Control Of The Internal Structure Of Electrospun Nanofibers And Its Application In Strain Sensor

Throughout the ages,things in nature have given us countless inspirations,especially some plants and animals with hierarchical structure.Spider silk,as a typical representative of the micro-level hierarchical structure in natural materials,it has the advantages of lightweight,elastic,extensibility,UV resistance,biocompatibility,and biodegradability,and especially has the well-known remarkable mechanical properties,which is unmatched by any industrial materials.This is closely related to the specific spinning mechanism of spiders.It is an important issue to investigate the reasons for the remarkable mechanical properties of spider silk and study the spinning mechanism of spiders,and then how to use this mechanism in the electrospinning process to improve some properties of nanofiber membranes.This thesis first studied the spinning mechanism of spiders and the structure of spider silk.Spider silk has a typical hierarchical structure,from the beginning ordered arrangement of ?-sheets nanocrystals,to the protein nanocomposite structure,then the nanofibers,and finally the spider silk.The composite and synergistic effect of such a hierarchical structure renders spider silk excellent properties,especially the outstanding mechanical properties.Besides,during spider spinning process,the spinning solution goes through a long tube,which is very important to the orientations and conformations of macromolecule chains,thus affecting the mechanical properties of spider silk.Hall-Petch effect,which is also known as the grain-boundary strengthening,is widely applied in materials science.As a special case of the Hall-Petch effect,the nano-effect can be used to clearly explain the reason why the spider silk with a hierarchical structure has remarkable mechanical properties.Then,starting from the specific spinning mechanism of spiders,the spinning solution flowed through a relatively long tube without clogging.It is due to the flow state of the spinning solution in the tube being laminar fluid.Thus,the macromolecular chains in the spinning solution can move forward in an orderly manner,reducing mutual resistance.The combination of laminar fluid,shear forces,pH changes,and other causes cause changes in the orientation and conformation of the macromolecule chains arrangement and ultimately make spider silk have extremely strong mechanical properties.Therefore,this thesis imitates the long tube of spider spinning,combines electrospinning and chooses longer needles in the electrospinning process,uses the laminar fluid theory to control the internal structure of nanofibers.According to the the velocity distribution of laminar fluid in laminar flow theory,the macromolecule chains will be gradually straightened during the movement in the needle,and will gradually be aligned alone the steamline under the effect of pressure.As the length of the needle in the electrospinning device becomes longer,more macromolecule chains will have sufficient time to be straightened and aligned.Therefore,the internal structure of the prepared nanofibers will be more and more ordered.However,the more ordered the internal structure of the nanofibers,the worse the mechanical property of the nanofiber membrane,but its electrical resistance will decrease.After that,in order to improve the mechanical property of nanofiber membrane prepared by electrospinning,an air vortex electrospinning device were designed inspired by the vortex spinning.In this thesis,for the first time,vortex was used in the electrospinning process,and the formed vortex was used to control the internal structure of nanofibers prepared by electrospinning,thereby improving the mechanical property of the nanofiber membrane.The effect of the vortex on the internal structure of nanofibers and the interaction between nanofibers was investigated by changing the intensity of the vortex.When the velocity of the pumped air flow is small,the intensity of the formed vortex is weak.At this time,the weak vortex only has an effect on the micro structure of the jet in electrospinning.The weak vortex causes the micro structures of the jet to entangle with each other,so that the internal structures of the prepared nanofibers are tangled,and the mechanical property of nanofiber membrane has been improved.When the intensity of the vortex increases to a certain value,the entanglement of the internal structures of nanofibers reaches the limit,and at the same time,the nanofibers begin to adhere to each other,forming a large pore size,and the mechanical property of nanofiber membrane will be greatly improved.Thus,this device provides an effective and simple method to prepare nanofiber membranes with high mechanical property and large pore size.By changing the angle of the pumped airflow,it is found that the internal structure entanglement of nanofibers and the adhesion between the nanofibers are the best when the angle between the pumped airflow and the needle is 90°,and the mechanical property of nanofiber membrane is the strongest.Finally,a combination of electrospinning and 3D printing was used to design and prepare a flexible bending sensor.The carbon nanofibers obtained after the heat treatment of nanofibers with ordered internal structure have better electrical conductivity.The hydrogel mixed with carbon nanofibers as a conductive core adhere well to the shell prepared by PDMS.Mixing of carbon nanofibers and hydrogels forms a stable three-dimensional conductive network,thereby controlling the variation of resistance through the contact and separation of the carbon nanofibers.The use of healthy non-toxic and biocompatible PDMS as raw materials and preparing as the shell of sensor via 3D printing widens application range of the sensor.The unique core-shell structure can be reused by replacing the core directly when the core is damaged,which achieves the purpose of sustainable use.At the same time,this sensor shows excellent comprehensive performance in sensitivity,delay,response time,and durability.It's sensing range up to 140o,and the signal remains stable after multiple cycles of bending/release test.Moreover,by adjusting the concentration of carbon nanofibers and the shape of the core channel,different applications of the sensors can be achieved.The sensors are placed in different parts of human body to monitor various body movements in real time,and the results show that the sensor shows excellent performance in monitoring human health.Based on the many advantages of the sensor,it can be used for human health monitoring,medical diagnostics,tactile sensors,humanoid robots,and wearable artificial skin,etc.,and has broad application prospects in the field of wearable electronics in the future.