Study On Flexible Pressure Sensors For Monitoring Human Physiological Signals

With the development of artificial intelligence and the Internet of things technology,wearable electronic devices have gradually entered our life.A large quantity of flexible electronics related technologies has been incorporated into daily wearable items,and have shown a wide application prospect in many fields such as medical treatment,military,industry and entertainment.Comparing to traditional electronic devices,the flexibility and ductility of flexible electronic devices make them capable to adapt to more complicated working environments.In particular,as a wearable device,the flexible electronics can be closely attached to the surface of the human skin ensuring the accuracy of signal acquisition while improving wearing comfort.The flexible wearable sensing system is an important part of wearable electronic devices.As one of the core components of wearable sensing system,flexible pressure sensors can monitor human motion and physiological signals,and play an important role in health monitoring,disease diagnosis and treatment,human-computer interaction and other applications.In recent years,with the research and breakthrough of materials and sensing mechanisms of flexible pressure sensors,a series of highperformance sensors have been able to achieve accurate measurement of different human physiological and motion signals.However,due to the special properties and application scenarios of flexible wearable devices,higher requirements have been put forward for the cost,manufacturing process,stability,wearability and biocompatibility.In this paper,selection of materials,construction methods and sensing mechanisms are studied on the sensors,and low-cost pressure sensors based on natural fiber materials,all-nanofiber-based pressure sensors with excellent breathability and heat dissipation ability,as well as biocompatible self-healing multifunctional e-skin were developed respectively.Additionally,the application of the sensor in physiological signals monitoring was explored.The main research contents are as follows:1.Aiming at the wearability and biocompatibility of the pressure sensors,paper and cotton sheets based on natural fibers were selected as the matrix materials of flexible pressure sensors to realize the design and fabrication of the following three sensors.(1)Firstly,a low-cost,disposable paper-based pressure sensor was assembled with pressure-sensitive rubber by using environmentally friendly printing paper as electrode substrate,which is easy to obtain and cut into certain shape.(2)In order to improve the wearing comfort of the sensor,the textile cotton sheet was used as the matrix of the sensitive material,and the surface of the cotton fiber was modified by the in-situ vapor growth of PPy,effectively avoiding the problem of poor structural stability exists in the method of combining conductive particles with fibers through physical adsorption.(3)Aiming at the instability of the initial resistance of sensors caused by the poor contact between electrodes and sensitive layers,a flexible pressure sensor with paper/cotton fiber composite structure based on asymmetric electronic transmission structure was designed to make the initial resistance stable and regulable.2.In view of the difficulty in regulating the size and structure of natural fibers,the electrospinning technique was used to realize the nano-structure of textile fibers and the controllable construction of fiber network,the ultra-light and ultra-thin allnanofiber electronic skin was obtained.An interpenetrating network composed of conductive fiber(PVP@PPy)and insulating fiber(PAN)was fabricated through precisely regulating the growth site of PPy,which had made a breakthrough in the sensitivity of resistive pressure sensors,and provided a new strategy for the construction of interpenetrating networks of heterogeneous nanofibers.Moreover,a capacitive pressure sensor was constructed by introducing ionic liquid in the sensitive layer,the sensitivity of the sensor is greatly improved while reducing the power consumption.In addition,the nanofiber network constructed by electrospinning process provides channels for water-heat transmission,and tests had been performed to prove that the sensor has excellent ability of air permeation and heat dissipation,and can avoid discomfort or skin inflammation caused by long-term wearing.3.In order to solve the problems of poor multi-unit integrating ability and bending failure of gel-type and thin-film self-healing flexible sensors,an electronic skin with self-healing ability of both the substrate and functional material was designed and implemented.The biocompatible cellulose nanofiber/polyvinyl alcohol(CNF/PVA)composite was chosen as the binder of self-healing substrate and functional material,which was compounded with carbon nanomaterials to realize the construction of selfhealing strain,temperature,humidity sensing unit and multifunctional electronic skin.Water was used as the only solvent in the device's fabrication process as well as the trigger of the self-healing function,avoiding the damage on human health caused by toxic and harmful solvent residues.In the application section,the electronic skin was combined with the Bluetooth test module,and the data was transmitted wirelessly to the smartphone for display and processing,so as to realize the real-time monitoring of human physiological signals,environmental temperature,and environmental humidity.4.The flexible pressure sensors were applied to monitor human physiological signals and motion signals.These pressure sensors constructed in the aforementioned researches were used to monitor the movement of joints and muscles at different positions,as well as a variety of physiological signals such as pulse,breathing,swallowing,cough,blinking and vocal cord vibration according to their different sensitivity performances and structural features.Moreover,as the further expansion of the application in pulse monitoring,a wireless test and transmission system was designed and built to display,analyze and store the pulse wave signal collected by the pressure sensors on the smart phone terminal.Finally,based on the performance and application of a single sensor unit,pressure sensor array integrated by multiple sensor units was designed and fabricated to realize the detection of spatial pressure distribution.