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Nano Energy

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Performance improvement of flexible piezoelectric energy harvester forirregular human motion with energy extraction enhancement circuit

Muhammad Bilawal Khana,1, Dong Hyun Kimb,1, Jae Hyun Hanb,1, Hassan Saifa, Hyeonji Leea,Yongmin Leea, Minsun Kima, Eunsang Janga, Seong Kwang Hongb, Daniel Juhyung Joeb,Tae-Ik Leec, Taek-Soo Kimc, Keon Jae Leeb,⁎, Yoonmyung Leea,⁎

a Department of Electrical and Computer Engineering, Sungkyunkwan University (SKKU), 2066 Seobu-ro, Jangan-gu, Suwon 16419, Republic of KoreabDepartment of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic ofKoreac Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

A R T I C L E I N F O

Keywords:Harvesting circuitFlexible PZTIrregular inputHuman movement

A B S T R A C T

Flexible piezoelectric energy harvesters (f-PEHs) have drawn attention for their potential use as power sourcesfor wearable electronics. However, the amount of power harvested from conventional f-PEHs is still insufficientfor achieving energy autonomy; hence, they are only used for limited applications. One of the effective ap-proaches to enhancing the energy extraction from an f-PEH is to optimize the harvesting circuits. In this article,an energy extraction enhancement circuit (EEEC) using an f-PEH based on piezoelectric (PZT) material is re-ported to improve energy harvesting from irregular human movement of a joint or limb. The proposed EEEC isoptimized for harvesting energy from random energy inputs with varying magnitudes and intervals, just likesporadic and irregular human movement. An f-PEH with a total thickness of 170 µm is utilized, which providessufficient flexibility for attachment on clothes or human skin. By minimizing the capacitive load experienced bythe PZT material during deformation, the EEEC maximizes the output voltage and increases the amount ofextracted energy with low static power consumption (1.15 nW). Compared with a conventional full bridgerectifier (FBR)-based harvesting circuit, energy extraction is enhanced up to 495%.

1. Introduction

The advent of the Internet of Things (IoT) has greatly increased theuse of wearable electronics in every aspect of daily life, which canprovide a variety of information related to personal healthcare [1–9].For instance, a wristband activity tracker can collect various fitness-related information such as heartbeat rate, sleep quality, walking dis-tance, and estimated calorie consumption [10,11]. To power wearableelectronics, commercial rechargeable batteries have been widely uti-lized, which require periodic recharging due to their limited energystorage capacity. However, repeated battery charging disrupts thecontinuous monitoring or diagnosis of the owner's conditions since thewearable devices typically have to be taken off during recharging. Toaddress these issues, wearable electronics have been integrated withenergy harvesting technology with the goal of achieving self-poweredsystems [12–23], where device operation can be sustained with har-vested energy.

Among various approaches geared towards creating a self-poweredsystem, flexible piezoelectric energy harvesters (f-PEHs) have attractedconsiderable attention as potential continuous power sources forwearable electronics [24–26]. f-PEHs have a remarkable capability togenerate electricity even from slight mechanical movements common inour environment, such as human activities [27,28], machinery vibra-tions [29,30], and even wind or ocean waves [31]. Several researchteams have reported f-PEHs based on a variety of piezoelectric mate-rials, including ZnO, BaTiO3, and Pb(Zrx,Ti1-x)O3 (PZT) [32–41], butthe relatively small amount of energy harvested from these f-PEHs re-stricts the range of applications. It is thus desirable to design an energyharvesting circuit that enhances the amount of energy extracted by an f-PEH.

Although there have been numerous circuits designed for efficientlyharvesting energy from PEHs, most of these earlier works focused onoptimizing harvesting circuit operation for cantilever-based structureswhere output is periodic sinusoidal due to the mechanical resonance

https://doi.org/10.1016/j.nanoen.2019.01.049Received 24 November 2018; Received in revised form 14 January 2019; Accepted 15 January 2019

⁎ Corresponding authors.E-mail addresses: keonlee@kaist.ac.kr (K.J. Lee), yoonmyung@skku.edu (Y. Lee).

1 These authors contributed equally to this work.

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Available online 16 January 20192211-2855/ © 2019 Elsevier Ltd. All rights reserved.

T

excited by external vibrations [42,43]. However, this approach is notapplicable for f-PEHs combined with wearable electronics since theoutput generated from human motion is irregular rather than periodicsinusoidal by nature. An f-PEH generates extremely inefficient outputpower with varying amplitudes at random intervals like human motion.The amplitude of an output pulse is proportional to the amount of de-formation applied to the f-PEH, and the interval of the output pulses isdetermined by the period of the motion. Recently, there was an attemptto implement a PZT energy harvester for pulsed inputs [44], but it couldonly be used for PZT harvesters with a large capacity since it required ahigh activation energy.

Herein, we report an energy extraction enhancement circuit (EEEC)with an f-PEH based on PZT material to improve energy harvestingfrom irregular energy input. A flexible PZT thin film on a plastic sub-strate was fabricated using inorganic-based laser lift-off (ILLO). Theresulting product can be attached to clothes or skin to harvest energyfrom human motion [45–48]. The EEEC was designed as an inductor-based circuit to minimize energy loss during the current flow from f-PEH to the battery through an inductive converter. To handle the highvoltage output (> 140 V) of the f-PEH efficiently, high-voltage-tolerantdiscrete components are utilized for most of the passive elements, whilestandard voltage (3.3 V I/O) process is utilized for integrated circuitfabrication for cost-effectiveness. By minimizing the capacitive loadseen by PZT material, the EEEC maximizes the output voltage of f-PEHand increases the amount of extracted energy. The minimum activationenergy (i.e. harvesting overhead) for the proposed EEEC is 13 nJ perpulse, which is very low compared with previously reported paper of 12μJ [44]. The energy extraction is enhanced up to 495% compared withconventional full bridge rectifier (FBR). The proposed EEEC also hasvery low static power consumption (1.15 nW), which is a very im-portant metric for energy-efficient harvesting operation with sporadicenergy inputs.

2. Experimental section

2.1. Preparation of a flexible piezoelectric energy harvester

The prepared PZT sol-gel (MEMS Solution, Inc.,) was spin-coatedonto a sapphire substrate at 2400 rpm for 30 s. To crystallize spin-coated PZT, heat treatment was conducted using rapid thermal an-nealing (RTA) at 650℃ for 60min in air atmosphere. Spin-coating andheat treatment were repeated 20 times to obtain 2-µm thickness. Anadhesive epoxy (ultraviolet sensitive polyurethane) was also spin-coated onto thin-film PZT, which was then attached to the flexiblepolyethylene terephthalate (PET) substrate (4 cm × 4 cm × 125 µm).The ultraviolet- (UV-) sensitive epoxy was optically cured by a UV lightfor 2000 s. The ILLO process (XeCl excimer laser, 4.03 eV photon en-ergy at λ=308 nm) was carried out to separate the sapphire substratefrom the PZT thin film. The lateral type interdigitated electrodes (IDEs)were deposited with Cr/Au (thickness of 10 nm and 100 nm, respec-tively) and patterned using conventional microfabrication.Encapsulation was performed with SU-8 passivation epoxy (10-µmthickness), and then Cu electrical wires were fixed on the IDEs usingconductive paste. Finally, a poling process was conducted to enhancethe piezoelectric properties at 120℃ by applying an electric field of80 kV/cm for 3 h.

2.2. Design and fabrication of the harvesting circuit

The harvesting circuit was designed and simulated in commercialstandard voltage CMOS process (180 nm, 3.3 V I/O, 6 metal layers).After functional verification simulations, the harvesting circuit's layoutwas designed and sent to an integrated circuits (IC) manufacturingcompany for fabrication. The fabricated IC was then packaged andwirebonded with a pin-grid-array (PGA) package so that it could beplugged into a socket on a printed circuit board (PCB). The PCB is

designed to support an automated test. The energy source (PZT har-vester) and all testing equipment are connected through the PCB, andthe harvesting circuit operation is controlled for proper measurement.Fig. S1 shows the block diagram of the test set up. Analog Discovery 2®was used to interface the test IC with a PC. Analog and digital testpatterns are applied to test IC through Analog Discovery 2® and the testpatterns is controlled with the LabVIEW®. The measurements werecarried out for different bending speeds and displacements applied onthe f-PEH by adjusting bending configurations with bending machinecontroller.

2.3. Characterization and electrical measurement

The PZT energy harvester was bent by a custom-designed linearstage. A Keithley 2612A sourcemeter was used to measure the open-circuit voltage and short-circuit current from the f-PEH as a result ofperiodic bending/unbending motions. A Keysight B2912A sourcemeasurement unit was used to measure the voltage and current withEEEC operation, and a Tektronix MSO5204B oscilloscope was used toobserve the digital signal and input/output voltage waveforms.

3. Result and discussion

Fig. 1a illustrates the conceptual scheme for energy harvesting fromsmall human movements using an f-PEH attached to a human elbow.The f-PEH was fabricated using a protocol similar to that found in ourprevious reports, as described in the experimental section [38]. Elec-trical signals obtained from the movement of various parts of the body,such as elbow, ankle, knee, or shoulder joint, have irregular andsporadic characteristics with varying magnitude due to random andintermittent human motion. The piezoelectric energy harvester can beapproximately modelled as a current source with instantaneous current(IP) and an internal capacitance (CP) in parallel [42]. As the PZT is bent,the generated IP charges the internal load CP and potential external load(CLoad), developing voltage (VP) at the output.

Fig. 1b shows the driving mechanism of the EEEC. For a givenamount of deformation, it can be assumed that a fixed amount of chargeis generated by the PZT harvester. During the deformation, the amountof energy dW deposited to load capacitance (CL = CP + CLoad) by thegenerated charge dq can be written as

=dW V dq

where V stands for the voltage at CL at the time of dq accumulation.Therefore, V can be calculated from the total charge accumulated on CL

at time t, q(t), and the capacitance value CL:

=Vq tC( )L

The total energy generated by the piezoelectric energy harvester canbe calculated by integrating dW for the duration of the bending op-eration:

∫ ∫= = =dWq

Cdq Q

CW 1

2Q

L L0

2

From the above equation, the total amount of energy generated by f-PEH is inversely proportional to CL (Fig. 1b-i). This implies that theenergy derived from a PZT harvester can be maximized by minimizingCL, i.e., setting explicit CLoad to 0. Therefore, one of the key approachesof this work is to minimize the load capacitance by removing explicitload capacitance at the harvesting circuit interface and isolating theenergy harvester from the rest of the EEEC during deformation.

As the load capacitance is reduced, more energy is extracted andhigher voltage is accumulated at the output, as shown in Fig. 1b-ii.Therefore, the harvesting circuit must be designed to handle very highvoltages (> 140 V) generated by the f-PEH. The proposed EEEC candeal with high voltages with pulsed input by using high-voltage-

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Fig. 1. (a) A schematic illustration of wearable applications using the flexible PZT energy harvester. Load capacitance seen by PZT is minimized by eliminatingexplicit load capacitance. (b) Driving mechanism of the EEEC. i) Maximum energy is extracted by minimizing load capacitance. ii) A harvesting circuit is required tohandle high-voltage pulse inputs. iii) Up to 495% improvement is observed compared with simple rectifying circuit (FBR). (c) Chip micrograph.

Fig. 2. (a) A photograph of the flexible PZTenergy harvester bent by tweezer. The insetshows the OM image of the IDEs. (b) XRDpattern of the PZT thin film on a PET substrate.The inset shows a cross-sectional SEM image ofthe flexible PZT energy harvester. (c) High-re-solution TEM image and FFT pattern (inset) ofa PZT thin film. (d) An OM image of thetransferred PZT thin film showing laser shottraces (top-left), and AFM images of each re-gion with single or multiple laser shots duringthe ILLO process. (e) The generated outputopen-circuit voltage and short-circuit currentfrom the f-PEH. (f) The charged voltage in 10µF, 22 µF, 33 µF, and 68 µF electrolytic capa-citors by the flexible PZT harvester. The insetpresents a schematic circuit diagram.

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tolerant discrete components for most of the passive elements. Whenthe peak voltage is detected at the end of PZT deformation, the EEECefficiently converts the high output voltage to low voltage to storeenergy on battery. With such a maximized energy extraction approach,significant improvement in the amount of harvested energy is observed.The amount of energy harvested from a single pulse with the proposedharvesting circuit (EHRV) is improved by up to 495% compared withthat obtained with a simple rectifying circuit (EFBR), as shown inFig. 1b-iii. The inductor-based harvesting circuit is composed of acontrol circuit and power switches to control generated charge flow.Fig. 1b-iii only shows the conceptual explanation of energy extractionenhancement circuit. However, complete measurement details arepresented in the later sections. Fig. 1c shows the micrographic image ofthe EEEC whose integrated circuits for control are fabricated in 0.18-µmstandard voltage (3.3 V I/O) process.

Fig. 2a shows that the f-PEH with lateral type IDEs has high flex-ibility and durability under bending. The open-circuit voltage gener-ated by the f-PEH is proportional to the inter-electrode gap on the PZTmaterials. Since the interval between the electrodes can be easily ad-justed through a photolithography process up to a few hundred mi-crometers, higher voltage can be obtained from the IDE-based energy

harvesters compared to those with a metal-insulator-metal (MIM)structure. As shown in the optical microscopy (OM) image of the f-PEH(inset of Fig. 2a), the IDEs were designed with an electrode width of100 µm and inter-electrode gap of 100 µm. The square-shaped marksshown in the OM image are the traces of the laser shots producedduring the ILLO process.

Fig. 2b presents the X-ray diffraction (XRD) and scanning electronmicroscopy (SEM) analysis. The XRD pattern was indexed with a re-ference pattern for tetragonal PZT (ICDD Card No. 33–0784) andrhombohedral PZT (ICDD Card No. 73-2022). The results revealed thatthe polycrystalline PZT with random orientation was well crystallizedinto a perovskite structure in which tetragonal and rhombohedralphases coexist. In order to test mechanical property of PZT film, theuniaxial tensile test was conducted. As shown in Fig. S2, PZT film onPET substrate shows ductile fracture with the mechanical strength of90.4 MPa and the Young's Modulus of 3.76 GPa. The SEM image (insetof Fig. 2b) shows that the PZT thin film with a thickness of 2 µm wassuccessfully transferred to a PET substrate without mechanical damagesuch as fracture, cracking, and waviness. A cross-sectional SEM imageof the magnified PZT layer (Fig. S3) was also observed to investigate thecompactness of PZT thin film, which clearly shows a high compact

Fig. 3. (a) Block diagram of proposed harvesting circuit. (b) Conceptual waveform of the proposed harvesting circuit operation. (c) Generated energy and peakvoltage as function of load capacitance.

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structure of the film. Fig. 2c presents the high-resolution transmissionelectron microscopy (HRTEM) image and the fast-Fourier transforma-tion (inset of Fig. 2c), which show the exact lattice configuration of theperovskite PZT particles. Fig. S4 depicts the P-E (Polarization-electric)hysteresis loops of PZT film with increasing applied electric field at afixed frequency of 100 Hz. The film exhibited symmetric hysteresis loopwith remanent polarization (Pr) of 63.7 pC cm−2 and coercive fieldstrength (Ec) of 167.3 kV cm−1 at maximum electric field of 300 kVcm−1. As shown in Fig. 2d, atomic force microscopy (AFM) imagesreveal the surface morphology of the PZT thin film transferred onto thePET substrate. The incident beam of the excimer laser is absorbed in thePZT layer while passing through the sapphire mother substrate due tothe difference in band gap of each layer during the ILLO process [38].Thus, the interface between the PZT and the sapphire was locally va-porized, which creates surface fluctuation in the region where a singlelaser shot is irradiated. In the double and triple shot regions, where thelaser track is overlapped, repeated laser shots melt the PZT interface,resulting in flattening as the fluctuation decreases. The root meansquare (RMS) roughness of single, double and triple laser shot regions isreduced to 23.6 nm, 14.3 nm, and 6.93 nm, respectively.

The electrical output performance of the f-PEH during mechanicaldeformation is shown in Fig. 2e. It generated an open-circuit voltage of165 V and a short-circuit current of 1.5 µA as a result of the continualbending/unbending motion induced repeatedly by a linear motor witha curvature of 0.5 cm−1, a strain rate of 2.3% s−1, and a frequency of0.4 Hz. Fig. 2f presents graphs of charging four different capacitors(with capacitances of 10 µF, 22 µF, 33 µF, and 68 µF) by periodicbending deformation of the f-PEH. The capacitors were charged by thePZT harvesting device from 0 V to 1 V within tens to hundreds seconds(48 s for 10 µF, 86 s for 22 µF, 141 s for 33 µF, and 288 s for 68 µF). AnFBR was utilized to rectify the electrical output to charge the capacitors,and the circuit diagram of the rectifier is shown in the inset of theFig. 2f.

Fig. 3a shows the operation details of the proposed harvesting cir-cuit. An inductor-based energy harvesting circuit is implemented sinceit can effectively handle varying input voltage with wide continuousrange of voltage conversion ratios, whereas capacitor-based circuitsonly support limited and discrete conversion ratios. The f-PEH is con-nected to the harvesting circuit through an FBR for rectification. Inconventional harvesting circuits, a buffer capacitor (CBUF) is added atthe output of the FBR (VRECT) to provide temporary energy storage andcontrol impedance. In addition to CBUF, the parasitic capacitance of theharvesting circuit (CPAR) is seen as load capacitance for f-PEH duringdeformation. In the proposed EEEC, CBUF is removed to minimize theload capacitance. Furthermore, by inserting switch S1 between the FBRand the harvesting circuit and keeping S1 turned off during deforma-tion, CPAR is isolated from the f-PEH, and hence the load capacitanceseen by the f-PEH is minimized. As the PZT is bent, current (IP) isgenerated by the PZT, and internal capacitance CP is charged, in-creasing VRECT output voltage (① in Fig. 3b). As VRECT exceeds a certainthreshold (VTHRS), a wake-up trigger circuit (WTC) activates the har-vesting circuit with TRIG signal. By setting an appropriate VTHRS level,losing energy by activating the EEEC for a small input can be avoided.As the TRIG signal activates the harvesting circuit, the peak voltagedetector (PVD) monitors the VRECT and detects if it reaches its peak,which indicates the end of the deformation process (② in Fig. 3b).

Once the peak is detected, the energy stored in CP is transferred toan inductor by connecting S1 and S2 (③ in Fig. 3b). Meanwhile, the peakdetection (PD) signal activates the inductor peak current detector(IPCD). As CP is discharged, the inductor current reaches its peak whenVL1 become 0 V, and this condition is detected by the IPCD. Then S1 andS2 are turned off, and S3 and S4 are turned on to transfer the energyfrom the inductor to the battery (④ in Fig. 3b). As the inductor currentbecome 0, a harvesting cycle is completed, and all circuits are turned offexcept WTC, which is needed to detect the next input.

Fig. 3c shows the measurement results for the generated energy and

voltage versus the explicit load capacitance. For these measurements,the load capacitance was connected to the PZT harvester while theEEEC was not connected, and the generated energy values at the end ofa single bending/unbending operation of the f-PEH were measured.This graph verifies that the energy generated by a single bending/un-bending operation of the PZT harvester decreases as the CLOAD seen bythe energy harvester is increased. The generated peak voltage also de-creases as load capacitance is increased, as discussed earlier.

Measurements were performed with the test setup described in theearlier sections. Current flowing from the inductor to the battery ismeasured to calculate the harvested energy per pulse, which is referredas EHRV in this paper. Energy consumed during the EEEC operation istermed as ELOSS. For comparison with the conventional circuits, a FBR-based harvesting circuit was utilized. In this kind of circuit, only FBR isconnected to the PZT to rectify its voltage and output of the FBR isdirectly fed to a battery. Energy harvested to the battery in this circuit istermed as EFBR in this paper. Referring to IBAT and VBAT shown in theFig. 3a, EHRV and EFBR can be defined as

∫

∫

=

=

E V I dt

E V I dt

HRV tt

BAT BAT

FBR tt

BAT BAT

if

if

where ti and tf stands for the time when the energy transfer to batterystarted and finished, respectively.

The f-PEH can be attached to any object, such as cloths and textiles,for powering wearable electronics. Therefore, the output of the f-PEHcan be random, as shown in Fig. 4a, with varying amplitude and po-larity at different instants of time. As highlighted in Fig. 4a, the pro-posed harvesting circuit is designed not to be activated for small inputpulses. This is because the overhead for harvesting circuit activationcould be significant and greater than the amount of energy it can har-vest from small pulses. In such a scenario, the net harvested energy canbe negative, and hence activation is prevented. The measured wave-form confirms that the EEEC is only activated when sufficiently largevoltage is accumulated at the output (VPZT), and the harvested energy istransferred to the inductor when VL1> 0 and transferred to the batterywhen VL2> 0.

A bending machine was used to mimic the human motion forbending the f-PEH, and its operation is illustrated in Fig. 4b. The flex-ible PZT harvester is gradually bent with displacement Δd for a shortduration of time Δt. Then, for the performance analysis, the averagebending speed S is defined as

=∆

∆

dt

S

The measurements were carried out for the bending speeds rangingfrom 0.64 cm/s to 2.6 cm/s due to the limited Δt ranging from 20ms to990ms with the bending machine. However, energy transfer to batteryat the end of bending operation, which can be detected by PZT outputvoltage peak detection with PVD, only takes a few µs to complete.Therefore, theoretically, harvesting with high frequency pulses as shortas a few hundreds of µs is possible.

Fig. 4c shows how the amount of harvested energy per pulse variesfor different bending speeds (S) and displacements (Δd) applied to thePZT material. The amount of harvested energy (without subtractingELOSS) increases in general as bending speed increases, since lesscharges leak away during bending process. Slight decrease in amount ofharvested energy is observed with the highest bending speed set up,since the harvesting circuit was configured for optimized operationwith medium bending speed. Harvesting with the highest bendingspeed could be also improved with adjusting circuit configuration butdetails are excluded here since the circuit detail is beyond the scope ofthis paper. Fig. 4d depicts the amount of energy harvested with theproposed harvesting circuit (EHRV-ELOSS) and a conventional FBR circuit(EFBR) for different bending displacements. In this figure, the amount ofenergy consumed for harvesting circuit operation is taken into account;

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hence, EHRV-ELOSS refers to the net energy harvested and stored to thebattery. Thanks to the maximized energy extraction of the proposedharvesting circuit, EHRV-ELOSS is higher than EFBR for the entire bendingdisplacement range. The amount of energy harvested by the proposedharvesting circuit is increased to 408% on average compared to thatobtained using an FBR. Peak efficiency is achieved when the bendingspeed is 2.6 cm/s and the bending displacement is 2.5 mm while using3 V battery voltage. While the amount of energy generated by FBR is0.262 µJ at this point, that of proposed EEEC is 1.3 µJ, which is en-hanced to 495%. Such improvement is significant compared with 205%reported in [42], and 269% reported in [43].

4. Conclusion

A new energy-harvesting circuit for an f-PEH is proposed to enableenergy harvesting from irregular human motion. Instead of using aconventional impedance-matching approach, energy extraction is sig-nificantly enhanced to 408% on average by minimizing the capacitiveload seen by f-PEH. The proposed EEEC also maximizes output voltageup to 101 V with extremely low static power consumption (1.15 nW).The f-PEH with total thickness of 170 µm is utilized, which gives en-ough flexibility for attachment on clothes or human skin. The f-PEHbased on PZT material generated an open-circuit voltage of 165 V and ashort-circuit current of 1.5 μA through mechanical bending motion on alinear stage. Thanks to the proposed EEEC, this result increases energyextraction up to 495% than that of conventional full bridge rectifier

Acknowledgement

This study was supported by the Center for Integrated Smart Sensorsfunded by the Ministry of Science and ICT as Global Frontier Project(CISS-2016M3A6A6929958) and National Research Foundation (NRF)of Korea funded by the Ministry of Science, ICT and Future Planning(Grant No. NRF-2017R1A2A1A18071765). This work was also sup-ported by KAIST Wearable Platform Material Technology Center(WMC) (NRF-2016R1A5A1009926). Authors thank Woosung Jo atKAIST for his contribution on PZT material mechanical property mea-surements.

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2019.01.049.

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[16] G.T. Hwang, Y. Kim, J.H. Lee, S. Oh, C.K. Jeong, D.Y. Park, J. Ryu, H. Kwon,S.G. Lee, B. Joung, D. Kim, K.J. Lee, Self-powered deep brain stimulation via aflexible PIMNT energy harvester, Energy Environ. Sci. 8 (2015) 2677–2684,https://doi.org/10.1039/c5ee01593f.

[17] G.T. Hwang, V. Annapureddy, J.H. Han, D.J. Joe, C. Baek, D.Y. Park, D.H. Kim,J.H. Park, C.K. Jeong, K. Il Park, J.J. Choi, D.K. Kim, J. Ryu, K.J. Lee, Self-powereddevices: self-powered wireless sensor node enabled by an aerosol-deposited PZTflexible energy harvester (Adv. Energy Mater. 13/2016), Adv. Energy Mater. 6(2016) 1–9, https://doi.org/10.1002/aenm.201670080.

[18] J.H. Han, K.M. Bae, S.K. Hong, H. Park, J.H. Kwak, H.S. Wang, D.J. Joe, J.H. Park,Y.H. Jung, S. Hur, C.D. Yoo, K.J. Lee, Machine learning-based self-powered acousticsensor for speaker recognition, Nano Energy 53 (2018) 658–665, https://doi.org/10.1016/j.nanoen.2018.09.030.

[19] J.H. Han, J.H. Kwak, D.J. Joe, S.K. Hong, H.S. Wang, J.H. Park, S. Hur, K.J. Lee,Basilar membrane-inspired self-powered acoustic sensor enabled by highly sensitivemulti tunable frequency band, Nano Energy 53 (2018) 198–205, https://doi.org/10.1016/j.nanoen.2018.08.053.

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[21] H.S. Wang, C.K. Jeong, M.H. Seo, D.J. Joe, J.H. Han, J.B. Yoon, K.J. Lee,Performance-enhanced triboelectric nanogenerator enabled by wafer-scale nano-grates of multistep pattern downscaling, Nano Energy 35 (2017) 415–423, https://doi.org/10.1016/j.nanoen.2017.04.012.

[22] C.K. Jeong, K.M. Baek, S. Niu, T.W. Nam, Y.H. Hur, D.Y. Park, G.T. Hwang,M. Byun, Z.L. Wang, Y.S. Jung, K.J. Lee, Topographically-designed triboelectricnanogenerator via block copolymer self-assembly, Nano Lett. 14 (2014)7031–7038, https://doi.org/10.1021/nl503402c.

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[25] C.K. Jeong, J. Lee, S. Han, J. Ryu, G.T. Hwang, D.Y. Park, J.H. Park, S.S. Lee,M. Byun, S.H. Ko, K.J. Lee, A hyper-stretchable elastic-composite energy harvester,Adv. Mater. 27 (2015) 2866–2875, https://doi.org/10.1002/adma.201500367.

[26] M. Lee, C.Y. Chen, S. Wang, S.N. Cha, Y.J. Park, J.M. Kim, L.J. Chou, Z.L. Wang, Ahybrid piezoelectric structure for wearable nanogenerators, Adv. Mater. 24 (2012)1759–1764, https://doi.org/10.1002/adma.201200150.

[27] Z. Li, G. Zhu, R. Yang, A.C. Wang, Z.L. Wang, Muscle-driven in vivo nanogenerator,Adv. Mater. 22 (2010) 2534–2537, https://doi.org/10.1002/adma.20090435.

[28] D.H. Kim, H.J. Shin, H. Lee, C.K. Jeong, H. Park, G.T. Hwang, H.Y. Lee, D.J. Joe,J.H. Han, S.H. Lee, J. Kim, B. Joung, K.J. Lee, In vivo self-powered wirelesstransmission using biocompatible flexible energy harvesters, Adv. Funct. Mater. 27(2017) 1–8, https://doi.org/10.1002/adfm.201700341.

[29] Y.H. Shin, I. Jung, M.S. Noh, J.H. Kim, J.Y. Choi, S. Kim, C.Y. Kang, Piezoelectricpolymer-based roadway energy harvesting via displacement amplification module,Appl. Energy 216 (2018) 741–750, https://doi.org/10.1016/j.apenergy.2018.02.074.

[30] C.K. Jeong, S.B. Cho, J.H. Han, D.Y. Park, S. Yang, K. Il Park, J. Ryu, H. Sohn,Y.C. Chung, K.J. Lee, Flexible highly-effective energy harvester via crystallographicand computational control of nanointerfacial morphotropic piezoelectric thin film,Nano Res. 10 (2017) 437–455, https://doi.org/10.1007/s12274-016-1304-6.

[31] S. Lee, S.H. Bae, L. Lin, Y. Yang, C. Park, S.W. Kim, S.N. Cha, H. Kim, Y.J. Park,Z.L. Wang, Super-flexible nanogenerator for energy harvesting from gentle windand as an active deformation sensor, Adv. Funct. Mater. 23 (2013) 2445–2449,https://doi.org/10.1002/adfm.201202867.

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[34] C. Dagdeviren, P. Joe, O.L. Tuzman, K. Il Park, K.J. Lee, Y. Shi, Y. Huang,J.A. Rogers, Recent progress in flexible and stretchable piezoelectric devices formechanical energy harvesting, sensing and actuation, Extrem. Mech. Lett. 9 (2016)269–281, https://doi.org/10.1016/j.eml.2016.05.015.

[35] K. Il Park, M. Lee, Y. Liu, S. Moon, G.T. Hwang, G. Zhu, J.E. Kim, S.O. Kim,D.K. Kim, Z.L. Wang, K.J. Lee, Flexible nanocomposite generator made of BaTiO3nanoparticles and graphitic carbons, Adv. Mater. 24 (2012) 2999–3004, https://doi.org/10.1002/adma.201200105.

[36] K. Il Park, C.K. Jeong, J. Ryu, G.T. Hwang, K.J. Lee, Flexible and large-area na-nocomposite generators based on lead zirconate titanate particles and carbon na-notubes, Adv. Energy Mater. 3 (2013) 1539–1544, https://doi.org/10.1002/aenm.201300458.

[37] C.K. Jeong, K. Il Park, J. Ryu, G.T. Hwang, K.J. Lee, Large-area and flexible lead-free nanocomposite generator using alkaline niobate particles and metal nanorodfiller, Adv. Funct. Mater. 24 (2014) 2620–2629, https://doi.org/10.1002/adfm.201303484.

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implantable biomedical applications, Nano Energy 1 (2012) 145–151, https://doi.org/10.1016/j.nanoen.2011.07.001.

Muhammad Bilawal Khan received his B.S. degree inElectrical Engineering from National University ofComputer and Emerging Sciences, Islamabad, Pakistan in2014. He is currently a M.S/Ph.D. candidate inSungkyunkwan University, South Korea. His research in-terests include energy harvesting system design and lowpower circuit design.

Dong Hyun Kim received his B.S. degree in MaterialsScience and Engineering (MSE) from SungkyunkwanUniversity in 2015 and M.S. degree from KAIST in 2017. Heis currently working toward his Ph.D. at KAIST under thesupervision of Prof. Keon Jae Lee. His doctoral researchinterests include energy harvesting system, flexible elec-tronics, and biomedical applications.

Jae Hyun Han received his B.S. degree in Materials Scienceand Engineering (MSE) from Sungkyunkwan University in2014 and M.S. degree from KAIST in 2016. He is currentlyworking toward his Ph.D. at KAIST under the supervision ofProf. Keon Jae Lee. His doctoral research interests includepiezoelectric and triboelectric energy harvesting, flexibleacoustic sensors, and laser-material interaction.

Hassan Saif received his B.S. degree in ElectricalEngineering at University of Engineering and Technology,Lahore, Pakistan in 2012. He is currently a M.S/Ph.D.candidate in Sungkyunkwan University, South Korea. Hisresearch interests include low power switch capacitor DC-DC converters and energy harvesting system design.

Hyeonji Lee received her M.S. degree in SemiconductorDisplay Engineering and B.S. degree in SemiconductorSystems Engineering in Sungkyunkwan University, SouthKorea. She is currently an image sensor circuit designer inSamsung Electronics, South Korea. Her research interestincludes ultra-low power energy harvesting system and CISdesign.

Minsun Kim received his B.S degree in information andcommunication engineering from the Hansung University,Seoul, South Korea in 2015. He received the MS degree inElectronic, electrical and computer engineering fromSungkyunkwan University, South Korea. His research in-terests include low-power circuit design and security circuitdesign.

Yongmin Lee received the B.S degree in biomedical en-gineering from Konyang University, Daejeon, South Korea,in 2016. He is currently pursuing the M.S degree inElectronic, Electrical and Computer Engineering fromSungkyunkwan University, Suwon, South Korea. His re-search interests includes low-power circuit design and di-gital VLSI design.

Eunsang Jang received his B.S. degree in semiconductorsystems engineering from Sungkyunkwan University, SouthKorea in 2016. He received his M.S. degree in electrical andcomputer engineering from Sungkyunkwan University,South Korea in 2018. His research interest includes lowpower integrated circuit and power management IC.

Seong Kwang Hong received his Ph.D. degree in ElectricalEngineering at Hanyang University in 2017. Currently, heworks with Prof. K. J. Lee as a postdoctoral research as-sociate in the Department of Materials Science andEngineering (MSE) at Korea Advanced Institute of Scienceand Technology (KAIST). His research interests includeenergy harvesting system, flexible electronics, and sensorapplications.

Daniel J. Joe received a B.S. degree in ElectricalEngineering at the University of Illinois at UrbanaChampaign (UIUC) in 2008 and a Ph.D. degree in Electricaland Computer Engineering at Cornell University in 2014.Currently, he is a BK21 Plus postdoctoral research associatein the Department of Materials Sciences and Engineering atKAIST. His current research interests involve various flex-ible thin-film materials and devices including energy har-vesters, acoustic sensors, pressure sensors, wireless powertransfer system, etc.

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Prof. Taek-Soo Kim received the B.S. degree in mechanicalengineering from Yonsei University, Seoul, South Korea, in2001, and the M.S. and Ph.D. degrees in mechanical en-gineering from Stanford University, Stanford, CA, USA, in2006 and 2010, respectively. He is currently an AssociateProfessor of mechanical engineering with the KoreaAdvanced Institute of Science and Technology, Daejeon,South Korea, and also the Director of the AdvancedPackaging and Thin Film Laboratory. His current researchinterests include the mechanics-related subjects of ad-vanced packaging and thin films: adhesion, deformation,fracture, fatigue, reliability, and stress- and strain-inducedmultiphysics phenomena.

Prof. Keon Jae Lee received his Ph.D. in Materials Scienceand Engineering (MSE) at University of Illinois, Urbana-Champaign (UIUC). During his Ph.D. at UIUC, he involvedin the first co-invention of “Flexible Single-crystallineInorganic Electronics”, using top-down semiconductors andsoft lithographic transfer. Since 2009, he has been a pro-fessor in MSE at KAIST. His current research topics are self-powered flexible electronic systems including energy har-vesting/storage devices, IoT sensor, LEDs, large scale in-tegration (LSI), high density memory and laser materialinteraction for in-vivo biomedical and flexible application.

Prof. Yoonmyung Lee received the B.S. degree in elec-tronic and electrical engineering from the PohangUniversity of Science and Technology, Pohang, SouthKorea, in 2004, and the M.S. and Ph.D. degrees in electricalengineering from the University of Michigan, Ann Arbor,MI, USA, in 2008 and 2012, respectively. From 2012–2015,he was with the University of Michigan as a ResearchFaculty. In 2015, he joined Sungkyunkwan University,Suwon, South Korea, as an Assistant Professor. His currentresearch interests include energy-efficient integrated circuitdesign for low-power high-performance very large scaleintegration systems and millimeter-scale wireless sensorsystems.

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