Double-helix multilayered tribo-metamaterials (DH-MTMs) for self-powered wireless monitoring systems

Because of the rapid development of sensing technologies in recent years, marine health monitoring has been considered as a promising solution to accurately identify working conditions, monitor structural damages, or make rational assessments over the safety, durability, and suitability of marine structures. Advanced sensing technologies form the backbone of marine health monitoring. Triboelectric nanogenerators (TENGs) have received a wide range of attention because of their performance in continuously and accurately self-powered sensing in different fields. One of the recent research interests is to improve their sensing response based on the structural strategy of mechanical metamaterials (i.e., controlling mechanoelectrical characteristics by structural design of periodically microunits). Tribo-metamaterials have been reported as effective devices for energy harvesting and marine health monitoring. Owing to their well controllability, wide range of material selection, high sensitivity, and simple fabrication, tribo-metamaterials have particularly been proposed for monitoring ocean engineering structures.

The DH-MTMs are designed with the double-helix structures to increase the contact area for higher sensitivity and monitoring accuracy, which are particularly consisted of multiple composite layers, including the conductive layer fabricated by the PLA-CB and the dielectric layer by the TPU. Each helix component is fabricated using the all-in-one 3D printing technology with the 2 nozzles that extrude the conductive and dielectric materials at the same time. The scanning electron microscope (SEM) images of the PLA-CB material shows that the PLA material (blue) and the CB material (yellow) are well mixed together, which gives the PLA-CB unique conductive properties.

The DH-MTMs are integrated into the wireless monitoring system for water flow and wind turbine anchor chain monitoring. The wireless monitoring system consists of several modules such as sensors (DH-MTMs), data acquisition, MCU, wireless data transmission, and data visualization. The sensors module is composed of the DH-MTMs, which efficiently convert mechanical excitations into electronic response signals. We test the monitoring effect of the DH-MTMs with the water flow velocities of 0.1, 0.2, 0.3, and 0.4 m/s. With the increase in the water flow velocity, the monitoring results gradually increase and remain stable over a period of 120 s, which proves the suitability of the DH-MTMs for monitoring the water flow velocity. The monitoring testing is conducted under the wave heights of 0.05, 0.1, and 0.15 m, and the wave frequency is fixed as 1 Hz. The monitoring data of the DH-MTMs are increased with the increase in the wave height under the wave period of 60 s, which demonstrates the ability of the DH-MTMs for placement monitoring under the surface and underwater multienvironmental marine conditions.

This study presented the DH-MTMs fabricated by the all-in-one 3D printing for multienvironmentally self-powered wireless monitoring systems. The proposed DH-MTMs were tested under the cyclic motions in the horizontal direction, horizontal–vertical coupling, and circular direction, respectively, and the testing results showed that the DH-MTMs generated different signal responses under various test conditions. Comparing with the electrical characteristics under the horizontal vibration, the DH-MTMs were validated under the horizontal–vertical coupling vibration with the accuracy of up to 90%. Eventually, the DH-MTMs were integrated into the self-powered wireless monitoring system for monitoring water flow velocity and deformation of the underwater structure. Experimental results showed that the reported wireless monitoring systems accurately captured the displacement of underwater structures and the water flow with the velocity of less than 0.1 m/s. The monitoring accuracy can be improved in 3 aspects: improving the material conductivity, improving the printing accuracy, and improving the data acquisition and transmission frequency. This study provides new design guidelines for standardized preparation of triboelectric metamaterials for real-time monitoring of the multienvironmental marine conditions.