Ergonomically designed replaceable and multifunctional triboelectric nanogenerator for a uniform contact

Abstract

We report a replaceable and multifunctional triboelectric nanogenerator (TENG) with a triangular prism shaped supporter to enhance the uniformity of contact and separation during walking. The supporter was ergonomically designed by considering the walking style, therby enhancing the uniformity of the contact between the Al and PDMS film inside the TENG. The TENG with the supporter generated a high output performance of 64 V and 55 μA when walking, showing an enhancement of approximately 600% when compared with the flat TENG. We also demonstrate a self-powered pressure distribution sensor to monitor the human gait patterns by pressing the TENG arrays when walking, a useful technology in reliable health monitoring systems.

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Introduction

The development of wearable devices is changing our industry for the better and giving us information for healthy lifestyle choices. However, the advanced development and production of wearable devices brings an increased demand on power supplies. Nowadays, as numerous wearable technologies such as Galaxy Gear, Google Glass and Smart Watch are available, the entire wearable device market is expected to grow at a strong rate. Innovative technologies such as smart shoes and insoles have also been developed rapidly since 2015. Most wearable devices are operated by lithium rechargeable batteries, limiting their lives and sustainable operation. For seamless integration of smart sensors or wearable devices into clothing, the elimination of battery replacement or recharging is required, and the question of how to supply power in a stable and reliable manner is one of the most important issues to be addressed in bringing about the perfect commercialization of wearable devices.

In an effort to solve this problem, a power generating device called the triboelectric nanogenerator (TENG) has been invented and has proven to be a low cost, simple and efficient technique for energy conversion.^1–10 Many applications of TENGs have been successfully developed such as self-powered sensors,¹¹ self-charging cells,¹² trace memory systems,¹³ distress signal emitters,⁸ self-electroplating technologies¹⁴ and mobile charging.¹⁵ For wearable technology applications, the mechanical energies from human motion can be utilized; however, there may be few body parts generating sufficiently large mechanical energies by friction. If the TENG is placed inside shoes and a force by weight is applied, useful quantities of energy can be generated when walking or running. Recently, there were some reports on a power-generating shoe insole with built-in TENG.^16–18 The mechanical energy generated when walking is assumed to be several watts. However, the output powers reported so far were not as large as expected. Ergonomical design of the TENG to match the foot-strike pattern may be required to increase the output power.

Herein, we report a replaceable and multifunctional TENG with a triangular prism shaped supporter to enhance the uniformity of contact and separation during walking. In general, people walk on the ground from heel strike to end of the toe like a water wave. When taking a step, the foot is reported to form an angle of approximately 25° with the ground, causing maximum foot pressure. When this is not considered in the design of the TENG, the energy output may be small due to the non-uniform contact inside TENG. A triangular prism shaped supporter was introduced to enhance the uniformity of contact. The supporters were sponge-like mesoporous polydimethylsiloxane (PDMS) films created by fused deposition modelling (FDM) 3D printed mold-casting method. Various TENGs were fabricated using different angles of the supporter. With the supporter, the output voltage and current increased to 64 V and 55 μA, respectively, under natural human walking, showing an enhancement of approximately 600% when compared with the flat TENG. We further show that the Al and PDMS inside TENG can be uniformly contacted with the supporter. Finally, we also demonstrate the self-powered pressure distribution sensor to monitor the human gait pattern by pressing the TENG arrays under human walking.

Experimental

Fabrication of a triangular prism shaped supporter and a triboelectric nanogenerator

Acrylonitrile Butadiene Styrene (ABS) filament (Makerbot Industry, USA) was used for the fabrication of the mold for the supporter using a FDM-based 3D printer on steady pace for 20 min. PDMS (Sylgard 184, Dow Corning) was mixed with deionized (DI) water and the mixture was stirred for 30 min. The suspension was poured into the mold and dried in atmosphere at 90 °C to remove DI water, creating air voids in PDMS. The mass ratio between DI water and PDMS was about 2 : 3, which was used to create porosity of 40% and enhance compressibility of the supporter. After the PDMS film layer was peeled off from the mold, it was placed on a 6 cm × 3 cm flexible PET/PI substrate, folded in half. The Al film as a bottom electrode and PDMS film as a dielectric were placed on the supporter. The Al film as a top electrode was then attached on top of the PI/PET substrate.

Characterization

For practical application, we measured the output voltage and current of the TENGs when walking as a function of weight from 74 to 92 kg. A Tektronix DPO 3052 Digital Phosphor Oscilloscope and low-noise current preamplifier (model no. SR570, Stanford Research Systems, Inc.) were used to support the electrical signals with the angle of the supporter. The morphologies of mesoporous supporters were characterized by a Nano 230 field-emission scanning electron microscope (FEI, USA).

Results and discussion

Fig. S1† shows the real images and schematic when walking on a rigid surface. It is clearly seen that the step starts with an angle of approximately 25° (Fig. S1b†). This may mean that the force exerted by the human body is initially applied with an oblique angle of 25° and not vertically. The pressure then moves from the heel to the toe with a wave-like pressure. This will result in non-uniform contact inside the TENG and a decrease of the output power. The triangular prism shaped supporter was introduced for calibrating the angle with the ground under natural human walking. Fig. 1a shows the schematic of the TENG embedded with the supporter. The supporter has a sponge-like mesoporous structure for making comfortable walking shoes, as shown in Fig. 1c. The supporter was fabricated by a mold-casting method using a FDM based 3D printer, as shown in Fig. 1b (see the movie provided in S2†). Furthermore, the supporter was attached at the bottom side of the curved PET/PI substrate. An Al film is then introduced onto the supporter and the top side of the substrate, followed by the attachment of PDMS film. The PU/PDMS sponge is also positioned at the inside of the insole for enhancement in the restoring force and to maintain an air gap of approximately 2 cm between the top and bottom parts of the insole.

Fig. 1 (a) Schematic of the fabrication process for the TENG with mesoporous PDMS supporter. (b) The image of the 3D printer (c) the SEM image of sponge-like mesoporous PDMS at low (left) and high (right) magnitude.

The supporter has an average area of 2 cm × 2 cm and was fabricated with various angles of 5° to 35° in order to find the optimized condition for high-output power generation. To observe the effect of the angle on the output power, we systematically measured the output voltage and current as a function of the angle from 0° to 35°, as plotted in Fig. 2. For practical application, an external pressure was generated by the human body while walking. The flat TENG without the supporter generated an output voltage and current of 8 V and 6 μA, respectively. When the supporter with an angle of 5° was used, the output voltage and current increased to 38 V and 30 μA, respectively. Decreased output signals were measured when the angle of the support was 35°. Further increase in the angle to 25° increased the voltage and current to 64 V and 55 μA, respectively, an enhancement of approximately 600% compared with the flat TENG. The change in the output power with the angle is explained via uniformity of the contact between the Al and PDMS. Fig. 3 shows how Al and PDMS is contacted when walking. Without the supporter, the wave-like contact is clearly observed in Fig. 3a. However, the supporter can make uniform contact possible, as shown in Fig. 3b. The uniform contact brings about higher output performance in TENG with the supporter. To support these results, cycled compressive force around 30 N with a frequency of 3 Hz was applied vertically to the TENGs with and without the supporter, and the output voltages and currents were measured. For the TENG with the supporter, an electrical output current of approximately 7 μA was generated, whereas 16 μA was generated in the flat TENG under the same mechanical force. The decrease of the output current in the TENG and supporter was due to non-uniform contact, as shown in the inset of Fig. 3c. The distance between the Al electrodes is also one of the parameters influencing the output power. In general, electric potential difference is reported to decrease as the distance decreases.¹⁹ This means that the output power of the TENG also decreased with the supporter because the distance decreased.

Fig. 2 (a) The output currents of flat TENG and TENGs with various angles from 0° to 35° of a triangular prism shaped supporter. (b) The voltage and current of TENG with the supporter as a function of angle.

Fig. 3 Optical images showing the contact mechanism of (a) the flat TENG and (b) the TENG with inclined supporter (25°) under human walking for comparing uniformity of contact. (c) The current signal of TENG with and without inclined supporter under vertical force using pushing tester.

Based on the abovementioned results, the optimum angle was chosen to be approximately 25° for the highest output power. Fig. S4† shows the stability and durability of the TENG. It is clearly seen that the output voltage is quite stable and does not appear to change significantly after tens of thousands of compression cycles. The output voltage was found to be significantly decreased within 1 min at high humid environment of 90%, as shown in Fig. S5.† This is very important because TENG is placed in the shoes. To solve this problem, we sealed the TENG with PDMS. After the sealing treatment, there was no significant decrease in the output voltage under high humidity. We also measured the output current of the TENGs with an applied weight from 74 to 92 kg, as plotted in Fig. 4a. The output voltages and currents increase with weight, which may be ascribed to an increase in the effective contact area.^20–22 At 92 kg, the output voltage and current reached 69 V and 65 μA, respectively. The output current as a function of walking speed from 1.2 m s⁻¹ to 2.4 m s⁻¹ was also measured and plotted in Fig. 4b. It is clearly seen that the output voltages and currents increase with the speed of walking. Enhancement in the output signals at higher frequency is reported to be due to the effective compensation effects of the electrons lost by scattering with the molecules in air.^23,24 To investigate the output power of the TENG with the supporter, resistors were used as external loads from 1 Ω to 1 GΩ. The instantaneous power of the external resistance for the TENG reaches a peak value of 1.5 mW at a resistance of 1 MΩ, as shown in Fig. 4c. To show the practical applications, we evaluated the charging characteristic of TENG, which was integrated with an AC to DC converting circuit, as shown in Fig. S6.† The converting circuit consists of one rectifier, three low capacitors (3 × 0.001 μF) and one capacitor (100 μF) to convert AC to DC output signal. When human weight of 74 kg under frequency of 1 Hz was applied, the capacitor was found to be charged up to 0.6 V. One white LED bulb connected to the output terminals of the circuit was continuously powered with comparable brightness for 5 seconds, as shown in Fig. 4d.

Fig. 4 The output currents of TENG with supporter under (a) various human weight from 74 kg to 92 kg and (b) various walking velocity from 1.2 m s⁻¹ to 2.4 m s⁻¹. (c) The measured output performance and instantaneous power of TENG with supporter (25°) as a function of external resistance. (d) The measured voltage of a commercial capacitor (100 μF) charged with AC to DC signal converting circuit, and the image of the brightest white LED connected the 100 μF capacitor and rectifier circuit continuously for 5 seconds.

We also demonstrated the self-powered pressure distribution sensor for monitoring the local pressure actions of the human foot. TENG arrays (5 × 5) with an area of 1 × 1 cm² were fabricated and the output power was measured when walking, as shown in Fig. S7.† In general, the patterns of the human gait are characterized by differences in walking velocity, kinetic and potential energy, and changes in the contact with the surface. Physical and psychological state will also influence the gait patterns; thus, understanding the fundamentals of gait can assist individuals with disabilities. Under a constant human weight of 74 kg, the insole containing TENG arrays reveals relative pressure distribution to extract and integrate each data sources when walking normally or abnormally such as in-toeing and out-toeing, as shown in Fig. 5a–c. Fig. 5d–f show a two-dimensional contour plot of normal gait, in-toeing gait and out-toeing gait, respectively. In the case of a normal gait, the pressure distribution across the foot is quite balanced during walking. However, the abnormal gaits induce changes in pressure distribution due to the differences in weight concentration points in the foot during walking. In particular, the highest pressure values were obtained at the medial forefoot and hallux region in the left-foot with in-toeing gait, as shown in Fig. 5e. In the case of out-toeing gait, as shown in Fig. 5f, the pressure distribution was concentrated at the lateral forefoot region. This study successfully shows one of the potential applications of this technology such as object recognition, ultra-sensitive e-skin, and self-powered health and activity monitoring system.

Fig. 5 (a–c) The photograph and (d–f) mapping figures obtained from the output voltage under several human gait: (a, d) normal gait, (b, e) in-toeing gait, (c, f) out-toeing gait.

Conclusions

In summary, we reported replaceable and multifunctional TENGs with a triangular prism shaped supporter to enhance the uniformity of contact and separation during walking. The supporter, which is ergonomically designed by considering the human walking style, was made from sponge-like mesoporous PDMS films fabricated using a 3D printed mold-casting method. When the supporter was applied with an angle of 25°, an output voltage and current of 64 V and 55 μA were generated, along with 1.5 mW in output power, showing an enhancement of approximately 600% compared to the flat TENG. The enhancement is due to uniform contact between Al and PDMS when walking. We also demonstrated a self-powered pressure distribution sensor for monitoring the local pressure actions of the human foot. The two-dimensional contour plot thus generated successfully showed a unique pressure distribution under three human gait patterns.

Acknowledgements

This study was supported by the Technology Innovation Program (10054640, Multi-energy charging IC & module development for long-lasting battery of wearable devices) and funded by the Ministry of Trade, Industry & Energy (MI. Korea), the KIST-UNIST partnership program (1.160097.01/2V04450), and the 2016 Research Fund (1.160034.01) of UNIST (Ulsan National Institute of Science and Technology).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17429a

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