Piezoelectric crystals commonly found in various applications include quartz and water-soluble crystals such as sodium potassium tartrate, diammonium ethylene tartrate, dipotassium tartrate, and potassium sulphate (Zu et al., 2016; Lutjes et al., 2021; Dong et al., 2022). For polycrystalline, notable piezoelectric materials include barium titanate, zirconium lead titanate, and lead niobium magnesium oxide, et al. (Okayasu and Watanabe, 2016; Wu et al., 2016; Kumar et al., 2023; Shi et al., 2023). In 1997, large S-E strain was found in lead based relaxor ferroelectric crystals, which is widely regarded as a significant advancement in piezoelectric materials (Xu et al., 2000). Sm-doped lead niobium magnesium oxide-lead titanate crystals exhibit high piezoelectric coefficients exceeding 4,000 pC/N and dielectric constants with value of 12,000. The uniformity of crystals rod properties is enhanced by leveraging the sub-condensation properties of Sm elements during crystal growth. This advancement establishes a solid basis for the development of piezoelectric crystals suitable for high frequency medical ultrasound probes and high precision actuators (Li et al., 2019).

Barium titanate (BaTiO₃, BT) presents high dielectric properties, which is extensively manufactured as high-frequency circuit components (Karvounis et al., 2020; Chen et al., 2022), as shown in Figure 2A. The thermal properties of BT can be enhanced by the process of Pb and Ca co-dopant (Hasan et al., 2023). The enhanced piezoelectric properties in BT-based ceramics can be achieved by use of ion-pair effect and A/B-site synergistic doping modification (Xie et al., 2022), as seen in Figure 2B. High d ₃₃ value (30.5 pC/N) and Curie temperature (Tc = 657°C) were found in Bi₄Ti₃-x (Zn_1/3Nb_2/3)xO₁₂ ceramics. Additionally, the aligned ferroelectric domains demonstrate remarkable temperature stability (Xie et al., 2022). Lead zirconate titanate (PZT) is a solid-state solution consisting of lead titanate (PbTiO₃) and lead zirconate (PbZrO₃), which is extensively used as transducers materials owing to consistent piezoelectric properties and high Curie temperature (Tan et al., 2019), as shown in Figure 2C. The ceramics exhibit various features, after small amounts of dopant, such as niobium, antimony, tin, manganese, tungsten, et al. (Koh et al., 2022; Chen et al., 2023b; Habeeb Khan et al., 2023). With the protection of environment, lead free piezoelectric materials are widely investigated. Quartz crystals integrated on silicon substrate is shown in Figure 2D. Figure 2E illustrates the schematic structure of AlN crystals (Fei et al., 2018). Commonly used lead-free piezoelectric systems include KNN-BNT, KNN-BT, BNT-BT, BKT-BT, BNT-BT-KNN, and BNT-BKT (Mayamae et al., 2017; Zhang et al., 2022a; Wang et al., 2022; Safari et al., 2023; Tai et al., 2023). The S-E strain in (K, Na) NbO₃ polycrystalline is enhanced by solid solution near phase border, which undergoes a phase transition, as shown in Figure 2F.

Polar polymers like polyvinylidene fluoride (PVDF) are good examples of polymer piezoelectric materials, which show low sound resistance, and can be made into thin parts (Wang et al., 2023a). Polymers can be used directly in actuator and sensor, owing to electrical and mechanical energy conversion (Takahashi and Tadokoro, 1980; Koseki et al., 2012; Bouad et al., 2022; Zhang et al., 2023c). Due to big dipole moments, β phases PVDF exhibit high dielectric and piezoelectric properties (Nasir et al., 2006; Satapathy et al., 2011; Huang et al., 2021). The d ₃₃ value can reach up to 62pC/N in β-phase PVDF films (Huang et al., 2021). Polytetrafluoroethylene (PTFE) is resistant to acids, alkalis, and a wide range of organic solvents (Dhanumalayan and Joshi, 2018; Niu et al., 2022). Neutral PTFE particles (d > ∼1–5 µm) can be turned into piezoelectric electrets using an easy-to-use ultrasound process at faster rate than piezoelectric catalysts (Wang et al., 2021).

Compared with the piezoelectric polymer, the inorganic piezoelectric ceramic particles have high piezoelectric constant and low breakdown strength. Therefore, through adding piezoelectric ceramics powder into polymers, the composite shows high piezoelectric coefficient and low poling field (Hema Malini et al., 2022). Compared with individual piezoelectric components, piezoelectric composite materials can overcome the temperature boundary of piezoelectric polymers and the inherent brittleness of inorganic piezoelectric biomaterials, while also allowing for large-scale manufacturing. The addition of barium titanate particles contributes the formation of β-phase in PVDF membrane (Khan et al., 2021), as shown in Figure 3A. Guan measured heel pressure in a new hybrid film made of lead zirconate titanate powder and micro fibrillated cellulose (PZT/MFC), which is a bendable film made with a polarization process (Guan et al., 2022), as presented in Figure 3B. Li made synthetic fabric films out of mixtures of PVDF and cellulose acetate (CA). Electrostatically spun PVDF/CA fiber membranes (EFMs) were used to make flexible nanogenerators (Li et al., 2022a), as shown in Figure 3C. For TiO₂/SiO₂/polymethyl methacrylate (PMMA)/PVDF composites, the β -phase content in the PVDF matrix increase from 19% to 43% as TiO₂ or SiO₂ contents increase from 0% to 5%, which made piezoelectric properties of PVDF better (Zhao et al., 2012; Li et al., 2013). A flexible ZnO/PVDF hybrid piezoelectric films with TiO₂ particles addition were used to make nanogenerator, showing a voltage is 2.3 times higher than that of pure PVDF nanogenerator (Kim et al., 2018). In a three-phase hybrid nanogenerator out of PVDF, ZnO, and BT nanorods, the output increase from 3 V to 12 V (Sabry and Hussein, 2019). The addition of nanoparticles can improve the crystallinity and the mount of β phase of the fiber films (Fakhri et al., 2019; Ye et al., 2021). Kar made a new bendable piezoelectric nanogenerator based on two-dimensional SnO₂ nanosheets and PVDF composites, and presented a high output rate with value of 16.3% (Kar et al., 2019). A three-dimensional hybrid nanostructure of MnO2/Gr/multi-walled carbon nanotubes was also used to enhance the electrical properties of PVDF (Yang et al., 2018). Recently, nano-piezoelectric materials, including quantum dots, two-dimensional materials and topological insulators, have been added to PVDF to improve the power output of self-powered wearable devices and achieve excellent output performance (Hoque et al., 2017; Biswas et al., 2019; Bagchi et al., 2020; Shi et al., 2020; Saikh et al., 2021; Sarkar et al., 2023a; Sarkar et al., 2023b). A 2D halide chalcogenide transverse heterostructures prepared by liquid-phase epitaxy, which strongly suppressed the in-plane ionic diffusion in 2D halide chalcogenides by doping rigid π-conjugated organic ligands (Shi et al., 2020).

Piezoelectric biomaterials are naturally suited for coupling mechanical and electrical forces in biological systems for real-time sensing, actuation, and power generation in vivo; however, large-scale synthesis and alignment of piezoelectric phases in bio-piezoelectric thin films still a major challenge (Swagata et al., 2017; Hoque et al., 2018; Lay et al., 2021; Sarkar et al., 2021; Das et al., 2022; Mondal et al., 2022). Piezoelectric biomaterials are low symmetry, highly ordered structures, lacking inversion centers. Therefore, linear electromechanical coupling is an inherent functional characteristic of most biomolecules. Piezoelectricity has been demonstrated in various biomaterials, such as wood and bone, as well as fibrillar proteins such as collagen, chitin, and elastin, which exist in highly ordered crystalline molecular forms in mammalian tissues. The classical piezoelectric principle has been applied to similar uniaxial oriented bioactive polymers, such as poly (lactic acid) (PLLA), poly (lactic acid) γ- Benzylglutamic acid (PBG) and cellulose 3. These biodegradable polymers have been used as piezoelectric implants to promote pure and composite forms of bone formation. As is well known, the surface charge and wettability of the scaffold control the interaction between the material cell interfaces. The shape of the bracket can be controlled through various manufacturing processes, and the additional functionalization of the material can be used to fix biochemical substances. The morphological characteristics, piezoelectric constant, and ferroelectricity of piezoelectric materials can be modified to meet specific requirements. Nanostructured PZT in the form of nanoribbons or nanowires has been developed and applied in the construction of biomedical and energy harvesting devices. However, due to its cytotoxicity, the application of PZT ceramics in tissue engineering is limited. Therefore, lead-free piezoelectric ceramics have been developed to reduce people’s concerns about lead exposure to toxic environments.

Two simple, portable, cost-effective, biocompatible, and environmentally friendly piezoelectric nanogenerators (PENGs) were designed using naturally biodegradable mud volcanic clay from the Andaman and Nicobar Islands in India. The output voltage of MPENG and BPENG is ∼85V, and the short-circuit current is ∼1.6 μA. The output voltage is∼125 V, and the short-circuit current is ∼1.9 μA. The power density is 4115 respectively μW/cm³ and 7187 μ W/cm³ (Das et al., 2022). Recently, a generator can produce electrical signals when squeezed by body movement, which will lead to a wide range of uses for muscle-powered electromechanical treatments (Chorsi et al., 2019). A self-assemble method using lysine as a piezoelectric generator has presented a way to make a product through chemical qualities of a material, as shown in Figure 3D. This fast self-assembly technology could greatly cut the cost of these kinds of gadgets and make them much easier to get and use.

A biodegradable poly nanofiber was designed to get chondrocytes and cartilage tissue grow back, which can create piezoelectric effect when stressed or loaded in a joint (Yang et al., 2021), as shown in Figure 3E. Without extra battery, cartilage can be regrown by implanting the disposable PLA support and producing microcurrents while walking. After 1–2 months of exercise, hyaline cartilage grown back in rabbits with serious osteoarticular flaws that had received piezoelectric scaffold grafts. The fully healed cartilage tissue was surrounded by a large number of chondrocytes and type II collagen. An active self-assemble method to shape piezoelectric biomaterial films is presented in Figure 3F. Nanoconfinement caused uniform nucleation, which will get rid of dependence on interface of films (Liu et al., 2022b). During the process of van der waals exfoliation, ultrathin films are thinned to effective piezoelectric domain thickness (Zhang et al., 2023d).

AlN thin films is a hot topic of research in RF field because they are often used in high frequency sound resonators. These films can be made by blasting with a magnetron. Zhao reported a high-quality bendable AIN piezoelectric film made with micro- and nano-fabrication technology (Zhang et al., 2022b). The Mo/AIN/Al structure on silicon {100} was made by blasting, and then ion etching was used to remove the silicon used as a support to get a bendable Mo/AIN/Al sandwich film (Zhao et al., 2020b). Wen used an RF magnetron sputtering method with aluminum-rich AlN (Al-AlN) targets to make AlN films on Si surfaces with a low number of defects (Wen et al., 2022). Al vacancies and O impurity defects in AlN films can be cut down with the help of modulation in AlN films. Patidar used a highly detailed oblique angle deposition method to make c-axis oriented AlN (0002) films with reactive metal ions that were timed with HiPIMS (Patidar et al., 2023). As shown in Figure 4A, combining HiPIMS with a small substrate bias of only −30 V greatly improves the crystalline quality and texture of the films. Process gas doping and point defect formation can be further reduced by synchronizing the negative substrate bias with Al. The films have a clear out-of-plane pattern and regular grain polarization.

One of the best ways to enhance piezoelectric response of AlN thin films is to mix it with other elements to make conformal films. By mixing transition metals (TMs, TM = Sc, Cr, Sr, Mo, Ru, and Rh, etc.), TM-N bonds become weak and move the TM atoms closer to the centers of the three nearby N atoms. It was found that the place of the TMs was strongly linked to their group number (Akiyama et al., 2009a; Akiyama et al., 2009b; Luo et al., 2009; Liu et al., 2013; Mayrhofer et al., 2015; Hu et al., 2018; Manna et al., 2018; Yanagitani and Jia, 2019; Fiedler et al., 2021; Feng et al., 2022; Lv et al., 2023a; Patidar et al., 2023; Zha et al., 2023). For Mo dopant, piezoelectric coefficient d ₃₃ of AlN: Mo (3.46%) films reached to 7.33 pm/V (Feng et al., 2022). In the past few years, research has been done to figure out how to make high-quality mixed epitaxial films of AlN. Using nanopatterned AlN/sapphire templates with regular hexagonal holes, the dislocation, etch pit density in AlN heteroepitaxial thin films was reduced to about 104 cm^-2, which is close to value in AlN bulk crystals, by controlling the separation and grouping of the columns (A strategy for obtaining AlN, 2023).

PZT piezoelectric thin films are widely used, which can be made by various methods, such as magnetron sputtering (Beklešovas et al., 2022), chemical vapour deposition (Aratani et al., 2001), molecular beam epitaxial growth, hydrothermal synthesis (Bian et al., 2016), pulsed laser deposition (Gatabi et al., 2017), and sol-gel (Lee et al., 2021).

High performance PZT films were obtained on fluorine-doped tin oxide (FTO)-coated aluminum borosilicate glass (AG) surfaces using a modified sol-gel method (Di Marco et al., 2023). Liu made Pb (Zr_0.52Ti_0.48)O₃ thin films with islands of Pb(Zr_xTi_1-x)O₃ solid species using the sol-gel method (Liu et al., 2022c), as shown in Figure 4B. The Pb(Zr_0.52Ti_0.48)O₃ films have been changed by Pb(Zr_xTi_1-x)O₃ crystalline seed islands with different Zr/Ti ratios, presenting higher dielectric constant, lower coercive electric field, and less leakage current density. Furthermore, PbZr_0.52Ti_0.48O₃ thin films were successfully made by the sol-gel method at ultra-low temperature (450 °C) in an oxygen plasma-assisted environment in Ref. (Li et al., 2023a), which can be used to make modern CMOS devices.

After fast thermal annealing at 620°C, a high energy storage density of 10.0 J/cm³ was obtained in PbZr_0.52Ti_0.48O₃ (PZT)/PbZrO₃ (PZ) hybrid films on a LaNiO₃/SiO₂/Si substrate synthesized by sol-gel method (Yang et al., 2023). Rhun grown thick PZT films on platinum-coated silicon plates using sol-gel method, and move PZT films and ITO electrodes onto glass substrates (Le Rhun et al., 2022). In visual range, the average amount of light that get through PZT stacks on glass was 70%, which makes it possible to make clear piezoelectric motors on glass for high-performance haptic devices and other new uses, like self-cleaning or making smart windows.

Flexible piezoelectric films composed of lead magnesium niobate and lead titanate (PMN-PT) and multi-walled carbon nanotubes (MW-CNTs) were prepared in polyvinylidene fluoride (PVDF) matrix for green energy harvesting and self-power supply (Das et al., 2017; Das et al., 2018). Fully artificial transparent Sm-doped Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (Sm:PMN-PT) thin films on mica substrates were fabricated by one step sol-gel process. The films show high d ₃₃ with value of 380 pm/V, indicating potential use for mechanical energy gathering, motion tracking, and human-computer interaction (Das et al., 2017; Lv et al., 2022).Using a self-flux method, large size of lead free 0.94 (Bi_1/2 Na_1/2) TiO₃ -0.06BaTiO₃ (BNBT) piezoelectric crystals were grown by self-flux method, as shown in Figure 4C.

Several methods are used to make KNN-based piezoelectric films, such as alkaline methods, pulsed laser methods, and sol-gel methods (Bai et al., 2011; Khorrami et al., 2017; Akmal et al., 2018; Sharma et al., 2019; Fast et al., 2020; Kovacova et al., 2020; Abdullah et al., 2021; Cheng et al., 2022; Nair et al., 2022). Because of the advances in flexible piezoelectric nanogenerators, alkali metal niobates have received a lot of attention and are considered an environmentally friendly choice for lead-based piezoelectric materials. Abu used potassium sodium niobate (KNN) to make an energetic film (Abdullah et al., 2021), as shown in Figure 4D. It was used to make copper plates for a piezoelectric nanogenerator (PENG). It was also shown that KNN-based energy films could be used for multipurpose uses (like force and pressure sensors) and lead-free energy harvesting. As shown in Figure 4E, Nair made a lead-free foldable piezoelectric nanogenerator out of KNN-PVDF nanocomposites (Nair et al., 2022). Cheng investigated the strengthening effect of CuO-doped KNN-based ceramics, and found that Cu doping strongly upset the ferroelectric ordering (Cheng et al., 2022). An idea of using solution synthesis to keep the alkali chemistry of the Mn-doped KNN films uniform was given in Ref. (Kovacova et al., 2020). A uniform grain size of 80 nm and a leakage current density of 2.8*10⁻⁸ A/cm² under an electric field of up to 600 kV/cm were obtained in chemically uniform KNN thin films.

ZnO thin films can be made by blasting with an RF magnetron (Li et al., 2018; Sonklin et al., 2022; Algün et al., 2023; Cuadra et al., 2023; Kahveci et al., 2023; Murthy et al., 2023; Toma et al., 2023). {002} oriented Li-doped ZnO thin films on SiO₂/Si was obtained using RF magnetron sputtering, when the sputtering power was 220 W and the Li-doped concentration was 5% (Li et al., 2018). As shown in Figure 4F, nanoparticles of undoped zinc oxide (ZnO) and Sr_xZn_1-x (x = 0.01, 0.02, 0.03, 0.04, and 0.10) were synthesized using the sol-gel method (Algün et al., 2023), as shown in Figure 4F. As shown in Figure 4G, high crystallinity ZnO thin films were synthesized by a spray pyrolysis method, show antibacterial qualities when exposed to UV light (Cuadra et al., 2023). Transparent conductive films were made by sputtering Ga-doped and (Ga + Nd)-doped ZnO films with an RF magnetron (Toma et al., 2023). It was found that undoped ZnO has a resolution of 85% in the visible range. Using sol-gel dip coating method, Murthy putted thin plates of aluminum and rubidium-doped ZnO on glass surfaces (Murthy et al., 2023), and found that the quality of crystals gets worse because of lattice stress after Rb particles added to a ZnO host lattice.

There are several ways to make PVDF films, such as electrostatic spinning, solution casting, solution casting, spin coating method, electro-spinning technique, stretching, vacuum evaporation, and homogenization, etc (Jin et al., 2021; Yen et al., 2022; Das et al., 2023; Ghosh et al., 2023; Mishra et al., 2023; Sarkar et al., 2024). The scratch spraying method was used to make polyvinylidene fluoride-trifluoro ethylene (PVDF-TrFE) copolymers (Singh et al., 2018; Bhunia et al., 2019; Gupta et al., 2019; Sapkota et al., 2022). In first step, cobalt ferrite (CoFe₂O₄) nanoparticles were made using acoustic chemistry. In second step, cobalt ferrite nanoparticles with different weight percentages (0, 2.5, 5, and 10%) were added to PVDF-TrFE to make nanocomposites. As shown in Figure 4H, an electrostatic spinning process was employed to make polyvinylidene difluoride matrix fiber membranes with modified lead zirconate titanate nanoparticles (PZT NPs) (Yuan et al., 2023). The inserted particles changed the distribution of polarized electric field, which helped to polarize PVDF. As shown in Figure 4I, thin films substrate is made of flexible hollow fiber polymer membranes sprayed with thin layers of photocatalyst (Zakria et al., 2023). Copper oxide/polyvinylidene fluoride thin film hollow fiber membranes (Cu_xO/PVDF TF HFM) were made by spraying Cu_xO on PVDF hollow fiber membranes (HFM) with a radio frequency (RF) magnetron. As much as 91% of the BPA was taken out of the cleaned wastewater by irradiation. After three rounds in a row, the efficiency of recycling hit about 71.3%. Flexible nanocomposite films were made by adding BFO powder to copolymers PVDF-TrFE and PVDF-HFP, showing high output voltage (Tripathy et al., 2023). Dielectric constant was reached up to 40 in an independent flexible hybrid film made of polyvinylidene fluoride (PVDF) and molybdenum disulphide (MoS₂) nanoflakes synthesized by sol-gel method, which was to be 5 times more than that of pure PVDF (Jangra et al., 2023). A lead-free, facile, low-cost, sol-gel-processed reduced graphene oxide (rGO)/P(VDF-TrFE) nanocomposite with multipurpose capability demonstration as a piezoelectric nanogenerator (PENG) and hybrid piezoelectric triboelectric nanogenerator (HPTENG) devices is presented in Ref.171. The maximum output power densities of hybrid piezo-triboelectric and piezoelectric devices are 0.28 W/cm3 and 0.34 mW/cm3, respectively. The triboelectric device demonstrates the direct illumination of 45 blue light-emitting diodes, which are connected in series to collect the mechanical energy generated by repeated finger taps (Bhunia et al., 2019).

Gathering energy from surrounding environment, like mechanical vibration, heat, fluid flow, electromagnetic radiation in form of light and radio waves (RF), and energy from body can provide clean power to run electronic devices like wireless sensor networks, mobile electronics, and wearable and implantable biomedical devices (Shirvanimoghaddam et al., 2016; Liu et al., 2018; Won et al., 2018; Ali et al., 2019). Mechanical energy is the most common type of energy that can be turned into useful power (Wang, 2012; Hu et al., 2019; Karan et al., 2019; Sun et al., 2019; Yan et al., 2019). Piezoelectric energy harvesting is a very easy way to turn mechanical energy in the environment into electrical energy. Because piezoelectric effect is based on intrinsic polarization of the material and does not need a separate voltage source, a magnetic field, or contact with another material, like electrostatic, electromagnetic, and friction electrical energy harvesting (Xie and Wang, 2015; Wang et al., 2018). Compared to other energy harvesting methods, their density output and voltage output are 3–5 times higher (Kim et al., 2011; Wu et al., 2015; Shi et al., 2018; Chen et al., 2019; Guan et al., 2020). Piezoelectric units are easy to integrate to microelectromechanical systems (Madinei et al., 2016; Zhou et al., 2020). It has been used in many fields, such as buildings, transportation, wireless electronics, MEMS, the Internet of Things (IoT), personal and internal healthcare devices (Siang et al., 2018).

Gao suggested a cantilever energy harvester using PIN-PMN-PT crystals, showing high power output with value of 102 W/m³. The designed energy generator worked well to power wireless devices for tracking and sending data, which could help to give power supply of IoT systems in a safe way (Gao et al., 2020). As shown in Figure 5A, a droplet-based generator (DEG) for gathering energy from the natural environment has been designed (Li et al., 2022b). The self-capacitance effect of upper electrode enables an ultra-high instantaneous peak output power with value of 765 W/m². As shown in Figure 5B, Petritz presents an energy harvesting system used ferroelectric polymer transducers and organic diodes (Petritz et al., 2021). These components are seamlessly integrated onto ultrathin substrates with 1 µm in thickness.

The progress of CMOS has significantly contribution to self-powered direct current (DC-type) energy harvesters operating at low input voltages, enhancing the overall performance of these energy harvesters (Wang and Li, 2016). Lei presented a surface engineering approach for transparent conductive membranes using self-assembled monolayers (SAMs) in conjunction with silver nanowires (AgNWs) for triboelectric nanogenerators (TENGs) and self-powered pressure sensors (Lei et al., 2023). The enhanced.

TENG has a notable capability to function as a pressure sensor array (4 × 4 pixels) for trajectory tracking, with a high sensitivity of 221 V-kPa-1. As shown in Figure 5C, Wu developed sensors using composite thin-film infrared sensor arrays for imaging human hand (Wu et al., 2023), indicating potential use of self-powered infrared sensor in wearable non-visual sensing and smart sensing applications. As shown in Figure 5D, piezoelectric sensors were designed for real-time monitoring of meteorological wind and rain (Li et al., 2023b). The ZnO@CF/PVDF composite thin film PNG demonstrates a maximum output power of 7.9 μW under an external load of 10 MΩ. Furthermore, the remarkable washing resistance, longevity exceeding 50,000 cycles, and sustained stability over a period of 12 months make the composite film as a viable candidate for deployment of weather sensor in autonomous vehicles. Finally, self-powered physiological monitoring devices have the capability to consistently monitor and transmit electrocardiogram (ECG), blood pressure, temperature, and exercise parameters of the human body (Yan et al., 2023).