Properties and Applications of Flexible Poly(Vinylidene Fluoride)-Based Piezoelectric Materials

: Poly (vinylidene ﬂuoride) (PVDF) is a kind of semicrystalline organic polymer piezoelectric material. Adopting processes such as melting crystallization and solution casting, and undergoing post-treatment processes such as annealing, stretching, and polarization, PVDF ﬁlms with high crystallinity and high piezoelectric response level can be realized. As a polymer material, PVDF shows excellent mechanical properties, chemical stability and biocompatibility, and is light in weight, easily prepared, which can be designed into miniaturized, chip-shaped and integrated devices. It has a wide range of applications in self-powered equipment such as sensors, nanogenerators and currently is a research hotspot for use as ﬂexible wearable or implantable materials. This article mainly introduces the crystal structures, piezoelectric properties and their applications in ﬂexible piezoelectric devices of PVDF materials.


Introduction
In 1880, French scientists Pierre Curie and Jacques Curie discovered the piezoelectric effect that applying force to crystals with non-centrosymmetric structures along a specific direction will generate electric charges on the surface of zinc blende, sodium chlorate, boracite, tourmaline, and crystal quartz [1]. The discovery opened the prelude to the research of piezoelectric materials. According to the structural symmetry, the crystal materials can be divided into 32 point groups. Among them, 21 kinds have non-centrosymmetric structures and exhibit piezoelectricity, except for the 432 point group with high symmetry. Piezoelectric materials are mainly divided into inorganic piezoelectric materials, such as piezoelectric single crystals, ceramics, metal oxides, organic polymer piezoelectric materials, such as nylon-11 [2], polylactic acid [3], polypropylene [4], poly(vinylidene fluoride) (PVDF) [5] and its copolymers, and organic-inorganic composite piezoelectric materials. Inorganic piezoelectric materials are a type of piezoelectric materials that have been discovered and applied earlier, but due to their brittleness, hardness and the complex processing, their applicability in the field of flexible, miniaturized, and integrated devices is limited [6]. Among the organic polymer piezoelectric materials, PVDF and its copolymers exhibit the best piezoelectric properties among the current piezoelectric polymers [7][8][9][10][11], considered to be one of the most feasible alternatives to piezoelectric ceramics.
PVDF is a kind of semicrystalline polymer. In 1969, Japanese scientist Kawai found that PVDF has stronger piezoelectricity after being mechanically stretched and high-voltage polarization [12]. Compared with other polymers such as poly(vinyl chloride) and polycarbonate, it has more outstanding piezoelectric activity, this discovery means that piezoelectric historical progress has taken place in polymer research. After that, the researchers also found that PVDF and its copolymers also have good pyroelectric and ferroelectric The molecular chain structure and lattice constant of the α phase and the δ phase are similar, and they are both in the helical conformation (TGTG') [40,42]. The TGTG' conformation is polar, but because of the inclination of dipoles to the molecular axis, it has components of the net moment both perpendicular and parallel to the chain that leads to a smaller polarity of the chain [5]. However, there is a difference in the arrangement of the molecular chains between the α phase and the δ phase. The two molecular chains in the Crystals 2021, 11, 644 3 of 21 unit cell in the α phase are stacked in an antiparallel manner, the dipole moments mostly cancel out each other, thus no net dipole moment remains [5]; while for the δ phase, the two molecular chains in the unit cell are arranged in an up-down pattern, the dipole moments of the molecular chains are parallel and show a distinct net dipole moment. Under the condition of applying an electric field, the transformation from α phase to δ phase can also be realized [40]. In the β phase, the molecular chain is in a plane zigzag all-trans conformation (TTT) [39,43], the -CF 2 -group and -CH 2 -group on the molecular chain are facing one side each, and the dipole moments are arranged in parallel and perpendicular to c axis, this conformation is the most highly polar conformation. Considering the steric hindrance of the fluorine atoms in the molecular chain, Hasegawa et al. [39] proposed an alternately deflected molecular chain structure, which can reduce the steric hindrance between fluorine atoms and lower the molecular potential energy. The γ phase crystal structure can be regarded as the deformation of the β phase crystal structure, which can be obtained by high-pressure heat treatment and casted in the organic solvent dimethyl sulfoxide or dimethylacetamide [41]. The polarity of the γ phase is second only to the β phase. Lovinger [44] found that the polar homologue of the α phase, the δ phase, is prone to phase transition at high temperature when studying the annealing behavior of PVDF films, resulting in a polar phase γ phase and a small amount of non-polar ε phase [45]. The mixture verifies the existence of the ε phase. The conformation of the ε phase and the γ phase is the same as T 3 GT 3 G', but the molecular chain stacking method is similar to that of the α phase, which is a non-polar phase. Due to the small difference in potential energy between the various conformations of PVDF, it is easy to undergo phase transition under external conditions such as heat treatment, pressure, and electric field. Figure 1 gives two common molecular structures of PVDF. mation is polar, but because of the inclination of dipoles to the molecular axis, it has components of the net moment both perpendicular and parallel to the chain that leads to a smaller polarity of the chain [5]. However, there is a difference in the arrangement of the molecular chains between the α phase and the δ phase. The two molecular chains in the unit cell in the α phase are stacked in an antiparallel manner, the dipole moments mostly cancel out each other, thus no net dipole moment remains [5]; while for the δ phase, the two molecular chains in the unit cell are arranged in an up-down pattern, the dipole moments of the molecular chains are parallel and show a distinct net dipole moment. Under the condition of applying an electric field, the transformation from α phase to δ phase can also be realized [40]. In the β phase, the molecular chain is in a plane zigzag all-trans conformation (TTT) [39,43], the -CF2group and -CH2-group on the molecular chain are facing one side each, and the dipole moments are arranged in parallel and perpendicular to c axis, this conformation is the most highly polar conformation. Considering the steric hindrance of the fluorine atoms in the molecular chain, Hasegawa et al. [39] proposed an alternately deflected molecular chain structure, which can reduce the steric hindrance between fluorine atoms and lower the molecular potential energy. The γ phase crystal structure can be regarded as the deformation of the β phase crystal structure, which can be obtained by high-pressure heat treatment and casted in the organic solvent dimethyl sulfoxide or dimethylacetamide [41]. The polarity of the γ phase is second only to the β phase. Lovinger [44] found that the polar homologue of the α phase, the δ phase, is prone to phase transition at high temperature when studying the annealing behavior of PVDF films, resulting in a polar phase γ phase and a small amount of non-polar ε phase [45]. The mixture verifies the existence of the ε phase. The conformation of the ε phase and the γ phase is the same as T3GT3G', but the molecular chain stacking method is similar to that of the α phase, which is a non-polar phase. Due to the small difference in potential energy between the various conformations of PVDF, it is easy to undergo phase transition under external conditions such as heat treatment, pressure, and electric field. Figure 1 gives two common molecular structures of PVDF. The TrFE (trifluoroethylene) and CTFE (chlorotrifluoroethylene) units are introduced into PVDF to obtain binary or terpolymers. Compared with the H atom, the F atom has a larger size, so the introduction of these units will produce greater steric hindrance, which will help the formation and stability of the all-trans chain structure (β phase) [5,46]. These copolymers mainly have three phase structures, the low-temperature (LT, a crystalline phase similar to the polar orthorhombic β phase) phase [47,48], the high-temperature (HT, the packing of conformationally disordered chains in hexagonal structure) phase, and the cooled phase (CL, a kind of superlattice consists of the domains of long trans chains linked together with the boundaries of disordered trans-gauche bonds), the phase transition occurs among them during thermal treatment process [49][50][51]. Phase transition is closely The TrFE (trifluoroethylene) and CTFE (chlorotrifluoroethylene) units are introduced into PVDF to obtain binary or terpolymers. Compared with the H atom, the F atom has a larger size, so the introduction of these units will produce greater steric hindrance, which will help the formation and stability of the all-trans chain structure (β phase) [5,46]. These copolymers mainly have three phase structures, the low-temperature (LT, a crystalline phase similar to the polar orthorhombic β phase) phase [47,48], the high-temperature (HT, the packing of conformationally disordered chains in hexagonal structure) phase, and the cooled phase (CL, a kind of superlattice consists of the domains of long trans chains linked together with the boundaries of disordered trans-gauche bonds), the phase transition occurs among them during thermal treatment process [49][50][51]. Phase transition is closely related to content of TrFE or CTFE units [17,52,53]. In P(VDF-TrFE) copolymers with 50~60 mol% VDF molar, the LT phase transforms into the CL phase during the heating process, and the CL phase continuously transforms into the HT phase. Transitions between the phases are definite; however, for the copolymer with lower amounts of VDF less than 40 mol%, there is a broad transition between the CL and HT phases, when the content of VDF up to 70~80 mol%, a clear and discontinuous first order transition between the LT and HT phases is observed and the CL phase does not practically appear [47]. It is worth mentioning Crystals 2021, 11, 644 4 of 21 that when the P(VDF-TrFE) undergoes annealing treatment above the Curie temperature and cooling back, a defective ferroelectric (DFE) phase would appear in addition to the ferroelectric (FE) phase [51]. Subsequently, the DFE phase can be transformed into the FE phase when polarized at room temperature, leading to a significant increase of the ferroelectric phase content [51]. P(VDF-TrFE-CTFE) copolymers with CTFE content from 0 to 10 mol% mainly exhibit ferroelectric or relaxor ferroelectric (RFE) properties at low temperatures whereas they all present paraelectric (PE) behavior at high temperatures [54]. What is more, the phase transitions are also affected by various amounts of CTFE units in the thermal evolution of this relaxor ferroelectric terpolymers. For the terpolymer with 9.7 mol % of CTFE, there is a continuous phase transition (RFE to PE), however, for the composition (4.4 mol % CTFE), a discontinuous transition occurs (FE to PE) and the RFE to PE and FE to PE phase transitions coexist [55]. Meanwhile, the thermo-mechanical and dielectric properties evolve continuously with CTFE content and their evolutions are linked to the structural properties [54].

Characterization Methods
The characteristic of crystallization of PVDF is the coexistence of crystalline and amorphous regions. The XRD [43,[56][57][58][59], FTIR [60][61][62][63][64][65][66] and DSC [24] are the methods commonly used for qualitative or quantitative analysis of different phase structures. In early research, XRD was used to analyze the structure of each phase of PVDF, to obtain information such as conformation, unit cell parameters, space group, etc. In subsequent studies, the phases and crystallinity are often judged based on the characteristic peaks in the diffraction pattern. PVDF containing a single β phase can be obtained by solvent crystallization, the diffraction pattern produces a sharp absorption peak at 2θ ≈ 20 • , which represents the sum of the diffraction intensity of (1 1 0) and (2 0 0) planes. Similarly, the characteristic diffraction peaks of α phase can be obtained, the absorption peaks at approximately 2θ = 17. There are three types of absorption peaks in the infrared spectra, such as the common absorption peaks of the three phases, the characteristic absorption peak of only one phase, or the double peaks produced by the two phases [66]. The characteristic absorption peaks can be used to distinguish each crystal phase, and then calculate the content of each phase in the mixture. However, the positions of some characteristic peaks are very close, especially the β phase and the γ phase which have very similar conformations, so there are many statements on the selection of characteristic peaks. For example, in some research, the positions of 510 and 840 cm −1 peaks can be used to characterize individual β phase or γ phase [61,62], or the common peak of the two [60,65]. Cai et al. [66] compared the characteristic absorption peaks of each phase and proposed to judge the existence of α phase by the peaks at 763 and 614 cm −1 , and the peak at 1275 cm −1 to judge the existence of β phase, the 1234 cm −1 to the γ phase. The characteristic peaks of β phase and γ phase are combined to determine the attribution of peaks at 510 and 840 cm −1 . For the quantitative calculation of each crystal phase, Gregorio et al. [62] used the Lambert-Beer law to determine the content of each phase in a polymorphic mixed sample, as shown in Equations (1)- (3).

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where A α and A β represent the absorption intensity at 766 and 840 cm −1 , respectively, determined by I 0 and I. I 0 is the intensity of incident light, I is the intensity of transmitted light; L is the thickness of the sample, C is the monomer concentration, and X is the crystal degree, K is the absorption coefficient under a certain wavenumber; the values of K α and K β are calculated as 6.1 × 10 4 and 7.7 × 10 4 cm 2 /mol using samples containing single α phase and β phase, respectively. Similarly, when the sample is a mixture of α phase and γ phase, the relative content of the two phases can be calculated using the same analysis method. A α and A γ represent the absorption intensity at 766 and 510 cm −1 , respectively, and the K γ is 5.8 × 10 4 cm 2 /mol [62]. DSC is also a commonly used method to analyze the melting point of materials and calculate crystallinity [24]. The melting point of the raw PVDF is around 148~177 • C [56].
Melting heat ∆H f was determined from the melting peak area. Crystallinity can be evaluated by Equation (4) [57].
where ∆X c represent the crystallinity, ∆H 100 = 104.7 J/g is the melting heat for a complete crystalline sample of PVDF [44]. However, when using one method for characterization, interference factors such as preparation method and choice of raw materials will often cause changes in intensity and shift of peak positions, which will interfere with the judgment. Therefore, multiple characterization methods are often combined to analyze the structure of the sample scientifically and accurately.

Piezoelectricity
The ferroelectricity and piezoelectricity of PVDF are closely related to its crystal structure. The antiparallel arrangement of molecular chains in α phase and ε phase makes the unit cell have no net dipole moment. In the other three crystal types, the dipoles in the unit cell are parallel to each other and perpendicular to the molecular chain axis, the unit cell has a net electric dipole moment and can be spontaneously polarized. Among them, the β phase has highest dipole moments per chain per repeat unit (2.10 D) [17], dielectric permittivity and piezoelectric coefficient. Therefore, the content of β phase has the most obvious effect on piezoelectric properties, which is the most ideal phase structure [67,68].
The magnitude of the electric polarization P produced by the piezoelectric effect is proportional to the applied stress σ (Equation (5)).
The stress state of the crystal is determined by the second-order tensor, the electric polarization intensity is described by a vector. The piezoelectric strain coefficient d that is the most commonly used piezoelectric parameter is a third-order tensor. Due to the symmetry of the unit cell, the piezoelectric constant matrix of the PVDF film can be simplified as Equation (6) [38,69].
The subscript "i" is defined as the direction of electric field or electric displacement, "j" is defined as the direction of stress or strain (simplified subscript), "d" is the third order tensor.

Piezoelectric Properties of PVDF-Based Materials
PVDF and its copolymers including the P(VDF-TrFE) and P(VDF-CTFE) are all ferroelectric materials with piezoelectric and ferroelectric properties. We know that quartz crystal has weak piezoelectric activity, and complex production process, lead based piezoelectric ceramics such as lead zirconate titanate (PZT) are toxic and barium titanate (BaTiO 3 ) has a low Curie temperature and a complicated high-temperature sintering process [70,71]. Compared with these inorganic piezoelectric materials, organic polymer materials have obvious advantages in terms of mechanical properties, light weight, good flexibility and chemical stability, easy processing, nontoxic and biocompatibility, becoming a type of piezoelectric functional material that developed rapidly in the last two decades. However, the α phase of PVDF has the lowest molecular potential energy among all phases [72]. The untreated PVDF mainly crystallize in the form of α phase, but its piezoelectricity mainly depends on the polarity to β phase. Although PVDF has great flexibility, its piezoelectric constant is still small compared to inorganic piezoelectric materials. Therefore, increasing the content of the β phase becomes the key means to improve the piezoelectric activity of PVDF. Table 2 shows the comparison of the piezoelectric properties of PVDF and its copolymers with some inorganic materials. Several methods have been performed to improve this shortcoming of PVDF. For example, by stretching and applying electric field, the transformation from α phase to β phase can be induced. By repeated unidirectional poling at 650 MV/m for at least 40 times, pure β phase crystals for a biaxially oriented PVDF film can be achieved with high piezoelectricity (d 33 = −62 pC/N) [84]. However, when high electric field polarization is applied, the preparation steps are complicated and time-consuming; when other structural units (TrFE, CTFE, or HPF) are introduced into PVDF, the copolymer will be directly converted to β phase without additional stretching [9,85]. For example, P(VDF-TrFE) polymer with TrFE units content higher than 20 mol% can completely convert the phase of PVDF into polar β phase crystal form with TTT conformation [86]; Inorganic or organic nanoparticles can also be used to compound with polymers, such as BaTiO 3 [87,88], TiO 2 [89], ZnO [27,90,91], MgO [92], Pd [93], graphene [94,95], clays [57,96], biomolecules [97] and other substances, the composite two-phase material has the advantages of low acoustic impedance and low dielectric constant. The mechanism is that the filler is usually a polar molecule or charged that can interact with the -CF 2 -or -CH 2 -groups of PVDF. Liu et al. [98] provide a chemical mapping of the interfacial coupling between the nanofiller and the polymer matrix in ferroelectric polymer nanocomposites by combining atomic force microscopy-infrared spectroscopy (AFM-IR) with first-principles calculations and phase-field simulations. It is confirmed that the addition of ceramic fillers into a ferroelectric polymer leads to augmentation of the local conformational disorder in the vicinity of the interface, resulting in the local stabilization of the all-trans conformation on a molecular scale, as show in Figure 2. However, the higher surface activity and larger surface area of nanoparticles usually cause the poor dispersion in polymer matrix, agglomeration will occur, and the crystallinity of the polymer will decrease. For example, compared with pure PVDF, the crystallinity of the PVDF/ZnO composite film without surface modification of PVDF will drop from 54.76% to 47.73% [90]. Therefore, the filler is often surface modified and then compounded with the polymer. Simultaneously measured topography (2 × 2 µm 2 ) and (f) AFM-IR chemical maps with irradiation by a laser at 1275 cm -1 in the BTO/P(VDF-TrFE-CFE) nanocomposites with BTO of 400 nm in diameter. Reproduced from [98] with permission from the John Wiley and Sons, copyright 2020.

Fabrication Methods
Different morphologies/structures of PVDF-based materials in the forms of dense or porous films, 3D scaffolds, patterned structures, fibers and spheres can be obtained by processing technologies including doctor blade, spin coating, printing technologies, non-solvent-induced phase separation (NIPS), temperature-induced phase separation (TIPS), solvent-casting particulate leaching, solvent-casting using a 3D nylon template, freeze extraction with a 3D poly(vinyl alcohol) (PVA) template, replica molding, and electrospinning or electrospray, etc. [71].
The method of melting crystallization heats the polymer to elastomeric state and extrudes the film with uniaxial or biaxial stretching and can continuously produce the film in a large area [99], but it requires a large amount of raw materials not suitable for laboratory study to explore new functional materials. Here we introduce several commonly used methods for film preparation, solution casting [100][101][102], doctor blade [71], spincoating [9,71,[103][104][105], and inkjet printing technologies [106][107][108][109]. Solvents with stronger polarity (dimethylformamide, methyl pyrrolidone or dimethyl sulfoxide, etc.) can help the dispersion of the polymer and generation of electroactive β phase when prepare a certain mass fraction polymer solution. String and ultrasonic dispersing can accelerate the dispersion of polymers. For solution casting method, it requires pouring the prepared solution into the mold, and a film with a certain thickness can be obtained after completely evaporating the solvent by heating. The template can be a simple petri dish for film preparation, in contrast, if 3D scaffolds are required, 3D nylon template or PVA template with different architectures and interconnected porosity have been used as templates for casting [110]. For doctor blade, with the movement of the squeegee above the substrate, the polymer solution spreads out on the substrate, the thickness of the film is controlled by polymer mass fraction and the gap size between the blade and the substrate [71]. The spin-coating method can be used to prepare even thinner films with better uniformity, but the newly formed layer will adversely influence the formerly prepared layers. After spin coating and heating, a dense film can be formed. The speed and time of spin coating, the mass fraction of the solution, and the number of spin coating layers will all affect the final thickness of the film [103]. Inkjet printing is a technology for the deposition on various substrates of high-definition patterns of different kinds of materials in a controlled and defined manner. These materials can be integrated into entirely inkjet-printed flexible electronic devices as the functional layers [106][107][108]. It should be noted that when preparing the P(VDF-TrFE) ink formulation, priority should be given to choose a solvent with a high boiling point and low vapor pressure to prevent the quick solvent volatilization during the printing process to form films and further block the inkjet printing [108]. Micron-level films can be obtained through multiple deposition of the ink formulation. In addition to the patterned film or electrode by inkjet printing, the photopatternable P(VDF-TrFE-CTFE) with dent azido groups can be cross-linked thermally and photochemically, such a kind of polymer has the characteristics of high dielectric constant and no need of external additives, which can be used as a negative photoresist to obtain high-quality patterned films [111]. After film preparation, annealing to reduce the residual thermal stress, and polarization to orient the dipoles should be considered.

Pressure Sensors
The PVDF-based piezoelectric materials are explored for pressure sensors, which have been used in various fields such as engineering, physiological testing and micromechanical elements. The state of the polymer is mainly thin film or nanofiber. The overall structure is mostly a sandwich structure, where the upper and lower layers are electrodes, and the middle layer is PVDF-based piezoelectric materials. Through the detected pressure change, the pressure sensor can be used to detect whether the pipeline valve is normally switched [112]. As the charge generated on the electrodes by straining the foil is discharging slowly, the sensor is suitable for the detection of dynamic low-frequency pressure changes. It has the advantages of good aging resistance at low temperatures and low pressures, low production cost, short response time, and wide operating temperature and pressure ranges, but the measurement accuracy is limited by the intersection of temperature, thermoelectricity and temperature sensitivity. In addition, the P(VDF-TrFE) copolymer can be obtained by spin-coating on the silicon substrate into thin film (1 µm). A thin film sensor with a metal-P(VDF-TrFE)-metal structure, which can achieve rapid recovery, was fabricated by a lithography process. Due to its compact form factor and biocompatibility, it can be integrated with catheters to detect blood flow direction and vascular pressure, making it a huge application potential in the field of implantable medical Crystals 2021, 11, 644 9 of 21 devices [20]. As shown in Figure 3a,b, the highly oriented P(VDF-TrFE) nanofibers prepared by electrospinning technology have high sensitivity, and the simple pressure sensor shows excellent response in a wide pressure range. The sensitivity is found to be 1.1 V/kPa within the pressure range of 0.4~2 kPa [21]. In addition to applying positive pressure to cause the material to bend and deform to produce piezoelectric response, PVDF can also be designed as a simple accelerometer, vibrometer, and orientation sensors, promising for uses in self-balancing robots and sensitive impact detectors.
PVDF-based pressure sensors also show a wide range of applications in the field of wearable devices for human-computer interaction. The skin-interactive metal-free spongy electrode in a piezoelectric sensor where highly aligned poly(vinylidene fluoride) (PVDF) nanofiber (NF) arrays are introduced as the piezoelectric active component and conducting polyaniline-(PANI-) coated PVDF (PANI−PVDF) NFs mats served as flexible electrodes (Figure 3c), this design effectively avoids the disadvantages of poor compatibility and fragility of metal electrodes, at the same time, shows excellent mechanical to electrical energy conversion property (Figure 3d) that enables to sense human finger touch with a high energy conversion efficiency being in the order of 53% [22]. Due to the piezoelectric and pyroelectric effects of PVDF, electrospinning nanofibers are also used to manufacture highly sensitive wearable pressure sensors and pyroelectric respiration sensors. The graphene doped PVDF materials are found to show better performances (Figure 3e) [94]. Such pressure sensors exhibit high sensitivity to the bending and stretching of fingers, wrists and elbows, and can detect a variety of human physiological signals (Figure 3f). In addition to graphene, the molecular ferroelectric (dabcoHReO 4, DH) is also used to compound with PVDF, and the hydrogen bond between them can be utilized to prepare a composite film with a high β crystal form content (95%), which can be applied for flexible electronic sensing equipment with excellent performance [114]. Core/shell PVDF/dopamine (DA) nanofibers with a very high β crystal form content (93%) and self-aligned polarization can be fabricated by electrospinning (Figure 3g), the all-fiber-based soft sensor fabricated and tested on human skin and in vivo in mice shows a high sensitivity and accuracy for detecting weak physiological mechanical stimulation from diaphragm motions and blood pulsation [97].
In addition to modifying the material itself, the patterned electrode can also effectively improve the sensitivity of the device [34,121,122]. A pair of dislocation interdigital electrodes are designed on the upper and lower surfaces of the P(VDF-TrFE) piezoelectric film by inkjet printing (Figure 3h). Then, by applying a high voltage, multiple alternating oblique polarization regions (Figure 3i,j) are generated inside the cross-sectional area, which exhibits enhanced sensitivity to an external stress stimulation [34].

Acoustic Sensors
Sound detection and recognition sensors (SDRs) based on electrostatic, capacitive, electromagnetic or electrical methods have the disadvantages of large size, internal power supply and low electro-acoustic conversion efficiency. In contrast, the PVDF-based piezoelectric sensor has the characteristics of small size, flexibility, self-powered, high acousticelectric conversion efficiency and high signal-to-noise ratio, which can meet the current requirements for SDRs applications [116].
Piezoelectric acoustic sensors require the sensing materials to possess the merits of good flexibility and high sensitivity. Electrospinning technology is a feasible method to fabricate PVDF nanofibers, which are easy to vibrate under sound waves. The PVDF piezoelectric nanofiber sensors prepared by electrospinning can distinguish sound waves of different frequencies even similar frequencies. Figure 4a shows the entire workflow of the sensor, and Figure 4b illustrates the proposed sound sensing mechanism [118]. As can be observed, such kind of device shows high sensitivity to sounds above 100 decibels. It is worth mentioning that there is a maximum voltage output when sound frequency is 220Hz (Figure 4c), which can be used for noise detection. In addition to the traditional

Acoustic Sensors
Sound detection and recognition sensors (SDRs) based on electrostatic, capacitive, electromagnetic or electrical methods have the disadvantages of large size, internal power supply and low electro-acoustic conversion efficiency. In contrast, the PVDF-based piezoelectric sensor has the characteristics of small size, flexibility, self-powered, high acousticelectric conversion efficiency and high signal-to-noise ratio, which can meet the current requirements for SDRs applications [116].
Piezoelectric acoustic sensors require the sensing materials to possess the merits of good flexibility and high sensitivity. Electrospinning technology is a feasible method to fabricate PVDF nanofibers, which are easy to vibrate under sound waves. The PVDF piezoelectric nanofiber sensors prepared by electrospinning can distinguish sound waves of different frequencies even similar frequencies. Figure 4a shows the entire workflow of the sensor, and Figure 4b illustrates the proposed sound sensing mechanism [118]. As can be observed, such kind of device shows high sensitivity to sounds above 100 decibels. It is worth mentioning that there is a maximum voltage output when sound frequency is 220 Hz (Figure 4c), which can be used for noise detection. In addition to the traditional resonant system sensor, a suspended, transparent, nonresonant acoustic sensor is prepared through the dynamic near-field electrospinning (dNFES) process, which has a wide frequency response range, high resolution and high fidelity, and can be applied for voice recognition and recording [123]. When the individual nanofibers vibrate under the acoustic field in a collective manner, an output voltage is generated (Figure 4d). The sensing device can detect acoustic signals in the frequency range of 200~5000 Hz (Figure 4e) which covers the most common audible sound frequency spectrum in daily life. Particularly, the PVDF based SDRs can differentiate two similar input sound frequencies (Figure 4f). Meanwhile, due to the high transparency and high permeability of the nanofiber network, it can be combined with optical and gas sensors to collect multidimensional information in the same smart device platform. Of course, traditional membranes can also be used as acoustic sensors. Ionic liquid in-situ electric field polarization can be used to obtain flexible, accurate and highly sensitive P(VDF-TrFE) film (Figure 4g) [116]. Sensors based on the film can identify and distinguish complex sound signals, such as those musical notes of different frequencies (Figure 4h), bass and treble (Figure 4i), and the frequencies produced by different species that have great potential in various applications such as microphones, biometrics, environmental protection, and artificial intelligence. resonant system sensor, a suspended, transparent, nonresonant acoustic sensor is prepared through the dynamic near-field electrospinning (dNFES) process, which has a wide frequency response range, high resolution and high fidelity, and can be applied for voice recognition and recording [123]. When the individual nanofibers vibrate under the acoustic field in a collective manner, an output voltage is generated (Figure 4d). The sensing device can detect acoustic signals in the frequency range of 200~5000 Hz (Figure 4e) which covers the most common audible sound frequency spectrum in daily life. Particularly, the PVDF based SDRs can differentiate two similar input sound frequencies (Figure 4f). Meanwhile, due to the high transparency and high permeability of the nanofiber network, it can be combined with optical and gas sensors to collect multidimensional information in the same smart device platform. Of course, traditional membranes can also be used as acoustic sensors. Ionic liquid in-situ electric field polarization can be used to obtain flexible, accurate and highly sensitive P(VDF-TrFE) film (Figure 4g) [116]. Sensors based on the film can identify and distinguish complex sound signals, such as those musical notes of different frequencies (Figure 4h), bass and treble (Figure 4i), and the frequencies produced by different species that have great potential in various applications such as microphones, biometrics, environmental protection, and artificial intelligence. Reproduced from [116] with permission from the Elsevier, copyright 2020.
Nondestructive testing is often used for performing process monitoring (PM) and structural health monitoring (SHM) to assess the quality of the produced composite parts  [116] with permission from the Elsevier, copyright 2020.
Nondestructive testing is often used for performing process monitoring (PM) and structural health monitoring (SHM) to assess the quality of the produced composite parts [124]. For the external nondestructive testing, such as radiographic testing, electromagnetic (EM) testing, acoustic emission (AE) testing, infrared thermography (IRT) testing, etc., they have to be instrumented on the surface or even out of the parts, and are done in a particular period inconstantly, in-situ SHM techniques can do continuous monitoring of the structure and can reduce the environmental interference due to the encapsulation [124,125]. PVDF devices have also been used as in-situ acoustic emission sensors [126][127][128]. Damage sensing and interfacial evaluation of single-glass or basalt fibers/epoxy composites can be monitored through AE [128]. Due to the limited sensitivity compared with PZT, only fiber fracture signals can be detected by the piezoelectric PVDF sensor with 30µm glass composite without matrix crack and interfacial debonding signals. The advantage of the PVDF is that it is diverse in the choice of contacting methods, piezoelectric PVDF sensors can be attached or embedded in the composite to monitor the composites without sensitivity loss contributed from their ductile flexibility. De Rosa et al. [127] performed an investigation based on AE with PVDF sensors as AE sensors, which are surface-mounted and embedded in glass/epoxy laminates. The sensors are used to monitor the laminates while being subjected to tension, three-point bending and post-impact three-point bending. Table 3 shows the performances of several kinds of PVDF-based piezoelectric sensors which are mentioned.

PVDF Nanogenerators
In 2006, Professor Zhonglin Wang proposed the use of ZnO nanowire (NW) arrays to convert micro-nano mechanical energy into electrical energy, that is, the strategy of nanogenerators [129]. In addition to the usage as a self-powered sensor, nanogenerators can provide electrical energy for micro-nano electronic devices and can also directly provide electrical energy for chemical reactions [26]. PVDF nanogenerators can collect mechanical energy from the environment [29,130,131] or human movement [28,[132][133][134] and convert it into electrical energy. Compared with traditional rigid and fragile piezoelectric ceramics, the flexible PVDF electronic devices are becoming a current research focus due to their excellent flexibility and easy processing technology.
According to the piezoelectric mechanism, when suffering from the dynamics of non-zero strain rate, the positive and negative charge centers will separate, which leads to polarization and changes in bound charge. The flow of free charge in the drive circuit is used to shield the bound charge, generating alternating current, and the net charge continuously generates voltage across the nanogenerator. Pi et al. [9] used spin-coating method to prepare P(VDF-TrFE) film on polyimide (PI) substrate as the functional layer of the nanogenerator (Figure 5a), and the short-circuit current and open-circuit voltage changes of the nanogenerator in the cyclic stretching-releasing process are explained through experiments and models (Figure 5b). tals 2021, 11, x FOR PEER REVIEW 13 o According to the piezoelectric mechanism, when suffering from the dynamics of no zero strain rate, the positive and negative charge centers will separate, which leads to p larization and changes in bound charge. The flow of free charge in the drive circuit is us to shield the bound charge, generating alternating current, and the net charge contin ously generates voltage across the nanogenerator. Pi et al. [9] used spin-coating meth to prepare P(VDF-TrFE) film on polyimide (PI) substrate as the functional layer of nanogenerator (Figure 5a), and the short-circuit current and open-circuit voltage chang of the nanogenerator in the cyclic stretching-releasing process are explained through periments and models (Figure 5b).  [135] with permission from the Elsevier, copyright 2021.
As mentioned in part three, the piezoelectric performance of the doped PVDF fi will be improved when comparing to the pure PVDF. Mao et al. [26] obtained PVDF/Z composite film by solution casting, and then etched ZnO with hydrochloric acid soluti to obtain mesoporous PVDF film (Figure 5c). This fabrication method circumvents requirement of large mechanical strain or high electric field for the formation of β pha PVDF, and using the electronic device's own weight to modulate its displacement a amplify its electrical output realizes a very simple system design (Figure 5d,e). This si ple-designed device is easy to integrate into other systems and is used in self-power electronic equipment. Recently, the lead-free piezoelectric ceramics were also used  [135] with permission from the Elsevier, copyright 2021.
As mentioned in part three, the piezoelectric performance of the doped PVDF film will be improved when comparing to the pure PVDF. Mao et al. [26] obtained PVDF/ZnO composite film by solution casting, and then etched ZnO with hydrochloric acid solution to obtain mesoporous PVDF film (Figure 5c). This fabrication method circumvents the requirement of large mechanical strain or high electric field for the formation of β phase PVDF, and using the electronic device's own weight to modulate its displacement and amplify its electrical output realizes a very simple system design (Figure 5d,e). This simple-designed device is easy to integrate into other systems and is used in self-powered electronic equipment. Recently, the lead-free piezoelectric ceramics were also used to compound with PVDF for use in wearable or implantable devices. The flexible composite film doped with 0.5Ba(Zr 0.2 Ti 0.8 )O 3 -0.5(Ba 0.7 Ca 0.3 )TiO 3 (BZT-BCT) can be prepared by the electrospinning (Figure 5f), which exhibits excellent voltage output performance, the output voltage as high as 13.01 V under cyclic tapping under 6 N at 10 Hz [135]. Besides, nanogenerator can respond to the activities of the human body to show their applications in wearable field (Figure 5g). Jella et al. [136] compounded methylammonium lead iodide (MAPbI 3 ) perovskite with PVDF and compared the output performance of the composite films with different thicknesses (36.9, 69.3 and 97.7 µm). It is reported that the composites with higher thickness exhibited more outstanding piezoelectric output performance than the thinner ones. Results show that the output voltage value and other parameters are closely related to the thickness of the electroactive layer. In addition to the inorganic materials, researchers have gradually turned their visions to the combination of PVDFbased materials with other organic materials for seeking the improved performance of the nanogenerators and meanwhile achieving better biocompatibility and degradability, for example, the biowaste crab shell-extracted chitin nanofiber (CFN) [137] and vitamin B 2 [29]. Table 4 shows the performances of PVDF-based piezoelectric nanogenerators

Biomedical Applications
Taking the advantage of biocompatible feature, the PVDF-based piezoelectric polymer is explored for biomedical applications. The PVDF based materials can be used for simulating the extracellular matrix environment and use in vivo physiological activities or environmental stimuli to generate in-situ electrical signals to promote bone [138,139], nerves [30,32] and muscle [33] regeneration as well as biological tissue engineering.
Genchi et al. [32] prepared a P(VDF-TrFE)/BaTiO 3 nanoparticle composite film (Figure 6a), which presents better piezoelectric properties than the pure P(VDF-TrFE) and can support the proliferation and differentiation of SH-SY5Y cells. What is more, the piezoelectricity of the film can be activated under ultrasound stimulation, so that the number of β3-tubulinpositive cells and the axon length of the cells on the P(VDF-TrFE)/BTNP composite film could be increased significantly (Figure 6b), thus, the film can be applied for use as wireless neuron stimulation devices. For the selection of piezoelectric materials, BaTiO 3 nanoparticles and P(VDF-TrFE) composite were applied [140], and polydopamine was introduced on BTO nanoparticles to achieve better interface compatibility between BTO nanoparticles and P(VDF-TrFE), as shown in Figure 6c. Solution casting method is chosen to prepare composite film and then conduct polarization treatment. It is reported that the material promotes the in vitro osteogenic behavior of bone marrow mesenchymal stem cells. Furthermore, it can also promote the continuous maintenance of the in vivo electrical microenvironment, rapid and extensive healing of bone defects (Figure 6d). Damaraju et al. [31] used electrospinning to prepare three-dimensional fiber scaffolds to stimulate the differentiation and tissue formation of mesenchymal stem cells (MSC). Cheng et al. [30] used the casting/annealing-solvent replacement method to composite the degradable material polycaprolactone (PCL) and PVDF to prepare a porous nerve tissue engineering scaffold (Figure 6f). Compared with a scaffold with a high elastic modulus prepared by electrospinning, such a design shows strong mechanical strength. In addition, the stent exhibits better biodegradability and biocompatibility, which can avoid the process of removing the implant material in the second operation. This kind of composite scaffold can achieve the function of promoting regeneration and recovery of a 15 mm rat with sciatic nerve defect. Figure 6e gives the average piezoelectric response amplitude induced by applied a.c. voltage. The slope of the piezoelectric response amplitude versus the applied voltage for PVDF/PCL scaffolds is dramatically increased. It needs to be pointed out that when exploring PVDF piezoelectric materials for biomedicine applications, more factors should be taken into account, such as biocompatibility and mechanical properties, etc.

Summary and Outlook
PVDF based piezoelectric materials have shown more and more application poten- Confocal fluorescence microscopy images of SH-SY5Y neuroblastoma cells at the end of a differentiation period of 6 days on P(VDF-TrFE), P(VDF-TrFE)/BTNP films, and on Ibidi film as control. Cells were either unexposed or exposed to chronic US stimulation. Reproduced from [32] with permission from the John Wiley and Sons, copyright 2016. (c) Schematic illustration of the preparation process of BTO/P(VDF-TrFE) membrane. (d) Bone defect repair in rat calvarial models after implantation of polarized nanomembranes with 5 vol% BTO content and polarized neat P(VDF-TrFE) membranes. Reproduced from [140] with permission from the American Chemical Society, copyright 2016. (e) Piezoelectric response of PVDF, 20% PVDF/PCL, PCL, in terms of the average values of amplitude responded to applied a.c. voltage. (f) SEM images of the multilayered and microporous structure for 20% PVDF/PCL scaffolds. Reproduced from [30] with permission from the Elsevier, copyright 2020.
Damaraju et al. [31] used electrospinning to prepare three-dimensional fiber scaffolds to stimulate the differentiation and tissue formation of mesenchymal stem cells (MSC). Cheng et al. [30] used the casting/annealing-solvent replacement method to composite the degradable material polycaprolactone (PCL) and PVDF to prepare a porous nerve tissue engineering scaffold (Figure 6f). Compared with a scaffold with a high elastic modulus prepared by electrospinning, such a design shows strong mechanical strength. In addition, the stent exhibits better biodegradability and biocompatibility, which can avoid the process of removing the implant material in the second operation. This kind of composite scaffold can achieve the function of promoting regeneration and recovery of a 15 mm rat with sciatic nerve defect. Figure 6e gives the average piezoelectric response amplitude induced by applied a.c. voltage. The slope of the piezoelectric response amplitude versus the applied voltage for PVDF/PCL scaffolds is dramatically increased. It needs to be pointed out that when exploring PVDF piezoelectric materials for biomedicine applications, more factors should be taken into account, such as biocompatibility and mechanical properties, etc.

Summary and Outlook
PVDF based piezoelectric materials have shown more and more application potentials in sensors, power generation and biomedicine, etc., due to their excellent physical and mechanical properties, chemical stability, and low production costs. Besides, the combination of piezoelectric properties with other properties such as acoustics, optics, and thermals to design multifunctional integrated systems is also becoming an important direction of the current research.
At present, there are many methods for preparing PVDF based polymer films, but pure PVDF requires stretching and polarization post-processing operations, and the process is complicated. Therefore, the improvment of the preparation process and the piezoelectric performance is one of the follow-up studies. In addition, research on composite piezoelectric materials based on PVDF materials is active. Exploring new composite films using materials with strong piezoelectric activity would improve the properties of the PVDF based composites. Choosing appropriate inorganic or organic materials to construct new structures will be more conducive to improving the corresponding level of piezoelectricity of the film and the applicable scope of the related device.
PVDF has a complex conformation, and the crystalline region and the amorphous region coexist, hence, the crystallization mechanism is complex. Though many studies to date focus on the design and application of related devices, the fundamental and deep studies on the intrinsic properties of PVDF materials are limited. How the transformation occurs between different conformations, which component is primarily responsible for the piezoelectricity, and how the composite films improve piezoelectric performance are worth further investigation. What is more, although the performance and application of PVDF-based materials have made important progress in laboratory stage, they still face many challenges for industrial applications, and some questions about the fatigue and the durability after composition need further investigations.
Author Contributions: L.X. and G.W. contributed equally to this work. Conceptualization, L.X., G.W. and F.Y.; methodology, G.W.; software, C.J.; writing-original draft preparation, L.X. and G.W.; writing-review and editing, F.Y. and X.Z.; project administration, F.Y. and X.Z. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.