Soha Mohammadpourfazeli†
a,
Shabnam Arash†b,
Afshin Ansari†c,
Shengyuan Yangd,
Kaushik Mallicke and
Roohollah Bagherzadeh†*ad
aAdvanced Fibrous Materials LAB, Institute for Advanced Textile Materials and Technologies (ATMT), Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran. E-mail: bagherzadeh_r@aut.ac.ir
bDepartment of Biomechanics, University of Nebraska Omaha, USA
cMaterial Engineering Department, Imam Khomeini International University (IKIU), Iran
dState Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, P.R. China
eDepartment of Chemical Sciences, University of Johannesburg, Auckland Park, South Africa
First published on 9th January 2023
Polyvinylidene fluoride (PVDF) is a favorite polymer with excellent piezoelectric properties due to its mechanical and thermal stability. This article provides an overview of recent developments in the modification of PVDF fibrous structures and prospects for its application with a major focus on energy harvesting devices, sensors and actuator materials, and other types of biomedical engineering and devices. Many sources of energy harvesting are available in the environment, including waste-heated mechanical, wind, and solar energy. While each of these sources can be impactively used to power remote sensors, the structural and biological communities have emphasized scavenging mechanical energy by functional materials, which exhibit piezoelectricity. Piezoelectric materials have received a lot of attention in past decades. Piezoelectric nanogenerators can effectively convert mechanical energy into electrical energy suitable for low-powered electronic devices. Among piezoelectric materials, PVDF and its copolymers have been extensively studied in a diverse range of applications dealing with recent improvements in flexibility, long-term stability, ease of processing, biocompatibility, and piezoelectric generators based on PVDF polymers. This article reviews recent developments in the field of piezoelectricity in PVDF structure, fabrication, and applications, and presents the current state of power harvesting to create completely self-powered devices. In particular, we focus on original approaches and engineering tools to design construction parameters and fabrication techniques in electro-mechanical applications of PVDF.
Both natural and artificial materials exhibit a range of piezoelectric effects.2 Although piezoceramics are one of the most efficient piezoelectric materials, a polymer exhibiting transducer characteristics has remarkable advantages over ceramics because of the less brittle nature and greater flexibility of polymers.3
Various ceramics, such as PZT,4 ZnO, BaTiO3, Na/KNbO3, and polymers like (PVDF) and its copolymers with hexafluoropropylene [P(VDF-HFP)] and tetrafluoroethylene [P(VDF-TrFE)], and poly(vinyl acetate) (PVAc) are used as piezoelectric materials. The drawbacks of these materials are brittleness, toxicity, non-biodegradability/non-biocompatibility and complicated fabrication processes. However, polymers show advantages, such as toughness, non-toxicity, and biocompatibility, making them suitable materials for applications requiring high bending and twisting and for biomedical devices.5 Piezoelectric polymers can be classified as amorphous, crystalline, or semicrystalline structures. Semicrystalline piezopolymers include polyvinylidene fluoride (PVDF).6 Crystallinity is based on the polymer's molecular structure and is determined by the ratio of semicrystalline to amorphous regions, which changes with fabrication methods and thermal history. Molecular dipoles are primarily responsible for the piezoelectric effect. Randomly oriented dipoles get arranged with thermal annealing and high voltage, and stretching the polymer speeds up the process. The electric voltage in semicrystalline polymers enhances the piezoelectricity, while in amorphous materials, poling near Tg is needed to align the dipoles and freeze them. The piezoelectric coefficient of amorphous polymers increases 4–5 times by poling.7 After PVDF-TrFE, in comparison with other piezopolymers, PVDF has demonstrated the highest dielectric constant and a high piezoelectric coefficient, as shown in Table 1.8
Polymer | Dielectric constant (1 kHz; 25 °c) | Piezoelectric coefficient (pC/N) |
---|---|---|
PLA | 3.0–4.0 | 9.82 |
Polyhydroxy butyrate PHB | 2.0–3.5 | 1.6–2.0 |
PVDF | 6–12 | 24–34 |
Poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) | 18 | 38 |
Polamide-11 | 5 | 4 |
As mentioned above, PVDF has a piezoelectrically active part that can be in the form of nanostructures, wires, fibers, ribbons, tubes, etc.,9 and has excellent mechanical strength, flexibility, chemical resistance, ruggedness, high hydrophobicity, biocompatibility, and thermal stability, compared to other commercialized polymeric materials. PVDF is used in energy harvesters, battery separators, sensors, and biomedical applications.10,11 There are no engineering fields in which piezoelectric materials have not found applications.12 Engineers are investigating enhancing the polarity of PVDF materials and further improving their mechanical–electrical conversion efficiency to increase the piezoelectric coefficient.1 However, improving the polarization characteristics of flexible piezoelectric materials has been challenging. In this respect, it has faced unsolved problems concerning lead perovskite based piezoelectrics, and a lot of attention has been paid to obtaining a self-powered generator and supercapacitor by the development of flexible lead-free perovskite devices.13
Bhavya et al.14 published a novel strategy to develop a self-charging supercapacitor using lead-free perovskite as a piezoelectric and polyvinyl alcohol–potassium (PVA–KOH) film. The fabricated supercapacitor can quickly self-charge to 200 mV by simple thumb impact, 155 mV by bending it to 150°, 160 mV by twisting it, and a high value of 266 mV by applying a compression of 20 N.
The piezoelectric coefficient of PVDF can be impressively amended by doping with inorganic piezoelectric materials, such as piezoelectric ceramics like PZT. In addition, Ag, BTO, nanoclay, graphene, and other nanomaterials can change the crystal structure of PVDF and increase the β-phase percentage. Researchers have utilized different ways to access the optimizes fraction of β-phase. In addition, some new fabrication methods can also amend the polarity of flexible piezoelectric materials. Techniques undertaken to achieve a self-polarized β-phase in PVDF include spin coating, phase-inversion techniques, and the Langmuir–Schaefer (LS) method, none of which need electrical poling.9 The trend towards smaller flexible electromechanical devices based on PVDF is predicted for the future, such as using the help of piezoelectric properties of biocrystals, to improve biocompatibility for the development of bionic devices.1
Fig. 1 The structure of PVDF: (a) structure of the all-trans conformations showing the planar carbon backbone with attached fluorine and hydrogen atoms; (b) the crystal structure of the β-phase.17 |
A wide of research has been done on the different crystal phases of PVDF, and α- and β-phases are of great interest due to their unique properties. The α-phase has a molecular chain combination of trans–gauche (TGTG), placing the F and H atoms alternately on each side of the chain, and is also the most thermodynamically stable phase, while the β-phase shows the highest dipolar moment per unit cell with the highest piezoelectric coefficient.10,18 The crystalline structures of these forms are shown in Fig. 2.19
Fig. 2 Crystalline structure of β, α, αP, and γ phases of PVDF projected onto the a–b plane of the unit cell. Large circles are fluorine atoms, small circles means dot carbon atoms, and hydrogen atoms are omitted.19 |
Fig. 3 shows multiple crystalline polymorphs, namely α, β, γ, δ, and ε related to PVDF.20 The crystallization of PVDF can be controlled by molecular weight, molecular weight distribution, polymerization method, cooling rate, and thermal history, and the crystallinity can range from 35% to 70%.21 Moreover, another factor affecting the morphology and crystal structure of the PVDF membrane is the coagulation medium. However, the crystallinity of all membranes remained intact, and all membranes exhibited a crystal-type structure. PVDF films are mostly biaxially or uniaxially rolled or stretched to ameliorate their electrical and mechanical properties or to effect conversion from one form to another. For instance, stretching α-samples will generate β-material since the molecular chains of form β are more extended than those of form α.11,20
Fig. 3 Chemical structure of the three main phases of PVDF.20 |
Wang et al.22 detected a similar outcome of coagulation bath temperature on the crystallinity structure. Better crystallinity is exhibited in a membrane fabricated from a 15 °C bath temperature than a 60 °C coagulation bath. They also reported that a membrane precipitated at 60 °C only appeared with a common crystal structure, while at the lowest temperature (15 °C), the PVDF crystallites consisted of a mixture of crystal forms. The coagulation medium is another factor besides precipitation temperature that affected the crystal structure and morphology of the PVDF membrane.
Lin et al.,20 intriguingly, noticed the formation of larger spherulites in membrane morphology by dissolving PVDF in DMF at disparate temperatures and immersing the cast membranes in a 1-octanol coagulation bath. However, the crystallinity of all membranes remained and all membranes exhibited an unaffected type of crystal structure. On the other hand, Wang et al.23 found astounding changes in the morphology of PVDF membranes prepared at several temperatures in N,N-dimethylacetamide (DMAc). They observed that the cross-sections of all membranes were composed of an interconnected structure with cavities, a type of crystal structure. However, by increasing the dissolving temperature, the size of the holes becomes more extensive.
Chen et al.23 discovered that 1-octanol in both the casting solution and coagulation bath formed a PVDF membrane influenced by the crystallization process. The particulate size decreases when the crystallization proceeds liquid–liquid demixing, with increasing 1-octanol content in doping solution.
Gregorio used Fourier transform infrared (FTIR) and X-ray diffraction (XRD) spectroscopy to determine the three crystalline phases formed in films prepared under different conditions and found a relationship between crystallization and membrane properties, such as morphology and mechanical strength.20 Some crystallographic data for the polymorphs of PVDF are listed in Table 2.
The unit cell (Å) | The unit (g cm−3) | Space group | Structure | Molecular chain | Electroactive properties | |
---|---|---|---|---|---|---|
α-phase | a = 4.96 | 1.92 | P21/c | Monoclinic | TGTG′ | Low polarity |
b = 6.94 | ||||||
c = 4.62 | ||||||
β-phase | a = 8.58 | 1.97 | Cm2m | Orthorhombic | TTTT | Largest polarity |
b = 4.91 | ||||||
c = 2.56 | ||||||
γ-phase | a = 4.96 | 1.92 | P21/cn | Orthorhombic | T3GT3G′ | Medium polarity |
b = 6.94 | ||||||
c = 4.62 |
Fig. 4 Schematic diagram of the mechanism of dehydrochlorination in PVDF: (a) formation of double bonds in the chain and (b) cross-linking of the polymer.16 |
D = dT+ εE | (1) |
X = sT + dE | (2) |
The piezoelectric constants of PVDF depend on the stretch ratio, temperature, crystallinity, and poling conditions.29 A piezoelectric matrix can characterize the piezoelectric properties of PVDF; the first matrix is for the film poled without stretching and the second is for the film after uniaxial stretching.
If the material is poled after uniaxial stretching:
The d-constant is the one most frequently used. The only non-zero components of the d-matrix are d31, d32, d33, d24. The first subscript defines the direction of displacement or the electric field, during the second, the direction of stress or strain.
The two principle modes of operation for energy harvesters are d31 and d33, which are based on the directions of polarization and stress (Fig. 5).30 The d31 coefficient is a transverse coefficient, which defines the electric polarization created toward the direction perpendicular to the applied stress direction. While the d33 coefficient is the longitudinal coefficient, which depicts the electric polarization produced in the same direction as the applied stress. The electrodes were arranged vertical to the poling direction in the d31 and d33 modes, which means the electric field is aligned with the polarization.20
Fig. 5 The piezoelectric d33 and d31 modes.20 |
Measuring piezoelectric constants in PVDF is very common, detecting the resulting voltage or current on its surfaces by applying a sinusoidal elongational force along with the polymeric film. Another method includes measures of strain or stress induced by a sinusoidal electric field or piezoelectric measurements at the resonant frequencies of electrically driven PVDF measures of strain or stress induced by a sinusoidal electric field. Static techniques for piezoelectric data generation involve applying hydrostatic pressure to PVDF films from an inert gas or employing an electric field, and measuring the resulting change in specimen thickness.19
Compared to other crystalline phases, the β-phase shows a higher dipole moment with higher electroactive (piezo, pyro, and ferroelectric) properties. Hence, improving the β-phase content in PVDF polymers with different methods is widely discussed.31 Some essential techniques are used to enhance the β-phase content in the polymer, which are different approaches to enhancing piezoelectricity in PVDF:
(3) |
The heating of the specimen may be effected conductively, dielectrically, or by the use of frequency-modulated lasers. Generally, the dependence of pyroelectricity on experimental parameters is the same as described for piezoelectricity.33 The variation of the pyroelectric constant with poling field and a comparison of such constants for PVDF was shown by Kenney and Roth. Pyroelectric coefficients have been studied down to low temperatures (6 K) by Glass and Negran, and as a function of pressure by Scheinbeim et al.35 Pyroelectricity in PVDF remains stable for years and is fully reversible with temperature change.34,35
(4) |
This equation is used to quantify the FTIR results. In fact, a formula is used to measure the transformation efficiency of the gauche state to the trans-state in PVDF samples. Here Xα and Xβ are the crystalline mass fractions of α- and β-phases and Aα and Aβ correspond to absorption bands at 763 and 840 cm−1, respectively. Table 3 shows that with an increase in deformation of the polymer chain in the β-phase with applied stress, the piezoelectric coefficient (d33) was enhanced, and the highest piezoelectricity for particular poling conditions is attained when the mechanical deformation provides a polymer with the highest polar β-phase.4
Stretching ratio | F(β) (%) | d33 (pC/N) |
---|---|---|
1 | 43 | 0 |
2 | 60.2 | 28 |
3 | 65.1 | 29 |
4 | 73.5 | 30.5 |
5 | 83.6 | 31 |
5.5 | 84.5 | 31.5 |
6 | 85.35 | 32.5 |
6.5 | 86.5 | 33 |
Thus, obtaining and increasing the β-phase is essential to having high piezoelectric and ferroelectric properties in PVDF synthesis. The mutual transformation between the three different three crystalline phases of PVDF can be controlled using different fabrication methods. The routes for increasing the amount of β-phase are shown in Fig. 6.4 Increasing the fraction of F(β) is the most important item in fabrication methods. As Fig. 6 shows, electric poling is one of the most important ways to obtain the β-phase. Other ways that can improve β-phase are cold stretching with high ratio, high-energy irradiation and annealing at high pressure. Fig. 7 shows % β-PVDF transformation from α-PVDF by different corona poling fields.
Fig. 6 Mutual transformation routes and fabrication methods of different β-PVDF.37 |
Fig. 7 Evolution of the additional α → β phase transformation due to the different corona poling fields.38 |
Turning to some FTIR peaks of PVDF, the unshared peaks for the β-phase are close to 445, 473, and 1275 cm−1; spectrum peaks at 840 and 510 cm−1 are used to define β- and γ-phases. Moreover, the β- and γ-phases show very close peaks in the range of 1428–1432 cm−1. Therefore, the peaks at 1431 and 1429 cm−1 can operate as the defined bands of the β- and γ-phases, respectively.4 Fig. 8 shows the FTIR spectra of polymorphs of PVDF.
Fig. 8 FTIR spectra of different crystalline polymorphs of PVDF: (a) α-phase, (b) γ-phase, (c) β-phase.39 |
A change in the orientation of the unit cell axes should also be perceived in X-ray diffraction measurements. Fig. 9 illustrates the XRD spectra of the main crystal phases of PVDF. The peaks at 2θ = 17.9°, 18.4°, 20.2°, 27.9°, 36.1°, and 39.0° are assigned to crystal planes 110, 020, 021, 111, 200, and 002, respectively, corresponding to the α-phase crystal plane. The diffraction peaks of the β-crystal phase are defined at 20.7°, 36.6°, and 56.9°, attributed to planes 110, 200, 020, 101, and 221, respectively.4
Fig. 9 XRD patterns of various planes of different crystalline phases of PVDF.40 |
The 110 plane is the leading and most common plane in PVDF phases, and this plane might be a reasonable way to identify the β-crystal phase after making a mutual transformation. Fig. 10A illustrates an XRD analysis of the PVDF structure with a mechanical process that shows an extreme diffraction peak at 20.6, corresponding to β-PVDF that is attributed to 200 planes. At the same time, this peaks' diffraction at 20.2 and 19.9 correspond to the γ-phase and α-phase, respectively. Fig. 10B shows the same peak shifting in the prominent peak of PVDF with a chemical process to enhance the β-phase, and there are many more examples of this peak shifting, due to chemical, and mechanical treatment, among others.
Fig. 10 XRD patterns of the main peak of the crystalline phases of PVDF: (a) oriented,38 (b) composited.41 |
Conventional characterization techniques such as wide-angle X-ray diffraction (WAXD) have been employed to identify the stability of the β-phase after orientation. Fig. 11 shows (a) non-oriented PVDF and (b) oriented PVDF. The non-oriented crystal lattices all show Debye rings for planes corresponding to the α-phase. For example, the 110 plane, expressed as 110(α), has a complete ring in the edge, through, and end patterns. In contrast, there are no β-phase complete Debye rings in the non-oriented sample. The oriented crystal lattices show robust and complete Debye rings for 200 planes expressed as 200(β). 020, which is expressed as 020(β), is the other plane that shows all patterns, which implies an oriented structure with the β-phase. Thus WAXD could be a reasonable way to identify the β-phase.5
Fig. 11 2D WAXD along the edge, through, and end directions of PVDF:6 (a) non-oriented along the edge direction, (b) non-oriented along the through direction, (c) non-oriented along the end direction, (d) oriented along with the edge direction, (e) oriented along the end direction.42 |
Fig. 12 Images of 3D-printed PVDF-TrFE structures: (a) optical image of a layered PVDF-TrFE on top of a thin film, (b) cross-sectional HIM image of the structure, and (c) the top of the structure.44 |
Another method of 3D printing is fused deposition modeling (FDM). This method improves the diffusion of nanoparticles in polymer matrix nanocomposites and prevents nanoparticle agglomeration. Two step of filament extrusion and 3D printing are followed by thermal poling to enhance the piezoelectric response.
FTIR results have proved that a solvent-cast film has more β-phase content than 3D-printed films. Although the current output in the 3D-printed films was higher because of the uniform diffusion of nanoparticles and lack of porosities and cracks. All the above-mentioned properties in 3D-printed films are due to the filament extrusion process conducted in this method.45 The advantages of the 3D-printing technique include an improvement in piezoelectricity, freedom of design, and large-scale manufacturing of active biomaterials for actuators and sensors.
PVDF is inherently hydrophobic and, as a result, it shows low cell attachment and spread. Surface modification is needed to activate the surface of PVDF and make it hydrophilic. Some surface modifications on PVDF are grafting chemical vapor deposition and plasma treatment. Plasma treatment has advantages over other methods because it does not need extra specialized equipment, and it modifies the surface homogeneously. Electrospinning and plasma treatment have been applied on PVDF surfaces for bone tissue applications. A Cellosaurus cell line was cultured on PVDF samples and control glass. There were no meaningful variations in the viability test and the spreading area measures. Comparing the results with the drop-casting method (Fig. 13 and 14) demonstrates no significant change in cell viability or spreading area measurements.45
Fig. 13 SEM micrographs of (a, c and d) electrospun vs. (b, e and f) drop-cast PVDF scaffolds. (c) and (e) show the contact angle of the scaffolds before oxygen treatment while (d) and (f) show it after oxygen treatment.45 |
Fig. 14 SEM micrographs of the behavior of Cellosaurus cells on (a, c and e) hydrophobic and (b, d and f) hydrophilic PVDF scaffolds. (a and b) are related to drop-cast, (c and d) to electrospun, and (e and f) to SEM-FIB cross-sections of electrospun scaffolds.45 |
The transient intracellular calcium concentration indicates growing osteoblasts on PVDF scaffolds with a ratio of 44:46 on hydrophilic to hydrophobic scaffolds. However, in scaffolds made by the drop cast method, the transients ratio was 15:18 on hydrophilic to hydrophobic samples, and on the glass control (5%). It has been concluded that hydrophilicity does not significantly affect the number of active cells in any of the samples (Fig. 15). This result proves that plasma treatment does not affect the piezoelectric properties and the scaffolds produce a local electric field that initiates cell growth.
Fig. 15 The percentage of activated cells on hydrophilic/hydrophobic electrospun and drop-cast PVDF scaffolds.45 |
The results also indicated that in electrospun scaffolds, the β-phase is predominant, while in drop cast scaffolds, the γ-phase is the primary phase.
Fig. 16 Illustration of the process of (A) embossed and (B) singular PVDF microstructures.51 |
Fig. 17 SEM images of (A and B) singular and (C and D) embossed columns, (E) crisscrossed, and (F) parallel channels.51 |
Fig. 18 Summary of soft lithography fabrication steps for bump-shaped films.50 |
Fig. 19 Schematic of a new mold-transfer method to trim the PVDF-TrFE piezoelectric polymer film and a new dome/bump-shaped tactile sensor module for a smart microcatheter.50 |
Fig. 20 Scheme and operating principle of the dome/bump-shaped flexible PVDF-TrFE tactile sensors: (a) dome shaped, (b) bump shaped.50 |
In the micro-embossing process, polymer actuators have deflection limitations. They are usually made by sticking a piezoelectric stripe to a substrate and making a composite. The composite interface must be kept without deformation or aging. One way to design and fabricate a piezoelectric polymer composite is to use the micro-embossing process used for temperature-sensitive, flexible polymers. Generally, microactuators are shaped as a unimorph cantilever simulated to optimize design parameters for large deflections (Fig. 21). A research group has developed a PVDF-based cantilever with a metalized nickel–copper alloy electrode layer. The tip deflection is related to the thickness of both the piezo and non-piezo layers. This technique can be applied to a wide range of piezoelectric microactuators.
Fig. 21 PVDF composite cantilever.52 |
Another way to prepare a composite is the layer-by-layer method using symmetric electrodes of a metal-core piezoelectric fiber known as SMPF printed on a matrix to make a vibration sensor. Bian et al.53 developed a composite with PVDF as the piezoelectric layer and molybdenum wire as the metal core to mimic the structure of a hair (Fig. 22). The vibration sensor has a cantilever beam structure, and the SMPF can detect vibrational amplitude, the direction of the matrix, the harmonic excitation frequency, and the vibration direction. They melted the PVDF particles and used an extrusion system with Mo-wire in the middle. By adjusting the extrusion speed of PVDF and molybdenum, the phase conversion rate of PVDF is kept under control. When the extrusion speed of Mo-wire is higher than that of PVDF, the molten PVDF will experience mechanical stretching, thus transitioning from α- to β-phase.53
Fig. 22 (a) The structure of the SMPF. (b) The cross-section of the SMPF and polarization distribution.53 |
In nano-filled (nanohybrid) composites we are able to take advantage of conventional filler technology with the help of nanoparticles to achieve enhanced mechanical, thermal, and electrical properties in materials. In a study conducted by Gaur et al., two types of PVDF-based nanohybrids were prepared (Fig. 23). The first one was compromised of PVDF and an eggshell membrane, and the other was a system made of same ingredients with nanoclay. Nanohybrid films were covered by aluminum as a conducting layer, and the electrodes were placed on them. The structure was then encapsulated in PDMS to prevent damage. Eggshell alone does not change the PVDF structure. However, nanoclay and eggshell combinations transform the matrix into a piezoelectric phase. The eggshell improved the energy harvesting capability of the nanohybrid composite. The produced nanohybrid was proven to be fully biocompatible in terms of cell viability, attachment, and propagation, and it is likely to be used as a nanogenerator for medical self-regulating, monitoring, and diagnostics.5
Fig. 23 Schematics of nano-hybrid structural orientations under different conditions with the presence of ESM, nanoclay, and a combination of both.4 |
As mentioned in Table 4, (P(VDF-TrFE)) is a ferroelectric material with remarkable chemical stability, piezoelectric effect, and biocompatibility and its nanofiber mats because of their flexibility, light weight, and low cost as a functional material for flexible tactile sensors, have attracted increasing attention recently. It is feasible to obtain a better piezoelectric response than PVDF thin films, with highly stretchable structures, and large effective working areas. Furthermore, the β-phase of P(VDF-TrFE) indicates preferential crystallization and high piezoelectricity. Even if PVDF generally presents a higher degree of crystallinity, the TrFE content facilitates crystallization in the polymer chain, resulting in a higher content of the β-phase. The organization and spherulite size of polymers and copolymers on the surface morphology are divergent. Meantime, P(VDF-TrFE) flexible piezoelectric fibers achieved aligned arrangements, in which the polymer chains exhibited a strong preferential orientation and the piezoelectric performance was enhanced. Thus, electrospun P(VDF-TrFE) mats were used as the core piezoelectric layer in a tactile sensor and friction-functional layers were fabricated on flexible films, which worked under piezoelectric and triboelectric hybrid stimulation.18 Baozhang Liu et al.,64 by doping AgNWs into PVDF via electrospinning, prepared PVDF/Ag nanowire (AgNW) composite nanofibers. They observed that by adding AgNWs, the content of β-phase in PVDF increased, and the piezoelectricity of PVDF nanofibers was greatly enhanced due to the interactions between the AgNWs and the PVDF matrix, which forces the PVDF chains to form β-phases. The β-phase and sensitivity were found to be enhanced when augmenting the content of AgNW. The sensitivity of PVDF/AgNWs nanocomposite nanofibers used for pressure sensors can achieve 30 pC/N.
Composite | Improved properties | Applications | Reference |
---|---|---|---|
Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) | Piezoelectric effect, chemical stability, and biocompatibility | Micro energy development in flexible, wearable electronic devices and wireless sensor networks | 57 |
Poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) | Flexibility, chemical stability, and biocompatibility | Electrodes were screen-printed on a multi-layered P(VDF-TrFE) structure | 58 |
Type of ferroelectric material with significant piezoelectric effect | |||
Poly(vinylidene cyanide-vinyl acetate) (P[VDCN-VAC]), and (vinylidene fluoride-tetrafluoroethylene) (P[VDF-TeFE]), and poly(vinylidene fluoridehexafluoropropylene) (P[VDF-HFP]) | Piezoelectric effect, light weight, flexibility, and low cost | Flexible tactile sensors | 20 and 59 |
No need for stretching or drawing of the polymer chain. Even if poling is necessary to increase the piezoelectric effect | |||
Polyaniline (PANI) – zinc ferrite – drop casting technique to prepare the ternary PVDF composite (80 mm) | PANI helps to prevent the internal resistance of the nanogenerator and stabilize the output voltage. Improves the homogeneity of filler distribution in the composite. Zinc ferrite nanorods improve the mechanical and piezoelectric properties | Nanogenerator and stabilize the output voltage | 60 |
Lanthanum-doped lead zirconate titanate (PLZT) – PVDF composite with PLZT | In 50 vol% CNT film the polar phase was obtained. Piezoelectric constant augmented to 30 pC/N for 50 vol% PLZT-PVDF. Energy density raised to 29.1 mg cm−3 | Energy-harvesting devices | 20 |
Piezoelectric output | |||
PVDF fibers were prepared using electrospinning – graphene oxide (GO) and reduced graphene oxide (rGO) | RGO and GO nanofillers help to improve the piezoelectric properties. The filler content gives a powerful interfacial interaction with the PVDF chain | Energy-harvesting devices, portable electronics, wearable devices and pressure sensors | 61 |
Ca3(PO4)2 nanorod-incorporated PVDF films have been prepared via an in situ flexible piezoelectric nanogenerator | The addition of different molar concentrations of Ca3(PO4)2 accelerated the nucleation of electroactive crystallization. Also achieves huge dielectric value by the in situ process (interfacial polarization) | Charge cell phones, small portable electronic gadget | 20 |
PVDF-ZnO nanofiber mats prepared with electrospinning | Modified in the crystal structure and improvement in the electrical output | As an elevated energy-scavenging interface, it can provide a simple, effective, flexible and cost-effective approach to self-powering microelectronics apparel for several applications | 62 |
BaTiO3–PVDF composite nanofibers | Improvement in the polymer phase crystallization and increase in piezoelectrically generated voltage | Offers a simple and flexible methodology to self-power microelectronics and nanogenerator on behalf of several applications in industry, medicine and others | 63 |
Recent development | Properties | Applications | Reference |
---|---|---|---|
Polyamide, polydioxanone and PVDF | Excellent physical and mechanical | Surgical sutures and meshes to replace polypropylene (PP) | 65 |
PVDF monofilament sutures | Superior biocompatibility and mechanical properties | Alternative suture material with barbs, used effectively for tendon repair | 66 |
PTFE-PVDF-PP membranes | Revealed no significant histopathological change | Eyeball implants in rabbits | 68 |
Composites of PVDF and HAP | No toxicity in vitro, and raised wettability | Bone regeneration applications, medical and dental implants | 69 |
a β-PVDF-based electrospun scaffold | Mechanical stability in adherence, differentiation of cardiomyocytes | Repairing damaged heart muscles and stem cells, cardiac patch | 71 |
Composites of PVDF-reduced graphene oxide | Reduced graphene oxide promotes β-phase in PVDF and improves the wettability to a large degree | More cell adherence, spread, and proliferation in the case of human umbilical vein endothelial cells | 72 |
Some cell lines in the human body, such as corneal, epidermal, retinal, vascular endothelial cells, and nervous systems, are controlled by electric signals. Piezoelectric signals are also found in parts of the body like bone, cartilage, DNA, ligaments, skin, and tendons.8 One reason why engineers are looking for active biomaterials is that cell lines such as osteoblasts and chondrocytes demand external mechanical or electrical stimuli for enhanced tissue repair. Conductive and piezoelectric biomaterials are a major category of active biomaterials used to fabricate ideal scaffolds for specific applications.50
PVDF is naturally hydrophobic, which limits its application in pharmacy and as a biomaterial. To overcome this drawback, several approaches have been applied. These approaches can fall into two categories: physical and chemical modifications. Coating, depositing, immobilizing, and cross-linking a hydrophilic polymer on PVDF is considered physical modification, while electron beam radiation, plasma treatment, and chemical reactions are chemical modifications.67
Yu et al.70 fabricated a mesoporous PVDF-based piezoelectric nanogenerator and encapsulated it within polydimethylsiloxane (PDMS). They implanted the aforementioned device in a rat's quadriceps femur muscle surface to test its in vivo biocompatibility. After extracting the films, there was no evidence of cellular toxicity or muscular atrophy, just limited inflammatory infiltrates. The spongy mesoporous film showed excellent flexibility and biocompatibility, and it did not lose its efficiency during this time. For long-term use of this device, it was exposed to an ultrasonic bio-fluid bath. They showed that sandwiched PVDF NG could serve as a long-life implantable powering system in the future.
In research conducted by Gallego et al.,51 soft-lithography-based techniques were used to make individual and embossed PVDF microstructures with delicate geometries. Tests showed that the material has biocompatibility, and intense interaction between cells and substrate occurred. A remarkable piezoelectric response was achieved due to poling and actuation. To resist gastric acid and intestinal juice, duodenal barriers must have thermal stability, corrosion, and abrasion resistance. PVDF composites seem to be best for these applications; moreover, they barely stick to the intestine wall, and due to their antibacterial properties, they demonstrated negligible adverse effects.68
Despite superior mechanical properties and chemical resistance, PVDF lacks microbial resistance. To overcome this drawback, quaternary pyridinium monomers have been grafted on PVDF and showed extreme antimicrobial properties.73
Another piece of research conducted to enhance the biocompatibility of PVDF was done via oxygen plasma and dopamine coating. The results show that oxygen plasma treatment for a short time improved PVDF biocompatibility up to four-fold, and dopamine coating created a strongly adhesive hydrophilic surface with a threefold enhancement in biocompatibility.74
Studying PVDF as a neural scaffold membrane indicates that neither pure PVDF nor one loaded with multiwalled CNTs has the potential to support neurite branching, but they are permissible for oligodendrocyte morphological differentiation.75
Interestingly, the addition of reduced graphene oxide to polyvinylidene fluoride promotes wettability and β-phase formation in PVDF, resulting in higher cell adhesion and proliferation and biocompatibility of the composite.72
It is known that piezoelectrical scaffolds generate electricity, facilitating angiogenesis and nerve restoration. In this concept, the biocompatibility of a composite of PVDF–PCL was tested with rat Schwann cells. Cell proliferation and differentiation were observed as well as biodegradation in 4 months.76 In vivo testing of nanogenerators may face some critical issues. Bio-fluid–NG interaction, ion diffusion in NG, detachment of tissue from NG, and the tissue response to high local electricity produced by piezoelectric materials are unknown.70
Piezoelectric materials can convey an electric motive without an external energy source; thus they are considered promising candidates in injured tissues. PVDF has high chemical stability for use in membranes, electrolytes, and catalysis for wastewater treatment77 and filtration technology, bioreactors, distillation, gas separation,78 separators for lithium-ion batteries,79 etc. Owing to its high piezoelectricity, pyro and ferroelectric properties as well as low density, toughness, and flexibility, it has applications in wearable functional devices,80 transducers81 and sensors (chemical, pressure, impact, audio, ultrasonic and biosensors), health monitoring systems, biomaterials, and flexible actuator devices.1 In electronic devices, such as speakers, LCDs, LEDs, watches, and telecommunication devices, piezoelectric ignition systems measure the mechanical and physical changes under shock loading.82
PVDF as a biomaterial has been used in bone,45 neural83 and muscle tissue regeneration,84 abdominal hernia repair,85 endothelialization, vascular surgery,66 and tissue sensors. PVDF copolymers and PVDF-TrFE have also been reported as bone regeneration, neural tissue, and wound healing fibers and membranes.85
PVDF-based piezopolymers have applications in bone because of their characteristic of power generation under small mechanical deformation. Crystalline micelles in collagen molecules induce piezoelectricity. Consequently, the extracellular matrix of cells is the reason for the piezoelectric effect in bone tissue. Hence, scaffolds similar to ECM are in demand in bone rehabilitation.46
PVDF is now being used in the medical industry as medical textiles and surgical meshes, drug delivery, tissue engineering, surgical sutures, and implants.66 PENG devices based on PVDF have the flexibility to achieve twistable, foldable characteristics, and stretchability, which is ideal for wearable electronics such as biomechanical monitoring units or energy supply units (Fig. 24).86
The knowledge discussed in this review on the design and fabrication of laboratory-scale piezoelectric PVDF-based materials provides a better understanding for prospective scientists and ongoing studies in the fields of energy harvesting and sensors. Indeed, even for a specific application, rarely can one cure provide all the appropriate properties. Although several studies have evaluated the efficacy of combined factors, there is still a pressing need to develop an ideal structure and fabrication methodology for different piezoelectric applications. So far, most studies have been conducted on small size models and fabrication techniques. It is crucial to evaluate the performance of factors at a larger scale before industrial trials. In addition, the ability to scale up PVDF fabrication strategies is still an important consideration that needs to be addressed.
PVDF is a significant processible inorganic material whose final properties depend on its fabrication method, which has been shown to have excellent ferroelectric properties, and which could be the savior of scientific progress in the fields of energy harvesting and medicine in the future.
Footnote |
† These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2023 |