Wearable Electronic Textiles from Nanostructured Piezoelectric Fibers

Wearable energy harvesting is of practical interest for many years and for diverse applications, including development of self‐powered wireless sensors within garments for human health monitoring. Herein, a novel approach is reported to create wearable energy generators and sensors using nanostructured hybrid piezoelectric fibers and exploiting the enormous variety of textile architectures. It is found that high performance hybrid piezofiber is obtained using a barium titanate (BT) nanoparticle and poly(vinylidene fluoride) (PVDF) with a mass ratio of 1:10. These fibers are knitted to form a wearable energy generator that produced a maximum voltage output of 4 V and a power density 87 μW cm−3 which is 45 times higher than earlier reported for piezoelectric textiles. The wearable energy generator charged a 10 μF capacitor in 20 s which is four and six times faster than previously reported for PVDF/BT and PVDF energy generators, respectively. It also emerges that the established knitted energy harvester exhibits sensitivity of 6.3 times higher in compare with the piezofibers energy generator. A knee sleeve prototype based on a PVDF/BT wearable device for monitoring real‐time precise healthcare is demonstrated. The developed processing method is scalable for the fabrication of industrial quantities of smart textiles.


Introduction
Garments traditionally perform social and protective functions, but the addition of wearable electronics provides the means to produce a new generation of smart garments.
Such affordable smart garments could fulfil diverse applications, ranging from work wear in specific industries to the almost infinite scenarios of personal use including energy harvesting/storage, force/ pressure measurement, porosity or color variation, and sensors (movement, temperature, chemicals). [1][2][3][4][5][6][7] However, performance, scalability, and cost problems have restricted the deployment of currently available smart textiles. To build smart textiles on an industrial scale, method of manufacturing and material selection are two important requirements. The approach of new energy materials and novel fabrication methods are essential to develop wearable technologies. 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. Large-scale production of energy harvester fibers using conventional manufacturing processes, however, is still a challenge.
Energy harvesting from environmental mechanical sources such as body movements including finger imparting, [8] pushing, [9] stretching, [10] bending, [11] twisting, [12] air flow, [13] transportation movement, [14] and sound waves [15] has attracted widespread attention to promote flexible self-powered devices. [16] The best common mechanical energy harvesting methods are based on piezoelectric materials. [17] Piezoelectric materials can be classified in three categories: piezoelectric ceramics, piezoelectric polymers, and piezoelectric composites. [18] Unlike the energy harvesters utilizing solar or thermal energy, performance of piezoelectric generators is generally not limited by environmental factors. [19] Piezoelectric generators have received massive interest in energy harvesting technology due to their unique ability to capture the ambient vibrations to generate electric signals. [20] The unique energy transduction of piezoelectric materials enables their applications in fields of energy harvesting, actuators, [21] sensors, [22] structural health monitoring, and use in biomedical devices. [23] Numerous approaches Wearable energy harvesting is of practical interest for many years and for diverse applications, including development of self-powered wireless sensors within garments for human health monitoring. Herein, a novel approach is reported to create wearable energy generators and sensors using nanostructured hybrid piezoelectric fibers and exploiting the enormous variety of textile architectures. It is found that high performance hybrid piezofiber is obtained using a barium titanate (BT) nanoparticle and poly(vinylidene fluoride) (PVDF) with a mass ratio of 1:10. These fibers are knitted to form a wearable energy generator that produced a maximum voltage output of 4 V and a power density 87 μW cm −3 which is 45 times higher than earlier reported for piezoelectric textiles. The wearable energy generator charged a 10 μF capacitor in 20 s which is four and six times faster than previously reported for PVDF/BT and PVDF energy generators, respectively. It also emerges that the established knitted energy harvester exhibits sensitivity of 6.3 times higher in compare with the piezofibers energy generator. A knee sleeve prototype based on a PVDF/BT wearable device for monitoring realtime precise healthcare is demonstrated. The developed processing method is scalable for the fabrication of industrial quantities of smart textiles.
have been used to fabricate piezoelectric generators, such as coating, [24] spinning, [25] depositing, [26] and printing. [27] Energy harvesting technology from human body movement is desirable due to new direction in power up portable electronic devices. [28] Among all of fiber generators or sensors, piezoelectric fibers that operate based on the piezoelectric effect as a response to applied strain are especially attractive, due to ubiquitous mechanical vibrations occurring in our daily life that can be harvested into electrical signals. [19] Among the variety of materials exhibiting piezoelectricity, polymers are of interest due to several enhanced properties desirable in flexible piezoelectric generators. Recently, much attention has been paid to poly(vinylidene fluoride) [29,30] (PVDF). This nontoxic material is a semicrystalline polymer that exists in four different crystalline forms (α, β, γ, and δ) [31] depending on the preparation conditions. [32] The β-phase is the desirable phase owing to its ferroelectric nature. [33] Spinning process such as wet spinning, [19] dry spinning, [34] melt spinning, [35] gel spinning, [36] and electrospinning [37] are the most primitive process for producing flexible piezoelectric generator with the ability for mass production. 3D fabric structure has been designed into fabric-based piezoelectric generator as the 3D spacer knitted fabric with the ability to provide power of 1.10-5.10 µW cm −2 under the impact pressures of 0.02-0.10 MPa at a frequency of 1 Hz. [38] Another study showed simple meltspun bicomponent filaments were developed by using PVDF and carbon black/polyethylene to supply power of 1 µW cm −2 at an impact pressure of 20 kPa. [39] While these piezoelectric textiles presented flexibility and energy harvesting performances, their piezoelectric performance (i.e., power output, sensitivity, durability) is still not satisfied because of the poor electrical connection between the piezoelectric fibers and electrodes.
Furthermore PVDF showed limited piezoelectricity and power output because their active material was based on ferroelectric polymers with high piezoelectric voltage constant but a low dielectric constant (ε ∼ 30). One potential method to increase the dielectric constant of the PVDF polymers is to introduce high dielectric constant materials such as inorganic piezoelectric materials (i.e., barium titanate (BT) nanoparticles) into the PVDF matrix. [14,40,41] However, inorganic piezoelectric materials, such as, lead zirconate titanate, [42] BT, [43] sodium niobate, [44] lead magnesium niobate lead titanate, [45] and zinc stannate (Zn 2 SnO 4 ) [46] have high significant piezoelectric performance but their rigid nature limit their application in flexible self-powered devices. [41] Hybrid piezoelectric composites for energy harvesting based on BT is attractive because of its high dielectric constant (ε ∼ 2000), low cost, natural abundance, and environmental friendliness. However, overcoming the limited flexibility and durability of the BT still remains an unavoidable challenge to be addressed for optimization of its energy harvesting performance. Consequently, several research groups have introduced some structural strategies for piezoelectric polymer design to enhance the piezoelectric efficiency by incorporating inorganic piezoelectric nanostructures as an effective piezoelectron pathway. [47] Using this strategy, the voltage and current outputs of the P(VDF-TrFE) nanofibers could be enhanced up to 200% by adding BT nanoparticles into the polymer matrix. [25] It was also reported that flexible piezoelectric energy generators based on PVDF-HFP/BT composite film exhibited high electrical output up to ≈75 V and ≈15 µA. [47] Furthermore, a novel strategy was developed to improve the interface effect of PVDF/ BT nanocomposites to enhance energy density from 6.5 to 9.01 J cm −3 . [48] Nevertheless, such polymer-based energy harvesters still have limitation of complex fabrication methods and low flexibility which could be problems to expand their applications in wearable electronics. To solve these problems, we explore a novel approach to develop high-performance textiles-based energy harvesting devices as next-generation wearable energy generators and sensors. Here, different variations of textile designs from melt-spinning, knitting, weaving, and braiding with the capability of low-cost, high production speed, and high performance are developed.
The high-performance PVDF piezoelectric nanocomposite fibers with and without BT nanoparticles were produced through a melt-spinning process. The effect of BT nanoparticles addition on the piezoelectric properties of hybrid PVDF/ BT fibers with different ratio of the BT nanoparticles was investigated by fabricated these fibers into triaxial braided energy harvesters, as previously reported. [49] The optimized PVDF/BT nanocomposite fibers were then used to create wearable woven and knitted energy harvesters and strain sensors.
The wearable energy harvesting textile fabrics were made from developed piezoelectric fibers (Figure 1). A seamless feed-in process for integrating conductive fibers as electrodes with piezoelectric fibers was established to allow continuous fabrication of the energy harvesting textiles. As-fabricated knitted, triaxial braided and woven energy harvesting devices provide better mechanical properties (i.e., durability, flexibility, and comfort) and piezoelectric performance in compare with PVDF fibers. The developed textile energy harvesting devices are durable, light, and flexible and it is expected to be practical for wearable devices with high performance in the near future.

Morphology and Characterization of PVDF and PVDF/BT Nanocomposite
Scanning electron microscopy (SEM) was applied to reveal the morphological variations of PVDF/BT films and fibers prepared with different ratios of BT to PVDF. As can be seen from Figure S1 (see Supporting Information for more details), an agglomerated structure occurred in the PVDF/BT nanocomposite film that was cast from solution and when the amount of BT nanoparticles was more than 10 wt% in the polymer matrix.
Both neat PVDF and hybrid PVDF/BT were prepared in a two-stage process involving film casting followed by melt spinning. Films were cast containing 0, 5, 10, and 20 wt% of the BT nanoparticles. The as-prepared ground PVDF/BT nanocomposite film was fed into an extruder to aid homogenization by shear forces and flow pressure. It was found that preparing nanocomposites with more than 10 wt% of BT was difficult due to the limited mobility of the polymer chains even at the molten state and this method was not able to disperse the aggregated BT particles within the polymer properly. [50] SEM micrographs of the as-spun PVDF and PVDF/BT nanocomposite fibers are shown in Figure 2. As can be seen from the surface morphology and cross-section (Figure 2a-d), both melt-spun PVDF and hybrid PVDF/BT fibers are very smooth and without any observable porosity or voids. SEM images of the cross-section of PVDF/BT nanocomposite fiber show a very dense structure with the uniform distribution of nanoparticles throughout the fiber (Figure 2b,c). The BT nanoparticles in the polymer matrix can be seen clearly in the fiber cross-section at the higher resolution ( Figure 2e).  shown from elemental mapping analysis in Figure 2g-i,ii,iii, the hybrid nanoscopic structure of the melt-spun PVDF/BT was confirmed through the overlapped Ti, Ba, F mapping images ( Figure 2g).
The phase transformations of the as-prepared PVDF and hybrid PVDF/BT fibers following different amounts of additive (BT) were scrutinized using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC), thermogravimetric analysis, and X-ray diffraction (XRD). Piezoelectric properties of PVDF fibers can be enhanced with a higher fraction of β-phase. The FTIR results (Figure 3a) indicated that the ratio of the BT nanoparticles in the polymer matrix could affect β-phase formation in the PVDF/BT nanocomposite fiber ( Figure 3a). The vibrational bands at 764 and 976 cm −1 are attributed to the nonpolar α-phase, whereas the characteristic peaks at 841 and 1276 cm −1 correspond to the electroactive β-phase. [51] The relative fraction of the β-phase in a sample containing just α and β-PVDF can be calculated from the following formula [52] 1.26 where A α and A β are the absorbance peaks at 766 cm −1 (α-phase) and 840 cm −1 (β-phase). The characteristic peaks of the α-phase severely decrease in the PVDF/BT fiber in compared with the pure PVDF which signify that adding BT nanoparticles is a highly efficient method of inducing fraction of the polar β-phase.
The variation of β-phase fractions for as-prepared PVDF and PVDF/BT nanocomposite fibers is shown in Figure 3b (also see Figure S2 in Supporting Information for the as-prepared PVDF/ BT films). As can be seen from Figure 3b, the as-prepared PVDF/BT nanocomposite fibers exhibited a higher proportion of β-phase compared to pure PVDF fiber. The β-phase content increases from 51% for pure melt-spun PVDF fiber and reaches a maximum value of 98% for as-prepared PVDF/BT nanocomposite fiber containing 10 wt% of BT nanoparticles. The β-phase content of the PVDF/BT nanocomposite fibers decreased as a result of increasing BT percentage for more than 10 wt%. This phenomenon could be explained through the effect of filler on the physical and mechanical properties of the polymer matrix. It was observed that noticeable increase in β-phase initiates from the interaction enhancement between the local electric field close to the nanoparticle filler and the PVDF dipoles. [53] However, increased defects for the case of too much BT content prevent segmental motion and asymmetrical β-phase formation which is due to a decrease in F(β) from PVDF/ BT(10 wt%) to PVDF/BT(20 wt%). [52] The XRD patterns in the 2θ range of 10°-60° for PVDF and PVDF/BT nanocomposite ( Figure 3c) show rise in the intensity ratio of the βto α-phase for as-prepared PVDF/BT nanocomposite fiber in compare with PVDF fiber. Moreover, the peaks related to crystallization plane of the (020) and (110) reflect β-phase formation at 20.6°, indicated the existence of αand β-phase. Peaks at 2θ ∼ 22.2° which correspond to the (001) and Adv. Mater. Technol. 2020, 5, 1900900 (100) in as-prepared PVDF/BT nanocomposite fibers are evident for tetragonal BT nanoparticles. [54] Due to the addition of BT, modifications in the crystallinity of PVDF have been investigated on the basis of the obtained X-ray pattern. As the concentration of BT increases, the intensity of standard peaks in the X-ray pattern of PVDF decreases and slightly displaces toward shorter angle. These peak shifts indicate the existence of specific interaction between the different phases of PVDF and BT. [55] In addition, thermal analysis of the PVDF and hybrid PVDF/BT fibers specified that the crystalline structure formation in the as-prepared piezoelectric fibers is 41%, 62%, 65%, and 48% for the as-prepared PVDF, PVDF/BT 5 , PVDF/BT 10 , and PVDF/BT 20 fibers, respectively (see Figures S3, S4 and Table S1 in the Supporting Information).
The as-prepared PVDF and PVDF/BT nanocomposite fibers have also been investigated to evaluate the effect of BT nanoparticles on the mechanical properties. During melt spinning and cold drawing process, fibers are continually under tension which brings tenacity and elasticity for the final fibers. [56] A comparison of the stress-strain curves for the various prepared fibers is given in Figure 3d. As can been seen from Figure 4 and Table S2 (see the Supporting information for more details), the ultimate tensile strength and elastic modulus of the prepared fibers have significantly increased for PVDF/BT 5 and PVDF/BT 10 compared to PVDF fiber. Young's modulus and tensile strength of the as-prepared PVDF/BT nanocomposite fiber with 10 wt% BT is 130% and 170% higher than pure PVDF fiber, respectively (Figure 4a). The elongation at break of the as-prepared PVDF and PVDF/BT 10 nanocomposite fibers were 137% and 80%, respectively. These values are 685% and 400% higher than previously reported for melt-spun PVDF fiber. [49,56] The use of 20 wt% BT nanoparticles in the polymer matrix produced PVDF/BT 20 fiber with significantly lower Young's modulus and tensile strength compared with the PVDF fiber. These results confirm the reinforcing role played by BT nanoparticles in the PVDF fibers. However, the decrease in mechanical properties of the PVDF/BT 20 fiber with addition of more than 10 wt% BT nanoparticles may be due to aggregation of nanoparticles and/or phase separation of the polymer (see Figure S1 in the Supporting Information). This phenomenon may introduce stress-concentration or low adhesion at the phase interface that would lower the tensile strength and modulus. [57][58][59]

Wearable Energy Generator and Sensor Performance
Initial evaluation of piezoelectric energy harvesting performance of the hybrid fibers used the triaxial braided textile structure described in our previously reported results. [49] The dielectric constant (ε r ) and dielectric loss (tan δ) of the asprepared braided wearable energy generators were measured at room temperature in a frequency range up to 10 6 Hz as shown in Figure 5a,b. It was reported [30,52,60] that piezoelectric properties of PVDF nanocomposite polymer could be significantly improved for the piezoelectric polymer with high dielectric constant and low dielectric loss due to enhancing its electroactive β-phase. The dielectric constant influences the performance of a piezoelectric power generator. Higher dielectric constants in harvester systems lead to larger power output. [60] It should be also noted that the piezoelectric coefficient is linearly proportional to the dielectric constant (ε) of the piezoelectric materials, i.e., d 33 ∼ ε Pr, where Pr is the remnant polarization. [61] The dielectric constant of the braids prepared with the hybrid fibers was decreased with increasing frequency for all samples. However, the dielectric constant values were higher in braids made with higher BT content fibers due to the large dielectric constant of BT ( Figure 5a). The gradual decrease in dielectric constant when measured at higher frequencies can be attributed to the reduction in dipole mobility where the dipoles are not sufficiently mobile to displace to the same extent when the frequency of the applied electric field exceeds the relaxation frequency. [62] On the other hand, the dielectric loss increased only slightly with increasing BT contents.
The dispersion of the BT nanoparticles into the polymer matrix increases the filler/polymer interfacial area and formation of the β-phase fraction and consequently increases the dielectric properties and decreases the loss tangent (Figure 5a,b). Electroactive β-phase formation can be explained by the surface charge/dipole interaction occurring between the BT nanoparticles and PVDF chains. [63] The BT nanoparticles surface charge contributes significantly in the electroactive β-phase nucleation procedure. [64] This step-like decrease in dielectric constant with frequency may be explained by the Maxwell-Wagner-Sillars interfacial polarization mechanism. [65] The voltage output of the braided PVDF and PVDF/BT nanocomposite fibers occurring as a result of mechanical   Figure 6 and Figure S7 in the Supporting Information. Mechanical stimulation of the braid occurred by compressing the braid between a solid flat surface and platen that was driven vertically by an oscillating cam attached to a reciprocating motor. The amplitude and frequency of the cyclic compression were initially kept constant to investigate the effect of fiber composition on the piezoelectric output of the braided energy generator. An example of the developed braided energy generator device is shown in Figure 6a. As can be seen from Figure 6c, the voltage output of the triaxial braided energy generator was significantly improved due to synergistic effects of piezoelectric BT nanoparticles incorporated into the PVDF piezoelectric polymer. As-spun PVDF fiber generated a maximum voltage output of 480 mV, while the voltage output of the braided PVDF/BT 10 fiber was found to be 1100 mV. This value is 229% and 250% higher, respectively, than PVDF fiber or previously reported for melt-spun hybrid PVDF/BT fiber with the same 10 wt% content of BT nanoparticles. [49,19] The voltage output of the braided PVDF/BT 20 with more than 10 wt% BT nanoparticles generated a maximum voltage output of 895 mV which is much lower compared to the braided PVDF/BT 10 fiber. The voltage output can be estimated using the following expression 33 V g YD ε = (2) in which voltage output (V), strain (ε), Young's modulus (Y), piezoelectric voltage constant (g 33 ), and the fiber diameter (D) have the above relation. The D and ε are similar in all of the asprepared hybrid PVDF/BT fibers, and the Y is higher in PVDF/ BT 10 than that in PVDF/BT 20 nanocomposite fibers (≈891 and 412 MPa, respectively). The measured V is higher in the asprepared PVDF/BT 10 fiber than that in PVDF/BT 20 fiber, but the combined analysis indicates the g 33 is much larger in PVDF/ BT 20 than in PVDF/BT 10 meaning the piezoresponse in PVDF/ BT 20 fibers is more sensitive to external stress. [52] Adding piezoelectric nanoparticles to the polymer matrix can negatively affect the piezoelectric performance of the composite because of the positive and negative piezoelectric co-efficient of BT nanoparticle and PVDF polymer and canceling their effect. [30,60,66] The voltage output of the braided PVDF/BT 10 energy generator device under cyclic impact was further investigated. The generated positive and negative pulse signals corresponding to the pressing and releasing process during a cyclic impact are shown in Figure 6b which was selected and magnified from Figure 6c in the colored background region.
A graph of instantaneous power at different load resistances is shown in Figure 6d. The power generated by the as-prepared PVDF and PVDF/BT nanocomposites fibers was calculated through where U(t) is the real-time voltage integrated over time t, R is the external load resistance, and T is the period of load application. [67] With further increasing load resistance, the voltage starts to increase up to 3 V at the load resistance of 1000 kΩ. As a power source, the developed triaxial braided PVDF/BT 10 piezofibers Adv. Mater. Technol. 2020, 5, 1900900 reach the maximum power output of ≈0.21 µW when the load resistance equals to 400 kΩ. Although fibers output impedance based on the measured voltage and current output is ≈1 MΩ but it is covered with internal resistance of the Keithley which was used for measuring these data. The amount for the load resistance is in the range according to the latest researches [68][69][70] which depends on the filler type and sample thickness.
To demonstrate the feasibility of harvesting energy using the flexible triaxial braided piezonanocomposite fibers, a bridge rectifier (four diodes of 1N5817) was placed in the circuit to feed a different capacitor under mechanical pressures (Figure 7a). While this was the more conventional approach adopted for this development stage, one can find a higher efficient circuit by Mirvakili et al. [71] Harvesting energy has a close relationship with the diode characteristics, and a diode with low reverse leakage current is favorable so that the Schottky diode 1N5817 is one the best options at this stage of development. For future, and more extreme target conditions, other more suitable options for higher frequency rates and lower drop voltages are recommended, e.g., Avago HSMS-285C or SDM03U40.
Different capacitors' (0.68, 2.2, 10, and 22 µF) charging performances were carried out upon mechanical force on the triaxial braided piezonanocomposite fibers (PVDF/BT 10 ) in an RC circuit ( Figure 7b) and it is observed that the buildup voltage increases exponentially and reaches a steady state as shown in Figure 7b. In order to calculate the voltage across the capacitor (as shown in Figure 7d) during charging capacitor, Equation (4) was used which is related to the RC circuit charging where C is the capacitance of the capacitor and R is the resistance in RC circuit. The time constant (τ = RC) in experimental and theoretical, has a good correlation with each other as shown in Figure 7d. A slight difference may be due to the power consumption by the measuring unit present in the device during the measurements. The comparison of braided energy generators prepared with different hybrid fibers indicated that the PVDF/BT 10 composition provided optimal performance and these fibers were then used to prepare knitted and woven textiles, as shown in Figure 8. The storage energy calculated for different asdeveloped energy harvesters in the capacitor 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). [72] The energy storage based on the as-fabricated textiles energy harvesters (woven, triaxial braided, and circular knitted structures in Figure 8 where E e is the total energy stored in capacitor (Equation (6), E s is the input mechanical strain energy during a single cycle, and n cy is the number of impact cycles to charge the capacitor. [73] Based on the equations and the charging process of the 10 µF capacitor, the energy conversion efficiency of the as-developed triaxial braided, circular knitted, and woven energy generators is calculated as 27%, 29%, and 40%, respectively (see Discussion S1 of the Supporting Information for more details). The obtained results suggested that the developed textiles energy harvester would establish the viability of such wearable piezoelectric energy generators in real-life applications. The knitted PVDF/BT 10 energy generator was able to charge a 10 µF capacitor in just 400 s under periodic impact and relaxation (Figure 8c 2 ). The obtained results are very promising compared to previous reported systems (see Table S3 in the Supporting Information). The circular knitted structure enables the ready integration of electrodes into the triaxial structure (inner and outer electrodes) which could enhance the collection of charge and energy conversion.
The force sensitivity and power output of the wearable energy harvesters based on PVDF/BT 10 textile structure were compared, as shown in Figure 9a. The sensitivity of the wearable energy generators was assessed by a ratio of voltage output to the applied force when the textiles were subjected to compress using repeated impact as shown in Figure S7 in the Supporting Information. [49] The voltage output of the piezoelectric textiles was proportional to the applied force. It was found that the force sensitivity of the textile energy harvesters was 3, 4, and 10 V N −1 for woven, braided, and knitted energy generators, respectively. The results revealed that the knitted energy generators exhibited an almost 6.3-fold increase in the value (10 V N −1 ) compared to the recent reported. [49] The power output of the wearable energy harvesters was found to be 36.2, 38.8, and 87 µW cm −3 under a periodic compression for woven, braided, and knitted energy generators, respectively. The power output density of the woven wearable energy generator was significantly enhanced and it was 294% and 4578% higher than previously reported for braided and woven piezoelectric energy generators, respectively [49,56] (see Table S4 in the Supporting Information). The novelty of circular knitting and braiding techniques for piezoelectric fibers is their packaging structures which interwinded fibers to each other due to more durability and flexibility. The experimental results in our previous work [49] confirmed the stability performance of the triaxial braided piezoelectric fibers in the bending test during 1000 cycles to a maximum strain of 50% at 0.6 Hz with no change in its performance.
The obtained results suggested that developed wearable energy generators would be able to charge the capacitors to a certain voltage under a specific time for the self-powered electronic devices. Consequently, the integration of the developed wearable energy generator with energy storage device (i.e., rechargeable battery or capacitors) can be of great potential for practical applications, including development of self-powered Adv. Mater. Technol. 2020, 5,1900900  wireless sensors within garments and monitor the status of human health.
In addition, developed wearable energy generator prototype device was used as the integration of garment for self-charging power a rechargeable battery or capacitors. Figure 10 shows the performance of the developed energy generator when integrated into garments for biomechanical energy harvesting and storage during walking and/or running. The voltage output of Adv. Mater. Technol. 2020, 5,1900900   the wearable energy generator could be tailored in the range of 300 (walking) to 1000 mV (running) as shown in Figure 10b. As can be seen from Figure 10c, the wearable energy generator could increase the voltage of the storage capacitor from 0 to 25 mV in 20 s so that the 10 µF capacitor was fully charged after ≈25 steps at a running frequency of 1.2 Hz. This fast charging rate is four and six times faster compared to previously reported for the nanofiber piezoelectric PVDF/BT energy generator and hybridized energy conversion and storage based on PVDF film. [25,74] The choice of 10 µF was sufficient to demonstrate full charging of a capacitor at this stage of development, with minimum input (25 steps), a higher storage (> 500 µF) is recommended for applications outside the laboratory, e.g., sports training demonstrators. In such cases, it is also recommended to increase the voltage of the output of the system through a selection of approaches, e.g., by implementing voltage multiplication, parallel charge-serial discharge techniques.
We have explored the potential applications of the developed wearable energy harvester as movement sensors. As demonstration examples, both wearable and portable textile sensors have been developed as shown in Figures 11 and 12. Wearable movement sensors are useful for real-time precise healthcare applications. Here, a knee sleeve prototype device from a woven PVDF/BT 10 nanocomposite fibers was established to support personal recovery after an injury ( Figure 11 and Movie S1, Supporting Information). A wireless data acquisition board could be assigned to the developed knee sleeve prototype device for data transmission of knee flexion resulting from tension and bending of the woven textile sensor. As can be seen from Figure 11a,b, the developed knee sleeve prototype device generated a voltage output that varied with knee bending angle from the initial (zero bending, Figure 11a Figure 11 a 2 ). A voltage divider is used to connect an expansion board (AnEx board) and a portable wireless device. The AnEx board can be attached to the knee sleeve and linked to it by an external connector. The collected data can be transferred via Bluetooth to a laptop for signal analysis.
To demonstrate potential applications of the developed circular knitted piezoelectric strain sensor for the detection of human and/or industrial activities, knitted wearable piezoelectric fibers were assembled into two wearable sensors with the required structures to track these activities as shown in Figure 12. The sensing performance of the developed knitted piezoelectric PVDF/BT 10 fibers based on assembling core-sheath structure has been evaluated as shown in Figure 12a. 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 12a 1 ,a 2 .
Furthermore, the versatility of circular knitted piezoelectric PVDF/BT10 sensors was demonstrated by knitting the textile with seamlessly integrated electrodes (Figures 9b  and 12b). Sequentially feeding silver-coated nylon, followed by PVDF/BT 10 fibers and then again silver-coated nylon into the knitting machine produced the textile structure shown in Figure 12b. This textile was coated with silicon resin to enhance its durability as well as creating new applications such as hydraulic and/or pneumatic pressure sensors. The performance of the silicon-coated knitted piezoelectric sensor under biomechanical pressure of periodic finger pressure and relaxation was shown in Figure 12b 1 ,b 2 . The response time of the silicone-coated knitted sensor (Figure 12b 2 ) was significantly improved (2.5 s) as compared to core-sheath knitted structure (5 s) in Figure 12a 2 . In addition, the capability of the silicone-coated knitted piezoelectric structure as hydraulic and/or pneumatic pressure sensors was demonstrated in Figure 12c,d and Movie S2 in the Supporting Information. As can be seen from Figure 12c,c 1 , the developed knitted sensor was subjected under hydraulic pressure where 2 mL water was pumped into the developed hollow structure (5 mm diameter and length of 20 mm) and it exhibited very fast response time (≈2 s) with higher voltage output compared to others (Figure 12a 2 ,b 2 ). Figure 12d-d 2 shows knitted sensor under pneumatic pressure of 20 kPa. The development of silicon-coated circular knitted piezoelectric structure capable of sensing pressure these stimuli is of great importance for various applications such as heart rate detection, pressure monitoring, strain gauges, robots, etc., due to its ability to response to bending, twisting, and compression motion. More importantly, developed processing method is scalable for the fabrication of industrial quantities of strain sensing and smart textiles.

Conclusion
In summary, smart textiles based on wearable knitted, braided and woven energy generators and sensors were developed from nanostructured piezoelectric nanocomposite fibers. Hybrid piezoelectric PVDF fiber with and without BT nanoparticles were developed through a melt-spinning process. It was found that high-performance hybrid PVDF/BT 10 piezofiber with 98% of the electroactive β-phase generated a maximum output open circuit voltage of 4 V and a power density 87 µW cm −3 during cyclic compression. These energy generator could charge a capacitor (10 µF) six times faster than previously reported. In addition, the wearable textile-based piezosensors were utilized and designed for various sensing response including hydraulic and/or pneumatic pressure sensors with tunable sensitivity.
The demonstrated processing method is scalable for the fabrication of industrial quantities of strain sensing and energy harvesting smart textiles.

Experimental Section
Materials: The piezoelectric polymer powder provided by brand name Solef 6010 was from Solvay Soleris (Milan, Italy). BT piezoelectric nanoparticles with the mean diameter of 50 nm and with 99.9% trace metals basis were purchased from Sigma Aldrich Company (China). N,N-dimethylformamide (DMF, >99.8%, Merck) as the solvent was used. Silver-plated polyamide yarn (235/36dtex 4 ply) was supplied from Shieldex USA. Woven and knitted conductive fabric, which was silver-plated nylon with weight 80 g m −2 was purchased from core electronics, Australia.
Nanostructured Hybrid PVDF/BT Films: To optimize the ratio of BT nanoparticle into PVDF polymer matrix for enhancing piezoelectric and mechanical properties of the hybrid PVDF/BT fibers, PVDF/BT nanocomposite films with different amount of BT nanoparticle were prepared. To prepare PVDF/BT nanocomposite, 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 overnight. To prepare PVDF/BT nanocomposites solution with different amount of BT nanoparticle wt% (i.e., 5, 10, 15, 20, and 25), the BT nanoparticles were 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 (15 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. Hybrid PVDF/BT nanocomposite films with different percentage of BT nanoparticles were obtained using film casting method with the thickness of 440 µm and dimension of 4 × 2 cm (see Figure S1 in the Supporting Information). All as-prepared samples were subjected to cold drawing process as post treatment at 80 °C which increased the sample length by 13%.
Melt-Spinning of PVDF and PVDF/BT Nanocomposites Fibers: The PVDF and PVDF/BT nanocomposites fibers were produced through melt-spinning process. As-prepared PVDF/BT nanocomposite films which were prepared through film casting were used to produce the met-spun fibers. The met-spinning of PVDF with and without BT was performed with a twin screw extruder (Barrel Scientific Ltd.) and a spinneret with a hole diameter of 3 mm as illustrated in Figure S5 and Movie S3 (Supporting Information). The grounded nanocomposite powder was kept at the temperature of 70 °C overnight and then fed into the extruder. In order to achieve uniform fiber, feeding powder to the spinneret was controlled. The temperature for the nine sequential zones of extruder was set from 180 to 220 °C. Melt-spun PVDF/BT nanocomposite fibers containing 5, 10, and 20 wt% of the BT were prepared (labeled PVDF/BT x , where x = 5, 10, 20 is the wt% of BT into the PVDF polymer (PVDF/BT 5 , PVDF/BT 10 , and PVDF/BT 20 )). The final diameter of the stretched PVDF and PVDF/BT nanocomposite fibers was ≈170 µm.
Fabrication of Wearable Energy Harvester: The scalability and tuneability of the textile fabrication approach was demonstrated for generating wearable energy harvesters that offered an extra design dimension in constructing and optimizing piezoelectric modules, yet this was a totally unexplored area. The textile structure determined the mode of deformation at the individual fiber level (bending, twisting, and/or stretching), allowing the tuning of fiber properties to maximize both mechanical and power-generating performance. Consequently, manufacturing of the wearable energy generator and sensors was carried out using conventional textiles production including knitting, breading, and weaving techniques.
Knitted Wearable Energy Harvesters: Knitted wearable sensors and energy generator based on as-prepared nanostructured PVDF and hybrid PVDF/BT fibers were developed using a Harry Lucas circular knitting machine with the head size of 1/12 in, gauge, 28 and 20 needles. A feeding tension setting of 10 and pickup tension of 32 was applied to fibers and knitted structures, respectively. The linear density of the as-prepared knitted structure was 0.034 g cm −2 . As can be seen from Figure 13 and Movies S4 (see the Supporting Information), as-prepared melt-spun piezoelectric fibers were shown superior mechanical properties and were comparable with commercially available fibers, which enabled to be utilized in knitting machine with the requirement of the applied mechanical stress and strain during the knitting process. To assemble the knitted wearable energy harvester device, the commercially available woven conductive fabric as inner and outer electrodes with thickness of 80 µm was embedded inside and outside of the knitted structure. The use of the circular knitting would be able to provide more protection (i.e., short circuit) for the knitted wearable devices due to surrounding inner electrode by knitted structure. In addition, stripes electrodes could be fabricated into knitted structure as needed and, e.g., a wearable knitted device with two knitted electrodes into the structure was developed (see Figure 9b).
Braided Wearable Energy Harvester: Wearable energy generator and sensors based on as-prepared PVDF and hybrid PVDF/BT fibers were developed using a braiding technique. Manufacturing of the triaxial braided energy harvester was accomplished using a Trenz-Export braiding machine in a multi-step process. The fabricated triaxial braided structure is illustrated in Figure 14. A silver-coated Nylon (235/36 dtex 4 ply thread) was used as the core electrode along the length of 12 braided PVDF fibers. Finally, to provide outer electrode, the whole structure was covered with 12 silver-coated nylon fibers using braiding machine. As previously reported, [49] the developed triaxial piezoelectric energy generator had the ability for mass production to supply required power. Moreover, novel packaging of the triaxial braided structure to protect PVDF fibers and electrodes provided more durability for the piezoelectric energy generator device.
Woven Wearable Energy Harvester: Wearable energy generator and sensors based on as-prepared PVDF and PVDF/BT nanocomposites fibers were developed through weaving process. The preparation was based on plain weave structure which was each weft yarn was passed above and below (riser and sinker) the warp yarns repetitively so formed a simple cross pattern (Figure 15).
In the plain weave, the short length of yarn intertwined between warp and weft yarn which led to have fabric with high density and consequently prevented short circuits between two electrodes.
Moreover, the plain weave structure had homogenous surface to provide moderate constant electrical properties. [75] The thickness of developed PVDF and PVDF/BT woven structure was 260 µm. The linear density of as-prepared PVDF and PVDF/BT woven structure was 0.057 and 0.029 g cm −2 , respectively.
To assemble the woven wearable energy harvester device, the commercially available woven conductive fabric as electrodes with thickness of 80 µm was attached to the top and bottom of as-prepared fabric using sewing machine ( Figure S6; see the Supporting Information for more details).
Poling Process: To enhance piezoelectric performance of the developed wearable energy harvesters, the electric poling procedure for both PVDF and PVDF/BT structures was carried out. For the wearable triaxial braided structures, the poling was done in radial direction (inner and outer electrode) for 25 kV DC at 80 °C, which was equal to the 37 Mv m −1 Figure 13. a) The process for producing a knitted structure by circular knitting machine. b) Circular knitted structure of PVDF/BT fiber, c) magnified image of loops formation in knitted structure, d) optical photograph of the circular knitted piezogenerator: i) conductive woven fabric as electrodes and ii) knitted PVDF/BT fiber as the middle layer. Figure 14. Micrograph of the as-developed triaxial braided piezogenerator: i) silver-coated nylon as electrodes and ii) PVDF fibers in braided structure as the middle layer. of the electric field as previously reported. [49] The poling process for the knitted and weaved structure was also carried out by applying voltage on conductive fabric as top and bottom of the as-prepared structure at the above conditions. Sample Excitation Method: An in-house setup was made for the performance assessment of all three piezoelectric textile structures which could apply periodic impact force. An illustrative graph of the measurement system and assembly details is shown in Figure S7 in the Supporting Information. In order to reduce the interference of triboelectric charges, the samples and impact head were covered with tape. Continues force was applied in frequency of 1 Hz. A Nema stepper motor run by Ultimate board was used as the impact power source, and the generated open-circuit voltage and short-circuit current were collected with a Keithley (2612B, USA) system simultaneously.
Characterizations: DSC (TA Instrument) at a heating rate of 10 °C min −1 was used to measure the melting temperature (T m ) and melting enthalpy (ΔH m ) of the fibers. The characteristic crystalline phases of PVDF fibers were examined through FTIR (Shimadzu IR, ATR mode) study over a range of 400-4000 cm −1 . XRD (GBC, MtriX SSD) was carried out to identify the crystal phase of materials. Surface morphology of the fibers was examined with the field emission SEM (JEOL 7500). Leica M205A stereo microscope was used to measure fibers' diameters. The mechanical properties of the fibers were measured using a Shimadzu tensile tester (EZ-S). The samples were mounted between two grips and were subjected to tensile test with the strain rate of 10 mm min −1 .
The electrical response of piezoelectric sample was measured by a Picoscope 4424 digital oscilloscope (Pico Technology) and 2612B Source Measure Unit (Keithley, 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.
The experiments involving human subjects have been performed with the full, informed consent of the volunteers.

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