Highly Stretchable Self‐Powered Wearable Electrical Energy Generator and Sensors

The ubiquity of wearables, coupled with the increasing demand for power, presents a unique opportunity for fiber‐based mobile energy generator systems. However, no commercially available systems currently exist with typical problems including low energy efficiency; short cycle life; slow and expensive manufacturing; and stiff, heavy or bulky componentry that reduce wearer comfort and aesthetic appeal. Herein, a new method is demonstrated to create wearable energy generators and sensors using nanostructured hybrid polyvinylidene fluoride (PVDF)/reduced graphene oxide (rGO)/barium‐titanium oxide (BT) piezoelectric fibers and exploiting the enormous variety of textile architectures. Highly stretchable piezoelectric fibers based on coiled PVDF/rGO/BT fibers energy generator and sensor are developed. It is found that the coiled PVDF/ rGO/BT enables to stretch up to ≈100% strain that produces a peak voltage output of ≈1.3 V with a peak power density of 3 W Kg−1 which is 2.5 times higher than previously reported for piezoelectric textiles. An energy conversion efficiency of 22.5% is achieved for the coiled hybrid piezofiber energy generator. A prototype energy generator and sensors based on a hybrid piezofibers wearable device for energy harvesting and monitoring real time precise healthcare are demonstrated.


Introduction
The advent of new energy materials and novel fabrication strategies are essential to develop wearable technologies. [1][2][3] Wearable energy generating devices that can be seamlessly integrated into garments are a critical component of the wearable electronics genre. Currently flexible fiber energy harvesters have attracted significant attention due to their ability to be integrated into fabrics, or stitched into existing textiles. [4,5] Largescale production of energy harvester fibers using conventional manufacturing processes, however, is still a challenge. There has been a lot of interest in flexi ble, lightweight, and high-power energy devices for wearable technologies and miniaturized electronic applications. To meet the demands for such applications, recent research has focused on dimension conversion of energy devices from three-or 2D types to 1D fibrous structure. Such a trend is well demonstrated in energy generation or conversion fields, for example, fiber photovoltaic cells, fiber piezoelectric generators, fiber thermoelectric generators, and fiber biofuel cells. [6] However, such fiber energy harvesters still suffer from complicated fabrication methods, complex structures, and rigidity or relatively low flexibility, which could be obstacles to scaling-up and wearable electronics applications. [7] In the recent decades, the piezoelectric energy harvesting devices have been considered for two important role, deal with global energy crisis and challenges toward integration of self-power devices into clothes. [8][9][10] The piezoelectric energy harvesting devices due to their special function for energy conversion from external stimulus gained considerable attention recently. [11] For decades mechanical energy has been concerned as a power source for energy harvesting application in frequently used electronic devices [12] including consumer wearable electronics [13] and wireless sensors. [14] The most key aspects for piezoelectric generators are material selections, structure designs and potential performance. Hence, recently research effort is to discover novel structures and materials to enhance output power of these generators. [15,16] Fibers are the smallest unit of a textile material, hence they are great strategic importance to fabricate piezoelectric generators from them. [16,17] The most common method to harvest energy from piezoelectric fibers is to embedded them between two electrodes [18,19] or using The ubiquity of wearables, coupled with the increasing demand for power, presents a unique opportunity for fiber-based mobile energy generator systems. However, no commercially available systems currently exist with typical problems including low energy efficiency; short cycle life; slow and expensive manufacturing; and stiff, heavy or bulky componentry that reduce wearer comfort and aesthetic appeal. Herein, a new method is demonstrated to create wearable energy generators and sensors using nanostructured hybrid polyvinylidene fluoride (PVDF)/reduced graphene oxide (rGO)/barium-titanium oxide (BT) piezoelectric fibers and exploiting the enormous variety of textile architectures. Highly stretchable piezoelectric fibers based on coiled PVDF/rGO/BT fibers energy generator and sensor are developed. It is found that the coiled PVDF/ rGO/BT enables to stretch up to ≈100% strain that produces a peak voltage output of ≈1.3 V with a peak power density of 3 W Kg −1 which is 2.5 times higher than previously reported for piezoelectric textiles. An energy conversion efficiency of 22.5% is achieved for the coiled hybrid piezofiber energy generator. A prototype energy generator and sensors based on a hybrid piezofibers wearable device for energy harvesting and monitoring real time precise healthcare are demonstrated. them in highly stretchable coil structure. [20] The coil structure mostly use for thermoelectrics application [21,22] and there is less report about its piezoelectric application. [20,23] Polyvinylidene difluoride (PVDF) and its copolymers as piezoelectric polymers are the alternative options to fabricate the piezo generator as a result of their biocompatibility, high chemical resistance, flexibility, lightweight, low cost and high piezoelectric properties. [24,25] These piezoelectric polymers due to their high energy density and sizeable electromechanical deformation are commonly operate as sensors, actuators, microrobotics, and artificial muscles. [26] Meanwhile, realizing high energy harvesting performance of piezoelectric fibers is another important issue. Especially for energy generators based on PVDF, a promising piezoelectric polymers with high piezoelectric voltage constant and environmental friendliness, overcoming the low dielectric constant, low flexibility and stretchability of the PVDF still remain an unavoidable challenge to be addressed for optimization of its charge generation performance.
Among five crystal phases (α, β, γ, δ, and ε) of PVDF as a semicrystalline fluoropolymer, β and γ are mostly responsible for its piezo, pyro, and ferroelectric properties. [27] To achieve polar phase of PVDF base polymers, number of methods including spin coating, [28] electrical poling, [29] electrospinning, [30,31] mechanical stretching, [32] solution casting, [33] and fillers like clay, cetyltrimethyl ammonium bromide (CTAB), carbon nanotube (CNT), graphene oxide (GO), rGO, and CoFe 2 O 4 are used. [34] Among these methods, use of fillers provides an attractive, interesting, and simple approach especially in the aspect of nanocomposite structure for the piezoelectric generators fabrication. [35] In fact, the main aspect of nanofillers is to enhance the functionalities of PVDF by stabilizing the γ/β-phase, which has a crucial role in improvement of energy harvesting properties of PVDF based nanocomposites. [33] Up to now, various piezoelectric materials have been discovered to enhance the energy harvesting performance through different polymer, ceramic, and conductive filler in a composite structure. [36] A wide range of fillers such as barium titanates, titanium dioxides, zinc oxides, and carbon nanotubes are utilized to improve the β-phase crystallinity of the PVDF. The disadvantage of these filler is that they are expensive, brittle, toxic, and nonenvironmentally friendly. [35,37] Piezoelectric polymers have low dielectric constant and high breakdown strength. [38] In contrast, ceramic materials provide low breakdown strength and high dielectric constant. Hence, the combination of flexible polymer and ceramic fillers [39] make it feasible to improve the charge generation and storage capabilities of composite structure and have been considered for charge generation and storage applications. [40] During poling process, due to lower dielectric constant of piezoelectric polymers in compare with ceramics, the electric field has less effect on the piezoelectric ceramic particles and they cannot be fully poled. To overcome this problem, the effective method is using conductive filler (graphene, carbon nanotubes, and metal particles) for creation a continuous electric flux path between the piezoelectric particles. [41] Graphene is an ideal nanofiller to enhance the electrical, mechanical, and thermal properties of polymers at very low loading contents. [22] Many researchers studied properties of GO or the reduction of GO (rGO) before its dispersion into polymer. [42] The oxygen including in functional groups of GO and the PVDF polymer chains interact with each other and improve the dispersibility. From another side, the incorporation of GO into PVDF deteriorates the mechanical and electrical properties of the polymer composite. Therefore, the common method is to reduce GO to rGO, which still has graphene properties but enhanced its dispersion in the polymer. [43] The basal and edges planes of graphene nanosheets have oxygen functional groups, therefore, rGO can act as a negative triboelectric material. Moreover, rGO can limit electric charges in composite structure due to the difference between electrical conductivity of rGO and the matrix. [44] Reduced graphene oxide stabilizes polar phase in PVDF and leads the charges easily toward the electrodes and enhances the energy harvesting properties. [33] The electrostatic interactions between the positive and negative charge centers of metal ions and negative charge clouds in rGO leads to enhancement in the piezoelectric polar phases. [43] The dielectric constant and ferroelectric behavior of the nanocomposite confirm remarkable increase for the PVDF/rGO (0.1 wt%) in compare with pure PVDF. [45] The prepared composite film of PVDF and rGO (0.05 wt%) show more β-phase formation and three times higher output voltage than pure PVDF film. [46] Herein, we explore a novel approach to develop high performance textiles-based energy harvesting devices as next generation wearable energy generators and sensors. A different variation of textile designs from melt-spinning, knitting, and weaving with the capability of low-cost, high production speed and high performance are developed. The high-performance PVDF, PVDF/barium-titanium oxide (BT), PVDF/rGO and PVDF/rGO/BT piezoelectric nanocomposites fibers were produced through a melt-spinning process. The effect of graphene and BT nanoparticles additions on the piezoelectric properties of hybrid PVDF fibers were investigated. [13] To develop highly stretchable energy harvesting textiles, the optimized PVDF/rGO/BT nanocomposite fibers were converted into the coiled structure (spring-like) by twisting and coiling of the as-prepared fibers as previously reported. [47] As developed coiled PVDF/rGO/BT fibers with a relatively high piezoelctic performance combined with an excellent elasticity of 100% strain used to create wearable woven and knitted energy harvesters and strain sensors (Figure 1). The coiled piezofiber energy harvester affords a practical potential candidate in the energy harvesting devices applications of wearable smart garments, self-powered wireless sensors within garments for human health monitoring, and finger sensor, etc. The hybrid piezoelectric fibers exhibited improved stretchability and piezoelectrical properties as compared with the pristine piezoelectric polymers material. The nanocomposite fibers energy generators and sensors can be used for applications where the energy harvesting, along with stretchability and excellent electromechanical properties, is of primary importance. BT ceramic were added into the PVDF matrix to improve the piezoelectric response. The optimized BT concentration has the value of 10 wt% based on our previous work. [24] Stable dispersions of rGO in DMF facilitated their mixing with the PVDF polymer, which can be readily dissolved in DMF as well. Figure 2b shows the images of the PVDF nanocomposites films. It can be seen that the introduction of either BT or rGO did not affect the formation of the PVDF films. The graphene nanosheets and the BT nanoparticles embedded homogeneously in the fibers and they are identifiable along the fiber axis ( Figure 2c). All prepared nanocomposite films and fibers are flexible. The flexibility of a PVDF/rGO nanocomposite film is shown in Figure 2d. The nanocomposite fibers also have the ability for the knitting, weaving, and braiding as reported in our previous research. [13] The surface morphological features of PVDF nanocomposite fibers were characterized by scanning electron microscope (SEM). The surface morphology and cross-section (Figure 3a-c) of nanocomposite fibers show that both meltspun PVDF and hybrid PVDF/BT fibers are very smooth and   Figure 3d shows low contents of rGO sheet with the dimension of 200-400 nm. The graphene nanosheets can be seen in Figure 3f. As described below, the addition of a low concentration (0.03 wt%) of rGO into PVDF enhanced the composite output performance. Using too high rGO concentration leads to agglomeration in the composites which create short circuits and cause charge leakage between electrodes. [37] The research show that only 0.1 wt% of rGO was sufficient to hamper α phase formation and promote a nearly complete β phase structure. [48] The energy harvesting performance and the high frequency capacitors improved for the rGO (0.3 wt%) and BT (35 wt%) of PVDF/rGO/BT nanocomposite film. [49] Hence, to evaluate the effect of rGO on the PVDF/BT nanocomposite and pure PVDF fiber, 0.5 wt% of the rGO selected for more consideration.

Morphology and Microstructures of the PVDF Nanocomposites
The piezoelectric performance of nanocomposites rely on the crystalline structure and the electroactive polar phase formation. [50] So, for clarification purposes of the polar crystalline phases present in the PVDF based nanocomposites, Fourier transform infrared (FTIR) spectral analysis was performed, as presented in Figure 4a. The α phase of PVDF has the peaks at 764 and 975 cm −1 . [51] These peaks nearly disappeared in the PVDF nanocomposite fibers with the presence of BT nanoparticles and graphene nanosheet which have an important role in the β phase formation. [52] The FTIR spectra of the melt spun fibers proves strong vibration peaks at 840 and 1275 cm −1 , which are related to the CH 2  wagging vibration and CF 2 -symmetric stretching and the presences of these peaks demonstrate the β-phase formation in the composite. [53,54] The reduced graphene causes formation of the piezoelectric phase in PVDF matrix as shown by peaks at 1173 and 1275 cm −1 as well as decreasing α-phase peak intensity. In fact, β-phase formation is mainly influenced by better interaction between the two components. [55] A high intensity peak at 1175 cm −1 in all nanocomposite fibers highlighted the high quality of the PVDF based fibers. [49] The intensity of vibration peaks is sensitive to the crystalline structure in polymer and the F(β) of fiber structures was calculated from the FTIR spectra to quantify the amount of β phase formed [56] 1.26 where A α and A β are the absorbance of vibration peaks at 766 cm −1 (α-phase) and 840 cm −1 (β-phase). The variation of β phase contents for as-prepared PVDF and its nanocomposites fibers and coils are shown in Figure 4b. More β phase formed in coil samples and this effect is likely due to more stretching and aligning of dipole along the fiber axis during coil fabrication. The highest β phase formation was present in the PVDF/ rGO/BT nanocomposite coil with 58% improvement when compared with its untwisted fiber. The high fraction of β phase in PVDF based fibers is desirable due to their enhanced piezoelectric response. The FTIR and X-ray diffraction (XRD) technique had been used to explore effect of rGO and BT fillers on the crystalline structures of PVDF fiber. Figure 4c demonstrates the XRD patterns of the PVDF fiber and its combination with BT nanoparticles and rGO nanosheets. Pure PVDF has the stable α-phase (TGTG) peaks at 2θ = 17.5° (100), 19.8° (110), and 26.5° (021). Other nanocomposite fibers have the same peak appearances owing to the existence of semi crystalline PVDF polymer. [57] The β phase (TTTT) appeared at 20° signifying the coexistence of α and β phase. The phase transformation from α to β in the nanocomposite fibers comes from the molecular chain stretching during the fabrication process. Another peak appears at 2θ ≈ 22° in BT nanocomposite fibers and is associated with the (001) and (100) reflections of tetragonal BT nanoparticle. [52] The layered graphite structure in rGO has diffraction peaks at 26.2° which is related to the (002) diffraction. [58] Also, an increase in the intensity of β phase peaks confirmed the good dispersion of the rGO in PVDF matrix. [51] The nucleation rate of the β phase in nanocomposites contained rGO increased gradually because of delocalization of π-electrons with remaining oxygen functionality in rGO which interact with the CH 2 , CF 2 dipoles of the PVDF polymer chain. [33] Basically the interaction of OH groups with the fluorine (F) groups in the PVDF support them to be aligned on one side. [59] The deconvolution method was used to classify the existence of β and γ-phases individually. The incorporation of 0.5 wt% rGO increases the crystallinity (χ ct ) from ≈68.4% in PVDF (α ≈ 20.9%, γ ≈ 47.5%) to ≈97% (β ≈ 94%, γ ≈ 3%) in PVDF/rGO (0.5 wt%) composite. The asprepared PVDF and its composite fibers have also been investigated to evaluate the effect of filler addition on the mechanical properties of the nanocomposite fibers. A comparison of the stress-strain curves for the various prepared fibers is given in Figure 4d. While graphene is used as the filler in a composite structure, it is expected to have superior mechanical properties due to its high intrinsic strength. As can been seen from Figure 5, the tensile strength and Young's modulus of the prepared composite fibers have significantly increased compared to PVDF fiber (see the Supporting Information for more details).
The Young's modulus and tensile strength of the as-prepared PVDF/rGO nanocomposite fiber are 208% and 163% higher than pure PVDF fiber, respectively. The tensile strength of the PVDF and PVDF/rGO nanocomposite fibers are three and six  times higher than fibers reported recently. [60] The enhancement in the β crystallites of the composite afford advancement in the mechanical properties (confirmed by FTIR measurements). [61] One of the reasons to have better mechanical properties in composite structures is a good dispersion of particles with strong interfacial interaction between the filler particles and the polymer matrix.
The BT nanoparticles have desirable electrostatic interaction/hydrogen bonding with PVDF chains due to their high dipole attraction. Therefore, BT nanoparticles act as a "bridge" to connected PVDF chains and rGO particles. [62] However, the intimate interaction between BT nanoparticles and rGO substrates can also limit the dispersion of BT nanoparticles in rGO sheets and cause aggregation formation. Addition of rGO/BT fillers into the PVDF matrix can lead to a crowding effect and these unexfoliated aggregates create stress concentration points during the propagation of the cracks and make lower tensile strength. [63] Thermal stability of the composite fiber was characterized by the thermogravimetric analysis (TGA) which was performed in temperature range of 30-900 °C and is presented in Figure 6. For temperatures lower than 400 °C, the thermal stability of the nanocomposite contained rGO increased. The increased thermal stability of the rGO was caused by the high thermal conductivity of rGO, which facilitates dissipation of the thermal energy very quickly. [64] For rGO, the slight mass loss at the range of 200-400 °C is attributed to the removal of residual oxygen containing groups on rGO surfaces. [65] The higher thermal conductivity of the rGO helps to transfer heat from the GO layers to the PVDF matrix. [66] Temperature related to 5% degradation is considered as the onset of thermal degradation (T 5% ). Both the T 5% and the temperature corresponding to 50% weight loss (T 50% ) increase by up to 43 and 44 °C, respectively, in the composite of PVDF/rGO/BT in comparison with pure PVDF. This enhancement in thermal stability comes from a variety of physicochemical interactions among composite components. [67] Pure PVDF fiber begins to decompose at about 370 °C, but the PVDF/BT and PVDF/ rGO/BT nanocomposite fibers appear to degrade at 450 °C. The strong interaction between nanoparticles and the PVDF matrix was indicated by the XRD patterns and FTIR spectra. Furthermore, the residual weight at 900 °C of PVDF nanocomposites increases clearly in comparison with the pure PVDF according to their concentration. The residual mass for pure PVDF is 33.15% and is 33.59% for the PVDF/rGO and this weight difference confirms the addition of 0.5% of rGO in the composite structure. Also, the residual mass for PVDF/ BT nanocomposite is 42.32% which confirms a 10% addition of BT in the composite (see supporting information for more details).

Ferroelectric Properties of Nanocomposite Films
The polarization-electric field loop measurements have been carried out at room temperature to discover ferroelectric behaviors in the PVDF and nanocomposite films under an electric field of 300 kV cm −1 . The P-E loops for pure PVDF, PVDF/BT, PVDF/rGO, and PVDF/rGO/BT composite films are presented in Figure 7. The electrode area was 0.1962 cm 2 and the film thickness was 150 µm. The formation of the loops comes from the presence of phase separation between voltage and charge. By increasing the applied electric field, the polarization gradually increases as well because of the alignment of molecular dipoles in one direction. [43] The PVDF/rGO/BT nanocomposite film shows a maximum polarization value of 0.71 µC cm −2 when the applied electric field is 300 kV cm −1 (Figure 7a). The PVDF/rGO/BT nanocomposite film shows the remnant polarization of 0.12 µC cm −2 , while it is 0.032 µC cm −2 for pure PVDF. The variation in remnant polarization could refer to molecular dipoles charge accumulation. This charge accumulation comes from the interactions between PVDF chains and the oxygen functional groups of rGO-BT nanocomposite. Therefore, the P-E hysteresis loops of the PVDF/rGO/BT nanocomposite film were assessed from 50 to 350 kV cm −1 until breakdown (Figure 7b). The remanent polarization (Pr) of the nanocomposite films increased from 0.088 for PVDF to 0.15 µC cm −2 for PVDF/rGO/BT at 350 kV cm −1 and 1 Hz, which indicated ferroelectricity enhancement in the nanocomposites (Figure 7c). Furthermore, remnant polarization is just in accordance with the piezoelectric response of material. [33] Whereas, the PVDF/rGO/BT nanocomposites has higher remnant polarization than PVDF, it reveal higher output voltages. Therefore, the nanocomposites are excellent for piezoelectric energy harvesting applications and also provide a high energy density capability. The real energy density of PVDF nanocomposites to determine the energy harvesting strength could be calculated from the P-E loop by the following Equation ( where E is the electric field and P refers to the polarization of the samples. [68] In fact, the energy density relies on the dielectric constant, dielectric breakdown strength, polarization, and the applied electric field. [69] The energy density greatly increased up to 79.6 61 mJ cm −3 at 300 kV cm −1 with nanocomposite containing PVDF/rGO/BT, which is 23% higher than pure PVDF (64.61 mJ cm −3 ). The strong interactions between the rGO/BT and PVDF matrix could be the main reason for the polarization properties. The energy density graph in Figure 7d shows that the charges at the interface for the PVDF/rGO/BT nanocomposite are much higher than the PVDF/BT due to the conductive nature of the rGO sheet. Figure 8 shows optical microscope images of coil samples fabricated from pure PVDF. The coil has a diameter (D) of ≈270 µm and its filament is 160 µm (Figure 8a,b). During meltspinning the filament tends to be elongated and oriented in the microfiber direction. A twist of about 12 000 turns m −1 was inserted  into the filaments until coil formed completely along the entire fiber length. The coil has a uniform structure along its length (Figure 8c). The coil bias angle (α c ) is the angle between the fiber and the coil's cross-section. The coils bias angle calculated from Equation (3) [47] ( )

Performance of Nanocomposite Coil Structures
where N is the coil turns and l is the length of the fiber which made the coil. The coil bias angle showing 30° from the coil's cross-section, which agrees with the calculated value from Equation (3). The optical microscope was used to observed coil bias angle (Figure 8d). The mechanical properties of nanocomposite samples improved due to continuous geometry of fiber and their stretchability. The mechanical properties of fabricated coils were explored through the uniaxial tensile test of samples (Figure 9a,b). The tensile extension in the longitudinal direction for the coil structure is shown in Figure 9c. To assess coil performance, the samples were vertically stretched several times. Figure 9d shows stress-strain curves found on loading and unloading filament over a 200% strain. The coil has significantly lower stiffness (200% strain at 40 MPa) compared with its fiber (50% strain at 100 MPa). Therefore, coil structures are more desirable for stretchy fabrics to monitor movement of robots, human, and prosthetics without limitation or everywhere that a continuous stretching is considered. It is also possible to use more than one fiber to make a coil in order to get a higher piezoelectric response and higher tensile strength. The diameter of the coil structure depended on the number of fibers used in the supply coil. The coil diameter for single and double 2-ply fibers are ≈270 and ≈500 µm (Figure 10a). In spite of a large amount of twisting, the fibers had no signs of failure and the fiber seems to be continuous. Although coils made from 2-ply fibers were highly stretchable, it was noted that deformation occurred by microbuckle unfolding rather than coil opening and this process lead to a greater variation in voltage output in the 2-ply coils in comparison with the single fiber coil especially as higher strain (Figure 10b). Although double fiber coil have poor regularity, but for all PVDF nanocomposites, the output voltage increased for 2-ply fiber coil in comparison with the single fiber coil. The higher amount of output voltage related to the PVDF/rGO double fiber coil with ≈90% increment. This higher amount of value can be described by the higher conductivity of PVDF/rGO nanocomposite which facilitate charge transfer from coil surface to both electrodes. It is shown that coils harvesters provided arbitrarily high voltages if multiple harvesters were combined in series. [70] All fabricated coils were exposed to stretching force and the effect of different fillers studied. Figure 11a,b shows the open circuit output voltage and short circuit current for all PVDF nanocomposite. Among all the samples, the PVDF/rGO/BT nanocomposite showed a maximum voltage output of 1240 mV at an applied force of 1 N. It can be seen that rGO contribution enhances the coils energy harvesting performance. The following description considers the rGO function in nanocomposite structure for power generation. The rGO filler has two roles in the nanocomposite structure: 1) providing dipole alignment and 2) prompting microcapacitor formation within the nanocomposite. The functional groups of oxygen including carboxyl and carbonyl in the rGO plane have the key roles in promoting polymer chain alignment through collecting fluorine atoms to one side. [49] After loading rGO to the composites, the β phase formation was enhanced and polymer chain alignment happened which was indicated according to the XRD results. The highly dispersed rGO sheets increases the availability of many free charges on their surface. In the PVDF/BT nanocomposite structure, both PVDF and BT dipoles accumulated free charges of rGO, and new dipoles have been generated in those specific regions. In comparison with other conductive fillers, rGO has higher surface area. Hence, more dipole would be formed and as a result the storage capability and charge generation will be enhanced. [36] The pure PVDF coil generated a maximum voltage output of 300 mV while PVDF/ rGO and PVDF/BT samples showed 400 mV and 700 m V maximum output voltages, respectively. The output voltage for PVDF/BT nanocomposite is higher than PVDF/rGO, this may  due to number of rGO sheets and many free charge of graphene sheets which did not find dipoles from composite and these free charges stay inside the composite and negatively affected coil performance. [49] The results show that the piezoelectric output of coils were more sensitive to the strain percentage than the loading rate. In frequencies higher than 1 Hz, the short equilibrium time does not let the coil to reach stable surface charge transference between electrodes and this negatively affected the generated signal. As illustrated in Figure 11c, there exists an output voltage of 300 mV when a strain of 10% was applied on PVDF/rGO/BT nanocomposite coil. When the strain increased to 100%, the output voltage reaches 1.2 V which is approximately four times higher than in the PVDF coil. Also the generated voltage by single coil structure during stretching is three times more than core-sheath piezoelectric microfiber under pressure. [71] Additionally, the nanocomposite coils has high stretchability up to 100%, which is significantly higher than recent report for piezoelectric fibers (<50%). [19] The relation between strain and output voltage is relatively linear for all nanocomposites. The mechanism to show voltage generation by coil structure is illustrated in Figure 11d. The power generated by the nanocomposites coils was calculated through Equation (4) where U(t) is the real-time voltage, T is the period of load application and R is the external load resistance (400 KΩ in here). [19] At frequency of 1 Hz and 100% strain, the maximum power output for 10 mm length PVDF/rGO/BT nanocomposite coil reached 3.6 µW, 0.42 µW cm −3 or 3 W kg −1 based on considering the diameter, length and mass of the coil. The energy conversion efficiency from mechanical energy into electrical energy was 22.5%, which was extracted from below Equation (5) 100 e m η = × E E (5) Where E e is the total output power energy (Equation (4)) and E m is the input mechanical strain energy during coil axial extension according to applied force on cross section of coil for specific strain percentage. This conversion efficiency for PVDF/ rGO/BT nanocomposite coil is ≈2.5 higher than latest reported harvesting electrical energy from coil structure of polyurethane microfibers. [21] The potential applications of the PVDF/rGO fiber have been explored in different application for movement sensor as can be seen from Figures 12-15. The PVDF/rGO fiber fabricated in the plain weave structure (4 × 10 cm with total weight of 3 g) with silver coated nylon as electrodes and polyester fibers to separate each unit of generators is shown in Figure 12a,b. The woven structure has four active units (4 × 1 cm, 0.25 g) and where the electrodes can be connected in series or in parallel connection. The woven generator was poled for serial and parallel connections. It was also found that the output voltage and current of the PVDF nanogenerator could be enhanced by serial and parallel connections, respectively. [72] For both series and parallel connections, the output peak voltage increases with increasing the number of active units. Woven piezoelectric fibers units in a series connection exhibit a higher output peak voltage and power than that of a parallel connection (Figure 12c). For the series connection, the maximum output peak voltage for four active unit is 1000 mV under periodic pressure and relaxation. A bridge rectifier (four diodes of 1N5817) was placed in the circuit to feed a different capacitor under mechanical pressures in frequency of 1 Hz. Different capacitors (22,33,47, and 100 µF) charging performances were carried out upon mechanical deformation on the woven fabric with serial connection of units ( Figure 12d) and it is observed that the build up voltage increases exponentially to eventually reach a steady state. According to this woven fabricbased piezoelectric fiber, it possible to have piezoelectric fibers woven in different parts of a garment as the sensor or connected them in serial connection to have higher voltage to charge personal electronic devices. The PVDF/rGO piezoelectric fiber knitted in a circular knitting machine as the set up described in our previous work [24] (Figure 13a). The storage energy calculated for woven fabric with four active unit which connected in series was determined using capacitor potential energy formula where C is the capacitor capacitance and V is the charging voltage across the capacitor under steady-state condition at a definite time (t). [24] The energy storage based on the woven fabric in 100 µF capacitor was found to be 50 µJ. Based on the Equations (5) and (6) the charging process of the 100 µF capacitor, the energy conversion efficiency of the woven energy generators is calculated as 26%. The wearable energy generator could increase the voltage of the storage capacitor from 0 to 1 V in 200 s and the 100 µF capacitor was fully charged. The power output of the woven structure is 28 µW cm −3 under a periodic compression. This power is equal to the film power output of PVDF/(rGO-Ag) and film nanogenerator of rGO/ PVDF-TrFE which recently reported [43,37] with this significant difference that PVDF/rGO fibers have capability for mass production and have flexibility to be integrated to the textiles (see Movie S1 in the Supporting Information).
To demonstrate potential applications of the developed circular knitted piezoelectric strain sensor for the detection of human and/or industrial activities, knitted wearable piezoelectric PVDF/rGO fibers were assembled in core-sheath structure as shown in Figure 13b. The performance of the potential real-life applications of the developed wearable sensor under biomechanical pressure of periodic finger pressure and relaxation was demonstrated in Figure 13c,d. In comparison with pure PVDF with the same structure, PVDF/rGO fibers shows 100% improvement in output voltage. The structure of a circular knitted sensor filled inside and covered outside by silicon rubber to protect it from environmental condition (dust and humidity) for different applications is shown in Figure 13e). The sensor output voltage was measured after different forces range from 0.5 to 10 N applied on 2 cm of sample by mechanical tester machine during compression test (Figure 13e,f). As can be seen from Figure 13f by increasing the force the output voltage increased and this sensor can be considered as a pressure sensor for wide range of applications (sensors in shoes, any part of clothes). As depicted in Figure 13f, the output voltage increases from 100 mV to 1200 mV in the pressure range of 8-160 kPa, and it demonstrates the sensitivity of 7.34 mV kPa −1 (R 2 = 0.97). To verify potential health care application, the PVDF/rGO coils sensor with a length of 2 cm was fixed to a finger with an adjustable band as a prototype device. Silver coated nylon on both ends of the coil were included as electrodes. The coils could accurately identify the movements when the finger was bent and straightened ( Figure 14a). A clear result was obtained when coils generates different voltage signals at different bending angles. The higher the bending angle is, the higher the voltage is ( Figure 14b). During the bending of the finger, the voltage increased and reached a peak value, whereas with the relaxation of the finger, the voltage decreased and returned to the initial position. The coil sensor can detect finger bending by generating voltage up to 730 mV. Here the frequency for the bending movement is approximately 0.5 Hz. The voltage response to the periodic motion of the finger validates that coil structure is stable in the monitoring process. The magnitude of the sensor signal (voltage and current) steadily increased almost linearly with the bending angle of the finger, thus representing the ability of the sensor to distinguish the degree of motion, as shown in Figure 14c. The results confirmed that maximum peak-to-peak power of about 7 nW or 0.03 µW cm −3 can be harvested from wearable piezoelectric coils by finger bending movements. This coil structure is suitable to motion monitoring without disturbing the body's movements and was established to support personal recovery after a surgery. Also, the generated voltage can be transferred to the computer for hand motion detections in the virtual reality environment for soft virtual reality glove systems. Moreover, the PVDF/rGO coil also has a potential to be integrated in a stretchable 2D fabric used in close contact with the human skin or attached to textile as a decoration (Figure 15a). PVDF/rGO coils set as warp along with elastomer fiber in a woven structure. Silver-coated nylon woven on both end as electrodes and acrylic fiber is weft direction made distance between two electrodes (Figure 15a 1 ,a 2 ). The poling voltage applied along the length of PVDF/rGO colis on woven silver coated nylons on both ends. The textile structure can be covered with soft and stretchable silicone material to provide protection from the environment including washing. The total weight of woven structure (2 × 5 cm) including the silver-coated nylon, coils and acrylic fibers is about 1 g.
The energy-harvesting woven structure can be a potential strategy for building up a self-power system, which can be used to drive wearable electronics sustainably. Hence, using the PVDF/rGO coils, we attempted to simultaneously detect the muscle contraction and human arm motion in different angles during lifting two different weights (1 and 5 kg). The AnEx board can be attached to the fabric and linked to it by an external connector. The collected data can be transferred via Bluetooth to a laptop of phone for signal analysis (Figure 15a). As can be seen from Figure 15c,d the generated voltage for two weights of 1 and 5 kg have significant difference in the position of 90° but for maximum lifting of weight in 180° this difference decreased.

Conclusion
In summary, the highly flexible nanocomposite piezoelectric fibers consisting of PVDF, BT, and rGO were developed. The fibers were fabricated though a melt-spinning process. The fibers were strong enough to be coiled by twist insertion and then endure axial extension up to 100% strain. A high energy density of 80 mJ cm −2 was identified in the PVDF/rGO/BT nanocomposite owning to the rGO presence. These results demonstrated that PVDF-based graphene nanocomposites had better dielectric and conductivity properties. It is clear that rGO sheet are ordered and aligned in a uniform orientation, accounting for the improved significant enhancement in Young' modulus of the PVDF/rGO nanocomposite fiber. It was found that high-performance PVDF/rGO/BT piezofiber with 84% of the electroactive β-phase generated a maximum voltage output of 1.3 V and a power density 3 W kg −1 during longitudinal extension at 1 Hz. The coils could be easily fabricated through twist insertion. The piezoelectric response would be increased by increasing the original fiber diameter, length and number. The coil structure from meltspun piezoelectric fibers is a novel method with ability of flexibility, lightweight and mass production capability. These coil structures have great potential application as motion detectors and self-powered biomedical and textile applications in daily life.

Experimental Section
Materials: Poly(vinylidene fluoride) (PVDF) in powder was purchased from Solvay Soleris (Milan, Italy), under the commercial name Solef 6010. The melt flow index of Solef 6010 is 2 g/10 min at a load of 2.16 kg (or 6 g/10 min at a load of 5 kg) at 230 °C. The cubic Barium titanate nanoparticles with a mean diameter of 50 nm and with 99.9% trace metals basis were purchased from Sigma Aldrich Company (China). DMF (>99.8%, Merck) as the solvent was used. Silver plated polyamide yarn (235/36dtex 4 ply) supplied from Shieldex USA.
Synthesis of Graphene Oxide: GO was synthesized from natural graphite powder through a modified Hummer method in two steps to reach improvement in oxidation of graphite. The experimental procedure was carried out under a fume hood and the reaction temperature was controlled precisely by a digital thermometer. The chemically converted graphene (CCG) with 1 mg mL −1 concentration dispersed in DMF following the reported method. [73] Briefly, an exfoliated aqueous graphene oxide dispersion (0.05 wt%) was chemically reduced by hydrazine and ammonia at 90 °C in two different steps. The H 2 SO 4 (5 wt%) added to the dispersion followed by filtration and drying resulted in the formation of agglomerated CCG powder. To prepare a stable dispersion, the graphene powder was dispersed in DMF using triethylamine by sonication cycles and centrifugation. The direct reduction of graphene oxide in solution helped to avoid rGO aggregation to enhance the rGO dispersion in the PVDF matrix as described in the next section. Moreover, there was no residual reducing agent in the dispersion used to prepare the composites.
Preparation of PVDF/Graphene Composite: To prepare CCG/PVDF composite with 0.5 wt% CCG content, an appropriate amount of CCG dispersion was added to a PVDF solution in DMF (15 w/v%) under constant sonicating and stirring (Figure 16). The mixture was further stirred and sonicated for two hours to ensure homogeneous dispersion of CCG nanosheets into the polymer matrix. After evaporation of the solvent, the resultant composite was chopped into small pieces, washed in ethanol and dried in a vacuum oven at 60 °C for 3 h. The composite was ground into powder manually in a mortar with liquid nitrogen.
Nanostructured Hybrid PVDF/BT Composite: To prepare PVDF/ BT nannocomposite, first 30 g of PVDF powder was dissolved into DMF (200 mL). A clear and transparent solution was obtained upon continuous stirring on heater in water bath at 70 °C during overnight. To prepare PVDF/BT nanocomposites solution amount 10 wt% of BT nanoparticle was dispersed into DMF (50 mL) using a probe sonicator for 60 min under nitrogen flow at 0 °C then dispersed BT was added to the as-prepared PVDF (10 wt%) solution and to form a stable suspension, sonicated for 30 min under nitrogen flow at 0 °C then stirred for 2 h. The suspensions were casted onto a clean glass plate and after evaporation of the solvent, the film was peeled off from the dish. The resultant composite was chopped into small pieces finally to have homogenous mixing, then the chopped film was ground finely and used for further characterizations. More details about BT concentration optimization described in the previous work. [24] To prepare the PVDF/ rGO/BT nanocomposite, the as-prepared solution of PVDF/rGO/DMF mixed with dispersed BT into DMF and sonicated for 30 min under nitrogen flow at −10 °C then stirred for 2 h. the nanocomposite film made the same procedure as describe above.
Melt Spinning of Nanocomposite Fibers: Fiber spinning was performed using a twin screw extruder (Barrel Scientific Ltd.) (see the Supporting Information for more details). The fabrication of circular knitted piezogenerator described in the previous work. [24] Coil Fabrication: To make coils from nanocomposite fibers (Figure 17) as the first step a length of precursor fibers (a 1 ) were cut off from the fiber spool. The top end of the fiber was joined to the power drill (a 2 ) by a paper clip (a 3 ) and a fixed weight (≈20 gm) (a 4 ) hanging on the other end which applied 2 MPa stress to the fiber. The fibers were held in tension but could not rotate due to the applied tension. The weight selection has a key role to have uniform and straight fibers and which also prevent snarl formation prior to coiling. The precursor fiber was twisted by rotating the powered drill in a clockwise direction (from the top view) to form a "S" twist. The fabricated coil sample (a 5 ) was mounted on a metallic stand (a 6 ) stretched for 8% with respect to the initial coil length. while both ends of coil were clamped to prevent twist loss, it was annealed at 120 °C which is above its glass transition (T g ), [74] in an isothermal heating oven. Heating at a temperature over T g helps to set the twisted shape permanently. After 30 min heating of the coil, it was removed from the oven and left to relax for 2 h at room temperature while still clamped. Figure 17 schematically shows the process of twist insertion and coil preparation from PVDF fiber.
Characterization: FTIR spectra of as-prepared PVDF and PVDF/BT nanocomposite fibers were carried out over a range of 400-4000 cm −1 using a Shimadzu IR prestige-21 Spectrometer equipped with Pike Technologies MiracleA germanium crystal ATR attachment. The melting temperature (T m ) and melting enthalpy (ΔH m ) of fibers were measured with a differential scanning calorimeter Phoenix from NETZSCH, Germany at a heating rate of 10 °C min −1 . TGA of samples was performed using TG 209 Libra from NETZSCH, Germany. The fibers diameters were measured with a Leica M205A stereo microscope. The diameter measurement for each fiber was performed at 10 different points along the fiber lengths. The mechanical properties of the samples were measured using a Shimadzu tensile tester (EZ-S). The samples were mounted between two grips and were subjected to tensile test with a constant rate of 10 mm min −1 . Tensile strength and elongation at break of the sample were recorded by TRAPEZIUMX software. The Young's modulus was calculated from the slope of the initial part of the curve, where the relationship between stress and strain was linear. The crystalline structures of samples were analyzed by XRD (GBC, MtriX SSD) using Cu Kα radiation (λ = 0.154 nm), with the generator working at 40 kV and 30 mA. Surface morphology of the as-prepared PVDF and PVDF/BT nanocomposite fibers were examined with the use of a JEOL 7500 SEM. Secondary Electron Imaging was used at an accelerating voltage of 10 kV and a probe current setting of 30 mV. The electrical response of piezoelectric sample was evaluated by a Picoscope 4424 digital oscilloscope (Pico Technology) and Keithley (2612B, USA). Dielectric loss and impedance properties were measured with a precision impedance analyzer (4294A, Agilent Technologies, Inc.) at room temperature with a frequency range from 10 2 to 10 6 Hz. To collect ferroelectric hysteresis loops, a ferroelectric test system (TF2000E, aixACCT, Germany) was used.
The experiments involving human subject have been performed with the full, informed consent of the volunteer, who is also a co-author of the manuscript. The device was covered by silicon resin to avoid direct contact with the skin and also protective gloves were worn.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.