Research On Multi-siganl Flexible Sensing Based On Low-dimensional Materials

Flexible sensors,which are flexible and lightweight,have been extensively utilized in various applications,including human health monitoring,human-machine interactions and robotics.Flexible sensors can capture a wealth of health-related physiological signals（e.g.,electrophysiological,physical,and chemical signals）that generated by human body with high accuracy.To improve the performance of flexible sensors,in recent years,low-dimensional nanomaterials have been widely employed as sensing materials of flexible sensors due to their exceptional physicochemical properties,such as electrical,mechanical,optical,and thermal conductivity properties.For instance,MXene,as a two-dimensional nanomaterial,can effectively detect gas molecules by its defect sites and functional groups,which enhances the sensitivity and limit of detection of flexible gas sensors.Despite these significant advances,current flexible sensors are still facing challenges when applied in real-world situations,as they are typically tested under laboratory settings that do not take into account the complexity of actual applications.There are still challenges with flexible sensors in real-world applications,e.g.,（Ⅰ）Low signal-to-noise ratio caused by sweat accumulation and non-conformable contact under multiple environments.（Ⅱ）The trade-off between sensitivity and sensing range limits the full range of human signal sensing.In this thesis,aiming to address these challenges in real-world applications,with the employment of low-dimensional materials,three design strategies（i.e.,asymmetric wettability strategy,ultra-conformal strategy and interlayer distances changed strategy）are proposed to enhance the performance of flexible sensors,including electrophysiological sensors with high signal-to-noise ratio as well as pressure sensors with both high sensitivity and a wide sensing range.The relevant detailed information is as follows:1.Aiming at the low signal-to-noise ratio issue of flexible sensors caused by sweating and breathability mismatch of human skin:Inspired by the active liquid transport phenomenon in nature,an innovative flexible sensor with an asymmetric wettability structure is developed based on silver nanowires/thermoplastic polyurethane/hydrolyzed polyacrylonitrile hybrid nanofibers by electrospinning and drop-casting.Its asymmetric wettability structure has two opposing wettability layers（i.e.,superhydrophilic and hydrophobic layer）,which can transport sweat directionally from the skin-sensor interface to the superhydrophilic layer of the sensor.The transported sweat can spread over the superhydrophilic layer for quick evaporation.Meanwhile,the hydrophobic layer of the sensor blocks the backflow of sweat and keeps the interface dry.By virtue of the large aspect ratio,high electrical conductance and superb mechanical flexibility of silver nanowires,the sensor shows high electrical conductivity without significant blockage of the gas permeable pathways.The high porosity endows the sensor with high water vapor transmission rate(WVTR,20 mg cm^-2 h^-1),which meets the breathable requirements of insensible skin perspiration.The directional sweat transport capability and high WVTR of the proposed sensor effectively address the issues resulting from sweat accumulation,which enhance the stability of the skin-sensor interface.Thus,the sensor can achieve reliable electrophysiological signal sensing with high signal-to-noise ratios,whether the human body is calm,sweating,or the sensor is been worn for a long term.Additionally,the proposed sensor shows the potential to be integrated into an intelligent electrophysiological sensing system for high accuracy and long-term continuous electrophysiological signals monitoring.2.Aiming at the low signal-to-noise ratio issue of flexible sensors caused by poor compliance at skin-sensor interface under multiple environments:Inspired by the hydrophobic-hydrophilic structure of human skin,a bilayer multifunctional hydrogel sensor based on MXene/chitosan/polyacrylic acid/aluminum nitrate is prepared by free radical polymerization and hydrophobic modification.As a two-dimensional material with metallic conductivity and dispersibility,MXene is utilized as a dynamic crosslinking agent instead of covalent crosslinking agents,which results in low modulus（4.5 kPa）and enhances conductivity of the sensor.Furthermore,the sensor also exhibits strong underwater adhesion to the skin（～388 kPa）and low swelling ratio（～3.8%）,which can seamlessly conformal adhere to the skin without water infiltration into the interface.Owing to the low contact noise and the stable skin-sensor interface,the sensor can achieve reliable electrophysiological signals sensing with high signal-to-noise ratio whether in dry,wet or underwater environments.In addition,the hydrophobic layer on surface of the sensor prevents the sensor from being affected by daily liquids and undesired adhesion under handling.Moreover,the sensor can also be utilized as a strain sensor for human motion detection and underwater communication.3.Aiming at the trade-off between sensitivity and working range of flexible sensor that unable to sense the full pressure range generated by human movement:A flexible pressure sensor based on multilayer MXene/cellulose nanofibers（CNF）is fabricated by vacuum filtration method.The composite sensing material,consisting of multilayers of MXene with large layer distances and mechanically flexible CNF,exhibits a hierarchical microstructure that allows continuous deformation over a wide pressure range.As a result,the sensor displays high sensitivity throughout the ultrawide pressure sensing range,which shows a sensitivity of 10.7 kPa^-1 for the first region（≤240 kPa）,34.6kPa^-1 for the second region（240 kPa to 640 kPa）and 16.6 kPa^-1 for the third region（640 kPa to 950kPa）.The high performance of the sensor allows the full range of human motion sensing from weak pressure to high pressure.The sensor also shows transient and biodegradable,which reduces the generation of electronic waste.Furthermore,by combining the pressure sensor with few-layered MXene nanosheets/CNF electrodes,an intelligent health monitoring system is constructed for multi-signals sensing,including sitting posture,sedentary time,ECG signals and heart rate.In summary,this thesis addresses issues and challenges in real-world applications of flexible sensors for health monitoring.Three design strategies for enhancing the performance of flexible sensors are proposed,and corresponding high-performance flexible sensors are fabricated.These strategies provide novel ideas and approaches for the development of flexible sensors.