Vibration Energy harvesting (VEH)
There is hardly a single place, or object, on Earth unaffected by groundborne vibrations. Noise sources are to be found everywhere: tides, oceanic storms and earth tremors are responsible for so-called microseismic noise in the low frequency (0.1-2 Hz) , and low amplitude (<10^-2 g) , band while human activity, and wind interaction with landscape, usually generates higher frequencies >10 Hz and higher amplitudes (~0.1-1g) [2,3]. Many vibrations are considered a nuisance, or a serious problem (as an example, groundborne vibrations in cities corrupting precise physical or medical device measurements), however the presence of these vibrations presents both an energy resource and opportunity. Fuelled by demands to reduce the power consumption of small electronic components, vibration-based energy harvesting (VEH)  has received considerable attention over the last two decades; it is clearly attractive to power devices using existing vibrational energy that reduces, or removes, costs associated with periodic battery replacement and the chemical waste of conventional batteries. Many applications arise: wireless sensor networks for civil infrastructure monitoring, unmanned aerial vehicles, battery-free medical sensors implants, and long-term animal tracking sensors among many others. The typical scheme of VEH is shown below: the building blocks are represented by energy source, piezoelectric transduction, power management and data storage ad transmission.
Vibration energy harvesting exploits ambient noise spectra to convert mechanical vibration into energy to power MEMS sensor and actuators in a large number of applications. MetaVEH will completely revisit the current complete harvesting system to make it drastically more efficient, sustainable, more portable and more integrated in a data-driven society.
To increase harvester efficiency, using ideas based around structuring surfaces, several approaches such as creating a parabolic acoustic mirror, point defects in periodic phononic crystals and acoustic funnels have been employed; lenses to concentrate narrow band vibrations have been proposed using phononic crystals and resonant metamaterials endowed with piezoelectric inserts have appeared very recently . Locally resonant metamaterials are characterised by resonating elements, analogous to Helmholtz resonators in acoustics and Fano resonances, enabling the creation of low-frequency band-gaps in structures relatively small compared to the wavelength; Tuning or spatially grading the resonators to broaden band-gaps and to focus elastic energy can augment these ideas. In particular, spatially graded metasurfaces, i.e. arrays of resonators with different resonant frequency distributed over an elastic surface, combine ease of manufacturing with the unique capacity to spatially select and focus different frequency bands. The specific illustration of how metasurfaces work in conjunction with energy harvester is presented in the figure above. The effect of metasurface is clearly visible in sub-plot (e), which show the dramatic power increase for the case of metasurface harvesting.
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Metamaterials for elastic wave control
The atomic structure of matter is usually responsible for defining the physical properties of the body, but in metamaterials those properties are controlled less by the microscopic structure and more by the macroscopic arrangement of resonating elements characterising the material . Those resonators are typically sub-wavelength, i.e. of size smaller than the propagating wavelength  and they are arranged over a distance that allows coupling between each other. The effective properties emerge due to specific interactions with wave fields. They can be driven either by Bragg-type scattering (as in phononic/photonic crystals) or by local resonance effects as in the metamaterial considered for this project. The length-scale varies from a few nanometers in optics/electromagnetic applications to tens of meters for seismic applications .
The effective properties of a metamaterial are exemplified by its wave-dispersion relationship, and are typically characterised by bandgaps, (frequency bands where waves cannot propagate), artificial anisotropy and low velocity zones, making them ideal for wave control.
Initially developed for electromagnetic and optical applications on a micro and nanoscale, metamaterials have emerged as a very popular concept used for the control of mechanical waves. Here a countless number of design and applications have been proposed, mainly for ultrasonic waves at small-scale. A metamaterial design made of a collection of rod-like resonators on an elastic surface and an arrangement of sprung masses (mass with elastic ligaments) are shown respectively in the figure below. Wave control is typically possible in a frequency band near the resonance frequency.
Currently there is no metamaterial design capable of working in the typical VEH band (<100 Hz) combining: compact dimensions (e.g. <1 m), limited weight to be considered portable and, very important, low dissipation (e.g. no polymers). Low frequency harvesting (on-body and moving vehicles applications) is currently an open problem that is mitigated using non-linear frequency-up converters. Depending on their kinematics, they are either resonant or non-resonant and magnetically or contact enabled; in most cases, such devices harness nonlinear phenomena.
Metamaterials and wave control
Recent advances in the field of metamaterials allow for the focussing and trapping of waves at specificlocations by exploiting local resonances. The process is controlled by the spatial distribution of the resonantelements and their resonant frequency. (a) Here we show a lens for guided waves, and (b) the dispersion curve ofthe metasurface. (c) Shows the local energy trapping phenomenon also underpinned by resonances and grading.(d) The dispersion curves for the surface acoustic wave along the metasurface.
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