Construction And Strain-insensitive Performance Of Bionic Polypyrrole-based Composite Fiber With Hierarchical Wrinkles Structure

With the large-scale development of the health industry,flexible electronics have replaced traditional silicon-based electronics with high Young's modulus as a hot topic in the scientific community and are widely used in personal health monitoring,electronic skin,implantable devices and wearable thermal therapy devices.As key components in flexible stretchable electronic devices,the design of stretchable structure of stretchable conductors is a new idea to realize the stable conductivity under large deformations.Nowadays,the existing stretchable structures mainly include spiral structure,wrinkle structure,serpentine structure,Kirigami-patterned shape,negative Poisson's ratio structure.Among them,the wrinkle structure fabricated in pre-strain strategy is widely applied for its advantages of the low cost,simple process and excellent performance.However most stretchable conductors with wrinkle structures were prepared by applying the pre-strain strategy to construct relatively rigid wrinkles on an elastic substrate,which may lead to large hysteresis due to the dramatical difference in mechanical properties between the conductive layer and the elastic core layer.Herein,inspired by the unique shape of maple leaf,a conductive fiber with hierarchical wrinkles was developed by using the surface modification,interfacial polymerization and modified pre-strain finishing methods to construct waterborne polyurethane(WPU)@polypyrrole(PPy)wrinkles on an elastic substrate to overcome the shortcomings of the existing wrinkles structure,developing a stretchable conductor with both excellent strain-insensitive performance and low hysteresis via adjustment and control of structure.Our study was carried out as follows:(1)A worm-shaped strain-insensitive conductive fiber with unitary macro wrinkles was developed using surface modification,interfacial polymerization and one-step pre-strain finishing method to construct WPU@PPy wrinkles on an elastic substrate,and the coating thickness,wrinkles density and conductivity of the fiber were optimized by adjusting the coating times of WPU and the polymerization time of PPy.Subsequently,the surface morphology,mechanical properties,electrical conductivity and strain-insensitive performance of the worm-shaped strain-insensitive conductive fibers fabricated by different pre-strain were systematically investigated.The results exhibited that the curve of the lower rising rate of relative resistance change corresponds to the higher prestrain a.Then the hysteresis and long-term durability along with surface morphology changes during stretching of the PU₃₀₀@WPU@PPy fibers possessing the optimal strain-insensitive performance were characterized.(2)Based on the aforementioned exploratory parameters,a maple leaf-inspired hierarchically wrinkled conductive fiber(HWCF)was developed by using two-step pre-strain finishing method to construct PPy micro wrinkles on the WPU elastic wrinkles.With the purpose to interpret the mechanism during the formation of the hierarchical wrinkles,buckling modes were simulated in FEM with the commercial software COMSOL Multiphysics.Then we kept the prestrain of b=300%unchanged and regulated wrinkles density by altering the prestrain of a.The mechanical properties,electrical conductivity and thermal properties,strain-insensitive properties along with dynamic conductivity of the PU₀@WPU@PPy fiber,PU₃₀₀@WPU@PPy fiber and PU₁₅₀@WPU₃₀₀@PPy fiber were then compared.Furthermore,the mechanism of outstanding strain-insensitive performance of PU₁₅₀@WPU₃₀₀@PPy fiber could be explained with the buckling efficiency,defined as ratio of effective extension length to prestrain.And the soaping resistance and friction resistance of the PU₁₅₀@WPU₃₀₀@PPy fiber were analyzed.The PU₁₅₀@WPU₃₀₀@PPy fiber exhibited the mild behavior of the relative resistance change versus the strain,significantly indicating the outstanding strain-insensitive performance as compared to the PU₃₀₀@WPU@PPy fiber(?8 times),and it exhibited a wonderful quality factor(Q)value(9.09)even though it was in a state of high strain(600%)as compared to many recently reported stretchable conductors based on spirally structure,buckling structure,worm-shaped structure and so on.(3)Finally,the applications as stretchable and bendable electrical circuits of the HWCF and their applitions in the smart wearable field were explored.Benefited from the bionic structure design,the LED was still illuminating even though the strain of the stretched fiber was as high as 600%,and it still proved a stable and periodic monitoring property even though the relative resistance change was as minimal as 0.05 during the bending-releasing cycles of the finger,wrist and elbow joints.In addition,the 1D HWCF with high degrees of freedom can be integrated into 2D complanate fabric or 3D stretchable tubular conductor employing embroidery or braiding,knitting and weaving processes,and the polypyrrole-based conductive fibers have good biocompatibility and will not cause damage to the skin as a wearable device.