Nano Energy 74 (2020) 104839

Available online 25 April 20202211-2855/© 2020 Elsevier Ltd. All rights reserved.

A triboelectric nanogenerator energy harvesting system based on load-aware control for input power from 2.4 μW to 15.6 μW

Karim Rawy a, Ruchi Sharma a, Hong-Joon Yoon b, Usman Khan b, Sang-Woo Kim b, Tony Tae- Hyoung Kim a,*

a School of Electric and Electronic Engineering, Nanyang Technological University (NTU), 50 Nanyang Ave, 639798, Singapore b School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon, 440-746, Republic of Korea

A R T I C L E I N F O

Keywords: Energy harvesting Maximum power point tracking (MPPT) Power conversion efficiency Power management circuits Triboelectric nanogenerator (TENG)

A B S T R A C T

This paper presents a triboelectric nanogenerator (TENG) energy harvesting system for ultra-low power appli-cations. We propose a load-aware control algorithm to improve the power conversion efficiency as well as the voltage conversion efficiency. The control algorithm minimizes the conduction and switching losses within a switched capacitor charge pump (SCCP) by modulating its switching frequency based on the load condition. Furthermore, a hysteresis input regulation control was developed for preventing breakdown. The overall system was optimized by utilizing a compact spice model from the physical mechanisms of the employed TENG. The fabricated test chip in 65-nm process technology provides a regulated output voltage of 1.2 V with power conversion efficiency of 88% at 30 Hz excitation frequency when the TENG output voltage is 2.5 V.

1. Introduction

Recent development of various emerging systems such as implant-able medical devices, wireless sensor nodes, and wearable electronics has opened a new era of applications such as Internet-of-Things (IoT) [1–6]. Self-sustainable, fully-integrated and low-cost power manage-ment is crucial in these applications. In this aspect, the use of energy harvesters as a primary energy source has been excessively studied to minimize the battery replacement/recharging cost and enhance the device lifetime [1–8].

Solar, thermal and mechanical energy harvesters have been used in many literatures as energy sources [5,8–13]. Among these energy sources, mechanical energy becomes promising due to its common ex-istence in our daily life; it can be harvested from human, vehicles, and objects vibrations. In the last decades, the piezoelectric transducer has been widely used for energy harvesting despite its complex fabrication and high cost [14]. However, lately, TENG was introduced as a new promising mechanical energy harvester [13–18]. It generates electric power based on the coupling of triboelectrification and electrostatic induction as explained in Ref. [14,15]. The continuous variation of capacitance between two materials generates current flow and hence electric power. The fabricated TENG details along with its charge’s

generation procedure is discussed and detailed in Supplementary Fig. 1. TENG becomes an attractive power source due to its fabrication simplicity, low cost, high flexibility, low weight, and small size [14]. Moreover, it can harvest energy from diverse vibration types in our daily life and can have various fabricated shapes (Supplementary Fig. 2), which makes it an adequate candidate for self-powered systems [14,15, 19–22]. Excessive studies, since 2012, have been conducted in the ma-terial properties and instantaneous output power of the TENG device from the perspective of material-engineering [13]. However, design of efficient TENG energy harvesting circuits has not been comprehensively investigated. TENG characteristics including its high peak output voltage (Fig. 1) and limited output power needs to be carefully consid-ered for designing efficient energy harvesting systems. These two bar-riers mainly stem from the high internal impedance (tens-hundreds of MΩ) due to its capacitive nature. The first integrated TENG energy harvesting circuit was presented in Ref. [13]. It used the fractional open circuit maximum power point tracking (MPPT) technique and achieved a peak tracking efficiency of 97%. However, the MPPT control circuit consumes relatively large power and the harvested power is only 51.1% of the total input power. In addition, the input voltage to the energy harvesting circuit is 70 V, which cannot be adopted in mainstream semiconductor process technologies whose supply voltage is just a few

* Corresponding author. E-mail addresses: karim002@e.ntu.edu.sg (K. Rawy), ruchi.nist@gmail.com (R. Sharma), yoonhj@skku.edu (H.-J. Yoon), usmanjadoon83@gmail.com (U. Khan),

kimsw1@skku.edu (S.-W. Kim), thkim@ntu.edu.sg (T.T.-H. Kim).

Contents lists available at ScienceDirect

Nano Energy

journal homepage: http://www.elsevier.com/locate/nanoen

https://doi.org/10.1016/j.nanoen.2020.104839 Received 19 February 2020; Received in revised form 2 April 2020; Accepted 9 April 2020

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volt. In this work, the power conversion efficiency is calculated by dividing the interface circuit output power (regulated output power) by the interface circuit input power (TENG output DC power).

This work presents a novel TENG energy harvesting system assisted with a smart, efficient and low power load aware control technique for enhancing power conversion efficiency and voltage conversion effi-ciency. It reduces the switching and conduction losses in the imple-mented switched capacitor charge pump circuit. It shows a 36% improvement in power conversion efficiency when compared with [13]. Furthermore, hysteresis-based input regulation is proposed. This mini-mizes the power losses and the tracking efficiency degradation and prevents the proposed energy harvesting system from breakdown due to

unacceptable high voltage. The remainder of this paper is organized as follow. Section 2 explains

the top architecture and the circuit implementation of the proposed TENG energy harvesting system as well as the proposed load aware control and input regulation techniques. Section 3 presents the mea-surement set-up and experimental results, followed by conclusions in section 4.

Fig. 1. (a) Representative operation modes of TENGs, (b) TENG AC output voltage waveform (Vteng) with uneven positive (Vpos) and negative (Vneg) peak values, and (c) Experimental P–V characteristic curves at different fex across different Zload values.

Fig. 2. Proposed TENG energy harvesting system top architecture including the proposed load aware control algorithm and the hysteresis-input regulation control.

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2. Proposed TENG energy harvesting system and circuit implementation

2.1. Electrical characteristics of TENG

Fig. 1(a) illustrates the operation of the employed TENG device based on the vertical contact mode. The top electrode consists of Per-fluoroalkoxy alkane (PFA) and Aluminum [24]. PFA is a well-known material for excellent stability to strong chemicals and corrosion pro-tection. These properties have been applied to TENGs [24]. The induced current flows when the TENG device is either pressed or released. Fig. 1 (b) shows the output waveform (Vteng) of the employed TENG when it is stimulated by vertical vibrations. The peak output voltage levels depend on the pressure and the materials, while the frequency of the pulses follows the frequency of the vibrations (i.e. excitation frequency (fex)). Unlike the piezoelectric transducers, Vteng can have different positive

and negative peak values. A dual-input bridge rectifier is reported in Ref. [13] to harvest positive (Vpos) and negative (Vneg) peaks separately. The rectified output voltage waveforms at no-load condition (TENG open-circuit characterization) as a function of the excitation frequency are shown in Supplementary Fig. 3. Fig. 1(c) depicts the relationship between the power and the TENG output voltage. The curves also show that at certain fex, an optimal voltage (i.e. Vmpp) exists where the output power is maximized.The power-voltage characterization curves shown in Fig. 1 (c) suggest that, the Vmpp varies with the excitation frequency, that depends, in the TENG case, determines the input power (Supple-mentary Fig. 3). The reason behind that is the variation of the TENG output impedance with the generated power (i.e. which depends on the excitation frequency). Hence, for efficient impedance matching, the Vmpp variation with the excitation frequency is expected a depicted in Fig. 1 (c). As shown in Fig. 1(c), Vmpp exceeds 30 V when fex is higher than 5 Hz. This requires high voltage process technology to regulate Vrect

Fig. 3. (a) Hysteresis-input regulation control and (b) timing diagram.

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near Vmpp as demonstrated in Ref. [13] using external voltage references due to the high voltage operation. However, this cannot be employed in mainstream process technologies where the supply voltage is just a few volt. Therefore, this work focuses on the operation region where the proposed TENG energy harvesting system can be directly employed as an on-chip power management solution. In this work, a conventional full-wave rectifier is employed as in Ref. [12,23] since the rectified voltage (Vrect) is much less than Vpos and Vneg so that no high voltage process technology is necessary.

Utilizing the TENG output voltage characterization (Fig. 1 and Supplementary Fig. 3), a TENG spice model was developed to study its characterization and optimize the design of a power management circuit that can efficiently harvest the mechanical energy harvested from the newly proposed TENG. The developed model is detailed in Supple-mentary Fig. 4. The developed spice model results verify the accuracy of the proposed model and the validation of the deduced equations for

Vteng (Supplementary Fig. 5).

2.2. Proposed energy harvesting system architecture

Fig. 2 illustrates the proposed TENG energy harvesting system ar-chitecture. It scavenges mechanical energy and manages it to supply current (IL) or store it in a super-capacitor (IE). The proposed architec-ture comprises four main blocks, namely a 0.5 � power converter to step down Vrect, dynamic latched comparators with hysteresis-based input regulation control (HIRC) and load aware control (LAC), an excess en-ergy storage circuit, and a digital core for supporting the LAC and HIRC schemes. First, upon the TENG vibration, the proposed HIRC regulates Vrect around 2.5V preventing breakdown. Then, Vrect is stepped down by the implemented SCCP to 1.2V supplying the IoT load. The losses within the SCCP are minimized using a proposed ultra-low power LAC circuit. It efficiently modulates the SCCP switching frequency according to the load condition (i.e. high, normal or light). Finally, using the HIRC output, the excess energy is stored using a supercapacitor, which can be used to supply the IoT load during the lack of input harvested energy.

2.3. Principle of hysteresis-based input regulation

As shown in Fig. 1, maximum power points (Vmpp) are formed at high voltage levels (e.g. > 30 V). However, mainstream process technologies prevent Vrect from being close to Vmpp due to the device reliability. Therefore, it limits the applications of the TENG energy harvesting systems. To address this, this work proposes an input regulation tech-nique that regulates Vrect within the maximum allowed voltage (Vmpp-

max) of the employed technology. However, this requires a part of the harvested energy to be lost at various operating conditions (e.g. fex, IL). The proposed hysteresis-based input regulation adopts an excess energy storage circuit that minimizes the harvested energy losses while main-taining the Vrect regulation close to the maximum allowed voltage.

As shown in Fig. 3, the Vrect level depends on the total output current (IT) and the vibration frequency (fex) at steady state. IT is composed of two components, the load current (IL) and the excess energy storage current (IE). The flow rate of IE is controlled by the digital core along with the excess storage circuit. Thus, the main idea of the proposed input regulation technique is to regulate Vrect near Vmpp-max by modulating IE with respect to IL and fex.

Fig. 3 illustrates the simplified diagram of the proposed hysteresis- based input regulation. It consists of two comparators with two external references (Vref-high and Vref-low), a 2-bit register, and a 5-bit counter. The comparator outputs (ФH and ФL) are stored in the 2-bit register and fed to the 5-bit-counter. The counter output (Rbits) modu-lates IE by altering the resistance (RM), which is implemented with a binary weighted resistor matrix. If Vrect-dic (a divided version of Vrect) lies between Vref-high and Vref-low, RM retains the value as well as IE. The operation of the proposed input regulation controlled by ФH and ФL is summarized in Fig. 3(a). Fig. 3(b) shows the operation of the proposed input regulation scheme. At high IL and/or low fex, Vrec-div goes below Vref-low (i.e. ФH ¼ 1, and ФL ¼ 0). The 5-bit counter output increments Rbits for increasing RM and decrease IE. Vrect recovers and maintains its regulation around Vmpp-max. Similarly, at light IL and/or high fex, Vrec-div goes above Vref-high (i.e. ФH ¼ 0, and ФL ¼ 1). Thus, the 5-bit counter decrements Rbits so that RM decreases to compensate for the decrease of IL. In both scenarios, the excess charges carried by IE are stored in a supper-capacitor. These excess charges can be used to feed a second load or the charge pump circuit in case of low harvested energy. A part of the stored energy is lost in RM due to the limited voltage of the process technology while maintaining Vrect near Vmpp-max.

2.4. Load-aware control algorithm

A switched capacitor charge pump (SCCP) with a gain of 0.5 is implemented to step-down Vrect. It is regulated at Vmpp-max (~2.5 V) and

Fig. 4. (a) Proposed load aware control flow-chart and (b) timing diagram.

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Fig. 5. (a) Proposed TENG energy harvesting system test setup and (b) measured power conversion efficiency.

Fig. 6. (a) Proposed load aware control operation at IL variation and (b) hysteresis-based input regulation control across different fex values.

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provides a regulated output voltage (Vout) of ~1.2 V. However, the TENG output power is limited around Vmpp-max as shown in Fig. 1. Thus, it is critical to minimize the conduction and switching losses in the charge pump for improving power conversion efficiency as well as voltage conversion efficiency. Note that the power conversion efficiency is calculated by dividing the interface circuit output power (regulated output power) by the interface circuit input power (i.e. TENG output power). In general, voltage conversion ratio (i.e. 0.5) must be kept constant across various load conditions. However, it deviates from 0.5 when load current varies. To prevent the undesirable deviation in the voltage conversion ratio, the switching frequency (fsw) of the charge pump must be controlled carefully depending on the load conditions. If fsw is higher than the required one, it increases switching loss. Similarly, if fsw is lower than the required one, it increases conduction loss. The proposed load aware control technique modulates the fsw of the charge pump with IL by monitoring the voltage conversion ratio.

As shown in Fig. 2, the proposed load aware control is implemented with a comparator and a 4-bit frequency divider. The comparator compares Vrect-div with Vout-div. Vrect-div and Vout-div are divided voltage levels of Vrect and Vout as indicated in Fig. 2. The comparator acts as a voltage conversion sensor, which indirectly reflects the IL condition. The 4-bit frequency divider controls the ring oscillator (ROSC) output fre-quency by a 4-bit digital value ‘N’. ‘N’ is set by the comparator output (Vcomp) to modulate fsw with respect to IL variations, maintaining the voltage conversion ratio around 0.5. Fig. 4 illustrates the flow chart of the proposed load aware control. It adopts an event/time driven tech-nique for modulating fsw across various load scenarios. Initially, ‘N’ is preset to an initial value by a power-on-reset (POR) block in Fig. 2. If Vout < 0.5Vin (i.e. Vcomp ¼ 0), fsw is increased by decrementing ‘N’ by one bit. Likewise, if Vout > 0.5Vin (i.e. Vcomp ¼ 1), the 4-bit frequency divider increments ‘N’ by one bit after a defined time td, decreasing fsw. For example, following the timing diagram depicted in Fig. 4, when IL abruptly increases, Vout goes below 0.5Vin (i.e. Vout-div < Vrect-div). Then, the proposed load aware control increases fsw by decrementing ‘N’ by one bit at every detected Vcomp negative edge for Vout recovery. Simi-larly, if Vout>0.5Vrect, fsw will be decreased by incrementing ‘N’ by one bit after a defined time ‘td’. At steady state, Vout oscillates around 0.5Vrect. Thus, by adjusting fsw to a proper value with respect to the load conditions, the power conversion efficiency along with the voltage conversion efficiency can be increasingly improved.

3. Measurements results

The proposed TENG energy harvesting system was designed and fabricated in 65-nm CMOS process. Fig. 5(a) shows the test setup of the proposed TENG energy harvesting system and the comparison table. The test chip occupies a silicon area of 0.394 mm2. A power amplifier is used to control the excitation frequency (fex) and the pressure of the vibra-tions applied on the fabricated TENG. These vibrations are generated by a commercial mini-shaker. It agitates one plate of the TENG while the other plate is fixed on a wall. A resistance matrix is used as Rload for characterization. The power conversion efficiency is recorded by calculating the power at Vin and Vout nodes (Fig. 5(b)) at each operating condition using current and voltage probes. Thus, the recorded power conversion efficiency (Pvout/Pvin) is improved by minimizing the energy losses within the power management circuit.

The slow TENG harvesting process (i.e. 100s of Sec) along with the big values of Cin and Cout (2 μF each) limits the bandwidth of the pro-posed control loops. Thus, the implemented dynamic latched compar-ators operate at a very low rate (<1 KHz) for ultra-low power consumption (<20 nW). Fig. 5(b) shows the power conversion efficiency at different fex when the output power of the proposed energy harvesting system changes. The test chip achieves a peak power conversion effi-ciency of 88% at fex ¼ 30 Hz, which is achieved by the minimized conduction and switching losses in the charge pump through the pro-posed load aware control. Note that the compared work in Ref. [13] is

using high voltage technology, which is not suitable for our target ap-plications such as IoT and on-chip power management.

Fig. 6(a) demonstrates the operation of the proposed load aware control. When IL increases, the comparator transmits pulses to the 4-bit frequency divider in the digital core (Fig. 2) to modulate fsw accordingly. fsw increases one step at every Vcomp negative pulse to restore Vout to its regulated value. This minimizes the conduction loss of the switched capacitor charge pump (SCCP) and maintains the voltage conversion ratio ~0.5. Thus, the proposed load aware control tracks the maximum power conversion efficiency after considering the load condition. The operation of the hysteresis-based input regulation is demonstrated in Fig. 6(b). In the proposed input regulation, two comparators outputs, ФH and ФL, monitor Vrect and maintain it between the two reference levels (Vref-high and Vref-low). They regulate it around Vmpp-max by modulating IE flow rate, which is used to store the excess charges, as explained in section III. With the decrease of fex (i.e. 30 Hz–20 Hz), assuming constant IL, Vrect goes below Vref-low. Hence, ФH turns to ‘0’, decreasing IE and keeping Vrect above Vref-low. Likewise, when fex increases from 20 Hz to 40 Hz, ФL turns ‘1’, increasing IE, maintaining Vrect below Vref-high.

4. Conclusion

This work proposes a triboelectric energy harvesting system for ultra-low power applications such as IoT and wearable devices. Here, nominal supply voltage is a few volt. To make TENG an attractive energy source, high power conversion efficiency and low output voltage compatible to integrated circuits are substantial. To achieve these, this work proposes a load aware control technique for minimize the con-duction and switching losses in the implemented switched capacitor charge pump, solving the limited harvested power issue. The proposed control technique successively achieves a maximum power conversion efficiency of 88% at fex 30 Hz, showing a 36% improvement compared with the prior art triboelectric energy harvesting system design in Ref. [15]. In addition, a hysteresis-based input regulation was developed to prevent the fabricated chip from breakdown, and store the excess harvested energy. The digital core complexity is implemented by using the digital integrated circuit design flow using hardware description language, which makes the proposed control techniques scalable.

Funding

This work was supported by the Singapore International Graduate Award and Ministry of Education, Singapore, under grant AcRF TIER 1- 2018-T1-002-105 (RG174/18 (S)).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Karim Rawy: Conceptualization, Methodology, Validation, Writing - original draft. Ruchi Sharma: Investigation, Validation. Hong-Joon Yoon: Investigation, Formal analysis. Usman Khan: Investigation, Formal analysis. Sang-Woo Kim: Conceptualization, Supervision. Tony Tae-Hyoung Kim: Conceptualization, Supervision, Writing - review & editing.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2020.104839.

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References

[1] W. Jung, S. Oh, S. Bang, Y. Lee, Z. Foo, G. Kim, et al., An ultra-low power fully integrated energy harvester based on self-oscillating switched-capacitor voltage doubler, IEEE J. Solid State Circ. 49 (2014) 2800–2811.

[2] J. Li, J.S. Seo, I. Kymissis, M. Seok, Triple-mode, hybrid-storage, energy harvesting power management unit: achieving high efficiency against harvesting and load power variabilities, IEEE J. Solid State Circ. 52 (2017) 2550–2562.

[3] X. Liu, E. Sanchez-Sinencio, A single-cycle MPPT charge-pump energy harvester using a thyristor-based VCO without storage capacitor, in: IEEE International Solid- State Circuits Conference, 2016, pp. 364–365.

[4] K. Rawy, F.K. George, D. Maurath, T.T. Kim, A time-based self-adaptive energy- harvesting MPPT with 5.1-μW power consumption and a wide tracking range of 10- μA to 1-mA, in: 42nd European Solid-State Circuits Conference, 2016, pp. 503–506.

[5] K. Rawy, F. Kalathiparambil, D. Maurath, T.T.H. Kim, A self-adaptive time-based MPPT with 96.2% tracking efficiency and a wide tracking range of 10-μA to 1-mA for IoT applications, IEEE Trans. Circ. Syst. I: Reg. Pap. 64 (2017) 2334–2345.

[6] W. Jung, S. Oh, S. Bang, Y. Lee, D. Sylvester, D. Blaauw, A 3nW fully integrated energy harvester based on self-oscillating switched-capacitor DC-DC converter, in: IEEE International Solid-State Circuits Conference, 2014, pp. 398–399.

[7] C.S. Wu, M. Takamiya, T. Sakurai, Buck converter with higher than 87% efficiency over 500nA to 20mA load current range for IoT sensor nodes by Clocked Hysteresis Control, in: IEEE Custom Integrated Circuits Conference, 2017, pp. 1–4.

[8] S. Bandyopadhyay, A.P. Chandrakasan, Platform architecture for solar, thermal, and vibration energy combining with MPPT and single inductor, IEEE J. Solid State Circ. 47 (2012) 2199–2215.

[9] A. Das, Y. Gao, T.T.H. Kim, A 76% efficiency boost converter with 220mV self- startup and 2nW quiescent power for high resistance thermo-electric energy harvesting, in: 41st European Solid-State Circuits Conference, 2015, pp. 237–240.

[10] X. Liu, E. S�anchez-Sinencio, An 86% efficiency 12 μW self-sustaining PV energy harvesting system with hysteresis regulation and time-domain MPPT for IOT smart nodes, IEEE J. Solid State Circ. 50 (2015) 1424–1437.

[11] M. Shim, J. Kim, J. Jeong, S. Park, C. Kim, Self-powered 30 μW to 10 mW piezoelectric energy harvesting system with 9.09 ms/V maximum power point tracking TimeTime, IEEE J. Solid State Circ. 50 (2015) 2367–2379.

[12] S. Stanzione, C.v. Liempd, M. Nabeto, F.R. Yazicioglu, C.V. Hoof, A 500nW batteryless integrated electrostatic energy harvester interface based on a DC-DC converter with 60V maximum input voltage and operating from 1μW available power, including MPPT and cold start, in: IEEE International Solid-State Circuits Conference, 2015, pp. 1–3.

[13] I. Park, J. Maeng, D. Lim, M. Shim, J. Jeong, C. Kim, A 4.5-to-16μW integrated triboelectric EnergyHarvesting system based on high-voltage dual-input buck converter with MPPT and 70V maximum input voltage, in: IEEE International Solid-State Circuits Conference, 2018, pp. 146–148.

[14] R. Hinchet, W. Seung, S.-W. Kim, Recent progress on flexible triboelectric nanogenerators for SelfPowered electronics, ChemSusChem 8 (2015) 2327–2344.

[15] Z.L. Wang, Triboelectric nanogenerators as new energy technology for self- powered systems and as active mechanical and chemical sensors, ACS Nano 7 (2013) 9533–9557.

[16] Y. Mao, D. Geng, E. Liang, X. Wang, Single-electrode triboelectric nanogenerator for scavenging friction energy from rolling tires, Nano Energy 15 (2015) 227–234.

[17] M. Wang, J. Zhang, Y. Tang, J. Li, B. Zhang, E. Liang, et al., Air-flow-driven triboelectric nanogenerators for self-powered real-time respiratory monitoring, ACS Nano 12 (2018) 6156–6162.

[18] Y. Tang, H. Zhou, X. Sun, N. Diao, J. Wang, B. Zhang, et al., Triboelectric touch-free screen sensor for noncontact gesture recognizing, Adv. Funct. Mater. 30 (2020) 1907893.

[19] W. Seung, H.-J. Yoon, T.Y. Kim, H. Ryu, J. Kim, J.-H. Lee, et al., Boosting power- generating performance of triboelectric nanogenerators via artificial control of ferroelectric polarization and dielectric properties, Adv. Energy Mater. 7 (2016), https://doi.org/10.1002/aenm.201600988.

[20] J. Zhong, Q. Zhong, F. Fan, Y. Zhang, S. Wang, B. Hu, et al., Finger typing driven triboelectric nanogenerator and its use for instantaneously lighting up LEDs, Nano Energy 2 (2013) 491–497.

[21] N. Zhang, C. Qin, T. Feng, J. Li, Z. Yang, X. Sun, et al., Non-contact cylindrical rotating triboelectric nanogenerator for harvesting kinetic energy from hydraulics, Nano Res. (2020), https://doi.org/10.1007/s12274-020-2654-7.

[22] B. Zhang, Y. Tang, R. Dai, H. Wang, X. Sun, C. Qin, et al., Breath-based human–machine interaction system using triboelectric nanogenerator, Nano Energy 64 (2019) 103953.

[23] S. Stanzione, C.v. Liempd, R.v. Schaijk, Y. Naito, R.F. Yazicioglu, C.v. Hoof, A self- biased 5-to-60V input voltage and 25-to1600μW integrated DC-DC buck converter with fully analog MPPT algorithm reaching up to 88% end-to-end efficiency, in: IEEE International Solid-State Circuits Conference, 2013, pp. 74–75.

[24] H.-J. Yoo, H. Ryu, S.-W. Kim, Sustainable powering triboelectric nanogenerators: approaches and the path towards efficient use, Nano Energy 51 (2018) 270–285.

Karim Rawy (S014) received the B.S. degree in electronics and communication engineering from Alexandria University, Alex-andria, Egypt in 2012. He received the Ph.D. degree in elec-trical and electronic engineering at VIRTUS, IC Design Centre of Excellence, Nanyang technological University, Singapore in 2018. From Aug. to Dec. 2017, he was a visitor Ph.D. student at the electrical and computer engineering department in Georgia Institute of Technology, Atlanta, GA, USA. At 2012, he joined the ASICs solution division for custom ASIC development and supply specializing in analog/mixed-signal and RF design, Si- Ware Systems, Cairo, Egypt.His research interests include ultra-low power analog/mixed-signal circuits, power manage-ments IC design, energy harvesting systems for IoT and biomedical applications.

Ruchi Sharma received the Maters of technology (M.Tech) degree in Electrical Engineering from Indian Institute of Tech-nology, Kanpur, India in 2012. From Aug. 2012 to Mar. 2017, She worked for Xilinx India Technology Services in Analog Mixed Signal Team. She worked on designing of Crystal Oscil-lators in 7 nm and Level Shifters in 16 nm and 7 nm Techno-logical Node. She developed the complete infrastructure and software for generation of I/O Buffer Information Specification (IBIS) files for 16 nm technological node. At 2017, She joined as a Research Associate in VIRTUS, IC Design Centre of Excellence, Nanyang Technological University, Singapore. Her research interests include Analog/Mixed-signal circuits, Radiation Resilient SRAM, Sub-threshold operations.

Hong-Joon Yoon is pursuing Ph.D. course under the supervi-sion of Prof. Sang-Woo Kim at School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU). His research interests are fabrications and characterizations of piezoelectric and triboelectric nanagenerators for energy har-vesting and their applications in self-powered devices.

Usman Khan is a research professor at the School of Advanced Materials Science &Engineering, Sungkyunkwan University (SKKU), South Korea. He received his Ph.D. degree from Uni-versity of Rome “Tor Vergata”, Italy, in May 2014. Afterwards, he pursued post-doc research at, initially, “Tor Vergata” (one year) and, then, at SKKU (three and a half). His research in-terests include design, simulation, fabrication and character-ization of 2D materials, MEMS and NEMS for energy harvesting, sensing and actuation.

Dr. Sang-Woo Kim is a professor in the Department of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). His recent research interest is focused on piezoelectric/triboelectric nanogenerators, sensors, and pho-tovoltaics using nanomaterials. He has published over 200 peer- reviewed papers (h-index of 54) and holds over 80 domestic/ international patents. Now he is a director of SAMSUNG-SKKU Graphene/2D Research Center and is leading National Research Laboratory for Next Generation Hybrid Energy Harvester. He is currently serving as an Associate Editor of Nano Energy and an Executive Board Member of Advanced Electronic Materials.

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Tony Tae-Hyoung Kim (M006-SM014) received the B.S. and M. S. degrees in electrical engineering from Korea University, Seoul, Korea, in 1999 and 2001, respectively. He received the Ph.D. degree in electrical and computer engineering from University of Minnesota, Minneapolis, MN, USA in 2009. From 2001 to 2005, he worked for Samsung Electronics where he performed research on the design of high-speed SRAM mem-ories, clock generators, and IO interface circuits. In 2007–2009 summer, he was with IBM T. J. Watson Research Center and Broadcom Corporation where he performed research on circuit reliability, low power SRAM, and battery backed memory design, respectively. On November 2009, he joined Nanyang Technological University where he is currently an associate professor.He received “Best Demo Award” at APC CAS2016, “Low Power Design Contest Award” at ISLPED2016, best paper awards at 2014 and 2011 ISOCC, “AMD/CICC Student Schol-arship Award” at IEEE CICC2008, Departmental Research Fellowship from Univ. of Minnesota in 2008, “DAC/ISSCC Student Design Contest Award” in 2008, “Samsung Humantec Thesis Award” in 2008, 2001, and 1999, and “ETRI Journal Paper of the Year Award” in 2005. He is an author/co-author of þ130 journal and conference papers and has 17 US and Korean patents registered. His current research interests include low power and high performance digital, mixed-mode, and memory circuit design, ultra-low voltage circuits and systems design, variation and aging tolerant circuits and systems, and circuit techniques for 3D ICs. He is an IEEE senior member and served as the Chair of IEEE Solid-State Circuits Society Singapore Chapter. He serves as an Associate Editor of IEEE Transactions on VLSI Systems and has served numerous conferences as a committee member.

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