Mechanical energy produced by human motion is ubiquitous, continuous, and usually not utilized, making it an attractive target for sustainable electricity-harvesting applications. In this study, flexible magnetic-Juncus effusus (M-JE) fibers were prepared from plant-extracted three-dimensional porous Juncus effusus (JE) fibers decorated with polyurethane and magnetic particles. The M-JE fibers were woven into fabrics and used for mechanical energy harvesting through electromagnetic induction. The M-JE fabric and induction coil, attached to the human wrist and waist, yielded continuous and stable voltage (2 V) and current (3 mA) during swinging. The proposed M-JE fabric energy harvester exhibited good energy harvesting potential and was capable of quickly charging commercial capacitors to power small electronic devices. The proposed M-JE fabric exhibited good mechanical energy harvesting performance, paving the way for the use of natural plant fibers in energy-harvesting fabrics.
The Internet of Things technology has been developing rapidly during the past decade, affecting all aspects of human life [1], [2]. Electronic devices are ubiquitous. The demand for wearables has been rapidly increasing with a focus on portability, comfort, and convenience [3], [4], [5]. Wearable electronics have been explored for health monitoring, healthcare, motion monitoring, sensors, and electronic skin with the potential to significantly enrich human lives [3], [6], [7].
Batteries and capacitors remain the main components of energy–storage equipment for wearable electronics. However, the development of portable energy–storage equipment has been limited by the following factors: ① Most energy–storage devices are rigid and bulky; ② most energy–storage units are produced using toxic chemicals that can harm humans and the environment; and ③ the limited lifetime of these devices and storage requirements precludes their use in remote areas such as deserts, mountains, and islands [8], [9]. Therefore, green, continuous, and convenient energy sources are highly desirable for wearable electronics [10]. Significant amounts of green and renewable energy are present in the environment, which can help resolve the problem of energy supply for wearable electronics with appropriate use [11], [12], [13]. Many scholars have addressed energy harvesting and proposed excellent solutions for the extraction of environmental energy such as thermoelectric [14], photoelectric [15], piezoelectric [16], [17], triboelectric nanogenerators [18], [19], and electromagnetic induction technologies [20], [21]. Electromagnetic induction was discovered by Faraday in 1831 and soon led to the second industrial revolution. It remains the most important source of electrical energy [22], [23]. Magnetoelectric generation technology is advantageous because of its stable electrical energy output and large current [21], [22], [24]. Although electromagnetic induction-based generators are common, magnets are typically bulky and rigid, severely limiting their application in flexible electronics [25], [26], [27]. Softening magnets and increasing their flexibility remain challenging issues. Although commercial magnetic rubber products and other magnetic fibers or film products are available, they typically exhibit low surface magnetism, which limits their use in energy harvesting [22], [28].
Textile products are indispensable to human life and are widely used in human production because of their flexibility [29], [30]. Fibers are the building blocks of textiles, and combining functional components with fabrics using modern textile technology is one of the most promising methods for preparing wearable energy-harvesting devices [9], [31]. Currently, the primary methods for preparing functional fibers include wet spinning [32], melt spinning [33], dip coating [34], and in situ synthesis [35]. Researchers have conducted numerous studies on harvesting environmental energy using these methods. However, these approaches have high costs, complicated preparation processes, and low loadings of functional substances [28], [32], [36]. Polyurethane (PU) is a linear multi-block copolymer consisting of hard segments, soft segments, and chain extenders (diphenylmethane diisocyanate, polytetramethylene glycol, and 1,4-butanediol). This characteristic structure is responsible for its high elasticity, toughness, biocompatibility, and processability. Since its discovery in 1937 by Bayer and his colleagues, as a versatile polymer, because of versatility in structure and properties, PU has found a vast vary array of applications including construction, automotive, electronics, textiles, coatings, and biomedicine [28]. PU-coated functional textiles refer to the preparation of coated fabrics with different functions by uniformly covering the surface of fiber textiles with polymer compounds or by changing the blended materials in the PU coating. In previous studies [37], [38], [39], [40], [41], PU was used to immobilize carbon black, carbon nanotubes/silica, graphene, MXene, and zinc oxide/silver to improve the material properties of electromagnetic shielding, shape memory, thermal conductivity, antifouling, and electrical conductivity. It can be seen that this strategy is widely recognized by researchers. PU is regarded as a versatile material and a sustainable problem solver for today’s challenges [42]. A strategy for the functionalization of Juncus effusus (JE) fibers through polymer adhesion was used in our previous works [43], [44], [45]. The use of polyvinyl alcohol, polyvinyl butyral, and polyvinyl pyrrolidone has enabled the application of JE fibers in dyeing wastewater purification, oil–water separation, and smoke filtration. JE fiber is a typical lignocellulosic fiber consisting of an interconnect [43], [44]. The high porosity (∼97.6%) results in a very low density (∼6.3 mg·cm−3) [43]. JE fibers are similar to natural aerogels, making them suitable for use as lightweight substrates. Moreover, the porous structure inside the fibers enables a large loading of magnetic particles. Previous studies on electromagnetic induction energy-harvesting devices can be divided into three main types: fibers, membranes, and fabrics [46], [22]. Fiber- and fabric-type electromagnetic induction energy-harvesting devices need to be prepared continuously using spinning technology, but precise solution push-pump systems are usually expensive. Membrane-type electromagnetic induction energy-harvesting devices, such as wearable textiles, cannot ensure basic permeability and comfort for long-term wear. Magnetic fibers can be prepared by dip coating without additional equipment. JE fibers form rich reticulated structures with extremely high porosity, which provide a natural advantage for loading functional substances and make them good substrates for preparing functional fibers [44], [45], [47]. Therefore, the use of JE fibers as lightweight carriers is simple. Magnetic fibers can be prepared by dip coating without additional equipment. Moreover, the flexibility of the fibers enables the formation of fabrics by weaving, which provides permeability and size expansion for better application potential.
In this study, we prepared magnetic-JE (M-JE) fibers and fabrics using a rich network structure of JE fibers and loaded them with Neodymium–Iron–Boron (NdFeB) magnetic particles, which collect the mechanical energy generated by the human swinging arm through electromagnetic induction with an induction coil. The effects of different processes on the properties of the prepared magnetic fibers were investigated, including PU concentration, magnetic particle content, JE pretreatment, and vacuum treatment. The M-JE fibers prepared by vacuum treatment with the PU solution at 25 wt% and a magnetic particle solid content of 80% exhibited outstanding magnetic properties. The magnetic field distribution of the M-JE fabrics and the working mechanism of energy harvesting in the M-JE fabrics were determined by numerical simulations using COMSOL software. The influence of various variables on the energy-collection characteristics of the prepared M-JE fabrics was systematically investigated. When the number of layers in a M-JE fabric was 5, the spacing was 0 mm, the number of coil turns was 2000, and the swing speed was 1.65 m·s−1, the prepared M-JE fabric exhibited the best energy harvesting performance, with the output voltage of 3 V and the current reaching 15 mA. The M-JE fabric and induction coil were affixed to the wrists and waists of the volunteers, and the mechanical energy generated by the volunteers’ swinging arms was harvested. When volunteers swung their arms quickly, the output voltage of the induction coil was 2 V and the current was 3 mA, which is sufficient for driving typical small electronic devices [1]. In summary, the prepared M-JE fibers and fabrics can effectively achieve biomechanical energy conversion with controllable cost, mass production, and high current, contributing to the development of flexible energy-harvesting fabrics.
2. Materials and methods
2.1. Materials
JE fibers were brought from Jiangxi Juncus effusus Co., Ltd., China. PU (9370AU) was provided by BAYER Co., Ltd., Germany. NdFeB magnet powder was bought by XINNUODE Co., Ltd., China. Pure polyester filaments were obtained from a local retail store. N, N-Dimethylformamide was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Deionized (DI) water was employed throughout the experiment. All the materials and reagents were used without further purification.
2.2. Design and fabrication of magnetic energy generator
M-JE experienced two stages including filling with magnetic solution and a coagulation bath for solidification. Before production, JE fibers were washed with surfactants to remove the surface flake structure, and the PU solution was prepared. The NdFeB microparticles were first mixed with PU solution to obtain the NdFeB magnetic solution, used to prepare M-JE fibers with different solid content (20%, 40%, 60%, and 80%) of magnetic particles. The magnetic solution and JE fiber were poured onto the watch glass and homogeneous mixing. Put the watch glass into the vacuum chamber and degassed for 5 min to ensure the full filling of the mixture in the JE fiber. After vacuum treatment, the JE fibers filled with the magnetic solution were put into water for a coagulation bath. This was followed by drying at 60 °C until the weight remained unchanged, obtaining M-JE fiber. To promote the practical application, M-JE fibers were manufactured into fabrics by the plain wave.
2.3. Measurements and characterization
The microstructure and element distribution of JE fiber and M-JE fiber was recorded by a scanning electronic microscope (SEM; GeminiSEM 300, ZEISS, Germany) and energy-dispersive X-ray spectroscopy (EDS) after gold coating. Three-dimensional (3D) microscope images were taken by an optical microscope (3D microscope; RH-2000, HIROX, Japan). The magnetic fabrics were magnetized using a commercial magnetizing machine (Magnetizer; M20-2040, Haiteli, China). Magnetic intensity on the surface of the M-JE and M-JE fabric was measured by a digital Gauss meter (Gauss meter; TM5100, TUNKIA, China). The magnetic hysteresis loop was tested by the physical property measurement system (PPMS; PPMS-9T, Quantum Design, USA). The voltage and current signals of the M-JE harvester were measured by the electrometer (6514, Keithley, USA) and digital source meter (2450, Keithley, USA) and high-speed data acquisition card (DAQ; NI-6210, National Instruments, USA). The M-JE fabrics were woven by a commercial semi-automatic weaving loom (TONYUA, China). The reciprocating motion of the M-JE fabrics were driven by linear reciprocating transverse motor (Linear motor; P01-37X120-C/C1100, LinMot, Switzerland). Washability testing of M-JE fabrics by commercial laundry machines (Washing Machine; FOM71, Electrolux, Sweden). The load-bearing capacity of magnetic fabrics is evaluated by connect resistors of disparate resistance values in series, and measure the voltage across the resistor and the current through it with the electrometer and digital source meter . The mechanical properties of the fibers were tested using a universal material-testing machine (Instron 5967, Instron, USA). Numerical simulation was calculated by COMSOL Multiphysics (COMSOL Multiphysics 6.0, COMSOL, Sweden) analysis software.
3. Results and discussion
3.1. Preparation and investigation of M-JE fibers and fabrics
The human body and its surroundings are rich sources of renewable energy generated through biomechanical motion, heat, and fluids [8]. The available power associated with biomechanical motion can reach 60 W [48]. Rational use of human energy can offer improved and sustainable solutions for wearable electronics. In this study, an M-JE fiber with high magnetism and high mechanical capacity was fabricated. The magnetic fabrics prepared using the proposed M-JE fibers could capture the mechanical energy of human motion and drive small electronic equipment such as light-emitting diodes, quartz watches, and calculators (Fig. 1(a)). Fig. 1(b) shows the fabrication process of the M-JE fibers and the M-JE fabrics prepared by M-JE fibers. The preparation of M-JE was as follows: ① NdFeB magnetic particles were added to the PU solution and stirred evenly; ② the JE fibers with hydrophilic treatment were immersed into the magnetic PU solution and fully absorbed by vacuum treatment; ③ the JE fibers filled with the magnetic solution were placed in water for a coagulation bath; ④ the M-JE fibers were woven into fabrics on a commercial SAG598 semi-automatic weaving loom. The cross-sectional structures of the JE and M-JE fibers are shown using SEM. The SEM images demonstrated that the pristine JE fibers are characterized by a consistent 3D hierarchical porosity structure with a smooth surface, and the internal aperture is about 45 μm (Fig. 1(c)). The connective structure of the JE fibers makes them rich in voids, which can accommodate sufficient padding. The SEM images in Fig. 1(d) show that the NdFeB magnetic particles are distributed on the internal networks and skeletons of the JE fibers. In the high-magnification SEM images, the NdFeB magnetic particles were affixed to the skeleton of the JE fiber by the PU. Furthermore, EDS was used to measure the elemental distribution of NdFeB magnetic particles attached to the M-JE fiber. As shown in Fig. S1 in Appendix A, it can be observed that the elements Fe and Nd are densely distributed on the particles. For the application of M-JE fibers, the presentation of mechanical properties is very important. The mechanical properties of the fibers were tested using a universal material-testing machine. First, the stress–strain curves of the JE and M-JE fibers were compared (Fig. S2 (a) in Appendix A). The JE fibers showed a tensile strength of 0.12 MPa and a strain of 9.6%. In contrast, the M-JE fiber tensile strength and strain increased 9-fold (1.13 MPa) and 51-fold (490%), respectively, which was attributed to the excellent mechanical properties of PU. In addition, durability was tested by repeatedly stretching and releasing the fibers over 50 cycles at different strains. As shown in Fig. S2(b) in Appendix A, the tensile strength of the M-JE fibers at 10%–50% strain remained stable after 50 cycles, demonstrating reliable mechanical properties. We also investigated the thermal decomposition performance of the M-JE fibers, as shown in Fig. S3 in Appendix A, the M-JE fibers begin to decompose at about 200 °C, which is sufficient for daily use.
In the magnetizing process, the magnetic dipoles in the NdFeB particles were rearranged regularly by a strong magnetic pulse; thus, the prepared M-JE fibers were magnetized [49]. Because the magnetism of the prepared M-JE fibers was due to the regular arrangement of magnetic dipoles in the incorporated NdFeB magnetic particles, the magnetic field strength of the prepared M-JE fibers was determined by the NdFeB particle content [21]. In this study, NdFeB magnetic particles were scattered in a PU mixture, and the mass concentration of the PU solution, solid content of NdFeB magnetic particles in M-JE fibers, and different preparation methods affected the content of NdFeB magnetic particles in the prepared M-JE fibers. Thus, the dependence of the magnetic field on the PU solution concentration, hydrophilic treatment of JE fibers, vacuum treatment, and solid content of NdFeB magnetic particles in M-JE fibers were investigated. The effect of PU concentration on the magnetic properties of the prepared M-JE fibers was mainly due to the effect of the content of NdFeB magnetic particles used in the configuration of the solution. To determine the effect of the PU concentration more accurately, NdFeB magnetic particle dispersions with different PU concentrations were configured in this study at 80% NdFeB magnetic particle solid content. However, the PU content of the solution increased with decreasing fluidity. To visualize fluidity variations in the magnetic PU solution, PU solutions of different concentrations were transfused into the sample bottle. As shown in Fig. S4 in Appendix A, 10 s after tilting the sample bottle, the magnetic solutions with 15%, 20%, and 25% PU almost flowed to the bottom, whereas the magnetic solution with 30% PU was only deformed. Fig. S5 (a) in Appendix A shows images of the cross-sections of the M-JE fibers prepared using PU solutions of different concentrations. As shown in Fig. S5(a), owing to the insufficient content of magnetic particles, the distributions of magnetic particles in the M-JE fiber cross sections prepared using the PU solutions at 15% and 20% were not uniform and exhibited delamination. The highly viscous flow state of the PU solution at 30% made adsorption by the JE fibers difficult. Therefore, NdFeB magnetic particles were present only on the outside of the M-JE fibers prepared using the PU solution at 30% (Fig. S5(a)). Thus, the magnetic properties of the M-JE fibers prepared using the PU magnetic solutions at 15%, 20%, and 30% were below those of the M-JE fibers prepared using the PU solution at 25% (Fig. S5(b) in Appendix A). In Fig. S5(b), the magnetic fibers prepared using the PU solution at 25% exhibited the highest magnetic field of 18.41 mT, which was higher than the field intensities obtained using the PU solutions at 15%, 20%, and 30%, owing to the higher percentage of magnetic particles and suitable viscosity flow state. After hydrophilic treatment, the hemicellulose of the JE fibers was removed, and the internal microfibrils shrunk to some extent [47]. Therefore, after the hydrophilic treatment, the JE fibers could adsorb higher concentrations of PU solutions than the original JE fibers. Fig. S6(a) in Appendix A shows the magnetic field of the M-JE fibers prepared using 20% of the PU solution and different treatments. The prepared M-JE fibers exhibited a high-intensity magnetic field (16.18 mT) following hydrophilic treatment. In contrast, vacuum treatment helped drain the gas from the JE fibers, enabling absorption of the magnetic PU solution. In Appendix A Fig. S6(b), for the PU solution at 25%, the prepared M-JE fibers with vacuum treatment exhibited an increase in the magnetic field intensity, from 18.41 to 24.01 mT. The amount of NdFeB magnetic particles in the prepared M-JE fibers affected their magnetic strength. Fig. 1(e) shows the magnetic hysteresis loops of the fibers prepared with different NdFeB magnetic particle contents. Increasing the NdFeB magnetic particle content resulted in higher magnetic field intensities, and the remnant magnetization increased from 15.04 to 60.39 A·m2·kg−1.
To illustrate the feasibility of the M-JE fibers, they were applied as weft yarns, and polyester yarns were applied as warp yarns to produce a plain weave fabric using a commercial SGA598 semiautomatic weaving loom (Figs. 2(a)–(c)). The prepared M-JE fabrics were then magnetized using a magnetizer with a north pole perpendicular to the sample surface. During the magnetization process, a powerful electromagnetic field was generated in the magnetization coil, and the magnetic dipoles in the magnetic particles were rearranged regularly under the action of the strong magnetic field, which made the JE fibers magnetic (Fig. S7 in Appendix A) [21], [49]. Weight and flexibility are key parameters of textiles. Therefore, the weight of the five-layer M-JE fabric was 34.2720 g, which is equivalent to approximately 1/3 of a kiwifruit (Figs. S8(a) and (b) in Appendix A). Furthermore, we demonstrated the flexibility of the M-JE fabric visually by twisting and bending. Owing to the apparent flexibility of the fabric in the warp direction, the display was performed along the weft direction (Video S1 in Appendix A). As shown in Fig. S8(c) in Appendix A, the M-JE fabric can generate arbitrary bends and returns, demonstrating its flexibility. The lightweight and flexible characteristics of the M-JE fabric allow it to be easily carried around, and it has good potential for application by combining it with clothing sewing. It is worth noting that M-JE fabrics can be mass-produced for the following reasons: ① JE fibers are natural plant fibers that are abundantly produced; ② PU, NdFeB magnetic particles, and polyester fibers used in the preparation of M-JE fibers are commercially available and can be purchased in large quantities, which can effectively reduce costs; and ③ the prepared M-JE fibers are sufficiently strong for use in industrial weaving machines, satisfying the continuous mechanical requirements of industrial weaving machines for continuous production.
To further investigate the magnetic field distribution in the proposed M-JE fabrics, the commercial analysis and calculation software COMSOL Multiphysics was used to analyze and calculate the two-dimensional (2D) magnetic field distribution in the prepared M-JE fabrics and the effect of the number of layers on the magnetic field distribution. The simulation conditions were as follows: ① The width of the M-JE fabric was set to 5 cm, and the diameter of the M-JE fibers was set to 25 mm; ② the M-JE fabric samples were magnetized in the y direction (y: the axes of the Cartesian coordinate system); ③ the residual magnetic flux density of the M-JE fibers was 25.81 mT; and ④ the environment of the calculation was ambient. The simulation results for the magnetic field distribution on the surface of the M-JE fabrics are shown in Fig. 2(d), where the colored bar (mauve to dark red) indicates a gradual increase in the magnetic field strength. As shown in Fig. 2(d-i), the magnetic field of the M-JE fabric samples was mainly distributed on the surfaces of the M-JE fibers and decreased rapidly in the y direction. We further explored the effect of the number of M-JE fabric layers on the magnetic field intensity on the M-JE fabric surface. Fig. 2(d) and Fig. S9 in Appendix A show the distributions of the surfaces of the M-JE fabric samples for different numbers of layers. The results clearly show that the magnetic field intensity varied with the number of layers on the surface of the M-JE fabric. The surface magnetic field intensity of the M-JE fabric increased with the increasing number of layers.
3.2. Investigation of the power generation performance
Five 5 cm × 5 cm samples of the prepared M-JE fabric were used to investigate their power generation performance. A stable testing platform was built to measure the power generation performance of the linear reciprocating transverse motor. The M-JE fabric and induction coil move relative to each other at a specific speed and a constant distance in the horizontal direction. Maxwell’s equations systematically describe the relationship between magnetic and electric fields (Eqs. (S1)–(S4) in Appendix A): ① The origin of the electric field in a time-varying magnetic field, whereby the changing magnetic field produces an electric field; ② an electric current and a varying electric field can produce a magnetic field; ③ the electric field is an active field with a divergence related to the charge density; and ④ the magnetic field is a passive field with zero divergence. By Faraday’s law in Maxwell’s equations, the voltage of a multiturn closed coil in a time-varying electric field can be solved using Eq. (1) [50].
In Eq. (1), emf(t) is a function of electromotive force (electromotive force is a physical quantity that reflects the ability of a power source to convert other forms) of energy into electrical energy in volts (V) with time (t), N is the number of coils, Ф is the magnetic flux through the coil, and the physical meaning of refers to the rate of change of magnetic flux in the coil with time. Ф can be calculated by Eq. (2):
In Eq. (2), B is magnetic flux density, and S is the coil area.
The current was calculated according to Ohm’s law (Eq. (3)).
In Eq. (3), I(t) is a function of current with time, and R is the resistance of the induction coil.
According to Eqs. (1), (2), (3), the theoretical equations for the relationship between output voltage, current, and magnetic field are derived as follows:
According to Eqs. (4), (5), the factors affecting the electric output include the rate of change with time of the magnetic flux density () and N. According to Faraday’s law, a variable magnetic flux in an induction coil induces a voltage. Therefore, when the M-JE fabric and induction coil move relative to each other, an induced voltage can be generated in the coil. This power generation process was simulated in detail using the commercial simulation and calculation software COMSOL. Fig. 3(a) and Video S2 in Appendix A illustrate the results of a computational simulation of the change in the magnetic field in the induction coil when the M-JE fabric moved relative to it. In Figs. 3(a-i) and (a-iv), the M-JE fabric is at the beginning and end of the cyclic motion, and its position is directly below the induction coil. The magnetic field strength in the induction coil was maximum. As the M-JE fabric moved away from the coil, the strength of the magnetic field in the coil decreased (Figs. 3(a-ii) and (a-iii)). Fig. 3(b) schematically shows the working mode of the energy-harvesting M-JE fabric and demonstrates the change in the current in the induction coil that occurs when the induction coil moves in a cycle above the M-JE fabric. The direction of the induced voltage in the coil can be described by the differential form of Faraday's law of electromagnetic induction in Maxwell’s equations: (Eq. (S1) in Appendix A) [50]. According to Eq. (S1), a rotating electric field can be induced in the presence of a varying magnetic field, with the direction of the electric field determined by the symbol “–”. Thus, when the induction coil was close to the magnetic fabric in the first half of the cycle, the magnetic flux in the coil increased and a counterclockwise current was induced. As the coil continued to move away from the magnetic fabric, the magnetic flux in the coil decreased and the induced current changed in the clockwise direction. In the second half of the cycle, the coil was close to the M-JE fabric again, and the current in the induction coil during the first half of the cycle exhibited the same rule of change. To verify this, we attached an induction coil to the electric reciprocating mechanism. The change in the current waveform was consistent with the inference associated with the induction coil cycle (Fig. 3(c)).
To optimize the mechanical-to-electrical energy conversion, several factors associated with the M-JE fabric were investigated, such as the diameter of the JE fiber, magnetic particle size, relative motion speed, number of turns of the induction coil, coil shape and connection method, gap between the M-JE fabric and induction coil, and number of M-JE fabric layers. Initially, the diameter of the JE fiber and the magnetic particle size were investigated, and the square coil area, relative motion speed, and turn count were 5 cm × 5 cm, 0.75 m·s−1, and 2000 turns, respectively. The M-JE fabric had the same area as the induction coil, and the distance between the magnetic fabric and the coil was 0 mm. The open-circuit voltage and short-circuit waveform for JE fibers of different diameters are shown in Fig. S10 in Appendix A. As the diameter of JE fibers increased, the peak open-circuit voltage increased from 0.26 to 0.52 V, and the peak short-circuit current increased from 1.20 to 2.39 mA. Owing to the limitation of the internal pore size of the M-JE fibers (Fig. 1(e)), the particle size of the magnetic particles may affect the mechanical–electrical energy conversion capability of the M-JE magnetic fabrics by affecting the distribution of the M-JE particles within the M-JE fibers. To systematically verify the effect of the magnetic particle diameter on the mechanical–electrical energy conversion performance of magnetic M-JE fabrics, we prepared different M-JE fabrics using magnetic particles with different particle diameters and tested their voltage and current output performances. Figs. S11(a) and (b) in Appendix A show microscopic images of magnetic particles with different mesh sizes. At the same magnification, the particle size of 100-mesh magnetic particles was significantly larger than the 400-mesh magnetic particles. Figs. S11(c) and (d) in Appendix A show the particle size distributions of magnetic particles with different mesh sizes. Statistically, 79% of the 400-mesh magnetic particles have a diameter distribution between 20 and 50 μm. In the 100-mesh magnetic particles, the particle size distribution is even wider, with 57% of the magnetic particles above 60 μm diameters. Fig. S11(e) shows a cross-sectional microscopic image of the M-JE fibers prepared with 100 mesh magnetic particles. A large number of magnetic particles were present on the outside of the M-JE fiber, while the inside was white. This indicates that owing to the limitation of the pore size inside the fiber (Fig. 1(c)), most of the magnetic particles were blocked on the outside of the fiber, and only a small number of smaller magnetic particles entered the fiber. We examined the energy output properties of M-JE fabrics prepared using two different particle sizes of magnetic particles. The open-circuit voltage and short-circuit current curves of the M-JE fabrics prepared with different mesh sizes of magnetic particles were compared, as shown in Figs. S11(f) and (g) in Appendix A, respectively. The peak of open-circuit voltage and the peak of short-circuit current of the magnetic M-JE fabric prepared by 100-mesh magnetic particles (0.42 V, 1.82 mA) were lower than those of the magnetic M-JE fabric prepared by 400-mesh magnetic particles (0.52 V, 2.39 mA). According to Eq. (4), the area of the coil is one of the factors affecting the electrical energy output performance of the M-JE fabric. The effects of the different shapes and areas of the coils on the voltage output of the M-JE fabrics were investigated. Three equal-diameter coils were wound and bent into three shapes (triangles, circles, and squares; Figs. S12(a) and (b) in Appendix A). We measured the voltage output performance of three different coil shapes at the same relative motion speed (0.78 m·s−1), the number of layers of M-JE fabric (five layers), and the same distance between the M-JE fabric and the induction coil (0 mm). As shown in Fig. S12(c) in Appendix A, the different coil shapes have slightly different output voltages, with the square coil having the highest peak voltage of 0.23 V and the triangular coil having the lowest peak voltage of 0.18 V. This is because of the different cross-sectional areas of the differently shaped coils. The area of the coil is mainly affected by the length of its sides. Three square coils with different side lengths were wound and their voltage output performances were tested. In Fig. S12(d) in Appendix A, as the side length of the coil increased, the peak output voltage also increased. The open-circuit voltage-time curves for different relative motion speeds are shown in Fig. 3(d), with higher speeds yielding higher output voltages. The maximum output voltage (0.39 V), achieved at a speed of 1.65 m·s−1. In Fig. S13(a) in Appendix A, the short-circuit current increased as the speed increased. An increase in the number of induction coils can influence the electric output. In Fig. 3(e), the open-circuit voltage of a single-layer M-JE rises from 0.32 to 0.96 V, while the short-circuit current drops from 5.6 to 3.5 mA when the number of turns of the induction coil is increased from 500 to 2000 (Fig. S13(b) in Appendix A). The connection of the coils in series allows for a change in the number of turns. We investigated the effect of the number of tightly connected coils on the voltage output performance. The two coils were connected in series with the same and different winding directions. As shown in Fig. S12(e) in Appendix A, connecting two identical coils in series in the same winding direction resulted in a higher output voltage. However, connecting them in opposite directions caused a significant decrease in the output voltage, which was even lower than the voltage induced by a single coil. This is because the voltage in the induction coil is in the direction. The direction of the induced voltage in the coil can be described by the differential form of Faraday’s law of electromagnetic induction in Maxwell’s equations (Eq. (S1)) [50]. According to Eq. (S1), the induced electric field has a direction, which is determined by the symbol “–”. Therefore, when the direction of the magnetic field is constant, coils with different orientations (clockwise and counterclockwise) will have voltages induced in opposite directions, causing the voltage output to drop significantly. The gap between the M-JE fabric and the induction coil affects the magnetic flux passing through the induction coil and influences the magnetoelectric conversion performance. In Fig. 3(f), as the distance between the M-JE fabric and the induction coil increases, the open-circuit voltage of the induction coil decreases from 0.97 to 0.16 V. The number of layers of the M-JE fabric has a large effect on the surface magnetic field strength (Figs. 2(a) and (d) and Fig S9), the output voltage of the induction coil increased from 0.97 to 3.09 V when the number of layers of M-JE fabric was increased from one to five (Fig. 3(g)). The distance between the M-JE fiber and the induction coil and the number of layers of the M-JE fabric also affected the short-circuit current. As shown in Fig. S13 in Appendix A, the short-circuit current decreased with increasing distance between the M-JE fabric and induction coil. The short-circuit current also increased gradually with the increasing number of M-JE fabric layers (0.9 mA increasing to 15 mA, Fig. S13(e)). This followed the same trend as the corresponding output voltage. We investigated the energy-output performance of the prepared M-JE fabrics. Resistors with different values were connected in series with an induction coil, and the voltages and currents of the resistors were determined in the same measurement environment using a Keithley 6514 electrostatic meter. The current within the load gradually decreases with increasing load resistance and increasing voltage across the load (Fig. S14 in Appendix A). According to the measured current and voltage values, the peak power of 10 mW was obtained at the load resistance of 500 Ω, corresponding to the power density of 4 W∙m−2 (Fig. S15 in Appendix A). After comparison with similar works, the M-JE fabrics in this study have significant advantages in terms of open-circuit voltage, short-circuit current, and power density (Table S1 in Appendix A).
The load of the PU resulted in M-JE fibers with good durability. To verify the stability and durability of the M-JE fabrics, their performances in extreme environments, washability, abrasion resistance, and stability were explored. We measured the voltage and current output performance by sequentially placing the M-JE fabrics (five layers), unprotected from any other source, in hot and ice water (Video S3 in Appendix A). Figs. 4(a) and (b) compare the open-circuit voltage and peak power variations of the M-JE fabrics in hot and ice water environments, respectively. The open-circuit voltage and peak power of the M-JE fabrics did not show a significant decrease in the hot and ice water environments, and the abrasion resistance of the M-JE fabrics by an abrasion labile tester (Figs. S16(a) and (b) in Appendix A). A piece of polyester fabric was secured to the rubbing head and driven by a reciprocating mechanism that constantly rubbed the M-JE fabric. After 100 rubbing cycles, the magnetic fabric did not exhibit any obvious signs of damage (Figs. S16(b) and (c) in Appendix A).
We tested the mechanical–electrical energy conversion properties of the M-JE fabrics before and after rubbing. After 100 times of rubbing, the open-circuit voltage and the peak power decreased slightly (Figs. 4(c) and (d)), which may be due to the fact that the M-JE fibers rotated in the fabric during the rubbing process, resulting in changes in the magnetic field of the M-JE fabric. Thus, the M-JE fabrics exhibited good abrasion resistance. To test the washing resistance of the M-JE fabrics, they were washed for 40 min with a commercial washing machine (Fig. S17 in Appendix A). After washing, the M-JE fabric did not exhibit any significant changes. We tested the mechanical–electrical energy conversion performance of the M-JE fabrics after washing. After washing, the peak open-circuit voltage and peak power decreased slightly (Fig. 4(c)), which may be due to the magnetic M-JE fibers rotating in the fabric during the washing process, resulting in changes in the magnetic field of the fabric. Thus, the M-JE fabrics exhibited good washability. M-JE fabrics have good stability and maintain stable mechanical–electrical energy conversion properties after long-term use. To verify the stability of the M-JE fabric, we tested the magnetic fabric continuously 3000 times by driving it at 0.78 m·s−1 through a linear reciprocating transverse motor. The open-circuit voltage waveform exhibited good repeatability during continuous movement (Fig. 4(e)). Therefore, the M-JE fabric exhibits excellent stability.
3.3. Application of M-JE fabrics
Owing to the excellent energy harvesting performance of M-JE fabrics, they have good application prospects in the field of human mechanical energy harvesting. In practice, M-JE fabrics and induction coils can be fixed to relatively moving limbs when the limbs move relative to each other, and biomechanical energy collection can be realized. In this study, the prepared M-JE fabric was affixed to the cuff of the volunteer’s clothes, and an induction coil was affixed to the waist (Fig. 5(a)). The swinging arm of the human body can be considered as a circular motion centered on the axis of the joints, in contrast to the relative motion in the horizontal direction. To investigate the effects of the swing angle and speed on the electrical energy output performance, we conducted tests using a swing-arm mechanism with a controlled swing angle and speed (Fig. S18(a) in Appendix A). In Figs. S18(b) and (c) in Appendix A, as the swing speed increases, the peak output voltage and current increase from 0.77 V and 2.03 mA to 2.03 V and 6.73 mA. Figs. S18(d) and (e) in Appendix A illustrate the relationship between the swing angle and energy output. As the swing angle increases, the output voltage also increases, from 0.74 V and 2.03 mA to 3.95 V and 6.73 mA. The energy harvesting performance of the M-JE fabric was tested when the volunteers swung their arms at running and slow walking speeds. Video S4 in Appendix A shows the energy conversion states of the M-JE fabric obtained as a volunteer swung an arm quickly and slowly. When the volunteer swung the arm rapidly, the M-JE fabric swept the induction coil located at the waist quickly. A light panel (100 LED bulbs connected in parallel) on the volunteer’s chest flashed as the arm swung (Fig. 5(b)). The induction coil was connected to an electrostatic meter (Keithley 6514) to record the voltage and current induced when the volunteers swung their arms. As shown in Figs. 5(d) and (e), the voltage and current waveforms were similar to the current waveforms shown in Fig. 3(b) and Fig. S13, the proposed M-JE fabrics are capable of generating a stable alternating current (AC) voltage and current output with a peak value of approximately 2 V and 3 mA (peak power is 6 mW, peak power density is 2.4 W·m−2) during arm swinging, demonstrating good energy harvesting capabilities sufficient to power common small appliances. A half-bridge rectifier circuit consisting of a diode converts the AC generated by the relative motion of the M-JE fabric with the induction coil into digital current (DC) [51]. To confirm the potential of the proposed M-JE fabric to serve as a power source, the prepared fabric was used for charging capacitors with different capacitances (220, 470, and 1000 μF) by connecting it to the AC section of the rectifier bridge (Fig. S19 in Appendix A). When the volunteer swung the arm rapidly, a cyclical AC was generated in the induction coil (Figs. 5(d) and (e)). After the diode rectification process, the AC is converted into a pulsed direct current, and electrical energy is stored in a capacitor (Fig. S20 in Appendix A).
Fig. 5(c) illustrates the voltage profiles of the different capacitors when storing the electrical energy generated when the volunteer swings the arm. As a volunteer rapidly swung arms, the voltage at both ends of the capacitor increased rapidly, charging a 1000 µF capacitor to 1.5 V, which should be sufficient for operating small appliances [1]. We also tested practical applications (Video S5 in Appendix A). During the experiment, wire connections were made according to the circuit diagrams shown in Fig. S21 in Appendix A. The diode is used as a rectifier device to convert the AC power in the induction coil to DC power, and the 1000 μF capacitor is used as an energy storage element to ensure that the electronics can work continuously. The circuit was connected to common electronic devices such as timers, calculators, electronic watches, and electronic temperature and humidity meters. As shown in Fig. 5(f) and Video S5, the timer was capable of maintaining the time and could be paused and started at any time as the volunteer’s arm continued to swing. The calculator performed a complete calculation, and the electronic watch displayed the time normally, as shown in Video S5. An electronic temperature and humidity meter displayed the current temperature and humidity of the environment (Fig. 5(g) and Video S5). Therefore, the proposed magnetic corduroy fabric can effectively collect the mechanical energy generated by human body motions and store it for normal use in common electronic devices.
4. Conclusions
In this study, M-JE fibers were prepared by loading NdFeB magnetic particles into JE fibers. We investigated the effects of PU concentration, NdFeB magnetic particle solid content, and different preparation methods on the magnetic properties of the prepared fibers. The best magnetic properties were achieved by the hydrophilic treatment of JE fibers using a PU solution at 25% solid content, vacuum treatment, and 80% solid content of NdFeB magnetic particles. Fibers can be woven into fabrics using commercial looms for the conversion and collection of mechanical energy through electromagnetic induction. This study investigated the effects of different variables on the collection of mechanical energy using the proposed magnetic fabric. The best output performance of the M-JE fabric energy harvester was achieved at the speed of 1.65 m·s−1 when the number of coil turns was 2000, the spacing was 0 mm, and the number of layers was five; the generated voltage was 3 V and the generated current was 15 mA. The output power was maximized when loaded with 500 Ω resistance, yielding the output power of 10 mW. Placing the magnetic fabric and induction coil on the wrist and waist of a volunteer facilitated the collection of mechanical energy when the volunteer swung arms. When the volunteer swung arms rapidly, the induction coil produced a voltage of 2 V and a current of 3 mA, which could charge a commercial capacitor and be used to drive small electronic devices, such as calculators, electronic watches, and timers. Energy harvesters have many attractive advantages, such as high output current, simple preparation, low cost of raw materials, and easy availability. This study describes a novel method for preparing flexible wearable energy-harvesting devices.
Acknowledgment
The authors acknowledge the National Natural Science Foundation of China (52303064 and U21A2095); the National Key Research and Development Program of China (2022YFB3805800); the Shandong Key Research and Development Program (2023CXGC010612). We thank the Analytical and Testing Center of Wuhan Textile University for the precise measurement.
Compliance with ethics guidelines
Junyao Gong, Chunhua Zhang, Liangjun Xia, Zhaozixuan Zhou, Weihao Long, Zhuan Fu, Sijie Zhou, Hua Ji, Lixin Du, and Weilin Xu declare that they have no conflict of interest or financial conflicts to disclose.
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