitivity to humidity,the film sensor can be also used to detect breathing of the human body with different ways（nose and mouth）and different degrees（normal,fast and deep）,and the response time and recovery time is only 0.41 s and 0.3 s.Furthermore,as a strain sensor,with high strain sensitivity factor（GF=9.38）and durability,it can monitor various joint movements and even tiny skin tissue activities.The hydrogel film sensor which integrates humidity,temperature and strain sensing improves the limited stretchability and complex construction of traditional devices,and also solves the problems of hydrogel sensor currently like the single sensing characteristics,low sensitivity and intolerability in harsh environment.The multifunctional all-in-one hydrogel film sensor provides a new design strategic and more possibilities for skin-like sensing devices,we firmly believe that it will have a broader application prospect in the field of flexible electronics.