Metamaterials and phononic crystals (PCs) with artificially designed periodic microstructures have attracted increasing research interests due to their unique properties that cannot be easily realized in natural materials. In recent years, the applications of metamaterials and PCs have been extended into the field of energy harvesting. A direct integration design strategy can yield a multifunctional system to suppress undesired vibrations/noise and convert them into electrical energy for providing power supply to widely distributed micro-electromechanical systems. Moreover, the defect state mode of metamaterials/PCs can localize the energy with an amplification effect, which has a great potential for improving energy harvesting efficiency. In addition, through tailoring the refractive index profile of metamaterials/PCs, the wave focusing phenomenon can be realized to boost the energy harvesting efficiency over a wide frequency range. This paper presents a comprehensive overview of the state-of-the-art advances in this direction over the past decade. According to the design strategies and working mechanisms, the existing studies in the literature on this topic are outlined and classified into three different categories. The advantages and disadvantages of various configurations are compared. The potential solutions to the existing drawbacks are discussed. An outlook on future prospects in this area is provided.

In light of the crucial role of marine ecosystems and the escalating environmental conservation challenges, it is essential to conduct marine monitoring to help implement targeted environmental protection measures efficiently. Energy harvesting technologies, particularly triboelectric nanogenerators (TENGs), have great potential for prolonging the lifespan and enhancing the reliability of sensors in remote areas. However, the high internal resistance, low current, and friction-induced abrasion issues of TENGs limit their performance in practical applications. This work presents a rolling mode triboelectric nanogenerator that utilizes multi-tunnel grating electrodes and the opposite-charge-enhancement mechanism to harvest wave energy efficiently. The device achieves significant instantaneous and root mean square power density of 185.4 W/(m³·Hz) and 10.92 W/(m³·Hz), respectively. By utilizing stacked devices and an exclusively designed power management module, a self-powered ocean sensing system including computing and long-range wireless communication (0.8 km) capabilities was developed. Laboratory and in-situ ocean tests were conducted to assess and validate the system. This work offers a potential solution for the challenging deployment of marine self-powered sensing nodes.

In recent years, additive manufacturing has been developing rapidly mainly due to the ease of fabricating complex components. However, complex structures with overhangs inevitably require support materials to prevent collapse and reduce warping of the part. In this paper, the effects of process parameters on printable bridge length (PBL) are investigated. An optimisation is conducted to maximise the distance between support points, thus minimising the support usage. The orthogonal design method is employed for designing the experiments. The samples are then used to train a neural network for predicting the nonlinear relationships between PBL and process parameters. The results show that the established neural network can correctly predict the longest PBL which can be integrated into support generation process in additive manufacturing for maximising the distance between support points, thus reducing support usage. A framework for integrating the findings of this paper into support generation process is proposed.

The paper proposes a modified metamaterial beam for simultaneous vibration suppression and energy harvesting. Local resonators in the modified metamaterial beam are alternately coupled, and each resonator is associated with a piezoelectric element for converting vibrations into electrical energy. First, the mathematical model of the modified metamaterial beam based piezoelectric energy harvester (PEH) is developed. The vibration suppression and energy harvesting performances of this system are analysed and compared with those of a conventional metamaterial beam PEH. The analytical results predict that not only the energy harvesting performance can be massively reinforced in the low frequency range, but also the vibration suppression performance can be slightly enhanced due to the appearance of an additional band gap. Subsequently, two finite element models, Models A and B, are developed. Model A is expected to be equivalent to the analytical model for validation and the local oscillators represented by lumped parameters in the analytical model are modelled by using cantilevers with tip masses. These tip masses are alternately coupled with ideal springs. The finite element analysis results in terms of both vibration suppression and energy harvesting show good agreement with the analytical results. Finally, to propose a more practical design of the internal coupling, Model B is established. Every two neighbouring tip masses are alternately coupled by using a beam connection. The finite element analysis results show that Model B is not completely equivalent to the proposed analytical model: no significant enhancement in terms of energy harvesting but a remarkably enhanced vibration suppression performance.