Auxetic Wearable Sensors Based on Flexible Triboelectric Polymers for Movement Monitoring

Abstract

The methods to enhance the output voltage of triboelectric nanogenerators based on polymeric dielectric materials, such as building micro−nano structure and chemical modification, probably lower the structural uniformity of their devices and then decrease the performance stability because of stress aggregation. Here, we proposed a strategy based on auxetic structures for improving the triboelectric performance, which could concurrently uniformize the stress transfer due to their excellent friction properties. Using the negative Poisson's ratio (NPR) structure of the "I"-shaped structure and rotating polygon structure, we prepared the dielectric layers of polyvinylidene fluoride (PVDF) and copper with controllable Poisson's ratio from 0.84 to −1.51. Then, the finite element analysis proved that the auxetic NPR structure with tri-axial and tetra-axial symmetricity offered more uniform friction−force distribution and thus less fluctuation in a contact area than the "I"-shaped structure. We further designed triboelectric devices with polydimethylsiloxane (PDMS) deformation and triboelectric properties, and they showed that the triboelectric output voltage reached the maximum when their Poisson's ratio was close to 0, which was related to the minimal change in the contact area between the NPR layer and the PDMS layer. The devices were then inserted into wearable sports sensors, which could monitor the movement at different places of bodies and also have great potential in sensorium smart socks and brake sensors.

Highly Flexible and Conductive Electrodes through Combining Honeycomb and Butterfly Pattern Bio‐Inspired Structure for ECG Signal Recording

“…This phenomenon can be explained from two aspects: one is excellent elasticity and good toughness of PA Film, the other one is the stronger shear resistance, fracture toughness and cracks propagation resistance of BSH-NPR structure. [28][29][30] It is particularly worth mentioning here that releasing the strain enables re-joining of electrode contacts (albeit incomplete) to recover conductivity when electrode is stretched until contact points completely separated and is non-conductive. This is enough to demonstrate that flexible electrodes prepared in this paper have certain self-recovery.…”

“…[1][2][3][4][5] However, conductivity of flexible electronic devices can't be guaranteed due to the lack of reliability of existing flexible electrodes under various deformation and many other fields due to unique auxetic properties and outstanding mechanical properties including stronger shear resistance, fracture toughness and cracks propagation resistance, etc. [27][28][29][30][31][32] It is worth mentioning that the NPR structure has been reported many times to have the ability to improve flexible electrode mechanical performance. [33][34][35][36][37] Weng et al compared between thin film electrode and NPR electrode, it is found that the stress level in the NPR electrode is relatively low due to the faster appearance of the inelastic deformation.…”

“…The negative Poisson's ratio (NPR) structure has been successfully employed in flexible electronic, biomedicine, sensor and many other fields due to unique auxetic properties and outstanding mechanical properties including stronger shear resistance, fracture toughness and cracks propagation resistance, etc. [ 27–32 ] It is worth mentioning that the NPR structure has been reported many times to have the ability to improve flexible electrode mechanical performance. [ 33–37 ] Weng et al.…”

Enhancing Output Signals of Sport Monitors Based on Triboelectric Porous PVDF Nanogenerators via Concaving Cells and Cell-Packing Structures

“…39−41 Research has proved that the meta-concave structures of negative Poisson's ratio (NPR) can improve the mechanical properties and output performance of piezoelectric and 2D triboelectric devices. 42 Poly(vinylidene fluoride) (PVDF) is a ferroelectric polymer whose β-phase PVDF has high electron affinity, high spontaneous polarization, and excellent polarization stability and is an excellent TENGs triboelectric material. Therefore, we propose a two-step strategy to concave the cell and cell-packing structure of porous TENGs based on PVDF for improving their triboelectric output and mechanical performance concurrently.…”

“…Fortunately, modification in the structural design of sensors, which can bring higher sensitivity, is free of component change, meaning it can avoid the reduction in mechanical properties induced by interfacial noncompatibility. − Research has proved that the meta-concave structures of negative Poisson’s ratio (NPR) can improve the mechanical properties and output performance of piezoelectric and 2D triboelectric devices . Poly­(vinylidene fluoride) (PVDF) is a ferroelectric polymer whose β-phase PVDF has high electron affinity, high spontaneous polarization, and excellent polarization stability and is an excellent TENGs triboelectric material.…”