arXiv:2101.09312v1 [cond-mat.mtrl-sci] 22 Jan 2021 Highlights presented. sect been have rema final topics concluding the research and At possible papers and review umbrella. p outlooks missing one review for nuder the recommendations re up by our all som summed suggested article, even systematically directions review or research been of overlaps review future similar present for many the dations In and papers, exist. review contradictions fi pro the existing future of for in parts several road prog missed However, recent the technology. on pave this light to of development shed and to fields, tried interdisciplinary va papers related in such points of design Most small-sca and applications. to ene approaches, pre piezo-based need modeling been of have piezomaterials, increasing aspects papers including different and review cover many to design researchers result, many in a piezoelec As flexility to paid generation. their been energy to has attention due explosive an vesters decade, last the In Abstract hswork. this rpitsbitdt Elsevier to submitted Preprint ehnclEgneigDprmn,Fclyo Engineerin of Faculty Department, Engineering Mechanical izeeti nryHretn:aSseai Review Systematic a Harvesting: Energy Piezoelectric ∗ • • orsodn uhr e. 9(4)30 12 l uhr con authors All 4142. 3305 +98(241) Tel.: author, Corresponding mi address: Email oprtv vriwo eiw ntempo iz-nryharve piezo-energy of map presented. the is in ing reviews of r overview are comparative harvesting A energy piezo-based of field the viewed. in papers review All aa Ghazanfarian Jafar [email protected] o 59-1,Zna,Iran. Zanjan, 45195-313, Box ∗ oamdMsaaMohammadi Mostafa Mohammad , fReviews of JfrGhazanfarian) (Jafar ,Uiest fZna,P.O. Zanjan, of University g, rbtdeulyto equally tributed aur 6 2021 26, January g harvesting, rgy l aebeen have eld pr have apers etdby sented commen- pcsof spects essin resses rchar- tric k on rks rious ion, st- e- le e • An extensive description of research lines for future research is provided.

• Statistics of publications and funding organizations are presented.

Keywords

Energy harvesting; piezoelectric; energy conversion; renewable energies; micro-electro-mechanical systems

2 Contents

1 Introduction 3

2 Reviews with non-focused topics 6

3 Design and fabrication 10 3.1 Materials ...... 10 3.2 Structure ...... 16 3.3 MEMS/NEMS-based devices ...... 22 3.4 Modelingapproaches ...... 29

4 Application 32 4.1 Vibrationsources ...... 32 4.2 Biologicalsources ...... 34 4.3 Fluids ...... 40 4.4 Ambientwasteenergysources ...... 44

5 Challenges and the roadmap for future research 47

1. Introduction

Due to recent developments of portable and wearable electronics, wire- less electronic systems, implantable medical devices, energy-autonomous sys- tems, monitoring systems, and MEMS/NEMS-based devices, the procedure of generation of a small-scale amount of energy may lead to a revolution in development of ultra-low-power technologies. Figure 1 demonstrates com- parison of output power versus power density generated by the piezoelectric system and some other direct energy conversion techniques [1]. It can be concluded from the figure that among various types of energy harvesting, the piezomaterials are the most common and general candidate of energy harvesting with a wide range of output voltage. On the other hand, advantages of piezoelectric harnessing include simple structure without several additional components, no need to moving parts or mechanical constraints, environment friendliness and being ecologically safe, portability, coupled operation with other renewable energies, no need to an external voltage source, compatibility with MEMS, easy fabrication with microelectronic devices, high output voltage, cost effectiveness, wide frequency range, scalibility, and an AC-type output. Hence, piezomaterials

3 Figure 1: Comparison of power density as a function of output voltage difference for different energy harvesting systems [1]. are an excellent candidate to replace batteries with short lifespan for powering macro to nanoscale electronic devices. Piezomaterials can extract power directly from structural vibrations or any other environmental waste energy source in biomedical systems, nano-devices, healthcare and medicine, remote and hostile applications, and they can be used as as transducers, actuators, and surface acoustic wave devices. Some disadvantages of piezo-harvesters are poor coupling of thin films, high output impedance, producing relatively high output voltages at low elec- trical current, discontinuous nature of energy harvesting, brittleness, flexible PVDF with low coupling, and charge leakage. The number of review papers on piezoelectric energy harvesting has been extensively increased in the recent decade. Due to the tremendous number of published review papers in this field, finding an appropriate review paper itself turns out to be a challenge. On the other hand ,there are overlaps, similarities, missing parts, and sometimes contradictions between different reviews in the map, which may lead to the confusion of readers. Therefore, the main motivation of the present paper was to present a systematic review of the review papers about the piezoelectric harvesters. We try to summarize all deficits, advantages, and missing parts of the existing review papers on

4 piezo-energy harvesting systems. An extensive search among electronic sources identified about 90 review papers in diverse applications related to the piezoelectric energy harvesting. As will be demonstrated later, such papers present many different conclud- ing remarks corresponding to area of usage, materials, design approaches, and mathematical models. We tried to perform a very detailed searching procedure with too many keywords and by several search engines to cover all published review papers or the review papers in which ”piezo” was not mentioned directly in the title. Statistics of publications during two recent decades excluding conference papers and conference reviews with the keyword ”piezo AND energy harvest- ing” extracted from SCOPUS are shown in Fig. 2. Results from SCOPUS in- cluded overall number of 4435 documents, containing 874 open access papers, 130 book chapters, and 36 books. The National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities, and the National Research Foundation of Korea were the most frequent funding sponsors. Most common subject areas were engineering, material sciences, physics and astronomy, chemistry, and energy. An extrapolation shown in the figure anticipates publications of a huge number of articles (about 5000 articles a year) during the coming decade. Due to interdisciplinary nature of piezoelectric energy harvesting, pre- diction of behavior of the piezo-generators are related to different thermo- electro-mechanical sciences as well as material engineering. We have illus- trated a systematic map of various aspects of piezo-energy harvesting and design in Fig. 3. Different branches of related sciences and applications in- clude fabrication methods, hybrid systems, performance evaluation, size, us- age methods, configurations, modeling aspects, economical points, energy sources, optimization, design of an electric interface, and selection of a proper material. All sub-branches in the figure will be discussed in subsections of the present paper. In all sections, the review articles have been presented in the order of the year of publication. This is so helpful to understand the rev- olutionary progressing level of the energy harvesting during the last decades. The outline of the paper is as follows. At the first section, the focus is on the reviews about the design process, structures, the material considerations, size effects, and the mathematical modeling challenges. At the second part of the article, the main theme of the review will be evaluating applications of the piezo-harvesters. The most common applications include vibrational energy sources, fluid-based harvesters, scavenging energy from ambient waste ener-

5 5000

4000

3000

2000 Number of publications 1000

0 2000 2005 2010 2015 2020 2025 2030 Year

Figure 2: Overall history and future estimation of publications on piezoelectric energy harvesting. gies, and energy harnessing in biological applications. In the last section, a summary of future challenges, research directions, and missing review topics will be presented.

2. Reviews with non-focused topics

There is a few number of general review articles without having a specific focal point. Here, we are to investigate some general points presented by such review papers. Mateu and Moll [2] presented an overview of several meth- ods to design an energy harvesting device depending on the type of energy available for microelectronics. They summarized the power consumption of the microelectronic devices and explained the working principals of piezoelec- tric, electrostatic, magnetic induction, and electromagnetic radiation-based generators. They mentioned that the piezoelectric generators already have most of advantages of inductive and electrostatic generators. Also, they have a robust structure, but due to some difficulties in micro-machining process of piezoelectric materials, miniaturization of the piezoelectric harvesters is somehow more difficult.

6 7

Figure 3: Strategic map of piezoelectric energy harvesting design aspects, modeling issues, and applications. Aton and Sodano [3] reviewed some general topics published between 2003 to 2006, discussing the efficiency improvement, configurations, circuitry and method of power storage, implantable and wearable power supplies, har- vesting from the ambient fluid flows, the micro-electro-mechanical systems, and the self-powered sensors. There is no specific classifications of articles in this review. They described that the future directions are the development of a complete self-powered device, which includes power harvester, storage, and application circuitry combined. Also, they declared that enhancement of energy generation and storage methods along with decreasing the power requirements of electronic devices may be a prime target at future. Khaligh et al. [4] addressed the piezoelectric generators suitable for human- powered and vibration-based devices as well as the electromagnetic genera- tors, including resonant, rotational, and hybrid devices. A brief information has been presented about the hybrid generators provided by an imbalanced rotor, which needs more in-deep investigations in future reviews. They con- cluded that the output voltage of piezoelectric systems is too high and the output current is too low. In contrary, the electromagnetic generators can generate high output currents, but the voltage is lower than needed. Sun et al. [5] made a review on applications of piezoelectric harvesters. However, they put everything in a nutshell. Obviously, such topics need more close considerations. Studying an autonomous system of piezoelectric energy harvesting from the ambient with higher performance, designing controllable vibrator, adjustable tuning circuit, and larger energy transforming efficiency are their suggestions as future plans. Taware and Deshmukh [6] briefly re- viewed a number of literature in the field of piezoelectric energy harvesting. They mentioned advantages and disadvantages of some of piezoelectric mate- rials. They explained cantilever based piezoelectric energy harvesters, their related design points and mathematical model. Calio et al. [7] reviewed the material properties of 19 types of piezo- materials, the piezo-harvesters operating modes, the resonant/non-resonant operations, the optimal shape of the beam, the frequency tuning, the ro- tational device configurations, the power density and bandwidth, and the conditioning circuitry. They tried to make a selection guide between piezo- electric materials based on the power output and operating modes. They concluded that the resonant d33 cantilever beam needs the optimization and the d15 harvester is still too complex to be fabricated but with great poten- tial. This paper is a good suggestion for beginners to start a research in the field of piezoelectric energy harvesting.

8 Batra et al. [8] reviewed mathematical modeling and constitutive equa- tions for piezomaterials, the lumped parameter model, mechanisms of piezo- electric energy conversion, and operating principles of the piezoelectric energy harvesters. They reported the dielectric, the piezoelectric, the mechanical, and the pyroelectric properties of some of piezo-materials. Applications of piezo-harvesting from jaw, shoe, wind, knee, leg, railways, and road move- ments have been explained. A 4-page review paper exist [9], which mainly has focused on some points about the history of piezoelectric effect, piezo- materials, and applications like harvesting from foot steps and roads. Safaei et al. [10] made a comprehensive review in the field of piezoelec- tric energy harvesters as an update of their previous review [3], to account for more recent papers published between 2008-2018. Their literature re- view included the lead-free piezomaterials, the piezoelectric single crystals, the high-temperature piezoelectrics, the piezoelectric nanocomposites, 0-3 composites, piezoelectric foams, nonlinear and broadband transducers, and micro-electro-mechanical transducers. They also discussed several types of piezoelectric transducers, the mathematical modelings, energy conditioning circuitry, and applications such as fluids, windmill-style harvesters, flutter- style harvesters, from human body, wearable devices, implantable devices, animal-based systems, infrastructure, vehicles, and multifunctional/multi- source energy harvesting. Several useful illustration have been presented in the paper, which sums up different technologies in a unified framework. How- ever, their brief recommendations for future horizons in the field is just lim- ited to fabrication of piezoelectric nanofibers, piezoelectric thin films, print- able piezoelectric materials, exploiting internal resonance of structures, and development of metamaterials and metastructures. Sharma and Baredar [11] analyzed the current methods to harvest en- ergy from vibration using a piezoelectric setup in low-range frequency zone, piezoelectric material properties, and the modeling and experimental inves- tigations. They also presented shortcomings of the piezoelectric harvesters with respect to solar, wind and biomass energy sources. They indicate that the disadvantages of the piezo-harvesters are depolarization, sudden break- ing of piezo layer due to high brittleness and poor coupling coefficient, poor adhesive properties of PVDF material, and lower electromagnetic coupling coefficient of PZT. They discussed that the design of high-efficiency energy harvesters, invention of new energy harvesting designs by exploring non- linear benefits, and design of portable compact-size systems with integrated functions are great forthcoming challenges.

9 In summary, although most of the aforementioned general review papers have almost similar titles, but their scientific depth and the extent of reviewed items are clearly different. For example, some of these review papers are in fact an introduction for the piezoelectric energy harvesting concept, the working principles and applications. Some papers like Ref. [7] has focused on design strategies of the piezoelectric energy harvesters, and is a try to present a guide for the selection of piezoelectric materials for harvesters. There are a few review papers (like the one written by Safaei et al. [10]) which the titles are a real representative of the context. Moreover, almost all the aforementioned reviews suffer from weak classifications stemming from generality of their topic of intersect.

3. Design and fabrication 3.1. Materials The choice of suitable piezoelectric material is a critical step in designing energy harvesters [12]. Thus, lots of the review papers in the field of energy harvesters less or more have addressed the piezoelectric materials. Different performance metrics have been used for comparing piezoelectric materials on diverse applications. In actuating and sensing applications, the piezoelectric strain and piezoelectric voltage constants are appropriate criteria. However, for energy harvesting, the output voltage, the power density, costs, opera- tional bandwidth, and functionality at the working situation are the most important factors. Bedekar et al. [13] presented a comparison between properties of different single-crystal piezomaterials in longitudinal mode operating at high tem- peratures. They found that the power density of soft PZT is about three orders of magnitude higher than that of the high-temperature crystals at the room temperature. Y Ca4O(BO3)3 and La3Ga5SiO14 crystals annealed at 600 retrogrades promise magnitude of power similar to the power at the room temperature. Lefeuvre et al. [14] reported the coupling coefficient and the mechanical quality factor of a number of piezo-materials and explained their advantages and disadvantages. The electrical circuits with one- to three-stage interfaces designed for optimizing the electrical power flow have been compared for novel techniques. They presented the figure of merit for energy conversion as the product of the squared coupling coefficient and the mechanical quality factor. They concluded that the soft ceramics have high coupling coefficients

10 and moderate mechanical quality factors. The hard ceramics have lower cou- pling coefficients and higher mechanical quality factors. Hence, the soft and hard ceramics have roughly the same figure of merit. Although the efficiency is one of the main topics of the review, indication of a clear relationship between efficiency evaluation and the reported information is missing. Mukherjee and Datta [15] provided a comparative study of properties of a few number of piezoelectric materials, which are related to energy har- vesting from cantilever-based vibrational motion. Six piezoelectric ceramics have bee considered. The coupling factor, the quality factor, the relative di- electric constant, and the Young’s modulus were defined as the performance Indicators of the piezoelectric materials. They concluded that EC-97 EC-97 lead titanate generates more power at optimal resistance and at resonant frequency compared to the other materials. Safaei et al. reviewed piezoelectric ceramics like soft and hard PZTs, piezoelectric polymers including PVDF, piezoelectric single crystals, lead-free piezoelectrics, high temperature piezoelectrics, piezoelectric nanocomposites, and also piezoelectric foams. For some of these materials they reported the coupling coefficient, and the maximum output voltage of the material without describing the geometry of the piezoelectric harvester. Brittleness of PZTs and existence of health risks in PZT ceramics due to the toxicity of lead are the most important challenges of using PZTs, which motivates the develop- ment of lead-free flexible and high-performance piezoelectric materials. They concluded that the need for enhancement of electro, mechanical, thermal, and biocompatible properties has led to the introduction of new piezoelectric ma- terials including new lead-free piezoelectrics, high-temperature piezoelectrics, piezoelectret foams, and piezoelectric nanocomposites. A detailed overview of the piezoelectric and ferroelectric materials in- cluding inorganic and compliant piezoelectric materials was given by Bowen et al. [16]. They mentioned some points about high-temperature harvesting related to the Curie temperature, light harvesting into chemical or electri- cal energy, and optimization algorithms. Their investigation contains pa- rameters like pyroelectric coefficient (harvesting from temperature fluctua- tions), the electro-mechanical coupling, the mechanical quality factor, the constant-strain relative permittivity, the constant-stress relative permittiv- ity, the piezoelectric coefficient, and the elastic constant. For high-strain applications, they suggested polymeric or composite-based systems. sug- gested future directions are understating and development of new materials and gaining strong scientific underpinning of the technology and reliable mea-

11 surements. Applications of artificial metamaterials in energy harvesting have been investigated by Chen et al. [17]. Such artificial materials exhibit exotic and unique properties like negative stiffness, mass, the Poisson’s ratio, and the re- fractive index, which cannot be achieved by natural materials. They declared that the wide bandwidth, the frequency self-adaptation, the development of active, controllable, non-linear metamaterials, and obtaining high efficiency are future research hot-spots in the field. They believe that metamaterials will bring a revolution on conventional energy harvesting approaches. Yuan et al. [18] introduced the dielectric electroactive polymers as promis- ing replacements for conventional piezoelectric materials. Electroactive poly- mers are lightweight, flexible, ductile, low-cost, with high strength-to-weight ratio, low mechanical impedance, and can endure large strains. The dielec- tric polymers need high voltage to realize energy cycles, which may lead to the breakdown of the device. Piezoelectric materials are employed in energy harvesters because of their compact configuration and compatibility. How- ever, these materials have inherent limitations including aging, depolariza- tion, and brittleness. In comparison, electrostrictive polymers are promising candidates for replacing piezoelectric materials in vibration energy harvest- ing cases. The challenge in design of electroactive polymer energy harvesters is to develop systems capable of ensuring a constant initial voltage on the polymer at small cost. Narita and Fox [19] reviewed three categories of energy harvesting piezo- electric ceramics/polymers, magnetostrictive alloys, and magnetoelectric mul- tiferroic composites. Their review includes describing the properties of PZT, PVDF, and ZnO and also they remarked some advantages and disadvan- tages of traditional piezoelectric ceramics, piezoelectric polymers and com- posites. They focused on the characterization, fabrication, modeling, sim- ulation, durability and reliability of piezo-devices. Based on their analysis, the future directions include the device size reduction to make them suitable for nanotechnology, optimization, and developing accurate multi-scale com- putational methods to link atomic, domain, grain, and macroscale behaviors. Investigation of temperature-dependence of properties, development of mate- rials and structures capable of withstanding prolonged cyclic loading, dura- tion of electro-magneto-mechanical properties, and fracture/fatigue studies are other recommendations for future research. Liu et al. [20] discussed various types of harvester configurations, piezo- electric materials, fabrication techniques, performance enhancement, hybrid

12 energy harvesting, and limited applications of the piezoelectric setups. Their review on piezoelectric materials includes explaining the main mechanical properties and power output of one- and two-dimensional nonostructured materials, piezoelectric ceramics and polymers along with the piezoelec- tric bio-materials. They advised that harvesting the low-frequency high- amplitude ocean wave energy using a piezoelectric system, utilization of in- organic/organic, bio piezomaterials, enhancing the low output performance and narrow bandwidth still need more close attention. They also mentioned some points on application of piezomaterials in internet of things. They concluded that with the current rapid development of the internet of things (IoTs), the energy harvesting will become an emerging technology for the development of smart cities, smart homes, smart health, smart agriculture, intelligent transportation, industry, security, and marine. Mishara et al. [21] focused on the basic theory and principles behind the piezoelectric energy harvesting devices, comparison of piezo-ceramics and piezo-polymers, structural configurations and fabrication. Some types of piezoelectric polymers such as polyvinylidene fluoride, polylactic acid, cel- lulose, polyamides, polyurea, polyurethanes, and their composites were ex- tensively addressed. They also presented a review of performance of the uni-morph/bi-morph cantilever beam, the circular diaphragm, the cymbal transducer, and the stacked structures. They announced that the design and fabrication of the device, the compatibility with advanced technologies, long durability by meeting performance requirements need to be considered more deeply. It is reported that the piezoelectric market is going to take 28.8% of the energy harvesting market by reaching over 800 million dollars experiencing a sharp trend. Priya et al. [22] summarized materials and resonator structures as well as the key metrics like the power density and the bandwidth of structures at low frequency input in microscale piezoelectric energy harvesting. They also gave an overall outlook of the fabrication methods, including the par- ticle/granule spray deposition techniques, aerosol-deposition (AD), granule spray in vacuum and circuits with the aim of minimal energy dissipation. Achieving higher power density and wider bandwidth of resonance are the two biggest challenges currently facing the technology of MEMS based en- ergy harvesters. They expect that in future, a coin size harvester will deliver about 100µ W continuous power below 100 Hz at less than 0.5 g vibration and at a reasonable cost. In this paper the main focus is on the MEMS-based piezoelectric energy harvesters and their related design aspects, however the

13 title refers to more general aspects. Gosavi et al. [12] addressed various materials that are in use for energy harvesting at the micro and nanoscales. They defined a systematic roadmap to select the piezoelectric materials for employing in micro and nanoscale energy harvesters. They pointed out that the ZnO thin film is the most widely used structure in micro and nanoscale harvesters, and can be eas- ily synthesized in the required sizes, shapes and are very economical to be fabricated. Zaarour et al. [23] summarized the energy harvesting technologies devel- oped based on piezoelectric polymeric fibers, inorganic piezoelectric fibers, and inorganic nanowire. The paper contains a complete review of piezo- electric fibers with respect to the peak voltage, the peak current, the active area, and their advantages and disadvantages. Standardizing the perfor- mance of the piezo-nanogenerator, developing effective packaging technol- ogy, packaging of nano-piezo-harvesters, commercializing product for harsh environments, finding a suitable approach for enhancing the electrical out- puts, and enhancing the durability and the output stability are some future horizons. Table 1 sums up related highlights and presents a list of different piezo- materials used in construction of the piezoelectric harvesters. Most of the review papers have tried to compare the piezoelectric materials and draw a roadmap for selecting an appropriate material for an energy harvester. However, the choice of material is strictly depended on the type of energy harvester, its working condition, the cost, accessability and ease of fabrica- tion/synthesis of the piezoelectric material. For example, Ullah Khan and Ahmad [24] who have reviewed vibration energy harvesters (VEHs) utilizing bridge oscillations, pointed out that in piezoelectric vibration energy har- vesters (PE-VEH), the main selection criteria are the dielectric constant, the Curie temperature, and the modulus of elasticity of the material. For instance, in high acceleration vibrations, the piezoelectric materi- als with high value of elastic modulus can be an appropriate choice. How- ever, the piezoelectric materials like lead lanthanum zirconate titanate, which has a very high value of dielectric constant will perform very well in low- acceleration vibration environments. Also, due to the easiness of in situ fabrication of lead zirconate titanate (PZT) with sol-gel technique, and its easy integration with the other microfabrication processes, PZT is largely utilized in most of the developed PE-VEHs. As another example, we can point out the selection of a desirable piezo-

14 Table 1: A summary of review papers related to the materials used in piezo-harvesters in the order of the year of publication. Materials Highlights Bedekar et al. [13] Soft P b(Zr, T i)O3 (PZT), Y Ca4O(BO3)3 In longitudinal mode configuration (YCOB), La3Ga5SiO14 (LGS), LiNbO3 (33-mode); annealed material; high- (LN) temperature operation. Lefeuvre et al. [14] Soft and hard ceramic PZT, PZT fibre com- In longitudinal (33) and transversal posites, PMN-0.25PT ceramic, PMN-0.25PT (31) modes. single crystal, BaT iO3, PVDF, Quartz Mukherjee and Datta [15] PSI-5H4E Lead Zirconate Titanate, EC-98 Output power; same geometrical de- EC-98 lead magnesium niobate, EC-57 bar- tails, the 31-mode. ium titanate, EC-97 EC-97 lead titanate, 804 lead zirconate titanate, 906 lead nickel nio- bate Bowen et al. [16] GaN, AlN, CdS, ZnO, α-quartz, BaTO3, Comparison of piezo/ferro/pyroelectric PZT-4 hard PZT, PZT-5H PZT, PMN-PT, materials; 33-, 31- and 15-modes. LiNbO3, PVDF Chen et al. [17] Metamaterials Unique properties; comparison with acoustic, phononic, electromagnetic harvesters. Yuan et al. [18] PZT, PVDF, P(VDF-TrFECTFE), silicone, Comparison with dielectric electroac- Acrylic tive polymers Narita and Fox [19] ZnO, P[VDF-TrFE], PVDF, BaT iO3, PZT, Comparison with magnetostrictive al- LiNbO3 loys, and magnetoelectric multiferroic composites, and pyroelectrics; 33- and 31-moads. Liu et al. [20] ZnO, MoS2, W Se2, PMN-PZT/PT, PZT, Peak power, peak voltage, peak cur- Alkaline niobate (KNLN), BaTo3, AlN, rent, power density PVDF, Cellular PP, PDMS piezoelectret, PET/EVA/PET piezoelectret, fluorinated ethylene propylene (FEP) paralleltunnel piezoelectret, M13 bacteriophage, fish swim bladder Mishara et al. [21] PZT, polyvinylidene fluoride, polylac- Focus on piezo-polymers, fabrication, tic acid, cellulose,15 polyamides, polyurea, configuration, comparative points for polyurethanes ceramic- and polymer-based systems. Priya et al. [22] PZT, AlN, potassium sodium niobate Methods, circuits, microscale; 33- and (KNN), P b(Zn1/3Nb2/3)O3 − P bT iO3, 31-modes; coefficient of thermal expan- P b(Mg1/3Nb2/3)O3 − P bT iO3, sion. P b(Y1/2Nb1/2)O3 − P bT iO3, PbSc0.5T a0.5O3 Zaarour et al. [23] SiO4, AlPO4, cane sugar, rochelle salt, Nanofibers, nanowires, piezo- topaz, tourmaline-group minerals, PVDF, nanogenerators. PTrFE, PTFE, Polyurea, Nylon II, PP, FEP, PAN, synthetic crystals, synthetic ceramics, lead-free piezoceramics electric material for walking energy harvesting applications. Based on a review performed by Maghsoudi Nia et al. [25], this application needs an incombustible, chemically resistant, low price material, which is unbreakable under harsh conditions. For example, these criteria have made PVDF more suitable than PZT for the most of piezoelectric harnessing from walking. Most of the review papers have contented themselves with reporting some electromechanical properties of piezoelectric materials, and provided a few information on the accessability, relative cost, chemical properties, ease of fabrication, and suitable working conditions of different piezoelectrics. This lack of information is demonstrative of the need for further research and also the necessity for making more comprehensive, and application-based reviews on piezoelectric materials.

3.2. Structure All the piezoelectric energy harvesters include a mechanical part (or transduction part) which converts the input mechanical energy into elec- tric charges on the piezoelectric material, and an electric part that keeps the electric charges and converts them into a suitable form of electric output like electric direct voltage. Design of the mechanical part of a piezoelectric energy harvester usually includes the determination of its size, configuration, working modes, and its materials in order to enhance some of its performance characteristics like the output electric energy, the conversion efficiency and the working bandwidth. The size of the piezoelectric energy harvesters vary from micro and nanoscale (lower than 0.01 cm3) to the macro-scale (75 cm3) [26]. Upon the literature, the piezoelectric energy harvesters can be classified from different points of view. Form the view point of operating frequency, they may divided into two main categories; the resonant type devices which operate at or near their resonance frequency, and non-resonant systems that do not depend on any specific frequency to operate. The piezoelectric en- ergy harvesters may harvest energy from motions in a unique direction or from multi-directions. Accordingly, they may be single-directional or multi- directional harvesters. Also, they may have a single or several vibration modes. The last one is named as multi-modal harvesters. From the view- point of governing dynamic models, the piezoelectric harvesters may be linear or non-linear [27]. As indicated in Fig. 3, their configuration may be classi- fied as cantilever type, stack type, cymbal type, circular diaphragm type, or the shell and film types.

16 Wardle et al. discussed several design considerations for microscale piezo- electric energy harvesters [26]. They stated that the power or energy sources can be divided into two groups: sources with a fixed energy density (e.g., batteries) and sources with a fixed power density (normally ambient energy harvesters). Figure 4 compares two strategies of fixed-energy and fixed-power density production of electric power during 1-year and 10-year operating in- tervals (using the data of seven references). Figure 4 not only provides a comparison based on the density of the output power, but also indicates that the power density of the fixed-energy density sources extensively drops after just 1 year of operation. So, they need maintenance and repair if possible. Designing an effective power normalization scheme, the strain cancelation due to multiple input vibration components, optimizing the minimum vibra- tion level required for positive energy harvesting, and the prototype testing to eliminate the proof mass are among their suggestions as future works.

Figure 4: Comparison of power density of different strategies of power generation [26].

Priya [28] gave a review of piezo-harvesters appropriate for light-weight flexible systems with easy mounting, large response, and low-frequency oper- ation; called the low-profile piezo-transducers in on/off-resonance conditions. A good discussion on piezoelectric polymers, energy storage circuit, and mi- croscale piezo-harvesting device is available in the article. They mentioned that the electrical power generated by the piezoelectric energy harvesters

17 is inversely proportional to the damping ratio, which should be minimized through proper selection of the materials and design. They also have summa- rized the conditions leading to the maximum efficiency in low profile piezolec- tric energy harvesters. Szarka et al. [29] focused on the electrical design of low-power (lower than 10 mW) piezoelectric and electromagnetic transduction mechanisms describing the power electronics circuits. The main review area includes the AC-DC conversion, the passive and active rectifications, the start-up issues, the harvester-specific interactions, the voltage conditioning, the DC- DC charge pumps, the power regulation, and the impedance matching. The proposed future challenges are developing the start-up topologies with the input voltages in the order of deci-volts, further improving the conversion efficiencies of switching converters at micro-watt power levels, the power efficient control techniques, and the energy harvesting systems for naturally occurring vibrations. Guyomar and Lallart [30] discussed the non-linear electronic interfaces for energy harvesting from mechanical vibrations using piezoelectric cou- pling. They classified the energy harvesting systems and compared them based on their performance characteristics and implementation issues. They demonstrated the use of intermediate energy tanks for decoupling and initial energy injection for conversion magnification. Limitations for the MEMS- based designs are for magnetic components like inductors and transformers, electronic commands, discrete components such as diodes and transistors, which need to be considered in advanced future designs. Ibrahim and Vahied [31] focused on tuning the resonant frequency as a solution to reduction of power output due to deviation from resonance fre- quency, which is one of the most important challenges of the piezoelectric system design. The other solution may be widening the operating frequency bandwidth. They categorized such methods into manual and autonomous tuning systems and reviewed different manual and automatic techniques for the frequency tuning. They commented that the most suitable method for frequency tuning depends on the intended application. Future plans may be the development of a truly autonomous standalone energy harvester, sup- plying the necessary power for the tuning mechanism, integration of the piezoelectric transducer, circuitry, power management, and energy storage into flexible thin film substrates. Li et al. [32] presented a review of configurations such as cantilever beam, discs, cymbals, diaphragms, circular diaphragms, shell-type, and ribbon ge-

18 ometries. They categorized the main types of piezoelectric materials based on the working frequency of the harvester. They concluded that the piezo- electric ceramics provide a higher power output, and the piezoelectric poly- mers generally generate the smallest power output. They also discussed the optimization methods like the matching frequencies, up converting, the band- width broadening of piezoelectric energy harvesters, utilizing the metal end caps to amplify the effective piezoelectric strain constant, and the bi-stable cantilever structure with white noise excitation. At the end, they reviewed some design points about three main components of circuits of harvesters, the rectifier, the regulator, and the energy storage device. They stated that the biggest challenges for the piezoelectric energy harvesters are the low in- put frequencies of the mechanical energy sources and the difficulty to get the piezoelectric harvesters to efficiently respond to them, and the perfor- mance limit of the piezoelectric materials. Future of the piezoelectric energy harvesting is dependent on lowering the energy consumption of electronic de- vices, unless new piezoelectric materials appear to enhance their performance to a higher level. Yildirim et al. [33] focused on design considerations to enhance the perfor- mance of ambient-based piezo-harvesters, including amplification techniques, resonance tuning methods and introducing the non-linear oscillations. Such design methods are frequency up conversion, employing mechanical amplifier, hybrid design of the harvester, increasing the degrees of freedom, preloading the piezoelectric harvester, multimodal arraying, using extensional modes, magnetic tuning, mono/bi-stable systems, mechanical stoppers, stochastic loading, and utilizing piezo-magneto-elastic materials. They categorised the power amplification techniques and frequency broadening techniques for the performance enhancement of the vibration-based energy harvesters. They discussed pros and cons of each technique from the need to extra space for mechanical amplifiers to preventing damage to piezoelectric elements due to shock forces in the mechanical non-linear stopper. Regarding large discrepancies in the definition and tested values of effi- ciency in literature, Yang et al. [34] tried to investigate different ways to cal- culate the energy conversion efficiency of the piezoelectric harvesters. How- ever, this article is not fundamentally a review paper, but there are fruit- ful classified information about computation of the non-resonance and the on-resonance efficiencies. They derived an analytical expression for the effi- ciency of the cantilever piezoelectric energy harvesters. They indicated that efficiency is related to the electro-mechanical coupling effect, the damping

19 effect, the excitation frequency, the electrical load, the light damping, and the excitation frequency. Uchino [35] starts his review by mentioning the historical background of the piezoelectric energy harvesting, and explaining several misconceptions by the current researchers. He reviewed the different design approaches followed by mechanical, electrical, and MEMS engineers. It is mentioned that the five important figure of merits in piezoelectrics are the piezoelectric strain con- stant d, the piezoelectric voltage constant g, the electromechanical coupling factor k, the mechanical quality factor Qm, and the acoustic impedance Z. Also, energy transfer rates for piezoelectric energy harvesting systems with typical stiff cymbals and flexible piezoelectric transducers were evaluated for three phases/steps including mechanical-mechanical energy transfer (as step 1), mechanical-electrical energy transduction (as step 2) and electrical- electrical energy transfer (as step 3). Moreover, a hybrid energy harvesting device which operates under either magnetic and/or mechanical noises was introduced. It is concluded that the remote signal transmission, energy ac- cumulation in rechargeable batteries, discovering a genius idea to combine nano-devices in parallel, and enhancing the energy density in medical appli- cations have been introduced as future research fields. Uchino, does not have a positive comment on future of NEMS and MEMS piezoelectric harvesters due to their very low energy levels (in the order of pW to nW). He believed that without discovering a genius idea on how to combine thousands of the nano-devices in parallel and synchronously in phase, the efforts on designing the NEMS/MEMS piezoelectric harvesters will be in vain just as a dream. Dell’Anna et al. [36] described the cutting-edge status of the research on power conditioning, qualitative working principle, adapted control schemes, and the employed components for the piezoelectric energy harvesters. Seven criteria were investigated, including efficiency, standalone operation, circuit complexity, adaptivity, micro-scale compatibility, start-up operation, and the minimum operating voltage. Ameliorating the performance, optimizing sys- tems with respect to high peak efficiency performance, the minimum op- erating voltage, the time-varying exciting frequency and amplitude in real applications need more close attention in future works Yang et al. [37] performed a comprehensive review of different designs strategies, the non-linear methods, the optimization techniques, and the har- vesting piezo materials in applications like shoes, pacemakers, tire pressure monitoring systems, bridge and building monitoring. They declared that high energy conversion efficiency, ease of implementation, and miniaturiza-

20 tion are the main advantages of such systems. However, authors state that enhancement of energy efficiency of the piezo-based harvesters is still an open challenge. They also made a systematic performance comparison on some of the energy harvesters. They pointed out that a considerable gap exists be- tween the achieved performance and the expected performance. Therefore, in situ testing, applying more realistic excitations, system-level investiga- tions on piezo-harvesters integrated with power conditioning circuits, energy storage elements, sensors, and control circuits are needed to be investigated. Maamer et al. [27] discussed some missing concepts in previously pub- lished papers about the main different improvement techniques and the de- sign concepts for the piezoelectric and electromagnetic transduction. They classified the mechanical energy harvester improvement techniques. In ad- dition to the general concepts such as improving efficiency and enhancing the output power, they reviewed some overlooked concepts such as the non- resonant and multi-directional systems, techniques for widening the operating frequency, conceiving a non-resonant system, frequency tuning, geometrical adjustment, multi-frequency, multi-modal and non-linear systems, frequency- up conversion, free moving object, and optimization techniques. Talib et al. [38] explained effective strategies and the key factors to en- hance the performance of piezoelectric energy harvesters operating at low frequencies, including the piezoelectric material selection, optimizations of the shape, size and structure, and development of multi-modal, nonlinear, multi-directional, and hybrid energy harvesting systems. This review paper is suitable for the beginners who want to get acquainted with the piezo- electric materials and some designs of piezoelectric energy harvesters. They concluded that the recent developments are inclined towards generation of more power from low-frequency and low-amplitude ambient vibrations with reduced required piezoelectric material. Adding a single DOF system in the form of an extension beam or a spring to the piezoelectric beam is a remark- able advise to enhance the power output. They showed that the multi-modal energy harvester exhibits a broader bandwidth when its multiple resonance peaks get closer. Brenes et al. [39] provided an overlook of existing energy harvesting cir- cuits and techniques for piezoelectric energy scavenging to distinguish be- tween existing similar solutions, which are different in practice. Such cate- gorization is helpful to ponder the advantages and drawbacks of each avail- able item. Their review is unique since they have classified piezo-systems based on adaptive versus non-adaptive control strategies, topologies, archi-

21 tectures, techniques form one hand, and electromechanical models from the other hand. The best system has been introduced with respect to the opti- mized power efficiency, the design complexity, the strength of coupling, the multi-stage load adaption, and the vibration frequency. Al-Yafeai et al. [40] presented a review of design considerations for en- ergy harvesting from car suspension system, including the piezomaterials, the mathematical modeling, the power dissipation, number of degree-of- freedoms, the road input, location of the piezo-system, and the electronic circuit. Dagdeviren et al. [41] highlighted essential mechanical to electri- cal conversion processes and the key design considerations of flexible and stretchable piezoelectric energy harvesters appropriate for soft tissues of hu- man body, smart robots and metrology tools. They declared that the de- velopment outlooks of such devices are piezomaterial designs, fabrication techniques, and device machine mechanics. Table 2 gathers together all de- sign parameters and descriptions of design points mentioned in the review papers.

3.3. MEMS/NEMS-based devices The largest number of reviews on piezo-harvesters have been written in the field of MEMS or NEMS piezoelectric harvesters. Micro and nanoscale energy harvesters are very useful for easy powering or charging of mobile electronics, even in remote areas, without the need for large power storage elements. MEMS-type devices include cantilever, cymbal and stack whereas NEMS type devices are wires, rods, fibres, belts and tubes. Generation of output electric current using piezoelectric energy harvesters faces many limi- tations and difficulties. Some of these limitations are low output power, high electric impedance, crack propagation in most of piezoelectric materials due to overloading, frequency matching of the harvester with vibrational energy sources, and fabrication/integration of piezoelectrics in micro/nanoscale [42]. Beeby et al. [43] gave a review of existing piezoelectric generators and micro generators, including the impact coupled, the resonant and human- powered devices, and the cantilever-based setup in comparison with two electromagnetic and electrostatic mechanisms. The coupling factor of each transduction mechanism is summarized in tables classified by the transduc- tion type. They commented that the piezoelectric generators are the simplest type of generator to fabricate and are capable of producing relatively high output voltages, but only at low electrical currents. Their conclusions may help readers to be familiar with the suitability of each technique.

22 Table 2: Details of design considerations of piezoelectric energy harvesting systems in the order of the year of publication. General topic Design parameters Al-Yafeai et al. [40] Car suspension Circuit, material, mechanisms. duToit et al. [26] MEMS, vibration The device size; the maximum tip displacement at maximum power output; operating volume; the mechanical damping ra- tio; the electrical load; the device mass; the input vibration characteristics. Priya [28] Low-profile piezo- Selection of the piezoelectric materials for on and off resonance transducers applications; analytical models. Szarka et al. [29] Power conditioning Complexity; efficiency; quiescent power consumption; startup behavior; utilization of the harvester compared to an optimum load. Guyomar and Lallart [30] MEMS Nonlinear electronic interfaces; small scale implementation; conversion enhancement principles; switching techniques Ibrahim and Vahied [31] Frequency tuning Resonant frequency, the center of gravity, the stiffness (by axial preload, extensional mode, magnetic/electric methods), dimension, mass (by adding a tip mass), electrical damping. Li et al. [32] Low frequency Geometry, piezoelectric material, matching the resonance fre- quency of the piezoelectric element to input frequency of the host structure, electronic circuits design. Chua et al. [81] Rain drop Circuit design. Khan and Ahmad [24] Bridge Resonant frequency; material, mechanical and electrical char- acteristics; the external mechanical and electrical load condi- tions. Yildirim et al. [33] Ambient vibration High-performance design. Yang et al. [34] Efficiency The non-resonance and the on-resonance efficiencies. Uchino [35] Developments Mechanical-mechanical energy transfer; mechanical-electrical energy transduction; electrical-electrical energy transfer. Dell’Anna et al. [36] Power management Comparison with respect to seven design criteria circuits Yang et al. [37] Applications High-performance; shoes, pacemakers, tire pressure monitor- ing systems, and bridge and building monitoring; high power output; broad operational bandwidth. Maamer et al. [27] General The excitation form, the operating frequency band, conceiving a non-resonant system, multi-directionality (2D case more re- trievable), possibility of integration in MEMS devices, multi- direction23 harvesting, lifespan, welding operation. Talib et al. [38] Low-frequencies Performance increment; multi-modal, nonlinear, and multi- directional design; material characteristics; optimized geome- try for high strain energy density; flexibility; high excitation amplitude; broad bandwidth. Brenes et al. [39] Maximum power Electrical tuning; circuit design; mathematical models; clas- point sification of technologies. Cook-Chennault et al. [44] provided an overview of strategies for power- ing MEMS via non-regenerative (batteries, microcombustors, turbine, heat engines, micro-fuel cells) and regenerative (solar cells, thermoelectrics, elec- tromagnetics, electrostatics, piezoelectrics) power supplies. Investigation of PMN, PZN, PMN-PT and PZN-PT/polymer composite fabrication, ana- lytical techniques for prediction of performance, temperature-dependence of the piezoelectric properties, development of hybrid piezoelectric/storage devices, enhancement of charge density, cyclic loading type and duration, nanoscale fabrication, and optimization of combinations of storage and gen- eration schemes are open areas worthy of exploration. They also made a comparison between electromagnetic, electrostatic and piezoelectric energy harvesting transduction mechanisms. Kang et al. [45] focused on recent progress of the bulk-type, MEMS-based, flexible piezoelectric energy harvesting technologies using PZT from view- point of material, fabrication, and unique design. The nonorods, nanofibers, nanoparticles, nonowires, and thin film piezo-harvesters have been investi- gated. Enhancing performance of the flexible piezoelectric energy harvesting technologies and the packing density of the active piezomaterials, generation of uniform and standard output voltage, current, and power are their advises as future directions. Muralt et al. [46] discussed the impact of composition, orientation, and microstructures on piezoelectric properties of perovskite thin films like PbZr1- xTixO3 (PZT) in applications such as low-voltage radio frequency MEMS switches and resonators, actuators for millimeter-scale robotics, droplet ejec- tors, energy harvesters for unattended sensors, and medical imaging trans- ducers. The article is mostly devoted to review of applications. they con- cluded that an important point in thin-film piezoelectric harvesters is the choice of the electrodes. also, it is much better to use interdigitated electrodes for harvesting with high permittivity piezoelectrics. Matching of effective re- sistance of the harvesting circuit to the internal piezoelectric resistance is the only recommended future issue for harvesters in the paper. Wang [47] gave an overall review of theoretical and experimental char- acterization methods for predicting and determining the piezoelectric poten- tial output and the representative models of nanogenerators with the source of high-frequency acoustic waves and low-frequency vibrations or frictions. Theoretical predictions have shown that in the nanometer range, flexoelec- tric effect plays a significant role in enhancement of the electromechanical coupling. They believe that harvesting ambient mechanical energy at the

24 nanometer scale holds great promises for powering small electronics. The flexoelectric effect in nanoscale electromechanical coupling considering the film properties of sub-100 nanometer harvesters including mechanical prop- erties, coupling between the piezoelectric and semiconductor properties, the piezoelectric nanomaterial fabrication, the device integration, and adapting large-scale roll-to-roll processing are the future challenges and opportunities. Nechibvute et al. [48] presented a review of the piezoelectric micro/nanogenerators as a renewable energy source to power wireless sensors as autonomous devices. Their conclusions and comments are reducing the power consumption of the wireless sensor node by duty cycling, experimental and theoretical model- ing and optimization of the piezoelectric and flexoelectric effects in single crystalline nanowires, coupling of piezoelectric and semiconducting effects, improving the fabrication methods and the associated device integration techniques, efficient design of interface circuits, advancing power manage- ment techniques, developing ultralow power wireless micro-controller units, and enhancing the power output of the piezoelectric generators to match the requirements of the wireless sensor devices. Kumar and Kim [49] presented a review on ZnO nanostructures as a good candidate for energy harnessing, due to its multi-functional characteristics such as excellent electrical and optical properties, inexpensiveness, relative abundance, chemical stability towards air, environmental friendliness, and a much simpler and wide range of crystal-growth technology. They also ex- plained its potential applications in photovoltaics, piezoelectric nano-energy generation, and hybrid systems. They claimed that there is a good potential for ZnO to be used in nanogenerators and hybrid devices to further improve the output performance, the neutralization of the the piezoelectric poten- tial screening effect, and optimization and localization of the free carriers in nanowires. Kim et al. [50] commented that for the elimination of chemical batteries and complex wiring in microsystems, a fully assembled energy harvester with the size of a US quarter dollar coin should be able to generate about 100 micro Watt of continuous power from ambient vibrations. In addition, the cost of the device should be sufficiently low. They reviewed challenges of the piezo- harvester, including the need to high power density and wide bandwidth of operation of the piezoelectric systems, the non-linear resonating beams for wide bandwidth resonance, and improvements in materials and the structure design. They concluded that the epitaxial growth and grain texturing of the piezomaterials, the embedded medical systems, the lead-free piezoelectric

25 MEMS-based materials, and the giant-piezoelectric coefficient materials are active research fields. They presented an extensive comparison of thin-film piezo-systems from various sources and concluded that the state-of-the-art of power density is still about one order smaller than what is needed for practical applications. Toprak and Tigli [51] conducted a review on piezoelectric harvesters based on their size; nanoscale, microscale, mesoscale, macroscale. They also pre- sented an interesting statistics that the number of publications between 2009 and 2014 on piezoelectric harvesting is more than twice the sum of publica- tions about the electromagnetic and electrostatic systems. They commented that the inherent reciprocal conversion capability is an important advantage of the piezoelectric energy harvesters, which allows them to have simpler ar- chitectures compared to their electromagnetic and electrostatic counterparts. It is declared that the bio-compatibility, the reconciliation with the CMOS technology, the rectification and storage losses, and enhancing the operation bandwidth are the most challenging issues about such systems. A discus- sion on validity of the classical constitutive relations for the piezomaterials in nanoscale is a missing section of the paper. Salim et al. [52] announced that to further increase the effectiveness of the MEMS-based harvesters from ambient energy, researchers started to venture into hybrid energy harvesters. They elaborated details of hybrid electromagnetic-piezoelectric and triboelectric/piezoelectric MEMS-based har- vesters and their privileges such as small features, ability for monolithic inte- gration with the integrated circuit in a single platform, robustness, and easy fabrication in bulk. Development of electronic interface circuits, employing the power produced from the electromagnetic part of a hybrid harvester, and designing a synchronized switch harvesting on inductor (SSHI) system are their recommendations as promising ways for power enhancement. Briscoe and Dunn [53] summarized works on nanostructured piezoelectric energy harvesters, including single and arrays of strained ZnO nanorods, flexible substrates, and alternative nanostructures. Their main focus was on ZnO-based devices and their paper contains many SEM images of such materials. They commented that a great advantage of ZnO nano-rods is that they can be grown in aligned arrays on plastic substrates. This offers the bending potential of the device by the substrate. Maximizing the rate of change of any strain delivered into a system to increase the polarization, improving the coupling of the device to the environment, and advancement of materials and device architectures are their recommendations for future

26 works. Wang et al. [54] provided a survey of piezoelectric nanogenerators, funda- mental piezoelectric theory, typical piezoelectric materials, the working mech- anisms, modeling, and the structure design concepts. Based on their discus- sion, typical piezomaterials are natural crystals, natural materials, synthetic crystals, synthetic ceramics, polymers, lead-free ceramics, organic nanos- tructures, and piezo-nanowires including the lead-free niobates, piezoelectric semiconductors, BaT iO3, and ZnSnO3. They described the fabrication and performance of vertical and laterally integrated nanogenerators. Future de- velopments are increasing the output power, the integration packaging of en- ergy storage unit with the nanogenerators, optimization of structural design for various working conditions, the long-term stability, mechanical strength, and chemical stability. Khan et al. [55] covered the electro-mechanical properties of thin film piezomaterials, including the lead zirconate titanate, the lead-free piezoma- terials, piezopolymer films, cellulose-based electro-active paper (EAPap) in applications such as sensory and actuation systems, energy harvesting, and medical and acoustic transducers. They reviewed the enhancement of piezo- electric response, advances in integration, applications as piezoelectric sen- sors, actuators, resonators, and energy harvesters of piezoelectric thin films. They suggested future challenges including the development of high-response thin film piezoelectrics, refined control of surface roughness, and exploration of nanoscale and microscale piezoelectric devices. Selvan and Ali [42] reported benefits, capacities, applications, challenges, and constraints of micro-power harvesting methods using thermoelectric, thermophotovoltaic, piezoelectric, and microbial fuel cell. Moreover, they highlighted outstanding breakthrough performances of each of the mentioned power generators. They reported that in comparison to other methods, a piezoelectric system has smaller size and a simpler mechanism, generates higher output voltage, is highly sensitive to applied strain, possesses higher frequency response, and experiences a longer life cycle. In the meanwhile, the piezoelectricity requires flexible and lead-free materials, a wider bandwidth, and increased operating frequency. The piezo-device’s impedance is high, and the material is prone to cracking or breaking. Similar to several other reviews, they recommended that more enhanced methodologies are required to further increase the energy conversion efficiency making them compact, wearable, small-scale, and flexible structures. Todaro et al. [56] reviewed the current status of the MEMS-based en-

27 ergy harvesters using piezoelectric thin films, and highlighted approaches and strategies. They commented that such harvesters are compact and cost-effective especially for harvesting energy from environmental vibrations. They believe that two main challenges of this topic to achieve high-performance devices is increasing the amount of generated power and the frequency band- width. They also introduced the theoretical principles and the main figures of merit of energy conversion in piezoelectric thin films. Other recommenda- tions for future research are developing proper materials, new device archi- tectures and strategies involving bimorph and multimorph design exploited for bandwidth and power density improvements, progressing in synthesis and growth technologies for lead-free high quality piezoelectrics, employing new flexible materials with tailored mechanical properties for larger displacement and lower frequencies, and exploiting the non-linear effects for obtaining a wider bandwidth and a higher efficiency. Jing and Kar-Narayan [57] reviewed nanostructured polymer-based piezo- electric and triboelectric materials as flexible, lightweight, easy and cheap to fabricate, being lead-free and biocompatible, and robust harvesters. They highlighted effects of growth parameters, nanoconfinement, self-poling, sur- face polarization, crystalline phases, and device assembly on energy harvest- ing performance. They indicated that most of previous research have fo- cused on certain bulk polymers. Extending such concepts to polymer-based nanocomposites, and searching for other polymeric candidates like hybrid materials are necessary. Future research topics are discovering new piezo- electric polymeric systems and crystalline phases usable as both piezoelectric and triboelectric harvesters. Gosavi and Balpande [12] discussed nanodevices, material synthesis tech- niques, fabrication processes, performance metrics, and device characteriza- tion of micro/nanoscale piezo-harvesting setups. They listed 15 properties of piezomaterials, including d31, d33, d15, relative permittivity, polling field DC, depoling field AC, the Curie temperature, the dielectric breakdown, density, open-circuit stiffness (E11, E33), compressive strength, compressive depoling limit, and dynamic/static tensile strength for PZT, PVDF, ZNO, ALN, BaT iO3. Also, there is a good discussion on commercial status of harvesters in the article. They suggested that a great scope for further en- hancements exists for conversion efficiency and operating frequency band- width, shifting from micro to nanoscale to boost growth of applications as implantable nanogenerators, design of hybrid energy devices, self-harvesters, biocompatible non-toxic and high output flexible energy harvesters based on

28 lead-free piezoelectric thin films. Table 3 presents a list of details of reviews on micro/nanoscale energy harvesters. In summery, almost all review articles discussed some great chal- lenges of development of MEMS/NEMS-based piezoelectric harvesters like the limited bandwidth and low output power. On the other hand, beside these type of harvesters, there are some competitive technologies like elec- tromagnetic, thermoelectric, and electrostatic energy harvesting, which are employed for scavenging the environment waste energy. Unfortunately, lim- ited systematic and reliable reviews have been done on comparison of these harvesting systems. Most of the comparative review papers have focused on output power and coupling coefficient of the harvesting systems and other important future like lifetime, capability of working in harsh environmen- tal condition, cost, commercial accessability and (TRL) technology readiness levels have been ignored.

3.4. Modeling approaches Most of review papers have described some of the existing mathemati- cal models in order to clarify the physical bases of the piezoelectric energy harvesters for the readers. There are a few number of review papers, which have focused on evaluation of different modeling approaches for piezoelectric energy harvesting. Erturk and Inman investigated mechanical [58] and mathematical [59] aspects of the cantilevered piezoelectric energy harvesters to avoid reuse of simpler and incorrect older models in literature. They reviewed the gen- eral solution of the base excitation problem for transverse and longitudinal vibrations of a cantilevered Euler-Bernoulli beam. They proved that the classical single-degree-of-freedom (SODF) predictions may yield highly inac- curate results, and is just appropriate for high tip-mass-to-beam-mass ratios. Damping due to internal friction (the Kelvin-Voigt damping), damping re- lated to the fluid medium, base excitation as a forcing function, and the backward piezoelectric coupling in the beam equation are among modeling parameters. Zhao et al. [60] compared different modeling approaches for galloping the piezoelectric wind energy harvester, including single-degree-of-freedom, single-mode and multi-mode Euler-Bernoulli distributed-parameter models (ignored in Ref. [59]). They concluded that the distributed-parameter model has a more rational representation of aerodynamic force, while the SDOF model more precisely predicts the cut-in wind speed and the electro-aeroelastic

29 Table 3: Details of review papers on piezoelectric energy harvesting for micro/nanoscale applications in the order of the year of publication. Main topic Extra descriptions and highlights Beeby et al. [43] Vibration-based Comparison with electromagnetic and elec- trostatic; micro-systems Cook-Chennault et al. [44] MEMS portable devices Comparison of non-regenerative and regener- ative power supply systems Muralt et al. [46] Sensors/actuators/harvesters Thin films; material properties; piezotronics

Wang [47] Nanogenerator Ambient energy harvesting; nanometer scale; theoretical and experimental characteriza- tion; the piezotronic effect Kumar and Kim [49] Semiconducting ZnO Comparison of its performance in photo- voltaics and the hybrid setup with piezoelec- tric harvester Kim et al. [50] Piezo-MEMS Grain textured and epitaxial piezoelectric films; lead-free piezoelectric films; nonlin- ear resonance-based energy harvesting struc- tures Toprak and Tigli [51] Size classification Nanoscale, microscale, mesoscale; CMOS technology Salim et al. [52] Hybrid MEMS-based Power comparison; architecture; mathemati- cal modeling; vibration-based micro harvest- ing; triboelectric, electromagnetic Briscoe and Dunn [53] Nanogenerators Single and arrays of ZnO nanorods; screening effect; SEM images Wang et al. [54] Nanowire Vertical-aligned nanowire arrays, lateral- aligned nanowire networks; nanowire- composite Khan et al. [55] Piezo-thin films Electro-mechanical proprieties of thin film piezo-materials; piezotronics; Selvan and Ali [42] Methodological performance Comparative points about thermoelectric, thermo-photovoltaic, piezoelectric, microbial fuel cell for micro-scale devoces Todaro et al. [56] Vibration MEMS; Approaches, strategies; thin films; wurtzite structure Jing and Kar-Narayan [57] Polymer-based30 Comparison with triboelectric; nanowires, PVDF, Nylon, polylactic acid, ZnO-polymer nanocomposite piezoelectric, polymer- ceramic hybrids. Gosavi and Balpande [12] Micro/Nanoscale Performance metrics; pyroelectric; commer- cial status; structural characterization tech- niques; material synthesis and deposition behavior. In addition, they performed a parametric study on the effect of the load resistance, wind exposure area, mass of the bluff body, and length of the piezoelectric sheet on the cut-in wind speed as well as the output power level of the GPEH. Abdelkefi [61] presented an overview of various types of aeroelastic vibra- tion mechanisms like the flutter in airfoil sections, the vortex-induced vibra- tions in circular cylinders, the galloping in prismatic structures (D-section, triangular, square), the wake galloping, the flapping-leaf/flags, T-beam, and micro-power generators. The strength of this study is the review of math- ematical models and experimental researches including pure experimental and combined (linear/non-linear) theoretical/experimental models. They an- nounced that the existence of two- or three-degree-of-freedom, electro-aero- mechanical aspects, the placement of piezoelectric sheets, characteristics of the layers, the mode shapes, and the wake effects, the tip effects, three- dimensionality, the unsteady wake and stall effects, representation of the galloping aerodynamic loads, influence of non-linearity, the cross-section ge- ometry, the Reynolds number, presence of gust and variations in the wind, the noise effects and disturbance, the small place of installation, and the prototype fabrication are design-modeling challenges especially for real har- vesters. Wei and Jing [62] presented a state-of-the-art review of theory, model- ing, and realization of the piezoelectric, electromagnetic, and electrostatic energy harvesters. The linear inertia-based theory and the non-linear mod- els have been described for three mentioned vibration-to-electricity convert- ers. They investigated some characteristics of the piezo-harvesters such as being unaffected from external/internal electromagnetic waves, simple struc- ture, depolarization, brittleness of the bulk piezo-layer, the poor coupling in piezo-film, and the poor adhesion with the electrode materials. Development of new piezoelectric materials, creation of new energy harvesting configu- rations by exploring the non-linear benefits, and design of efficient energy harvesting interface circuits are among their suggestions as future prospects. They concluded that the non-linearity is an important and effective param- eter in terms of performance enhancement, and theoretical modeling of the non-linear system with keeping reliability and stability is a challenging task. Kim et al. [63] mentioned that the theoretical modeling of piezoelectric energy harvesters should include not only its structure, but also the piezo- electric coupling effect as well as the electrical behavior. They performed a literature review on various mathematical models, which have been employed

31 for the purpose of piezoelectric energy harvesting.

4. Application

4.1. Vibration sources Vibration is the most common source of energy for piezoelectric har- vesters, since there is no need to convert the input energy to the mechani- cal energy to produce electricity in the piezomaterial. Also, its abundance, accessibility and ubiquity in the environment, in addition to multiple pos- sible transduction types have made it more attractive for energy harvesting applications. The response of the piezoelectric materials to the employed vibrations depends on their electromechanical properties like the natural fre- quency, their geometry, the electromechanical coefficients, and the damp- ing characteristics. The design strategies for these types of harvesters, per- formance enhancement methodologies, behavior of the energy harvesters in harsh environment, their fatigue life, and failure mode, and the conditioning electric circuits are some of the important issues, which should be addressed in the review papers. Sodano et al. [64], as one of the earliest reviewers of the field, discussed the future goals that must be achieved for power harvesting systems to find their way towards the everyday use, and to generate sufficient energy to power the necessary electronic devices. They mentioned that major limitations in the field of power harvesting revolve around the fact that the power gener- ated by the piezoelectric energy harvesters is far too small to power most electronic devices. Increasing the amount of energy generation, developing innovative methods of accumulating the energy, use of rechargeable batter- ies, optimization of the power flow from a piezoelectric setup, minimizing the circuit losses, identifying the location of power harvesting and the excitation range, proper tuning of the power harvesting device are their predictions for future prospects of the vibration-based piezo harvesters at that time. The review papers like the one presented by Zhu et al. [65] are the result of an explosive utilization of the vibration-based micro-generators in powering wireless sensor networks. They demonstrated an overall review of the princi- ples and the operating strategies to increase the operational frequency range of the vibration-based micro-generators. This point is the most fundamental shortcoming of such systems, referring to intermittent and continues elec- tromechanical tuning and the bandwidth widening. Enhancing the tuning

32 efficiency, designing an extra closed loop system to control the tuning pro- cess, development of miniature actuators and the non-linear generators with bi-stable structures are their recommendations as future possible research topics. Harb [66] reviewed a brief history of all energy harvesting methods includ- ing the vibration-based, the electromagnetic-based, the thermal or radioactive- based, with the source of pressure gradient, the solar and light-based, biolog- ical, and micro-water flow systems. They also presented the state-of-the-art of energy harvesting techniques, power conversion, power management, and the battery charging. It is concluded that it is hard to make a fair comparison between different methods, due to importance of many number of parameters that affect the performance. However, it is advised that the different types of vibrations are the most available and the highest power provider sources. Kim et al. [63] summarized the key ideas and performances of the piezo- electric energy harvesters based on vibration, various types, the piezoelectric materials, and the mathematical modeling of vibrational energy harvesting devices. They listed 17 important electro-mechanical characteristics of PZT- 5H, PZT-8, and PVDF piezomaterials and described various configurations such as the cantilever type, the cymbal type, the stack type, and the shell type. They advised that the future opportunities for research are develop- ment of high coupling coefficient piezoelectric materials, giving the ability to sustain under harsh vibrations and shocks, development of flexible and re- silient piezoelectric materials, and designing efficient electronic circuitry for energy harvesters. Bian and Yang [67] introduced a review of theory of vibration energy harvesting based on piezoelectric materials, and the state of the vibration energy harvesting using piezomaterials, including the structure, the material selection, the electrical interface of the energy conversion, the technology of energy storage, and the application of energy harvesting technology. Saadon and Sidek [68] presented a brief discussion of vibration-based MEMS piezoelectric energy harvesters. They summarized various designs of harvesters and experimentally obtained results in the last 3 years before the date of publication of the paper. They focused on the working modes and maximum output power of the MEMS piezoelectric energy harvesters. They mentioned that two common working modes for MEMS harvesters are 33 mode and 31 mode. It is concluded that the delivered power output of the MEMS-based devices is still inadequate to be used as a DC power supply to power mobile electronic devices, remote sensors, and other medical

33 monitoring devices. Designing and fabrication of a novel vibration-based MEMS micro-power harvesting device, which can provide an optimal desired DC output power characteristic with high efficiency and may maintain an output power for wireless sensor networks are the future works. Harne and Wang [69] reported major efforts and findings documented in literature about common analytical frameworks and principal results for bi-stable electromechanical dynamics, and a wide variety of bi-stable energy harvesters. They summarized various manifestations of the bistable energy harvesters and their bistability mechanisms. The bi-stable systems may pro- duce dramatically larger power output, and possess the non-linear behavior and a broader frequency bandwidth. Based on their discussion, the remain- ing challenges of such systems are maintaining high-energy orbits, operation under a stochastic vibratory condition, designing the coupled bi-stable har- vesters, and defining proper performance metrics. Siddique et al. [70] provided a literature review on vibration-based mi- cropower generation using electromagnetic and piezoelectric transduction systems and hybrid configurations. They reported some performance char- acteristics of the piezoelectric energy harvesters with different materials and configurations. They claimed that most of the recent research has been de- voted to modification of the generator size, shape, and to introduce a power conditioning circuit to widen the frequency bandwidth of the system. Further research topics are development of the MEMS-based energy harvesters from renewable resources and making the miniature electric devices more reliable. Maamer et al. [27] categorized the common sources of mechanical energies based on their frequency level. They stated that the harvested power den- sity from different sources is related mainly to their motion frequency and magnitude. They made a comprehensive review on different optimization techniques to harvest more energy from mechanical vibrations. In summery, different configurations of the piezoelectric cantilevers, their power output and the performance enhancement strategies have been cov- ered by the review papers well. However, a systematic comparison of different configurations of the piezoelectric energy harvesters, and also their ability to sustain harsh vibrations and shocks, their fatigue life, their cost and access- ability have not been considered by the reviews.

4.2. Biological sources Biomechanical energy harvesting presents an important alternative to electrical energy supplied by batteries for portable electronic devices. An

34 average amount of energy used by the body is 1.07 × 107 J per day. This amount of energy is equivalent to approximately 800 AA (2500 mAh) batter- ies with the total weight of about 20 kg. This considerable amounts of human energy opens the road of development of energy harvesting technologies for powering electronic devices [71]. Internal charging of implantable medical devices (IMD’s) is the other im- portant biological application of piezoelectric energy harvesters. Extending the lifespan of IMD’s and their size minimization have become a main chal- lenge for their development. For such devices, energy from the body move- ment, muscle contraction/relaxation, cardiac/lung motions, and the blood circulation is used for powering medical devices [72]. In addition to bio- compatibility problems, the main challenges in developing of these types of energy harvesters lie in constructing a device that can harvest as much energy as possible with minimal interference with the natural function of the body. Also, the device should ideally not increase the amount of energy required by a person to perform his/her activities. Specially for IMD’s, the lifetime and efficient power output of energy harvesters are of outmost importance. Figure 5 illustrates magnitude of harvestable energy sources from the human body organs. Similar values can be predicted more or less from organs of animals in related applications. Riemer and Shapiro [71] investigated the amount of electricity, which can be generated from motion of various parts of the body such as heel strike, ankle, knee, hip, shoulder, elbow, arm, leg, the center of mass vertical motion as well as the body heat, using the piezoelectrics and electrical induction generators. They obtained the amount of positive and negative muscle work in each case, and the motion where energy is lost to the surroundings (e.g., the heel strike). They claimed that such technologies are appropriate for the third world countries, which is doubtful refereing to its low performance and high cost of fabrication. Utilizing all phases of the negative work during the gait cycle, designing a high gear ratio, and improving the effectiveness of the required control system are among their recommendations as future works. Mhetre et al. [73] gave a brief review of micro energy harvesting techniques and methods using the piezoelectric sensors from the limb movements for drug delivery, dental applications, and the body heat using the piezoelectric transducers. They just announced that the main challenge is to enhance the energy output using proper electronic circuit designs. Much more research is required to harvest energy from other biological parameters such as the body temperature and respiration.

35 2-Breathing 100mW 12-Shoulder 2.2W

1-Upper limbs 10mW

5-Bodyheatemission 13-Backpack 100-525 W 50mW

3-Typing 1mW 11-Elbow 2.1 W

9-Center ofmass 10-Hip 20W 38 W

7-Knees 36 W 4 4-Walking 8- Ankles 1W,2W 66 W

6-Heelstrike 2-20W

Figure 5: Available sources of energy from the human body organs illustrated on Reza Abbasi’s ”Prince Muhammad-Beik” drawing. The data number 1-4 from Refs. [7], number 4-13 from [71].

36 Xin et al. [74] reviewed shoes-equipped piezoelectric energy harvesters. They described advantages and limitations of the current and newly devel- oping piezoelectric materials, including the flat plate type, the arch type, the cantilever type, the nanocomposite-based, the photosensitive-based, and the hybrid piezoelectric-semiconductors technologies. They announced that en- hancing the coupling coefficient of the piezoelectric materials and optimizing the structure of the energy harvester and the energy storing circuit require further researches. Hwang et al. [75] addressed the developments of flexible piezoelectric energy-harvesting devices by using high-quality perovskite thin film and in- novative flexible fabrication processes. They commented that the bulky types of the energy harvesters have limited utility as implantable energy sources inside the human body due to incongruent contact with the corrugated and curved surfaces of organs such as eye, brain, lung, and heart. In addition, the energy harvesting devices with thick and rigid substrates are unsuitable for responding to the movements of internal organs and muscles. Therefore, extremely slim, lightweight, and pliable energy harvesters on plastic films such as polyimide (PI), polyethylene terephthalate (PET), and polyethylene naphthalate (PEN) are required for conformal settlement and energy harvest- ing on organs. They reviewed and compared different types of flexible thin film piezoelectric harvesters and sensors including flexible BaTiO3, PZT and PMN-PT thin-film energy harvesters, which were the candidates for biomed- ical applications like the self-powered cardiac pacemaker, the biomimetic artificial hair cells, in vivo energy harvester driven by organ movements, the direct stimulation of the living heart, and detecting nanoscale cellular deflec- tions. They commented that the electric power harvested from the bending motion of a flexible thin film was sufficient to stimulate heart muscles. Also, Easy bendability, higher conversion efficiency, more sensitive sensing capabil- ity in nanoscale, self-energy generation and realtime diagnosis/therapy capa- bilities are among advantages of such systems. It seems that such strategic devices may have many applications in diverse industries as well as biomedi- cal cases. This area needs to be more demonstrated in future review studies. Zheng et al. [72] presented an overall review of the piezoelectric en- ergy devices in comparison to the triboelectric harvesters with the source of body movement, muscle contraction/relaxation, cardiac/lung motions, and the blood circulation. They proposed that future opportunities are fabrica- tion of intelligent, flexible, stretchable, and fully biodegradable self-powered medical systems for monitoring biological signals, in vivo and in vitro treat-

37 ment of various diseases, optimization of the output performance, obtaining higher sensitivity, elasticity, durability and biocompatibility, biodegradable transient electronics, intelligent control of dynamic properties in vivo, im- proving the operating lifetimes, and the absorption efficiency. Surmenev et al. [76] described novel techniques in fabrication of hybrid piezoelectric polymer-based materials for biomedical energy harvesting ap- plications such as detection of motion rate of humans, degradation of organic pollutants, and sterilization of bacteria. They described the different meth- ods, which are employed for the improvement of the piezoelectric response of polymeric materials and scaffolds. They also reviewed biomedical devices and sensors based on hybrid piezo-composites. Similar to most other reviews, increasing the performance is one of proposed future works. Others are align- ment of nanofiller particles inside the piezopolymer matrix, developing com- mon standards for consistently quantifying and evaluating the performance of various types of piezoelectric materials, and investigation of structural parameters. Ali et al. [77] discussed possibilities of the piezo-based energy conver- sion from the source of muscle relaxation and contraction, the body move- ment, the blood circulation, lung and cardiac motion in applications such as pacemakers, blood pressure sensors, cardiac sensors, pulse sensors, deep brain simulations, biomimetic artificial hair cells, active pressure sensors, and active strain sensors. The piezoelectric materials containing nanowires, nanorods, nanotubes, nanoparticles, thin films, the lead-based ceramics, the lead-free ceramics, the polymer-based materials, the textured polycrystalline materials, and the biological piezomaterials have been evaluated. They pro- posed several challenging problems such as flexibility to fit into the shape of an organ, proper management of power, selection of media for electrical con- nection, enhancing the biological safety, designing the interface between the body tissue and the implanted piezomaterial, efficient encapsulation, further miniaturization, and conducting related experiments on small/large animal and human cases. The reviewed articles about the biological applications have focused on highlighting new materials and structures of biological energy harvesters and their power outputs. Bio-compatibility, interference of the devices with bio- logical organs, reliability of the device along with its lifetime and economical issues are open topics in the field. Table 4 summarizes different highlights and descriptions of reviews related to biological applications.

38 Table 4: Details of review papers on piezoelectric energy harvesting in biological applica- tions in the order of the year of publication. Review period #Refs. General topic Extra description and highlights Riemer and Shapiro [71] 1977-2009 38 Biomechanical Human motion and body heat; com- parison with electrical induction gener- ators. Mhetreetal.[73] 1990-2010 29 Human ener- Dental applications; drug delivery. gies Hwang et al. [75] 1955-2014 71 Biomedical Flexible thin film nanosensors and har- cases vesters; harvesting from slight and tiny movements. Zheng et al. [72] 1955-2017 107 Biomedical Comparison with triboelectric har- vester. Surmenev et al. [76] 1974-2018 235 Biomedical Hybrid piezocomposites; polymer- based harvesters; antibacterial effect.

Ali et al. [77] 1962-2018 240 Human body Mechanical, thermal, and chemical bioenergies; details of piezo-devices.

39 4.3. Fluids Various forms of renewable energies such as sunlight, heat, wind, ocean waves and tidal currents are obtained from energies in our planet. For ex- ample Truitt and Mahmoodi [78] reviewed effects of wind-based energy har- vesting from vibrations generated by different bluff bodies and aeroelastic instabilities (fluttering and galloping). They presented an overall study of energy generation density and the peak power outputs versus bandwidth. After a limited review of dynamics of piezoelectric energy harvesters, theo- ries and principles, energy densities and output powers, they concluded that the balance of efficiency-cost-manufacturability is the future horizon of the topic. They suggested the use of PVDFs in fluid excitation applications due to their increased flexibility over PZTs. They concluded that the flutter- and galloping-based methods generate a higher output power, but with a narrower frequency bandwidth in comparison to the vortex induced meth- ods. Also, the final frontier in energy harvesting seems to be active energy harvesting in which the system dynamics was actively changed in real time to meet changing environmental dynamics. Results of the review conducted by Wong et al. [79] implies that the piezo- electric harvesting form rain drop has privileges such as simple structure, easier fabrication, reduced number of components, and direct conversion of vibrations to electrical charge. They presented a summary of different ma- terials, mechanisms, dimension, velocity and kinetic energy of the raindrop, type of impact, and the load impedance. They stated that the main challenge in this field is to design and optimize the raindrop harvester for outdoor use, being resistant against sunlight, wind, the impact force of larger drops, being waterproof, showing appropriate sensitivity to drops, supplying constant-rate energy over long periods of time, and optimum power efficiency. Some of con- clusions and challenges presented in the paper are general characteristics of all piezo-based harvesters. Hence, a more close focus on aspects related to the raindrop energy production is needed. McCarthy et al. [80] reviewed the research done on piezoelectric en- ergy harvesting utilizing the fluttering. They introduced the mathematical terms used across the literature to define the performance of the flutter har- vester. They discussed effect of the Strouhal number as a function of the Reynolds number, the wind characteristics, and the atmospheric boundary layer (ABL). They declared that the ultra-low power densities, long return period on investments, and quantification and alleviation of the fatigue dam- age are the most challenges for fluttering energy harvesting. In their opin-

40 ion, determining the fatigue life and some metrics of a piezoelectric flutter, weather and precipitation effects are active research fields. Chua et al. [81] reviewed different types of raindrop kinetic energy piezo- electric harvesters, including the bridge-structure, the cantilever structure with the impact point near the free-end, the cantilever structure with six im- pact points at varies surface locations, the cantilever structure with impact point at the center, the PVDF membrane or the PZT edge-anchored plate, and the collecting diaphragm cantilevers. Also, they presented a brief sum- mary of hybrid harvesters. It is stated that the best parameter to compare different harvesters is efficiency rather than the output peak power. Then based on this criterion, it is found that the cantilever-type and the bridge- type energy harvesters made of PZT are the best choices. This is in contrast to the recommendations of Wong et al. [79]. In addition, an overview of the critical points about the circuit design of piezomaterials for harvesting en- ergy from the raindrop kinetic energy has been presented. Despite of unique review of circuits for such applications, a limited number of future directions of the hybrid systems have been expressed, including enhancement of the efficiency of hybrid energy harvesters and developing innovative methods to harvest more types of energies within one hybrid system. Viet et al. [82] first compared three energy harvesting methods, includ- ing electrostatic, electromagnetic and piezoelectric technologies to indicate privileges of the piezoelectric harvesting in power generation, transmission, structural installation, and the economic costs. Then, they reviewed different design methodologies of harvesting energy from ocean waves. Effects of lon- gitudinal, bending, and shear couplings have been discussed. It is concluded that due to higher energy generation density, higher voltage generation capa- bility, simpler configuration, and more economical benefits, the piezoelectric technology is superior to two other methods. They recommended magnifi- cation of the induced stress and strain in piezoelectric materials, designing piezoelectric harvesters, which utilize d33 d15 coupling modes, and opti- mization of energy conversion without energy leakage based on an excited frequency as future directions. Elahi et al. [83] studied the fluid-structure interaction-based, the human- based, and the vibration-based energy harvesting mechanisms by qualita- tively and quantitatively analyzing the existing piezoelectric mechanisms. They reviewed the vortex-induced vibration, fluttering, galloping, as well as the human-related structures. They commented that significant amount of research has been conducted on aeroelastic energy harvesters, but aerody-

41 namic models can be improved by taking into account steady, quasi-steady, and unsteady aerodynamics. Performing experiments on under-water piezo- materials, using multiple flexible piezoelectric patches, design of the laptop keyboard or the keypad of a cellular phone with piezoelectric patches, devel- oping the doped piezoelectric patches with higher coupling coefficients, and utilizing the specialized amplifiers are the future research guidelines. Hamkehdar et al. [84] presented a review of energy harvesting from fluid flows. Despite the general topic of the paper, the piezo-energy harvesting from blood as a liquid has been ignored. However, they have performed a literature review on vortex induced vibration, the vortex street, the flutter induced motion, the galloping, and the waves with water and air as working fluids. Also, there is a short discussion on modeling challenges. Moreover, different types of energy harvesting mechanisms have been studied in or- der to identify their advantages and weaknesses. Increasing the frequency of harvesters using converters with two buoys, investigating fundamentals of biomimetic in future energy harvesting devices, and harvesting from oil pipelines considering the safety issues are their recommendations for the ex- tension of the topic. Wnag et al. [85] categorized the FIVs for energy harvesting into four categories based on different vibration mechanisms: vortex induced vibra- tion, galloping, fluttering, and buffeting. They discussed the vortex-induced vibrations and buffeting (as forced vibration cases), galloping and flutter (as limit-cycle vibration items) using electromagnetic, piezoelectric, electro- static, dielectric and triboelectric methods as well as the corresponding nu- merical and experimental endeavors. They presented a fruitful summary of the current research status on flow-induced vibration hydro/aero energy harvesters. It is concluded that the flow pattern around bluff bodies, the size limitations, estimation of costs of equipment, the maintenance costs, the lifespan, protection of equipment in the case of extreme weather, possible environmental impacts, the non-linear modeling, the intelligent regulating elements such as artificial neural network, implementation of hybrid multi- purpose energy harvesters, and development of new materials need to be further studied. However, a distinct conclusion about the piezoelectric en- ergy harvesting in comparison to other techniques has not been provided in the paper. Table 5 presents the details and highlights of review papers on fluid-based piezo-energy harvesting.

42 Table 5: Details of review papers on piezoelectric energy harvesting from fluids in the order of the year of publication. Review period #Refs. General Extra description and highlights topic Truitt and Mahmoodi [78] 1945-2013 62 Wind The best material choice, the best har- vesting methodology Wong et al. [79] 1948-2014 87 Raindrop Raindrop size, output power. McCarthy et al. [80] 1935-2015 96 Wind Comparison with different method of energy harvesting from fluttering; noise level; performance metrics; fatigue damage. Abdelkefi [61] 1990-2015 224 Aeroelastic Review of modeling methods, power conditioning, realistic prototypes. Chua et al. [81] 1976-2015 73 Raindrop Interface circuits, light/moderate/heavy/violent rain, the best material choice, optimization, different shapes. Viet et al. [82] 1998-2016 96 Ocean Structural installation; economic costs; waves comparison with electrostatic and elec- tromagnetic methods. Elahi et al. [83] 1959-2018 256 FSI Comparison with human-based and vibration-based piezo-mechanisms. Hamlehdar et al. [84] 1987-2019 199 Fluids Includes just water and air; a brief re- view of modeling approaches. Wang et al. [85] 1990-2020 125 Flow- More detailed mathematical models; induced demonstration of the current research vibration field; investigation of electromagnetic, piezoelectric, electrostatic, dielectric and triboelectric.

43 4.4. Ambient waste energy sources Pillai and Deenadayalan [86] presented a review of acoustic energy har- vesting methods and piezoelectricity as a promising technology in this cat- egory due to being sensitive and efficient at high frequency excitations. They described the working principle of the acoustic resonator energy har- vester, the sonic crystal energy harvester, the allied acoustic energy harvester, the hydraulic pressure energy harvester, and some special types of thermo- acoustic energy harvesting. They investigated matching of the resonant fre- quency of the piezoelectric crystal and the resonant frequency of the resonator tube, optimization of the system, and the electrodynamic arrangement of the piezoelectric material. They also discussed the thermo-acoustic energy har- vesting, which generates electrical power from traveling and standing waves with high conversion efficiency, reliability, environmental friendliness, and high fabrication cost. Based on their conclusions, optimization of the res- onator and the coupling of thermo-acoustic engine to the acoustic-electricity conversion transducer is an open research fields. Khan and Izhar [87] reviewed the recent developments in the field of electromagnetic- and piezoelectric-based acoustic energy harvesting. They reported sound pressure levels of various ambient acoustic energy sources. They focused on the Helmholtz’s resonator and also the membrane piezoelec- tric energy harvesters. They compared electromagnetic- and piezoelectric- based acoustic energy harvesters and concluded that the size of both sys- tems are comparable in millimeter scale, and the resonant frequencies of the electromagnetic system is lower (143-470 Hz versus 146 Hz-16.7 kHz). A set of useful data about the sound pressure level (dB) and the frequency of various acoustic energy sources have been reported. They declared that researchers were focusing on enhancing the performance of the piezoelectric membrane through novel fabrications and optimized geometrical configura- tions. Obtaining an optimum design of the Helmholtz’s cavity and an efficient transduction mechanism are future horizons in this field. Performance of the electromagnetic- and piezoelectric-based vibration en- ergy harvesters for energy production from bridges has been evaluated by Khan and Ahmad [24]. Their review includes unimorph cantilever type PE- VEH, bimorph cantilever type PE-VEH, and membrane-type piezoelectric PE-VEH. They have expressed that the majority of current harvesters are constructed based on the electromagnetic effect, but the piezomaterials are commercially available and are easy to be developed. The resonant frequency

44 is a critical parameter in such narrow-band low-frequency applications, which is a privilege of the electromagnetic systems. Duarte and Ferreira [88] made a comparative study on road pavement energy harvesting technologies. Their study was performed on solar energy harvesting including photovoltaic, thermoelectric, and ASC technologies, and vehicle mechanical energy harvesting including piezoelectric and electromag- netic technologies. They compared the aforementioned technologies based on the installed power (per area or volume), conversion efficiency, power density and the energy generation of the technology in normal operating conditions. Also, they classified the harvesting technologies based on their TRL (technol- ogy readiness levels) values. It is demonstrated that the piezoelectric tech- nology is at high TRL grades. But, it delivers insufficient energy production rate with low economic characteristics. On the other hand, quantification of the conversion efficiency of the technologies, unfixed harvesting systems, use of generated electrical energy to directly supply electrical equipment near the railway, optimization and testing of systems in real environments, injection of energy into the electrical grid, simultaneous use of harvesting and stor- age, and the effect of railway dynamics on the developed systems need more careful attention. Guo and Lu [89] discussed recent advances in application of thermoelec- tric and piezoelectric energy harvesting technologies from pavements. They found out that a pipe system cooperating with a thermoelectric generator is superior in terms of cost effectiveness and electricity output to piezoelec- tric transducers (fabricated with PZT) in such applications. Based on their recommendations, impact of the mentioned energy harvesting facilities to pavement performance, life cycle assessments, optimization with respect to traffic conditions and solar radiation, and the change in vehicle fuel consump- tion due to additional vehicle vibration or resistance should be evaluated in future works. Maghsoudi Nia et al. [25] presented different technologies of converting the kinetic energy of the human body during walking to electricity by locating a harvesting system on the body or inserting a harvester in the floor. They found that the performance of the body-fitted case is highly dependent on the location of the piezo-generator, the average acceleration and frequency of the motion, and the physiological parameters. But, the generated electrical power in the pavement version is more reliable. In contrast to the results of Guo and Lu [89], it is recommended that the piezoelectric harvester is a better choice for such applications, due to simplicity and flexibility, regardless

45 of lower power output. Duarte and Ferreira [90] presented a comparative study of photovoltaic, thermoelectric, electromagnetic, hydraulic, pneumatic, electromechanical, and piezoelectric harvesting technologies. Evaluation parameters to compare studies are the conversion efficiency, the maximum generated power, the installation method, and their TRL. They declared that the essential eco- nomic data of products are not yet available. No more detailed suggestions for future directions and open research fields has been presented. Yildirim et al. [33] reviewed amplification techniques, resonance tuning methods, and non-linear oscillations in applications involving the ambient vibration harvesting, based on piezoelectric, electrostatic, and electromag- netic conversion methods. The investigated sources of energy scavenging are the car engine, the blender casing, the clothes dryer, a closing door, HVAC systems, windows, compact disk driver, and the second story floor of an office. They offered several design advises to enhance the performance characteristics before such devices can be used in industrial or everyday-life applications. Wang et al. [91] illustrated applications of photovoltaic cells, solar col- lectors, geothermal, thermoelectric, electromagnetic, and piezoelectric en- ergy extraction systems from bridges and roads in terms of energy output, benefit-cost ratio, and technology readiness level. They employed the con- cept of levelized cost of electricity (LCOE) to evaluate the cost required to produce the same unit quantity of electrical energy, which was defined as the total costs divided by total electrical energy produced. They implied that the fatigue failure analysis of the mechanical loading, the stress concentration de- pending on the packaging integrity, the initial installment, the maintenance and repair, the operating costs during the service life, and assessing the en- vironmental impact of the energy harvesting systems are potential research fields. Based on their conclusions, the grade of support of the piezoelectric harvesters by governments is low to medium, while the solar and geothermal systems are strongly being supported. Al-Yafeai et al. [40] reviewed the methodologies to convert the dissipated energy in the suspension dampers of a car to electricity, along with discussing the mathematical car models and respective experimental setups. The dis- advantages of the piezo-generator in comparison to other methods are poor coupling, high output impedance, charge leakage, and low output current. However, advantages are simple structure, no need to external voltage sources and mechanical constraints, compatibility with MEMS-based devices, high

46 output voltage, and having wide frequency range. They declared that in- vestigation of harvesting energy from a full car model, the direction of the exerted force and optimization of the harvester could be addressed in future investigations. Table 6 presents details and highlights of the review papers on ambient and waste energy piezo-harvesting methods.

5. Challenges and the roadmap for future research

Table 7 illustrates the number of published review papers on each field, the year of the first and the last published review paper, and the research fund sources. It is obvious that energy harvesting from ambient energies, the MEMS/NEMS-based, the fluid-based harvesting, and material consid- erations, respectively possess the highest rate of publication of the review papers. The largest number of reviews is written about the MEMS-based harvesters and the design issues. The numbers in parenthesis also demon- strate the number of funded review papers. Although, it is predictable that some supporters prefer to remain anonymous, it is seen that about 46% of papers have been supported by a non-university organization. The last col- umn of the table presents a list of organizations and respected countries, which have devoted a full/partial financial support to the review papers on piezomaterials. It is expected that the new review papers focus on more specific top- ics than general reviews. However, they may still contain some degree of generality. Due to the multidisciplinary nature of the field, it is vital to give comprehensive reviews on very detailed aspects of the piezoelectric har- vesters. Publication of review papers with general topics is not very welcomed anymore. Also, the rate of publication of the review papers on biological applications is less than the expected rate. Due to the rapid progress of piezoelectricity in biomedical engineering, increasing the number of reviews in related fields is inevitable. We suggest the researchers to present some state-of-the-art articles with specific topics including progress in piezoelec- tric materials, new applications of piezoelectric energy harvesters, and new developments in MEMS and NEMS piezoelectric harvesters. The results of comparative researches on energy harvesters for the railway demonstrated that, even in macroscale energy harvesting, the piezoelectric energy harvesters are not very successful with respect to other harvesting technologies. This situation may be worst for micro and nanoscale harvesters. We predict that the single (non-hybrid) piezoelectric energy harvesters would

47 Table 6: Details of review papers on piezoelectric energy harvesting from ambient waste energy in the order of the year of publication. Review period #Refs. General topic Extra description and highlights Pillai and Deenadayalan [86] 1968-2013 80 Acoustics Thermo-acoustic engines with standing and traveling waves; resonant frequency. Khan and Izhar [87] 1964-2014 54 Acoustics Comparison with electromagnetic harvester; the resonant frequency; size of harvesters. Khan and Ahmad [24] 1985-2014 49 Bridge Comparison with the electromag- netic harvester; resonant frequency.

Xin et al. [74] 1969-2015 53 Shoes Different types of systems developed for shoes. Duarte and Ferreira [88] 1995-2015 34 Railway tracks Comparison with electromagnetic, electromechanical actuation, the hy- draulic actuation technologies; tech- nology readiness level (TRL) report.

Guo and Lu [89] 1944-2016 65 Pavement Comparison with the thermoelectric system; cost-effectiveness analysis. Maghsoudi Nia et al. [25] 1995-2015 42 Walking, pave- Comparison with electromagnetic ment harvesters; comparison of body- fitted and pavement harvesters. Duarte and Ferreira [90] 1979-2016 97 Road pave- Comparison with photovoltaic, ments thermoelectric, ASC, electro- magnetic, hydraulic, pneumatic, electromechanical, and MEMS harvesting technologies. Yildirim et al. [33] 1996-2015 105 Ambient Car engine, blender casing, clothes vibration dryer, closing a door, HVAC sys- tems, windows, compact disk driver, and the second story floor of an of- 48 fice. Wang et al. [91] 1982-2018 120 Roadway and Comparison with photovoltaic cell, bridge solar collector, geothermal, thermo- electric, electromagnetic; TRL re- port; government support. Al-Yafeai et al. [40] 1969-2020 166 Car Suspen- Location of the piezomaterial; com- sion parison with electromagnetic and electrostatic harvesters; circuit de- sign. be the true choice only in some specific applications in which other harvesting systems have inherent limitations. Thus, there is an essential need for making fair comparisons of all types of energy harvesters for specific applications. On the other hand, we encounter the growing number of publications on piezoelectric energy harvesters with respect to other harvesting methods. It should be noted that although the researchers have an strange tendency for developing these types of harvesters, the real world will select energy harvesting systems with higher performances and lower costs. Based on the data listed in Table 7, we may detect three types of research lines, 1. Pioneering topics, which are still under consideration: general reviews (2005-2019), the design key points (2005-2020), the material-related studies (2009-2019), the MEMS-based devices (2006-219), 2. Pioneering topics without recent publication of review papers: the mod- eling approaches (2008-2017), the vibration-based harvesters (2004- 2015), sensors and actuators (2007-2016), 3. The newly developed topics: fluids (2013-2020), ambient waste energy (2014-2020), the biological applications (2011-2019). The missing topics and concluding future research topics, which need more close investigations to demonstrate their state-of-the-art are 1. Development of hybrid multi-purpose energy generators to completely harness energy of any kind and with any characteristics combining the piezo-pyro-tribo-flexo-thermo-photoelectric technologies. 2. Investigation of the mathematical models, the analytical and numeri- cal solution techniques especially in nanoscale geometries in which the classical continuum mechanics principal fails or in stochastic and non- linear situations. Some modified constitutive relations may need to be developed in non-continuum regimes. Also, the second law analysis and investigation of such systems from the thermodynamic viewpoint are missing topics. The ab initio first principal simulations with an atomistic nature are other challenging aspects of the nanoscale piezo- harvesters. Development of opensource codes like OpenFOAM and LAMMPS to include solvers involving the piezoelectric effect may be another future research topic. 3. Application of piezomaterials in energy saving or reducing the energy demand of a system rather than generation of energy requires a com- prehensive review. An example of such energy reduction is the delay

49 Table 7: Statistics of review papers published on different topics related to piezoelectric energy harvesting. The numbers in parenthesis demonstrate the number of funded review papers in each field. Topic #reviews Period # reviews per year Non-university research fund sources 1 General 8(4) 2005-2019 0.53 National Science Foundation (USA), National Natural Science Foundation (China), Spanish Ministry of Science and Technology and the Regional European Development Funds (European Union), NanoBioTouch European project/Telecom Italia/Scuola Superiore SantAnna (Italy). 2 Design 15(5) 2005-2020 0.94 Texas ARP (USA), U.S. Department of Energy Wind and Water Power Technologies Of- fice (USA), Ministry of Higher Education (Malaysia), Natural Science and Engineering Research Council (Canada), National Natural Science Foundation (China)/EU Erasmus+ project/Bevilgning 3 Material 11(7) 2009-2019 1.00 M/s Bharat Electronics Limited (India), National Nature Science Foundation (China), Of- fice of Basic Energy Sciences, Department of Energy (USA)/Center for Integrated Smart Sensors funded by the Korea Ministry of Science (Korea), National Natural Science Foun- dation (China)/Shanghai Municipal Education Commission and Shanghai Education Devel- opment Foundation (China), European Research Council/European Metrology Research Pro- gramme/ UK National Measurement System, National Natural Science Foundation (China), China scholarship Council/China Ministry of Education/Institute of sound and vibration 4 Modeling 5(3) 2008-2017 0.5 Air Force Office of Scientific Research (USA), Air Force Office of Scientific Research (USA), a NSFC project of China 5 Vibration 8(1) 2004-2015 0.67 Energy Efficiency & Resources of the Korea Institute of Energy Technology Evalua- tion/Creative Research Initiatives 6 Biology 6(5) 2011-2019 0.67 Russian Science Foundation/Alexander von Humboldt Foundation/European Commission, National Key R&D Project from Minister of Science and Technology (China), Basic Science Research Program (Korea)/Center for Integrated Smart Sensors as Global Frontier Project, R&D Center for Green Patrol Technologies through the R&D for Global Top Environmental Technologies program funded by the Korean Ministry of Environment, Paul Ivanier Center for Robotics and Manufacturing Research/Pearlstone Center for Aeronautics Research 7 Sensors 5(4) 2007-2016 0.5 Spanish Ministry of Education and Science, NSSEFF/fellowship/ NSF/ Ben Franklin Tech- nology PArtners/the Center for Dielectric Studies/ARO/DARPA/the Materials Research In- stitute/U.S Army Research Laboratory, Converging Research Center Program by the Ministry of Education Science and Technology (Korea), Basic Science Research Program through the National Research Foundation of Korea 8 MEMS/NEMS 15(7) 2006-2019 1.07 National Science Foundation (China), the Basic Science Research Program, through the Na- tional Research Foundation of Korea, European Research Council, Ministry of Education (Malaysia), Office of Basic Energy Sciences Department of Energy (USA), International Re- search and Development Program of the National Research Foundation of Korea 9 Fluids 8(3) 2013-2020 1.00 Ministry of Higher Education (Malaysia), National Natural Science Foundation (China), Aus- tralian Research Council/FCSTPty Ltd 10 Ambient 11(4) 2014-2020 1.57 Center for Advanced Infrastructure and Transportation (USA), Portuguese Foundation of Science and Technology, European Regional Development Fund, National Natural Science Foundation of China

50 in decaying disturbances and delaying transition to turbulence using piezo-actuators placed on the surface of bluff bodies. 4. Due to the multi-physics nature of the piezoelectric effect, it is highly recommended to prepare review papers on optimization methods as a complicated task in research roadmap of the piezo-harvesters. 5. Commercialization of the piezo-based harvesters and enhancing the technology readiness level need a serious attention. Perhaps, the next decade is the decade of extensive commercialization of the piezo-harvesters. 6. Plenty of patents have been published in recent years. Even some review papers should be devoted to investigation of patents presented in the field. 7. Focused reviews are needed on vibration-based piezo-harvesters in four recent years, development of piezotronics, and design of complete self- powered autonomous systems. 8. The overall design of devices including all parts, integrating the whole device in thin films, accumulation in rechargeable batteries, and taking into account the energy consumption needed to store the harvested energy. 9. Optimization of device architecture and size reducing configurations for portable applications, flexible wearable compact embedding im- plantable devices. 10. In situ prototype testing and design of harvesters coupled with en- vironment and realistic applications to face with sunlight in outdoor applications, naturally occurring stochastic vibrations, the wind speed variation, dust, noises, required flexibility to fit the shape of human organs, and waterproofness. 11. Quantification of the figure of merit for piezomaterial properties such as energy transforming or conversion efficiency and standardizing the performance of piezo-based devices. 12. Reducing the maintenance cost, enhancing the lifespan, ameliorating the performance, analysis of government supports, the cost-benefit bal- ance, and investigations of piezo-harvesting from energy policy view- point. 13. Thermal design of piezo-systems including the temperature-dependence properties and high-temperature harvesting limitations. 14. Fabrication of new piezomaterials with the non-linear behavior, larger displacements, lower frequency, wider operation bandwidth, and the frequency self-adaptation capabilities.

51 15. Use of metamaterials, non-toxic, biocompatible, printable piezomateri- als, nanofibers, lead-free and high-piezoelectric coefficient materials to present new piezomaterials. 16. Improving design of electrical circuitry and managing rectification and storage losses. 17. Modified structural designs including fracture-fatigue studies to in- creases reliability, stability, and durability of the device. 18. Design of efficient control techniques. 19. Extending the application of piezomaterials in novel fields such as in- ternet of things. However, the research on piezoelectric energy harvesting is not mature enough and many interdisciplinary active research fields are currently avail- able. It should be mentioned that the progress of small-scale devices with very low power need is tightly tied to the revolution in design of efficient high-output power piezoelectric energy harvesters.

Acknowledgment

This research was supported by the Iran National Science Foundation (Grant number 98017606).

References

[1] Erturk, A., Inman, D. J. (2011). Piezoelectric energy harvesting. John Wiley & Sons.

[2] Mateu, L., Moll, F. (2005, June). Review of energy harvesting techniques and applications for microelectronics. In VLSI Circuits and Systems II (Vol. 5837, pp. 359-373). International Society for Optics and Photonics.

[3] Anton, S. R., Sodano, H. A. (2007). A review of power harvesting us- ing piezoelectric materials (2003-2006). Smart materials and Structures, 16(3), R1.

[4] Khaligh, A., Zeng, P., Zheng, C. (2009). Kinetic energy harvesting using piezoelectric and electromagnetic technologies-state of the art. IEEE transactions on industrial electronics, 57(3), 850-860.

52 [5] Sun, C. H., Shang, G. Q., Tao, Y. Y., Li, Z. R. (2012). A review on application of piezoelectric energy harvesting technology. In Advanced Materials Research (Vol. 516, pp. 1481-1484). Trans Tech Publications Ltd.

[6] Taware, S. M., Deshmukh, S. P. (2013). A review of energy harvest- ing from piezoelectric materials. IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE), 43-50.

[7] Calio, R., Rongala, U. B., Camboni, D., Milazzo, M., Stefanini, C., De Petris, G., Oddo, C. M. (2014). Piezoelectric energy harvesting solutions. Sensors, 14(3), 4755-4790.

[8] Batra, A. K., Alomari, A., Chilvery, A. K., Bandyopadhyay, A., Grover, K. (2016). Piezoelectric power harvesting devices: An overview. Ad- vanced Science, Engineering and Medicine, 8(1), 1-12.

[9] Sharma, S., Sharma, N. C., Upadhayay, A., Hore, D. (2018). An Inno- vative Strategy of Energy Generation using Piezoelectric Materials: A Review. ADBU Journal of Engineering Technology, 7.

[10] Safaei, M., Sodano, H. A., Anton, S. R. (2019). A review of energy harvesting using piezoelectric materials: state-of-the-art a decade later (2008-2018). Smart Materials and Structures, 28(11), 113001.

[11] Sharma, P. K., Baredar, P. V. (2019). Analysis on piezoelectric energy harvesting small scale device: a review. Journal of King Saud University- Science, 31(4), 869-877.

[12] Gosavi, S. K., Balpande, S. S. (2019). A Comprehensive Review of Micro and Nano Scale Piezoelectric Energy Harvesters. Sensor Letters, 17(3), 180-195.

[13] Bedekar, V., Oliver, J., Zhang, S., Priya, S. (2009). Comparative study of energy harvesting from high temperature piezoelectric single crystals. Japanese Journal of Applied Physics, 48(9R), 091406.

[14] Lefeuvre, E., Sebald, G., Guyomar, D., Lallart, M., Richard, C. (2009). Materials, structures and power interfaces for efficient piezoelectric en- ergy harvesting. Journal of electroceramics, 22(1-3), 171-179.

53 [15] Mukherjee, A., Datta, U. (2010, December). Comparative study of piezoelectric materials properties for green energy harvesting from vi- bration. In 2010 Annual IEEE India Conference (INDICON) (pp. 1-4). IEEE.

[16] Bowen, C. R., Kim, H. A., Weaver, P. M., Dunn, S. (2014). Piezoelectric and ferroelectric materials and structures for energy harvesting applica- tions. Energy & Environmental Science, 7(1), 25-44.

[17] Chen, Z., Guo, B., Yang, Y., Cheng, C. (2014). Metamaterials-based enhanced energy harvesting: A review. Physica B: Condensed Matter, 438, 1-8.

[18] Yuan, X., Changgeng, S., Yan, G., Zhenghong, Z. (2016, September). Application review of dielectric electroactive polymers (DEAPs) and piezoelectric materials for vibration energy harvesting. In Journal of Physics: Conference Series (Vol. 744, No. 1, p. 012077). IOP Publishing.

[19] Narita, F., Fox, M. (2018). A review on piezoelectric, magnetostrictive, and magnetoelectric materials and device technologies for energy har- vesting applications. Advanced Engineering Materials, 20(5), 1700743.

[20] Liu, H., Zhong, J., Lee, C., Lee, S. W., Lin, L. (2018). A comprehensive review on piezoelectric energy harvesting technology: Materials, mech- anisms, and applications. Applied Physics Reviews, 5(4), 041306.

[21] Mishra, S., Unnikrishnan, L., Nayak, S. K., Mohanty, S. (2019). Ad- vances in piezoelectric polymer composites for energy harvesting appli- cations: A systematic review. Macromolecular Materials and Engineer- ing, 304(1), 1800463.

[22] Priya, S., Song, H. C., Zhou, Y., Varghese, R., Chopra, A., Kim, S. G., ... Polcawich, R. G. (2019). A review on piezoelectric energy harvesting: materials, methods, and circuits. Energy Harvesting and Systems, 4(1), 3-39.

[23] Zaarour, B., Zhu, L., Huang, C., Jin, X., Alghafari, H., Fang, J., Lin, T. (2019). A review on piezoelectric fibers and nanowires for energy harvesting. Journal of Industrial Textiles, 1528083719870197.

54 [24] Khan, F. U., Ahmad, I. (2016). Review of energy harvesters utilizing bridge vibrations. Shock and Vibration, 2016.

[25] Nia, E. M., Zawawi, N. A. W. A., Singh, B. S. M. (2017) A review of walking energy harvesting using piezoelectric materials. In IOP Confer- ence Series: Materials Science and Engineering 291(1), 012026.

[26] Dutoit, N. E., Wardle, B. L., Kim, S. G. (2005). Design considerations for MEMS-scale piezoelectric mechanical vibration energy harvesters. Integrated ferroelectrics, 71(1), 121-160.

[27] Maamer, B., Boughamoura, A., El-Bab, A. M. F., Francis, L. A., Tounsi, F. (2019). A review on design improvements and techniques for mechan- ical energy harvesting using piezoelectric and electromagnetic schemes. Energy Conversion and Management, 199, 111973.

[28] Priya, S. (2007). Advances in energy harvesting using low profile piezo- electric transducers. Journal of electroceramics, 19(1), 167-184.

[29] Szarka, G. D., Stark, B. H., Burrow, S. G. (2011). Review of power conditioning for kinetic energy harvesting systems. IEEE transactions on power electronics, 27(2), 803-815.

[30] Guyomar, D., Lallart, M. (2011). Recent progress in piezoelectric con- version and energy harvesting using nonlinear electronic interfaces and issues in small scale implementation. Micromachines, 2(2), 274-294.

[31] Ibrahim, S. W., Ali, W. G. (2012). A review on frequency tuning meth- ods for piezoelectric energy harvesting systems. Journal of renewable and sustainable energy, 4(6), 062703.

[32] Li, H., Tian, C., Deng, Z. D. (2014). Energy harvesting from low fre- quency applications using piezoelectric materials. Applied physics re- views, 1(4), 041301.

[33] Yildirim, T., Ghayesh, M. H., Li, W., Alici, G. (2017). A review on performance enhancement techniques for ambient vibration energy har- vesters. Renewable and Sustainable Energy Reviews, 71, 435-449.

[34] Yang, Z., Erturk, A., Zu, J. (2017). On the efficiency of piezoelectric energy harvesters. Extreme Mechanics Letters, 15, 26-37.

55 [35] Uchino, K. (2018). Piezoelectric energy harvesting systems Essentials to successful developments. Energy Technology, 6(5), 829-848.

[36] Dell’Anna, F. G., Dong, T., Li, P., Wen, Y., Yang, Z., Casu, M. R., ... Berg, Y. (2018). State-of-the-art power management circuits for piezo- electric energy harvesters. IEEE Circuits and Systems Magazine, 18(3), 27-48.

[37] Yang, Z., Zhou, S., Zu, J., Inman, D. (2018). High-performance piezo- electric energy harvesters and their applications. Joule, 2(4), 642-697.

[38] Talib, N. H. H. A., Salleh, H., Youn, B. D., Resali, M. S. M. (2019). Comprehensive Review on Effective Strategies and Key Factors for High Performance Piezoelectric Energy Harvester at Low Frequency. Interna- tional Journal of Automotive and Mechanical Engineering, 16(4), 7181- 7210.

[39] Brenes, A., Morel, A., Juillard, J., Lefeuvre, E., Badel, A. (2020). Max- imum power point of piezoelectric energy harvesters: a review of opti- mality condition for electrical tuning. Smart Materials and Structures, 29(3), 033001.

[40] Al-Yafeai, D., Darabseh, T., Mourad, A. H. I. (2020). A State-Of-The- Art Review of Car Suspension-Based Piezoelectric Energy Harvesting Systems. Energies, 13(9), 2336.

[41] Dagdeviren, C., Joe, P., Tuzman, O. L., Park, K. I., Lee, K. J., Shi, Y., ... Rogers, J. A. (2016). Recent progress in flexible and stretchable piezo- electric devices for mechanical energy harvesting, sensing and actuation. Extreme mechanics letters, 9, 269-281.

[42] Selvan, K. V., Ali, M. S. M. (2016). Micro-scale energy harvesting de- vices: Review of methodological performances in the last decade. Re- newable and Sustainable Energy Reviews, 54, 1035-1047.

[43] Beeby, S. P., Tudor, M. J., White, N. M. (2006). Energy harvesting vibration sources for microsystems applications. Measurement science and technology, 17(12), R175.

[44] Cook-Chennault, K. A., Thambi, N., Sastry, A. M. (2008). Powering MEMS portable devices: a review of non-regenerative and regenerative

56 power supply systems with special emphasis on piezoelectric energy har- vesting systems. Smart materials and structures, 17(4), 043001. [45] Kang, M. G., Jung, W. S., Kang, C. Y., Yoon, S. J. (2016, March). Re- cent progress on PZT based piezoelectric energy harvesting technologies. In Actuators (Vol. 5, No. 1, p. 5). Multidisciplinary Digital Publishing Institute. [46] Muralt, P., Polcawich, R. G., Trolier-McKinstry, S. (2009). Piezoelectric thin films for sensors, actuators, and energy harvesting. MRS bulletin, 34(9), 658-664. [47] Wang, X. (2012). Pieogenerators Harvesting ambient mechanical energy at the nanometer scale. Nano Energy, 1(1), 13-24. [48] Nechibvute, A., Chawanda, A., Luhanga, P. (2012). Piezoelectric energy harvesting devices: an alternative energy source for wireless sensors. Smart Materials Research, 2012. [49] Kumar, B., Kim, S. W. (2012). Energy harvesting based on semicon- ducting piezoelectric ZnO nanostructures. Nano Energy, 1(3), 342-355. [50] Kim, S. G., Priya, S., Kanno, I. (2012). Piezoelectric MEMS for energy harvesting. MRS bulletin, 37(11), 1039-1050. [51] Toprak, A., Tigli, O. (2014). Piezoelectric energy harvesting: State-of- the-art and challenges. Applied Physics Reviews, 1(3), 031104. [52] Salim, M., Aljibori, H. S. S., Salim, D., Khir, M. H. M., Kherbeet, A. S. (2015). A review of vibration-based MEMS hybrid energy harvesters. Journal of Mechanical Science and Technology, 29(11), 5021-5034 [53] Briscoe, J., Dunn, S. (2015). Piezoelectric nanogenerators: a review of nanostructured piezoelectric energy harvesters. Nano Energy, 14, 15-29. [54] Wang, Z., Pan, X., He, Y., Hu, Y., Gu, H., Wang, Y. (2015). Piezoelec- tric nanowires in energy harvesting applications. Advances in Materials Science and Engineering, 2015. [55] Khan, A., Abas, Z., Kim, H. S., Oh, I. K. (2016). Piezoelectric thin films: an integrated review of transducers and energy harvesting. Smart Materials and Structures, 25(5), 053002.

57 [56] Todaro, M. T., Guido, F., Mastronardi, V., Desmaele, D., Epifani, G., Algieri, L., De Vittorio, M. (2017). Piezoelectric MEMS vibrational en- ergy harvesters: Advances and outlook. Microelectronic Engineering, 183, 23-36.

[57] Jing, Q., Kar-Narayan, S. (2018). Nanostructured polymer-based piezo- electric and triboelectric materials and devices for energy harvesting applications. Journal of Physics D: Applied Physics, 51(30), 303001.

[58] Erturk, A., Inman, D. J. (2008). On mechanical modeling of cantilevered piezoelectric vibration energy harvesters. Journal of intelligent material systems and structures, 19(11), 1311-1325.

[59] Erturk, A., Inman, D. J. (2008). Issues in mathematical modeling of piezoelectric energy harvesters. Smart Materials and Structures, 17(6), 065016.

[60] Zhao, L., Tang, L., Yang, Y. (2013). Comparison of modeling methods and parametric study for a piezoelectric wind energy harvester. Smart materials and Structures, 22(12), 125003.

[61] Abdelkefi, A. (2016). Aeroelastic energy harvesting: A review. Interna- tional Journal of Engineering Science, 100, 112-135.

[62] Wei, C., Jing, X. (2017). A comprehensive review on vibration energy harvesting: Modelling and realization. Renewable and Sustainable En- ergy Reviews, 74, 1-18.

[63] Kim, H. S., Kim, J. H., Kim, J. (2011). A review of piezoelectric en- ergy harvesting based on vibration. International journal of precision engineering and manufacturing, 12(6), 1129-1141.

[64] Sodano, H. A., Inman, D. J., Park, G. (2004). A review of power harvest- ing from vibration using piezoelectric materials. Shock and Vibration Digest, 36(3), 197-206.

[65] Zhu, D., Tudor, M. J., Beeby, S. P. (2009). Strategies for increasing the operating frequency range of vibration energy harvesters: a review. Measurement Science and Technology, 21(2), 022001.

58 [66] Harb, A. (2011). Energy harvesting: State-of-the-art. Renewable En- ergy, 36(10), 2641-2654.

[67] Bian, Y., Yang, C. (2011). A review of current research for energy har- vesting based on vibration of piezoelectric materials. Yadian yu Sheng- guang, 33(4), 612-622.

[68] Saadon, S., Sidek, O. (2011). A review of vibration-based MEMS piezo- electric energy harvesters. Energy conversion and management, 52(1), 500-504.

[69] Harne, R. L., Wang, K. W. (2013). A review of the recent research on vibration energy harvesting via bistable systems. Smart materials and structures, 22(2), 023001.

[70] Siddique, A. R. M., Mahmud, S., Van Heyst, B. (2015). A comprehensive review on vibration based micro power generators using electromagnetic and piezoelectric transducer mechanisms. Energy Conversion and Man- agement, 106, 728-747.

[71] Riemer, R., Shapiro, A. (2011). Biomechanical energy harvesting from human motion: theory, state of the art, design guidelines, and future directions. Journal of neuroengineering and rehabilitation, 8(1), 22.

[72] Zheng, Q., Shi, B., Li, Z., Wang, Z. L. (2017). Recent progress on piezo- electric and triboelectric energy harvesters in biomedical systems. Ad- vanced Science, 4(7), 1700029.

[73] Mhetre, M. R., Nagdeo, N. S., Abhyankar, H. K. (2011, April). Micro energy harvesting for biomedical applications: A review. In 2011 3rd International Conference on Electronics Computer Technology (Vol. 3, pp. 1-5). IEEE.

[74] Xin, Y., Li, X., Tian, H., Guo, C., Qian, C., Wang, S., Wang, C. (2016). Shoes-equipped piezoelectric transducer for energy harvesting: A brief review. Ferroelectrics, 493(1), 12-24.

[75] Hwang, G. T., Byun, M., Jeong, C. K., Lee, K. J. (2015). Flexible piezoelectric thin-film energy harvesters and nanosensors for biomedical applications. Advanced healthcare materials, 4(5), 646-658.

59 [76] Surmenev, R. A., Orlova, T., Chernozem, R. V., Ivanova, A. A., Bartasyte, A., Mathur, S., Surmeneva, M. A. (2019). Hybrid lead- free polymer-based scaffolds with improved piezoelectric response for biomedical energy-harvesting applications: A review. Nano Energy, 62, 475-506.

[77] Ali, F., Raza, W., Li, X., Gul, H., Kim, K. H. (2019). Piezoelectric energy harvesters for biomedical applications. Nano Energy, 57, 879- 902.

[78] Truitt, A., Mahmoodi, S. N. (2013). A review on active wind energy harvesting designs. International Journal of Precision Engineering and Manufacturing, 14(9), 1667-1675.

[79] Wong, C. H., Dahari, Z., Manaf, A. A., Miskam, M. A. (2015). Harvest- ing raindrop energy with piezoelectrics: a review. Journal of Electronic Materials, 44(1), 13-21.

[80] McCarthy, J. M., Watkins, S., Deivasigamani, A., John, S. J. (2016). Fluttering energy harvesters in the wind: A review. Journal of Sound and Vibration, 361, 355-377.

[81] Chua, K. G., Hor, Y. F., Lim, H. C. (2016). Raindrop kinetic energy piezoelectric harvesters and relevant interface circuits: Review, issues and outlooks. Sensors & transducers, 200(5), 1.

[82] Viet, N. V., Wu, N., Wang, Q. (2017). A review on energy harvesting from ocean waves by piezoelectric technology. Journal of Modeling in Mechanics and Materials, 1(2).

[83] Elahi, H., Eugeni, M., Gaudenzi, P. (2018). A review on mechanisms for piezoelectric-based energy harvesters. Energies, 11(7), 1850.

[84] Hamlehdar, M., Kasaeian, A., Safaei, M. R. (2019). Energy harvesting from fluid flow using piezoelectrics: A critical review. Renewable Energy, 143, 1826-1838

[85] Wang, J., Geng, L., Ding, L., Zhu, H., Yurchenko, D. (2020). The state- of-the-art review on energy harvesting from flow-induced vibrations. Ap- plied Energy, 267, 114902.

60 [86] Pillai, M. A., Deenadayalan, E. (2014). A review of acoustic energy harvesting. International journal of precision engineering and manufac- turing, 15(5), 949-965.

[87] Khan, F. U. (2015). State of the art in acoustic energy harvesting. Jour- nal of Micromechanics and Microengineering, 25(2), 023001.

[88] Duarte, F., Ferreira, A. (2016). Energy harvesting on road pavements: state of the art. Proceedings of the Institution of Civil Engineers-Energy, 169(2), 79-90.

[89] Guo, L., Lu, Q. (2017). Potentials of piezoelectric and thermoelectric technologies for harvesting energy from pavements. Renewable and Sus- tainable Energy Reviews, 72, 761-773.

[90] Duarte, F., Ferreira, A. (2017, June). Energy harvesting on railway tracks: state-of-the-art. In Proceedings of the Institution of Civil Engineers-Transport (Vol. 170, No. 3, pp. 123-130). Thomas Telford Ltd.

[91] Wang, H., Jasim, A., Chen, X. (2018). Energy harvesting technologies in roadway and bridge for different applications A comprehensive review. Applied energy, 212, 1083-1094.

61