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International Journal of Bioprinting logoLink to International Journal of Bioprinting
. 2022 Nov 24;9(1):647. doi: 10.18063/ijb.v9i1.647

PVDF/AgNP/MXene composites-based near-field electrospun fiber with enhanced piezoelectric performance for self-powered wearable sensors

Cheng-Tang Pan 1,2,†, Karishma Dutt 1,†, Amit Kumar 3, Rahul Kumar 1, Cheng-Hsin Chuang 3, Yi-Ting Lo 4, Zhi-Hong Wen 5, Chien-Shu Wang 4,*, Shiao-Wei Kuo 2,6,7,*
PMCID: PMC9947487  PMID: 36844238

Abstract

336MXenes, as highly electronegative and conductive two-dimensional nanomaterials, are extensively studied for their use in sensors and flexible electronics. In this study, near-field electrospinning was used to prepare a new poly(vinylidene difluoride) (PVDF)/Ag nanoparticle (AgNP)/MXene composite nanofiber film as a self-powered flexible human motion-sensing device. The composite film displayed highly piezoelectric properties with the presence of MXene. Scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy revealed that the intercalated MXene in the composite nanofibers was evenly spread out, which not only prevented the aggregation of MXene but also enabled the composite materials to form self-reduced AgNPs. The prepared PVDF/AgNP/MXene fibers displayed exceptional stability and excellent output performance, enabling their use for energy harvesting and powering light-emitting diodes. The doping of MXene/AgNPs increased the electrical conductivity of the PVDF material, improved its piezoelectric properties, and enhanced the piezoelectric constant of PVDF piezoelectric fibers, thereby allowing the production of flexible, sustainable, wearable, and self-powered electrical devices.

Keywords: Near-field electrospinning, Piezoelectric fibers, Sensing elements, Conductive fillers, Composite materials

1. Introduction

337The internet of thingsyp (IoT) will benefit from the ubiquitous use of sensor networks in our daily lives, such that each person and everything will be connected. Wearable electronics, for example, can be positioned on skin, clothes, or accessories, allowing users to monitor and receive feedback on their health and physical activities[1–3]. While being an essential aspect of the IoT, these devices would likely function as personal assistants[4–8]. Wearable electronics should be light in weight, inexpensive, small, flexible, and sustainably powered[9]. These requirements cannot be met by existing batteries, as they are too bulky and have relatively short lifespan. Nevertheless, various pressure sensors, including resistive, capacitive, piezoresistive, and piezoelectric sensors, are available in the market[10–12]. In most cases, the output of these sensors is an electrical signal that varies depending on the amount of force or pressure applied[13–15]. Practical sensing applications require not only the conversion of mechanical signals into electrical signals but also an instantaneous, linear, and stable response. It is generally difficult to achieve an instantaneous, linear, and stable response with piezoresistive pressure sensors; moreover, in the interest of energy and environmental sustainability, renewable energy sources should ideally be used to power such devices. Piezoelectric materials have been extensively researched on in recent years for their capacity to transform mechanical energy into electrical energy, and vice versa. In particular, there has been a great deal of interest in the use of organic piezoelectric materials for fabricating flexible and wearable electronics due to their low density, high sensitivity, biocompatibility, stretchability, wide detection range, low to zero power consumption, high output, rapid response, flexibility, and ease of processing[16–20].

Piezoelectricity is a phenomenon of electromechanical conversion between mechanical and electrical energy. Although non-centrosymmetric material is not a sufficient requirement, it is necessary to display this effect[21–24]. With the exception of cubic class 432, all non-centrosymmetric crystallographic classes display piezoelectricity[25]. Pierre and Jacques Curie were the first to discover that the asymmetric movement of ions or charges in piezoelectric materials is caused by deformation via the changes in electrical polarization[26–28]. Due to their relatively high piezoelectric coefficients and natural flexibility, poly(vinylidene fluoride) (PVDF) and its copolymers have been widely used in the production of organic piezoelectric materials[29–32]. Kawai et al. discovered that PVDF has five main phases: alpha (α), beta (β), gamma (γ), epsilon (ε), and delta (δ)[32]. The main electroactivity of PVDF arises from its crystalline ferroelectric phases (γ- and β-phase), with the γ-phase being less electroactive than the β-phase[33]. Since the piezoelectric coefficients of PVDF materials are substantially lower than those of piezoelectric ceramics, PVDF materials have not been employed as self-powered sensors in actual applications. In order to enhance the piezoelectric capabilities of PVDF and its copolymers, two techniques have been employed: increasing the piezoelectric phase content and enhancing the residual polarization[34]. It is generally difficult to achieve a high content of the β-phase through solution processing and conventional melting[35], but there are several strategies to do so, including incorporating fillers[36], mechanical stretching, heat treatment, and polarizing[37,38]. Electrospinning has been recognized as a feasible approach to converting the non-electroactive phase into the electroactive phase (γ- and β-phase). The electrospinning of a PVDF solution allows the conversion of the nonpolar α-phase into the polar β-phase.

Near- and far-field electrospinning can be used to enhance the piezoelectric characteristics of PVDF. Electrospinning, as an excellent approach for fabricating polymer fibers, was first patented in the United States in 1902[39]. Until the 1990s, this technique was often neglected, but it re-emerged as a leading technique following the rapid development and applications of nanofibers. It is possible to control the diameters and morphologies of electrospun fibers by modifying the process parameters, particularly the molecular weight, solution properties (e.g., surface tension, conductivity, and viscosity), flow rate, electric voltage, and distance between the needle and the collection plate. In this study, the parameters of near-field electrospinning (NFES) [40–42] were optimized to develop flexible, dynamic, and soft sensing elements of PVDF piezoelectric fibers incorporating silver nanoparticles (AgNPs) and transition metal carbide/nitride (MXene). NFES was performed in conjunction with a rolling collection system and its XY-axis digital control platform. In order to optimize the piezoelectric fibers, the needle and rotating collector were vertically aligned to collect the fibers produced by electrospinning. The stainless-steel needle was connected to a positive high-voltage source. Under the effects of gravity, the vertical collection method makes it easy to break through the spinneret to collector surface. With the software-controlled platform moving back and forth, continuous piezoelectric fibers were obtained through electrospinning. Through the combined effects of the conductivity, viscosity, and contact angle of the best precursor solution, a Taylor cone was formed, when the electrostatic force, provided by the electric field, was greater than the droplet surface tension, resulting in a large amount of orderly material collected on the rotating collector with parallel electrospinning[43–46].

338Transparent electrodes, which are often used for organic light-emitting diodes (OLEDs)[47], touch screens, and thin-film silicon solar cells, have thrived along with the advancements in smartphone technology[48]. Although indium tin oxide (ITO)[49] has been the predominant material in transparent electrodes, it is a brittle material that requires a vacuum environment for production, making its thin-film fabrication process costly and energy-consuming. Furthermore, the limited interfacial area between the molecular chains of PVDF and typical conductive fillers poses a challenge to achieving adequate filler–dipole interactions and increasing dipole polarization during the electrospinning of piezoelectric PVDF or its copolymers. The incorporation of conductive fillers into polymer solutions has the potential to increase the intensity of the electric field, thus resulting in an increase in the polarization of polymer dipoles at a constant voltage[50–53]. Several conductive active nanofillers have been employed to build conductive networks on various substrates, such as graphene (rGO)[54], carbon nanotubes (CNTs)[55], AgNPs[56], MXene[57], and silver nanowires (AgNWs)[58], showing excellent conductivity and transmissivity; all of which have been developed as alternatives to ITO[59,60]. The use of nanocomposites strategy, with an emphasis on the inclusion of conductive nanofillers, represented by graphene and carbon nanotubes[53,61], is a direct approach to improving piezoelectricity, and therefore the sensitivity of PVDF. By using a small amount of conductive nanofillers, it is possible to increase the piezoelectric phase development, produce strong interfacial coupling effects, and preserve the high flexibility of PVDF, all of which are useful for increasing the sensitivity of nanocomposites.

Silver is an attractive material on account of its antibacterial properties, electrical conductivity, and relatively low cost. Although AgNPs have enormous surface areas that are rich in electrons, their vulnerability to oxygen (O2) prevents them from being employed directly. A biocompatible and reducible PVDF has been created to prevent the oxidation of AgNPs and enhance their electrical stability[62,63]. Furthermore, AgNPs have been widely used as fillers in polymer matrices in view of their excellent ability to improve the crystal β-phase structure of PVDF[64,65]. A β-phase content of approximately 80% can be achieved with the addition of AgNPs to PVDF. The interaction between electron-rich AgNPs and the F atoms in PVDF increases the piezoelectricity and the β-phase content, resulting in electrically conductive particles in nanofibers upon electrospinning, and thereby improving polarization[62,65–69].

MXenes are novel two-dimensional (2D) materials displaying excellent aqueous dispersion and electrical conductivity as well as abundant surface functional groups. MXenes have garnered widespread attention due to their distinctive electrical and mechanical characteristics, which make them ideal for supercapacitors, lithium-ion (Li-ion) batteries[70,71], and use in various applications[72], including strain sensors, energy storage[73,74], catalysis[75], electromagnetic interference (EMI) shielding[76], and healthcare[76–78]. MXene is an attractive alternative for nanocomposites engineering due to its nanometer-scale dimensions, huge aspect ratio, and great electric conductivity. While the majority of published works have focused on improving dielectric, electrochemical, and thermoelectric characteristics, a number of studies have looked into MXene’s capacity to modulate the piezoelectric capabilities of piezoelectric polymers because of its exceptional conductivity, layer structure, and active surface. Nevertheless, it is important to select an appropriate matrix to achieve high levels of flexibility, porosity, and permeability. Research has revealed that combining an MXene with PVDF would result in a composite with high permittivity and compatibility. Since conductive fillers and a high dielectric constant can improve the piezoelectric properties of a polymer, it is anticipated that MXenes would be excellent materials for increasing the piezoelectric capabilities of PVDF and its copolymers[79–81]. MXene can promote the formation of the piezoelectric phase, bring about a high interfacial coupling effect, and provide an improved piezoelectric response in PVDF, much like the conductive reinforcements, represented by graphene and carbon nanotubes. Additionally, it should be highlighted that MXene nanosheets have a stronger reinforcing effect than graphene and carbon nanotubes at the same loading amount. For instance, the maximum reported piezoelectric coefficient d33 of MXene/PVDF hybrid film is 43 pC/N, compared to that of carbon nanotubes/PVDF, which is 23 pC/N. Meanwhile, it has been discovered that the presence of MXene nanosheet improves the mechanical performance, i.e., the young’s modulus, of PVDF. MXene/PVDF hybrid films have a greater voltage sensitivity Sv of up to 0.0480 V/N in detecting force when compared to traditional PVDF-based force sensors (Sv = 0.0221 V/N) due to their outstanding piezoelectric response and high Young’s modulus[82].

The intercalated MXene in composite nanofibers is spread out evenly, which not only solves the problem of MXene aggregation but also enables the composite materials to self-reduce AgNPs in situ. More importantly, wearable electronics incorporating MXene/AgNP composites have significant EMI shielding properties, allowing them to protect the human body from potentially dangerous electromagnetic radiation. A new generation of multipurpose wearable electronic devices based on MXene/AgNP materials is expected to be launched[83–86]. 339The porous structures and high specific surface areas of electrospun PVDF composite fibrous films engender them as ideal substrates for producing low-cost films[87,88].

In this study, we prepared new types of PVDF/AgNP/MXene composite nanofibers (Scheme 1A–C), characterized them using various techniques (Fourier transform infrared [FTIR] spectroscopy, X-ray diffraction [XRD], scanning electron microscopy [SEM], and piezoelectric measurements), and employed them in self-powered wearable devices (Scheme 1D and E). These composite fiber materials displayed extremely high conductivity as well as good mechanical and piezoelectric capabilities. The integration of AgNP/MXene composites into the PVDF matrix developed the electroactive β-phase and improved the piezoelectric properties of PVDF. These are potentially advantageous for future self-powered flexible and wearable optoelectronic devices. We encapsulated interdigitated electrodes (IDEs), comprising copper (Cu), PVDF/AgNP/MXene piezoelectric fibers, and flexible poly(ethylene terephthalate) (PET) substrates, into poly(dimethyl siloxane) (PDMS). These piezoelectric devices were packaged in PDMS to enhance their durability (Scheme 1F). We achieved a high-voltage output from the PVDF/AgNP/MXene piezoelectric fibers with the desired morphology, thus enabling them to turn on an LED (Scheme 1G).

Scheme 1.

Scheme 1

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Experiment process for fabricating PVDF/AgNP/MXene composite fibers through NFES: (A) Electrospinning the solution, (B) Testing the conductivity, (C) NFES, (D) Packing the sensor, (E) Measuring the output voltage, (F) Harvesting energy, and (G) Turning on an LED.

2. Materials and methods

2.1. Synthesis of MXene

MXene (Ti3C2) was synthesized by eliminating an intermediary element (Al atoms) from the MAX phase (Ti3AlC2) using minimally intensive layer delamination (MILD) etching. Commercial lithium fluoride (LiF) powder (2 g) was dissolved in concentrated hydrochloride (HCl; 9 M, 30 mL) and deionized (DI) water (10 mL), and continuously magnetically stirred (300 rpm) at room temperature for 10 mins. Ti3AlC2 powder was added slowly and carefully, and the mixture was continuously stirred (300 rpm) at 35°C for 36 h. The resultant suspension was centrifuged (5 min per cycle) with DI water (3500 rpm), and then decanted until the pH of the supernatant neared 6. At the end of the process, the products were washed with ethanol and dried overnight under vacuum at 60°C to obtain a stable liquid-free delaminated MXene in powdered form. The overall process is illustrated in Figure 1A.

Figure 1.

Figure 1

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(A) Schematic representation of the synthesis of MXene. Electrospinning solutions of (B) pure PVDF, as well as (C) PVDF/AgNP and (D) PVDF/AgNP/MXene composites. Digital images of (E) pure PVDF, (F) PVDF/MXene, and (G) PVDF/AgNP/MXene fiber sheets. SEM images of (H) PVDF/MXene and (I, J) PVDF/MXene/AgNP fibers.

2.2. AgNP/Mxene hybrid composites

MXene was dispersed in DMSO. The mixture was sonicated for at least 30 min to break up any agglomerates and was then magnetically stirred at 250 rpm for 30 min on a heater. AgNO3 (content kept at 10 wt% relative to the weight of MXene) was introduced into the suspension. After sonication for another 30 min, the solution appeared black, and the mixture was stirred magnetically for 30 min.

2.3. Conductivity measurements

In order to measure the conductivity, a clean CON30 conductivity meter was used.

2.4. Morphological characterization

The surface morphology of the synthesized Ti3C2 MXene was examined under ultrahigh resolution using transmission electron microscopy (TEM); its selected area electron diffraction (SAED) patterns were also obtained. 340The JEOL JSM-6380 scanning electron microscope was used to analyze the surface appearance of the PVDF piezoelectric fibers and the PVDF/Mxene and PVDF/AgNP/MXene composite fibers.

2.5. Electrospun polymer fiber membrane

The electrospinning solutions were prepared as shown in Figure 1B–D. The PVDF (Figure 1B) and PVDF/AgNP (Figure 1C) solutions were prepared using a previously reported procedure[89,90]. The optimal operating parameters were as follows: a PVDF content of 18 wt%, a direct-current (DC) voltage of 14 kV, and a tube rotation velocity of 1900 rpm. The PVDF powder was dissolved in acetone and DMSO (1:1, v/v). In order to prepare the PVDF/AgNP/MXene (Figure 1D) composite fiber membrane, PVDF (18 %wt) was dissolved in acetone, and the desired amounts of MXene-AgNP/DMSO suspensions were added, respectively, under magnetic stirring; the mixtures were stirred for 12 h at room temperature in an airtight container to form homogeneous solutions. As illustrated in Figure 1C and D, the AgNP/DMSO and AgNP-MXene/DMSO solutions appeared pale-yellow and black, respectively. A research laboratory-scale NFES apparatus was employed, comprising a positive power supply (0–40 kV), a negative high-power supply, a precision flow infusion pump, a needle, a rotating collection device, and a dual-axis platform controller. Scheme 2A is a schematic representation of the NFES device; its three primary elements were a metal spinning needle, a metal collector with reverse polarity, and a high-voltage power source (Scheme 2B). NEFS was performed by loading the prepared solution into a syringe with a stainless-steel needle tip. The propelling syringe was filled with the configured polymer solution and attached to the precision flow propulsion pump. The stainless-steel needle was connected to a power supply with high-voltage electric field (positive electrode) to function as the NFES nozzle. The grounding end was a glass tube (outer diameter: 20 mm; thickness: 0.50 mm) that contained the piezoelectric fibers, a direct current (DC) motor, and a negative high-voltage power supply (negative). Copper tape (thickness: ca. 0.10 mm) was wrapped around the glass tube to ensure that the electric field makes contact with the ground (negative electrode). When a high voltage was applied to the needle, an electric field was formed between the spinning needle and the collector, resulting in the continuous spinning of the needle. The moving platform was controlled by a computer to move the needle laterally to collect the orderly and non-small area fibers with a specific stretching direction. Fibrous membranes were collected using a grounded, revolving collector. The fibers were dried at room temperature overnight.

Scheme 2.

Scheme 2

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(A) NFES equipment setup, (B) Schematic representation of NFES, (C) Equipment for the electrical measurement of voltage, and (D) A fabricated piezoelectric sensor.

2.6. Mechanical properties

The mechanical properties of the PVDF/AgNP/MXene composites were evaluated using a universal testing equipment (AGS-50KNXD, Shimadzu, Japan).

2.7. Sensor package and electrical characteristic measurement

2.7.1. Fabrication of a wearable sensor

This study investigates the complex piezoelectric responses of PVDF and PVDF-based composite fiber 341structure. Firstly, the piezoelectric fiber and conductive copper tape were cut into the desired dimensions, and Ag paste was applied on the copper electrodes and conductive silver-plated wires in it, which served as the negative and positive electrodes of the piezoelectric device. Piezoelectric fiber sheets with a width of 10 mm were placed on a piece of a parallel copper interdigital electrode (IDE). Subsequently, flexible PET was tightly bonded to the IDE to achieve a large displacement, and the sensor was packaged with PDMS on both sides. Approximately 1,400 fibers were used, and PDMS was used as the package’s outmost material to create a flexible sensor. Then, the PET with the Ag glue applied on the copper electrodes and silver-plated wire in it was placed in a vacuum ball to extract the vacuum, as shown in Scheme 2D. This step involves removing the bubbles created during the vacuum process and heating them for 15 min on the heating plate.

2.7.2. Electrical measuring instrument

After that, the fiber was encapsulated by PDMS and attached to the electrical measurement device. The sensor was distorted instantly by tapping it with a rotating beater. The GW Instek GDS-2104A oscilloscope was used to measure the electrical properties. The piezoelectric sensor was deformed to perform a reliability test. The deformation of the piezoelectric fiber generated the voltage data. As the voltage measured in the experiment was easily interfered by noise, such as impact and flapping, when performing the voltage measurement experiment, it was necessary to measure the positive and negative poles, respectively, to prove that the measured signal was piezoelectric. When the polarization direction changed the shape of the piezoelectric fibers, the electrons inside the material were pushed out and became free electrons. The piezoelectric d33 mode turned the mechanical strains that were spread along the fibers into an alternating voltage; this procedure is illustrated in Scheme 2C. In the design of piezoelectric energy harvesting, piezoelectric coefficient d33 may be used as a parameter to determine the appropriate piezoelectric material. Only piezoelectric coefficient d33 needs to be tested in the experiment. The short-circuit charge collected on the electrodes of piezoelectric energy harvesting per unit applied mechanical stress is the physical meaning of d, indicating that the greater the d, the larger the possible charge collected in the short circuit.

There are several methods for determining the d33 coefficient of piezoelectric materials, including utilizing a d33 meter[82] and a pneumatic pressure rig[91]. It would be straightforward and simple to characterize PVDF with a d33 meter, but it has been reported that d33 meters may not provide the most accurate readings on thin films or very thin samples[92]. d33 must be tested according to the formulation of mathematical equations[93]. Since there is no direct apparatus to measure d33, a comparative technique was adopted, and a representative PVDF material with an identical d33 value was created for reference[82].

3. Results and discussion

3.1. MXene

342The as-synthesized MXene Ti3C2 powder was characterized using TEM, SAED patterns, and energy dispersive spectrometry (EDS). Figure 2A and B presents low- and high-magnification TEM images, revealing the flake-like appearance of nanosheets following exfoliation, along with apparent layered structures of the MXene material. The associated SAED pattern (Figure 2C) reveals the hexagonal symmetry of the carbide layers, inherited from Ti3AlC2. The EDS spectrum in Figure 2D shows the atomic percentages of the residue of Ti3C2 MXene; a large proportion of the char residue featured the anatase phase of Ti3C2, with O and F atoms represented in comparatively small proportions (ca. 23.2% and 5.9%, respectively).

Figure 2.

Figure 2

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(A) Low-magnification TEM image of the 2D MXene Ti3C2, (B) High-magnification TEM image of the synthesized stacked layers of the bulk Mxene, (C) SAED pattern, and (D) EDS spectrum and atomic percentages of the residue of Ti3C2 MXene.

3.2. PVDF/Ag-NP/MXene electrospun fibers

We prepared PET-based piezoelectric devices, featuring new PVDF/AgNP/MXene electrospun fibers, and compared the behavior of electrospun fibers formed from PVDF, PVDF/AgNP, PVDF/Mxene, and PVDF/AgNP/MXene. The decoration of Ag atoms on the MXene sheets was achieved through AgNO3 self-reduction, as demonstrated using morphological, functional, and structural characterizations. Figure 1 displays the fabrication process of the all-fiber structures. Doping stretchable polymer fibers with high-conductivity NPs allowed the development of novel functionality, including the ability to detect physical deformations. Since its metallic conductivity is better than those of other solution-processed nanomaterials, 2D titanium carbide MXene is a promising material for producing conducting and stretchy fibers. MXenes are a broad classification of 2D materials that have extraordinarily high electrical conductivity (up to 104 S/cm). We assumed that the inclusion of this MXene would improve the conductivity of the polymer solution; thus, in order to develop fibers that have a unique mix of electrical conductivity and stretchability, a better understanding is required of the interactions between MXene and the polymer material. In a previous study, we found that an 18 wt% PVDF solution had the desired viscosity and surface tension for NFES[94]; hence, we measured the conductivity of composites prepared from 18 wt% PVDF and AgNP/MXene. Here, we found that the electrical conductivity increased from 40 to 1,148 µS/cm following the addition of MXene/AgNP composite to PVDF solution, primarily because the inclusion of AgNPs in MXene improved the electrical conductivity of PVDF composites.

In order to study the influence of the solution concentration, the NFES process parameters were adjusted as follows: a 20 G needle (outer diameter: 0.91 mm; inner diameter: 0.61 mm), the speed of the XY dual-axis digital control platform at 2 mm/s, a high-voltage input of the needle at 14 kV, molecular weight, and the distance between the needle and the roller collection device at 1 mm. The nanofibers prepared from PVDF/AgNP/MXene with long-lasting piezoelectricity were directly etched on the glass tube. Figure 1E–G presents the images of pure PVDF, 343PVDF/MXene, and PVDF/AgNP/MXene fiber sheets, respectively; as expected, the color varied from white to blackish upon increasing the concentration of MXene.

We used SEM to investigate the morphologies of MXene composite fibers in greater detail. A well-aligned fiber at the contact would favor the production of well-oriented β-form extended-chain crystallites at the interface. The diameters of the fibers were determined from SEM images recorded at four different locations along their length. The electrospinning of MXene composite fibers resulted in fibers that were perfectly aligned with one another. Although the PVDF/AgNP/MXene fibers were continuous and had porous network topologies, there was no beading throughout the fibers. Figure 1H–J shows high-magnification SEM images of PVDF/MXene/AgNP composite fibers. Thinner fibers were formed in the presence of MXene, most likely because of the increased static electrical field caused by the higher solution conductivity. There were a few minor bumps, resulting in unsmooth fibers, as well as micron-sized MXene, which resulted in slightly uneven fiber diameters; the PVDF/MXene fiber diameters ranged from 0.39 to 4.50 µm (Figure 1I). With the addition of AgNP, the fibers became clear and dry, presumably because the AgNPs enhanced the thermal conduction, thereby enhancing solvent evaporation during the fly process in the electrical field and following the deposition on the rotating disk. The diameters of the PVDF/AgNP/MXene fibers ranged from 1.0 to 6.5 µm (Figure 1J). Nevertheless, the fiber surfaces had uneven structures (surface roughness and bumps), presumably due to regionally nonuniform MXene distributions produced by slower solvent evaporation, as a result of interactions between the hydrophilic MXene and the humid atmosphere (Figure 1H).

We used FTIR spectroscopy and XRD to determine the chemical and crystalline structures of the materials used in this study. Figure 3A presents the FTIR spectra of MXene, AgNPs, and PVDF in range from 1,700 to 750 cm−1. Signals at 840, 1,071, 1,275, and 1,402 cm−1 are typical of the β-phase. After the addition of MXene, the β-phase became more apparent, and the width became narrower. The sample containing AgNPs had the highest content of the β-phase. Signals for Ti–O, C–O, and C–F bonds in the MXene were also evident. The signal for Ti–O bonds moved from 1,624 to 1,644 cm−1, revealing that the C–F bonds of PVDF and the surface groups of MXene interacted through dipole–dipole interactions.

Figure 3.

Figure 3

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(A) FTIR spectra of pure PVDF, PVDF/MXene, and PVDF/AgNP/MXene composite materials and (B) their corresponding tensile stress-strain curves.

The (002) diffraction peak of MXene was evident in the XRD patterns of PVDF/AgNP/MXene fiber films. In addition, the signals for the (111), (200), (221), and (311) planes of the Ag crystal were all present at values of 2θ of 38.14, 44.4, 64.4, and 77.6°, respectively (Figure 4A). The usual (110) diffraction peak of α-phase PVDF, appearing at a value of 2θ of 20.2°, became less prominent following the increase in concentration of MXene in the films. Similarly, the (021) diffraction peak of β-phase PVDF appeared at a value of 2θ of 20.8°. These changes in crystalline form suggested that the PVDF chains were nucleated by MXene in a very heterogeneous manner and indirectly confirmed that there were strong interactions at the interface between MXene/AgNP and PVDF.

Figure 4.

Figure 4

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XRD patterns of (A) PVDF/AgNP/MXene composite membrane as well as, (B) pure PVDF and PVDF/AgNP/MXene composite membranes.

When mechanical forces are applied to piezoelectric materials, they deform and change their polarization density almost instantly. This behavior causes a voltage to appear in the polar direction, and vice versa. The composite products exhibited an outstanding mechanical performance, owing to strengthening mechanisms. The practical applications of composite materials are limited by poor mechanical properties. Therefore, the mechanical properties of piezoelectric materials are crucial factors when attempting to increase the piezoelectric response. However, they are often overlooked when engineering piezoelectric materials. Although Young’s modulus has been studied in relation to the piezoelectric characteristics of piezoelectric polymers, there is still a need to investigate the effects of other mechanical qualities, including compression modulus and flexibility[83,95-97]. Polymer composite materials are not useful if they have poor mechanical qualities[51–53]. Therefore, the mechanical characteristics of PVDF as well as PVDF/MXene and PVDF/AgNP/MXene composites were investigated (Figure 3B). As observed in our experiments, the AgNP- and-MXene-based piezoelectric fiber displayed superior mechanical characteristics. The mechanical properties of the two types of fillers were different due to their varying volume fractions. The following observation demonstrated that the quasi AgNPs and MXene worked together to improve the mechanical qualities: tensile strength of PVDF/MXene/AgNP > tensile strength of PVDF/Mxene > tensile strength of PVDF. The maximum tensile stress improved from approximately 2.0 to 10.5 MPa in the presence of 5 wt% MXene, as a result of the interactions between the polymer chains and the MXene with AgNPs, thereby strengthening the electrospun fibers. Notably, incorporating MXene into AgNP/PVDF fibers led to much greater strain to failure and strength, relative to those of pure PVDF fibers. The toughness was a measure of the nanofibers’ stretchability. Indeed, the stress–strain curves of the PVDF/AgNP/MXene composite fiber revealed increased toughness. Compared with pure PVDF fibers, the mechanical properties of PVDF fibers incorporating AgNPs and MXene were effectively enhanced. In particular, when compared with pure PVDF and PVDF/AgNP composit fibers, the tensile strength, young’s modulus, and toughness of the PVDF/AgNP/MXene fiber film were superior, verifying the synergistic impact of PVDF, AgNPs, 344and MXenes on the mechanical characteristics of the fibers, as shown in Table S1.

3.3. Piezoelectric characteristics of PVDF/AgNP/MX-ene composite membrane

Our objective for this study was to achieve a high-voltage output from PVDF/AgNP/MXene piezoelectric fibers. Prior to analyzing the piezoelectric properties of the fibers, we confirmed the piezoelectric effect. The data from the positive and negative two-pole positive and reverse electrical measurement devices showed opposing peaks, as seen in Figure 5.

Figure 5.

Figure 5

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Piezoelectric properties and voltage output of the PVDF/AgNP/MXene composite: (A, B) forward and (C, D) reverse connections.

The electrical tapping tests (Figure 6) confirmed that the output voltage of the PVDF/AgNP/MXene composite piezoelectric fiber was significantly higher than that of pure PVDF as well as PVDF/AgNP and PVDF/MXene composites. Therefore, doping the PVDF piezoelectric fiber with AgNP/MXene would increase the voltage. The piezoelectric property is dominated by the young’s modulus and piezoelectric coefficient of piezoelectric materials. Piezoelectric materials would achieve higher piezoelectric performance with a larger piezoelectric coefficient and a smaller young’s modulus. The ability of piezoelectric materials to withstand changes in length would significantly affect the piezoelectric property[98]. In contrast to the substantial study and application of PVDF piezoelectric films, the present research on PVDF piezoelectric fibers is limited. In 2010, Pu et al. reported that the piezoelectric strain coefficient d33 of PVDF piezoelectric fiber was 57.6 pm/V, while the that of commercially available PVDF piezoelectric film was 25 pm/V[99]. In this study, we used a unique technique to create piezoelectric PVDF composite films with much higher d33 values than previously reported values. The piezoelectric coefficient (d33) of the PVDF/AgNP/Mxene sample was verified, in which the d33 value was found to be 52.23 pC/N. The results are given in Table S2 for a better comparison. Tapping tests were also conducted to evaluate the sensitivity of the fabricated pressure sensor based on the PVDF/AgNP/MXene hybrid fiber film. Figure S1 (1.2MB, pdf) shows that the output voltage was tested at several low frequencies ranging from 1, 2, 3, 4, 5, and 7 Hz. Sensors based on PVDF/AgNP/MXene are sensitive to variations in the applied frequency. The voltage response of the sensor based on PVDF/AgNP/MXene, as shown in Figure S1 (1.2MB, pdf) , increases with increasing frequency, with a maximum voltage sensitivity of 0.31428 V Hz-1, which is larger than the that of PVDF/MXene (0.242 V Hz-1) and PVDF (0.010 V Hz-1) sensors[44]. High stability and reliability are also necessary for the practical deployment of flexible force sensors. A cantilever vibration test was conducted to assess the operating stability and reliability 345of the sensor, in which the schematic representation of the test is shown in Scheme 2C. Figure S2 (1.2MB, pdf) shows the output response of the PVDF/AgNP/Mxene hybrid sensor at 1 Hz driving frequency. The sensors exhibit superior stability and durability as long-term effective and durable sensors as they can run consistently for over 1 week (one cycle of 1,000 s per day) without performance deterioration.

Figure 6.

Figure 6

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Piezoelectric response of (A) pure PVDF, (B) PVDF/AgNP, (C) PVDF/MXene, and (D) PVDF/AgNP/MXene.

We investigated the potential of using PVDF/AgNP/MXene composite fiber as a self-powered wearable sensor for the detection of human motion, with regard to the exceptional flexibility and energy-harvesting capabilities of the device. Figure 7 shows the voltage outputs of the sensor detecting the clapping of hands, moving of arms, moving of fingers, and walking.

Figure 7.

Figure 7

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Applications of PVDF/MXene/AgNP composite as a self-powered sensor for the detection of human motion. Voltage outputs of the sensor detecting the (A) clapping of hands, (B) moving of arms, (C) moving of fingers, and (D) walking.

346The gloves were first attached with fabricated nanofiber mats, and the voltage output was then monitored when clapping hands, thereby converting hand pressure into electricity (Figure 7A). The voltage output was measured when clapping hands at different frequencies. Figure 8 reveals that the consecutive clapping of hands resulted in voltage responses that were recoverable and steady. More notably, when a person clapped his or her hands, first slowly, and then quickly, the output voltage frequency reacted in a manner consistent with the movement, indicating good synchronization. Subsequently, sensors were connected to the underarm of clothing and to the medial elbow (Figure 7B). The sensors detected the movement of the arms and converted the arm pressure into electricity, resulting in a series of voltage signals. When we attached the sensor to a finger, it detected finger movements, converting finger pressure into electricity (Figure 7C). In addition, walking was detected by a sensor attached to the ground, converting leg pressure into electricity (Figure 7D). The output voltages provided repeatable, steady, and synchronous signals.

Figure 8.

Figure 8

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(A) Digital images of the sensor while clapping hands, and (B-D) voltage outputs of the sensor detecting the clapping of hands at various frequencies, i.e., 6, 8, and 9 Hz, respectively.

The presence of AgNP/MXene composite in PVDF also enhanced the electrical conductivity of the material, thereby improving the piezoelectric characteristics of the whole fiber. The output voltages when clapping hands, moving arms, moving a finger, and walking were 15, 7.5, 4, and 11 V, respectively. When the hands were moved, as displayed in Figure 8C and D, voltage peaks of up to 14–15 V were recorded, as a result of the pressure imparted to the sensor site while being touched. In view of the irregularity of the forces applied, there were variations in the sizes of the voltage peaks. The PVDF/AgNP/MXene fibers exhibited exceptional stability and excellent output performance, enabling their use for energy harvesting and powering LEDs. Figure 9 reveals the ability of the sensor to power an LED through the touch of a finger.

Figure 9.

Figure 9

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(A) Circuit schematic representation of LED which is incorporated into the circuit. (B) Image of the LED before it was lit up. (C) Image of the LED that was lit up when the sensor is touched

In order to evaluate the sensing ability of the optimized PVDF/AgNP/MXene composite piezoelectric sensor, we coupled it with a circuit integrated with an Arduino microcontroller (Mega 2560). Arduino is a hardware platform used for the development of applications based on Atmel microcontrollers. The Mega 2560 model, which we applied in this study, is a standard commercial board entrusted to acquiring and recording inputs. The 347microcontroller was employed as an open-source electronic platform with C language programming for electrical signal processing and counting control. The voltage generated by the PVDF/AgNP/MXene composite piezoelectric sensor was used to light an LED. The Arduino, LEDs, piezoelectric sensors, and a power management circuit were placed on a breadboard to turn on the LED when the sensor was touched. Figure 9A is a schematic representation of the circuit. In this study, the positive terminal of the LED was connected to a resistor set to 1 MΩ, and it was then connected to pin4 of the Arduino, the voltage output channel of the sensor. The generated electrical voltages were further limited by a 1 MΩ resistor to protect the microcontroller from electric shocks. The LED would glow after the piezoelectric sensor had reached the threshold voltage when applying the force. Figure 9B and C reveals that a commercial red LED could indeed be lit up.

4. Conclusion

The addition of AgNP/MXene composite increased the electrical conductivity of PVDF, improving the piezoelectric properties of its entire fiber. FTIR spectra revealed that the contents of the α-phase of PVDF were lower, and the contents of the β-phase increased after being polarized by high electric field through NFES in PVDF as well as PVDF/AgNP, PVDF/MXene, and PVDF/AgNP/MXene composites. Electrical tapping experiments revealed that the output voltages of the energy harvester based on PVDF/AgNP/MXene composite piezoelectric fiber was higher than those of devices based on pure PVDF and PVDF/AgNP and PVDF/MXene composites, thus confirming that the addition of AgNP/MXene increased the output voltage of PVDF piezoelectric fibers. We investigated the potential use of PVDF/AgNP/MXene composite fibers as self-powered wearable sensors for the 348detection of human motion, based on their exceptional flexibility and energy-harvesting capabilities. The sensor voltage outputs, depending on whether the sensor was detecting the clapping of hands or the moving of arms and fingers, ranged from 4 V to 15 V as a result of the pressure imparted to the sensor upon being touched. In view of the irregularity in the forces applied, there were variations in the sizes of the voltage peaks. The PVDF/AgNP/MXene fibers demonstrated exceptional stability and excellent output performance, enabling their use for energy harvesting and the powering of LEDs.

Acknowledgements

The authors thank the staff at National Sun Yat-sen University for their assistance with the TEM (ID: EM022600) experiments.

Funding

This work was supported by the grant KAFGH_A_111002 from Kaohsiung Armed Forces General Hospital and financially supported by the Ministry of Science and Technology, Taiwan, under contracts MOST 110-2124-M-002-013 and 111-2223-E-110-004.

Conflict of interest

The authors declare no conflicts of interest.

Author contributions

Conceptualization: Cheng-Tang Pan, Karishma Dutt, Amit Kumar

Data curation: Karishma Dutt, Rahul Kumar, Zhi-Hong Wen

Formal analysis: Karishma Dutt, Yi-Ting Lo

Funding acquisitions: Cheng-Tang Pan, Shiao-Wei Kuo

Investigation: Karishma Dutt, Amit Kumar

Methodology: Cheng-Tang Pan, Karishma Dutt

Project administration: Karishma Dutt

Resources: Cheng-Tang Pan, Shiao-Wei Ku, Cheng-Hsin Chuang

Software: Karishma Dutt, Chien-Shu Wang,

Supervision: Cheng-Tang Pan, Shiao-Wei Kuo

Visualization: Karishma Dutt, Shiao-Wei Kuo and

Writing – original draft: Karishma Dutt

Writing – review & editing: Cheng-Tang Pan, Karishma Dutt, Shiao-Wei Kuo

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data

Not applicable.

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