Low-temperature 3D Printing Preparation Of Highly Sensitive Conductive Hydrogels With Bionic Structures And Investigation Of Their Conductivity-sensing Mechanisms

With the rapid development of modern science and technology and electronic information technology,flexible strain sensors,with their unique advantages of flexible deformation and many degrees of freedom,are now widely used in human health detection,electronic skin,implantable devices,soft robots,motion detection and other fields.Piezoresistive flexible strain sensors have been the main application form of flexible strain sensors because of their low cost,simple design and preparation,high sensing sensitivity,wide detection range,low energy consumption,and low difficulty in integrating and outputting data.As a typical piezoresistive flexible sensing material,conductive hydrogel combines the electrochemical properties of conductive polymer materials with the softness and biocompatibility of hydrogel,which is an ideal material for constructing highly sophisticated flexible electronic products such as wearable electronic devices and tissue engineering,and has become an important direction for the rapid development and application of piezoresistive flexible strain sensor materials.The wide application of conductive hydrogels also puts forward higher and more complex requirements on their mechanical properties,conductivesensing functions,and structural forms,such as high strength,high conductivity,high sensitivity and linearity,wide strain sensing range,high durability and reliability,high molding accuracy,and so on.Therefore,how to balance the mechanical,functional and structural properties of conductive hydrogels has become a bottleneck problem to be solved in the design and preparation of conductive hydrogel-based piezoresistive flexible strain sensing materials.3D printing technology with material universality,convenient customizability and high efficiency of continuous production is now widely used in the field of flexible sensing materials manufacturing,and its open path structure design provides an important technical basis to innovatively solve the key problems of conductive hydrogel in design and preparation.The chelate part of the mantis shrimp is characterized by high mechanical strength and high fatigue resistance.The chelate is composed of highly mineralized hydroxyapatite,mineralized chitinous fibers,and mineralized α-chitinous fibers,and constitutes an interpenetrating bicontinuous network of organic and inorganic phases,with an overall high-density layered form and an internal helical structure,forming the material and structural basis for high mechanical properties.The hierarchical spiral structural feature of the chelate provides effective technical insight for enhancing the mechanical strength of 3D-printed conductive hydrogels through a structural design method.In this thesis,polyvinyl alcohol and sodium lignosulfonate are used as 3D printing conductive hydrogel base material system.Based on the characteristic that the rheological properties of the system change with decreasing temperature,the low-temperature 3D printing technology,is innovatively adopted and the hierarchical spiral structure of chelate is applied to the low-temperature 3D printing path design to realize the low-temperature 3D printing of conductive hydrogel matrix material(sodium lignosulfonate/polyvinyl alcohol composite hydrogel)with high mechanical strength and multi-structural form characteristics.Based on the permeability of the matrix material to silver ions,the dense silver elemental particles layer formed on the outer surface of the hydrogel by reduction reaction realizes the function of high electrical conductivity and high sensing sensitivity of low-temperature 3D printing bionic structured conductive hydrogel.By analyzing the effect of conductive process parameters on the microscopic morphology of the silver layer and the effect of the printed structure on the upper surface on the sensing performance,the conductive mechanism and sensing mechanism of low-temperature 3D printing bionic structure of highly sensitive conductive hydrogel are revealed,providing a new and effective method to solve the bottleneck problem of balancing mechanical,functional and structural properties of conductive hydrogel.The specific research and main conclusions of this thesis are as follows:(1)Sodium lignosulfonate/polyvinyl alcohol composite hydrogels are synthesized by freeze-thaw cycling and solvent replacement,and silver elemental particles are reduced in situ on their surface by a conductive process to verify the effectiveness of the conductive process and low-temperature 3D printing highly sensitive conductive hydrogel matrix materials’ synthesis process,revealing the optimal composition ratio of matrix materials and the optimal conductive process parameters,based on which,realizing conductive-sensing functions such as high conductivity,high sensitivity,wide strain sensing range,high linearity,and effective monitoring of subtle vibrations.(2)Self-encoded structural features of the hierarchical layer spiral of shrimp chelate are designed as structural models for low-temperature 3D printing paths.The feasibility of lowtemperature 3D printing of the sodium lignosulfonate/polyvinyl alcohol hydrogel reaction solution is verified based on the gradual increase of its viscosity and the existence of sol-gel transition point with decreasing temperature.On the basis of having highly shaped structural characteristics,the low-temperature 3D printing bionic structured conductive hydrogel matrix has the highest tensile strength when the angle of adjacent layers of the low-temperature 3D printing path structure is 30°,which verifies the feasibility of further enhancing the mechanical strength of the low-temperature 3D printing highly sensitive conductive hydrogel matrix from the structural design perspective and reveals the strength gain mechanism.(3)The low-temperature 3D printing bionic structured highly sensitive conductive hydrogel matrixes can be reduced by conductive process with in situ silver elemental particle layer on their surface,which verifies the validity and feasibility of the process logic of lowtemperature 3D printing followed by conductive functionalization.The printed path structures on the upper surface of low-temperature 3D printing bionic structured highly sensitive conductive hydrogel have a significant effect on their sensing performance.The lowtemperature 3D printing bionic structured highly sensitive conductive hydrogel has the best linearity when the upper surface printed path structure is 0°;the low-temperature 3D printing bionic structured highly sensitive conductive hydrogel has the widest strain sensing range when the upper surface printed path structure is 90°.The low-temperature 3D printing of bionic structured highly sensitive conductive hydrogels effectively balance mechanical strength and high sensitivity characteristics.(4)The conductive-sensing mechanism is revealed by analyzing the action regularity of conductive process parameters on the microscopic morphology of silver layer on the surface of low-temperature 3D printing bionic structured highly sensitive conductive hydrogel and the microscopic morphological characteristics of silver layer under different strains.The conductive mechanism of low-temperature 3D printing bionic structured highly sensitive conductive hydrogel is the conductive pathway mechanism.When the distance between silver elemental particles is small,adjacent silver elemental particles lap or contact each other,forming efficient electron movement paths,the conductivity of low-temperature 3D printing bionic structured highly sensitive conductive hydrogel is high.When the distance between silver elemental particles is large,adjacent silver elemental particles separate from each other,which hinders the efficient movement of electrons,and the conductivity of the lowtemperature 3D printing bionic structured highly sensitive conductive hydrogel is low.Due to the large modulus difference between the outer surface silver layer and the inner hydrogel of the low-temperature 3D printing highly sensitive conductive hydrogel,the sensing mechanism is the crack extension mechanism.Moreover,the sensing mechanism of low-temperature 3D printing bionic structure highly sensitive conductive hydrogel with different upper surface print path structures is influenced by the specific gravity of the mechanical load on the silver layer at the print path and the silver layer at the gap between adjacent print paths simultaneously.When the silver layer at the print path is mainly subjected to mechanical loads,its relative resistance changes mainly in linear correspondence with the applied strain;when the silver layer at the gap between adjacent print paths is mainly subjected to mechanical loads,its relative resistance changes mainly in nonlinear correspondence with the applied strain.