Ferroelectric Polymer PVDF-Based Nanogenerator

This chapter deals with the development of ferroelectric polymer polyvinylidene fluoride (PVDF)-based nanogenerators. Due to its inherent flexibility, PVDF has been studied for application in nanogenerators. We first introduce PVDF and its copolymers, and briefly discuss their properties. Then, we discuss fabrication methods, including solution casting, spin coating, template-assisted method, electrospinning, thermal drawing, and dip coating. Using these methods, a wide variety of ferroelectric polymer structures can be fabricated. In addition to the performance enhancements provided by fabrication methods, the performance of PVDF-based nanogenerators has been improved by incorporating fillers that can alter the factors affecting the performance. Next, we review energy sources that can be exploited by PVDF-based nanogenerators to harvest electricity. The abundant energy sources in the environment include sound, wind flow, and thermal fluctuation. Finally, we discuss implantable PVDF-based nanogenerators. Another advantage of PVDF is its biocompatibility, which enables implantable nanogenerators. We believe that this chapter can also be helpful to researchers who study sensors and actuators as well as nanogenerators.


Introduction
All electronic devices need electrical energy to operate. Although fossil fuels have been the primary sources of that electrical energy to date, alternatives are emerging. This is particularly important given the ongoing proliferation of portable devices. For example, as "big data" has become increasingly important for monitoring structures, healthcare services, smart cites, and so on, there has been an explosion of sensors to collect that information [1][2][3]. These sensors can even be located in the human body, beneath the human skin, and inside personal wearable devices [4][5][6][7]. In such cases, batteries cannot be easily charged or repeatedly replaced. To meet the energy requirements of remote or inaccessible application like these, one of the most promising alternatives to conventional batteries is a nanogenerator, which can convert mechanical energy or thermal fluctuations in the ambient environment into electricity.
Polyvinylidene fluoride (PVDF) and its copolymers, ferroelectric polymers, are ideal candidates for use in nanogenerators. Their unique properties include their high flexibility, lightness, chemical stability, and relatively simple manufacturing  [14] PVDF/Ba(Ti 0. 9

PVDF and its copolymers
Semicrystalline PVDF has at least four crystalline modifications: α, β, γ, and δ [26]. Generally, the α-phase is the most stable crystal phase of PVDF in ambient conditions. The conformation of α-PVDF, displayed in Figure 1a, is trans-gauchetrans-gauche (TGTG'). The α-phase is nonpolar because of its centrosymmetric symmetry and can be easily obtained from melt crystallization at atmospheric pressure. On the other hand, the β-phase is classified as a ferroelectric and exhibits the largest remnant polarization of ~ 13 μC/cm 2 among the phases. Ferroelectricity in the β-PVDF is directly correlated to its macroscopic dipole moment. In the β-phase, the conformation is all trans (TTTT) as shown in Figure 1b. The transition from the PVDF α-phase to the β-phase can be induced by stretching the polymer. The γ-phase is also polar and its conformation is trans-trans-trans-gauche-trans-trans-transgauche (T 3 GT 3 G) as shown in Figure 1c. The γ-phase is also ferroelectric. However, the γ-phase is less frequently observed because it requires extreme temperature control and high pressures to develop. The δ-phase has the same configuration as the α-phase. The difference is that the δ-phase is a ferroelectric. Crystallization of the δ-phase can be achieved by electroforming from the bulk α-PVDF in a high electric field of about 170 MV/m. Recently, M. Li et al. demonstrated that a δ-PVDF thin film can be made at elevated temperatures by applying a short pulse of about 250 MV/m [27]. They confirmed the existence of the δ-phase via combined GI-XRD and FTIR measurements on pristine and electroformed capacitors.
Due to its electroactive properties, high β-phase content is desirable for applications. This can be achieved physically or chemically. The most common chemical derivatives of PVDF available are polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) and polyvinylidene fluoride-hexafluoropropene (P(VDF-HFP)). Despite its high cost, P(VDF-TrFE) is often preferred over PVDF. The main advantage of P(VDF-TrFE) is that the β-phase can easily develop without mechanical stretching or incorporating fillers. The mechanism behind the development of the β-phase is based on the introduction of additional fluorine atoms within a certain amount, which allows steric hindrance to occur. P(VDF-HFP) has received a lot of attention because of its extremely high electrostrictive response [28]. As with PVDF, the β-phase in P(VDF-HFP) can be obtained by mechanical stretching [29]. Another way to develop the β-phase in P(VDF-HFP) is casting from solution in dimethylformamide (DMF) [30].
Since the discovery of piezoelectricity in PVDF [31], the mechanism behind the piezoelectric response has been a subject of debate. Recently, L. Katsouras et al. identified the piezoelectric effect in semicrystalline PVDF and its copolymers via in-situ dynamic X-ray diffraction, from measurements with P(VDF-TrFE) capacitors [32]. The piezoelectric effect is attributed to the change in lattice constant produced by the electromechanical coupling between the intermixed crystalline and amorphous regions as well as the electrostrictive response of the crystalline part. Both the electrostrictive Q 33 and the additional, d coupling , are negative; hence, the piezoelectric coefficient, d 33 , of P(VDF-TrFE) is negative. Understanding the fundamental basis of piezoelectricity in ferroelectric polymers is essential to further technological innovations.
In order to identify the phases of PVDF, analysis techniques based on electromagnetic radiations such as X-ray and infrared have been widely used. The crystalline structure in PVDF and its copolymers can be confirmed via X-ray diffraction (XRD). In general, the nonpolar α-phase and the polar β-and γ-phases of PVDF appear in XRD patterns (Figure 2a) [33]. There are peaks at 18.4, 19.9, and 26.6° corresponding to (020), (110), and (021) reflections of the monoclinic α-phase, respectively. The peak at 20.6° is associated with the crystalline (200) and (110) of the orthorhombic β-phase. For the γ-phase, the dominant peaks appear at 18.5 and 20.2° corresponding to (020) and (110), respectively. In the case of P(VDF-TrFE), the peak corresponding to the (200) and (110) planes of the β-phase crystalline phase is represented at 19.7°. The position and width of peaks can change depending on experimental conditions and the ratio between VDF and TrFE. Therefore, the diffraction patterns can be differently observed in the literature.
The ferroelectric phase formation of PVDF and its copolymers have been also confirmed using Fourier transform infrared spectroscopy (FTIR). However, there exists a conflict on spectrum peaks corresponding the phases. Recently, X. Cai  light on this issue [33]. Furthermore, they analyzed the FTIR results and suggested the relative fraction of the β-and γ-phase calculations in terms of crystalline components as shown in Figure 2b. Since the absorption band at 840 cm −1 corresponds to the β, γ, or both phases, the fraction of the β-and γ-phases can be calculated as follows: where I EA and I 763 are the absorbance at 840 and 763 cm −1 , respectively, and K 840 and K 763 are the absorption coefficients at the respective wave numbers, whose values are 7.7 × 10 4 and 6.1 × 10 4 cm 2 mol −1 , respectively. If PVDF materials contain both phases, another method is needed to quantify individual β-and γ-phases. The quantification of β-and γ-phases can be demonstrated as follows: where ∆ H β′ and ∆ H γ′ are the height differences (absorbance differences) between the peak around at 1275 cm −1 and the nearest valley around at 1260 cm −1 , and the peak around 1234 cm −1 and the nearest around at 1225 cm −1 , respectively.

Fabrication techniques
PVDF and its copolymer-based nanogenerators consist of a layer of PVDF or its copolymers sandwiched between two electrodes, like a capacitor. As previously noted, PVDF and its copolymer-based nanogenerators have advantages over piezoelectric ceramics-based nanogenerators, because of their flexibility and high piezoelectric voltage coefficient (g 33 ). One important consideration is the choice of a suitable technique for the fabrication of ferroelectric polymer-based nanogenerators. A great variety of methods to make ferroelectric polymer-based nanogenerators has been developed for a few decades: solution casting, spin coating, template-assisted method, electrospinning, thermal drawing, and dip coating, as XRD patterns (A) and FTIR spectra (B) of PVDF membranes [33]. described in Figure 3 [9,12,[34][35][36][37][38]. The particular fabrication technique can affect the crystallinity and phase of ferroelectric polymers, influencing their electroactive and mechanical properties. This section covers conventional and recently developed fabrication techniques involving ferroelectric polymers in greater detail.

Solution casting
Usually, PVDF and its copolymer-based nanogenerators are prepared by solution casting because it is a simple and low-cost process and allows large-scale production. The casting method is also suitable for preparing composites of fillers and a polymer matrix because it is relatively insensitive to the viscosity and density of solution. As a result, solution casting has been used in most research on compositebased nanogenerators [16,18,22,23,39]. The thickness of the film can be controlled by the type of organic solvents employed and the concentration and amount of the ferroelectric polymer solution. Since evaporation speed can differ depending on the organic solvents used, the morphology of the ferroelectric polymer films can also be affected [40].

Spin coating
Spin coating is a universally used technique to fabricate uniform thin polymer films using the centrifugal forces induced by a spinning substrate. The thickness of the films can be controlled by a few parameters, such as solution concentration, viscosity, spin speed, and spin time. Generally, a low concentration polymer solution (<10%) is used for uniformity. In addition, the thickness of spin-coated ferroelectric polymer films can be controlled by the number of successive spin coatings.
Spin coating has been successfully used to prepare nanogenerators based on thin P(VDF-TrFE) films. For example, Z. Pi et al. reported a flexible nanogenerator based on P(VDF-TrFE) thin film produced by spin coating. A 2% concentration of P(VDF-TrFE) copolymer solution was repeatedly spin coated on the substrate to reach the required thickness [12]. In another study, Lee et al. successfully developed a nanogenerator based on micropatterned P(VDF-TrFE) polymer (thickness ≈ 7 μm) [41].  They evaluated the improved stretchability, mechanical durability, and robustness of the line-based micropatterned nanogenerator, compared to a flat (nonpatterned) nanogenerator.

Template-assisted method
Nanoconfinement involves confining dimensional geometries in a nanosized region. Typically, the template-assisted method from a melt or solution has been employed to fabricate one-dimensional nanostructures [8,9]. In the templateassisted method, a polymer melt or solution is used to wet the surface of a porous template. It permeates into the pore of the template due to the high surface energy of the template walls. Finally, polymer nanotubes or nanowires form with a uniform size distribution. This technique has been reported to improve the performance of polymer-based nanogenerators. For example, V. Bhavanasi et al. prepared P(VDF-TrFE) nanotubes using a template-assisted method, with a P(VDF-TrFE) melt and anodic aluminum oxide (AAO) templates with a pore diameter of 200 nm [42]. The resulting P(VDF-TrFE) nanotubes demonstrated high power generation under dynamic compression pressure compared to that of P(VDF-TrFE) films. This improved performance was attributed to the enhanced piezoelectric coefficients, strain confinement in the 1D geometry, and improved surface area.
The nanoconfinement via templates induces ferroelectric polymer nanowires to retain preferential orientation with the b-axis parallel to the long axis of the template. This orientation leads to a vertical direction of the polarization parallel to the channel axis. Y. Calahorra et al. investigated the electromechanical interactions in individual P(VDF-TrFE) nanowires via piezoresponse force microscopy (PFM) [43]. Their results indicate that P(VDF-TrFE) nanowires grown via templateassisted methods possess piezoelectric properties without electrical poling process, which is called as "self-poled." However, the d 33 coefficient of nanowires without poling was lower than that of nanowires with poling. After tip-induced electrical poling, the d 33 of the nanowires increased from 7.7 pm/V to 21.6 pm/V. This phenomenon can be attributed to the direction of polarization parallel to the substrate before electrical poling.
When a template-assisted method is used, in-situ poling can be applied at the same time. This is known as the template-assisted electricity-grown method. X. Chen et al. fabricated vertically well-aligned P(VDF-TrFE) nanowire arrays with a preferential polarization orientation [44]. The contents of the β-phase in P(VDF-TrFE) nanowires and bulk films were analyzed via Fourier transform infrared (FTIR) measurement. The percent content of β-phase in the P(VDF-TrFE) nanowires increased to 83.5%. This value was higher than the bulk films (75.2%). Without further poling, the P(VDF-TrFE) nanowire array delivered an output voltage about 9 times higher than conventional spin-coated bulk films.

Electrospinning
Electrospinning is another common method used to fabricate β-phase ferroelectric fibers without further mechanical stretching or electrical poling processes [45]. The electrospinning technique utilizes an electrical force to obtain fibers. The basic principle of conventional electrospinning is as follows: When a DC electric field is applied between the spinneret (metallic needle) and the collector (grounded conductor), a conical object called a Taylor cone forms at the tip of the needle. When the electric field rises above a threshold value, the resulting electrostatic force can overcome the surface tension and viscous force of the polymer solution. An electrified liquid jet ejected from the nozzle undergoes a stretching and whipping process and then splits into threads with diameters ranging from hundreds of micrometers to tens of nanometers.
In 2011, D. Mandal et al. prepared P(VDF-TrFE) nanofiber webs via electrospinning [37]. They found evidence of the preferential orientation of CF 2 dipoles in the P(VDF-TrFE) nanofiber web via analyses using polarized FTIR and piezoelectric signal. They also demonstrated that output current signal could be enhanced by stacking several tens of layers of the electrospun web with the same polarization.
Unlike conventional electrospinning, near-field electrospinning using a short needle-to-collector distance enables superior location control, to produce orderly nanofiber patterns over large areas [46]. Notably, C. Chang et al. utilized a directwrite technique by means of near-field electrospinning (NFES) to produce and place piezoelectric PVDF nanofibers on working substrates [47]. They measured the piezoelectric response of single PVDF nanofibers to calculate their energy conversion efficiency (ECE). The ECE was defined as where κ is the electromechanical coupling coefficient of the piezoelectric material. The electric energy W e was estimated by integrating the product of the output voltage and current of the PVDF nanogenerator when stretched. The elastic strain energy was estimated using where E is the Young's modulus of the material, A is the cross-sectional area, ε is the strain applied to the material, and L 0 is the length of the material. It was found that the energy conversion of the nanofiber-based nanogenerator (average 12.5%) was higher than experimental PVDF films (0.5-4%) and conventional PVDF thin films (0.5-2.6%).

Thermal drawing
Another interesting method to make ferroelectric polymer fibers is thermal drawing, as reported by S. Egusa et al [38]. The advantage of thermal drawing is that all components (piezoelectric layer, electrode layers, and wrapping layer) are extracted a preform made of poly(vinylidene fluoride) with metal electrodes and an insulating polymer, making the process simpler compared to other methods. However, they encountered some challenges. During thermal drawing, there is a susceptibility to break up and mixing due to flow instabilities and variation in layer thickness. To address such challenges, they used carbon-loaded poly(carbonate) (CPC) and P(VDF-TrFE). CPC has high viscosity (10 5 -10 6 Pa s) at the draw temperature and thus can be used to confine the low-viscosity crystalline piezoelectric layer and ensure adequate conductivity (1-10 4 Ω m). In the case of P(VDF-TrFE), the β-phase can form without any mechanical stress compared to PVDF. Therefore, they were able to make multimaterial piezoelectric fibers with a uniform diameter using these two materials. They also demonstrated the benefits of drawing ferroelectric polymer fibers for application in acoustic transducers and capacitors [38,48]. However, they did not mention the scavenging ability of P(VDF-TrFE) fiber prepared by thermal drawing.
Due to flexibility of the thermal drawing, it has been successfully demonstrated for fabricating composite fibers such as BaTiO 3 -PVDF, Pb(Zr,Ti)O 3 (PZT)-PVDF, and carbon nanotube (CNT)-PVDF [49]. In that study, the piezoelectric performance of the fabricated fibers was systematically compared while bending and releasing. For a BaTiO 3 -PVDF fiber, the generated open-circuit voltage and shortcircuit current were 1.4 V and 0.8 nA, respectively. For a PZT-PVDF fiber, the corresponding values were 6 V and 4 nA. A CNT-PVDF fiber generated 3 V and 1.2 nA. These results were attributed to the high piezoelectric coefficient (BaTiO 3 and PZT) or the induced β-crystallization (CNT).

Dip coating
Dip coating is one of the most effective methods of depositing ferroelectric polymers on a 3D substrate. For example, D. Kim et al. developed a three-dimensional spring-type nanogenerator using a dip-coating and multidirectional electrode deposition [35]. The nanogenerator had a bi-layered structure with a top electrode and a P(VDF-TrFE) layer on a conventional spring that had two roles. It served as both the core electrode and the mechanical substrate for the ferroelectric polymer. In another study, J. Ryu et al. fabricated a P(VDF-TrFE)/polydimethylsiloxane (PDMS) stretchable hollow fiber using dip coating [36]. To fabricate the fiber, a cylindrical carbon rod was used as a template, onto which each of the layers was sequentially coated. The diameters of the P(VDF-TrFE)/PDMS layer and PDMS layer were controlled by varying the withdrawal speed during dip coating.

Nanogenerators based on pure PVDF films
The typical type of nanogenerator is based on ferroelectric polymer films without any further processing or incorporation of fillers. Conventional PVDF film-based nanogenerators can be simply fabricated and easily stacked to improve performance. In addition, the flexible nanogenerators can be designed using various structures. For example, J. Zhao and Z. You fabricated a nanogenerator based on specially designed sandwich structure that was compatible with a shoe [50]. The structure was composed of multilayered PVDF films and two wavy surfaces of a movable upper plate and a lower plate. The PVDF film layers were connected in parallel for high output current. Due to the structural design and stacked PVDF films, the nanogenerator provided an average output power of 1 mW while walking at a frequency of ~1 Hz. In another study, W.-S. Jung et al. prepared a curved piezoelectric nanogenerator consisting of multilayered PVDF films and a polyimide substrate [13]. They also connected all PVDF films in parallel to increase the output current. The curved structure enhanced the output power by distributing effectively applied stress. The authors integrated the nanogenerator into a shoeinsole and to a watch strap and attached them to long-sleeved shirts, and then evaluated the PVDF-based nanogenerators as a power source for wearable sensors and biomedical devices.

Nanogenerators based on composites
In order to enhance the electrical and mechanical properties of ferroelectric materials, many research groups have employed the concept of composites, typically made of a polymer and a ceramic. Composite is a common approach in piezoelectric-based applications. Ultrasonic transducer is a typical example. The composites are used for impedance matching. Since piezoelectric ceramics like PZT, BaTiO 3 , and Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) have a relatively high acoustic impedance compared to water or human tissue, impedance matching is required. Such matching results in broad bandwidth and increased sensitivity.
Composites can also be utilized for nanogenerators based on PVDF and its copolymers to boost their performance. The outputs of nanogenerators depend on a variety of factors, which include the piezoelectric coefficient, elastic modulus, and dielectric constant. The output voltage generated by nanogenerators can be expressed as where ε (l) is the strain, E is the elasticity modulus, and g 33 is the piezoelectric voltage constant, given by g 33 = d 33 _ ε 0 ε r with d 33 being the piezoelectric coefficient, ε r being the dielectric constant, and ε 0 being the permittivity of free space [15]. Hence, a higher piezoelectric coefficient and elastic modulus, and a lower dielectric constant result in higher nanogenerator performance. In this section, we will discuss the effect of fillers on composite-based nanogenerators. By varying the concentration, size, and morphology of fillers, the factors of composites can be controlled.

Piezoelectric coefficient
A piezoelectric coefficient is the most critical factor affecting the performance of nanogenerators. The piezoelectric coefficient (d 33 ) of bulk PVDF and its copolymers is remarkably low compared to that of bulk ceramics such as PZT (700 pC/N) [51], BaTiO 3 (350 pC/N) [52], and Pb(Zn 1/3 Nb 2/3 )O 3 -PbTiO 3 (PZN-PT) (2500 pC/N) [53]. As already mentioned in Section 2, the piezoelectric coefficient of PVDF and its copolymers is related to their crystallinity. In other words, the performance of nanogenerators can be improved by increasing the crystallinity of the ferroelectrics.
Recent studies have revealed that the crystallinity of ferroelectric polymers can change with fillers including organics, inorganics, and metals [16,23,39,54]. The amount of β-phase in ferroelectric polymers can be enhanced by imbedding fillers into the ferroelectric polymer. For example, B. Dutta et al. reported a piezoelectric nanogenerator based on a NiO@SiO 2 /PVDF nanocomposite [23]. To prevent agglomeration, they coated nonconductive SiO 2 on the NiO nanoparticles, which resulted in the homogeneous dispersion of the nanofillers in the polymer matrix. FTIR results indicated that the β-phase in the PVDF increased with the increasing loading fraction of NiO@SiO 2 as shown in Figure 4a. This phenomenon occurred due to the electrostatic interaction between the negatively charged surface of the NiO@SiO 2 nanoparticles and the positive -CH 2 groups of PVDF.
In addition to considering the content of fillers, the geometric structures of fillers should be also considered for their effect on the extent of β-phase crystallization and their influence on piezoelectric performance in the composites. A recent example was demonstrated in Ag/PVDF composites, where the effect of mixing silver fillers with different morphologies into P(VDF-TrFE) was investigated [54]. When Ag nanoparticles were added to P(VDF-TrFE), the crystalline transformation of P(VDF-TrFE) was not observed (Figure 4b). Similarly, work conducted by H. Paik et al. also found that the effect of adding Ag nanoparticles has an insignificant effect on the internal structures of the film [55]. On the other hand, when Ag nanowires with a diameter of 120 nm were mixed with P(VDF-TrFE), some α-phase transformed into β-phase, which indicates that the piezoelectric response can be enhanced as shown in Figure 4c. This was attributed to the fact that the molecular structure of P(VDF-TrFE) can be easily packed in a form that favors the formation of β-phase due to the planar surface of the Ag nanowires.
Interestingly, many research groups have used ferroelectric materials as fillers to enhance the performance of their nanogenerators. Even though ferroelectric fillers Ferroelectric Polymer PVDF-Based Nanogenerator DOI: http://dx.doi.org /10.5772/intechopen.90368 such as particles, nanowires, and nanorods have a poor piezoelectric coefficients, compared to that of bulk ceramics, they are still good candidates for piezoelectric nanogenerators. For example, PMN-PT nanofiber has a piezoelectric coefficient (d 33 ) of 50 pC/N [56].
However, PVDF and its copolymers have a negative piezoelectric coefficient, as previously mentioned. If PVDF and its copolymers are used as matrices, the conflict of piezoelectric constants can limit its advantages and result in lower output. Previous reports indicated that PZT/P(VDF-TrFE) 0-3 composites with 50 volume percent ceramic did not exhibit a piezoelectric response because they had the opposite sign piezoelectric coefficient, thus canceled out [57]. To circumvent this effect, different poling process can be adapted. For example, C. K. Jeong et al. demonstrated an enhanced nanogenerator based on BaTiO 3 nanowires and P(VDF-TrFE), which was poled using double-side poling process to remove the piezoelectric discrepancy between BaTiO 3 and P(VDF-TrFE) [21]. These examples further highlight why the mechanism behind the combination of piezoelectric nanofillers and piezoelectric polymers should be comprehensively explored.

Elastic modulus
It is also important to consider elastic modulus, the resistance to elastic deformation under load, according to the equation (Eq. (6)). A recent publication showed that Young's modulus plays an important role in a piezoelectric nanogenerator [58]. In that work, H. S. Kim et al. used paraelectric BaTiO 3 nanoparticles as fillers to exclude the effect from the piezoelectricity of the fillers. The Young's modulus increased from 2.17 to 3.03 GPa when the BTO content increased from 0 to 10 wt% and then decreased to 2.15 GPa as the content further increased to 30 wt% (Figure 4d). They found that the BTO content-output relationship was directly related to Young's modulus. Clearly, the optimal content for achieving the maximum Young's modulus, which corresponds to the highest piezoelectric output, should be identified.

Dielectric constant
Dielectric constants are an important parameter that also can affect the performance of nanogenerators. In composites, the dielectric constant can change with the volume of fillers. In a BaTiO 3 -PVDF composite film, the dielectric constant increased from 8 to 31.8 at a frequency of 10 4 kHz when the volume fraction of fillers was increased from 0-30% [59]. Furthermore, the size of the nanoparticles can modify the dielectric properties. When the BaTiO3 filler size was increased from 10.5 to 34.6 nm, the dielectric constant at a frequency of 10 4 kHz increased from 20.1 to 31. In another study, P. Kim et al. systematically studied the dielectric constant and dielectric breakdown strength of a composite comprising P(VDF-HFP) and phosphoric acid surface-modified BaTiO 3 nanoparticles [60]. The dielectric constant increased as the nanoparticle volume fraction was increased from 0-60% and then decreased as the nanoparticles volume fraction increased above 60% (Figure 4e). The drop in dielectric constant at high volume fractions was attributed to the effect of the porosity of the composites. The dielectric breakdown strength decreased when the nanoparticle volume fraction increased, due to percolation of the BaTiO 3 nanoparticles and air voids (Figure 4f).

Energy sources
In addition to mechanical deformation, such as uniaxial compression and bending, energy sources such as air flow, sound, and thermal fluctuation are also available. A typical example of ambient natural sources is wind. Ferroelectric polymer-based nanogenerators can easily yield electricity from wind energy because of their considerable flexibility. For example, Li et al. fabricated nanogenerators based on P(VDF-TrFE) films to harvest mechanical energy from low speed wind, which is very close to winds commonly observed in the natural environment [34]. The experimental setups are illustrated in Figure 5a. They evaluated the wind energy extraction capability by comparing three instinct operating modes. Each operating modes exhibited different piezoelectric energy harvesting behaviors, including mode frequency, output voltage, and power. The authors concluded that a nanogenerator operating where the wind is incident to the side of the film delivered high output power compared to other modes within the wind speed range of 1.7 and 4.7 m/s (Figure 5b). These results may be attributed to the huge deflection that occurred over the entire area of the nanogenerator.
Besides wind, sound is also an abundant energy source that can be found in the environment. PVDF films can be used as an active layer for harvesting sound waves. A variety of polymer-based nanogenerators have been developed to scavenge energy from sonic inputs. For example, S. Cha et al. presented nanoporous arrays of PVDF fabricated by a lithography-free, template-assisted preparation method (Figure 5c) [61]. They demonstrated that the porous PVDF nanogenerators generated an output voltage and current of 2.6 V and 0.6 μA, respectively. These output values were 5.2 times and 6 times higher than bulk PVDF film nanogenerators, respectively (Figure 5d). The piezoelectric potential was enhanced in the nanoporous structure as a result of the geometrical strain confinement effect.
More recently, nanoweb-type nanogenerators were developed to convert sound wave into electrical energy [62,63]. The benefit of using nanowebs in nanogenerators compared to films is their high power generation because they are more flexible and easier to vibrate under the same acoustic waves. DOI: http://dx.doi.org /10.5772/intechopen.90368 Pyroelectricity is a property of certain materials, which develop an electric field across the polar axis when there is a temperature change. According to the pyroelectric theory [64], the pyroelectric current I is expressed as where Q , S , λ , and T are the induced charge, the electrode surface of the material sample, the pyroelectric coefficient, and the temperature, respectively. Ferroelectric ceramics like PMN-PT and PZT have high pyroelectric coefficients above 550 μCm −2 K −1 compared to those of ferroelectric polymers (33μCm −2 K −1 ). As a result, a PZT-based nanogenerator delivered more current density than that of ferroelectric polymers. However, PVDF and its copolymers have also been employed to harvest thermal energy from temperature fluctuations, where they provide the benefits of light weight, low-cost, lead-free, flexible, and transparent  properties [41,65,66]. A number of research groups have focused on improving the performance of such nanogenerators by changing their designed conditions. For example, D. Zabek et al. reported a simple modification of the electrode design in a PVDF-based nanogenerator [67]. The partially covered electrode was developed by photo-lithography and a wet etching process. The modification of the electrode design enabled a significant increase in pyroelectric voltage (380%), current (480%), and pyroelectric energy harvesting transformation (1080%).Their findings showed that the electrode area coverage could be optimized by balancing the ratio of the area of aluminum electrode required to get the free charge to the area of exposed PVDF to improve heat transfer. In another study, they also demonstrated that interconnected graphene nanoplates (GNPs) can be used as an electrode for high thermal radiation absorbance and high electrical conductivity as shown in Figure 5e [68]. The GNPs were easily deposited by a screen print technique. The graphene ink/PVDF/Al system showed better performance than a system with conventional aluminum electrodes (Figure 5f).

Biocompatibility
Lead (Pb) is considered one of the core materials in modern society because it is inexpensive and has high density and resistance to corrosion. However, it is also toxic and can harm multiple human body systems. Legislation has been adopted in many countries to prevent and reduce the use of Pb in many communities [69]. Currently, many of the materials with high piezoelectric performance are leadbased materials, that is, PbTiO 3 [69]. Nevertheless, it is urgent to develop lead-free piezoelectrics that can perform as well as or better than PZT.
From the biocompatibility point of view, one of the most promising alternatives is a ferroelectric polymer. The unique advantage of PVDF and its copolymers lies in their good stability, similar to PTFE. They are chemically inert and resistant to sunlight [65,73,74]. There have been reports on the biocompatibility of PVDFbased nanogenerators. For example, Y. Yu et al. demonstrated biomechanical energy harvesting in a living organism using PVDF-based nanogenerator, where a DOI: http://dx.doi.org/10.5772/intechopen.90368 mesoporous PVDF film was encapsulated by polydimethylsiloxane (PDMS) [75]. They inserted the nanogenerator under the skin of a mice's leg and observed that there were no signs of incompatibility induced by the NG. In addition, an output voltage of 200 mV was stably generated from the gentle motions of the rat's leg for 5 days as shown in Figure 6a. In another work, J. Li et al. studied PDMS and PDMS/ Parylene-C packaged PVDF nanogenerators implanted inside a female ICR mouse for up to 6 months [76]. They observed no signs of toxicity or incompatibility via pathological analyses and blood and serum test. In addition, there was no issue with reduction in electrical outputs over the entire implantation period (Figure 6b). Therefore, it is conceivable that PVDF-based nanogenerators can be used in a biological environment.

Conclusions
In this chapter, we summarized recent studies of ferroelectric polymer PVDFbased nanogenerators. PVDF and its copolymers are attractive materials for nanogenerator applications. The materials are flexible, transparent, chemically stable, easy to process, and biocompatible. Because of these advantages, PVDFbased nanogenerators can be placed anywhere, including bones, human skins, and wearable devices that usually have curved surfaces. In addition, they can harvest electricity from a variety of energy sources.
Considerable effort has been expended by numerous research groups to enhance the performance of PVDF-based nanogenerators using various fabrication methods, designing device structures, and incorporating fillers. However, studies on PVDF-based nanogenerators are fairly limited. The mechanisms leading to the enhanced performance after the incorporation of fillers should be comprehensively surveyed. Especially, in the case of ferroelectric fillers, a careful approach to the enhancement should be taken, since a piezoelectric effect also occurs with fillers. To optimize nanogenerator performance, circuits for nanogenerators should be developed. Their performance depends on the circuits and components, including load resistors, capacitors, and wiring. In order to realize the commercialization of PVDF-based nanogenerators, packaging of the devices is critical to prevent mechanical fatigue. The selection of substrates and electrode materials can also improve the mechanical properties of these devices. Therefore, the relationship between a ferroelectric polymer layer and other layers should be considered.
We hope that this chapter will help readers to better understand principles of PVDF-based nanogenerators. Furthermore, several new approaches that have been in this chapter can be adopted for other applications, such as sensors, actuators, and field-effect transistors.