Graphene Nanoplatelet Integrated Thermally Drawn PVDF Triboelectric Nanocomposite Fibers for Extreme Environmental Conditions

Triboelectric nanogenerators (TENGs) utilize the synergetic effect of triboelectrification and electrostatic induction to guide electrons through an external circuit, enabling low‐frequency mechanical and biomechanical energy harvesting and self‐powered sensing. Integrating 2D material with a high specific surface area into flexible ferroelectric polymers such as polyvinylidene difluoride (PVDF) has proven to be an efficient strategy to improve the performance of TENG devices. Scalable fabrication of graphene‐integrated PVDF nanocomposite fiber using thermal drawing process is demonstrated for the first time in this study. The open‐circuit voltage and short‐circuit current show 1.41 times and 1.48 times improvement with the integration of 5% graphene in the PVDF fibers, respectively. The TENG fabric shows a maximum power output of 32.14 µW at a matching load of 7 MΩ and a power density of 53.57 mW m−2. The fibers exhibit excellent stability in harsh environmental conditions such as alkaline medium, high/low temperature, multi‐washing cycle, and long‐time usage.


Introduction
The progress in nanoscience and nanotechnology has led to significant advancements in flexible and wearable electronic sensors and equipment.[3][4] The utilization of textiles and textile-based wearable devices holds particular significance due to their capability to gather substantial amounts of psychological data. [5]everal textile-based sensors have been reported for continuous monitoring, namely capacitive, piezoresistive, optical, and DOI: 10.1002/aelm.202300643nanogenerator-based. [6][7][8][9][10][11][12][13][14] However, a key challenge for the sustainable application of wearable devices remains the necessity of low-profile power systems, which is particularly daunting in implanted and textile-based wearables to untether them from rigid and heavy power sources. [15,16]In this respect, nanogenerator-based energy sources that work on several principles, including piezoelectric (PENG), [17] triboelectric (TENG), [18] thermoelectric (TEG), [19] magnetoelastic(MEG) [20] and electromagnetic (EMG) [21] are capable of harvesting mechanical motion eliminating the need of additional power source for the operation.TENGs are low-cost, lightweight, and highly efficient at low frequency compared to other nanogenerators. [22]ENGs have been utilized for different applications by exploiting the wide range of material choices and available operational modes. [23,24]rrespective of TENG's application as an energy harvester or a self-powered sensor, the high energy conversion efficiency desired for practical applications is still far from achieved due to the low surface charge density.[34][35] However, these methods require complex fabrication procedures, special materials, and expensive equipment, which raise the total manufacturing cost, rendering them unsuitable for widespread applications. [28,29]Thus, a material with high charge affinity and electron trapping property is desirable for enhanced triboelectric output.
Polyvinylidene difluoride (PVDF), one of the most widely used ferroelectric polymers, has distinctive polymorphs (, , , , and ), and it stands out from other polymers due to its excellent flexibility, piezoelectric coefficient, electronegativity, spontaneous yet stable and tunable polarization, polarization stability, and high electron affinity. [36][39] Several methodologies have been investigated to improve the triboelectric output of PVDF, including surface functionalization, [40] ion injection, [41] spontaneous polarization, [42] addition of charge trapping interlayer, [43] and dielectric constant increment. [44]The addition of 2D materials that provide charge-trapping sites is an effective way to tune composite materials' dielectric properties. [45]ow-dimensional materials are crystalline materials with strong covalent bonds that provide in-plane stability.Their weak interlayer interaction allows exfoliation to an atomically thin layer that exhibits exceptional electrical, optical, thermal, and mechanical properties. [46]2D materials such as transition metal dichalcogenides (TMDs), transitional metal carbides/nitrides (MXenes), graphene, and graphene oxide have been utilized for triboelectric nanogeneration due to their unique electronic properties. [47]53] Previous studies of graphene derivative-integrated PVDF nanocomposite TENG demonstrated the fabrication of PVDF nanocomposite membrane/film by solution processing [54] or preparation of nonwoven nanocomposite matt by electrospinning. [55,56]Both methods require the deposition of electrodes on the surface of the PVDF nanocomposite layer in a separate process.Additionally, membrane/film-based TENGs are incompatible with textile fabric manufacturing.Even though electrospinning can produce continuous electro-spun fiber, the requirement of electrode material deposition in a separate process is still challenging and time-consuming.In this respect, the thermal drawing process (TDP) can preserve the design of a multi-material preform from the macroscopic level into the fibers at the microscopic and even nanoscopic length scales.Moreover, TDP can overcome the existing challenges by continuously co-drawing electrode and triboelectric material to kilometer-long fiber at one draw within a few hours with an appropriate preform.
Herein, we have demonstrated the scalable fabrication of graphene nanoplatelet (GNP) integrated thermally drawn PVDF nanocomposite fiber for the first time.A comprehensive study on the fabrication and characterization of GNP-PVDF under harsh environmental and wear conditions was performed.The addition of GNP and subsequent thermal draw of nanocomposite fiber improves the  phase transformation of PVDF up to 13% compared to unprocessed PVDF films, an indicative parameter for the potential of improved triboelectric performance.The fabric sensor exhibited a power density of 53.57mW m −2 , and the open-circuit voltage (V OC ) and short circuit current (I SC ) output increased to 1.41 and 1.48 times compared to pristine PVDF fiber, respectively.The fiber demonstrated stable output during multi-cycle heating, cooling, washing, and long-time usage.

Figure 1a
,b shows a detailed schematic for fabricating triboelectric nanocomposite film and fiber.First, PVDF sheets were dissolved into dimethylformamide (DMF) by continuous stirring on a hot plate, and an ultrasonicated GNP-DMF solution was added to the PVDF-DMF solution.The stirring was continued until a uniform GNP-PVDF nanocomposite solution was obtained.The solution was then cast on a glass plate, and DMF was allowed to evaporate to form a GNP-PVDF nanocomposite film.The dry film was peeled off from the glass plate and rolled over a CPE rod (prepared by hot pressing).Finally, a sacrificial PC layer was rolled over the tribo-negative layer to assist during the thermal drawing.By this process, four separate fibers were prepared for comparative study-PVDF fiber without GNP, 1% GNP-PVDF fiber (PGr-1), 3% GNP-PVDF fiber (PGr-3), and 5% GNP-PVDF fiber (PGr-5).This method promotes the electroactive phase transition of  phase to  phase, enhancing triboelectric charge generation (Figure S1, Supporting Information). [57]n the TDP, a macroscale preform is drawn to a long-length fiber at elevated temperature by axial stress application. [58,59]The diameter of the fiber is governed by the feed speed, drawing speed, and preform Dia, which is expressed by the following equation: [58] where v feed is the feed speed and v draw is the drawing speed.Complex shear viscosity, storage modulus, and loss modulus are the important criteria during the multi-material thermal draw. [60]omplex shear viscosity ( ☆ ) is a rheological attribute of a material that describes the relation of stress to strain rate in viscoelastic fluid in the case of oscillatory shear stress. [61]A material's elastic behavior is presented by storage modulus (G'), described as the ability of a material to store and recover energy during deformation.On the other hand, the material's viscous behavior is expressed by loss modulus (G") and described as the material's ability to dissipate energy during deformation.For thermoplastic polymers, G' dominates over G" at room temperature.A stable draw can be achieved in the vicinity of a temperature range where the G" slowly increases with the increase of temperature and the G' decreases rapidly. [62]Thus, choosing a material with similar glass transition temperature and comparable complex shear viscosity is essential for the multi-material thermal draw.Moreover, multi-material fiber draw experience axial and radial velocity mismatches due to the parabolic temperature field experienced by preform in the radial direction during thermal drawing, which tends to disrupt uniform flow profile. [63]Thus, the drawing temperature was needed to be optimized.For this drawing, optimum fiber draw parameters were determined at 240 °C and 80 g tension, facilitating mostly uniform fiber draw.This process made it possible to continuously draw tens of meters long fiber (Figure S2, Supporting Information).The scanning electron microscope (SEM) and laser scanning confocal microscope (LSCM) image of the fiber showed the presence of a three-layer structure of the preform: CPE core (650 μm), GNP-PVDF nanocomposite (diameter 970 μm, thickness 160 μm) and PC (diameter 1170 μm, thickness 100 μm) (Figure 1c,d).

Materials Characterization
Raman spectroscopy is used to acquire information about the vibrational modes of a molecule. [64]It is an effective tool for determining carbon-based material's D-band, G-band, and 2D band. [65]Raman spectroscopy of GNP proved the presence of D band, G band, and 2D band, respectively, for wavenumbers 1350, 1570 , and 2700 cm −1 (Figure S3, Supporting Information).The presence of the G band and 2D band confirmed the presence of graphene. [66,67]Additionally, it was possible determine multilayered nature of GNP by analyzing the I 2D and I G peaks.The intensity ratio of I 2d and I G decreases with the increase of layer number. [68]I 2D / I G for single-layer GNP is 3.3; its value is 0.5 for seven-layer GNP. [68]The I 2D /I G value obtained was 0.29, proving the multilayer GNP structure's presence.The characteristic peaks of the three crystal phases of PVDF (, , and ) in FTIR spectra were detected as  phase peaks at 763795, 854, and 870 cm −1 , insignificant  phase peaks at 510, 840, and 1234 cm −1 and  phase peaks at 833 and 1279 cm −1 (Figure 2a). [69]Post-thermal drawing FTIR spectroscopy of the pure PVDF fiber to PVDF nanocomposite fibers of varied concentration demonstrated a decrease of  peak intensity and an increase of  peak intensity, manifesting the transition of  phase to  phase. phase can be calculated using the following formula [70] : where F() represents the relative fraction of  phase content; A  and A  are the absorbance at 763 and 840 cm −1 , which are associated with non-polar  phase and electroactive  phase, respectively; K a (6.1 × 10 4 cm 2 mol −1 ) and K b (7.7 × 10 4 cm 2 mol −1 ) are the absorption coefficients at the respective wavenumbers.The  phase is calculated in the commercially available PVDF film as 30% (Figure S4, Supporting Information).After the thermal drawing of only PVDF-TENG fiber, the  phase increased to 35%.This 5% increment can be attributed to thermal drawing at 80 g wt drawing tension. [71]However,  phases of thermally drawn nanocomposite fibers demonstrated significant improvement and were increased to 37.6%, 39.5%, and 43.3% for PGr-1, 3, 5% TENG fibers, respectively.The additional contribution of the  phase increment directly came from the addition of GNP at different concentrations. [72] phase transition in polymer melt occurs due to the application of tensile stress as higher tensile stress promotes chain re-orientation from  phase to the electroactive  phase of PVDF.Higher tensile stress induces higher -phase content. [73]Further increment of the  phase is restricted due to the polymeric material's viscoelastic nature, which changes nonlinearly with increasing applied stress. [74]Each component's viscosity must be comparable at optimum drawing parameters for multilateral thermal drawing.At very high drawing stress, viscosity changes rapidly, resulting in discontinuous thermal drawing hampering the continuous long-length TENG fabrication at our desired fiber diameter.
Moreover, XRD spectra of  phase PVDF locates at diffraction angles: 2 = 17.7°, 18.3°, 19.9°, and 26.5°, corresponding to the ( 100), ( 020), (110), and (021) planes while high polarity  phase demonstrated peak diffraction angle 2 = 20.2°corresponding to 110/200 plane. [75]The intensity of the  phase peaks showed a decreasing trend with the increased GNP percentage.On the contrary, the intensity of the  phase peak at 2 = 20.2°increased with the addition of GNP (Figure 2b).Even though, both FTIR and XRD proved the increment of the  phase, the  phase does not diminish completely as it is the most stable phase with the lowest free energy. [76]Thus, it coexists with an increased  phase percentage.Adding low-dimensional material like GNP acts as a nucleating agent, which assists the formation of the  phase during solution casting without any chemical modification or functionalization. [72,77]During the ultrasonication and magnetic stirring process, localization of static charge occurs on the GNP surface.Due to this static charge generation, a weak electrostatic attraction occurs between the PVDF and GNP.This interaction re-orients the polymer chain of PVDF into a more crystalline structure by converting to  phase. [77]Evaporation of the solvent induces a polarization-locking effect, which prohibits the spontaneous regeneration of  phase from  phase. [78,79]Furthermore, the thermal drawing of PVDF below its melting temperature exerts uniaxial stress on the polymer chain, aligning it towards the direction of the polymer flow, which increases the degree of crystallinity and assists in conversion to the  phase. [80]astly, surface roughness also assists in triboelectric charge generation.Since triboelectrification is a phenomenon induced by physical contact between two surfaces, contact mechanics between two materials interface affects the generation of tribo charges. [81]Depending on the surface energy and material's mechanical properties, surface roughness can increase or decrease triboelectric output. [82]The real contact area between two materials can be maximized when a soft material is pressed against another soft material (Figure S5, Supporting Information).In the case of our system, PEI and PVDF are soft and flexible polymeric materials (Young's modulus ≈ 3 GPa) that allow an increase of triboelectric output with the increase of surface roughness due to the increment of real contact area. [83,84]Furthermore, the sur-face profile was measured for pure PVDF and PGr-5 fibers along the fiber's longitudinal axis by the maximum height of profile (Rz) and arithmetical mean height (Ra), (Figure S6a,b Supporting Information). [85]Pure PVDF fiber demonstrated Ra = 0.029 μm, Rz = 0.285 μm while PGr-5 demonstrated increased surface roughness (Ra = 0.031 μm and Rz = 0.322 μm) (Figure S6c,d Supporting Information).LSCM imaging of the pure PVDF fiber and PGr-5 further demonstrated the increased surface roughness as well as uniform distribution of GNP at the surface of PGr-5 fiber (Figure S7, Supporting Information).

Electrical Characterization
The working mechanism of the fabricated TENG fiber was based on the fundamental single electrode (SE) mode, and the coupling effect of contact charging and electrostatic induction produces the output. [86]In this SE mode, only one triboelectric layer is directly in contact with the electrode, leaving the opposite layer free.The contact and separation between two layers create an electrostatic imbalance that induces charge exchange between the ground and the electrode. [87]At the initial state (non-contact state), the system is in a balanced state, i.e., there is no charge transfer between PEI and PVDF and no potential difference generated between electrode and ground (Figure 3a-i).When PEI comes in contact with PVDF due to external mechanical force, charge transfer occurs between two materials depending on their triboelectric polarities. [88]Electrons from PEI get injected into PVDF, creating a net negative charge on PVDF and a net positive charge on the PEI surface.As the two opposite charge is confined to the surface at the compressed stage, it resulted in a net zero electrical potential difference.Thus, the system is still balanced state as the PEI carries a positive charge and PVDF carries the same amount of negative charge (Figure 3a-ii).Polymeric materials are also insulators, and can retain the charge for a long time. [89]However, as the separation between PEI starts to take place, V OC starts to increase until it reaches a maximum value.Thus, the electrostatic balance of the system breaks and a potential difference is created under the open circuit condition, which triggers instantaneous electron flow from CPE to the ground due to electrostatic induction (Figure 3a-iii).After, electron is released from CPE, electrostatic balance is established and no electron flow is observed at this stage (Figure 3a-iv).Next, the top PEI starts to lower again towards the PVDF surface, and as a result, equilibrium is lost again as the primary electrode possesses a lower electrical potential.Thus, electron will move back from ground to electrode producing instantaneous current from opposite direction (Figure 3a-v).When two-layer touches again, electrostatic equilibrium is re-established.By the nature of electron flow in two opposite direction, an AC signal is generated after the completion of one contact separation cycle.
Electrical characterization was carried out using a custommade tapping device, an oscilloscope and a current preamplifier (Figure S8, Supporting Information).The output of a TENG can vary depending on the materials, contact area, and the frequency of the applied mechanical motion. [90]Figure 3b,c represent V OC and I SC for PVDF fiber with different GNP concentrations at a 3 cm contact area a 5 Hz frequency.PGr-5 fiber demonstrated V oc up to 18.4 V, which was 1.41 times higher than pure PVDF fiber (V oc for PVDF only fiber is up to 13 V).PGr-1 and PGr-3 showed similar trends increasing V OC up to 1.13 and 1.29 times, respectively.I SC also presented a similar trend by increasing 1.09, 1.34, and 1.48 times for PGr-1, 3, and 5, respectively.Several factors contribute to the increase of the triboelectric output for an increased percentage of GNP.The following equations gives the open circuit voltage (V OC ), short circuit current (I SC ) and short circuit charge transfer (Q SC ) by this cyclic contact separation process [91] : where  is the average surface charge density of the materials, S is the contact area of the device, and C 0 is capacitance using vacuum as the dielectric. [92]Equations ( 3) and ( 4) reveal the dependency of the triboelectric output on contact area and surface charge density.Moreover, the charge density of the surface depends on the capacitance and the dielectric property of the functional surface and is expressed as follows [93] : here, C is the capacitance,  0 is the permittivity of the vacuum, ɛ r is the relative permittivity, V is the fiber's surface potential, and t is the thickness of the fiber.Thus, the performance enhancement of a TENG depends on improving the functional surface's capacitive property, dielectric property, and surface charge density.Pure PVDF only facilitates electron displacement polarization and dipole orientation polarization.However, along with electron displacement and dipole orientation polarization, the GNP-PVDF nanocomposite also demonstrates the enhancement of the interfacial polarization, further increasing the composite's dielectric property. [83]Evenly dispersed GNP in PVDF matrix forms microcapacitor where the electroactive  phase of PVDF is sandwiched between parallel GNP layers forming a parallel plate capacitor setup (Figure S9, Supporting Information). [83,94,95]urthermore, surface charge density in a TENG device is determined by the combined effect of surface charge generation and charge decay.Charge decay can occur in three modes: air breakdown, triboelectric charge drift, and diffusion (Figure S10, Supporting Information). [96]Air breakdown occurs when the triboelectric layer surface is discharged due to interaction with N 2 gas in the air, which renders them ionized. [97]Triboelectric charge drift occurs when surface charge follows the electric field created between positive and negative surfaces which promotes triboelectric charge to escape the negative surface and get neutralized.Last, charge diffusion occurs due to the electron concentration gradient.Thus, electrons diffuse into the bottom electrode. [98]Stable output is observed when a dynamic equilibrium is reached through the simultaneous effect of charge generation and decay. [96]The microcapacitors in GNP-PVDF nanocomposite acts as a charge trapping site which improves the capacitive property and surface charge density. [99]Furthermore, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels also affect triboelectric output.Deformation of molecular orbital (lowering LUMO energy level) can increase triboelectric output as it improves the electron-accepting ability of the material by facilitating electrons in a lower energy state. [100]Graphene and graphene-based materials are known to decrease LUMO energy levels in Gr-PVDF nanocomposite materials. [101]Figure S11, Supporting Information, represents the V OC and I OC for PGr-5 fiber for different contact lengths at 5 Hz frequency.For 1 cm contact length, the output voltage reached 10.2 V.With the increase of contact area, output voltage increased successively and reached 23.2 V for a contact length of 4 cm.I SC followed a similar trend increasing from 11.6 μA for 1 cm contact length to 23.6 μA for a contact length of 4 cm.Contact area determines the amount of charge transferred between two materials.An increase in contact area increases the charge density by increasing the total amount of charge transfer between two materials. [102]The effect of contact frequency on triboelectric output is shown in Figure S12, Supporting Information.For PGr-5, V OC was observed at 7.6 V for 1 Hz tapping frequency with 3 cm contact length.With increased tapping frequency, output voltage increased successively, demonstrating a value of 24 V at 7 Hz frequency.I SC also followed a similar trend with an output current of 26.4 μA at 7 Hz.A faster contact separation cycle enhances triboelectric charge density during a highfrequency tapping cycle by keeping high surface charge density as it allows lower charge decay time.On the contrary, lower contact frequency leads to increased charge decay during the contact separation cycle, which affects the total triboelectric output.Figure 3d represents the peak power output of PGr-5 and pure PVDF TENG fibers when subject to tapping at 5 Hz with a 3 cm contact area.The PGr-5 fiber showed a peak power of 12 μW, which is 2.08 times higher than pure PVDF fiber.The overall performance of the fiber indicated its potential in self-powered sensing and energy harvesting due to human motion.
Flexible TENG fibers have the potential to be integrated directly to the clothes through weaving, knitting or braiding process for continuous monitoring of human motion and harvesting energy generated during human motion.For the quantitative assessment on the sensing and energy harvesting capability of the fabricated PGr-5 TENG fiber, 1 m of TENG fiber was weaved into a hybrid cotton-synthetic fabric structure where cotton yarn was used as the warp yarn and continuous TENG fiber was employed as a weft yarn to make a 5×2 cm woven plain fabric structure (Figure S13, Supporting Information).Figure 3e and Figure S14a, Supporting Information, demonstrate the fabricated sensor's V OC and I SC waveform with different contact frequencies.With the increasing frequency, the V OC increased from 4.8 V for 1 Hz to 32.8 V for 7 Hz, while I SC followed a similar trend and increased up to 36 μA for 7 Hz frequency.An increase in contact area also increases V OC and I SC .For the 2 cm 2 contact area, the V OC and I SC were 16 V and 16.4 μA, respectively while for an increase in contact area up to 8 cm 2 , the V OC and I SC increased to 28.4 V and 30 μA, respectively (Figure 3f and Figure S14b, Supporting Information).Regarding the variation of contact frequency and contact area, fabric TENG followed an output trend similar to fiber, as evident in electrical characterization.However, the increment of contact total area does not result in a proportional increase of triboelectric output of fabric as compared to fiber.The marginal performance improvement of the devised TENG fiber occurred due to increasing the total continuous fiber length for fabric TENG, which also increased the equivalent series resistance of the electrode, as evident by Ohm's law. [103]Thus, the effect of contact area increase on triboelectric output is suppressed.This shortcoming can be addressed by attaching short lengths of TENG fiber in an equivalent parallel circuit.However, it will diminish the advantages of scalability, a significant advantage of the TDP.One method of performance improvement of the polymer electrode can be adopted using highly conductive filler materials.Metallic and carbon-based materials are widely used to impart conductive properties on polymer electrodes.Metallic fillers are reported to demonstrate higher concompared to carbon-based materials. [104]Thus, integrating metallic nanomaterials can significantly improve the performance of the electrode.Another approach can be considered using a highly conductive core by convergence drawing of metal wires that will limit the flexibility of the fiber. [105]Lastly, liquid metal can be an option for an ideal core material to preserve the fiber's flexibility.However, liquid metals may need supercooling to maintain their liquid state over the entire service temperature range. [106,107]he output power of the Fabric TENG was investigated by varying the load resistance in series from 1 to 100 MΩ.The output power of the fabric sensor can be calculated as P = V 2 /R.Considering the inherent capacitive behavior of the sensor, the power output can be represented as [106] : here, Z c is the magnitude of its internal impedance.Peak power is achieved in matching load conditions.Maximum power of 32.14 μW was obtained at a matching load of 7 MΩ.Thus, the impedance of the fabric sensor was estimated to be 7 MΩ (Figure 3g).Next, to quantify the effect of support warp yarn's thickness on triboelectric output, three fabric sensors were fabricated with three different support yarns (287, 440, and 897 tex) (Figure S15a Supporting Information).With the increasing thickness of the warp yarn, the power density and current density of the fabric TENG decreased rapidly.TENG fabric with 287 tex warp yarn showed a peak current density of 6.67 mA m −2 and a power density of 53.57mW m −2 .These values decreased successively with increasing cotton thickness and reached a peak current density of 1.16 mA m −2 and power density of 3.36 mW m −2 for 897 tex warp (Figure S15b,c, Supporting Information).The power output also decreased with increasing warp yarn thickness (Figure S16, Supporting Information).This occurred due to the fact that increasing cotton yarn thickness resulted in wider spacing between warp and weft thread, decreasing picks per inch (PPI) that describes the number of weft threads per inch of woven fabric. [108]The ratio of PPI to EPI for fabric woven with 287 tex yarn was 4.62.But, this ratio dropped to 1.16 for fabric woven with 897 tex cotton yarn.This effectively reduced the triboelectric output by decreasing the total available contact area.

Durability Test
Throughout its lifetime, a textile TENG sensor may need to undergo thousands of tapping cycles due to cyclic contact separation movement between the sensor and the human body by maintaining mechanical stability when it is utilized as a wearable device.The fabric sensor was subjected to tapping cycles up to 75 000 times to determine its performance.No crack or deformation was visible over the fiber surface even after 75 000 tapping cycle (Figure 4a).Moreover, the V OC of the fabric sensor was tested up to 75 000 tapping cycles, demonstrating a relatively constant output voltage of 30 V (Figure 4b).PVDF is known for its superior physical and chemical durability as it shows excellent resistance to degradation to various chemical and environmental conditions. [36]Chemically, PVDF is resistance to oxidation and corrosion, including organic acids, salts, weak bases, and aliphatic, aromatic, and chlorinated solvents, proving its potential as an ideal material for wearable applications. [36]PVDF and GNP-PVDF nanocomposites are inherently hydrophobic materials that exhibit low wetting ability. [45,109]nother major requirement of a TENG fabric sensor is washability, including multiple washing cycles, being subjected to friction with constituent support yarn (cotton, polyester, wool, etc.).Moreover, the fabric sensor must be resistant to the mildly alkaline environment (pH 10) created by a standard detergent solution, which constitutes anionic groups, long-chain hydrocarbons, and a single non-ionic group. [110]To exemplify its ability to withstand washing conditions, the PGr-5 fiber was first subjected to a soap solution test in an alkaline medium (Figure S17, Supporting Information).A 12 cm long TENG fiber was immersed in 0.5% detergent solution and stirred continuously at 60 °C for 2 h.The output voltage of the fiber was measured after each washing cycle which was repeated for six times.The washing cycle produced fairly constant output indicating the ability of PGr-5 fiber for stability in alkaline medium.Furthermore, during high-speed rotation inside a washing machine, fabric faces centrifugal force (pushes the fabric outwards against the walls of the drum during the spin cycle), friction force (opposes the motion of the fabric against the walls of the drum), G-force (due to spinning of the drum) and water pressure. [111]The fabric sensor's ability to withstand these forces to sustain constant output was manifested by subjecting it to high-speed rotation-induced stress inside a washing machine at 60 °C with 1000 rpm for 2 h and output voltage was measured.The cycle was continued for a total of 12 h.In both cases (soap solution and washing machine test), the fiber and fabric sensor produced fairly constant output during multiple washing cycles (Figure 4c,d).In addition, during day-today usage, the fabric sensor has to get in contact with sweat.The long-term stability of the triboelectric output should not be degraded due to contact with sweat, which contains electrolytes like dissolved sodium and chloride secreted from sweat glands. [112]To determine its salt water stability, PGr-5 fiber was immersed in 4% salt water solution and after 100 h of immersion, the output remained fairly constant (Figure 4e).Multiple cooling and heating cycles also produced sustained out-put (Figure 4f,g).This work proved the durability of wearable TENG in harsh weather conditions.Further details of the tests were provided in the experimental section.In addition, PGr-5 TENG fiber's superior flexibility made it compatible with basic textile processing, such as bending and knotting which are necessary requirements to weave or knit fabric structure.The fabric sensor preserved flexibility in the horizontal and vertical directions even after weaving (Figure S18, Supporting information).

Temperature Dependence of the Triboelectric Output
Wearable devices are frequently subjected to outdoor environments, often experiencing freezing cold or extreme heat.Temperature can fluctuate rapidly at different hours throughout the day.Moreover, the temperature inside a shoe can increase up to 8 °C after 15-20 min of running. [113]Thus, standardizing TENG output at a wide temperature range is necessary before integrating it with wearable equipment.In PGr-5 TENG, charge transfer from PEI to GNP-PVDF composite film occurs due to triboelectrification. On the contrary, reverse charge transfer from PGr composite to PEI occurs due to thermal fluctuation. [114]Constant charge quantity in the triboelectric surface is achieved after a dynamic equilibrium is established due to electron' s forward and backward movement.At high temperatures, this backward movement tends to rise significantly, reducing the triboelectric surface's charge density and overall triboelectric output. [115,116]For determining temperature dependence on the triboelectric output of PGr-5 fiber, a small custom motor-driven tapping device (2 cm × 1 cm contact area) was used inside a closed heated chamber (Figure S19, Supporting Information).PGr-5 fiber showed high triboelectric output at room temperature due to the small extent of this thermal fluctuation.V OC and I SC output remained stable until 80 °C, and further increase in the temperature caused lower outputs (Figure 5a,b).The power output at room temperature was significantly higher than at 100 °C (Figure 5c,d).The peak power output at 100 °C decreases by 46% (1.18 mW) compared to the room temperature peak power output of 2.2 mW (Figure 5e).This phenomenon occurred due to the greater extent of thermal fluctuation, which promoted the backward movement of energetic electrons to surpass barrier height. [117]At low temperatures, the electron cannot acquire sufficient energy to surpass the barrier; thus, produced stable triboelectric output over a large temperature range for PGr-5 TENG.

TENG Fabric for Self-Powered Sensing and Energy Harvesting
Evaluating the fabric TENG performance in the case of basic human motion like walking, hand movement, arm bending, and grasping is a way to estimate its applicability for real-world application.The ability of pattern detection is essential for any physical movement monitoring sensor as it helps to analyze and understand deviation from the desired motion of interest in case of an anomaly. [118]For real-world applications, first, the lower panel of heel portion of a sock was replaced with the flexible PGr-5 fabric sensor demonstrating its ability to integrate with textiles (Figure 6a).The devised smart sock was successfully employed for gait analysis for a regular gait cycle which include heel contact, toe contact, heel leave, and toe leave. [119]During a complete walking cycle, the first two peaks represent heel contact and toe contact causing successive positive peaks.The following two peaks represent heel separation and toe separation causing successive negative peaks.As depicted in Figure 6b, the smart sock detected all four movements during a walking cycle separately, demonstrating its superior sensitivity and selectivity.Thus, the sensor can be used to study abnormal movement patterns caused by a variety of neurologic or orthopedic such as Parkinson's, [120] runners dystonia [121] and musculoskeletal abnormality, and it can be utilized for initial disease detection and subsequent rehabilitation process.Furthermore, the smart sock detected movement at various speeds characterized by the variable output voltage.At walking speed, the force exerted over the sensor was much smaller.This corresponds to a lower contact area and lower voltage output. [122]However, in the case of jogging and running, the exerted force increased the overall output voltage (Figure 6c).The sensor also detected the freezing of gait (FOG) characterized by a sharp bottom peak with high negative amplitude and successive irregular peaks without generating any high positive peak indicating that the feet are still off the ground (Figure S20, Supporting Information).Other information related to walking movement, like cadence [123] (number of steps taken per second, for slow walk ≈2, for fast walk ≈4), can be analyzed from the signal generated by the smart sock through the period of walking cycle (Figure S21, Supporting Information).Moreover, human footsteps can be counted by attaching the sensor on both socks, as each peak in the voltage output corresponds to a footstep during human motion (Figure 6d).
After that, two fabric sensors were fitted between the forearm and upper arm region on both hand which demonstrated detection ability of arm movement separately (Figure S22, Supporting Information).Furthermore, the sensor was subjected to hang grasping by fitting the sensor in the palm of two hands and a higher grasping force was applied for right-hand grasping.The sensor could detect the difference of applied pressure by larger voltage output, as evidenced by Figure S23, Supporting Information.These demonstrations prove the fabric TENG's potential of direct integration with human clothing in the form of socks, gloves, knee sleeves, elbow sleeves, and so on for physiological monitoring.However, due to non-intimate irregular contact of the fabric sensor with the body, the voltage output presents a high amount of noise.
TENG sensors integrated with clothing provide an excellent opportunity to scavenge mechanical energy from human motion and convert it to electrical energy.Figure S24, Supporting Information, shows that full hand tapping on the fabric sensor can lead to an V OC of 110 V and a I SC of 100 μA.However, TENG produces alternating current and a rectifier circuit is essential for AC to DC conversion as AC output is unsuitable for many electronic devices. [124]For this reason, a bridge rectifier was connected to the fabric sensor for AC to DC modulation.Figure S25, Supporting Information, the conversation of the AC signal generated from the sensor to the DC signal as desired which can be utilized to charge commercial capacitors.The fabric sensor was connected to a 10 μF capacitor through a bridge rectifier to confirm its ability to charge energy storage devices.The fabric sensor demonstrated sufficient energy generation due to human motion to run a portable electronic device (table clock).Hand tapping of the sensor for 1 min stored enough charge in the capacitor (10 μF) to run the clock for 7 sec (Figure S26, Supporting Information, and Video S1, Supporting Information).

Conclusion
In this study, successful fabrication of GNP integrated PVDF triboelectric fiber through thermal drawing was presented.In this process, we can fabricate potentially kilometer-long fiber that proves its applicability in industrial-scale production.Moreover, the fiber output increases with increased GNP content as 2D materials enhance PVDF's  phase and surface roughness.The peak power output of the PGr-5 increased by 208% compared to pure PVDF fiber.The fabricated sensor can generate a power density of 53.57mW m −2 , enough to power up electronic devices like table clocks.The power density was higher compared to recently devised textile based TENGs in SE mode (Table S1, Supporting Information).Additionally, the fiber is durable enough to endure textile processing and rough weather conditions, maintaining relatively constant output.The fiber demonstrated long-term stability in various harsh weather conditions.We have shown that the thermally drawn GNP-integrated PVDF TENG fiber has the potential to be directely integrated with textiles for psychological monitoring and energy harvesting.We envision that this work will contribute to functional fiber processing, energy harvesting, sensing and pave the way for advancing low-dimensional material-integrated triboelectric fiber and textile-based electronics.
Sensor Fabrication: Sensor fabrication starts with the fabrication of nanocomposite film.25 wt% PVDF was dissolved in dimethylformamide (DMF) by constant mechanical stirring in the hot plate at 60 °C for 1 h.At the same time, graphene was subjected to ultrasonication in DMF solution for 1 h.Afterward, the graphene-DMF solution was transferred to the PVDF solution and the mechanical stirring process continued for 7 h.After that, the PVDF-graphene solution was cast onto a glass petri dish and the DMF solution was evaporated by treating at 80 °C for 8 h to get graphene-PVDF nanocomposite film of 1%, 3%, and 5% concentrations.The electrode used for the fabrication was CPE in continuous film form.It was subjected to mechanical compression inside a mold at 170 °C to get a cylindrical-shaped CPE rod of 12 mm.Then, PVDF nanocomposite film was rolled over the CPE to get the desired tribonegative layer in the preform stage.An additional sacrificial PC layer was rolled over the PVDF nanocomposite layer to ensure uniform thermal drawing.The final diameter of the preform was 15 mm without the PC layer and 25 mm with the PC layer.The prepared preform was annealed at 90°C in a vacuum for 24 h.Then, it was consolidated using a three-stage process inside a custombuilt furnace-i) annealing at 150 °C in a vacuum for 2 h, ii) consolidating at 190 °C for 45 min and iii) subsequent cooling at room temperature.Fiber draw was carried out in a custom-built thermal draw tower at 240 °C.At the same time, applying controlled tension by capstone pulling ensured uniform diameter enabling continuous draw of tens of meters of fiber.Later, the PC sacrificial layer was dissolved in DCM to get grapheneintegrated PVDF TENG fiber.The fabric sensor was woven together with cotton yarn using a handloom.
Materials Characterization: The morphological structure of the fiber was investigated using FEI QUANTA 200F field emission SEM.FTIR spectroscopy was performed using Bruker ALPHA Platinum-ATR FTIR 256 scan steps with a scan resolution of 0.5 cm −1 .Laser DIC images were taken using Keyence VK-X100 3D Laser Scanning Confocal Microscope.The XRD analysis was performed with Pananalytical X'pert Pro XRD using Cu K radiation.Raman spectroscopy was performed using WITec al-pha300RSplus.
Electrical Characterization: For the electrical measurement of the drawn fiber, 12 cm of fiber was cut, and 2 cm of the electrode was exposed and wrapped with conductive tape.Open-circuit voltage was measured in a single electrode (SE) set up using an oscilloscope (Agilent Technologies DSO1014A) and short circuit current was measured using a low noise current preamplifier (Stanford Research Systems SR570).A custom-made tapping device of varied contact areas (2/4/6/8 cm 2 ) was used for cycling tapping.
High-Temperature Electrical Characterization: The 12 cm cut length of PVDF-graphene TENG fiber was placed over a hot plate inside a closed chamber to characterize TENG fiber at elevated temperature.Each fiber was allowed to be heated for two hours before the measurement.A small custom-made tapping device of contact area (2 cm 2 ) was used, which was powered by a Waveform generator (Agilent 33210A).
Durability Test: To test the washability of fiber, a 12 cm long PGr-5 TENG fiber was cut, immersed in a 0.5% commercial detergent solution, and subjected to continuous stirring at 60 °C for 2 h.Then, the fiber was removed from the solution, rinsed in DI water twice, followed by drying in the oven for 10 min, and then its output voltage was measured.The cycle continues for five more times.Further, the fabric sensor is subjected to high-speed rotation-induced stress inside a washing machine (Arcelik 7100 M, at 1000 rpm and 60 °C).After that, the fabric sensor was rinsed in DI water twice, dried in an oven at 75 °C for 1 h, and the output voltage was measured.The cycle was repeated five more times.
Fiber durability against salt water was tested at room temperature by immersing the fiber in 4% salt solution (4 gm table salt in 100 mL DI water).The fiber was rinsed three times with DI water and dried before measuring the OCV output after immersion.
The cooling cycle was performed by cooling the fiber in −80 °C refrigerator.After cooling, the fiber was allowed to relax at room temperature for 10 min, and output voltage was measured.
The heating cycle was conducted by keeping the sample inside a furnace at 100 °C.After heating, the fiber was allowed to relax at room temperature for 10 min, and output voltage was measured.
Surface Roughness Measurement: The surface roughness of the triboelectric fiber was measured using Keyence VK-X100 3D Laser Scanning Microscope.The surface images were taken using 0.01 z pitch at 100x focus.Area roughness was measured using a cutoff value  c of 0.25 mm

Figure 1 .
Figure 1.Fabrication procedure for GNP-PVDF triboelectric fiber.a) Solution processing of GNP and PVDF to produce a dry nanocomposite film.b) Schematic of the thermal drawing process for continuous TENG fiber.c,d) SEM image of the drawn fiber.Scale bar: 500 μm.d) LSCM image of the drawn fiber's cross-section.Scale bar: 100 μm.

Figure 2 .
Figure 2. Material characterization of thermally drawn fibers.a) FTIR spectra of the PVDF fiber and GNP-PVDF nanocomposite fibers with various wt%.b) XRD spectra of the PVDF fiber and GNP-PVDF nanocomposite fibers with various wt%.

Figure 3 .
Figure 3. Electrical characterization of thermally drawn TENG fiber and fabric.a) Working principle of a single electrode (SE mode) TENG fiber.b) Open-circuit voltage (V OC ) and c) short circuit current (I SC ) of pristine and PGr-(1-5%) TENG fiber (contact length: 3 cm and frequency: 5 Hz).d) Peak power output for PGr-5 fiber and only PVDF fiber with equivalent circuit connection for measuring power output.e) Output voltage PGr-5 TENG fabric at varied frequencies (1, 3, 5, and 7 Hz) with a 3 cm contact length.f) Output voltage PGr-5 TENG fabric at varied contact areas at 5 Hz frequency.g) The power output of PGr-5.

Figure 4 .
Figure 4. Durability test for PGr-5.a) PGr-5 fiber's surface image at different stages of thousands of tapping cycles Scale bar: 10 μm.b) Reliability of output voltage for thousands of tapping cycles.Washability test for c) fiber and d) fabric.Both fiber and fabric demonstrated stable output during multiple washing cycles.e) Saltwater stability for PGr-5 fiber.Voltage output of the PGr-5 fiber during multiple f) cooling and g) heating cycles.

Figure 5 .
Figure 5. Temperature dependence of triboelectric output.a) V OC and (b) I SC of the PGr-5 fiber at different temperatures.Power output for PGr-5 c) at room temperature and d) 100 °C.e) Peak power output for PGr-5 at room temperature and 100 °C.

Figure 6 .
Figure 6.Demonstration of real-world applications with TENG sensor.a) Smart TENG sock prepared by TENG fabric attachment at the heel region.b) Detection of walking pattern using TENG output.c) Variation of open circuit voltage at varied human movement speed (sensor was attached on right sock only).d) Step count measurement (sensor was attached to both socks).