Triboelectric Nanogenerator-Based Vibration Energy Harvester Using Bio-Inspired Microparticles and Mechanical Motion Amplification

Abstract

In this work, the novel design of a sliding mode TriboElectric Nano Generator (TENG)—which can utilize vibration amplitude of a few hundred microns to generate useful electric power—is proposed for the first time. Innovative design features include motion modification to amplify relative displacement of the TENG electrodes and use of biological material-based micron-sized powder at one of the electrodes to increase power output. The sliding mode TENG is designed and fabricated with use of polyurethane foam charged with the biological material micropowder and PolyTetraFluoroEthylene (PTFE) strips as the electrodes. Experimentations on the prototype within frequency range of 0.5–6 Hz ensured peak power density of 0.262 mW/m², corresponding to the TENG electrode size. Further numerical simulation is performed with the theoretical model to investigate the influence of various design parameters on the electric power generated by the TENG. Lastly, application of the proposed TENG is demonstrated in a wearable device as an in-shoe sensor. Conceptual arrangement of the proposed in-shoe sensor is presented, and numerical simulations are performed to demonstrate that the real size application can deliver peak power density of 0.747 mW/m² and TENG; the voltage will accurately represent foot vertical force for various foot force patterns.

1. Introduction

TriboElectric Nano Generators (TENGs) are being widely investigated for harvesting electrical energy from human motions, ambient mechanical machine vibrations and wave motions. The TENG utilizes the pervasive phenomenon of electric charge generation in case two different materials come in contact with each other and are operated in sliding or contact separation modes. The sliding mode TENG is preferred for in-plane and low frequency vibrations [1]. The TENG has advantages of lightweight, economical construction and easy material availability and is used to operate wireless sensors.

The TENG gives better performance for low frequency vibrations and can be used for human motion and wave energy harvesters [2]. TENG construction is comprised of electron-accepting material moving relative to that of the electron donor material, where optimum selection of the material is essential to ensure better charge density and output electric power [3,4,5,6,7]. Performance of the TENG is influenced by environmental conditions, including humidity and pressure, that can be improved by optimum choice of the related parameters [8,9]. Implementation of nano-textured surfaces, surface modifications and controlling environmental conditions improve electric power output from TENG devices [10,11,12].

TENG devices can operate with linear or rotary modes and are designed to operate bio-sensors and ensure power densities up to 7.5 mW/m² [13,14,15]. Hu et al. proposed a low cost TENG with use of PolyEthylene Terephthalate (PET) and kapton film with the electrode surfaces having friction interfaces, where the output energy was used for lighting a number of light-emitting diodes [16]. There are attempts to modify the mechanical structure of TENG electrodes for improving output power density. Xu et al. fabricated TENG electrodes in the form of a number of spiral-shaped beams to deliver 12–20 V of open circuit voltage when the relative displacement amplitude at the electrode was 5–15 mm [17]. Lu et al. proposed a stretchable TENG with polyacrylamide-based material for biomedical applications where displacement of a few millimeters is used to operate the self-powered sensors [18]. Wu et al. designed a multimodal structure for improving the wide-band power output of a wearable TENG device used for biomedical application [19]. Corrugated textile material-based TENG proposed by Choi et al. has silk and rubber electrodes [20]. The device is operated with the motion of human body parts generating peak power of 56.2 µW. Tian et al. incorporated a hollow, tubular structure for increasing the contact area of TENG electrodes, such that 300 N of force is utilized to illuminate 31 light-emitting diodes [21]. Kim et al. used a core-shell and helical shape for TENG electrodes to utilize human body movements for powering biomedical sensors [22].

Incorporation of nano-texture and corrugations with chemical plating and electro-plating results in an increase of the contact area to improve output power density up to 15 mW/m². Surface modifications include the use of alternate layers of neutral-active strips on the electrode surface in order to accumulate the electric charge and to increase power output [23,24]. In order to increase power density of TENG, multi-layer construction is preferred over that of single-layer construction. Fabrication of TENG in multiple layers involves manufacturing the electrodes in the form of strips, where the device ensures up to 400% increase in power output in comparison to that of single-layer construction [25,26]. Zhang et al. used a MXene-embedded polyvinylidene fluoride (PVDF) composite to enhance dielectric permittivity and to achieve 350% improvement in electric power delivered by the TENG [27].

Use of bio-inspired sustainable material in the form of fish gelatin and animal hair can ensure power densities of TENG up to 100 µW/m² and current up to 4 µAmp [28,29]. Ingredients like amino acids in the bio-inspired materials act as an active layer and increase power output. Zhang et al. used a porous structure of carbon powder and manganese dioxide Nano composite to deliver open-circuit voltage of 63 V [30]. A typical application of TENG involves waste energy recovery by utilizing interaction between tire and road that can ensure peak voltage up to 38 [31,32,33].

TENGs are used in biomedical sensors and are designed to power healthcare sensors by utilizing human motion [34,35]. The biomedical TENG proposed by Wang et al. is comprised of stainless steel foil of 0.05 mm thickness and ethyl cellulose film as electrodes [36]. Li et al. designed a triboelectric generator that utilizes respiratory motion for powering healthcare sensors and uses polytetrafluoroethylene (PTFE) and kapton films [37]. Parandeh et al. fabricated a TENG based on silk and grapheme oxide layers [38]. Authors investigated the effect of graphene oxide surface modification and concluded that the controlled surface roughness improves power output. Biocompatible polymer sheets are used in the design of TENG to drive several light-emitting diodes [39]. A biocompatible TENG proposed by Liu et al. used compressive force in a human artificial joint to power sensors used for wear debris detection [40]. A flexible, structured, wearable TENG designed by Liao et al. used silicon and aluminum material as electrodes [41]. The device is mounted on hand gloves to utilize finger-bending movement that ensured peak open circuit voltage of 4.5 V. A low-cost and self-powered acceleration sensor based on liquid–metal TENG ensures sensitivity of 800 mV/g with a detection range of 0–100 m/s² [42]. A mechanically regulated, low-cost, self-powered lighting control system uses a hybrid nanogenerator with frequency up-conversion to deliver maximum power of 108 mW [43].

Energy harvested by TENG has been used to illuminate an array of light-emitting diodes, where power density of up to a few micro-Watts per cm² has been demonstrated by measuring open circuit voltage [44,45,46,47]. Furthermore, there are attempts to use the harvested power for operating force/pressure sensors and storage in an energy storage element [17,31,48,49,50].

Piezoelectric material-based energy harvesters generated electric power when applied with mechanical strain and had the advantage of higher power density. Kouritem and Altabey performed theoretical simulations for a piezoelectric energy harvester with an arrangement of automatic resonance tuning mechanism to deliver peak voltage of 12 V for frequency range of 27–137 Hz [51]. Piezoelectric energy harvesters used in a civil structure for health monitoring comprise a number of piezoelectric beams with proof masses [52]. Theoretical simulations with an arrangement of cantilever beam and an adjustable mass ensured peak power of 2.28 mW within frequency range of 7–18 Hz [53]. Authors have estimated power recovery of 52.4 mW from a real size wearable device. Finite element and analytical models are investigated to demonstrate that the energy harvesters can generate peak voltage of 1.0 V for vibration amplitude of 18 mm at 2 Hz frequency. However, these devices use costly piezoelectric material and have lower durability in context to presence of shock and vibrations. Furthermore, the piezoelectric harvesters need an optimum voltage conditioning circuit, and they generate lower power at less voltage [54].

Literature identifies that the TENG can deliver peak power density of 0.1–2.76 W/m², where the input motion driving the generator electrode is having an amplitude of a few millimeters [55]. However, for the applications where the vibration amplitude is limited to a few hundred microns, the power density is limited within 0.20–0.68 mW/m² [56,57]. There is a need to improve power density of TENG devices for these applications by incorporating motion modification arrangement to operate the TENG electrodes with amplified relative displacement and velocity over that of the input vibrations. In addition to the stroke length, electric power harvested by the TENG depends on material charge density. Human hairs have amino acids and are one of the materials with highest positive charge density. There have been a few attempts to utilize macro-sized animal hairs in the TENG to ensure better electric power [30]. However, it is necessary to investigate the application of micro-sized human/animal hairs due to the fact that micro-sized hairs form results in higher contact area than that of the macro-sized form to ensure better performance.

In the presented work, design and analysis of a TENG suitable for energy harvesting, a case of vibration amplitude of a few hundred micrometers and within the frequency range of 0.5–6 Hz is proposed. Important highlights of the presented work are as follows:

• In corporation of displacement amplification with the fluid link to ensure higher relative displacement of the TENG electrode, even if the input vibration amplitude is smaller. Absence of gears for motion amplification results in a robust and reliable construction.
• Use of human hair powder with particle size lower than 25 µm to increase surface charge at the positive electrode to ensure peak power density of 0.262 mW/m² and increase in the peak power of up to 403.7%.
• Detailed mathematical modeling supported with experimental validation.
• Demonstration of application of the energy harvester for an in-shoe sensor to deliver peak power density of 0.747 mW/m² and ability to detect walking anomaly and perform step counting.

The paper is organized as follows: Section 2 explains materials and methods that contain the prototype and experimentation details. Section 3 explains details of mathematical modeling to estimate voltage and electric power harvested by the TENG. Lastly, Section 4 includes a discussion on the experimental and simulation results for voltage and electric power. Effect of important design parameters on the performance of the energy harvester are discussed along with demonstration of the device for use in an in-shoe sensor.

2. Materials and Methods

2.1. Prototype and Experimentation Details

2.1.1. Prototype Details

The prototype sliding mode TENG is designed to utilize very small vibration amplitude into useful electric power. Principle components and working of the prototype energy harvester are illustrated in Figure 1. Initially, the vibration amplitude is applied at input of the fluid amplification link, which ensures that the input displacement is amplified and the sliding mode TENG generator operates with increased relative displacement and velocity than that of the input vibrations. Output terminals of the TENG electrodes are connected to an external load resistance (i.e., a resistance mesh), and the electric power across the load resistance is measured.

Arrangement of the components in the prototype energy harvester is shown in Figure 2, and the CAD model is shown in Figure 3. The arrangement in the fluid amplification link is illustrated in Figure 4, which is comprised of two cylinders and pistons with bigger and smaller diameters. Input vibrations are provided at the bigger piston, which oscillates in linear direction. Movement of the bigger piston makes the oil flow between the two cylinders, making the smaller piston move with amplified displacement and velocity. There is a provision of a guiding piston after the smaller piston to ensure precise motion at output of the fluid amplification link. Care has been taken while manufacturing the cylinders to ensure minimum friction at the piston and cylinder with the precaution that the oil does not leak and sufficient pressure is maintained. It is shown in Figure 4 that the input vibrations are provided to the fluid amplification link at point “A” whereas output motion is taken out from point “B”, to drive the TENG electrodes. A photograph of the prototype is shown in Figure 5.

Construction of the TENG used in the prototype is illustrated in Figure 6, which is motivated from previous research [58,59]. The PolyTetraFluoroEthylene (PTFE) strip with 0.5 mm thickness is used as one of the electrodes since the material ensures higher negative surface charge density. A polyurethane type of foam charged with microparticles of human hairs is used as the other electrode due to higher positive surface charge density. The PTFE electrode is supported with another 2.0 mm thick PTFE support plate and is held stationary. On the other hand, a foam and hair powder electrode is carried by a wooden plate and is driven with output motion from the amplification mechanism. The smaller piston and guiding piston in the amplification link ensure that the moving electrode in the TENG undergoes precise linear motion. Copper plates of 0.05 mm thickness are used below the positive and negative electrodes to collect the electric charges that are generated, as the electrodes move relative to each other. Experimentation is performed with 1, 2 and 4 sets of electrode pairs.

Figure 7a shows a photograph of the foam used in the prototype, whereas Figure 7b shows force deflection characteristics of the foam. The initial, uncompressed thickness of the foam was 4 mm, which has been compressed to 0.5 mm after clamping in the prototype. The microparticles of human hair have been uniformly spread across the foam volume before clamping. During the experimentation, iterations are performed with mass ratio of hair powder to foam varied between 0.0, 0.10 and 0.21. It is noticed that the maximum mass fraction in the foam for the hair powder is 0.21, beyond which the foam cannot hold the hair particles. Human hairs are collected from the Indian male population in the age group of 25–40. The black hair samples are converted into the micro-sized powder form through a patented technology and used in the present study. Further the microparticles are sieved to obtain hair powder particles of size less than 25 µm. A photograph of the hair powder is shown in Figure 8a, whereas a Scanning Electron Microscopy (SEM) image of the hair particles is shown in Figure 8b. The SEM images reveal that the hair particles are sized less than 25 µm and are tubular in shape with average length of 20 µm and diameter of 5–10 µm. Further chemical composition of the micropowder has been investigated by using Energy-Dispersive X-ray Spectroscopy (EDX), which reveals that it mainly contains carbon. Detailed chemical composition of the micropowder is given in

Table 1. Results for EDX analysis.

Element	Mass Percentage (%)
Carbon	59.37
Oxygen	34.19
Sulphur	2.04
Calcium	1.55
Iron	1.47
Silicon	1.39

.

During working of the TENG prototype, the PTFE electrode is stationary, and the other electrode (foam + hair micropowder) moves relative to the PTFE electrode. It can be noted that the moving TENG electrode is driven by the fluid amplification link and moves with amplified relative displacement and velocity over that of the input vibration amplitude. Copper strips at the top of negative electrode and below the positive electrode collect the charge induced at the electrodes and are connected with arrangement as illustrated in Figure 9 along with an external resistance mesh. Voltage across the load resistance is measured by using an oscilloscope in order to determine the electric power generated by the energy harvester.

Details of the prototype energy harvester are as follows:

• PTFE and foam thickness: 0.5 mm
• Diameter of bigger piston in the fluid amplification link: 32 mm
• Diameter of smaller piston in the fluid amplification link: 4 mm
• Dimensions of the foam and PTFE electrode:40 mm in Width and 30 mm in Length
• TENG electrode relative displacement: 32 mm (Displacement amplification of 64.0)

2.1.2. Experimentation Details

The experimental set up is designed to provide harmonic vibrations to the prototype energy harvester with an amplitude of 0.5 mm, within frequency range of 0.5–6 Hz. The frequency range and amplitude refers to few applications of wearable applications in human motion [60]. Further, the test conditions are close to low frequency machine vibrations in vehicle operator cabin and process equipment where the acceleration amplitude is within 0.03–0.5 g for frequency range up to 4 Hz [61,62].

A photograph of the setup is shown in Figure 10. It is comprised of the lower reciprocating mass driven by a scotch yoke mechanism and DC electric motor to provide vertical vibrations (Motor power—5 HP). The lower mass is guided along two vertical pillars to provide accurate vertical vibrations. Upper mass is fixed in position and is clamped to the vertical guide bars. The energy harvester is fixed in the setup such that the lower reciprocating mass drives input element of the fluid amplification link. Further, the fluid amplification link drives the TENG electrode with amplified relative displacement and velocity. An external electric resistance mesh is connected across the TENG electrode, and an oscilloscope (make: Tektronix) is used to record the voltage wave form for different resistance values within the test frequency range.

3. Mathematical Modeling

The mathematical model of the prototype energy harvester is developed to estimate voltage and electric power across the load resistance. Figure 2 shows overall arrangement of components in the energy harvester. It can be noted that the excitation force is applied at input of the fluid amplification link, which further drives the TENG electrode with increased relative displacement and velocity.

Equation of motion for the moving electrode of TENG is given as:

$$m\ddot{x}=\left[F\right]\left[{\left(\frac{d}{D}\right)}^2\right]-F_{df}-F_{pf}-F_{ff}-kx$$

(1)

where

• F: force at the input of fluid amplification link;
• d: diameter of the smaller piston in the fluid amplification link;
• D: diameter of the bigger piston in the fluid amplification link;
• F_pf: Piston friction at the fluid amplification link;
• F_df: Friction force between the sliding dielectric materials of TENG;
• F_ff: Force due to fluid friction inside the cylinders of fluid amplification link;
• k: Restoring spring stiffness; and
• x: Displacement of the TENG moving electrode.

Friction force due to oil flow inside the fluid amplification link is derived from reference after neglecting the effect of friction inside the bigger cylinder and is given as [63]:

$$F_{ff}=\frac{16\pi \rho \upsilon L_{eqv}{d}_h{}^2\dot{x}}{4d^2}$$

(2)

where

• ρ: oil density;
• ʋ: oil kinematic viscosity;
• L_eq: equivalent length of pipe;
• d_h: hydraulic diameter of the smaller cylinder; and
• d: diameter of the smaller cylinder.

The sliding mode TENG operates with two dielectric materials moving relative to each other. The following voltage-charge-displacement (V-Q-x) correlation reported in the references is used for determination of voltage and current [64,65].

$$R\frac{dQ}{dt}=\frac{-1}{w{\epsilon }_0\left(l-z\right)}\left(\frac{d_1}{{\epsilon }_{r1}}+\frac{d_2}{{\epsilon }_{r2}}\right)Q+\frac{\sigma x}{{\epsilon }_0\left(l-zx\right)}\left(\frac{d_1}{{\epsilon }_{r1}}+\frac{d_2}{{\epsilon }_{r2}}\right)$$

(3)

where

• R: external load resistance;
• Q: charge induced at the electrodes;
• W: width of the electrode;
• z: separation distance;
• d₁: thickness of PTFE electrode;
• d₂: thickness of foam electrode;
• ε₀: free space permittivity;
• l: electrode length;
• ε_r1: relative permittivity of PTFE;
• ε_r2: relative permittivity of foam and hair powder; and
• σ: surface charge density.

Separation distance between the electrodes is given as [64]:

$$\begin{gathered} z=\frac{1}{2}\ddot{x}{t}^2 \mathrm{for} t<\sqrt{\frac{x_{max}}{\ddot{x}}} \\ z=x_{max}-\frac{1}{2}\ddot{x}{\left(2\sqrt{\frac{x_{max}}{\ddot{x}}}-t\right)}^2 \mathrm{for} \sqrt{\frac{x_{max}}{\ddot{x}}}\le t<3\sqrt{\frac{x_{max}}{\ddot{x}}} \\ z=\frac{1}{2}\ddot{x}{\left(4\sqrt{\frac{x_{max}}{\ddot{x}}}-t\right)}^2 \mathrm{for} 3\sqrt{\frac{x_{max}}{\ddot{x}}}\le t<4\sqrt{\frac{x_{max}}{\ddot{x}}} \end{gathered}$$

(4)

Voltage across the load resistance can be calculated from the charge by using the following equation:

$$V=R\frac{dQ}{dt}$$

(5)

Similarly, current is calculated from the charge by the following equation:

$$I=\frac{dQ}{dt}$$

(6)

Electric power across the TENG electrodes is given as:

$$P=\frac{V^2}{R}$$

(7)

Equations (1)–(7) are used to estimate voltage, current and power across the electrical load resistance. The effect of fluid amplification link pistons and dielectric material friction has been neglected in the analysis.

4. Results and Discussion

Working of the TENG with the two electrodes sliding with respect to each other is depicted in Figure 11a–d. The conversion of mechanical to electric energy is achieved with contact electrification and electrostatic induction. Figure 11a shows the initial position of two overlapping electrodes, and Figure 11b shows outward displacement of the positive electrode. During the outward displacement, the contacting area between the electrodes reduces. The resulting in-plane charge disintegration leads to higher potential at the positive electrode, making the current flow from the negative to the positive electrode. On the other hand, inward displacement of the moving electrode, shown in Figure 11c, results in increasing the contacting area, and the current flows from the positive to the negative electrode. As the two electrodes reach overlapping condition, there is no flow of charge between them, which refers to the initial condition depicted in Figure 11a,d.

Initially, experimentation is performed to investigate variation in the electric power harvested by the prototype energy harvester with different values of hair powder mass fraction carried by the foam. Harmonic excitations were given at the input of the fluid amplification link at 5 Hz with 0.5 mm input displacement amplitude. Experimental results shown in Figure 12 reveal that electric power increases with load resistance up to a certain value and reduces thereafter. In case of the foam with hair powder mass fraction of 0.21, maximum electric power of 0.314 µW is obtained with load resistance of 130 MΩ. Maximum power density with the prototype TENG electrode dimension is 0.262 mW/m². In case of foam without the hair powder, maximum electric power is 0.062 µW at load resistance of 90 MΩ. It is revealed that the incorporation of the human hair powder increases electric power by 403.7%. Variation of the peak voltage with load resistance is shown in Figure 13. Voltage increases with the load resistance and slope of the voltage-resistance curve reducing once the TENG reaches maximum power. It is evident from Figure 12 that the load resistance for deriving peak power from the TENG increases with higher hair powder mass fraction, which is attributed to the reason that the increased hair micropowder mass results in higher impedance of the circuit.

It was revealed that the prototype harvester delivers maximum power of 0.314 µW for more than 50,000 numbers of cycles, for hair powder mass fraction of 0.21. Experimentations were performed with the hair powder mass fraction higher than 0.21, and power delivered by the energy harvester for measured. However, for the hair powder mass fraction higher than 0.21, the prototype could not deliver the maximum peak power consistently for the higher number of cycles. Therefore, it is recommended to use the powder mass fraction of 0.21 for the foam sample presently used in the prototype.

Increase in voltage with incorporation of hair powder is attributed to the fact that the fine particles of the human hair ensure higher surface contact area with the PTFE electrode. This observation is in agreement with the earlier reported results that macro-sized human hairs can acquire higher positive charge and ensure higher electric power when used in TENG [66,67].

Figure 14 and Figure 15 show variation of the peak voltage and power within the frequency range of 0.5–6 Hz with the load resistance of 110 MΩ. It can be seen that peak voltage and power increase with frequency. Peak power density corresponding to the prototype electrode size is 0.262 mW/m², whereas the peak power density for the overall size of the prototype is 0.122 mW/m². Figure 16 compares experimental and simulation results for voltage plot in time domain for load resistance of 110 MΩ and hair mass fraction of 0.21. It can be observed from Figure 14, Figure 15 and Figure 16 that the experimental and simulation results for voltage and power follow the similar trend and exhibit maximum error of 5.05%.

During assembly of the electrodes, the initial foam thickness of 4 mm is compressed and fitted against the PTFE electrode plate. Higher compression of the foam increases the contact area between the two electrodes resulting in improved voltage and power. The effect of foam-compressed thickness on the peak voltage generated by the TENG is investigated with experimentation by measurement of peak voltage. It was observed that the lower compressed thickness results in increased voltage. However, thickness of the foam cannot be reduced below 0.60 mm, and voltage reduces significantly for the foam-compressed thickness lower than 0.6 mm. Reduction in the voltage for lower compressed foam thickness is due to the fact that the large compressive force locks the foam, restricting the relative displacement of electrodes.

The effect of increasing the number of electrode pairs on voltage output is investigated with experimentation on the prototype. It is observed that the peak voltage reduces with an increased number of electrode pairs. The peak voltage reduces by 44% and 91% with use of 2 and 4 electrode pairs, respectively. The reduction in voltage at higher number of electrode pairs is attributed to the fact that, with more electrode pairs, the edge effect as well as the circuit capacitance increases. This result in the reduction of overall charge transferred between the electrodes leads to reduced voltage output. This observation is in consonance with earlier findings reported in the literature [68,69].

It can be noted that the displacement amplification ensures that the vibration amplitude of a few hundred microns is used to operate the TENG electrodes with increased displacement and velocity. Displacement amplification in the present prototype is 64.0. The theoretical model of the TENG is used to investigate the effects of displacement amplification on the voltage and power output. The simulation results shown in Figure 17 indicate that incorporation of the displacement amplification ensures significant increase in voltage and power. Therefore, it is desirable to operate the TENG with largest possible displacement amplification to ensure better electric power output.

It can be noted that, although the amplification mechanism magnifies input displacement, force at the output of the mechanism is lower than that of the input force (F_out ≈ (F_in/Displacement amplification) − F_ff − restoring spring force). Therefore, the upper limit for the displacement amplification may be limited due to the fact that the net force on the moving electrode needs to be sufficient to move the electrode against the fluid friction force, restoring spring force and compressive force between PTFE and foam.

The application of the proposed energy harvester is demonstrated for use as a self-powered in-shoe sensor. The proposed sensor can be used for various applications, including step counting, walking pattern recognition and walking anomaly detection. In line with the related reference for in-shoe sensor, location of the sensor is proposed at inside and outside metatarsals, as illustrated in Figure 18 [70]. The sensor will utilize the vertical force encountered at the specified location and will be contained in the working volume of 14,600 mm³ (overall dimensions: length—78 mm, width—26 mm, height—18 mm).

The proposed arrangement in the sensor is illustrated in Figure 19, and the details of the TENG are shown in Figure 20. Initially, the foot force will be applied at the moving block, which is designed to move vertically with maximum displacement of 0.5 mm. The moving block will slide along the fixed block at an angle of 30° and perform the first stage of displacement amplification. Further, the moving block will operate the bigger piston in the fluid amplification link in a horizontal direction. Displacement of the bigger piston will drive the smaller piston with amplified motion. The smaller piston is connected to the moving electrode of TENG (PTFE), whereas the other electrode (memory foam and hair micropowder) is fixed to the outer side of the cylinder in the amplification link, as illustrated in Figure 20. In order to ensure compact construction of the device, moving and fixed electrodes are located outside the fluid amplification cylinder and are shaped cylindrically concave and convex, respectively. The arrangement will ensure that the PTFE electrode will slide with respect to the foam electrode along the axes of the cylinder. Both the electrodes will be connected to the support along with electrical insulating arrangement and provision of copper plate for the charge collection. The Creo model of the in-shoe sensor is shown in Figure 21.

There is a provision of load resistance connected between the copper plate charge collectors of the moving and fixed electrode, along with a microcontroller having a Wi-Fi data transmission module. Voltage generated across the load resistance is a function of relative movement of TENG electrodes, which again depends on the vertical force applied at the moving block. Therefore, the voltage generated across the load resistance can be used for the required sensing purpose. A microcontroller (ESP-8266) can be used to measure the voltage generated along the load resistance. The microcontroller will perform the task of voltage measurement, analogue to digital conversion and data transmission to cloud/server through the inbuilt wireless data transmission module.

Variation of the vertical force acting at the shoe with human motion is referred from the literature and shown in Figure 22 [71]. The force variation refers to the elderly people, and three different walking patterns are used in the simulation as shown in

Table 2. Vertical force acting on the shoe.

	F1	F2	t1 (s)	t2 (s)
Pattern 1	100 N	80 N	0.5	1.7
Pattern 2	80 N	64 N	2.1	5.7
Pattern 3	70 N	60 N	8.2	15.1

.

The mathematical model of the energy harvester discussed in Section 2 is used for estimation of voltage across the load resistance with use of the force data shown in Figure 23 and Table 2. A Creo model shown in Figure 21 is imported in Matlab—2018 for rigid body and numerical simulation. The CAD files of the individual components are imported in Matlab Simscape—Multibody, and the inertial properties are defined. The inertial properties for the individual components include mass and inertia tensor with respect to the Matlab coordinate system. Stationary parts in the assembly include bigger and smaller cylinder, PTFE electrode, microcontroller and the lower support. The Matlab model of the in-shoe sensor is shown in Figure 23. Details for the in-sole sensor used in the simulation are as follows:

• Thickness of PTFE and foam electrodes: 0.5 mm;
• Smaller and bigger piston diameter in the fluid amplification link: 16 mm and 4 mm;
• Surface area of TENG electrode: 640.56 mm²;
• Optimized load resistance for maximum power of 0.3550 mW: 140 MΩ;
• Recommended load resistance: 400 MΩ; and
• Restoring spring stiffness: 500 N/m.

Simulation results for vertical force and the voltage along the load resistance for walking pattern 1 are shown in Figure 24. Further maximum voltage in case of walking pattern 2 and 3 are 5.8 V and 5.2 V, respectively. Peak power in case of pattern-1 with voltage plot shown in Figure 24 is 0.311 mW, which ensures power density of 0.747 mW/m² for the dimensions illustrated in Figure 19. Correlation between the force and voltage vectors shown in Figure 24 is 0.986. It can be noted that the voltage generated by the proposed in-shoe sensor will be well within the measurement range of the microcontroller. (Input voltage range of Analogue to Digital conversion pins of ESP 8266 is 0–3.3 V.) Further, the voltage variation illustrated in Figure 24 can be used for various applications, including step counting, walking anomaly detection and pattern recognition.

The manufacturing cost of the proposed in-shoe sensor will be 470 INR (USD 6.01). Further, the design is simple to manufacture and will have a better reliability due to absence of parts like mechanical gears and bearings.

Power density, materials and working stroke for existing TENG devices reported in the literature, operating with vibration amplitude starting from 1.0 mm, have been compared with that of the energy harvester presented in this work.

Table 3. Comparison with existing reported work.

Sr. No.	Reference	Materials Used	Power Density	Total Displacement of Electrodes
1	Zhao et al., 2019 [59]	Acrylic sheet and FEP	0.25 mW/m2	1 mm
2	Yang et al., 2013 [72]	PTFE and Aluminium operating in sliding mode	350 mW/m2 for 100 MΩ electric load	16 mm
3	Liu et al., 2020 [45]	Carbon fiber reinforced composite lamina and papers operating in sliding mode	4.2 mW/m2 for 100 MΩ electric load	4 mm at 5 Hz
4	Vivekananthan et al., 2019 [73]	Paper and Kapton films in sliding mode	1.07 mW/m2 for 16,110 Ω electric load	10 mm
5	Pang et al., 2019 [74]	PTFE balls and Acrylic shell	544 mW/m2	60 mm at 2 Hz
6	Zou et al., 2021 [12]	FEP film and Aluminium in sliding mode	190 mW/m2	50 mm by hand motion
7	Jiao et al., 2020 [75]	Bread and natural vegetable	45 µW open circuit power	20 mm
8	Long et al., 2021 [76]	PTFE and Nylon film in rotary mode	13 nm/m2	360° rotations of the electrode
9	Li et al., 2021 [77]	Katpon and Silicone in contact separation mode	5 nm/m2	Not reported
10	Nie et al., 2021 [78]	Cellulode nanofibrils and Trirthoxy-tridecafluore-octylsilane in contact separation mode	13 mW/m2 for 55 MΩ electric load for 50 N force	Not reported
11	Guo et al., 2017 [79]	Polymide and Fluorinated ethylene propylene in contact separation mode	17.29 mW/m2 for 65 MΩ electric load for 10 N force	Not reported
12	Askari et al., 2017 [80]	Polyurethane and Kapton layers in hybrid mode	63 mW/m3 for 100 MΩ electric load	35 mm
13	Xu et al., 2022 [81]	Metamaterial structure with PTFE and nylon	Open circuit voltage up to 12 V	2.2–15 mm
14	This work	PTFE and Memory foam with human hair micropowder	Prototype: 0.262 mW/m2 for 130 MΩ of electric load Real size design in-shoe sensor: 0.747 mW/m2 for 140 MΩ of electric load	1.0 mm

reports the comparison that indicates the incorporation of motion amplification and use of micro-sized hair powder results in significant improvement in power density, even if the energy harvester is operated with very small displacement amplitude.

5. Conclusions

In summary, a novel design of the TENG is proposed to deliver peak power of 0.747 mW/m² from vibrations having amplitude of a few hundred micron. Novel features in the presented energy harvester include the use of fluid link for mechanical motion amplification and human hair powder with maximum particle size of 25 µm at the positive electrode. Experimental and numerical simulations with single electrode pairs delivered peak voltage of 6.6 V and power density of 0.262 mW/m². The effect of significant parameters, including the number of electrode pairs, amplification factor and load resistance has been investigated with the experimental and theoretical simulation methods. Finally, application of the energy harvester is demonstrated in case of an in-shoe sensor that can deliver peak voltage of 9.9 V and can be used for wearable applications, including step counting, walking anomaly and walking pattern recognition.