A Review on Wearable Electrospun Polymeric Piezoelectric Sensors and Energy Harvesters

In recent years, wearable sensors and energy harvesters have shown great potential for a wide range of applications in personalized healthcare, robotics, and human–machine interfaces. Among diﬀerent types of materials used in wearable electronics, piezoelectric materials have gained enormous attention due to their exclusive ability to harvest energy from ambient sources. Piezoelectric materials can be utilized as sensing elements in wearable sensors while harvesting biomechanical energy. Electrospun piezoelectric polymer nanoﬁbers are extensively investigated due to their high ﬂexibility, ease of processing, biocompatibility, and higher piezoelectric property (in contrast to their corresponding cast ﬁlms). However, as compared to piezoceramic materials, they mostly exhibit relatively lower piezoelectric coeﬃcients. Therefore, considerable eﬀorts have been devoted to improving the piezoelectricity of electrospun polymer nanoﬁbers recently, resulting in signiﬁcant advances. This review presents a broad overview of these advances including new material, structure designs as well as new strategies to enhance piezoelectricity of electrospun polymer nanoﬁbers. The challenges in achieving high mechanical performance as well as high piezoelectricity are particularly discussed. The main motivation of this review is to examine these challenges and highlight eﬀective approaches to achieving high-performance piezoelectric sensors and energy harvesters for wearable technologies.


Introduction
Wearable electronics generally refer to electronic devices integrated with clothing, accessories, or directly mounted on the body. [1,2]Over the last two decades, wearable electronics have capability. [26]However, many piezoresistive sensors show performance relying on mechanical stability of the conductive network and are susceptible to hysteresis and variable sensitivity as the network may continuously changes over time. [27]While piezoelectric and triboelectric sensors are similar in capability of higher frequency and a broad range of frequency detection, these two types of sensors are very different in the design.Triboelectric sensors generally show high output power but have much more sophisticated design and need to involve contact-separation process of two surfaces having different charge affinities. [20,21][30] Among the different materials, piezoelectric materials are particularly appealing and have been widely explored for making self-powered sensors and energy harvesters as they are easy to be made into diverse forms, cost-effective, and lightweight. [4,31]oreover, there are a variety of piezoelectric materials with diverse properties available.Dynamic mechanical energy can be converted into electric energy by piezoelectric materials with high power density and durability.Meanwhile, based on the electric charge output frequency and magnitude, piezoelectric materials could be used as sensors for detecting mechanical deformation.[34] Piezoelectricity has been observed in single crystals (such as ammonium dihydrogen phosphate, quartz, etc.), ceramics, polymers, and composites, [4,7,8,20,35,36] or even natural materials such as onion skin. [37]9] As external stress is applied to a piezoelectric material, the center of electrical charges for anions and cations changes.[40][41] Piezoelectric ceramics, e.g., lead zirconate titanate (PZT), [4,6,8,35,36] zinc oxide (ZnO), [3][4][5][6][7][8]20,35,36,39,42] and barium titanate (BaTiO 3 or BTO), [3][4][5][6][7][8]20,35,36,39,42] have high piezoelectric coefficients.[3,4,35,36] Also, they are chemically inactive and have high stability against humidity.But their inherent brittleness limits their applications in wearable electronics. On the other hand, piezoelectricpolymers like poly(vinylidene fluoride) (PVDF), [3][4][5][6][7][8]20,33,35,36,39,42] odd-numbered nylons, [39] polyacrylonitrile (PAN), [6,36,[43][44][45] etc. have exceptional mechanical properties, yet have relatively lower piezoelectric coefficients (e.g.,PVDF has a d 33 of ≈30 pC N −1 ) compared to piezoelectric ceramics (PZT has a d 33 of ≈500-600 pC N −1 ).[6,35] While some materials are inherently piezoelectric, there are certain materials that need to be poled to show piezoelectric properties.In this process, the gas breakdown in the materials results in the generation of electric dipoles and thereby inducing the piezoelectric effects.[39] Since all human tissues are intrinsically flexible and soft, wearable or implantable devices that are expected to use on or in the human body should be flexible and stretchable to gain precise and reliable information in interaction with human skin or in-ternal organs of the human body.Piezoelectric ceramics are brittle and cannot withstand large deformations.In contrast, piezoelectric polymer thin films are much more flexible and their stretchability can be tuned by using nano additives and appropriate fabrication methods.For instance, the electrospun PVDF nanofiber membrane containing boron nitride showed significantly increased elongation at yield up to 110%.[46,47] There are a variety of techniques for fabricating polymeric piezoelectric wearable electronics.The fabrication method is chosen based on the selected raw materials, the dimension, and the final applications of the sensors.[48] Some common fabrication methods include photolithography, [49] printing techniques, [50,51] laser printing, [52] casting, [19,53] coating, [54,55] and electrospinning.[45,56,57] Among these methods, electrospinning is one of the most common ones because of its ease of operation, continuous nanofiber formation, adjustable porosity of electrospun structure, and flexibility in various shapes and sizes. [6,40] Meanwhile, under58][59][60][61] Electrospinning is a particularly popular method for making piezoelectric polymer nanofibers such as PVDF and its derivatives.
64][65] For instance, in 2012, Chang et al. [62] summarized the recently developed nanofiber-based nanogenerators with their working principles and discussed some issues such as energy harvesting efficiencies and potential false artifacts during characterizations.More recently, Li et al. [65] reviewed major advancements in biodegradable piezoelectric materials.However, none have provided an in-depth survey of the piezoelectric and mechanical properties of the electrospun piezoelectric polymer nanofibers and key strategies to improve these properties.Herein, we present a comprehensive review of the electrospun polymeric nanofiber-based piezoelectric wearable sensors/energy harvesters and summarize recently reported various strategies to improve their mechanical and piezoelectric properties.This review is organized into five main sections.The first section discusses the piezoelectric polymers commonly used for making electrospun nanofiber-based piezoelectric wearable devices and their properties while the second section focuses on reviewing different strategies recently developed to improve the mechanical and electrical performance of these devices.Afterward, a brief discussion is presented on the various parameters affecting the quality of the electrospun nanofibers in the electrospinning process, followed by a discussion of some demonstrated applications of these piezoelectric nanofibers.The last section highlights the current challenges and future opportunities.

Piezoelectric Polymers
A key requirement for wearable devices is that they need to be comfortably worn.In other words, they should be mechanically compatible with the soft and curved human body to have better contact with the skin for reliable measurements.Hence, piezoelectric polymers, owing to their flexibility and deformability, are widely used to fabricate wearable sensors and biomechanical energy harvesters.There are generally two categories of piezoelectric polymers: semicrystalline or amorphous.The  [107] Copyright 2018, Multidisciplinary Digital Publishing Institute (MDPI).b) The internal crystalline structure of PVDF-TrFE, Reproduced with permission. [88]Copyright 2020, Royal Society of Chemistry.c) Schematic of the molecular structure of PAN.d) Cellulose's I (monoclinic) crystalline structure.Reproduced with permission. [68]Copyright 2020, American Chemical Society.

PVDF and Its Copolymers-Based Nanofibers
Among all the above-mentioned piezoelectric polymers, PVDF is the most prominent one in diverse applications, particularly in wearable electronics.Although, like other piezoelectric polymers, PVDF has shortcomings such as possessing a relatively lower dielectric constant and piezoelectric constant (d 33 ) than ceramic-based piezoelectric materials, [4] it exhibits high flexibility, high stability against mechanical stresses, high chemical resistance, and thermal stability. [3,4,6,8,36,69]72][73][74]

PVDF Homopolymer-Based Nanofibers
Fluorine atoms in PVDF have a large van-der-Waals radius and electronegativity, which leads to a dipole moment perpendicular to the monomer chain. [4]PVDF is a linear semicrystalline polymer possessing five different crystalline phases, i.e., , , , , and .The  and  phases show trans-gauche-trans-gauche (TG + TG − ) chain conformations, while -phase has all trans conformation (TTTT) and  and  phases have trans-trans-trans-gauche-transtrans-trans-gauche (T 3 G + T 3 G − ) chain conformations.The first three crystalline phases of PVDF are shown in Figure 1a.Among all these phases, the -phase has the highest spontaneous polarization and thereby better piezoelectric properties than other phases.In contrast, the  phase is the most stable one since it has the lowest energy. [3,4,6,7,33,36,40,75,76]The piezoelectricity of PVDF is dependent upon its -phase content as well as crystallinity.The higher the -phase content and its crystallinity, the higher the piezoelectric coefficient is.[79] The -phase content (F()) is usually calculated from the Fourier-transform infrared (FTIR) spectrum data, as follows: [80] where A  and A  are the absorbance intensities at 766 and 840 cm −1 , respectively, corresponding to  and -phases of PVDF.Different factors including the molecular weight of the polymer, electrospinning parameters, and additives can affect the formation of the -phase and its degree of crystallinity. [40,81,82]For example, Wu et al. [79] demonstrated that randomly oriented electrospun PVDF nanofibers with a -phase content, F(), of 79 ± 3%, have a d 33 of 16.8 ± 1.4 pC N −1 while the aligned electrospun nanofibers of PVDF showed a d 33 of 27.4 ± 1.5 pC N −1 with F() being 88 ± 1%.Zaarour et al. [83] examined the influences of molecular weight on properties of electrospun PVDF nanofibers and found that by increasing the molecular weight from 180 000 to 530 000 g mol −1 , -phase content increases due to the increased viscosity of polymer solution.Moreover, the increased -phase content leads to strengthening effects, i.e., by increasing the M w from 180 000 to 530 000 g mol −1 , the tensile strength was enhanced from 7.9 ± 0.92 to 10.78 ± 1.23 MPa, the elongation at break was improved from 35.01 ± 3.12% to 48.80 ± 5.81%, and Young's modulus was enhanced from 32.6 ± 2.97 to 82.73 ± 6.28 MPa.

PVDF-TrFE-Based Nanofibers
PVDF-TrFE is a semicrystalline polymer prepared by polymerization of VDF and TrFE.The copolymer can easily crystallize from either its melt or solution by pouring into an electroactive phase with the same structure as the -phase in PVDF without the need for stretching. [72,84]The internal crystalline phases of PVDF-TrFE are shown in Figure 1b.PVDF-TrFE is a semicrystalline polymer with different crystalline phases including (, , , , and ).The piezoelectric coefficient (d 33 ) of PVDF-TrFE is reported to be 25-40 pC N −1 .PVDF-TrFE with a molar content of 70/30 or 75/25 (major portion of PVDF and minor portion of TrFE) is the most promising piezoelectric material in fabricating wearable sensors and nanogenerators, due to its tunable mechanical flexibility and increased -phase content. [58,59,70]ompared with PVDF, PVDF-TrFE containing 20-35 mol% TrFE shows higher piezoelectricity as they can easily crystallize.][87][88][89][90][91] PVDF-TrFE has a higher crystallinity degree than PVDF (e.g., 50%), which can be increased up to 90% when annealed between the Curie and melting point.The enhanced chain mobility of PVDF-TrFE under elevated temperatures increases the lamella thickness.Moreover, the combination of PVDF-TrFE copolymer chains, if the molar content of TrFE is higher than 20%, causes the deformation of TG + TG − to all-trans (T m > 4) and the conversion of the  phase to the  phase. [72]Additionally, it is noticed that due to the greater distance between the chains in the -phase of the copolymers, dipole rotation is also easier in the main axis of the copolymer.
Apart from higher piezoelectricity, PVDF-TrFE nanofibers also have higher mechanical properties as demonstrated by Zhang et al. [84] The Young's modulus for aligned PVDF-TrFE and PVDF nanofiber is reported to be 91.8 ± 5.64 MPa and 75.1 ± 4.37 MPa, respectively.While the elongation at break for aligned PVDF-TrFE and PVDF are reported to be 70.1 ± 4.91% and 50.32 ± 3.84% correspondingly.These higher mechanical properties are likely due to the higher crystallinity of PVDF-TrFE.The intermolecular bonding between molecular chains in the crystalline phase is stronger, leading to a highly oriented chain configuration when the polymer is deformed.Similar results were reported by Kenney, [92] who revealed that copolymers have higher mechan-ical properties due to the higher crystallinity.It is also noteworthy that a terpolymer of PVDF with TrFE and chlorotrifluoroethylene (CTFE) demonstrates higher polarization and higher dielectric constant than those of the copolymer of PVDF-TrFE.d 33 of P(VDF-TrFE-CTFE) terpolymer is twice higher than PVDF-TrFE copolymer due to saturation polarization. [93]1.3.PVDF-HFP-Based Nanofibers PVDF-HFP is another type of piezoelectric PVDF-based block polymer.[94] Same as PVDF, the essence of piezoelectricity in PVDF-HFP is mainly because of its crystalline structure.The polar crystalline  and especially  phases contribute to the piezoelectricity in PVDF-HFP rather than the  phase.The piezoelectric coefficient of PVDF-HFP was reported to be around 18 pC N −1 .[73,74] The major advantages of PVDF-HFP over PVDF are higher solubility, enhanced hydrophobicity and free volume, and better mechanical properties, which arise from the HFP block in the polymer chain.The tensile strength and failure strain for PVDF-HFP nanofibers are 16.04 MPa and 130.98% respectively.[73,74,95] But, at the same time, HFP groups reduce the crystallinity degree in the PVDF structure. To ehance the -phase content in this polymer, several methods including electrical polling, thermal treatment, stretching, and creating composites with a wide range of nanofillers have been employed.[96]

PAN-Based Nanofibers
PAN is an amorphous polymer having cyano groups (-CN) in each repeating unit.It has been widely used as textiles, packaging materials, purification membranes, and precursors for producing carbon fibers.Researchers found that solid-state PAN typically has two conformations: planar zigzag (also called "sawtooth") and 3 1 -helical.The zigzag conformation has an all-trans (TTTT) structure with a dipole moment of 3.5 Debye, which is larger than that of the -phase of PVDF (2.1 Debye).This makes PAN an outstanding piezoelectric material for sensor application.The internal molecular structure of PAN is shown in Fig- ure 1c.Electrospun PAN nanofibers possess higher zigzag conformation content and an oriented structure, which is the reason for their high piezoelectricity.Compared with PVDF, PAN has lower dielectric loss and is thermally more stable and more costeffective.99][100] It was shown that at the same sound frequency (Hz) and sound pressure level (dB), the PAN nanofiber membrane has a voltage output of 398.72 mV compared with 185.21 mV for PVDF nanofiber. [97]In another work by Shao et al., [100] application of PAN nanofibers for human voice recognition was reported.The sensor can generate 19 mV under 60 dB and 40.7 V under 115 dB.Moreover, the device shows a sensitivity of 23.4 VP −1 to 70 dB sound at 90 Hz.However, it is found that PAN usually has relatively low piezoelectricity even though the presence of cyano groups results in a large dipole moment.][103] Moreo, the piezoelectricity of PAN can be improved by incorporating charge carriers or aligning cyano groups.
From the mechanical property point of view, PAN is more rigid than PVDF and studies have shown that the PAN nanofiber membrane showed a higher rigidity than PVDF nanofibers. [104]he tensile strength and Young's modulus for the PAN nanofiber membrane were measured to be 4.6 and 86.1 MPa, respectively, while the elongation at break wass 64.1%.The much higher stiffness is one of the reasons for their sound sensing capability.Additionally, the high-temperature stability of PAN allows applications at high temperatures. [104]

Cellulose-Based Nanofibers
Cellulose is a newly discovered renewable piezoelectric material.The source of cellulose is affluent and it has desirable properties such as biodegradability, high elastic modulus, good mechanical strength, superior optical characteristic, and fair prices. [105]n general, by using various chemical and physical methods, nanocellular structures can be extracted from primary sources of cellulose.The nanocellulose productions are divided into two main categories, namely cellulose nanofiber (CNF) and cellulose nanocrystal (CNC).CNFs are commonly long and flexible with the nanoscale diameter and microscale length while the length of CNCs is nanoscale (≈ 100-500 nm) and needle shaped. [105]As shown in Figure 1d, the cellulose structure has a 2 l screw axis which is a polysaccharide composing the linear chain of many  (1-4) linked D-glucose units.Also, to form the cellulose with the axially stiff fibril, the linear configuration of the cellulose chain is required.Therefore, in this regard, the intrachain hydrogen bonding between hydroxyl groups (-OH) and oxygens in adjacent ring molecules stabilizes the linkage and leads to a linear conformation in cellulose structure. [68]The most important factors affecting the piezoelectric property of CNFs are their dipolar alignment and crystallinity degree.The crystalline structures in cellulose nanofibers are non-centrosymmetric and it is the main cause of piezoelectric properties in CNFs.There are several techniques to improve the alignments and orientation of CNFs such as electrical poling, magnetic poling, mechanical stretching, hydrodynamics, and electrospinning. [68,105,106]Different degrees of alignment and different electrical output voltages can thus be achieved.Generally speaking, the output voltage for aligned CNFs is higher in comparison with randomly oriented CNFs.The piezoelectric constant of aligned CNFs has been revealed to be d 31 = 14.5 pC N −1 which is twice the value in randomly oriented CNFs. [105]When the CNFs are subjected to the electrical poling, due to the OH groups in cellulose structure being sensitive to the applied electrical field, these groups rotate to the out-of-plane direction causing more OH groups to be placed on the surface of cellulose film which leads to a higher piezoelectric coefficient in that direction. [68]he alignment also influences the mechanical properties of cellulose.For example, Kafy et al. [106] used wet spinning to align the CNFs and found that the alignment enhances the mechanical properties.For a single cellulose nanocrystal, the elastic modulus is ≈ 150 GPa in the longitudinal direction and 18-50 GPa in the transverse direction.Higher spinning speed results in a higher degree of alignment and higher strength.The tensile strength of fibers prepared at a spinning speed of 5 mL min -1 and 10 mL min −1 is 234.6 ± 10.3 and 249.7 ± 10.4 MPa, respectively, while the corresponding strain at break is 10.2 ± 0.4 and 9.2 ± 0.4%. [106]

PLA-Based Nanofibers
Poly(L-lactic acid) (PLLA) is a good example of a synthetic environmentally friendly polymer. [35]PLA is a semicrystalline polymer, a byproduct released through the human metabolic process, and can be derived from the fermentation of corn starch. [108]PLA experiences hydrolytic degradation and breaks into two segments when it exposes to microbial attack or has an interaction with water.The final byproducts caused by the degradation process are molecules such as water and carbon dioxide.Therefore, this polymer is completely environmentally friendly.Lactic acid has two naturally occurring enantiomers, L-lactic acid, and D-lactic acid.As a result, two forms of lactic acid, poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) are produced during condensation polymerization. [65]PDLA and PLLA are both optical isomers of polylactic acid and both have a helical structure but are different from each other.The PLLA helical structure is right-handed while PDLA is left-handed and they have mirror symmetry. [109]he molecular chain of these polymers has a chiral structure and carboxylate (-COO) polar groups are linked to an asymmetric carbon atom and arranged in a helical configuration, endowing them with piezoelectricity. [65,108,110]The value of the piezoelectric coefficient of PLLA and PDLA are equal but opposite.[113][114] Compared with PVDF, PLLA exhibits a higher shear piezoelectric coefficient value (d 14 ) of ≈12 pm V −1 . [115][118] The internal rotation of polar groups related to asymmetric carbon atoms is responsible for the shear piezoelectricity in PLLA, in which the piezoelectric constant can be defined only by d 14 , as d 25 =d 14 due to the symmetry.PLLA shows a piezoelectric response without polarization.[121][122] For example, Curry et al. [122] fabricated a piezoelectric transducer based on PLLA nanofiber to investigate its piezoelectric performance.The nanofibers were developed by electrospinning and the rotating speed of the drum ranged from 300 to 4000 rpm while other electrospinning parameters were kept constant.The as-spun nanofiber was amorphous.The samples were then annealed at 105 and 160.1°C, and then slowly cooled down, leading to a crystallinity degree of ≈70-88%.Moreover, it was noted that the increase in rotating speed improved their alignment.The largest electrical output, when subjected to a 30 N impact force, was about 0.9 nC for the sample collected at 4000 rpm.In contrast, the sample collected at 300 rpm exhibited very small output (≈ 0.1 nC).The d 14 for sample collected at a speed of 4000 rpm is ≈ 19 pC N −1 , higher than that of bulk PLLA film (≈ 12 pC N −1 ).This shows that the electrospinning process, annealing, and cooling treatments can significantly improve the shear piezoelectric response of PLLA samples by improving their crystallinity.The mechanical and thermal properties of PLLA are crucial parameters that should be considered during the fabrication process.One of the drawbacks of PLLA is its low thermal resistance with a softening point about 60 °C.[125] Studies have demonstrated different efficient methods to enhance the mechanical properties of PLLA nanofibers. [123,126,127]For example, Baheti et al. [123] managed to improve the mechanical properties of PLLA by incorporating wet waste milled Jute nanofibers (JNF).Their result showed that the elastic modulus of pure PLLA was increased by 39.42%, 217.30%, and 5.76% when adding 1, 5, and 10 wt% of JNF, respectively.A similar trend was observed for the tensile strength.Neat PLLA exhibits a tensile strength of 25.98 MPa, which increases to 70.3 MP for PLLA/JNF (5 wt%), followed by a drop for PLLA/JNF at 10 wt%.However, the elongation at break for the nanocomposite thin films was decreased from 4.84% for neat PLLA to 1.68% and 1.69% for PLLA/JNF 5 wt% and PLLA/JNF 10 wt%, respectively.
In summary, the different piezoelectric polymers discussed above have shown broad piezoelectric and mechanical properties.Table 1 summarizes their key features.It can be concluded that PVDF and its copolymers have higher piezoelectricity compared with other polymers.However, other polymers show some unique properties that PVDF and its copolymers lack.For example, PAN is thermally more stable and can be used at high temperatures while cellulose and PLA are more environmentally friendly.Moreover, while PVDF-TrFE has higher piezoelectric performance, it is costlier than PVDF.

Strategies to Improve the Piezoelectric Performance
As mentioned above, one main disadvantage of piezoelectric polymers, as compared to piezoelectric ceramics, is their relatively low piezoelectric coefficient.Therefore, considerable efforts have been devoted to improving their piezoelectricity.Figure 2 summarizes some key progresses made in understanding the fundamentals and finding effective strategies to improve the output of piezoelectric nanofibers.Since the discovery of high piezoelectricity of electrospun PVDF nanofibers in early 2000s, great efforts have been spent on studying the effects of different electrospinning parameters and nanofillers as well as different structure designs on its piezoelectricity.This section mainly focuses on common strategies that aim at modifying the structure (molecule level and macroscale) to improve the piezoelectricity of the electrospun polymer nanofibers.More specifically, the section is organized into two subsections: one discusses the incorporation of additives while the other reviews the different structure designs of the electrospun fibers.

Design of Polymer Nanocomposites/Polymer Blends
The addition of a second material into piezoelectric polymers is expected to modify their properties.In most cases, these additives interact with the polymers and change the polymer chain configurations, thereby modifying the crystalline structure, molecular dipoles, and their orientations.[132] Three categories of additives are included in this section including piezoelectric fillers, non-piezoelectric fillers, and polymers.

Piezoelectric Fillers
Flexible piezoelectric nanofiber mats with high piezoelectricity have been prepared by combining piezoelectric ceramics with different polymers.For instance, barium titanate (BaTiO 3 ), [45,133,134] zinc oxide (ZnO), [43,[135][136][137][138] and boron nitride (BN) [46,139,140] in different forms, such as nanoparticles, [45,85,86,[133][134][135][136][137] nanorods, [43,138] nanoflakes, [139] nanofibers, [89] nanosheets, [46,140] and nanowires [141,142] have been combined with polymers to develop flexible piezoelectric devices.43] For example, BaTiO 3 nanoparticles have been utilized to improve the piezoelectricity of PAN nanofibers by promoting the transformation of the 3 1 -helix into the planar zigzag.[45] Monolayer boron nitride, due to its strong in-plane piezoelectric properties, has demonstrated great performance in improving the piezoelectricity of PVDF.[46,140] A third example worth mentioning is the use of ZnO nanospheres to increase the piezoelectricity of PVDF nanofibers. Figure 3ashows the schematic illustration as well as optical and SEM images of the PVDF/ZnO nanofibers.It was shown that the addition of ZnO can increase the -phase content and the crystallinity degree of PVDF, thereby increasing the piezoelectricity.The negative charge on the ZnO nanospheres and the positive charge of the CH 2 group of PVDF interact with each other, which leads to the arrangement of dipoles in the polymer chain into the -phase form, thus enhancing the -phase content.Figure 3b,c presents the output voltage and current for the composite nanofibers with various contents of ZnO nanospheres (0-5 wt%).It is shown that the sample with 5 wt% ZnO demonstrated the highest opencircuit voltage (V oc ) of up to ≈ 11 V and short circuit current (I sc ) of ≈ 520 nA.[137] The sensitivity of the nanocomposite was 0.33 V kPa −1 .
][146] For example, Su et al. [41] modified BaTiO 3 with polydopamine (PDA) which was then added to PVDF solution for electrospinning at different weight contents (3, 5, 7, and 10%).The results showed that the sensor containing 5 wt% of PDA@BTO exhibited a maximum ≈ 11 V voltage under a force of 2 N. Figure 3d illustrates the mechanism of piezoelectricity enhancement, i.e., the greatly enhanced piezoelectricity arises from strong hydrogen bonding between PDA and PVDF. Figure 3e,f clearly shows that the piezoelectric performance (voltage and current) of composite nanofibers rises and then drops with increasing the PDA volume fraction.Apart from PDA, surface modification with carbon has also revealed significantly improved piezoelectricity.For instance, the composite nanofiber of ZnO@C/PVDF showed much higher output voltages. [144]igure 3g illustrates the formation of the -phase and output enhancement mechanism affected by both piezoelectricity and  [137] Copyright 2019, Elsevier.d) Schematic illustration of the interfacial interaction between BaTiO 3 and PVDF matrix after being modified by polydopamine.e) Output voltage, and f) output current of nanocomposite fibers with different concentrations of PDA under a mechanical force of 3 N. Reproduced with permission. [41]Copyright 2021, Elsevier.g) Schematic illustration of the -phase formation in ZnO@C/PVDF nanofiber.h) The output voltages of nanocomposite nanofibers containing different contents of ZnO@C.Reproduced with permission. [144]Copyright 2021, Elsevier.
triboelectricity.In this study, ZnO@C enhanced the -phase in PVDF, leading to an increased piezoelectric coefficient.Moreover, the modified nanoparticle can negatively shift the surface potential of the PVDF to a higher value, which leads to increased triboelectricity.The output voltage of the sample with 5 wt% carbon-coated zinc oxide was 37 V, which was about five times that of the pure PVDF, as shown in Figure 3h.
Piezoelectric fillers can be used along with other fillers, such as conductive carbon-based nanoparticles.In a study done by Shi et al., [143] BaTiO 3 and graphene nanosheets were dispersed in Reproduced with permission. [90]Copyright 2019, Elsevier.d) The phase fraction and e) the output voltage of the PVDF/nanoclay nanofibers.f) Schematic illustration of the charge separation mechanism after adding nanoclay.g) Stress-strain curves of neat PVDF-TrFE nanofibers (P) and its nanocomposite fibers (C15) with an inset bar chart showing their modulus and toughness.Reproduced with permission. [168]Copyright 2019, Elsevier.
PVDF solution with various ratios and electrospun to form hybrid composite nanofibers.It was shown that the nanofiber mat containing 15 wt% BaTiO 3 and 15 wt% graphene nanosheet has the highest output voltage of ≈11 V.The electrospinning process results in the alignment of dipoles along the electric field direction in BaTiO 3 nanoparticles and PVDF.Therefore, BaTiO 3 nanoparticles generate additional piezoelectric potential under external stress, leading to improved total outputs.On the other hand, the graphene nanosheet enhances the output by inducing the -phase nucleation and reinforcing composite nanofibers.The nanofiber mats show higher strength, which helps to improve the local stress transfer.Moreover, forming the conductive network of graphene nanosheets reduces the internal resistance, resulting in better transfer of the induced charges.

Nonpiezoelectric Fillers
Apart from piezoelectric nanofillers, electrically conductive nanomaterials such as carbon-based nanomaterials (MWCNTs [90,[147][148][149][150] and graphene, [23,[151][152][153] ) metal nanomaterials (silver nanowires [133,154] and gold nanoparticles [155] ), MXenes [156,157] and magnetic materials [146] have also been used to increase the piezoelectric output of the piezoelectric polymers.The addition of conductive nanofillers can enhance the -phase content of piezoelectric polymers and improve the transfer of the induced charges, thereby enhancing their piezoelectric performance.For example, Zhao et al. [90] proved that the electric output of PVDF-TrFE nanofibers could be improved by incorporating MWCNTs.The addition of conductive nanomaterials can enhance the distribution of inductive charges during elec-trospinning, which increases the crystallinity of PVDF-TrFE as well as polar -phase content.Moreover, the free charges injected in the electrospinning can be trapped by MWCNTs and then be gathered at the interfaces, as illustrated in Figure 4a-i,ii.However, higher loadings of conductive nanofillers can weaken and even prevent crystallization in PVDF-TrFE electrospun nanofibers (Figure 4a-iii).It was shown that the nanocomposite with 3 wt% fillers showed a maximum V oc of 18.23 V (Figure 4b) and I sc of 2.14 μA and a power density of 6.53 μW cm −2 .However, the mechanical performance, i.e., elongation at break and the young modulus, decreases as the content of MWCNT increases, as can be seen in Figure 4c.Similarly, Abbasipour et al. [23] indicated that graphene oxide, graphene, and halloysite nanotubes all enhanced the piezoelectricity of PVDF with graphene oxide showing the highest efficiency.The sample with 1.6 wt% graphene oxide showed the highest piezoelectricity, i.e., d 33 of 24 pC N −1 , g 33 of 14.22 × 10 −5 Vm N −1 , V oc of 2.39 V, I sc of 70 nA, and power density of 29 mW cm −2 .These fillers enhanced the piezoelectric properties by increasing the -phase content in the nanofibers.Wang et al. [156] investigated the piezoelectric performance of MXene/PVDF-TrFE composite.MXene has a lot of surface functional groups that can interact with PVDF-TrFE dipoles.Moreover, the high electrical conductivity of MXene leads to an increase in further polarization of PVDF-TrFE during electrospinning.Under 20 N applied at a frequency of 1 Hz, the composite nanofibers generated a V pp of 1.58 V, which was four times that generated by pure PVDF-TrFE (0.5 V).
For instance, Mokhtari et al. [158] introduced a polar inorganic salt LiCl to PVDF and found that the nanofiber mats with LiCl showed increased elongation at break and -phase content.A maximum electric output voltage of 3 V and output current of 0.5 μA with a power density of 0.3 μW cm −2 at the frequency of 200 Hz was achieved.Another example reported by Varposhti et al. [164] indicated that the piezoelectricity of the PVDF nanofibers can be improved by adding an ionic liquid.In their study, three different samples containing 0.25, 1, and 4 wt% of ionic liquid were prepared, and the one with 4 wt% of ionic liquid showed ≈98.6% -phase content, resulting in a maximum of ≈7.89 V under 125 mN pressure force.Apart from the above-mentioned materials, metal-organic frameworks with a non-centrosymmetric crystal structure have also exhibited great potential in improving the piezoelectricity of polymers due to their large specific surface area, tunable pore size, high porosity, and wide range of unsymmetrical ligands and metal ions available to choose. [80]Moghadam et al. [80] found that 5 wt% of microporous zirconium-based metal-organic framework could improve the piezoelectric coefficient of PVDF up to 248 mV mm −1 .The nanofibrous composite showed a V pp of 600 mV under an applied force of 5 N.
][168][169] For instance, PVDF nanofibers with 15 wt% nanoclay demonstrated significantly higher performance, i.e., 90% of the -phase (Figure 4d), V pp of 70 V (Figure 4e), and power density of 68 μW cm −2 . [168]Figure 4f illustrates the alignment and charge separation in PVDF nanofiber composites with nanoclay.-phase chains of PVDF aligned facing nanoclay with their CH 2 groups having a partial positive charge, whilst CF 2 groups having partially negative charge stay away from the nanoclay surfaces.The results also showed that the nanoclay could enhance the elastic modulus of composite nanofibers (Figure 4g).
Tables 2-4 summarize the piezoelectric performance of various recently reported piezoelectric nanocomposite fibers: Table 2 for PVDF, Table 3 for PVDF-TrFE, and Table 4 for PAN nanofibers.Firstly, it is shown that different types of fillers can be used to enhance the piezoelectric performance of polymer nanofibers.Secondly, the property improvement depends on a variety of factors including the filler type, concentration, surface functionality, etc.Although the property enhancement varies between different composite nanofibers, two general conclusions can be drawn: 1) There is an optimum concentration of fillers, in other words, the piezoelectricity increases upon increasing filler content to a certain level but mostly decreases when the content further increases.2) Hybrid nanofillers, in most cases, show more profound improvement as compared to single fillers.

Polymer Blending
Adding a second polymer has also been demonstrated as an effective way to improve the properties (piezoelectric and mechanical) of polymer nanofibers.173][174][175] Elnabawy et al. [171] investigated the piezoelectric and mechanical properties of PVDF/TPU mixture with various blending ratios (1:3, 1:1, and 3:1).The results demonstrated a significant improvement in mechanical properties by increasing the content of TPU.The failure strain of pure PVDF was 12.5%, while the maximum failure strain of PVDF/TPU was 85% at a ratio of 1:3.In contrast, the piezoelectric properties decreased with the increase of the TPU content.The maximum voltage for an impulse load of 300 gr was 1240 mV for neat PVDF, 830 mV for PVDF/TPU (3:1), 680 mV for PVDF/TPU (1:1), and 400 mV for PVDF/TPU (1:3).Another study by Sathiyanathan et al., [172] however, found that blending PVDF with TPU would improve the tensile strength without comprising their piezoelectricity.Apart from the elastic polymers, conducting polymers can not only enhance the mechanical properties but also the piezoelectric responses. [173]For example, Sengupta et al. [173] compared the output voltages of pure PVDF and a blend of PVDF and polycarbazole (PCZ) and found that the output voltage of pure PVDF was 0.37 V, which increased to 2.3 V for PVDF/PCZ.Moreover, by adding PCZ, Young's modulus of the samples increased from 17.1 to 20.3 MPa.However, elongation at break was reduced from 30% for pure PVDF to 24% after adding PCZ.

Structure Designs
Apart from designing the materials, researchers have devoted efforts to improving the mechanical and piezoelectric properties of fibrous-based nanogenerators and sensors by engineering their structures. [176,177]In this section, different structural designs including coaxial nanofibers, coated nanofibers, and some hierarchical structures are discussed.

Coated Nanofibers
Most of the nanofillers are incorporated into the polymer nanofibers by being dispersed directly into the polymer solution before electrospinning.The content of the nanofillers can be limited due to the high viscosity of the solutions when high loading of fillers is added, which results in low processability.][184][185][186] One study by Kim and Fan [186] showed that higher output (245.63 nW cm 2 ) was achieved for the PVDF nanofiber mat electrosprayed with ZnO NRs compared to those prepared by dispersing ZnO NRs into the PVDF solution before electrospinning (5.69 nW cm 2 ).Also, the ZnO NRs can be incorporated in other different ways, e.g., by pasting the ZnO NRs onto the collecting drum of the electrospinning machine for making PVDF nanofiber mats [170] or grow nanoparticles onto individual nanofibers. [184,185]It is shown that growing ZnO NRs on the PVDF nanofibers can enhance the sensitivity of the piezoelectric sensor to 3.12 mV kPa −1 , which was nearly 6 times higher than that of neat PVDF with a linear response over a broad range from 1.8 to 451 kPa.Moreover, the bending sensitivity (16.89V mm −1 ) was about 41 times higher than that of neat PVDF. [184]Additionally, apart from piezoelectric nanoparticles, Li et al. [185] coated electrospun PVDF-HFP nanofibers with conductive MWCNTs.A copper frame (used as a collector) floated in the MWCNT solution and nanofibers were collected on the copper frame while being coated by MWCNTs.The resultant nanofibers showed an output voltage of 0.62 V under 15 N, high bending stability over 10 000 bending cycles, as well as superior wearability, i.e., washability and breathability.Moreover, it is found that increasing the contact area between the nanofiber mat and the electrodes by coating conductive nanomaterials can help the charge transfer.This was demonstrated by a study that carbonpainted nanofiber mat can have a higher output voltage of 4.5 V and a current of 25 nA, compared to conventional nanofiber mats which only generate 1.6 V and 1.5 nA. [182]

Coaxial Nanofibers
][189][190][191][192] Coaxial electrospinning utilizes two concentric needles with different polymer solutions instead of a single nozzle utilized in uniaxial electrospinning.Through the combination of complementary polymers in the core and shell, nanofibers with better mechanical and piezoelectric properties can be achieved. [93]The key challenge to achieving high-quality core-shell nanofiber is to control the viscosity of the two solutions and their interfacial tension. [191]] It has been reported that, in the core-shell structure, the nanofibers are better aligned.Polymer density in neat nanofibers is lower than that of polymers in coaxial core-shell structure, which may be related to the strong interaction between core and shell layers.The mechanical properties of coaxial coreshell nanofibers are also related to polymer materials' chemical and physical interactions between the core and shell layers, the type of polymers, the degree of nanofiber alignment, and the nanofiber diameter.Overall, in coaxial core-shell structures, the higher degree of nanofiber alignment and more tightly packed structure contribute to a more significant enhancement in tensile properties. [176,193]Moreover, it was shown that coaxial structure enhances the -phase of PVDF and its copolymers, [150,176,188,189] which is due to the molecular interaction of the core and the shell of the nanofibers. [176,188]For example, Han et al. [176] fabricated a core-shell nanofiber, consisting of PVDF-TrFE as shell and PE-DOT:PSS as core, with different shell layer thicknesses.Their results indicated that the thinnest shell layer with the thickness ratio of shell to the core of 1:4 showed the best output voltage of ≈8.76 V, which was 10 times more than that of neat PVDF-TrFE nanofibers.Wang et al. [188] designed a core-shell nanofiber using silk fibroin as core and PVDF as shell (SF/PVDF).The contents of silk (7 wt%, 14 wt%, and 20 wt%) were controlled by the feed rate ratio of the two syringes.Results showed that the sample with 14 wt% silk exhibited the highest Young's modulus and the highest voltage of 16.5 V and current of 290 nA, which was more than sixfold that of PVDF nanofibers due to the increased -phase content.The schematic illustration showing the process of the coaxial electrospinning and the TEM images of the coreshell SF/PVDF nanofibers are given in Figure 5a,b, respectively.As illustrated in Figure 5c, silk fibroin has strong interactions with PVDF chains via hydrogen bonding, which enhances the -phase content, as indicated in FTIR spectra in Figure 5d. Figure 5e shows the output voltage of the SF/PVDF nanofibers tested at different frequencies.The output voltage increases upon increasing the frequency, which may be due to the increase in the speed of accumulation and evanishment of charges. [188]he core and shell can be both made of piezoelectric polymers. [93]One example is the core-shell nanofibers made of PVDF-TrFE and PVDF-TrFE-CTFE as the core and shell.The core-shell nanofibers of PVDF-TrFE-CTFE/PVDF-TrFE exhibited a much higher piezoelectric coefficient (50.5 pm V −1 ) than PVDF-TrFE (14.6 pm V −1 ) and PVDF-TrFE-CTFE nanofibers (30.2 pm V −1 ).Interestingly, even made of the same polymers, core-shell PVDF-TrFE/PVDF-TrFE-CTFE nanofibers showed a higher piezoelectric coefficient (50.5 pm V −1 ) than core-shell PVDF-TrFE-CTFE/PVDF-TrFE (15.4 pm V −1 ), which may be attributed to the presence of the crystalline RFE and FE domains with highly aligned dipole moments.
Coaxial electrospinning can also be applied to fabricate coreshell nanocomposite nanofibers.For instance, coaxial nanofibers consisting of PVDF/BTO as core and PVDF/GO as the shell were reported by Zhu et al. [190] The SEM and TEM images of (PVDF/0.1 wt% GO-PVDF/10 wt% BTO) nanofibers (Figure 5f) showed the surface and internal structures of nanofibers with nanofiller.The output voltage and piezoelectric coefficient were shown in Figure 5g,h, indicating the enhanced piezoelectricity as compared to neat PVDF.

Other Hierarchical Designs
Other hierarchical designs that consist of electrospun nanofibers as the key sensing elements have also been reported recently.][196][197][198][199] For instance, Ji et al. [57] fabricated a wearable energy harvester based on coreshell piezoelectric yarn.BNT-ST (0.78Bi 0.5 Na 0.5 TiO 3 -0.22SrTiO 3 ) and PVDF-TrFE nanofibers were firstly fabricated by electrospinning (Figure 6a).The nanofibrous layer was then manually twined around a conductive thread (Figure 6b) which acts as the inner electrode.The core-shell yarn was then braided with another string of conductive yarn, which acted as the external electrode (Figure 6c).Finally, the whole braided yarn was stitched to pieces of fabric with different lengths and stitching intervals.An output voltage up to 19.1 V during bending at 1 Hz was generated from the stitched yarn of 15 cm in length, 3 cm in width, and 0.15 cm intervals.Figure 6d,e shows that such yarn can be stitched onto gloves and insoles, respectively.A similar work by Gao et al. [194] presented a novel structured yarn consisting of a conductive core (a nylon filament coated by silver, as the inner electrode) with nanofibers of PVDF wrapping around the core.Then a layer of PVDF was coated on that yarn by passing it through a tank of PVDF.Finally, a silver layer was deposited, via electron-beam evaporation, on the yarn to act as the external electrode.Under cyclic compression of 0.02 MPa at a frequency of 1.85 Hz, the resultant yarn produced an average peak voltage of 0.52 V, a current of 18.76 nA, and a power density of 5.54 μW cm −3 , which is significantly higher than that of the yarn made solely from PVDF nanofibers.
In addition to the yarn structure consisting of a piezoelectric nanofiber mat winding around a conductive electrode, the  [188] Copyright 2021, Elsevier.f) TEMEM image, g) output voltage, and h) piezoelectric constant (d 33 ) of the core-shell piezoelectric nanofibers consisting of PVDF/0.1 wt% GO-PVDF/10 wt% BTO.Reproduced with permission. [190]Copyright 2020, Elsevier.Photograph of the piezoelectric yarn stitched on a glove and e) the shoe insole.Reproduced with permission. [57]Copyright 2019, Multidisciplinary Digital Publishing Institute (MDPI).f) Schematic of fabrication processes for piezoelectric nanofabric.Reproduced with permission. [197]Copyright 2021, Wiley-VCH GmbH.
piezoelectric nanofiber mat can also be twisted into yarns for piezoelectric fabrics. [197]Forouzan et al. [197] investigated the properties of PVDF and PVDF-TrFE woven fabrics produced from aligned nanofibrous yarns.Figure 6f shows the fabrication process of the twisted yarns and woven fabrics.It was found that, due to the high collection speed used during electrospinning, the -phase content in the aligned nanofibers was significantly higher than that of the randomly oriented nanofibers.Moreover, the piezoelectric response can be tuned by varying the linear yarn density, the twist per unit length, the thicknesses, and the density of the fabric.The highest voltage of 2.5 V and power density of 80 nW cm −2 were achieved for the yarn twist of 3000 twists per meter when subjected to a mechanical force of 280 mN.In the twisting process, a mechanical stretch was applied, resulting in the formation of -phase crystalline and enhanced strength.Therefore, yarns with higher twists per meter showed higher failure strength, modulus, and toughness.The enhanced interaction between nanofibers in the yarns results in a lower porosity and continuous geom-etry.As a consequence, higher mechanical properties were achieved.
Apart from the 1D yarn design, laminar configurations have also been reported. [200,201]Li et al. [200] designed a piezoelectric device based on a 3D multilayer assembly of PVDF nanofibers and beads.There are five layers including two layers of PVDF nanofiber mats coated with polypyrrole as the top and bottom electrodes, and two layers of CsPbBr 3 @PVDF beads electrosprayed onto the two sides of a neat PVDF nanofiber mat which was then sandwiched between two electrodes.Both the piezoelectric layers (PVDF nanofibers and CsPbBr 3 @PVDF beads) have high -phase content (83.5% and 94%, respectively).The globular CsPbBr 3 @PVDF beads enhanced the piezoelectric outputs with a V oc of 10.3 V and I sc of 1.29 μA under the pressure of 6 kPa and could also detect pressure as low as 7.4 Pa.This design yielded the elongation at break of 95.5%.Similarly, Sang et al. [201] created a three-layer structure based on flexible piezoceramic nanofibers, consisting of two piezoelectric energy harvester modules as the top and bottom layers and a triboelectric energy  (5, 10, 20, 30) of micropatterns at one electrode.c) Photographs of the actual PENG and its bending mode.d) Output voltages of PENGs with different 3D interdigital electrodes.Reproduced with permission. [87]Copyright 2019, Elsevier.e) The schematic of fabrication process of core-sheath fiber containing a copper core, piezoelectric PVDF-TrFE sheath, and the outer Cr/Au electrode.f) The voltage output and sensitivity of the core-sheath fiber under different forces.g) A photo of a piece of clothing stitched with the piezoelectric fiber, attached to a piece of garment.h) Detection of knee bending by the garment.Reproduced with permission. [196]Copyright 2022, Elsevier.
harvester module as the middle layer.The final hybrid module showed higher charging speed and superior durability and a maximum voltage of 253 V.It is worth noting that it takes only 40s for a 0.1 μF capacitor to be charged to 25 V.The device showed the capability to power 42 LED bulbs as well as a small fan.
Another unique design consisting of nanofibers deposited onto interdigital electrodes has recently been reported by Zhang et al. [87] An interdigital electrode was used to collect PVDF-TrFE nanofibers during electrospinning (Figure 7a).The 3D electrodes facilitated the stress excitation of the nanofibers and also increased the contact area with the nanofibers and thereby enhancing the electric outputs.The density of the microstructures of 3D interdigital electrodes can be easily adjusted, as shown in Figure 7b.When the size of interdigital electrodes was the same, the decreased perpendicular distance between micropat-terns and the increased densities lead to larger contact area between nanofibers and electrodes.Therefore, the output voltage increased (Figure 7d).By applying a mechanical force on a 1 cm 2 surface at 1.5 Hz, the maximum output voltage and power density of the PENG with a density of 20 reached about 50 V and 8.75 μW cm −2 , respectively.The devices can withstand bending, as shown in Figure 7c.Lu et al. [196] deposited electrospun PVDF-TrFE nanofibers directly onto a copper wire (inner electrode), followed by depositing a thin layer of Cr/Au (external electrode), then encapsulated with PDMS to form a fine fiber as illustrated in Figure 7e.The electrical output of the core-shell fiber was shown to be up to ≈ 900 mV, with a sensitivity of 60.82 mV N −1 (Figure 7f).This yarn can be stitched into a knitted fabric (Figure 7g) and then attached to clothing.The device could successfully detect knee joint bending, as shown in Figure 7h.

Parameters Influencing the Properties of the Electrospun Nanofibers
The properties of the polymeric nanofibers are dependent on a variety of parameters including polymer solution parameters, electrospinning parameters, and ambient parameters. [40,202,203]The influences of these parameters on nanofiber properties have been extensively studied.As an example, this section will summarize how these parameters affect the properties of PVDF and its copolymer nanofibers because they are the most popular piezoelectric polymers.
As discussed above, PVDF is a semicrystalline polymer with five polymorphs and the polar -phase has the highest dipole moment in each unit cell and contributes most to its piezoelectricity.During electrospinning, the high voltage and the high stretching ratio of the jet enables the PVDF instant dipole chains to line up in the direction of applied stress and as a result, the phase transformation happens by a strong and constant elongation flow in the polymer jet.This favors the formation of the -phase, leading to the higher piezoelectric property. [35]Electrospun fibers can form beads when the surface tension of the polymer solution changes.Fibers without beads can be formed by reducing the surface tension which can be controlled by selecting appropriate solvents.It should be noted that the nanofibers with smaller diameters possess a larger specific area, higher flexibility, and also a more sensitive response to mechanical stimulus.In addition, they have superior directional strength, making them an excellent material for energy harvesting. [6]

Solution Parameters
The viscosity, polarity, vapor pressure, and surface tension of the polymer solutions for electrospinning are critical factors, which are influenced by PVDF molecular weight, PVDF concentration, and type of solvent.

Molecular Weight
Zaarour et al. [83] investigated the effects of molecular weight on crystalline structure, surface characteristics, and piezoelectric as well as mechanical properties of PVDF nanofibers.Nanofibers from PVDF with a molecular weight of 180 000, 275 000, and 53 4000 g mol −1 were electropsun.Different -phase contents, i.e., 80.38%, 85%, and 88.84%, were obtained, respectively.As the molecular weight of PVDF increases, the viscosity of the polymer solution increases.Hence, longer time is required for the PVDF solution to flow from the nozzle to the collector.Therefore, the polymer jet has adequate time to evaporate the solvent, and the molecular chain of PVDF is highly stretched to form the  crystalline phase.It should be noted that to avoid possible clogging at the nozzle with a highly viscous polymer solution, a needle with a larger diameter is usually required for electrospinning PVDF with molecular weights of 275 000 and 53 4000 g mol −1 . [40]

Concentration
The polymer concentration has a proportional relationship with polymer solution viscosity, i.e., a higher concentration results in higher viscosity which leads to parabolically an increase and then a decrease in F().Moreover, beads are usually observed for low concentration solutions with relatively low viscosity.The concentration usually used in electrospinning PVDF is in the range of 10-25 wt%, which also depends on the polymer's molecular weight.For example, a lower concentration would be used for PVDF with a higher molecular weight. [40,202]

Solvent
The polarity and volatilization of the solvent are two key factors to consider when preparing PVDF nanofibers.A polar solvent such as dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and N-methyl-2pyrrolidone (NMP) with a proper amount of acetone is a good solvent for PVDF. [40]For example, Gee et al. [204] used three different solvents (DMF/acetone, DMSO/acetone, and NMP/acetone with a volume ratio of 6:4) to dissolve PVDF for electrospinning.It is found that the nanofibers produced by using DMF have the highest F() of 90.9% while 87.4% and 81.9% were achieved by using DMSO and NMP, respectively.Meanwhile, a proper amount of acetone plays an important role, facilitating the volatilization of the solvents and the -phase formation.Moreover, acetone can reduce surface tension due to its relatively low surface tension.Hence, the force applied by the electrical field can more easily overcome the solution's surface tension, resulting in more uniform nanofibers.However, an excessive amount of acetone would result in the formation of beaded nanofibers. [40]

Electrospinning Parameters
Nanofiber properties are largely dependent on electrospinning parameters, including applied voltage, tip-to-collector distance, flow rate, and collector type (metal plate, circle electrode, rotating drum, rotating disk, wire drum). [40,202]Table 5 summarizes the effects of these parameters on the morphology and properties of the nanofibers, which are also discussed in more details below.

Voltage
Firstly, studies have shown that there is an optimal voltage to generate a suitable Taylor cone to control the morphology, mechanical, and piezoelectric properties of nanofibers. [40,202,205]For example, Jiyong et al., [205] found that the optimum voltage for spinning PVDF nanofibers was 20 kV.A high electrical field is required so that a stretching force can be applied to align nanofibers.However, when the applied voltage exceeds the optimum value, the flying time is reduced and polymer chains do not have enough time to appropriately orient.Therefore, both the F() and the piezoelectric response decrease.

Feed Rate
Secondly, the flow rate of polymer solution has demonstrated deep impact on the formation of a Taylor cone, morphology, • Ease of use • Random fibers deposition • High nanofiber diameter [40,79,202]   Circle electrode collector (static) • High productivity and uniformity • Low F() • Random nanofibers deposition • High nanofiber diameter Drum collector (dynamic) • Ease of use • Ease of use • Highly aligned nanofiber • Challenging to produce thick nanofiber mats degree of crystallinity, and F() of the nanofibers.Some reports suggest that the average diameter of PVDF nanofibers increases when flow rate increases because the stretch of the electrospinning jet decreases.In addition, high flow rate results in low crystallinity due as the solvent does not have enough time to evaporate.F() was found to be higher at a low flow rate because of the higher stretching of jet. [40,82,202]However, some contradictory results have been reported in the literature.For example, Singh et al. [82] demonstrated an increase in the -phase content when increasing the flow rate.They attributed this to an amplified shear force by the needle on a viscous polymer solution at high flow rates.In addition, it was found that the F() increased until a threshold value (2 mL h −1 ) and then decreased.Therefore, an optimum flow rate is necessary to generate an appropriate Taylor cone and form high-quality uniform PVDF nanofibers without beads.

Distance between Tip-to-Collector
Most studies have shown that the relationship between tip-to-the collector distance (DTC) and F() is not linear.This is because DTC has two contrary effects on nanofibers formation: the increase of DTC leads to an increase in time needed to travel from the tip to the collector so that it has adequate time to stretch and evaporate the solvent.On the other hand, with the increase of DTC, the electrical force decreases, which is unfavourable for fabricating high-quality PVDF nanofibers. [40,202,206]Zheng et al. [206] demonstrated that when the DTC increased from 10 to 20 cm, the amount of F() decreased.Generally, the DTC is set to be around 15 cm.

Type of Collector
Lastly, the type of collector also affects the structure and properties of PVDF nanofibers.Static and dynamic collectors are the two main categories of collectors.The dominant force in the static collector is Coulomb forces which are exerted by the applied voltage.In contrast, when dynamic collectors are used, in addition to Coulomb forces, the mechanical stretch forces are applied, which align and orient the fibers to the longitudinal direction of fibers. [40,79,202]Therefore, dynamic collectors with a proper speed result in higher F() in the nanofibers. [40]For example, Wu et al. [79] studied the effects of rotating collectors and rotating speed on the content of -phase in PVDF nanofibers and found raising the rotating speed to 1500 rpm can apply more stretching and elongation force on nanofibers and produce aligned nanofibers.Meanwhile, the -phase content enhances from 84% to 84.96%.However, a further increase in rotating speed to 2000 rpm results in a decrease in the -phase content to 78% due to the high stretching force resulting in breakage and fraction in nanofibers.

Ambient Condition
Due to the difficulty in controlling the ambient parameters, limited studies have been reported on the effects of the environmental parameters on the electrospun piezoelectric nanofibers.However, some have found that temperature and relative humidity can affect the -phase formation in the PVDF nanofibers. [40,202]

Temperature
The increase in working temperature reduces the solution's surface tension and viscosity and increases the volatilization rate of the solvent.Some studies showed that higher temperatures could lead to the formation of nanofibers with thinner diameters.However, at very high temperatures (e.g., ≈ 60 °C), the  phase is more likely to form than the -phase.Therefore, a temperature of ≈ 25 °C is usually suggested. [40,202]

Relative Humidity (RH)
RH mainly affects solvent volatilization, which affects the phase formation and the surface morphology of the fibers.Under low humidity conditions, the faster solvent evaporation leads to the disruption of the traveling jet.It was shown that higher RH (≈ 60-70%) could lead to smoother nanofibers with higher -phase content. [40,202]n summary, all the different parameters (Figure 8) discussed above have deep impacts on the morphology, formation of phase, and thereby piezoelectricity of PVDF nanofibers.Each of them has an optimum range and selecting one parameter often depends on other parameters chosen for the process.For example, as stated before, a lower concentration should be adopted for polymers with a higher molecular weight.Moreover, when using a high electrospinning voltage, it is encouraged to increase the tip-to-collector distance.

Applications of Piezoelectric Nanofibers
Electrospun piezoelectric polymer nanofibers have demonstrated great potential in sensor applications [119,167,188,190] or energy harvesters. [135,186,194,195,197,200,201]This section introduces some examples.

Wearable Sensors
Wearable sensors have demonstrated broad diagnostic and monitoring (health and sports performance) applications.Piezoelectric sensors can detect mechanical deformations such as pressure [137,186] and stretch [31] associated with various body movements and activities.Examples include joint bending, [188,190,210] respiration, [25,41,189] facial expression, [41] swallowing force, [44] and human voice recognition. [95,100]For instance, Yang et al., [184] demonstrated a 3D hierarchically interlocked piezoelectric sensor by growing ZnO NRs onto an electrospun PVDF nanofiber surface as shown in Figure 9a.The sensor can detect wrist pulse and muscle movements by placing it on three parts of the calf muscles, including gastrocnemius, soleus, and anterior tibias.Deformations of these three muscles occur and can be detected while walking forward, left, and right.A system for detecting gait has been successfully developed, as illustrated by the schematic diagram in Figure 9b. Figure 9c shows signals of the anterior tibias, gastrocnemius, and soleus to detect gait events during walking forward, left, and right.Figure 9d shows the output signals of the left leg when walking left while Figure 9e illustrates the ratio of calf response amplitude (right leg) to the other calf response amplitude (left leg) in different walking directions.The sensors are also placed on other parts of the body (Figure 9f) to study the feasibility in detecting breathing patterns and wrist pulses.Figure 9g and h indicate that different breathing modes including normal breathing, deep breathing, and gasping, as well as pulse frequency can be successfully measured.The expanded pulse wave with detailed tidal wave (T-wave), percussion wave (Pwave), and diastolic wave (D-wave) (Figure 9i) can be used in clinical application, including the detection of hypertension and diagnosis or prevention of cardiovascular disease.
Another example is sensors based on core-shell piezoelectric nanofibers reported by Li et al. [189] High sensitivity and accuracy in detecting physiological mechanical deformations from diaphragm and heart pulses have been demonstrated by placing the sensor on neck, chest, and wrist, as shown in Figure 9j. Figure 9k indicates that the systolic peak (P1) and dicrotic wave (P2) can be detected, which are important to the diagnosis of arterial stiffness, coronary artery disease, and myocardial fraction.Moreover, the sensor was implanted inside the mouse body (on the diaphragm, around the femoral artery) to study the in-vivo application potential (Figure 9l).The sensors were utilized to monitor the diaphragm motion and arterial pulsation, from which, it is possible to identify the breath patterns and arterial stiffness at different physiological states, from anesthesia to overdose anesthesia, and then to euthanasia.The results (Figure 9m) show that as the physiological states transferred from anesthesia (green) to overdose anesthesia (red), respiration and heart pulses were reduced.The same trend is observed in the following continuous overdose anesthesia state, which indicates the potential application of this sensor.

Energy Harvesters
Piezoelectric nanofibers can be used to harvest the biomechanical energy associated with movements of human body.Walking, [211] talking, [212] breathing, [213] and heartbeat [214,215] are a few examples.In addition, other forms of mechanical energy such as airflow and vibration can also be harvested by piezoelectric nanofibers. [216]part from high output, another prominent feature of nanogenerators is their applicability in flexible and stretchable electronics.][219][220][221] For example, Ji et al. [201] designed a piezo-triboelectric device based on BNT-ST/PVDF-TrFE (0.78Bi 0.5 Na 0.5 TiO 3 -0.22SrTiO 3 /PVDF-TrFE) nanofibers (Figure 10a).The hybrid energy harvesting module, consisting of piezoelectric and triboelectric layers can charge the 10 μF capacitor up to 6.5 V in 100 s as shown in Figure 10b.Figure 10c shows the charge-discharge behavior of a 0.1 μF capacitor and it is seen that the capacitor can be charged up to 25 V in ≈ 40 s.The device can successfully power 42 LEDs (Figure 10d).Moreover, after charging a 10 μF capacitor for 250 s, it is possible to power a mini fan with a 3.5 W power consumption (Figure 10e).Venkatesan et al. [222] developed a piezoelectric energy harvester using PVDF nanofibers doped with the optimum amount of inorganic perovskite quantum dots and cellulose nanocrystals as an active layer that was sandwiched between aluminum foils as electrodes.The nanogenerator was used in the charging/discharging circuit connected to an LED.The finger tapping can produce 1.6 V kPa −1 outputs.Moreover, fist beating in which a pressure of 24.97 kPa was applied can generate a 20.3 V output voltage (Figure 10f).The power density up to 29.0 μW cm −3 was achieved when load resistance of 8 MΩ was used, as can be seen in Figure 10g.
The harvested energy is considered as one of the potential renewable energy sources to replace conventional batteries.For example, He et al. presented a wearable piezoelectric-driven selfpowered patterned electrochromic supercapacitor. [223]Piezoelectric materials have a relatively low power output, so a rechargeable battery or supercapacitor is needed for storing the harvested energy.To be able to accumulate more energy, it is possible to connect storage cells in series to enlarge the voltage range.The storage device voltage plays a key role in energy harvesting efficiency.To connect the piezoelectric device to the storage cell, a rectification circuit is needed.As the current source is alternating, a diode bridge rectifier is generally utilized as an AC-DC rectifier. [224]

Conclusion and Future Perspectives
In conclusion, this paper reviews recent advances in the development of flexible, high-performance piezoelectric polymer nanofibers with a focus on the latest advances in technologies to enhance their piezoelectric and mechanical properties.Figure 11 summarizes the key aspects associated with the development of high-performance piezoelectric polymer nanofibers that have been covered in this review.Although great progress has been made in this burgeoning area, there are still challenges for future developments and some representative ones are discussed below.

Low Piezoelectricity
As mentioned before, piezoelectric polymers have lower piezoelectric coefficient than piezoelectric ceramics.However, The ratio of calf response amplitude of the right leg to that of the left leg in different walking directions.f) An illustration of the pressure sensor assembly on the chest (I), wrist (II), and three calf muscles (III).g) The electrical outputs of different breathing patterns (normal breathing, deep breathing, and gasping).h) A healthy person's wrist pulse.i) The expanded pulse wave showing three peaks, i.e., P-wave, T-wave, and D-wave.Reproduced with permission. [184]Copyright 2020, Elsevier.j) Schematic of the PVDF-based sensor attached to chest, neck, and wrist and k) the corresponding output voltage.l) Schematic for in vivo applications of PVDF-based sensors inside mice body and m) output voltage signals induced by diaphragm motions and blood pulsing in different physiological states, including i: anesthesia; ii: overdose anesthesia; iii: continuous overdose anesthesia.Reproduced with permission. [189]Copyright 2021, Wiley-VCH GmbH.e) The photograph of the mini fan with a power consumption of 3.5 W powered by the energy harvester device.Reproduced with permission. [201]Copyright 2020, American Chemical Society.f) The maximum output voltage of the nanogenerator under fist beating.g) Output voltage and power density of nanogenerator under different load resistances.Reproduced with permission. [222]Copyright 2020, Elsevier.piezoelectric polymers show advantages such as flexibility, stretchability, low density, and ease of fabrication.Therefore, the use of piezoelectric polymers in wearable electronics is auspicious.New methods to improve the piezoelectric properties of polymer nanofibers are vital for broader applications.As discussed in Section 3, different strategies have been investigated to enhance the properties of the piezoelectric nanofibers.Nanofillers can enhance both the mechanical and piezoelectric properties of polymer nanofibers.However, further investigation is needed to determine how the fillers can be incorporated to maximize the enhancement.Although it is known that the structure of the polymer nanofiber mats can be tuned to optimize the performance, the structure-property relationship is not well-understood and worth further exploration.

Stretchable Electrodes for Piezoelectric Devices
Electrodes play a vital role in the piezoelectric device.Besides high electrical conductivity and high flexibility and stretchability, minimum resistance variation under static and dynamic tensile strains is critical for the practical applications of electrodes for stretchable piezoelectric devices.Two key strategies based on stretchable materials and structural designs have been applied to develop stretchable electrodes.Conventional conductive materials such as metals are primarily rigid and brittle.Stretchable structures are therefore widely utilized to improve their stretchability.Some commonly used structures include serpentine, [225,226] wavy, [227] coiled, [228] and kirigami. [229,230]Porous sponge networks have also recently been reported. [231]Changing the electrode shapes can stabilize the electrical conductivity during the stretching process in this method.However, complicated processes (such as photolithography) are required for patterning, limiting their applications in electronic devices. [232,233]Therefore, tremendous efforts have been devoted to creating new high-performance stretchable electrodes based on stretchable materials.[236][237][238][239] Despite these advances, challenges remain, including 1) relatively low electrical conductivity of the electrodes, 2) considerable variation of electrical conductivity under deformation, and 3) difficulty in seamlessly combining the stretchable electrodes with the active polymer nanofiber sensing layer.These need to be addressed to develop stretchable piezoelectric devices. [226,240,241]

Practical Application and Integration of Wearable Devices
Even though significant research has been done to develop and optimize the efficiency of wearable piezoelectric sensors, there are challenges to utilizing sensors in wearable accessories and garments for real-world applications.One of these challenges is that wearable devices need to be directly in contact with the skin or used regularly.The environment, such as mechanical deformation, sweat, temperature, humidity, and oxygen, may affect the sensing performance.Therefore, it is required to consider the stability of wearable devices under different environmental conditions.Other challenge includes achieving functionalities such as biocompatibility and biodegradability which are necessary to extend sensor applications.Moreover, integrating wearable sensors with the energy storage and data collection and processing units is a significant challenge.

Figure 2 .
Figure 2. Timeline of the progress in developing piezoelectric nanofibrous devices.

Figure 3 .
Figure 3. a) Schematic illustration, the optical and SEM image of the PVDF/ZnO nanofiber composites.b) The output voltage and c) current of ZnO/PVDF nanocomposite with different contents of ZnO.Reproduced with permission.[137]Copyright 2019, Elsevier.d) Schematic illustration of the interfacial interaction between BaTiO 3 and PVDF matrix after being modified by polydopamine.e) Output voltage, and f) output current of nanocomposite fibers with different concentrations of PDA under a mechanical force of 3 N. Reproduced with permission.[41]Copyright 2021, Elsevier.g) Schematic illustration of the -phase formation in ZnO@C/PVDF nanofiber.h) The output voltages of nanocomposite nanofibers containing different contents of ZnO@C.Reproduced with permission.[144]Copyright 2021, Elsevier.

Figure 4 .
Figure 4. a) SEM images and schematic illustration of fibers of i: neat PVDF-TrFE and its composites containing ii: 3 wt% MWCNTs, and iii: 9 wt% MWCNTs.b) The output voltage and c) tensile stress-strain curves of the PVDF-TrFE/MWNCTs nanocomposite with different fractions of MWNCTs.Reproduced with permission.[90]Copyright 2019, Elsevier.d) The phase fraction and e) the output voltage of the PVDF/nanoclay nanofibers.f) Schematic illustration of the charge separation mechanism after adding nanoclay.g) Stress-strain curves of neat PVDF-TrFE nanofibers (P) and its nanocomposite fibers (C15) with an inset bar chart showing their modulus and toughness.Reproduced with permission.[168]Copyright 2019, Elsevier.

Figure 6 .
Figure 6.a) Photograph and SEM image of the electrospun nanofiber mat.b) Schematic illustrating the inner electrode (conductive yarn) wrapped with nanofibers, c) Schematic and photograph of the design of yarn-based piezoelectric generator braided with outer electrode (conductive yarn).d)Photograph of the piezoelectric yarn stitched on a glove and e) the shoe insole.Reproduced with permission.[57]Copyright 2019, Multidisciplinary Digital Publishing Institute (MDPI).f) Schematic of fabrication processes for piezoelectric nanofabric.Reproduced with permission.[197]Copyright 2021, Wiley-VCH GmbH.

Figure 7 .
Figure 7. a) The fabrication process of PENG based on a 3D interdigital electrode.b) 3D interdigital electrodes with different densities(5, 10, 20, 30) of micropatterns at one electrode.c) Photographs of the actual PENG and its bending mode.d) Output voltages of PENGs with different 3D interdigital electrodes.Reproduced with permission.[87]Copyright 2019, Elsevier.e) The schematic of fabrication process of core-sheath fiber containing a copper core, piezoelectric PVDF-TrFE sheath, and the outer Cr/Au electrode.f) The voltage output and sensitivity of the core-sheath fiber under different forces.g) A photo of a piece of clothing stitched with the piezoelectric fiber, attached to a piece of garment.h) Detection of knee bending by the garment.Reproduced with permission.[196]Copyright 2022, Elsevier.

Figure 8 .
Figure 8. Summary of the parameters influencing the morphology, phase formation, and the piezoelectricity of the PVDF nanofibers.

Figure 9 .
Figure 9. a) Schematic showing the microstructure of the core-shell PVDF/ZnO nanofibers.b) Schematic diagram of the gait recognition system: gastrocnemius (GAST), soleus (SOLE), and anterior tibias (ANT TIB).c) ANT TIB, SOLE, and GAST signals for detecting i. forward, ii.right, and iii.left gait states, respectively.d) Signal outputs when walking left with the left leg.e)The ratio of calf response amplitude of the right leg to that of the left leg in different walking directions.f) An illustration of the pressure sensor assembly on the chest (I), wrist (II), and three calf muscles (III).g) The electrical outputs of different breathing patterns (normal breathing, deep breathing, and gasping).h) A healthy person's wrist pulse.i) The expanded pulse wave showing three peaks, i.e., P-wave, T-wave, and D-wave.Reproduced with permission.[184]Copyright 2020, Elsevier.j) Schematic of the PVDF-based sensor attached to chest, neck, and wrist and k) the corresponding output voltage.l) Schematic for in vivo applications of PVDF-based sensors inside mice body and m) output voltage signals induced by diaphragm motions and blood pulsing in different physiological states, including i: anesthesia; ii: overdose anesthesia; iii: continuous overdose anesthesia.Reproduced with permission.[189]Copyright 2021, Wiley-VCH GmbH.

Figure 10 .
Figure 10.a) SEM image of PVDF-TrFE and BNT-ST nanofibers and schematic illustration of the hybrid energy harvesting module (HEHM).b) Charging time for a 10 μF capacitor by the fabricated piezo-, tribo, and hybrid energy harvesting modules.c) Charge and discharge behavior of a 0.1 μF capacitor by hybrid energy harvesting module.d) The photograph of 42 LEDs powered by the energy harvester device in real-time.e)The photograph of the mini fan with a power consumption of 3.5 W powered by the energy harvester device.Reproduced with permission.[201]Copyright 2020, American Chemical Society.f) The maximum output voltage of the nanogenerator under fist beating.g) Output voltage and power density of nanogenerator under different load resistances.Reproduced with permission.[222]Copyright 2020, Elsevier.

Figure 11 .
Figure 11.Key aspects associated with the development of highperformance piezoelectric nanofibers.

Table 2 .
Piezoelectric performance of some recently reported PVDF composite nanofibers.

Table 3 .
Piezoelectric performance of some recently reported PVDF-TrFE composite nanofibers.

Table 4 .
Piezoelectric performance of some recently reported PAN composite nanofibers.

Table 5 .
A summary of the effects of electrospinning parameters on fiber properties.