Nanofiller‐Induced Enhancement of PVDF Electroactivity for Improved Sensing Performance

Piezoelectric self‐powered sensors are promising platforms for wearable portable devices. Poly(vinylidene fluoride) (PVDF) and its copolymer derivatives are extensively explored as a soft piezoelectric material owing to their high piezoelectric coefficient, chemical thermal stability, biocompatibility, lightweight, and excellent flexibility. It is proved that the dominance of the electroactive (EA) β‐phase crystals versus the non‐electroactive α‐phase crystals is one of the key parameters to obtaining high piezoelectric performance of PVDF. Conventional methods, such as mechanical stretching, electrical poling, and high‐temperature annealing, are investigated to enhance the fraction of the β‐phase. Recently, embedding nanoscale fillers in the PVDF matrix has been investigated to further increase the β‐phase fraction and achieved considerable advances. The introduction of nanofillers is also advantageous in terms of improving the electrical conductivity and dielectric properties of PVDF, which are not readily obtained through conventional methods. This review introduces the principles of EA phase transformation in the presence of nanofillers and summarizes recent advances achieved by introducing various fillers, such as perovskites, oxide semiconductors, and 2D chalcogenides. The potential sensor applications of the PVDF nanocomposites responding to temperature, light, acoustic, and mechanical stimuli are reviewed. This review ends with the outlook of this new approach.


Introduction
In recent years, self-powered wearable sensors that can monitor external stimuli, such as pressure, [1] strain, [2] vibration, [3] temperature, [4] light, [5] sound, [6] and electromagnetic field, [7] have received intensive attention due to their important role in medical diagnosis, [8] human health care monitoring, [9] human motion sensing, [10] and robotics applications. [11] Considerable efforts have been employed to find new materials and sensing DOI: 10.1002/adsr.202200080 mechanisms. [12,13] In this regard, piezoelectric materials have been gaining increasing interest owing to their high sensitivity to subtle vibration, fast response, excellent durability, and most importantly operability in a self-powered manner. [14][15][16] Piezoelectricity is determined by the polarization of the material, originating from the separation of constituent electrical charges, in which the net polarization is changed upon an external mechanical stimulus. To date, several inorganic piezoelectric materials, such as lead zirconate titanate (PZT), BaTiO 3 (BTO), ZnO, CdS, and GaN have been investigated for their application in wearable electronics. [17][18][19] However, their mechanical rigidity, toxicity of the constituent elements, and high-cost processing route have restrained their implementation in wearable sensors. Organic piezoelectric polymers are advantageous in terms of mechanical flexibility, biocompatibility, and cost-effectiveness in production. Poly(vinylidene fluoride) (PVDF), which has a molecular formula of ─(CH 2 -CF 2 ) n ─, is a well-known piezoelectric polymer having a low melting point (≈177°C) and a relatively high room temperature polymer density (1.77 g cm −3 ). [20,21] Moreover, extraordinary flexibility, mechanical strength, high dielectric constant, chemical resistance and weather resistance, and high thermal stability make it a unique polymer to be used in wearable electronics. [22] Besides, the piezoelectric behavior of PVDF is also known to bring pyroelectricity and other nonlinear optical properties. [23,24] The piezoelectric performance of PVDF depends on the net polarization originating from the alignment of the dipoles in the same direction: [25] the higher the polar phase, the better the response. Since commercially available PVDFs often suffer from a low fraction of the electroactive (EA) phase, [26] the development of PVDF copolymers has been widely investigated to improve polarization. Although the unit cell of the copolymer crystal shows less polarity than those of pure PVDF, some copolymers exhibit improved crystallinity, which leads to higher piezoelectric responses. [27] Poly(vinylidene fluorideco-hexafluoropropylene) (P(VDF-HFP)), poly(vinylidene fluorideco-trifluoroethylene) (P(VDF-TrFE)), poly(vinylidene fluoride-cotetrafluoroethylene) (P(VDF-TFE)), and poly(vinylidene fluorideco-chlorotrifluoroethylene) (P(VDF-CTFE)) are commonly-used Figure 1. Scheme of the chemical structure and the corresponding unit cells of the -, -, and -phases of PVDF. Reproduced with permission. [52] Copyright 2021, American Institute of Physics.
PVDF copolymers. These copolymers enhanced many other properties such as stability, elasticity, melting point, permeability, chemical reactivity, and glass transition temperature. However, in the present article, we have mainly focused on pure PVDF-based systems.
Over the years, different techniques have been developed to increase the proportion of the electroactive phase in PVDF including mechanical stretching, [28][29][30] high pressing, [31] thermal annealing, [32] fast cooling, [33,34] electrical poling, [35,36] and nanofiller incorporation. [37,38] Among the various methods, the transformation to the electroactive phase enhanced by nanofiller incorporation in PVDF has been intensively studied in recent years because the process is effective and easy for large-scale production. Moreover, it has been observed that the incorporated nanofillers can induce several unique sensing properties, for instance, temperature and light sensing, which are not achieved through the other conventional processes. [39,40] In addition, micropatterning of the PVDF composites with different nanofillers requires different etching conditions. Such conditions can be found in several literatures. [41,42] Several review articles have been published on PVDF-based piezoelectric materials and their applications. For example, Chen et al. [43] and Bae et al. [44] reviewed the PVDF-based electronic sensors, nanogenerators, and actuators. Lu et al. [45] and Pusty et al. [46] reviewed the PVDF-based composite materials for energy harvesting applications. Very recently, Gebrekrstos et al. reviewed the phase transformation in PVDF for application in energy harvesting, electromagnetic interference shielding, and membrane applications. [47] Despite those several reviews, the specific interaction mechanism between the fillers and the host polymer for the formation of the electroactive phase has not been reviewed clearly and improvement of the sensing performance resulting from the filler-induced electroactive phase has not been reviewed.
In this review article, we have focused on the mechanism of the electroactive phase transformation in PVDF through the incorporation of various nanofillers and the experimental characterization techniques of the phase transformation. The correlation between the enhanced electroactive phase formation with the improved sensing performance for various external stimuli (mechanical, acoustic, optical, and thermal changes) has been discussed in detail together with recent advancements.

Crystal Phase Characterization of PVDF
It is well documented that PVDF exists in five different crystalline phases ( , , , , and ). [48][49][50] The -, -and -phases are the commonly observed polymorphs, and the other two phases ( and ) are difficult to isolate through conventional processing methods. [51] The molecular structures and the unit cells of the -, -, and -phases are depicted in Figure 1. [52] Since the -phase takes an alternating trans-gauche (TGTG′) conformation, [53] making the molecular dipoles packed anti-parallel in the crystal unit cell, the net dipole moment is zero, thus non-electroactive (NEA). The -and -phases have a TTTGTTTG′ and TTTT conformation, respectively. They have parallel dipole arrangements in the unit cell, producing a non-zero dipole moment, thus electroactive in nature. The all-trans (TTTT) conformation of thephase leads to higher dipole moments (8 × 10 −30 C m) compared to the -phase. In addition, it has all the molecular chains arranged in a particular direction of a crystal unit cell, which brings the alignment of the dipoles along the crystallographic b-axis with a spontaneous polarization of ≈130 mC m −2 in the unit cell. [54][55][56] Consequently, the -phase is considered as a desirable phase for obtaining enhanced piezoelectric and pyroelectric properties. Crystallinity and the crystal phase of PVDF are commonly characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy is also employed to quantify the relative fraction of the -, -, and -phases ( Figure  2). [26,57] This section briefly introduces the characterization by the above-mentioned techniques.
The distinct XRD fingerprints of the -, -and -phases of PVDF are shown in Figure 2a. The -phase exhibits a main characteristic peak at 19.90°with a pair of peaks at 17.66°and 18.30°, and a broad peak at 26.56°. [58] The -phase shows a subtle shift of the main characteristic peak to 20.04°and the pair of the peaks in the -phase merge to form a single broad peak centered at ≈18.50°. The XRD pattern of the -phase is noticeably different. It shows a broadened characteristic peak at 20.26°, as well as a weak shoulder peak at a lower angle of around 18.50°. [59] The intensity ratio of the peaks at 20.26°and 18.3°(I 20.26°/ I 18.3°) is obtained from the XRD pattern to calculate a relative fraction of the -phase (F ) in PVDF, thus XRD analysis serves as an effective way for qualitatively distinguishing the -phase. [48,60,61]  Reproduced with permission. [26] Copyright 2014, Elsevier Science Ltd. c) Raman. Reproduced with permission. [57] Copyright 2014, SPIE.
FTIR spectroscopy is widely used to provide valuable information about the crystal phase ( Figure 2b). [48,62,63] Existence of the vibration bands at 766, 795, and 976 cm −1 denote the non-polar -phase. [63] These peaks are related to the CCC skeletal vibration, the in-plane bending or rocking vibration, and the CH 2 rocking vibration, respectively. The peaks at 833 and 1234 cm −1 are the signatures of the -phase. The electroactive -phase has two main characteristics peak at 840 and 1279 cm −1 . [64] The peak at 840 cm −1 originates from the CH 2 rocking vibration, whereas the peak at 1279 cm −1 is from the C─F stretching vibrations. [49] The relative fraction of the -phase (F ) can be qualitatively calculated using Equation (1) [33] where A and A represent the absorbance of the bands at 766 and 840 cm −1 , featuring the -and -phases, respectively. K and K indicate the absorption coefficients at the respective wavenumbers having the values of 6.1 × 10 4 and 7.7 × 10 4 cm 2 mol −1 , respectively. [65] Many authors have argued that Equation (1) cannot determine the exact proportion of thephase due to the overlapping of the 840 cm −1 peak with the 833 cm −1 peak appearing from the -phase. [66] Therefore, it is recommended that the equation should be modified to determine the total fraction of the electroactive phases ( -and -phases combined). In this modification, the total electroactive fraction (F EA ) is expressed using Equation (2) [67] F EA = I EA ( where I ΝΕΑ and I EA represent the intensity of the NEA 766 cm −1 band and the EA 840 cm −1 band, respectively. Followed by the works of Constantino et al. [68] and Riosbaas et al., [57] Raman spectroscopy has been accepted as an alternative method for discriminative determination of the crystal phases in PVDF. The Raman fingerprints of the -and -phases are shown in Figure 2c, which shows clear differences in the spectra. The region of 700-900 cm −1 is the main fingerprint of the crystal phases. [68] The -phase PVDF shows a sharp peak at 794 cm −1 , which is due to the rocking of the CH 2 bond, whereas the -phase corresponding to the CH 2 rocking exhibits a shift to 839 cm −1 . Moreover, an additional peak at 812 cm −1 is responsible for the -phase. [69] The relative fraction of the -phase can be obtained through the following equation [57]

Filler-Induced Phase Transformation in PVDF
The -phase of PVDF is desirable for high-performance electroactive responses. The addition of nanofillers in the PVDF matrix has been recently considered to be an effective approach to increase the fraction of the -phase because of the methodological simplicity and a variety of material choices. The nanofillers provide nucleation sites for the formation of the -phase crystals. As illustrated in Figure 3, pure PVDF is predominated by the -phase, hence electrical poling under a high external electrical field is necessary to orient the dipoles. [70] Unfortunately, depolarization takes place as soon as the electric field is turned off or gradual depolarization happens by the relaxation of the dipoles. On the other hand, surface charges and different chemical bonds of the nanofillers can interact with the dipoles (─CF 2 ─/─CH 2 ─) and ions of the polymer so that the dipoles can self-orient themselves without external field and generate a net dipole moment of the PVDF film. This section reviews the mechanism of the selfpolarization and characterization of the structural change.

Mechanisms of the Nanofiller-Induced Phase Transition
The interaction between the nanofillers and PVDF can be divided into two processes; electrostatic interaction and hydrogen bonding (Figure 4). [71,72] Sultana et al. reported enhanced piezoelectric properties by blending methylammonium lead iodide (CH 3 NH 3 PbI 3 :MAPI) perovskite and PVDF. [71] The electrostatic interaction between the negative charge of the PbI 3 − framework and the ─CH 2 ─ dipoles of PVDF is responsible for the nucleation of the -phase (Figure 4a). High -phase content was also Figure 3. Scheme of conventional polarization by electrical polling and nanofiller-induced self-polarization. The aligned dipoles by electric poling relax to the initial random alignment, whereas the alignment of the self-polarized dipoles by the electrostatic interaction with the nanofillers is locked. Reproduced with permission. [70] Copyright 2022, Royal Society of Chemistry.
obtained by incorporation of 2H-MoS 2 nanosheets. [73] It was observed that the electrostatic interaction of the negatively charged MoS 2 surface with the ─CH 2 ─ dipoles facilitate the formation of the -phase in the nanocomposites. Nucleation of the -phase also takes place through hydrogen bonding. Hydrogen bonding has been found to have more influence on the phase transformation of PVDF due to the stronger interaction. For example, Sasmal et al. found that hydroxylated BiFeO 3 (BFOH) has much more influence than the non-hydroxylated BiFeO 3 (BFO) in the phase transformation of the polymer. [74] The hydrogen bonding between the ─OH group of the BFOH surface and the ─CH 2 ─ dipole of PVDF was found to be much stronger than the electrostatic interaction between the BFO and the polymer, thus leading to higher polar phase content in the presence of BFOH than that of the BFO. Ghosh et al. introduced the hygroscopic rare earth ytterbium (Yb 3+ )-salt in PVDF in order to achieve a higher content of the -phase. [72] Their results suggested that Yb 3+ absorbs the atmospheric moisture due to its hydrophilic nature and further surrounds F − of PVDF chains, allowing a strong hydrogen bonding between the water molecules and PVDF (Figure 4b). As a result, the Yb-salt acts as a nucleation site and the strong hydrogen bonds (O─H···F─C) at the interface between Yb 3+ and PVDF molecules lead to locally-oriented packing of ─CH 2 ─/─CF 2 ─ dipoles in the -phase in PVDF. Following this direction, Jana et al. also reported -phase formation induced by hydrogen bonds through the incorporation of hygroscopic Mgsalt (MgCl 2 ⋅6H 2 O). [75] In another study, Li et al. developed dopamine (DA)/PVDF nanocomposite (PN) via electrospinning, and they reported an increase in the piezoelectric performance. [76] They found that the increase in the -phase is due to the hydrogen bonding (between the ─NH 2 of DA and ─CF 2 of PVDF) and the dipole interactions. There are also several instances when both types of interaction occur between the nanofillers and PVDF. The incorporation of silk fibroin (SF) in the PVDF matrix could efficiently increase thephase content through the hydrogen bonding, as well as the electrostatic interaction between the SF core and PVDF surface. [77] The abundant surface functional groups (─COOH, ─OH, and ─NH 2 ) of MXene and graphene oxide (GO) also allow them to make both types of interaction with PVDF, leading to the enrichment of the -phase crystals. [78,79]

Nanofiller-Induced Structural Change and the Dielectric Properties of PVDF
In addition to the higher -phase content, the dielectric properties of PVDF also play a crucial role in determining the efficacy of PVDF-based electronic devices. The dielectric permittivity determines the charge-holding capacity and the dielectric loss indicates the power loss over the power recovered in each cycle. Therefore, high dielectric permittivity and low dielectric loss are essential for efficient energy conversion. [80] Incorporation of the nanofillers can effectively increase the dielectric permittivity, Figure 4. Schematic outlining of the formation mechanism of the electroactive -phase in PVDF. a) Example of electrostatic interaction between the MAPI perovskite and the polymer dipoles. Reproduced with permission. [71] Copyright 2018, American Chemical Society. b) Example of hydrogen bonding between hydrolyzed Yb-salt and PVDF. The water molecules form hydrogen bonding. Reproduced with permission. [72] Copyright 2016, Elsevier Science Ltd.
however higher concentration of the nanofillers can enhance the dielectric loss and hamper the stability and flexibility of PVDF. Therefore, it is required to investigate the structural changes and dielectric properties in the presence of optimized nanofiller concentration.
XRD and FTIR are effective in terms of phase determination according to the concentration and species of the nanofillers in PVDF. For instance, Xue et al. demonstrated the effect of CsPbBr 3 perovskite NPs concentration on the structure and the dielectric property of PVDF ( Figure 5). [70,81] The XRD ( Figure 5a) and FTIR (Figure 5b) of the nanocomposite exhibited the structural transformation with increasing CsPbBr 3 concentration (2-8 wt%). Their study suggested that CsPbBr 3 can efficiently facilitate the transformation of the -phase to the -phase without affecting the stability and flexibility of the host polymer. The peak intensity of the -phase grew with the increase of CsPbBr 3 concentration, and the peak shifted to a higher angle until the concentration reached up to 4 wt%. The proportion of the -phase decreased when the concentration was above 4 wt%, hence the -phase content reached the highest value of 94.6% at 4 wt% of CsPbBr 3 concentration. Agglomeration and precipitation of CsPbBr 3 at high concentration was considered to be responsible for the decrease in the -phase content. The dielectric permittivity of the CsPbBr 3 /PVDF composite increased with increasing CsPbBr 3 concentration because the difference in dielectric properties between the polymer matrix and the filler enhances the Maxwell-Wagner-Sillars interfacial polarization. [82] However, the dielectric loss of the composite also increased with the CsPbBr 3 concentration because of the increased conductive paths caused by the CsPbBr 3 agglomeration. Since a lower nanofiller concentration is more desirable for flexible electronic applications, massive research has been performed in order to investigate the phase transformation efficacy of various nanofillers. [47] The phase transformation in PVDF according to the types of nanofillers and the enhanced piezoelectric performance is summarized in Table 1.

Sensor Applications
PVDF-based piezoelectric materials have been proven to be useful for a variety of applications such as energy harvesting, actuators, transducers, EMI shielding, dielectrics, and membrane technology. [47,92] In this section, we focus on the applications in sensors that are responsive to external stimuli of pressure, strain, sound, light, and temperature. Special attention has been imposed on understanding the specific interaction between the nanofillers and PVDF, which is responsible for the enhanced sensing performance.

Mechanical Sensor
Wearable sensors based on piezoelectricity have evolved rapidly due to their ability to continuously monitor various physiological signals including pulse rate, [93] respiration rate, [94] blood pressure, [93] and fitness. [95] Figure 6a illustrates the mechanism of the PVDF-based mechanical sensors. [96] At the initial stage, polarization density is low inside the material due to the coincidence of cationic and anionic charge centers (Figure 6a(i)).  [81] Copyright 2020, Multidisciplinary Digital Publishing Institute. Filler-induced changes in b) XRD, c) FTIR, and d) dielectric properties of PVDF. Reproduced with permission. [70] Copyright 2022, Royal Society of Chemistry.  . Piezoelectric mechanical sensing applications. a) Scheme of the working mechanism of the PVDF piezoelectric sensor. Reproduced with permission. [96] Copyright 2019, Wiley-VCH. b) Open-circuit voltage from the sensor made of the core-shell ZnO/PVDF NFs under various pressures. c,d) Applications of the self-powered wearable sensor for real-time monitoring of breathing behaviors (e) and wrist pulse. e) The single wrist pulse signal shows the three clear peaks of P-wave, T-wave, and D-wave. Reproduced with permission. [97] Copyright 2020, Elsevier Science Ltd. f) Scheme showing the working principle with the piezoelectric-based vehicle passage sensor, which was composed of TiO 2 -ZnO/PVDF composite NFs. Reproduced with permission. [111] Copyright 2021, American Chemical Society. g) Piezoelectric voltage signals produced by blood pulsing and diaphragm movements during various physiological states of a mouse. Anesthesia is the first condition, followed by overdose anesthesia and persistent overdose anesthesia (for diaphragm: agonal respiratory; for femoral artery: articulo mortis). Reproduced with permission. [76] Copyright 2021, Wiley-VCH.
Upon external compression, the polarization density increases to result in the enhanced piezopotential between the electrodes. Usually, the external force cannot change the direction of the oriented dipoles, hence pre-polarized dipoles in the PVDF film present the piezoelectric property. In a close circuit condition, the piezopotential causes the electrons to flow through the external circuit in order to screen the piezopotential and reach a new equilibrium state (Figure 6a(ii)). In this way, mechanical energy is converted into electricity. The highest polarization density is achieved at the maximum pressed state (Figure 6a(iii)). As the external force is released, electrons start to flow in the reverse direction in order to rebalance the charge induced by the strain release in the short-circuit condition (Figure 6a(iv)). The magnitude of the output voltage is proportional to the strength of the external stress due to the enhanced polarization effect in the nanocomposite. This proportionality can provide quantitative information on the applied mechanical stress. Yang et al. prepared ZnO/PVDF core-shell electrospun nanofibers (NFs) and demonstrated a pressure sensor for monitoring human physiological signals. [97] They observed that the ZnO nanorods (NRs) radially grown along the surface of the PVDF NFs could potentially enhance the -phase content up to 80%. The space charge polarization at the interface of the PVDF NFs and the ZnO NRs was responsible for such increment of the -phase. The piezoelectric device showed excellent pressure sensing performance having a linear open circuit voltage (V oc ) with external pressure ranging from 1.8-451 kPa (Figure 6b). The pressure sensitivity (S P =ΔV/ΔP ) of the sensor was 3.12 mV kPa −1 , which was six times higher than that of neat PVDF NFs. Moreover, the voltage amplitude was stably maintained during 5000 pressing cycles, showing good mechanical durability. Enhancement in the piezoelectric performance could also be obtained through the direct incorporation of ZnO nanoparticles (NPs) into the PVDF matrix. [89] The electroactive phase content (F EA ) was enhanced to 70% and the piezoelectric charge coefficient (d 33 ) was improved to −32 pC N −1 in the presence of 0.5 wt% of ZnO. This improvement was owing to the electrostatic interaction between the positive surface charge of the ZnO NPs and the ─CF 2 ─ dipoles of the PVDF chain. The sensor showed a good pressure sensitivity (S P = ≈1 kPa −1 ) and obtained the physiological information for vocal cord vibration, wrist pulse, finger motion, and sports movements. Human motion signals and physiological signals (respiration, heart rate, and gait recognition) were successfully collected from different body parts (Figure 6c,d). The sensor clearly distinguished the different modes of breathing such as normal breathing, deep breathing, and gasping (Figure 6c), and it successfully recorded the wrist pulse (Figure 6d) which clearly contained three distinct peaks of percussion wave (P-wave), tidal wave (T-wave), and diastolic wave (D-wave) (Figure 6e). When the sensors were attached to the leg for monitoring gastrocnemius (GAST), soleus (SOLE), and anterior tibials (ANT TIB) calf muscles, they could be used for gait recognition.
Although various nanoparticles promoted the electroactive phase in PVDF, simultaneous achievement of a high piezoelectric coefficient together with high dielectric permittivity is still difficult. In this regard, various ceramic NPs having individual high permittivities, such as CaTiO 3 , BTO, BiFeO 3 , and ZnSnO 3 , are preferred to improve the effective permittivity of the composite. For example, Panda et al. studied the effect of different concentrations (2-10 wt%) of CaTiO 3 on the piezoelectric and dielectric properties of PVDF. [98] They reported maximum enhancement of the -phase, as well as a nearly two times increase in the dielectric permittivity in the presence of 8 wt% of CaTiO 3 NPs with a peak-to-peak output voltage of 20 V and a current of 250 nA. The authors demonstrated the pressure sensing and human motion sensing capability of the sensor. Sasmal et al. observed an increase in the polar -phase, as well as dielectric permittivity with an increase in the content of Ba 2+ -doped BiFeO 3 NPs. [99] The dielectric permittivity and electroactive phase content were proportional to the NP concentration, which was attributed to the enhanced nucleation of the -phase owing to the strong electrostatic interaction between the filler and host. The addition of the BTO NPs led to an improved pressure sensitivity of up to 6 from 1.88 mV N −1 for the pure PVDF NFs. [100] Dispersion of the nanofillers in the polymer matrix plays a crucial role in determining the output performance. In this direction, Cho et al. modified the BiTiO 3 NPs by coating with PVDF-TrFE through a non-solvent polymer precipitation method to enhance the dispersion in the PVDF-TrFE matrix. [101] The enhanced interaction between the NPs and the polymer resulted in a higher -phase fraction and improved sensor performance.
Recent studies have shown the efficacy of low-dimensional (1D and 2D) materials, such as multiwalled carbon nanotubes (MWCNTs), GO, reduced graphene oxide (r-GO), and MXene, in the nucleation of the -phase in PVDF owing to their high surface-to-volume ratio and rich surface functional groups. The incorporation of MWCNTs has also received wide attention due to their superior electrical properties and great impact on the -phase formation even at a very lower concentration. Eun and co-workers prepared MWCNT-PVDF electrospun NFs with different concentrations of MWCNT. [102] Increased -phase (85%) and improved piezoelectric response were observed at 0.008 wt% of MWCNTs. In several other studies, MWCNTs have been used as a third agent to increase the piezoelectric response of PVDF nanocomposites. [103,104] For instance, 0.1 wt% of MWC-NTs increased the overall conductivity and the piezoelectric properties of potassium sodium niobite-PVDF electrospun NFs. [105] Kim et al. demonstrated a highly sensitive piezoelectric skin sensor using MXene (Ti 3 C 2 T x )/PVDF 3D porous structure. [106] A remarkable enhancement of the -phase content up to 99% was found after incorporating 10 wt% of Mxene in PVDF. The nanocomposite-based device further exhibited enhanced piezoelectric sensitivity of 11.9 in low pressure (<2.5 kPa) and 1.4 nA kPa −1 in high pressure (2.5-100 kPa). The sensor showed good performance in monitoring the radial artery pulse when attached to the wrist. GO and rGO are considered as promising nanofiller for boosting the mechanical, thermal, and electrical properties of PVDF. [107,108] In this direction, Yang et al. separately prepared GO-PVDF nanofiber composite and rGO-PVDF nanofiber composite by electrospinning. [109] The maximum -phase content and piezoelectric response were achieved at 2 wt% of GO and rGO in the polymer matrix. The rGO-PVDF sensor exhibited better sensing performance than the GO-PVDF sensor. The higher electrical conductivity and abundant surface charges of rGO were responsible for the better piezoelectric performance.
Hybridization of two materials often improves the characteristics compared to the use of individual materials. For instance, ZnO NPs decorated with ZnSnO 3 microcubes showed higher influence in the piezoelectric enhancement of PVDF (F = ≈88%) compared to the use of ZnO NPs or ZnSnO 3 microcubes, [110] hence the pressure sensor exhibited higher sensitivity for human motion sensing. Azimi et al. demonstrated that 3 wt% of TiO 2 -ZnO hybrid NPs loaded in PVDF NFs enhanced the -phase content up to 92%. [111] The hydrogen bonding between the surface hydroxyl groups of TiO 2 and F − of PVDF, as well as the electrostatic interaction between the ZnO surface and the polymer, was responsible for such enhancement of the electroactive phase in the polymer composite. They fabricated a piezoelectric pressure sensor and installed it on a speed bump to monitor the passage of the vehicles (Figure 6f). The sensor successfully identified the passing vehicles and demonstrated remarkable mechanical strength and endurance. Another study reported that PVDF composite containing SnO 2 nanosheets encapsulated with SiO 2 showed the highest -phase fraction (F = 78%) when the filler concentration was 15 wt%. [92] The study demonstrated superior piezoelectric skin for low-frequency biomedical signal sensing, UV protection, self-cleaning, and microwave shielding. Recently, Li et al. embedded ZnO@C core-shell NPs in PVDF NFs. [112] Increased -phase content (F = 88%) and enhanced piezoelectric coefficient (d 33 = −39.5 pC N −1 ) were obtained in the presence of 5 wt% of filler content.
Organic compounds are gaining growing interest due to their wide species of sources, biocompatibility, as well as biodegradability. In this direction, Li et al. introduced organic 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FDTS) droplets as liquid fillers for the piezoelectric enhancement of PVDF. [91] The addition of the droplet filler led to a maximum -phase content (F = 83.6%) when 10 wt% of filler content in the concentration range of 5-15 wt%, and brought a highly sensitive pressure sensor. The strong intermolecular hydrogen bonding between the FDTS nanodroplets and PVDF chains was responsible for such phase transition. DA-containing (1 wt%) PVDF NFs promoted the spontaneous alignment of the -phase in PVDF crystals due to the strong intermolecular interactions between the ─NH 2 groups of dopamine and ─CF 2 groups of PVDF. [76] The as-received DA/PVDF NFs showed outstanding stability and biocompatibility along with enhanced piezoelectric performance. They further fabricated a fiber-based soft sensor and tested it on mice in vivo and on human skin. The piezoelectric sensor demonstrated high sensitivity and accuracy for detecting weak physiological mechanical stimulation from blood pulsations and diaphragm motions. (Figure 6g). Previously, poly(methyl methacrylate) (PMMA) was also used as an organic filler for the electroactive phase transformation in PVDF. [113] In a recent study, Wang et al. observed incorporation of 5 wt% of PMMA could efficiently increase the electroactive phase content in PVDF. [114] Authors further developed PVDF-based ferroelectric/semiconducting ternary resistive film to demonstrate good resistive switching performance.

Acoustic Sensing
Since sound wave generates pressure on the device, the principle of sensing acoustic vibration with the PVDF-based piezoelectric sensor is similar to that of mechanical sensing, therefore, a higher electroactive phase is desirable for efficient sensing of low sound pressure levels (SPL). For instance, Roy et al. prepared a PVDF nanofiber composite containing a metal-organic framework (MOF) [Cd(II)-μ-I 4 ]. [115] The composite NFs showed 97% of the electroactive phase content with an enhanced piezoelectric coefficient (d 33 = −41pC N −1 ) and an open circuit voltage of ≈22 V. The interfacial interaction, as well as hydrogen bonding between the MOF and ─CH 2 ─/─CF 2 ─ dipoles, along with the extension of the PVDF chain, were supposed to be responsible for the enhanced electroactive phase. The sensor demonstrated superior capability in sensing human motion, as well as acoustic waves. When the sensor was placed in front of a sound speaker (Figure  7a), an open-circuit voltage of 6 V was obtained at a sound wave frequency of 120 Hz (Figure 7b) and 110 dB of SPL was achieved at 120 Hz and 6 V (Figure 7c). It was further observed that the output voltage increased when a circular hole was made in the central part of the electrode, which was supported by a theoretical model (Figure 7d). The sensor could distinguish the waveforms generated from various musical instruments (Figure 7e), as well as recorded alphabetic characters (Figure 7f). Metal oxide NPs (TiO 2 , ZnO, NiO, etc.) have also been used for acoustic sensing. [116,117] PVDF composite NFs with 0.5 wt% of TiO 2 NPs increased the -phase content (≈93%) and enhanced sensing performance of nearly 17.5 V of peak-to-peak open circuit voltage at 90 dB of SPL. [100] In a recent study, the introduction of Ag 2 CO 3 facilitated the conversion of the -phase to the semi-electroactive -phase, which obtained the maximum value of 93% at 0.5 wt% of the filler concentration. [118] The peak-to-peak open circuit voltage was enhanced with increasing SPL values in the range of 40-80 dB. The acoustic sensitivity was ≈13 V Pa −1 .
Recently, several hydrated metal salts were used as the filler to increase the electroactive phase content in PVDF. [119][120][121] Incorporation of 1 wt% of hydrated magnesium (Mg)-salt (MgCl 2⋅ 6H 2 O) efficiently increased the total electroactive phase up to 84%. [90] It was found that the metal ions (Ni 2+ , Fe 3+ , Co 2+ , Mn 2+ , etc.) formed a strong hydrogen bonding with the F − of the polymer through the absorbed water molecules (i.e., O─H─F─C), thereby the ions could act as a nucleation site for the generation of the electroactive phase and facilitated the conversion of the -phase to the -phase. The sensor made of the Mg-PVDF composite NFs improved the performance for pressure and acoustic sensing. The output voltage with peak values of 2 and 3 V was obtained under 80 and 120 dB of SPL, respectively. The acoustic sensitivity (S a ) was calculated to be ≈10 V Pa −1 , which was superior to the neat PVDF NFs-based sensor (S a ≈266 mV Pa −1 ).

Light Sensing
Flexible photodetectors have received increasing attention for their use in a wide range of wearable and portable applications, however, photodetectors often suffer from poor photoelectric conversion efficiency. Compositional engineering and interface engineering have been adopted to increase the charge separation and extraction in the interface. [122,123] The strong and permanent dipoles of piezoelectric polymer could also modulate the recombination, transport, and extraction of the photogenerated charges. [124] Moreover, the strain-mediated piezoelectric potential could act as a gate voltage to modulate the fate of  and at different SPL at a fixed sound wave frequency of 120 Hz (c). d) Finite element method (FEM) theoretical analysis on the distribution of the piezo-potential and dependency on SPL due to the acoustoelectric conversion. It compares the piezo-potential when there is a circular hole and no hole. e) Output voltage of the sensor with response to different musical instruments under an SPL of 110 dB. f) Output voltage of the sensor with response to different recorded alphabetic characters (<80 dB of SPL) from a sound speaker. Reproduced with permission. [115] Copyright 2021, American Chemical Society. the carriers. [125] In recent years, this new mechanism, named as piezo-phototronics effect, has shown great potential in a series of optoelectronic devices such as photodetection, strain mapping, and optical communication. [5,126,127] Additionally, the polymer matrix improves the mechanical stability, as well as overall protection, of the guest photo-sensitive materials leading to enhanced photodetection.
It is now well understood that the higher -phase crystals in PVDF enhance the piezo-phototronic effect. Although the inclusion of optically active fillers in the PVDF matrix is a promising way to make the combined system suitable for the application in piezo-phototronic devices, [71,128] this review article focuses only on the filler-induced enhancement of the optically active phase in PVDF and its correlation with enhanced photodetection. Recently, 2D transition metal dichalcogenides (TMDCs) have received growing interest due to their large surface-to-volume ratio, tunable bandgap, high carrier mobility, and good mechanical stability. [129][130][131] Till now, several studies have been performed to demonstrate the efficacy of different 2D TMDCs as an efficient filler material in PVDF. [132][133][134] Apart from the piezoelectric enhancement, these materials could also lead to enhanced photodetection. For instance, Bhattacharya et al. prepared 2D WS 2 nanosheet/PVDF composite films (Figure 8a) with different ratios of WS 2 (0.037-0.262%) via solution casting. [84] The Figure 8. Piezo-phototronic effect-based light sensing applications. a) Scheme of probable interaction between WS 2 and PVDF. b) Transient response of the WS 2 /PVDF composite sensor under ≈0.75% strain in the dark. It compares with the results obtained under the light illumination with/without ≈0.75% strain. c) Current on-off ratio under light illumination of different wavelengths when the sensor was bent for ≈0.75% strain. d) Transient response of the sensor under periodic illumination with ≈0.75% strain. Reproduced with permission. [84] Copyright 2021, Royal Society of Chemistry. e) Scheme of the piezo-phototronic effect when the optically-active filler was incorporated in PVDF. Reproduced with permission. [136] Copyright 2022, Elsevier Science Ltd.
addition of WS 2 led to a maximum -phase content (F = 85%) at 0.187 wt% filler loading and brought enhanced piezo-phototronic photodetection (Figure 8b-d). The sensor showed enhanced output current under external compressive stress (≈0.75% strain) upon light illumination without any external bias, whereas it did not show any photocurrent under bending conditions, indicating that the WS 2 nanosheets were responsible for the photo-response (Figure 8b). The current ratio between the stressed state (for 0.75% strain) to the relaxed state was further studied under dark, as well as under different illumination wavelengths (Figure 8c). The results revealed a much higher current on-off ratio under illumination compared to the dark, which resulted from the strong optical absorption of the WS 2 nanosheets. When 635 nm light was illuminated under 0.75% strain, the photocurrent quickly approached a saturated value and returned to the initial value as soon as the light was turned off (Figure 8d). The enhanced piezo-phototronic effect was due to the increased -phase content in the nanocomposite. A similar trend was observed by the same group when g-C 3 N 4 nanosheets decorated with Ag NPs were incorporated in PVDF. [135] The mechanism of the enhanced piezo-phototronic effect in the PN is schematically presented in Figure 8e. [136] In the Ag/PN/ITO device structure with ohmic contacts between the metal-semiconductor interfaces, the Schottky barrier height plays a crucial role in determining the charge transport, as well as charge extraction efficiency, in the metal-semiconductor interface. [136] When the device was strained, a positive piezoelectric potential can reduce the height of the Schottky barrier and leads to an ohmic contact, whereas a negative piezoelectric potential can increase the Schottky barrier. [137][138][139] A piezoelectric potential was generated by compression in an unstrained and light-off state, thus the left-side energy band (Ag/PN) was shifted down and the right-side band (PN/ITO) was lifted up. When the device was illuminated, charge carriers were generated due to the excitation of the optically active molecules in the polymer composite. The strain-induced piezoelectric potential helped the separation of the photogenerated electron-hole pairs, resulting in the increase of output current of the device.
Over the last decades, different halide perovskite nanocrystals have received wide attention in the scientific and engineering communities due to their unique optical and electronic properties. [140][141][142][143] Recent studies showed the capability of increasing the nucleation of the -phase crystals in PVDF. Maity et al. prepared a CsPbI 3 /PVDF composite film via solution casting. [136] The proportion of the electroactive phase was 96% at 2 wt% of filler content. The strong interaction between negatively charged PbI 2 and the dipoles of the polymer was responsible for such enhancement. The device exhibited superior light sensing capability under a 10 V bias. The photocurrent increased with light intensity from 10 to 100 mW cm −2 . The piezo-phototronic effect was demonstrated under light illumination at positive and negative strains (0% to ± 2.2%). The photocurrent increased under a positive strain, whereas a decrement of the same was observed under a negative strain. Sultana et al. demonstrated enhanced piezoelectric performance and improved photodetection in a MAPI/PVDF composite. [71] In another study, a nearly similar trend in photodetection was observed through the mixing of CsPbI 3 /rGO binary composite in PVDF. [144] Recently, lead-free perovskite nanocrystals are emerging as potential candidates in various optoelectronic applications owing to their low toxicity and superior optoelectronic properties. Stable lead-free Cs 3 Bi 2 I 9 was incorporated in the PVDF matrix in order to obtain enhanced mechanical sensing and photodetection. [88] The piezo-phototronic effect due to the high electroactive phase (92%) resulted in superior light sensing performance and good switching performance under periodic light illumination at 30 V of an external bias. The rise and decay times were calculated to be ≈141 and ≈278 ms, respectively. The photocurrent increased under light illumination at positive (0 to +2.2%) tensile strain, meanwhile, it decreased at negative (0 to −2.2%) tensile strain. In another study, Mallick et al. prepared a Cs 2 SnI 6 /PVDF composite film via solution casting and reported that the relative contents of the -and the -phases in the polymer composite were 55% and 45%, respectively. [145] They demonstrated its light sensing capability and observed a similar tensile-photocurrent behavior as reported in other stud-ies; 6.5 times enhancement in photocurrent at 1% positive tensile strain. This is because a positive strain may induce positive piezoelectric potential that can effectively decrease the Schottky barrier, facilitating the separation of electron-hole pairs and leading to the enhancement of photocurrent. [136,146,147]

Temperature Sensor
In addition to the piezoelectric effect, PVDF is also known for its pyroelectric properties, which are the generation of charges with temperature change. [148] The mechanism of temperature sensing in PVDF is shown in Figure 9a. [149] The polarization resulting from the aligned dipoles in the polymer leads to the generation of bound charge on each surface of the material and the pyroelectric response originates from the change in the degree of polarization with the material temperature. When a pyroelectric material (e.g., PVDF) is heated (dT/dt > 0), the net polarization of the material decreases as the molecular dipoles lose their orientation due to the thermal vibrations, thus causing a reduction in the number of free charges bound to the surface of the material. In an open circuit condition, those free charges remain at the electrode surface and an electric potential is generated across the material. [148] While, in short circuit conditions, an electric current flows between the two polar surfaces reducing the number of charges induced on the electrodes. As the material is cooled down (dT/dt < 0), the spontaneous polarization increases due to the decrease of thermal vibrations, which enhances the induced charges on the electrodes leading to the flow of electrons in the opposite direction. [150] The short circuit current (I sc ) and open circuit voltage (V oc ) can be expressed through the following equations where P*, , A, dT/dt, ΔT, and t denote the pyroelectric coefficient, relative permittivity of the material, area of the electrode, rate of change in temperature, and change of temperature and thickness across the polarization direction, respectively. It is well understood that the pyroelectricity is increased with the content of the electroactive phase. Roy et al. prepared GO/PVDF NFs with F EA = 96% by adding 1 wt% of GO. [85] The temperature sensing capability of the sensor was further evaluated by assessing the maximum open circuit voltage (V OC ) and maximum short-circuit current (I SC ) under different heating and cooling cycles (Figure 9b,c). The GO/PVDF-based sensor (PVGD) showed an output peak current of ≈45 and 100 pA under temperature fluctuation (ΔT)/switching frequency of 22 K/0.01 Hz and 6 K/0.1 Hz, respectively (Figure 9d,e). In addition to the short circuit current, the sensor brought an enhanced open circuit peak voltage of ≈60 and 100 mV at a lamp switching frequency of 0.01 and 0.1 Hz, respectively. The pyroelectric coefficient was 27 nC m −2 K −1 , which was almost six times higher than the neat PVDF (4 nC m −2 K −1 ) sensor (PVD). It was concluded that the incorporation of GO enhanced the electroactive phase content, IR absorption, and thermal conductivity, which was responsible for Figure 9. Pyroelectric effect-based temperature sensing applications. a) Schematic depicting the mechanism of pyroelectric effect-based temperature sensing. Reproduced with permission. [149] Copyright 2017, Royal Society of Chemistry. Time-dependent change of temperature (ΔT), as well as differential temperature (dT/dt) at lamp switching frequency of b) 0.01 Hz and c) 0.1 Hz. Short-circuit current and open-circuit voltage of the GO/PVDF NFs-based temperature sensor at d) 0.01 Hz and e) 0.1 Hz of switching frequencies. Reproduced with permission. [85] Copyright 2019, American Chemical Society.
the improved pyroelectric performance. In a similar direction, Li et al. demonstrated enhanced temperature sensing performance with a composite film of polyethyleneimine (PEI)-modified rGO (rGO-PEI)/PVDF. [151] A maximum short circuit current (I SC = 229 mA m −2 ) was achieved for the irradiation frequency of 25 mHz. MWCNTs were also used to enhance the pyroelectric performance of PVDF. [152] It was observed that the incorporation of 0.2 wt% MWCNTs could efficiently enhance the electroactive phase of the PVDF NFs up to ≈87% from 67% obtained from pure PVDF NFs. The pyroelectric coefficient (60 nC m −2 K −1 ) of Adv. Sensor Res. 2023, 2, 2200080 www.advancedsciencenews.com www.advsensorres.com Figure 10. Schematic presentation of nanofiller-induced phase transformation in PVDF for enhanced multifaceted sensing applications. the composite was much higher than that of the pure PVDF (4 nC m −2 K −1 ). I SC and V OC were 71 pA and 250 mV at a relatively low switching frequency (0.01 Hz) and ΔT = 14.3 K, while the values were 83 pA and 135 mV at a higher switching frequency (0.1 Hz) and at a lower temperature fluctuation (ΔT = 5.4 K). PVDF composites with ceramic NPs (WO 3 , ZnO, etc.) also exhibited enhanced pyroelectric performance; high coefficient of 40 μC m −2 K −1 with WO 3 composite film, [140] and reliable I SC and V OC of 15 nA and 0.71 V with ZnO/PVDF NFs. [153] Perovskite nanocrystals are also known for their IR sensitivity, as well as being considered an efficient candidate in the phase transformation of PVDF. [154] MAPI perovskite nanocrystals were incorporated in the PVDF NFs in order to enhance the piezoelectric, as well as pyroelectric properties. [154] The enhanced electroactive phase (F = 95%) at 0.5 wt% MAPI perovskite nanocrystals showed reliable pyroelectric performance under repeated cycles of heating and cooling. Positive output current and voltage (15.7 pA, 40.92 mV) were obtained when the temperature changed from 298 to 336 K at an IR light irradiation frequency of 0.01 Hz, and negative output current and voltage (18.2 pA, 41.78 mV) were observed while the temperature decreased to 298 K. The pyroelectric coefficient of the composite material was calculated to be 44 pC m −2 K −1 . Recently, lanthanide ions, such as Ce 3+ , Eu 3+ , and Yb 3+ , have been employed to enhance the pyroelectric performance of PVDF, [155,156] which is because they are superior in IR. [157][158][159] Ghosh et al. prepared a PVDF film modified with Er 3+ by adding hygroscopic Er-salt (ErCl 3 ⋅6H 2 O). [160] The water molecules in the Er-salt made a strong hydrogen bond with the F − of PVDF, driving the polymer molecules to arrange in all trans (TTTT) configuration (F = 74%). The pyroelectric coefficient of Er-PVDF was 33 μCm −2 K −1 . An increase in the output current from 12.5 to 15.5 nA was also observed when the temperature difference (ΔT) was increased from 14 to 24 K. The enhanced -phase, IR absorption, and heat transfer induced by the Er 3+ ions were responsible for improved temperature sensing performance.

Summary and Outlook
PVDF and its copolymer derivatives have attracted considerable interest in the development of wearable sensors owing to their biocompatibility, piezoelectricity, as well as pyroelectricity. However, relatively low electrical conductivity and low electroactive phase content have restrained their applications. In this context, the incorporation of nanofillers has been gaining growing interest owing to their capability of enhancing the content of the electroactive phases. An overview of the nanofiller-based strategies is schematically presented in Figure 10. The nanofillers include ceramic NPs (WO 3 , ZnO, CaTiO 3 , BTO, BiFeO 3 , ZnSnO 3 , etc.), carbon-based low dimensional materials (rGO, GO, CNT, C 3 N 4 , MXenes, etc.), van der Waals nanosheets (MoS 2 , WS 2 , etc.), hydrated metal salts (MgCl 2⋅ 6H 2 O, lanthanide ion (Ce 3+ , Eu 3+ , Yb 3+ ) salts), perovskite NPs (CsPbI 3, Cs 2 SnI 6 , CH 3 NH 3 PbI 3 , MOF, etc.), metal nanoparticles (Ag), and organic molecules www.advancedsciencenews.com www.advsensorres.com (dopamine, silk fibroin, etc.). The review provides the mechanism of electroactive phase formation in PVDF in the presence of the nanofillers and quantitative experimental characterization of the electroactive phases. Applications of the composites for piezoelectric, acoustic, piezo-phototronic, and pyroelectric applications are overviewed.
Although considerable advances have been made in understanding the enhanced polarization mechanisms and the possible applications, achieving simultaneously a high dielectric constant and a high electrical conductivity along with an improved piezoelectric coefficient is still considered as a key challenge. The flexibility of the PVDF nanocomposites is negatively impacted by a high degree of nanofiller loading which is needed to obtain a large dielectric constant. Another worry is that the breakdown strength decreases as the loading is increased. Therefore, surface modification of the nanofillers should be optimized so that the electroactive phase can be enhanced with the least amount of nanofillers. Modeling and simulation are needed to create a realistic picture for a better understanding of interfacial interaction. Since dispersion and orientation of the nanofillers play an important role in determining the efficacy of PVDF composites, more studies should be conducted on the control of the filler-PVDF interfaces and the directionality of the fillers. It has been observed that most of the PVDF-based piezoelectric mechanical sensors exhibit a linear response within a certain range of pressure. However, the sensing performances are influenced by several external stimuli, such as humidity, vibration, and temperature, [161,162] which is not desirable for the production of reliable commercial products. Advanced device fabrication should be developed to meet the requirements of the industry. A lot of research has been performed on wearable and implantable sensors. Although PVDF-based polymers are highly biocompatible and considered as good implantable candidates, there is still a long way to go for practical application to human beings in terms of safety concerns. Novel phenomena, such as piezo-phototronic and enhanced pyroelectric effect, are useful for slow external stimuli only so far, hence dynamic responses for faster stimuli should be investigated. Despite numerous obstacles limiting the advancement of flexible PVDF-based piezoelectric devices, there is a growing interest in the development of multifaceted wearable sensors. With emerging technologies in synthesizing new nanomaterials and device fabrication, it is believed that PVDF-based flexible sensors can be a good material platform for diverse sensing applications.