Construction And Application Of Flexible Wearable Multimode Mechanical Sensors Based On Nanofiber Textiles

In the emerging field of wearable electronics,flexible wearable mechanical sensors have attracted increasing attention due to their function of mimicking human skin to transform external mechanical stimuli into processable electrical signals,thereby enabling great potential applications in mobile medical monitoring,human motion detection,soft robot and human-machine interfaces and human-machine interaction.Smart wearable devices need to be in direct contact with human skin,commoditised textiles are the best product form and responsible platform to develop smart wearable sensors.In recent years,the development of wearable sensors with high sensitivity,fast respond speed,and wide sensing range based on advanced materials and structural designs is highly desirable according to the demand of monitoring personal health signals and environmental changes.Textiles render three-dimensional porous mesh interlace structure consisting of numerous slender fibers and yarns.These features of textile facilitate it to incorporate different kinds of functional nanomaterials.Depositing or embedding conductive nanomaterials such as carbon nanotubes?CNTs?,graphene,metal nanoparticles and nanowires onto traditional textile fiber substrates is the common approach to achieve fiber functionalization,thereby enabling the construction of the textile-based wearable sensors.Electrospun nanofibers have the dual advantages of nanomaterials and fibrous materials.Due to their unique merits of fine diameter,large surface area,controllable structures in conjunction with easy doping and functional modification,electrospun nanofibers as an ideal material have been widely used in the various intelligent wearable fields,such as flexible sensors,electronic skin,photoelectric devices,energy collection and storage.In this thesis,based on the remarkable advantages of nanofibers and textiles in material properties and structural engineering,a series of wearable multimode mechanical sensors based on nanofiber membranes,yarns and fabrics were designed and constructed by using elastic porous electrospun PU nanofibers as flexible substrates and conductive polymer,carbon nanotubes and graphene as sensing elements.We also studied the sensing performance of these as-prepared wearable sensors in sensing different mechanical stimuli,including pressure,tension and bending,and environmental stimuli,such as temperature,further exploring their wearable application performance in monitoring human health and motions.This thesis provided some new strategies for the development of wearable electronic devices and smart textiles.The main research contents of this thesis include the following aspects:?1?Firstly,stretchable conductive nanofiber membrane with core-shell structure was designed and prepared by using the electrospun GO-doped PU nanofiber membrane with three-dimensional elastic porous structure as the flexible substrate and in situ polymerization of conductive polymer PEDOT with thermoelectric performance.Then,a stretchable multimode mechanical sensor based on PEDOT conductive nanofiber membrane was constructed.The three-dimensional porous elastic composite nanofibrous substrate and the continuous self-assembled conductive pathway offer more contact sites,a larger deformation space,and a reversible capacity for pressure and strain sensing,which provide multimodal mechanical sensing capabilities with high sensitivity and a wide sensing range.The nanofiber-based sensor demonstrates a high-pressure sensitivity(up to 20.6 kPa^-1),a broad pressure sensing range?1 Pa-20 kPa?,excellent cycling stability and durability?over 10,000cycles?,as well as a high strain sensitivity?Gauge factor,GF 10.1-193.2?over a wide tension strain range?0.5%-550%?and good cycling stability.The nanofiber-based sensor also has a highly sensitive and stable response to bending strain.Finally,the nanofiber-based sensor has been proved to be a wearable electronic skin for pulse monitoring,expression and speech recognition,and human movement and touch monitoring.?2?Secondly,based on the advantages of weavable,wearable and diverse structural design of yarn-based flexible sensors,a weavable functional nanofiber core-spun yarn was designed and prepared by coating the functional nanofiber structures on the surface of the flexible yarn as the sensor platform.The weavable nanofiber core-spun yarn was obtained by coating a flexible Ni-coated cotton yarn with CNTs-doped PU nanofibers using a simple conjugate electrospinning technique.Then,a high-sensitive wearable textile pressure sensor was constructed by using textile weaving technology.Among them,the flexible Ni-coated cotton yarn was used as the sensing electrode,and the CNTs-doped PU nanofiber layer was used as the force-sensitive piezoresistive sensing layer.In our design,one dimensional coaxial structure of nanofiber core-spun yarn,the hierarchical fiber-bundled structure of the conductive Ni-coated cotton yarn electrode and three-dimensional elastic porous nanofiber structure of the force-sensing layer provide the sensor with a relatively large surface area,and a sufficient surface roughness and elasticity.This leads to rapid and sharp increases in the contact area under stimuli with low external pressure.As a result,the textile pressure sensor exhibited the advantages of a high-pressure sensitivity(16.52 N^-1),wide sensing range?0.003-5 N?,short response time??0.03 s?and good cycling stability.In addition,based on the one dimensional flexible weavable yarn structure,this textile sensor can be worn on the body for non-invasive human motion and physiological signal monitoring and the detection of dynamic tactile stimuli.?3?Thirdly,in order to optimize the structure of the above-mentioned textile-based wearable sensor,a stretchable multimodal textile sensor was further designed and developed by introducing a high stretchable conductive electrode.With a high stretchable fiber-shaped electrode coated with CNTs as the core yarn,a stretchable nanofiber core-spun yarn was developed by coating the stretchable core yarn with CNTs-doped PU nanofibers using a simple conjugate electrospinning technique.Then,a stretchable multimodal textile mechanical sensor was constructed by using textile weaving technology.The delicate hierarchical structure of this textile sensor ranges from the 1D macroscopic yarns to the submicron elastic nanofibers to the internal nanoscale CNTs percolation networks.This structure offers a relatively larger contact area,multiple contact sites and larger deformation space,which enabling the obtained textile sensor with a high-pressure sensitivity(12.3 N^-1),a broad sensing range??0.001-5 N?,and excellent cycling stability.Meanwhile,thanks to the highly stretchable conductive electrode substrate and one-dimensional core-sheath structure of the nanofiber core-spun yarn,this textile sensor exhibited high strain sensitivity with the average GF from114 to 720 and a larger stretching sensing range?0.1%-220%?as well as high sensitive bending sensing performance.In addition,based on the different structural deformation induced by different mechanical stimuli,the textile sensor can distinguish various mechanical stimuli by different resistance responses.Thanks to the merits,this textile sensor can be conformally attached on the human skin or integrate into smart textile for monitoring the human movements and the physiological signals as well as spatially mapping the pressure and strain distribution.?4?Finally,in order to further realize the multifunction integration and structure optimization of the textile sensor,a simple new method of simultaneous conjugate electrospinning and electrostatic spraying was proposed.In combination with chemical reduction technology,the conductive and temperature-sensitive reduced graphene oxide?RGO?flakes were embedded into the elastic PU nanofiber matrix,and then a stretchable RGO/PU composite nanofiber yarn with temperature and strain sensing performance was innovatively designed and prepared.A temperature-sensitive stretchable yarn/textile multimode mechanical sensor was constructed by using textile weaving technology to integrate into three-dimensional smart textile.The elastic PU nanofibers and RGO flakes were wrapped around each other to form an efficient and stable three-dimensional conductive network structure,which endows the yarn sensor with excellent stretchability and conductivity.The yarn sensor demonstrated excellent temperature sensitivity of up to1.78%/in the temperature range of 20 to 70 and can quickly and stably detect surface temperature changes of contact objects as well as ambient temperature changes such as hand proximity,breath and ice proximity.Meanwhile,the contact area between RGO flakes in elastic PU nanofiber matrix was changed according to the applied tensile strain.The yarn sensor exhibited high strain sensitivity with average GF from 21.5 to 5865 in the tensile strain range of 0.1%to 300%and excellent cyclic stability.The yarn sensor was successfully applied to monitor small muscle movements such as gesture changes and vigorous joint movements such as walking.Finally,the textile sensor prepared by weaving techniques showed excellent wearable and high sensitive performance,can monitor the subtle pressure stimulation in daily life,such as touching,wrist bending motions and expiratory stimulation coupled with temperature and pressure signals,which has great application potential in the field of wearable smart textiles and flexible electronic devices.