Metamaterials’ Application in Sustainable Technologies and an Introduction to Their Inﬂuence on Energy Harvesting Devices

: The realm of sustainable technologies and metamaterials represents a vibrant ﬁeld of inquiry, and, upon closer examination, a fascinating correlation emerges. Metamaterials, being artiﬁ-cially engineered substances, exhibit diverse characteristics, depending on their speciﬁc composition. Remarkably, they hold immense potential in various sustainability-driven applications, such as energy harvesting, puriﬁcation, and noise control. For instance, a straightforward approach involves the implementation of electromagnetic metamaterial absorbers in energy harvesting systems. As the scope of environmental concerns continues to expand, this proposed solution demonstrates its universal applicability, addressing a growing number of environmental challenges.


Introduction
Electromagnetic metamaterials (EM MM) are artificial materials that have properties not found in naturally occurring materials [1][2][3].They are engineered to have specific electromagnetic properties, such as a negative refractive index, which allows them to bend light in unusual ways [4].This makes them useful in a wide range of applications, including sustainable technologies.Terms, such as IoT and sustainability, are increasingly conquering the industry.Emerging technologies are increasingly aiming to identify with them, and the use of metamaterials can help them achieve this.This work addresses the development of metamaterials in terms of contributing to the creation and application of sustainable technologies.It briefly describes the concept of sustainable development, selected types of metamaterials, and it shows their mutual influence.

The Application of Metamaterials
The term sustainability has become popular in policy-oriented research as an expression of what public policies ought to achieve [5,6].The original meaning is where sustainability is concerned with the wellbeing of future generations and, in particular, with irreplaceable natural resources-as opposed to the gratification of present needs, which we call wellbeing [6].Discussing how we can explore the concept of sustaining a process or activity over a prolonged period without causing harm to the environment is important.This involves utilizing resources in a responsible manner that avoids depletion and safeguards future generations.When applied to the realm of technology, this term encompasses the development and the implementation of devices that minimize their environmental impact throughout their entire life cycles.This includes their usage in renewable energy sectors or reducing their negative influence on the environment.These devices are primarily designed to be energy-efficient, to employ renewable or recyclable materials, and to have an extended lifespan.Achieving a sustainable future entails not only harnessing sustainable energy sources, but also coupling them with environmentally friendly technologies for their ultimate application [7].
Appl.Sci.2023, 13, 7742 2 of 14 The use of unique structures of metamaterials has attracted researchers from many different fields, so there is a wide range of terminology associated with them [8,9].With reference to their wide application for sustainable use, several main categories can be distinguished: electromagnetic MM, chiral MM, mechanical MM, or acoustic MM.It is important to note that there are unusual effects that are achieved in three-dimensional metamaterials, and they are recognized in this article, and they can be also found in two-dimensional structures, known as metasurfaces or flat optics.
Electromagnetic materials, alternatively called negative-index metamaterials, comprising purposefully crafted inclusions, offer a realm of engineered substances that manifest extraordinary electromagnetic properties, surpassing those found in their individual constituent elements.These exceptional material characteristics, absent in naturally occurring substances, have captivated the scientific community, propelling groundbreaking advancements in the realm of electromagnetism [6].In conventional materials, the index of refraction is always positive.However, in MM, the refractive index for an electromagnetic wave has a negative value over some frequency range.Such materials have potential applications in a wide range of areas, such as electromagnetic cloaking [10], superlensing [11], and antenna design [12][13][14].
What is brought to chiral metamaterials are the control of the optical activity and the ellipticity of a medium.These lead to deeper understanding of the light matter interaction in composite metallic structures.Combining chiral metamaterials with nonlinear materials has opened new possibilities in nonlinear chirality, as well as providing the foundation for switchable chiral devices [15].Unique properties of chiral MM arise from their chiral structure and the interaction with electromagnetic waves.They exhibit circular dichroism, which actually means that they absorb left and right circularly polarized light differently.Potential applications can be related to sensing [16][17][18][19], communication [18], and optics [15,19,20].For example, in sensing, they can detect chiral molecules, which are difficult to detect using conventional methods.More recent advancements in chiral metamaterials cover full Stokes polarization perfect absorption, femtomolar bio-detection or optical control of nanomechanical Brownian motion eigenfrequencies [21][22][23].
Within the realm of three-dimensional periodic metamaterials, an expansive array of possibilities unfolds, encompassing electromagnetic, optical, acoustic, mechanical, as well as transport and stimuli-responsive properties [24].A closer examination of threedimensional mechanical metamaterials reveals their remarkable potential in acoustic and filter applications.These acoustic metamaterials possess the extraordinary ability to manipulate and govern sound waves, transcending the limitations of conventional materials.Particularly intriguing are metamaterials exhibiting zero or negative refractive indices for sound, opening up new frontiers in acoustic imaging [25,26] and enabling the precise control of sound at subwavelength scales.The rich tapestry of three-dimensional periodic metamaterials thus promises a fascinating journey into uncharted realms of scientific exploration [27].
Multifunctional metamaterials exhibit multiple and often unusual properties, simultaneously, which cannot be found in naturally occurring materials.These materials are designed to manipulate electromagnetic waves, sound waves, and other types of waves to create desired properties, such as negative refraction, cloaking, and superlensing.Multifunctional MM takes this a step further by combining different types of MM to create materials with multiple functionality.For example, MM with a negative refractive index can be combined with MM, which is designed to absorb electromagnetic waves.Another combination can be energy harvesting material with vibration control parameters [28] or which absorbs and senses possibility [29].Hierarchical structures with constituents over multiple length scales are found in various natural materials, such as bones, shells, or spider silk.All displays enhanced quasi-static mechanical properties, such as high specific strength, stiffness, and toughness [30].Introducing the concept of hierarchy in elastic metamaterials leads to periodic structures, capable of generating multiple, highly attenuative, and broadband BGs at multi-scale frequencies, including deep sub-wavelength regime [31].Hierarchical bio-inspired metamaterials can provide advantages for dynamic responses as compared to the conventional materials [30].
The diverse array of metamaterial types showcased here (Figure 1) exemplifies the transformative potential they hold in fostering environmental wellbeing and advancing sustainability.Whether it be their unique compositions, carefully chosen materials, or the latest strides in research, metamaterials offer distinct avenues through which positive environmental impacts can be realized (look Table 1).In the realm of sustainable development, these remarkable materials have already demonstrated their prowess in propelling green energy initiatives, promoting biodiversity conservation, and facilitating advanced filtration systems.It is through the judicious application of metamaterial technologies that we can forge a more sustainable future, one where in innovation is harmonized seamlessly with ecological stewardship.metamaterials leads to periodic structures, capable of generating multiple, highly attenuative, and broadband BGs at multi-scale frequencies, including deep sub-wavelength regime [31].Hierarchical bio-inspired metamaterials can provide advantages for dynamic responses as compared to the conventional materials [30].
The diverse array of metamaterial types showcased here (Figure 1) exemplifies the transformative potential they hold in fostering environmental wellbeing and advancing sustainability.Whether it be their unique compositions, carefully chosen materials, or the latest strides in research, metamaterials offer distinct avenues through which positive environmental impacts can be realized (look Table 1).In the realm of sustainable development, these remarkable materials have already demonstrated their prowess in propelling green energy initiatives, promoting biodiversity conservation, and facilitating advanced filtration systems.It is through the judicious application of metamaterial technologies that we can forge a more sustainable future, one where in innovation is harmonized seamlessly with ecological stewardship.

Solar Energy Cells
One of the most captivating realms where electromagnetic metamaterials (EM MM) have garnered significant attention is solar energy generation.These ingenious materials offer a pathway to highly efficient solar cells by exerting precise control over the absorption and conversion of light into electricity [32].Through the strategic manipulation of light absorption and scattering using metamaterials, scientists have achieved notable breakthroughs in surpassing the efficiency of traditional solar cells [33].META Materials Acoustic MM X

Solar Energy Cells
One of the most captivating realms where electromagnetic metamaterials (EM MM) have garnered significant attention is solar energy generation.These ingenious materials offer a pathway to highly efficient solar cells by exerting precise control over the absorption and conversion of light into electricity [32].Through the strategic manipulation of light absorption and scattering using metamaterials, scientists have achieved notable breakthroughs in surpassing the efficiency of traditional solar cells [33].META Materials Inc., a pioneering firm, has already taken this concept to a production scale by incorporating metamaterials into solar energy films.These films enhance efficiency by effectively collecting and absorbing light.An exemplary study by Raj Kumar et al. [34] showcased a remarkable achievement in the realm of broadband metamaterial solar spectrum absorption.Their proposed absorber, featuring a three-layer structure of metal-dielectric-metal, exhibited exceptional properties, including a wide incident angle and polarization-insensitive absorption.Through meticulous design and the utilization of a silicon dioxide dielectric layer between the petal-shaped metasurface and lower tungsten metal layers, the unit cell structure demonstrated an astounding 99% absorption across wavelengths, ranging from 456.62 nm to 677.73 nm.This remarkable utilization of metamaterials in solar energy applications showcases their potential to revolutionize the field and to drive sustainable advancements in renewable energy technology.

Wind Turbines
Metamaterials, with their remarkable capabilities, have found applications beyond solar energy, extending into the realm of wind turbines, as well.Traditional wind turbine blades are typically designed to prioritize aerodynamics, which can inadvertently limit their efficiency in capturing the full potential of the wind's energy.However, metamaterials offer a transformative solution by altering the flow of wind around the blades, enabling them to more effectively harness and capture energy.This breakthrough has sparked growing interest in wind farms, with researchers actively exploring new avenues to enhance and to optimize various associated technologies.
Zehai Zhang et al. [35] have made notable strides in the field of proposing groundbreaking metamaterial absorbers (MAs) specifically engineered to strongly absorb microwave frequencies at 2.46 GHz.By effectively engineering strongly absorbed incident microwaves and converting their energy into heat, this MA holds immense potential for microwave deicing applications in wind turbine blades.Through comprehensive multi-physical simulations, Zhang demonstrates the feasibility and effectiveness of the MA in generating heat, distributing thermal energy, and facilitating controlled temperature rises during microwave irradiation.
Moreover, the application of metamaterials extends beyond energy optimization in wind turbines to address the issue of noise disruption.Ana Azevedo Vasconcelos and her team at MTT (Metamaterial Technologies, Inc.) have pioneered the development of metamaterial interfaces, which are specifically designed to mitigate the adverse effects of noise pollution associated with wind turbines.By implementing large-scale unit cells of acoustic metamaterials, they have successfully reduced the noise emissions during wind turbine installation.This innovative solution not only contributes to minimizing noise disturbances, but it also holds promise in safeguarding marine biodiversity with regards to underwater noise pollution.
This convergence of metamaterials and wind turbine technology represents a compelling frontier, showcasing how these remarkable materials can revolutionize sustainable energy production.From improving energy capture efficiency to mitigating noise disruptions and promoting ecological harmony, metamaterials continue to unveil transformative potential in enhancing the sustainability and effectiveness of wind energy solutions.

Energy Storage
Important and growing in popularity application of electromagnetic and mechanical metamaterials is in energy storage.MM can be used to create batteries that are more efficient and have a higher energy density than traditional batteries.This could lead to the development of smaller and more powerful batteries, which would be useful in a wide range of applications, including electric cars and portable electronics.
J You et al. [36] demonstrated how to design an energy-storage metamaterial with enhanced mechanical properties and battery safety simultaneously.Mechanical properties of MM could be simultaneously manipulated via their architectures.In the same study, he proposed multifunctional metamaterials, possessing both load-bearing capacity and storage capability, comprising multi-phase lattice MM and cylindrical battery cells.Printed metamaterials were assembled with battery cells and compresses.Experimental results showed that the void in lattice MM could guide deformation mode away from the internal battery cell that postponed the emergence of battery short-circuit.They can absorb greater energy after defective phase incorporation.
Y Huang et al. [37] faced a problem with batteries in electric vehicles.The challenge is to make the design of lightweight and safe battery pack a critical issue.He proposed a multifunctional battery system comprising cylindrical battery cells and surrounding lightweight lattice metamaterial.Lattice density distribution was optimized to minimize stress on the battery during compression where compression testing of the assembled structural battery system revealed that the stronger lattice units in the X-shaped pattern resisted deformation and helped delay of a battery short circuit.

Energy-Efficient Buildings
Metamaterials are also being used to develop new types of energy-efficient building.By using MM to control the way that heat and light pass through a building's walls, scientists have been able to create buildings that use less energy, which would be beneficial for the environment.
Z. Viskadourakis et al. [38] proposed a method to develop metasurfaces in construction materials (plasterboard and wood).He showed that application of electromagnetic sprayprinted MS, integrated on construction materials with larger areas (such as walls or doors) can potentially be used for EM applications, for buildings, for power self-efficiency, and for management.

Water and Air Purification
Metamaterials can be used to create more efficient water and air filtration and assessment systems.In water systems, they can be an aspect of controlling the way that light passes through the water.This can make it easier to remove pollutants from the water, making it safer to drink.In air systems, they can be a part of process, removing pollutants from the air.By using MM to control the way air flows through the purifier, it is possible to remove more pollutants from the air.
Hong at el. [39] combined the cell structure design of metamaterials with ordinary materials, which leads to the creation of many new materials of different characteristics.This provides more possibilities for material research.He pointed out that water and air are usually polluted by various substances, such as organic matter, inorganic matter, and various microorganisms.Usually, filters with small critical sizes have good filtering effects, but their filtration resistance is large, and their flux is low.On the other hand, a large critical size filter will have a poor filtering effect.To overcome it, Hong pointed out that using a plate structure to make the filter screen can lead to an adjustable critical size filter.Using such a filter screen will greatly improve the controllability of the filtration process.

Thermal Management
Metamaterials can be used to create devices that can more efficiently manage heat.This can be used to create more energy-efficient buildings and electronic devices.
Li et al. [40] faced demand for sophisticated tools and approaches in heat management and control, which triggered the fast development of a field that includes conductive thermal metamaterials, nanophononics, and far-field and near-field radiative thermal management.He offered a unified perspective on control heat transfer toward manipulation of physical parameters and the realization of unprecedented phenomena in heat transfer using MM.Whereas, J. Wang et al. [41] confirmed and demonstrated that metamaterials have amazing properties in heat transfer beyond naturally occurring MM.The idea of thermal MM has completely subverted the design of thermal functional devices and makes it possible to manipulate heat flow at will [42].

Antennas
Metamaterials are more and more used in creating antennas that are more efficient at transmitting and receiving signals.This can create more efficient wireless communication systems, which would be beneficial for a wide range of applications.The progress of technology in consumer electronics demands an antenna having a compact size, high gain and bandwidth, and multiple antennas at the transmitter to receiver junction to enhance channel capacity [42].In the progress of new designs and optimization processes, one such technique is the use of metamaterials in antenna design [43].
Kumar et al. [42] pointed out that loading an antenna with one, two, and three-dimensional MM structures, comprised of a periodic subwavelength unit cell, exhibits RLC resonant structures and allows us to manipulate electromagnetic waves in the antenna system.Krzysztofik et al. [44] presented applications of metamaterials in design to enhance antenna parameters and confirmed that MM can be applied within the environment.He pointed out that, depending on the parameters of the desired antenna, MM can be applied to improve bandwidth, power gain, or to create compact, multifrequency band antennas.The effectiveness of improving the parameters depends on the structure, size, quantity, and method of use in the unit cell of MM.The application of MM in an antenna can increase their gain ≥ 2 dB, bandwidth ≥ 100%, reduce size ≥ 50%, or create additional frequency bands for multicommunication system-operated antennas [45][46][47].

Transfer and Harvesting Energy Metamaterials
Energy harvesting and transfer belongs to the terminology that recently began to have more and more popularity in a metamaterial scientific pilar.Energy transfer refers to the process of transmitting energy from one location to another, typically through the use of electricity from power plants to consumers via high-voltage power lines, or the distribution of natural gas through pipelines.Energy harvesting, on the other hand, refers to the process of capturing and storing energy from the environment in order to future usage.This includes capturing solar energy from solar panels, capturing wind energy through the use of wind turbines, or capturing wind energy through the use of devices, such as motion-powered generators.In summary, energy transfer is the process of "moving" energy from one place to another, while energy harvesting is the process of capturing and storing energy from the environment for later use.

Transfer Energy Metamaterials
Metamaterials can be used in energy transfer in a few different ways, and the number of applications is still growing.Their specific parameters and the ability to change them in different applications leads to the fact that these materials are very desired.MM can be used in high-efficiency transmission lines.They can lead to creating high-efficiency power transmission lines.This can be performed by using metamaterials to control the way that electromagnetic waves travel through transmission lines [47].By controlling the way that waves pass through the lines, it is possible to reduce the amount of energy lost during transmission, which would make the transmission lines more efficient.
Tian et al. [45] reviewed focuses on the recent advances in MM for simultaneous wireless information and power transmission technology.He introduced the potential technologies and also pointed out wireless transmission technologies, even concluding with perspectives on the rapidly growing transmission technology requirement for 6G.
Looking for other potential applications of metamaterials in the transfer energy process, the following can be noted: power line conditioners, smart grids, and wireless power transfer [45,46].
There are many control system ideas for power line conditioners, such as using AI [48].Power line conditioners are referred to to improve the quality of the electrical power that is transmitted over power lines.Whereas, metamaterials can be used to create conditioners that are even more efficient by means of their properties, which can improve and optimize the whole process.Smart grid systems are used to control the flow of electricity through power lines.Now, we are facing a huge transformation of power systems all over the world.
Escobar et al. [49] made a taxonomy of a large number of technologies in smart grids and their applications in scenarios of smart networks, neural networks, blockchain, the industrial Internet of Things, etc.He presented challenges and opportunities for smart grids.The biggest challenge that can be faced is the growth of the massive exposure of algorithms.This is due to the fact that the number of users increases day by day, requiring more data speed and bandwidth to avoid channel propagation delay [49].Let us not forget that the main factor of smart grids is communication between all of the local energy market participants.It is a part of the Internet of Things world, where metamaterials can be applicated to make more efficient connections for delivering huge masses of important data.
In terms of wireless communication, we can also find applications of MM in wireless power transfer [46], where they can be used to create them.These systems can transfer energy wirelessly over a distance.By using metamaterials, we can control the way that energy is transferred, and it is possible to create systems that are more efficient than traditional ones.Typically, a wireless power transfer system is a suitable alternative for power transmission, where conventional wired power transfer faces geographical challenges.In a long-range wireless power transmission system, power is transmitted through microwaves, and a highly directive antenna is required for an effective transmission through this system [46].
Singh et al. [46] presented a circularly polarized microstrip rectangular patch antenna incorporated with a metamaterial slab.Thanks to the application of metamaterial, this antenna can enhance the amount of transferred power to wireless sensors network with less radiation loss and improve other important antenna parameters.For example, it can reduce beam width radiated power.

Energy Harvesting Metamaterials
Energy harvesting is the process of capturing and storing energy from the environment in order to use it later.Metamaterials, due to their specific electromagnetic properties, have shown great potential in the field of energy harvesting.They can be used to create devices that can more efficiently capture and convert energy from sources, such as solar [50], wind [51], kinetic [52], or thermal [51,53].Metamaterials in such devices control the way that energy is absorbed and scattered.In this paper are listed a few applications of MM in the energy harvesting field, such as solar cells, wind turbines, or thermal management, but these are just a few of many more.
Most energy harvesting technologies are now focused on the areas of RF signals and microwaves.Metamaterials can be used to create devices that convert EM signals into electricity.These devices can be used to harvest specific RF signals or microwaves, but they can also take lost signals from sources, such as wireless networks or television signals.What makes these technologies sustainable is that taking the lost radiation from the environment makes the economy circular.
Microwave energy harvesting devices usually use antennas and other components to capture and convert energy from microwaves to electricity [54,55].Microwave energy harvesting devices typically use rectifying antenna (rectenna).These rectennas consist of a patch antenna and a diode.The patch antenna captures the microwave energy, and the diode rectifies it into DC electricity [56].The way to harvest energy from microwaves through the use of metamaterials is to apply them to the system, where MM can control the way that energy is absorbed and scattered.Let us note that the amount of energy that can be harvested from microwaves is relatively small, and the efficiency of the energy harvesting devices is still relatively low.Therefore, it is mainly used as a supplementary source of energy, rather than a primary one.
Fowler et al. [57] demonstrated a RF energy harvesting rectenna design based on a metamaterial perfect absorber.With the embedded Schottky diodes, which are often used in wireless communication systems, the rectenna converts captured RF waves to DC power.He pointed out that optimal RF energy harvesting occurs when the metamaterial is constructed, such that a high Q-factor resonance occurs at the same frequency as the external radiation source.Fowler confirmed that later, in [58], and Fowler pointed out that a perfect metamaterial absorber makes a promising solution for collecting ambient RF energy in low-power density environments because it is tunable, highly efficient, and electrically small [58].
Nowak et al. [56] conducted research, which proved that energy high-frequency electromagnetic field absorbers, based on resonance MM, constitute a competitive alternative to conventional antenna systems.They achieve efficiency exceeding 90%, which is more than five times higher than the conventional dipole-type antenna arrays.He also pointed out that practical application of MS structures is easier than classic antenna systems due to greater flexibility in the geometry.Resonant MM can also be used as high-efficiency couplers and receivers in the microwave wireless power transfer [59].
The amount of energy that can be harvested from RF or microwaves depends on several factors, such as the intensity of the signal, the energy efficiency of the device, and the size of the energy storage device.In general, it is possible to harvest enough energy from RF or microwaves to power small electronic devices, such as wireless sensors, but it would not be sufficient to power larger devices or to charge a battery.The harvested energy is usually used as a supplementary source of energy, rather than primary one.

Energy Harvesting Devices in Big Cities
The term Internet of Things is more and more common.IoT is rapidly evolving in big cities, as more devices are able to communicate with each other.This has led to the creation of smart cities, where everything is connected and can be controlled and monitored remotely [60,61].This not only improves the efficiency and sustainability of these systems, but it also enables new services and applications.The evolution of IoT is happening quickly due to advances of technology, such as the miniaturization of sensors and the increasing availability of radiation (5G).The demand of smart city solutions is growing as cities become more populous and face challenges related to a few United Union goals [62].
"Microwave" refers to a range of electromagnetic waves with frequencies between 300 MHz and 300 GHz.In the context of communication technology, microwave is often used to refer the band of frequencies between 1 GHZ and 100 GHz [63], which is used for various types of communication: wireless [64], satellite [65], or local wireless areas (WLANs) [66].In cites, they have a wide range of application: wireless communication (such as phone networks and wireless internet) are used to transmit and to receive signals for voice and data communication, TV and radio broadcasting, radars, remote sensing (satellite imagery), industrial heating [67], or medical applications (cancer treatments and diagnostic techniques) [68].All of the above are common, and their use is increasingly similar to IoT.
While harvesting energy from microwaves in a city, this could potentially interrupt or weaken microwave signals.This is because harvesting devices would capture some of the energy from microwave signals, which could reduce their strength.Let us not forget that, in general, the strength of signals used for communication is relatively high, so this should require a large number of energy harvesting devices to have a significant impact on the signals.Additionally, energy harvesting devices designed properly will not interfere with the signals, and they will work alongside them.Creating energy harvesting devices to capture microwaves in a city requires proper design and optimization to minimize the impact of the surrounding environment.While it is possible to capture energy and apply it to power sensors where in need, this is only a small portion of the microwave energy, so this will not weaken the signal.On the other hand, one can also apply the filtering technique to ensure that the energy harvesting device only captures specific frequency while avoiding other frequencies (from communication).
In terms of making an economic circuit and capturing lost signals, microwaves in the use of energy harvesting processes are typically highly directional, which means that they are focused in a specific direction in order to reach their intended destination.This minimizes the amount of lost signals.However, some can be lost due to reflection, scattering, or atmospheric conditions.Some signals are not captured by receivers, which represents a bigger problem in big cities.Devices, which could be able to capture and store energy from environment, can be a sustainable alternative to traditional methods of energy transfer.Moreover, in sustainability, one of the main advantages of energy harvesting is reducing the need for large-scale power transmissions and distribution systems that can be costly and have significant environmental impact.
When it comes to metamaterials and their impact on the environment, it is important to consider more things: the entire life cycle of the materials, from extraction of raw materials to production, as well as the time of living, use, and disposal.In terms of carbon footprint, typically the production of structures and products has significant impact on the environment.Most of this is related to the usage of energy, as well as the pollution that comes with it.In this case, what is important to also consider is the usage of raw materials and the place where they come from.Transport and the place of mining create a huge impact on the environment.Considering the closed-circuit economy, metamaterials can play a role in reducing the use of resources by increasing the efficiency of energy transfer and harvesting.Typically, energy efficient devices can be used for a longer period of time before needing to be replaced.

Monitoring Systems with Wireless Energy Harvesting Sensors
This work focuses its attention on possible venues for the use of metamaterials as environmentally sustainable materials.We wish to highlight, in this detailed section with energy harvesting, two outstanding uses for this application.The use of sustainable technologies is a response to a need in the technology market and to the 17 sustainable goals of the UN.The proposed solutions focus their attention on several of them: clean water and sanitation; affordable and clean energy; industry, innovation, and infrastructure; sustainable cities and communities and climate action.Help in those areas can be delivered using a monitoring system with wireless energy efficient sensors.
Desired areas for application similar system can be on offshore wind farms.A whole wind farm is a complicated system with desired stages of use of multiple different sensors [69,70].The sensors used on offshore wind farms are typically powered by a combination of on-board batteries and external power sources, but, even now, there are ideas of piezoelectric materials in the sensors [71,72].Sensors, such as anemometers and meteorological sensors, are powered by batteries.The batteries are recharged using small solar panels or other renewable energy sources.Others, such as LIDAR, require more consistent and reliable power sources.Those sensors are powered by electrical cables that are connected to the wind farm's substation or by a standalone generator [73].Anemometers and meteorological sensors typically require very low power inputs, typically in the range of a few milli amperes.Specific vibration sensors can be powered in the range of few milliamperes to a few amperes, too.One potential advantage of proposing energy harvesting technology into this monitoring system is that it eliminates the need for traditional power sources, such as batteries or external electrical cables.This can reduce the overall cost and complexity of the offshore wind farm, as well as reduce the risk of power failures or other issues.Wind farms are still undergoing a tremendous amount of application of new technologies, and the question of how to reduce energy costs and effectively monitor the system using sustainable solutions is still being asked.
The next proposed area is in sewer monitoring systems.Especially, in Poland, the number of hydrologic events and floods in 40 years increased four times, and, due to climate changes, it can be worse.The monitoring of sewer systems, underground drainage, or sewage is an active area of work by scientists.It can prevent urban floods and result in good water quality.Haswani et al. [74], for example, proposed a similar web-based real-time monitoring service with the use of a wireless sensor.These are only two of possible areas wherein to use monitoring systems containing several wireless sensors to highlight the importance of smart cities and IoT.

Application of Energy Harvesting Metamaterials
In this paper, we propose an application of an energy harvesting device with the use of metamaterials to capture and store microwaves for sensor power (look Figure 2).It should contain metamaterial structure, antenna, rectifying circuit, power management, as well as a storage, control, and monitoring system.
energy into DC electrical energy [76].It can be designed to be omnidirectional, meaning it can absorb energy from microwaves coming from any direction [77].
An energy harvesting circuit should be responsible for converting the microwave energy absorbed by the rectenna into usable DC electrical energy.The circuit can include components, such as diodes, capacitors, and voltage regulators, to ensure a stable output voltage.
In addition to the proposed system, there can be sensors detecting the presence of microwaves, thereby triggering the energy harvesting process.They can be designed to activate the rectenna and the energy harvesting circuit when they detect the specific frequency range of microwaves that the metamaterial absorber is designed to absorb.In another way, they can also be considered tunable metamaterial absorbers [78][79][80][81][82].
The device can be designed to be small and lightweight, making it suitable for use in discussed applications, where energy is needed, but it may be difficult or impractical to run wires or replace batteries.The main component of the energy harvesting wireless sensor is a metamaterial component.AM Hawkes et al. [75] presented design and experimental implementation of a power harvesting metamaterial.He demonstrated that the maximum harvested power occurs for a resistive load close to 70 Ω in both simulation and experiment.He also highlighted that it can be suitable for use in devices integrated with metamaterials.The rectenna is a device that combines a microwave antenna and a rectifier circuit to convert energy into DC electrical energy [76].It can be designed to be omnidirectional, meaning it can absorb energy from microwaves coming from any direction [77].
An energy harvesting circuit should be responsible for converting the microwave energy absorbed by the rectenna into usable DC electrical energy.The circuit can include components, such as diodes, capacitors, and voltage regulators, to ensure a stable output voltage.
In addition to the proposed system, there can be sensors detecting the presence of microwaves, thereby triggering the energy harvesting process.They can be designed to activate the rectenna and the energy harvesting circuit when they detect the specific frequency range of microwaves that the metamaterial absorber is designed to absorb.In another way, they can also be considered tunable metamaterial absorbers [78][79][80][81][82].
The device can be designed to be small and lightweight, making it suitable for use in discussed applications, where energy is needed, but it may be difficult or impractical to run wires or replace batteries.
As shown in Figure 1.The metamaterial absorber is placed on top of the rectenna, with the conductive elements of the absorber facing downward towards the rectenna's antenna elements.This arrangement would allow the absorber to interact with the incoming microwaves and absorb their energy, which would then be collected by the rectenna's antenna and passed on to the rectifier circuit.The additional sensor is placed in proximity to the metamaterial absorber and rectenna.When the sensor detects the presence of microwaves within this frequency range, it activates the rectenna and energy harvesting circuit to begin collecting and converting the microwave energy into electrical energy.The energy harvesting circuit is integrated into the same device as the metamaterial absorber, rectenna, and sensor, and it should be designed to provide a stable output voltage to power various electronic devices.
We want to highlight two recommended areas for applying such a system.For application on offshore wind farms, it can be a very desired technology in terms of separately placed wind turbines.Offshore wind farms need to be highly automated with good communication and real-time monitoring.The sensors used on offshore wind farms are typically powered by a combination of on-board batteries and external power sources.Some sensors, such as anemometers and meteorological sensors, are powered by batteries.Other sensors, such as LIDAR, require a more consistent and reliable power source.They are usually powered by an offshore substation.Anemometers and meteorological sensors typically require very low power inputs, even in the range of few mA.For vibration sensors, the power requirements can vary, depending on the size and complexity of the sensor, but they are typically in the range from few mA to a few amperes.Especially, these sensors with low power input are well suited to apply energy harvesting devices with metamaterials to them.One potential advantage of using energy harvesting technologies in this area is that they eliminate the need for traditional power sources, such as batteries or external electrical cables.This can reduce the overall cost and complexity of the offshore wind farm monitoring system, as well as reduce the risk of power failures.
The next possible application is an underground sewer monitoring system.In big cities, there can be seen noticeable increases in smart technologies.Binding that process with the increasing popularity of IoT, the use of sensors is increasing, too.That makes the application of energy harvesting devices even more important.An underground sewer monitoring system would provide potential protection against events, such as urban flooding, which are becoming more and more common.Such a consideration of even storing lost microwave radiation in the environment could apply for energy harvesting devices for underground sensor powering.Solutions such as these could be able to store energy for later usage.

Conclusions
Currently, the popularity of the term sustainability is rapidly growing and so are sustainable technologies.Metamaterial applications show great use in this area.In this article, we presented a brief overview of available and categorized metamaterial technologies, which provide a closer look at possible sustainable solutions.Different types of metamaterials can be seen in different applications.For example, mechanical structures show great potential in areas where new technologies are causing damage for biodiversity.Electromagnetic MM are an active area of work and can be categorized in more specific pillars.Where this review focuses on application in sustainable ways, electromagnetic metamaterials are showing great opportunity for the transfer and the harvesting of energy.Sustainable technologies should focus on simplicity and universality.The proposed harvesting system is simple and possible to apply within a metamaterial absorber.The whole idea of it is quite universal, and one can find more examples of such application than mentioned.For example, for different frequencies, different metamaterial structure can be used or even combined in a system to form a tunable metamaterial for widening the range of harvested waves.This solution could open new possibilities and ways of thinking about sustainable technologies.Metamaterials, with their extraordinary properties and engineered designs, offer immense potential for shaping a sustainable future.In this article, we presented compelling arguments highlighting the pivotal role of metamaterials in advancing sustainable practices across various domains.From energy harvesting to environmental sensing, these remarkable materials have the capacity to revolutionize industries and pave the way for innovative solutions to pressing global challenges.