Carboxylated Polystyrene Conducting Nanospheres For Constructing Fiber-based Piezoresistive Sensers And Performances

As one of the core components of flexible wearable devices,resistive sensors are favored in the field of human activity and physiological monitoring because of their simple structure,low energy consumption and easy readout mechanism.The resistive sensor is mainly composed of substrate,conductive material and electrode material,and its research focuses on improving the sensitivity of the sensor.The construction of microstructure on the surface of flexible film is one of the conventional methods to prepare highly sensitive piezoresistive sensing composites,but it often requires the use of relatively complex template method and etching method to complete the construction of microstructure.Compared with elastic film,fiber material is very popular in the construction of resistance sensor for human activity and physiological monitoring because of its high flexibility and easy forming.Its application mainly involves pressure sensing,strain sensing and humidity sensing.However,there are relatively few studies on the construction of microstructure on the surface of fiber materials to prepare high-performance piezoresistive sensing materials.At the same time,the resistive sensor is gradually showing a trend of multi-functional development.It has certain scientific research and practical application value to realize the multi-functional monitoring of human activity and physiological signals by cooperating with other conductive materials after designing the microstructure of the fiber surface.Using thermoplastic polyurethane electrospun fiber film（Thermoplastic polyurethane electrospun membrane,TPUEM）as substrate,a series of fiber-based piezoresistive sensors with multi-layer microstructure conductive network are constructed by combining carboxylated polystyrene nano-conductive spheres with graphene,carbon nanotubes and Ti₃C₂Tx MXene as conductive materials.Their applications and mechanisms in the fields of pressure sensing,pressure/strain sensing,pressure/humidity sensing and electromagnetic shielding/pressure sensing are explored respectively,which provides some new ideas for the design of multi-functional fiber-based flexible wearable devices.Firstly,a kind of carboxylated polystyrene（Poly（styrene-co-methacrylic acid）,Poly（St-co-MAA））nanospheres is prepared,and its effects on the particle size and polymer dispersity index（PDI）of Poly（St-co-MAA）nanospheres are investigated by adjusting the concentration of Styrene（St）,Methacrylic acid（MAA）,Ammonium persulfate（APS）and stirring rate.The results show that monodisperse Poly（St-co-MAA）nanospheres with an average particle size of 237.6 nm（PDI=0.082）can be prepared when the concentrations of St,MAA and APS are114.6 g/L,17.2 g/L and 0.75 g/L,respectively,the stirring rate is 350 r/min,70 ^oC and 8 h in nitrogen environment.Since the surface potential of Poly（St-co-MAA）nanospheres is-36.24m V,a series of Poly（St-co-MAA）@Polyaniline（PANI）and Poly（St-co-MAA）@polypyrrole（PPy）nanospheres are prepared by electrostatic adsorption and in-situ polymerization.Under the same process conditions,the conductivity of Poly（St-co-MAA）@PPy nanospheres is better than that of Poly（St-co-MAA）@PANI nanospheres,showing a lower percolation threshold（1.11 vol%）.When the molar ratio of Poly（St-co-MAA）nanospheres to pyrrole monomers is 20:10 in the in-situ polymerization,the surface of Poly（St-co-MAA）@PPy（20:10）nanospheres（PPNs）is completely coated by dense PPy.PPNs is selected to combine graphene,carbon nanotubes and MXene to construct different piezoresistive sensors.To investigate the role of PPNs adsorbed on the surface of reduced graphene oxide（rGO）in enhancing the comprehensive performance of pressure sensors,TPUEM is used as the substrate and homemade graphene oxide（GO）is selected as the precursor and reduced after adsorption on the surface of TPU electrospun fibers.The prepared reduced graphene oxide（rGO）/TPUEM has a typical hierarchical conductive network structure.The rGO/TPUEM is impregnated in PPNs ethanol dispersion for ultrasonic treatment,and PPNs is adsorbed on the surface of rGO/TPU electrospun fibers.During the compression process of PPNs/rGO/TPUEM,the resistance of the composite decreased by changing the contact point density and contact area between PPNs and rGO.The PPNs/rGO/TPUEM piezoresistive pressure sensor exhibits the highest sensitivity of 13.65 k Pa^-1 and the fastest response time of 37 ms in the pressure range of 0～62.50 k Pa under the working voltage of 1.0V,which indicates that the conductive structure constructed by using PPNs in synergy with rGO inside TPUEM can achieve a balance between high sensitivity,wide sensing range,and fast response.In addition,the pressure sensor can effectively monitor the signals generated by human exhalation intensity,wrist pulse and grasping objects.In the process of using pressure sensors to monitor human activities and physiological signals,strain sensors are also needed to supplement,especially in the monitoring of human joint motion.Therefore,it is of great significance to prepare a high-performance composite material which can take into account both pressure sensing and strain sensing.TPUEM is an ideal substrate for the preparation of pressure/strain sensing materials due to its excellent compressive and tensile properties.The homemade carboxy-modified carbon nanotubes（c-MWCNTs）are anchored on the surface of TPU electrospun fibers using ultrasonic treatment to form a dense conductive network.Then PPNs/c-MWCNTs/TPUEM composites are prepared by vacuum pumping different concentrations of PPNs ethanol dispersions into c-MWCNTs/TPUEM.PPNs and c-MWCNTs formed a multi-layer level conductive microscopic network structure on the surface of TPU electrospun fibers.The composite is assembled into a pressure sensor with a sensitivity value of up to 14.16 k Pa^-1 in the pressure range of 8.18 k Pa to 27.68 k Pa and a sensing time of 223 ms.The PPNs/c-MWCNTs/TPUEM composite is assembled into a strain sensor by coating it with a low concentration of PDMS with a GF of up to 2.03 in the strain range of 80.0%,and the strain sensor exhibited stable strain sensing performance even after cyclic stretching in the strain range of 50.0%.At the same time,the strain sensor exhibits a certain degree of hydrophobicity（Contact angle,CA=138.6^o）due to the multilayer structure of the surface and PDMS,which helps to reduce the effect of human sweat or humid environment on its strain sensing performance.The humidity sensor can be an effective complement to pressure sensor-based respiratory state monitoring during human inhalation and exhalation,thus providing a more comprehensive strategy for monitoring human activity and physiological information during human movement and physiological signals.The PPNs ethanol dispersion is vacuum pumped into TPUEM,and a large number of PPNs are adsorbed on the fiber surface to form a microstructure similar to octopus tentacles.The homemade MXene dispersion is sprayed on the PPNs/TPUEM surface to prepare MXene/PPNs/TPUEM composite.The pressure sensor assembled by this composite shows a sensitivity of 16.72 k Pa^-1 and a response time of 145 ms in the pressure range of 0 to 32.1 k Pa.The pressure sensor can be used to monitor human respiration,wrist pulse,finger clicking,joint motion and sit-up motion monitoring.Benefiting from the large number of micro-gaps between MXene and PPNs in MXene/PPNs/TPUEM and the good humidity responsiveness of MXene,the humidity sensor assembled by MXene/PPNs/TPUEM shows fast humidity sensing characteristics（response/recovery time=1.69 s/0.81s）and can monitor humidity changes during human exhalation.The complex circuits and miniature electrical components used by piezoresistive pressure sensors in the application of human activity and physiological information monitoring will produce certain electromagnetic radiation,which is not only harmful to human health,but also interfere with the normal operation and service life of the sensing equipment.Inspired by the pod structure,a kind of TPUEM composite material（MXene/PPNs/MXene/TPUEM）with MXene/PPNs/MXene sandwich structure is prepared by vacuum filtration and spraying,which shows a certain electromagnetic shielding function（SE_T=20.2d B）.At the same time,the pressure sensor assembled by MXene/PPNs/MXene/TPUEM displays a sensitivity of 177.3 k Pa^-1 in the range of 2.0 to 8.5 k Pa.It also has the ability to monitor human joint motion,speech recognition,wrist pulse and spatial distribution of tactile stimuli.The method of constructing MXene/PPNs/MXene sandwich structure proposed in this research work can be extended to other textile materials to prepare electromagnetic shielding/pressure sensing composites and devices.