Structure Design And Performance Study Of The Novel Flexible Electrode With Negative Poisson’s Ratio Characteristics

With the rapid development of flexible electronics,the flexible electrode,it is core component,also become the focus of academia and industry.Currently,flexible electrodes have been widely utilized in wearable electronics,electronic skin,energy storage devices and other fields due to their wearable,light-weight and deformable characteristics.Nonetheless,the conductive layer will often appear structural damage(i.e.fall off or appear micro-cracks under the action of external force)due to the alternating stress under repeated stretching,bending,folding and other load conditions in the process of use,thereby affecting its conductivity and service performance.This structural damage problem has become the main bottleneck restricting the development and application of high-performance flexible electrodes.To overcome this bottleneck,in this paper,from the perspective of mechanics,a new optimization scheme considering both the conductive layer topology configuration and the substrate material of the flexible electrode is proposed to prevent the conductive layer structural damage of the flexible electrode,thereby boosting its conductivity and service performance.Flexible electrodes of conductive layers with different NPR structures are constructed and a series of researches are carried out from the aspects of structural optimization design of conductive layer,material selection of substrate layer,performance testing and application of the device.The main research contents and results are as follows:(1)To boost the tensile,bending and fatigue properties of the conductive layer of the flexible electrode,the inner-concave honeycomb negative Poisson’s ratio(I-CHNPR)structures are first introduced into the conductive layer of the flexible electrode based on the topological optimization result of island-bridge(I-B)structure.A novel I-CHNPR structure-C flexible electrode with excellent mechanical properties is obtained by further improvement of the structure.Polyamide(PA)thin-film substrate flexible electrodes with I-B structure and different I-CHNPR structure conductive layers are fabricated by screen printing technology.Relevant performance analysis results show that the maximum stretching stress and bending stress of flexible electrodes with I-CHNPR structure-C are reduced by 30 and 28%,respectively,and the fatigue life is boosted by 18-fold relative to the existing I-B structure.Flexible electrode with I-CHNPR structure-C indicates good conductivity under different bending,tensile and fatigue load conditions due to its excellent mechanical properties,and it has advantages of wide application range and reuses many times in the monitoring of human electrocardiogram(ECG)signals.(2)To further explore the effect of the conductive layer structure on the mechanical properties of the flexible electrode,a U-shaped NPR structure flexible electrode with excellent mechanical properties and good conductivity is developed,so that it can adapt to more complex operating conditions and environments.The related properties of PA thin-film flexible electrodes with chiral structure,anti-chiral structure and U-shaped structure conductive layer are compared and analyzed.The results show that the optimized flexible electrode with U-shaped NPR structure can effectively improve the mechanical properties of the electrode compared with the chiral NPR structure flexible electrode(the maximum stretching and bending stresses are reduced by 67 % and 71 %,respectively).The resistance of flexible electrode with U-shaped NPR structure is only 2.3 Ω under 180° bending load,4.9Ω after 1000 fatigue cycles and 20.2 Ω at 20% strain,and it still has conductivity at 40%strain,exhibiting excellent tensile,bending and fatigue properties.In addition,the increase of preheating process in screen printing greatly improves the printing quality and efficiency of the electrode.Compared with conventional wet electrodes,the ECG signals from the flexible electrode with U-shaped in plane and curve surface environments exhibit more distinct signal characteristics and larger amplitude SNR(signal-to-noise ratio)and can be monitored for long-term monitoring(SNR can reach 34.9 d B at 30 min).(3)On the basis of verifying that the structural optimization of conductive layers can effectively improve the mechanical properties of flexible electrodes,the influences of PI thin-film,thin PET thin-film and PA thin-film substrate materials on the mechanical properties of flexible electrodes are explored,and a new idea of combinatorial bionic design is proposed and verified.Based on this,a new flexible electrode with butterfly-shaped honeycomb(BSH)negative Poisson’s ratio(NPR)structure through combining honeycomb and butterfly pattern bio-inspired structure is designed.Relevant analysis results show that combined bionic thinking can effectively enhance the mechanical properties of the conductive layer structure.Compared to the other two thin films BSH-NPR flexible electrodes,the BSH-NPR structure flexible electrode with PA thin-film substrate has higher bending-resistance(resistance is only 4.1 Ω at 180°),fatigue-resistance(only 4.9 Ω after 1000cycles),tensile-resistance(only 20.3 Ω at 20% strain)and certain self-recovery(releasing strain after fracture enables re-conductive),demonstrating excellent matching between PA thin film and BSH-NPR structure conductive layer.Compared with the rigid electrodes and commercial wet electrodes,the BSH-NPR flexible electrode with PA thin-film substrate exhibits a smaller electrode-skin contact impedance,and can monitor the ECG signal of the human body for a long time under the bending and repeated bending(dynamic)environment.The SNR is 30.5 d B at 30 min and 31.2 d B at 60 min,which is far more than the other two types of electrodes.To sum up,this paper adopts the structural optimization method to solve the structural damage faced by the conductive layer of flexible electrodes as the goal,and reveals the advantages of conductive layer structure with different NPR characteristics in improving the performance of the flexible electrode.On this basis,BSH-NPR structure flexible electrodes with different thin-film substrates are designed,which verified the feasibility of considering both the conductive layer topology configuration and the substrate material of the flexible electrode.Simultaneously,the flexible electrodes with different negative Poisson’s ratio structures are utilized to monitor human ECG signals under different conditions and environments,and the advantages of its performance are verified.The strategy proposed in this paper has certain guiding significance for the development and application of high performance flexible electrodes.