Sustainable energy harvesting and breath sensing with electrospun triboelectric nylon-6

A high-performance triboelectric nanogenerator (TENG) has been developed for breath sensing applications, utilizing tribopositive electrospun nylon-6 nanofibers and tribonegative fluorinated ethylene propylene (FEP). The optimization toward the development of electrospun nylon-6-based TENG includes a range of factors such as the applied force and frequency on tribo responses, the thickness of the fiber mat, the concentration of nylon-6 in the fiber mats, and the selection of the tribonegative material for pairing with nylon-6 nanofiber. Among these parameters, the nanofiber prepared with 18 wt% nylon-6, characterized by a uniform fiber distribution, the highest surface area of 55.69 m2 g−1, and an optimal thickness of 0.169 mm, demonstrated excellent TENG performance, among others. The TENG module constructed using nanofiber in a 4 cm2 area showed the TENG responses of more than 30 μA short-circuit current, 200 V open-circuit voltage, and 90 nC charge when hand-pressed. It achieved a substantial power density of 890 mW m−2 at 20 MΩ by applying a constant force of 10 N at a 10 Hz frequency. Charging a 1 μF capacitor to approximately 30.1 V in just 30 s highlights the potential of electrospun nylon-6 as a promising material for nanogenerator energy harvesting and sensing applications. The TENG device was found to be sufficient to power small, portable electronics such as LEDs and digital watch displays. A wearable belt was fabricated to showcase its breath-sensing capabilities by pairing it with FEP. The microcontroller connected to the TENG in the wearable belt is used to analyze the output produced through breathing patterns, subsequently activating a buzzer and LED by the nature of the breathing.


Introduction
Over the past few years, wearable electronics and sensors such as smart watches, smart masks, hearing aids, smart textiles, etc. have been developed to monitor the health and related activities of patients with chronic diseases and older adults, without interfering with our daily lives [1,2].For instance, the continuous monitoring of breath is crucial for individuals experiencing acute respiratory problems like asthma, pneumonia, and lung diseases, as these conditions can rapidly escalate to become life-threatening.In wearable electronics, the inclusion of rigid, heavy batteries with limited lifespan and maintenance requirements has impeded their user-friendliness of them.Hence, extracting energy from environmental sources, including mechanical vibrations, biomechanical movements, solar energy, thermal energy, etc. offers a viable solution for meeting the energy demands of the portable and wearable systems.Among the various techniques employed to self-power wearable electronics, triboelectric nanogenerators (TENGs) have gained attention due to their capability to harvest energy from ambient sources and biomechanical motions [3][4][5].TENGs are based on contact electrification and electrostatic induction.Contact electrification, or triboelectricity, occurs when two materials acquire an electric charge when brought into contact and separated [6].The materials involved in contact electrification have different electron affinities, causing one material to become positively charged (losing electrons) and the other to become negatively charged (gaining electrons).When these oppositely charged materials are separated, it creates an electric field between them and causes the accumulation of positive charges on one side and negative charges on the other.The electrical potential difference (voltage) is thus created between two electrodes in the TENG, via electrostatic induction.When an external circuit is connected, electrons can flow through the circuit, creating an electrical current in the TENG [7].By integrating TENG modules into textiles or wearable items such as masks, belts, watches, and shoes, it becomes feasible to create wearable electronics capable of detecting motion, pressure, and respiration, among other functions [8][9][10][11][12].The primary features required for these wearable electronics include flexibility, comfort, softness, and lightweight.In this regard, electrospun polymer-based materials are a noteworthy focus for TENG applications since electrospinning allows us to prepare the materials with high surface roughness, porosity, flexibility, and lightweight in a large scale in an easy way [1,3,13].
Selecting appropriate materials for TENG applications is crucial in designing an efficient and effective energy-harvesting or sensing device.The triboelectric series ranks materials based on their tendency to gain or lose electrons when in contact with other materials.Materials positioned at opposite extremes of the triboelectric series show significant variances in electron affinity, rendering them appropriate for TENG applications, primarily due to the enhanced electric charge transfer between them.The additional requirements of the materials for being used in TENG are surface roughness to enhance the contact between the materials and to increase the charge transfer, durability/wear resistance to withstand repeated contact and separation without significant wear or degradation, flexibility to conform to various shapes for specific geometries, environmental stability/considerations in which the TENG operate, cost/availability for commercial applications.Nylon, a synthetic material with exceptional mechanical characteristics, stands out as a noteworthy among materials with positive triboelectric properties [7].The amide repeating units present in the nylon structure contains nitrogen atoms with lone pairs of electrons, which are easily transferred or displaced during friction or contact and separation with other material [14,15].This electron transfer from nylon creates areas of localized positive charge on the surface of nylon and localized negative charges on the other material, which generates static electricity.It is important to note that the triboelectric properties of nylon-based materials can vary depending on several factors, including the specific type of nylon, its surface condition, and the materials it is in contact with.A study showcased the application of an electrospun nylon-6,6-based TENG to harvest vibrational energy and detect seat occupancy.This TENG exhibited a short-circuit current of 300 µA, an open-circuit voltage of 350 V, and a remarkable power density of 550 mW m -2 by palm pressing on a 4 cm * 4 cm sized tribo material (nylon-6,6) and copper (Cu) electrode [16].In a textile-based TENG, the electrospun nylon-6,6 nanofibers were introduced to enhance the tribopositive properties of silk.In contrast, an electrospun poly(vinylidene difluoride) (PVDF) layer was applied to increase the tribonegativity of polyethylene terephthalate (PET).This significantly boosted TENG output response approximately 17 times compared to the silk/PET configuration in the optimized force and frequency of 8 N and 8 Hz, respectively, using the tribomaterials in 2.5 cm * 2.5 cm dimension.This improvement in tribopositivity (and tribonegativity) in the contact layers can be attributed to the improved effective contact area achieved by incorporating electrospun nanofibers [15].There are reports of surface and bulk modifications of triboelectric materials aimed at boosting the output of nylon-based TENG.For instance, a TENG device comprising tribonegative MXene-doped PVDF nanofibers and the antibacterial Ag nanoparticles modified nylon-6,6 nanofiber tribopositive layer yielded an output voltage of 362 V and an output current of 38.5 µA [17].Tribopositive nanofiber membranes made using TiO 2 /nylon-11 have been engineered in wearable TENG applications and shown good attributes including antibacterial capabilities.Under a 10 N force and 10 Hz frequency, when paired with copper as another tribo-material, TiO 2 /nylon-11 based TENG module exhibited a short circuit current of 5.1 µA and an open circuit voltage of 56 V for samples measuring 2 cm × 2 cm dimension [14].Similarly, an e-skin, which is breathable, sensitive, and self-powered, relies on TENG for real-time monitoring of respiration and the diagnosis of sleep apnea was developed.They use polyacrylonitrile and nylon-6,6 nanofibers as contact pairs, and deposited gold electrodes to create a contact-separation in the nanofibrous TENG-based e-skin.This e-skin boasts impressive attributes, including a peak power density of 330 mW m −2 with a high-pressure sensitivity of 0.217 kPa −1 [8].The electrospinning of nylon 6 on a melt-blown nonwoven polypropylene (PP) was tried to prepare triboelectric layers and Ni-coated fabric was used as electrodes in the TENG module [18].The use of commercially available nylon cloth as triboelectric material has also been reported.A study used titanium-functionalized molybdenum disulfide interspersed PP cloth and nylon cloth to develop a respiration sensor and self-powered ammonia gas sensor [19].These studies explore the importance of electrospun nanofibers in TENG applications.While there are reports on electrospun nylon-6,6 and nylon-11-based systems, the systematic approach to optimize the electrospinning process for nylon 6, specifically for triboelectric energy harvesting and sensing applications, is minimal.The optimization of the electrospinning process for developing high-performance, flexible, and durable nylon-6-based TENG has relevance in the field of nanogenerators.The electrospun nylon-6 with a high surface area can lead to higher charge generation and improved TENG performance compared to commercial nylon or solution-cast films.Hence the current investigation was aimed at a systematic approach for optimizing TENG responses in electrospun nylon-6 polymer in vertical contact separation mode.The optimized TENG modules have been demonstrated for energy harvesting and breath-sensing applications.

Preparation of nylon-6 nanofibers by electrospinning
About 0.75 g of nylon-6 granules (Sigma Aldrich) were dissolved in 5 ml 85% formic acid (Merck) and the resultant solution (15 wt% nylon-6) was introduced into the electrospinning equipment via a 10 ml syringe with a steel needle having an inner diameter of 800 µm (figure 1(a)).During the electrospinning process, the nylon solution was dispensed at a constant flow rate of 500 µl per hour while applying a voltage of 30 kV.The needle tip was consistently maintained at a distance of 10 cm from the collector.To optimize the performance of TENG responses and the quality of nanofibers, we experimented with varying the concentration of the nylon-6 solution and the spinning duration while keeping the electrospinning conditions such as applied voltage, distance from the needle tip to the collector, flow rate, and temperature consistent.Different samples were prepared by altering the weight percentages of nylon-6 dissolved in formic acid, denoted as Ny 6 15 wt%, Ny 6 18 wt%, Ny 6 20 wt%, and Ny 6 23 wt%.The spinning time was adjusted within the range of 8-12 h to assess the thickness of the resulting fiber mat on TENG output.The electrospun nanofiber prepared was enclosed in aluminum foil, and later extracted for utilization in constructing TENG devices and for various characterizations.

Set-up for TENG performance analyses of electrospun nylon-6
To facilitate contact and separation of the triboelectric materials, we employed a custom-made setup capable of varying frequency and force.A 1 cm × 1 cm piece of tribopositive nylon-6 was paired with a tribonegative material of the same dimensions.With respect to nylon-6, the tribonegative materials studied include the commercially purchased films of polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), PVDF, indium tin oxide-coated PET (PET-ITO) and copper.To measure the TENG responses, the tribopositive and tribonegative materials were affixed to the copper electrode using adhesive tape.Subsequently, these electrodes, along with the triboactive materials, were attached to polymeric substrates.The tribopositive unit was positioned at the top of the movable piston, while the tribonegative unit was placed at the stationary bottom part of the setup.Connecting the electrical leads from the top and bottom electrodes with an electrometer made it possible to acquire TENG measurements during the continuous contact and separation of the materials using the setup figure 1(b).Due to the influence of environmental conditions on TENG measurements, the TENG output has been assessed at a humidity level ranging from 46% to 48%.

Fabrication of TENG device for energy harvesting and breath sensing applications
For the construction of the TENG-based device, a 4 cm × 4 cm nylon 6 fiber mat and FEP film were utilized after an optimization study.The tribo materials were affixed to the electrodes of the same dimensions, which were subsequently secured to a polymer substrate, as explained in the previous section.To enable the contact and separation of the top and bottom layers, squeezable spacers and an adhesive tape with a height of 7 mm were employed, ensuring a consistent gap between the top and bottom tribo layers within the device, as depicted in figures 1(c) and (d).This device was employed to generate power for portable electronics and charge capacitors through palm pressing.The schematic in figures 1(e) and (f) illustrates the structure of the breath sensor utilizing contact separation mode TENG.The wearable respiratory sensor aims to monitor the user's live breathing patterns during their everyday activities.To provide to breath sensing requirements, the triboelectric belt has been fabricated by attaching a 2 cm 2 TENG module to a flexible, adjustable belt tailored for comfortable placement around the abdomen of an individual (as depicted in figures 1(e) and (f)).The negative tribo-material was a 250 µm thick FEP film, while the positive tribo-material used was 170 µm thick electrospun nylon 6 mat.Two copper tapes each with a thickness of 50 µm were used as the conductive electrodes.Two flexible PET substrates were used as support to maintain the flatness of the dielectric materials and Cu tape electrode.The Cu tape electrode with the tribonegative material (FEP) was securely affixed to a flexible PET substrate using adhesive tape, and this assembly was then sewn onto the elastic belt.Subsequently, the positive triboelectric layer and the Cu foil electrode attached to the flexible PET sheet was positioned over the tribonegative layer, with layers of sponges serving as spacers to facilitate controlled contact and separation between tribopairs during the breathing process.

Characterization of electrospun samples
The electrospun nanofibers underwent structural, thermal and morphological characterization by employing various analytical techniques.Fourier transform infrared (FTIR) spectroscopy (Thermo Fisher, Nicolet is10) and x-ray diffraction (XRD, Rigaku, Ultima IV) analyses were employed for structural characterization.The thermal stability and morphology of the nylon-6 samples were assessed using thermo gravimetric analysis (TGA, TA Instruments, STD Q600) and field emission scanning electron microscopy (FESEM, Carlzeiss, Gemini 300), respectively.Specific surface area and pore-size distribution were determined through Brunauer-Emmett-Teller (BET) analysis (Anton Paar Quantachrome C-XR).Electrospun fiber mat thickness was measured using a Mitutoyo LCD micrometer.The triboelectric responses were measured with an electrometer (KEITHLEY 6517b) and the application of continuous contact and separation was performed using a customized setup capable of exerting a maximum force of 10 N and operating at a maximum frequency of 10 Hz.

Morphological, structural and thermal analyses of nylon-6 nanofibers
Electrospinning of nylon-6 was performed using formic acid solutions with varying weight percentages of nylon-6, including 15 wt%, 18 wt%, 20 wt%, and 23 wt%.The nanofibers were prepared at a flow rate of 500 µl per hour at an applied voltage of 30 kV.The fibers were taken for TENG analysis after 10 h of spinning.Figure (2) presents the FESEM micrographs of the electrospun nylon-6 along with the fiber diameter distribution and fractured surfaces of the nylon mat with an 18 wt% nylon-6 sample.In the SEM image of the 15 wt% Ny 6 sample, a notable presence of sprays and beads was observed, which led to the exclusion of preparing nanofiber samples with nylon-6 concentrations below 15 wt%.The average fiber diameter was calculated by taking 100 random nanofibers in the SEM micrographs and these were 32, 40, 56 and 65 nm for 15 wt% (figures 2(a) and (b)), 18 wt% (figures 2(c) and (d)), 20 wt% (figures 2(e) and (f)), and 23 wt% (figures 2(g) and (h)), of Ny 6 samples, respectively.As the polymer concentration increases, the fiber diameter tends to increase as reported in the literature [20][21][22].The main reason for this phenomenon is the increased viscosity of the polymer solution with higher polymer concentrations.When the solution has a higher viscosity, it resists elongation during the electrospinning process, resulting in thicker fibers.So, in the case of nylon-6 as well, the influence of the concentration of polymer solution on increasing the fiber diameter was confirmed through SEM analysis.The internal structure and morphology of nanofibers were analyzed by taking SEM images of the fractured surface of the Ny 6 18 wt% sample.Figures 2 (i) and (j) show the well-stacked layers of nanofibers with fewer defects.
The structural characterizations of electrospun nanofibers were carried out by FTIR analysis.Figure 3(a) shows the FTIR spectra of nanofibers prepared by varying the weight percentages of nylon-6.The N-H stretching of nylon-6 exhibits the characteristic features of a polyamide, displaying a peak at 3309 cm −1 and a secondary peak at 3075 cm −1 [16,23,24].The peak corresponding to the aliphatic C-H stretching vibration in the polymer chain was observed at 2933 cm −1 in nylon-6 samples.Generally, in the FTIR spectra of polyamides, amide I (∼1650-1655 cm −1 ), II (∼1540-1550 cm −1 ), III (∼1240-1350 cm −1 ) and V (∼800-690 cm −1 ) bands are observed [24].Amide I can provide information about the degree of hydrogen bonding and the conformation of the polymer chain.In contrast, amide II is sensitive to hydrogen bonding and can reflect the secondary structure of the polymer.Amide I mode is composed of contributions from the stretching of the C=O bond, the stretching of the C-N bond, and the deformation vibrations involving C-C-N bonds.The amide II mode encompasses a blend of vibrations, including the in-plane bending of N-H bonds, the stretching of C-N bonds, and the stretching vibrations of C-C bonds.A large contribution in the amide I band arises from C=O stretching at 1638 cm −1 , and the major contributor in the amide II band is N-H in-plane deformations at 1544 cm −1 .These bands were present in all electrospun samples.Amide III band is a due to the C-N stretch at 1350 cm −1 observed in electrospun samples.Further, the bending vibrations of aliphatic C-H groups in the amorphous regions of the nylon 6 structure were observed in the 1220-1240 cm −1 range.Polyamides exhibit a diffuse band at 690 cm −1 which has been assigned to the NH out-of-plane deformation vibration and is termed the amide V band.This band was present in all electrospun nylon-6 samples [24].In the FTIR spectra (figure 3(a)), the intensity of peaks associated with nylon fibers is increased when the concentration increases from 15 wt% to 23 wt% of nylon-6.This is because a higher polymer concentration in the electrospinning solution results in more polymer molecules in the resulting fibers, leading to stronger FTIR signals for characteristic functional groups in the polymer.The details about the type of polymorphs formed in nylon-6 during electrospinning were previously reported [25].The primary polymorphs found in commercially available nylon-6 are the α-nylon and γ-nylon forms, with the γ-nylon being the more commonly observed result of electrospinning in this material [25,26].The α-form is characterized by a flat zigzag configuration in which antiparallel chains form hydrogen bonds.At the same time, the γ-form arises from hydrogen bonding between parallel chains, causing a misalignment of hydrogen-bonding sites.The XRD α form displays distinct reflections at 2θ = 20.5 • and 2θ = 23.7 • , while the γ form exhibits characteristic reflections at 2θ = 11 • and 2θ = 21.3 • [27].The XRD patterns (figure 3(b)), of all electrospun nylon-6 nanofibers prepared at various polymer concentrations and spinning times exhibit a distinct diffraction peak at 2θ = 21.3 • (d-spacing 0.41 nm), which is characteristic of γ form of nylon-6 [27].However, a change in the intensity of α and γ forms was observed when the concentration of nylon 6 in the fiber mat increased from 15 wt% to 23 wt% and the spinning time was changed from 8 to 12 h.Increasing the polymer concentration and spinning time leads to the formation of α forms as evidenced by the formation of shoulder peak at 2θ values of 20.4 • and 23.7 • (d-spacing 0.43 nm and 0.37 nm) in addition to the γ form.Moreover, the absence of the peak at 2θ = 11 • , a characteristic reflection of the γ form in electrospun mats prepared with 20 wt% and 23 wt% of nylon-6, indicates a transition from the γ to α form in nylon-6 mats.Therefore, XRD analysis indicates that increased polymer concentrations and extended spinning durations in electrospun nylon-6 fibers result in the emergence of α phases along with the prominent γ phase, which is attributed to the formation of various hydrogen bonds during the electrospinning process.Furthermore, the XRD findings align with prior reports indicating that rapid solvent evaporation during electrospinning leads to the creation of the less structured γ phase, while a partial shift toward the α phase in the structure occurs at prolonged spinning [28].
The polymer degradation and nanofiber interaction in different electrospun samples were analyzed and the TGA plots are as in figure 3(c).The TGA plot reveals one-step degradation for all samples in the inert nitrogen atmosphere.A significant degradation has been observed at ∼430 • C for the samples due to the amide bond cleaves in nylon-6.However, the onset decomposition temperature for each sample was slightly different, as shown in the insert in figure 3(c).The nanofibrous mat obtained from the 15 wt% and 18 wt% solutions of nylon-6 showed an onset degradation starting at 432 • C, while a temperature shift to 437 • C in the sample prepared 23 wt% of nylon-6 solution.The increase in the onset decomposition temperature with increasing polymer concentration may be due to the formation of stronger hydrogen bonds between the polymeric chains.The electrospun samples were subjected to BET analyses to assess their surface area, pore size distribution, and pore volume, and the results are depicted in figures 3(d) and (e).Figure 3(d) illustrates the adsorption-desorption behavior of the electrospun samples, while figure 3(e) provides understanding how the concentration of the electrospinning solution affects surface area and pore volume.Additionally, the pore size distribution within the samples is presented in an inset in figure 3(d).As indicated by the morphological analysis, the concentration of the electrospinning solution exerts an influence on the nanofiber diameter.Higher concentrations of the polymer solution result in larger fiber diameters, which, in turn, impact the surface area too.Thicker fibers exhibit a reduced surface area per unit mass compared to thinner fibers.The BET analyses corroborated this observation.Among the various electrospun samples, the Ny 6 18 wt% sample displayed the highest surface area, measuring 55.69 m 2 g −1 .In contrast, the electrospun samples prepared using higher polymer solution concentrations, namely Ny 6 20 wt% and Ny 6 23 wt%, demonstrated decreased surface areas, measuring 41.56 m 2 g −1 and 36.84 m 2 g −1 , respectively.Notably, the Ny 6 15 wt% sample, despite having a smaller fiber diameter, exhibited a lower surface area (42.2 m 2 g −1 ) than the Ny 6 18 wt% sample due to the presence of polymer particles and beads (figure 2(a)).The packing density of nanofibers within the mat can also be affected by the concentration of the solution.Higher concentrations may lead to denser packing, consequently reducing the effective surface area available for external interactions, while lower packing densities may offer a more accessible surface area.The morphological analysis reveals that higher concentrations result in thicker fibers, thereby increasing packing density and lowering the surface area in Ny 6 20 wt% and Ny 6 23 wt% samples.The pore volume in the samples can be correlated to the fiber diameter as well.As the fiber becomes more and more thick, there can be decreased pore volume in the samples, which is evidenced in the figure 3(e).Also, the inset in figure 3(d) suggests the presence of mesopores in the electrospun samples.

Triboelectric performances of electrospun nylon-6 nanofibers
As explained in section 2.2, the upper part of the TENG consists of the substrate material, copper electrode, and the nylon-6 fiber mat attached to a mobile piston in the custom-made setup.In contrast, the bottom part comprises tribonegative material affixed to the copper electrode and the substrate attached to the stationary part of the setup.The mobile piston, along with the tribopositive unit, experiences continuous contact and separation with the stationary lower part that contains the tribonegative material.The electric current generation during the contact and separation of triboelectric materials is explained as follows (figure 4(a)).Initially, no charge generation or transfer occurs within the system because the upper tribopositive part is not in contact with the bottom tribonegative part.Charge separation initiates when the top and bottom parts of the tribomaterial surfaces come into contact.When the two parts are in contact, positive charges accumulate on the tribopositive material, while negative charges accumulate on the tribonegative material.However, when triboelectric materials are separated, an electric field is induced on both electrodes, causing a potential difference.Consequently, to equalize the potential difference, electrons flow from the bottom to the top electrode and create an electric current pulse.When the separation between the materials is at maximum, the potential drop reaches zero, and the charge flow will cease.Conversely, when the top electrode is pressed onto the bottom electrode, a potential drop reoccurs due to varying induced electric fields.To restore potential equilibrium, electrons move from the top to the bottom electrode, creating an electric current pulse with an opposite polarity.Hence, the periodic application and removal of a constant force give rise to positive and negative charge pulses in the system [16,[29][30][31].
The effect of force and frequency on the TENG responses were analyzed using an electrospun mat (1 cm × 1 cm dimension) of Ny 6 18 wt% sample that was prepared through an electrospinning process lasting 10 h, with a consistent flow rate of 500 µl per hour and an applied voltage of 30 kV.The initial TENG assessments were carried out using electrospun nylon-6 as tribopostive and FEP as the tribonegative material.Primarily, the effect of frequency on the contact and separation of tribo-active materials has been analyzed.In figures 4(b)-(d), the relationship between triboelectric responses, such as open-circuit voltage (V oc ), short-circuit current (I sc ), and charge, is depicted in relation to the applied frequency while maintaining a constant force of 10 N. The TENG responses were increased with an increase in the frequency of contact and separation because frequent contact and separation events can generate a significant number of triboelectric charges, resulting in higher TENG outputs.The values of V oc , I sc , and charge experienced an increase when the frequency was adjusted from 2 Hz (24 V, 0.8 µA, and 8 nC) to 10 Hz (43.8 V, 3.8 µA, and 13.4 nC).In addition, the electrical output of TENG demonstrates a clear dependence on the contact force; typically increasing with an increase in the force applied to press the tribo-contact surfaces together (figures 4(e)-(g)).This electrical output exhibits a linear relationship with force in certain instances [32], while in others, it reaches a saturation point at higher force levels [33,34].The tribo-charges can only transfer at regions of actual contact, so it logically follows that an increase in the real contact area due to contact pressure will result in a proportional rise in the electrical output [34].The values of V oc , I sc , and charge experienced an increase when the force was varied from 1 N to 10 N, rising from 33.34 V, 2.1 µA, and 9 nC to 43.8 V, 3.8 µA, and 13.4 nC, respectively.The custom made setup for bring contact and separation has frequency and force limit of 1 Hz and 10 N and thus higher frequencies and forces were not analyzed.After initial assessments, a frequency of 10 Hz at a force of 10 N was fixed to evaluate subsequent TENG properties.
To investigate the impact of nanofiber mat thickness on TENG responses, nylon-6 samples were prepared with different electrospinning durations (8 h, 10 h, and 12 h) while maintaining constant spinning parameters.The TENG responses of the nanofibers (1 cm × 1 cm dimension) were analyzed at 10 Hz and 10 N, as depicted in figures 4(h)-(j).The thicknesses of 0.107 mm, 0.169 mm and 0.219 mm were measured for the fiber mats prepared with different electrospinning duration of 8 h, 10 h, and 12 h, respectively.The values of V oc , I sc , and charge exhibited an increase when the spinning hour changed from 8 (31.34 V, 2.5 µA, and 9.5 nC) to 10 (43.8 V, 3.8 µA, and 13.4 nC).However, extending the spinning time to 12 h resulted in a decrease in TENG performance, with values dropping to 35.34 V, 3.5 µA, and 11.2 nC.In TENGs, V oc and surface charge densities are dependent on various factors including the time-dependent distance between two triboelectric friction layers (x(t)), the charge density (σ 0 ), air permittivity (ε 0 ), contact area (A), capacitance (C), thickness of the dielectric layer (d), dielectric constant (ε r ), and external voltage across the capacitor (∆V) via the relations given below [29,35] While varying the thickness of the mat, the time-dependent distance between two triboelectric friction layers (x(t)) and the charge densities can be varied.Based on these factors, the mat with a thickness of 0.169 mm prepared using a spinning time of 10 h showed higher TENG responses than others.Other than this, the quality of the nanofibers formed during the electrospinning can also affect the final output responses.The SEM images and the fiber diameter distribution of fiber mats prepared by varying spinning hours are given in figures S1(a)-(d) in the supplementary document.The nanofiber diameter distribution patterns of nylon-6, prepared with an extended spinning time of 12 h, exhibited a broader range of fiber diameters (<50 nm) in comparison to samples produced with 8 h (∼36 nm) and 10 h (∼40 nm) of electrospinning.Notably, the nanofibers of Ny 6 18 wt% in figure 2(d) displayed a uniform distribution, which is spun for 10 h, with an optimal mat thickness (0.169 mm) and surface area (55.69 m 2 g −1 ), resulting in the maximum TENG output responses.
The impact of nylon-6 concentration in the nanofiber on TENG responses was also assessed.Figures 4(k)-(m) depicts the changes in TENG output responses across various systems.When paired with FEP, the 18 wt% electrospun Ny 6 sample displayed a higher TENG response compared to the others.This behavior can be attributed to the physical characteristics of the nanofibers formed during the electrospinning process.As illustrated in the SEM micrographs in figure 2, the 18 wt% Ny 6 samples displayed a uniform distribution of nanofibers with a superior surface area, as depicted in figure 3(d).By examining the results of surface area (figure 3(d)) in relation to the output responses of the samples (figures 4(k)-(m)), the influence of surface area on the TENG responses in the samples becomes apparent.The surface area follows the order of Ny 6 18 wt% (55.69 m 2 g −1 ) > Ny 6 15 wt% (42.2 m 2 g −1 ) > Ny 6 20 wt% (41.56 m 2 g −1 ) > Ny 6 23 wt% (36.84 m 2 g −1 ), and the TENG responses also exhibited a similar trend.
Following the fundamental principle of the triboelectric effect, a material's polarity, denoting its ability to either gain or lose electrons, is directly correlated with the magnitude of charge produced during contact electrification [36,37].The influence of a triboelectric contact pair on an electrospun nylon-6 sample was examined.In this study, a tribopositive Ny 6 18 wt% sample was brought into contact with commercially acquired films of PTFE, FEP, PVDF, PET-ITO, and copper, acting as the tribonegative counterpart, with a force of 10 N applied at a frequency of 10 Hz.Figures 4(n)-(p) revealed that FEP exhibited a higher degree of triboelectric responsiveness when compared to other materials exhibiting triboactivity.According to the quantified triboelectric series proposed by ZL Wang, PTFE and PVDF had triboelectric charge densities (TECDs) of approximately −110 µCm −2 and −85 µCm −2 , respectively [36].In a subsequent update to the triboelectric series, FEP emerged as the most tribonegative material [38].Both FEP and PTFE are fluorinated polymers, suggesting that their TECDs would be closely aligned.Conversely, other tribonegative materials such as PVDF, PET-ITO, and copper demonstrated lower TENG responses in comparison to the FEP-based TENG system, primarily due to their lower TECDs, as indicated in the triboelectric series.To summarize, the sequence of triboelectric responses corresponded to the order FEP > PTFE > PVDF > PET-ITO > Cu, in accordance with the TECDs of these materials when paired with electrospun nylon-6.Figures S2(a)-(c) illustrates the triboelectric responses of nylon-6 samples when paired with PTFE.These results exhibit a comparable trend to that observed in the FEP paired system.
The power density was assessed using a 1 cm 2 nylon-6 sample with the highest TENG output performance (Ny 6 18 wt%, spun for 10) by pairing with an FEP.The load resistance was systematically varied from 1 MΩ to 1 GΩ, and the triboelectric current and voltage were measured under a constant force of 10 N at 10 Hz, and the results are shown in figures S3(a) and (b).For the power measurements, the product of peak current and voltage across different load resistors were calculated and plotted as in figure 5(a).As the resistance was increased from 1 MΩ to 1 GΩ, the current exhibited a decrease, ranging from 3.8 µA to 0.57 µA and the voltage was increased from 9 V to 43.8 V within the same resistance range.The device's power density is depicted in figure 5(b), revealing that the most effective power transfer occurred at 20 MΩ, reaching a value of 890 mW m −2 .The power density value is significantly higher when compared to the previous reports in nylon-based TENGs, which hints at the material's utility for energy harvesting and sensing.

TENG applications using electrospun nylon-6
The assessment of TENG responses employing nylon-6 has proven its suitability for energy harvesting and sensing applications.To showcase its real-time capability for charging electronic devices and portable gadgets, a larger TENG module was fabricated following the procedures outlined in section 2.3.This TENG module, measuring 4 cm 2 , was paired with FEP while accommodating flexible spacers to facilitate proper contact and separation during manual tapping or pressing.The voltage, current, and charge generated from hand-pressing on the TENG module's top surface were measured and illustrated in figures 5(c)-(e).A voltage of approximately 200 V was generated with hand pressing, with a current of 30 µA and charge of 90 nC.To conduct the energy harvesting applications, such as capacitor charging and providing power to illuminate LEDs, a sizable TENG device was assembled by seamlessly integrating four distinct modules of 4 cm 2 dimension into a single panel, as shown in the inset of figure 5(f).The TENG device's charging capabilities were evaluated by the continuous application of pressure by the palm of a hand.The charging profiles of capacitors with capacitance ranging from 1 µF to 100 µF under constant palm press are shown in figure 5(f).The device demonstrated significant charging rates, achieving 1.01 V s −1 at 1 µF, 0.133 V s −1 at 10 µF, 0.093 V s −1 at 22 µF, 0.051 V s −1 at 47 µF, and 0.016 V s −1 at 100 µF (calculated for palm pressing for 30 s) and thereby highlighting the self-sustaining capability of the proposed system.The same TENG modules were used to power blue LEDs connected in series as in figure 5(g) and video S1.The ability of the proposed system to power up simple electronic devices was demonstrated using a TENG having 4 cm 2 dimensions (figure 5(h)).To power the watch display, a 22 µF capacitor was initially charged by hand pressing on the device, and the watch was connected across it.Soon after the watch connected, the display started on and continued till the discharging of the capacitor.The watch was connected after charging the capacitor up to 5.5 V and started to display and then turned off as the hand pressing stopped and discharge began.Video S2 shows the charging of the capacitor by hand press to turn on the display of the watch.The durability of electrospun nylon-6 nanofibers was assessed, and figure S4 provides detailed current measurements for over 12 000 cycles.
To illustrate the material's sensing capabilities, a wearable breath/respiration monitoring belt was developed and the details of the fabrication has been described in the section 2.3.This belt incorporates a breath sensor utilizing TENG technology.Figures 1(d) and (e) provides a visual of the belt designed to be worn on the abdomen or chest.During thoracic respiration, the abdominal cavity undergoes intermittent expansion and contraction during exhalation and inhalation.The design presented in figure 6(a) facilitates efficient contact and separation between the triboelectric pairs, in response to the expansion and contraction of the abdomen.For flexibility and comfort, copper tapes were used instead of copper films.To capture the triboelectric signals generated by the device, a pair of copper wires is affixed to the copper tapes.When exhalation starts and tribo-pair begin to contact each other with the expansion of abdominal cavity, leads to the formation of the triboelectric charges on the surface of the materials.At the onset of inhalation, as the abdominal cavity contracts, the tribo-pairs begin to separate, creating a potential difference.This difference is balanced by the flow of electrons through the external circuit, a process elaborated in section 3.2 (figure 4(a)).Therefore, free electrons will move from one electrode to the other through the external circuit, resulting in the generation of a pulse of output voltage with positive amplitude.Once the tribo-pair reaches the maximum separation during the inhalation process, there will be no electron transfer through the circuit due to the drop of potential difference.Following this inhalation process, the tribo-pair re-establishes contact during exhalation, causing a potential drop.Subsequently, the charges on the electrodes flow back to equalize this potential drop, resulting in a pulse of output voltage with negative amplitude.This cycle of contact and separation of tribo-pairs persists through multiple exhalation and inhalation processes, generating signals from the triboelectric belt.
This triboelectric belt demonstrates a strong connection between the voltage produced and the nature and rate breathing.As breathing patterns transition from normal to rapid and then to deep, there is an associated change in the TENG responses recorded from the belt, as illustrated in figure 6(b).During regular breathing, the extent of contact separation that occurs between the triboelectric pairs is minimal, resulting in imperceptible responses from the belt.However, during irregular or abnormal breathing, abdominal movement varies to a significant extent.When breathing quickly, the belt registers higher frequency pulses as the continuous motion of the abdomen leads to increased contact and separation between tribo-pairs.Similarly, deep breathing is identifiable by wider pulse shapes, attributed to the prolonged contact time of the tribo materials.Hence as the level of contact and separation within the TENG module fluctuates, producing distinct TENG responses as depicted in figure 6(b).These results can be harnessed to set alarms based on an individual's breathing style.Also, conducted multiple real-time breathing tests on an individual and figure S5 in the supporting information shows the voltage generated (with the error range) in various breathing pattern of an individual.The data reflecting the wearer's breathing rhythm is transmitted via electrical signals originating from the TENG sensor.This TENG device functions as a tactile input sensor, integrated with an Arduino controller board, to activate the attached buzzer and LED.The Arduino board receives the triboelectric voltage generated by the wearer's breathing pattern when using the smart belt, and this voltage is transmitted to the A0 pin of the Arduino, as illustrated in figure 6(c).The microcontroller detects analog signals within the 0-5 V range.As evident from the triboelectric data, the output varies for normal, deep, and fast breathing patterns (figure 6(b)).During fast breathing, the microcontroller analyzes the analog signal, and when it surpasses a certain voltage threshold, it triggers the buzzer and LED to operate for 0.5 s.In the case of deep breathing, the microcontroller processes the input analog data and activates the buzzer and LED, which illuminate for 1.5 s.Video S3 shows the working of the smart belt for breath analysis.This combination of the TENG technology with a microcontroller, buzzer, and LED creates a good responsive breath monitoring system while also harnessing energy.The proposed device is pivotal in offering comprehensive healthcare solutions for individuals in remote locations.The elastic material in the belt accommodates abdominal expansion during breathing and provides the necessary support during contraction in the inhalation process.Additionally, the simple structure and choice of materials with good TENG responses ensure the practical application of the breath-sensing belt.
In this study, we have explored electrospun nylon 6's capability in both triboelectric energy harvesting and sensing applications.Recent research efforts have been concentrated on wearable devices like belts and masks, particularly evident in table 1. Notably, the focus on wearable devices for respiration and apnea has surged due to COVID-19.The device proposed in this study stands out in its ability to distinctly register voltage output alterations when transitioning from normal to abnormal breathing patterns (specifically, deep and fast breaths).For normal breathing, the voltage response measures at 0.3 V, whereas it exceeds 1 V for abnormal breathing patterns.This feature holds promise for setting up alert systems to enhance patient care.While most research reports similar voltage values for normal versus abnormal breathing [9,11,12] wearable belt reported here excels in sensitivity for analyzing breath patterns, as indicated by the notable change in the voltage output profile.While our current material choices for the wearable belt require enhancements for prolonged usability, the outcomes produced by the TENG module during breath analysis remain competitive in comparison to current cutting-edge technology.

Challenges and opportunities for enhancing TENG device P
The TENG device suggested for breath sensing boasts a simple design and exceptional sensitivity to human breathing patterns.However, there is room for enhancement in future iterations.The device's response may vary based on factors like age, weight, and other physical conditions of individuals.Conducting real-time breathing tests on a diverse range of people, considering differences in abdomen cavity and breathing rhythms, can provide deeper insights into the belt's response nature.Additionally, implementing wireless transmission capabilities could enable seamless use of the belt during physical activities, facilitating an intelligent health monitoring system by transmitting the gathered information.Ensuring sensor accuracy involves precise measurement of breathing patterns without external interference, where even subtle abdomen position changes can impact measurements.Expanding the TENG module area to a larger size than the 2 cm 2 used in the study could counter this issue.Despite a larger belt size, the device's thin and flexible materials should maintain comfort and wearability, allowing extended wear without discomfort or impeding natural movements.Fine-tuning data interpretation and analysis is vital for distinguishing diverse breathing patterns and delivering valuable insights to users or healthcare professionals.As TENGs might encounter varying environmental conditions like temperature and humidity fluctuations, it is crucial to develop TENGs resilient to these changes to maintain consistent performance.A significant challenge faced by breath-sensing TENG masks is their reduced sensitivity caused by humidity from the user's breath.Belts, in contrast, do not encounter this particular issue.Amid the COVID-19 era, creating breath-sensing devices with easy fabrication, cost-effectiveness, zero power consumption, and portability holds immense potential for clinical use.While considerable research is underway in the field of TENG, several aspects require addressing before commercializing products based on this technology.One of the main challenges is improving the efficiency of TENGs.Currently, the energy conversion efficiency of TENGs is relatively low, and efforts are needed to increase their power generation capabilities to make them practical for a wider range of applications.TENGs may be exposed to various environmental conditions, such as temperature and humidity fluctuations.Developing TENGs that can withstand these conditions without a significant drop in performance is crucial.Efficient voltage-boosting and power management systems are needed to ensure the harvested energy can be effectively stored and used.Integrating TENGs into products and systems requires careful mechanical design and engineering to ensure that the mechanical motion is effectively harnessed to generate power without interfering with the normal operation of the device.As TENG-based products emerge in the market, there will be a need for standardization and regulation to ensure safety, quality, and performance consistency.

Conclusion
Electrospinning of nylon-6 was systematically optimized for enhanced TENG output by varying the factors such as applied force and frequency, fiber mat thickness, and nylon-6 concentration in the fiber mats.The materials used as tribonegative pairing were varied and the TENG responses were monitored.The nanofiber prepared using 18 wt% nylon-6 with uniform fiber distribution, an average fiber diameter of 40 nm, a surface area of 55.69 m 2 g −1 , and an optimum thickness of 0.169 mm, showed the best performance than other systems when paired with FEP.Forming the fiber mat in an optimum thickness and pore volume with uniform fiber distribution and high surface area augmented the final output of the TENG.A TENG module, constructed using nanofiber in a 4 cm 2 area, exhibited notable TENG responses when hand-pressed, including over 30 µA of short-circuit current, 200 V of open-circuit voltage, and a charge of 90 nC.By applying a constant force of 10 N at a 10 Hz frequency, it achieved a significant power density of 890 mW m −2 at 20 MΩ.The powering efficiency using the TENG was demonstrated by charging various capacitors from 1 µF to 100 µF, displaying the digital watch and several LEDs under hand pressing on the TENG device.Further, the breath sensing using the TENG integrated wearable belt was demonstrated to analyze different breathing patterns of humans and, the regular-fast-deep breaths led to different TENG responses.The output generated via breathing patterns was detected and analyzed by the microcontroller attached to the TENG, which triggers a buzzer and LED based on the breathing nature.While improvements are necessary for the long-term durability of the materials used in crafting the wearable belt, the outcomes obtained from the TENG module during breath analysis are impressing.Moreover, the live electrical signal produced by the attached TENG module could integrate with a wireless transmitter and signal processor, creating the foundation for an intelligent health monitoring system.By incorporating more advanced designs and materials into specific segments of the belt, there is potential for its future utilization in tracking the breath of patients significantly impacted by respiratory issues.

Figure 1 .
Figure 1.Electrospinning process for nanofiber preparation (a); setup used for vertical contact separation in samples (b); TENG device fabricated for energy harvesting applications (c) and (d); wearable belt designed for breath monitoring application (e) and (f).

Figure 4 .
Figure 4. Working mechanism of TENG (a); effect of frequency on TENG responses (b)-(d); effect of force on TENG responses (e)-(g); effect of thickness or spinning time on TENG responses (h)-(j); effect of concentration of nylon-6 on TENG responses (k)-(m); the effect of contact pain on TENG responses (n)-(p).

Figure 5 .
Figure 5. Voltage and current profiles at various load resistances (a); power density profile at various load resistors (b); responses obtained by hand pressing on TENG device: voltage (c); current (d); charge (e); capacitor charging (f); LED powering (g); powering display of the digital (h).

Figure 6 .
Figure 6.Design of the breath sensing wearable belt fabricated using electrospun nylon-6 and FEP (a); voltage obtained for different breathing patterns (b); circuit diagram for integrating microcontroller to the TENG incorporated wearable belt (c); photographs of real time breath sensing using the wearable belt (d).No LED and warning alarm for normal breathing (right) while LED with warning alarm for abnormal breathing (left).

Table 1 .
Details of recently developed respiration sensing devices.