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Nano Energy

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Wireless powered wearable micro light-emitting diodes

Han Eol Leea,1, Daewon Leea,1, Tae-Ik Leeb, Jung Ho Shina, Gwang-Mun Choia, Cheolgyu Kimb,Seung Hyung Leea, Jae Hee Leea, Yong Ho Kima, Seung-Mo Kanga, Sang Hyun Parka, Il-Suk Kangc,Taek-Soo Kimb, Byeong-Soo Baea,⁎, Keon Jae Leea,⁎

aWearable Platform Materials Technology Center, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of KoreabDepartment of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of KoreacNational Nanofab Center, 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:Wireless power transferWearable microLEDMicroLEDWearable display

A B S T R A C T

Wearable displays are considered as a bilateral communication tool in the hyperconnected era. Although severalelectronic clothing display was demonstrated, high power consumption issue still remained. Here, we presentwireless powered wearable μLEDs (WμLEDs) with excellent stability. A custom-designed wireless power supplysystem operated a 30× 30 WμLED array on a fabric. The WμLED stability was intensively investigated underbending, stretching, 85 °C/85% relative humidity and artificial sunlight conditions, showing the suitability ofWμLED outdoor usage. Finally, a passive-matrix WμLED display successfully emitted a brilliant red glow on afabric.

1. Introduction

With the upcoming internet of things (IoT) era, wearable smartdevices have been spotlighted as a powerful info-communication toolfor new human-machine interfaces (HMIs) [1–8]. Wearable displayshave been proposed as a key technology for a bilateral visual commu-nication in hyper-connected society [4–6,9,10]. For example, electronicclothing displays are a potential candidate for ideal wearable platforms,because of human-friendly, ubiquitous and hands-free properties[11–13]. Several researchers have explored electronic clothing displaysusing organic light-emitting diodes (OLEDs) [14–17], weavable light-emitting fiber [18], and packaged light-emitting diode chips [12,19].However, these displays have drawbacks including high power con-sumption and low device stability (e.g., mechanical, thermal, andchemical instability) in outdoor usage. Recently, inorganic thin-filmmicroLEDs (μLEDs) have attracted significant attentions for next-gen-eration displays, due to their high power efficiency and excellent sta-bility [20–24]. Our research group has developed high-performanceflexible vertical μLEDs (f-VLEDs) for display and biomedical applica-tions, such as an optogenetic stimulator and a trichogenic treatmentdevice [25–27]. Nevertheless, the μLED power efficiency and stabilities(e.g., mechanical, chemical, thermal, humidity, and photo resistances)

have not been investigated under harsh wearable and environmentalconditions.

Flexible energy sources and self-powered systems are the crucialfactor for wearable applications, considering their battery-free, light-weight, flexible and portable properties [28–31]. Various flexiblepower sources such as piezoelectric nanogenerators (NGs) [32–36],triboelectric nanogenerators (TENGs) [37–42], and flexible bat-tery [43] have been reported for providing energy to flexible electronicsystems. However, these methods had obstacles of self-sufficient effi-ciency and limited operation time. Another approach for flexible powersource is a wireless power transfer, which has been demonstrated inflexible optoelectronics [27], drug delivery devices [44] and healthcaresensors [45]. However, the suitability of a wearable wireless energysource has not been studied on fabrics, which is essential for next-generation wearable energy and self-powered system.

Herein, we report wireless powered wearable μLEDs (WμLED) withoutstanding device stability. A wearable wireless energy transfer systemwas custom-designed for WμLEDs, and fabricated on a 100% cottonfabric, integrated with a 30×30 WμLED array. A thin-film based μLEDarray, which was transferred by transparent elastomeric adhesive(TEA), was successfully operated on a fabric for flexible wireless elec-trical power, stored in a portable battery. The TEA-based WμLED

https://doi.org/10.1016/j.nanoen.2018.11.017Received 19 September 2018; Received in revised form 22 October 2018; Accepted 6 November 2018

⁎ Correspondence to: Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu,Daejeon 34141, Republic of Korea.

E-mail addresses: bsbae@kaist.ac.kr (B.-S. Bae), keonlee@kaist.ac.kr (K.J. Lee).1 These authors contributed equally to this work.

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transfer process was theoretically investigated by a finite elementanalysis (FEA) simulation to optimize the compressive force withoutdevice breakdown. The mechanical stability of WμLEDs was evaluatedby a peel-off test, a 10%-stretching test, and a periodic bending fatiguetest. In addition, our WμLEDs showed excellent stability under 85 °C/85% relative humidity (RH) and artificial sunlight conditions, which isbeneficial for outdoor usage of WμLEDs. Finally, a passive-matrix (PM)WμLED display stably emitted a brilliant red light on a lab coat, ex-pressing the letters “K”, “A”, “I”, “S” and “T”. These results indicate thatthe hybrid power supply, which combines this wireless power transferwith self-powered energy source, can play an important role for self-sustainable and wearable electronic system.

2. Results

2.1. Fabrication of wireless powered wearable microLEDs (WμLEDs)

Battery-free μLEDs on fabric are significant for a wearable display indaily life without the periodic power recharging. To supply electricalpower to WμLEDs, a wearable wireless power transmission/receptionsystem was constructed by an exclusive circuit design, consisting offlexible antennas (f-antennas), a peak rectifier, and a voltage regulator,as depicted in the circuit diagram of Fig. 1a. Wireless energy transfer ona fabric was enabled by resonant inductive coupling of two f-antennas,using a frequency of 13.56MHz, which is generally used in industrial,scientific and medical (ISM) applications [27,28,30]. The alternatingcurrent (AC) energy source supplied radio frequency (RF) power to thewearable transmitter antenna (w-transmitter) with 13.56MHz fre-quency. Wireless power from the w-transmitter was collected by thewearable receiver antenna (w-receiver), and simultaneously trans-mitted to the peak rectifier for converting AC to direct current (DC)power. The converted DC power was applied to WμLEDs through thevoltage regulator, which prevents device breakdown by excessiveelectrical power. Fig. 1a schematically illustrates the fabrication pro-cess and wireless operation of WμLEDs on a fabric substrate. The de-tailed procedure is as follows: i) Thin-film AlGaInP μLEDs were fabri-cated on a rigid GaAs wafer, as described in the Experimental section[25]. The μLEDs and f-antenna were completely bonded with a 100%cotton fabric by a transparent elastomeric adhesive (TEA, thick-ness= 25 µm), which had been used for final passivation material indisplay industry. The pressure-sensitive TEA strongly attached theμLEDs and f-antenna to the fabric using compressive transfer equip-ment. The attached μLED devices were revealed by selective wetetching of a GaAs mother wafer by a citric acid (C6H8O7, 40 wt%)/hydrogen peroxide (H2O2, 15 wt%) solution. ii) Top contact holesand Au electrodes were deposited on the top surface of the fabric-bonded μLEDs. Finally, WμLEDs were passivated by an epoxy-basedpolymer resin with 5 µm thickness. iii) WμLEDs were interconnectedwith a wearable antenna, enabling wireless power transfer from anexternal portable energy source.

Fig. 1b presents the demonstrated wireless 30×30 μLEDs on afabric, which were stably operated by the power supply system withnear-field inductive coupling. As shown in Fig. 1c, the transmissionpower efficiency was analyzed as a function of the distance between thef-transmitter and f-receiver. The normalized power transmission ratiowas nonlinearly reciprocally proportional to the interdistance of twoantennas (EN =VRX/VTX×100, where EN is the normalized efficiency,VRX is the voltage of f-receiver, and VTX is the voltage of f-transmitter).After wireless power transmission at 1 cm antenna-antenna distance,the input AC energy with an amplitude of 12 V and a frequency of13.56MHz was successfully transformed to DC energy with an ampli-tude of 4 V by the AC-DC power conversion, as shown in the inset ofFig. 1c. These results indicate that our wearable wireless power supplysystem is suitable to freely operate wearable μLEDs without direct in-terconnection of additional batteries.

2.2. WμLED transfer to fabric using transparent elastomeric adhesive (TEA)

For rapid, large-scale and continuative fabrication of a WμLEDdisplay, a μLED transfer apparatus was customized with a triaxialtranslational stage, a sub-10 µm alignment system, and a load cell forfine control of applied pressure, as shown in Fig. 2a. The TEA-basedWμLED transfer was conducted by transfer equipment as follows: i) ATEA-laminated fabric was loaded on a lower translational stage, andμLEDs on a rigid wafer were attached to an upper load-cell by vacuum.A special microscope, which was placed between the stage and loadcell, detected the exact position of the μLEDs and fabric by simulta-neously overlapping the upper/lower images (an error margin within5 µm). The x- and y-axis location of the fabric substrate was reposi-tioned by a translational stage with a sub-10 µm error range, based onthe analyzed two axis displacement by a special microscope. ii) Highpressure of 90 kPa was applied to the aligned μLEDs and TEA-attachedfabric for 1min, enabling strong, stable, and reliable bonding. The TEAtransfer/bonding medium was carefully analyzed in terms of its optical,thermodynamic, and mechanical properties. As shown in the ultra-violet-visible (UV–vis) spectrum of Fig. 2b, the TEA exhibited superioroptical transparency of> 97.4% in the entire visible light region(wavelength of 400–700 nm), due to its ultrathin 25 µm thickness andhigh clarity of the acrylic TEA material. The inset image is a photographof a TEA-bonded textile, showing the distinct black color of the textilebehind the TEA. Fig. 2c presents the thermogravimetric analysis (TGA)profile of the TEA, which was measured with a ramping rate of 5 °Cmin−1 under a N2 atmosphere. The TEA exhibited a noticeable thermaldecomposition after 245 °C, and its 5% weight loss temperature (Td5%)was 284 °C. This result implies that the thermally stable TEA can retainits bonding between the fabric and the heated μLEDs without anythermal deformation or delamination under high temperature en-vironment.

Mechanically stable bonding of the WμLEDs on a fabric is essentialfor daily self-powered wearable applications without device break-down, cracking or exfoliation. To enhance the bonding stability of thewearable wireless μLED system, the TEA-based WμLED transfer/bonding process was optimized on the basis of a finite element analysis(FEA) simulation. Fig. 2d(i) shows a schematic illustration of thepressure-based WμLED array transfer, which was modeled for the the-oretical calculations. Fig. 2d(ii) depicts the calculated stress distribu-tion of 10×10 WμLEDs (50×50 µm2 chip size) during the TEAbonding process. When WμLEDs with TEA were compressed by theexerted pressure of 90 kPa, the stress on a unit WμLED chip rose to anaverage of ~ 0.4 GPa and a maximum of ~ 0.7 GPa, as shown in Fig. 2d(iii) (a WμLED at the right-top edge). Fig. 2e presents the calculatedstress profile of the right-top edge WμLED in the 10×10 array. Com-pression force of ~ 0.7 GPa was concentrated at the edges of a LED chip.Despite the maximum stress value of 0.7 GPa, the WμLEDs maintainedtheir shape without any cracks or destructions, due to the high fracturestress of the LED chips of about 2 GPa [46]. Fig. 2f is a photograph of aWμLED device on a fabric substrate, emitting a stable red glow inbending condition. The inset of Fig. 2f presents a cross-sectional scan-ning electron microscopy (SEM) image showing that the compressedTEA was deeply embedded in the fabric, binding the thin-film WμLEDchip with numerous threads. According to these results, the TEA caninduce powerful bonding for the fabric WμLED and the self-powereddevices by the optimized transfer/bonding process without breakdownor exfoliation.

2.3. Mechanical stability of WμLEDs

The mechanical properties of the WμLED system were scrutinizedfor wearable applications by several analyses, including a tensile test,peel adhesion test, DIC analysis and bending fatigue test. Fig. 3a(i) is aphotograph of the tensile test, pulling the rectangular-shaped unitWμLED by two grips uniaxially. Fig. 3a(ii) is the engineering stress-

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strain curve of the bare fabric (100% cotton) sample and a WμLED,showing typical curves of textile materials [19,47]. According to themeasurement results, the WμLED had a tensile strength of 54.4MPa andan elastic modulus of 0.4 MPa, indicating no significant mechanicaldegradation during the TEA-based WμLED fabrication process (TableS2). Bonding reliability of the unit WμLED was evaluated by a peel-offtest to observe the TEA peeling force from the fabric, as shown inFig. 3b. The upper inset of Fig. 3b is a setup image of the custom-de-signed 90 degree tape peel-off test, where the TEA is perpendicularlydetached with two actuators with a constant speed of 1mm/sec. Theaverage force of the TEA/fabric delamination was 5.27 N during thepeeling process, resulting in adhesion strength of the TEA-bonded fabricof 3.29 N/cm, as presented in the lower inset of Fig. 3b. This resultimplies that the TEA has high bonding strength with the fabric, which iscomparable with those of the conventional flat substrates such as glassand polyethylene terephthalate (PET). To verify the stretching stabilityof WμLEDs, a digital image correlation (DIC) analysis was experimen-tally performed. Fig. 3c(i) shows the three-dimensional DIC

measurement setup for quantifying the stretching deformation values ofunit WμLED. This method is an advanced visual analysis tool for ex-amining the device deformation degree by comparing the digital imagesbefore and after reliability test [48,49]. The unit WμLED device wasrandomly marked by speckle patterns as a reference indicator beforestretching, and uniaxially stretched in the x- (Fig. 3c(ii)) and y- direc-tions (Fig. 3c(iii)). The DIC algorithm was utilized to calculate and vi-sualize the applied strain by analyzing the positional changes of thespeckle patterns. The stretched WμLEDs exhibited negligible axial strainof< 2% in both x- and y-directions without any device breakdown ordelamination. The stretching stability of our WμLED indicates thatwireless-powered μLEDs can be applied to commercial clothing andself-powered system with various stretching motions, including waist,elbow, and knee motions. Fig. 3d presents the remarkable mechanicaldurability of the WμLEDs during the bending/unbending fatigue testwith a bending curvature radius of 2.5 mm. Despite harsh 100,000bending cycles, the forward voltage (Vf) increased by 0.76 V, and theirradiance (E) decreased only by 1.33mWmm−2 without device

Fig. 1. (a) Schematic illustration of wireless powered wearable μLEDs. The bottom inset is a schematic circuit diagram of the wireless power transmission/receptionsystem composed of flexible transmitter/receiver antennas, peak rectifier, and voltage regulator. (b) A photograph of 30×30 thin film-based WμLEDs stitched on afabric substrate. (c) The normalized power transfer efficiency of the wireless power supply system as a function of the antenna-antenna distance. The inset is a graphof time versus the input AC power (frequency of 13.56MHz) and the transferred DC power.

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exfoliation, cracking or breakdown. This outstanding mechanical sta-bility is attributed to the powerful adhesion force among the wirelesspowered μLED system, TEA and fabric substrate. On the basis of theseresults, the WμLEDs with superior mechanical stability are suitable forhuman-interface applications, such as wearable healthcare sensors,phototherapeutic devices and μLED displays operated by self-poweredflexible energy harvester.

2.4. Chemical, thermal, humid and photo stabilities of WμLEDs

Fig. 4a exhibits the I-V characteristics of WμLEDs with LED chips of

different size ranging from 50×50 to 300×300 µm2. As shown inthese curves, the Vf of the different-sized WμLEDs was ~ 2.8 V on fabricwithout degradation of electrical properties, indicating sufficient op-eration voltage by either wireless power transfer or self-powered energyharvester [23–27]. The electrical property changes of the WμLED weregenerally related to thermal degradation, internal resistance and LEDchip size [23–26]. Regardless of the chip size, however, our WμLEDscan minimize the device thermal degradation with low operatingpower, due to the porous nature of the fabric that enables continuousair circulation cooling of the heated LEDs [50–52]. Fig. 4b is a photo-graph of 30× 30 WμLEDs in a detergent solution, emitting a high-

Fig. 2. (a) Photographs of the customized μLED transfer equipment, comprising a triaxial translational stage, sub-10 µm alignment system, and high-pressure loadcell. i) The μLEDs and TEA-laminated fabric are attached by vacuum on the upper load cell and lower stage, respectively. ii) The μLEDs and TEA-laminated fabric arecompressed by high-pressure of 90 kPa, and bonded with each other by the deformed TEA. (b) UV–Vis spectrum of the TEA. The inset image presents the TEA-attached fabric, displaying the transparency of the TEA. (c) TGA curve for the TEA under a N2 atmosphere. Note that the Td5% for the TEA is indicated by dotted lines.(d) i) Schematic illustration of pressure-induced transfer/bonding process for transferring the μLED array to the fabric substrate. ii) Stress distribution by FEMsimulation when the 10× 10 μLED array is transferred to the TEA-laminated fabric. iii) Magnified image of the μLED stress distribution, which is at the right-top edgeof the μLED array. (e) Calculated stress profile as a function of the thin-film μLED chip dimensions (in the x-direction). The inset image is a cross-sectional stressdistribution image of the compressed μLED chip, showing the pressure concentration at two apexes. (f) A photograph of a thin film-based WμLED on 100% cottonfabric, emitting red light under the bent state. The inset shows a cross-sectional SEM image of the WμLED.

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power red glow with high electrical energy efficiency. The chemicallystable TEA maintained powerful bonding between the thin-film μLEDand fabric in the laundry detergent solution for ~ 40min, indicatingthat our wireless-powered μLEDs can be washed using ordinary laundrydetergents.

The WμLED system under humidity, heat, and light is an importantfactor for outdoor applications, including wearable displays, smart self-powered devices, and biomedical sensors. The stability of the WμLEDswas experimentally investigated by an accelerated stress tests with85 °C/85% relative humidity (85/85 test) and an artificial sunlight (Q-SUN test). WμLEDs were driven in a temperature/humidity-controlledchamber, increasing the erosion and oxidation rate of the WμLEDs[53–55]. Fig. 4c displays the 85/85 test results of the WμLEDs during12 consecutive days, showing slight changes in both Vf and irradianceduring the initial six days. As the 85/85 test proceeded, Vf pro-portionally increased to 5.3 V, while the irradiance decreased to19mWmm−2. Theses accelerated humidity/thermal tests indicate thelifetime of WμLEDs was ~42% longer than that of our previously re-ported flexible μLEDs [27]. Fig. 4d presents the Fourier-transform in-frared (FT-IR) spectra of the WμLED during the 85/85 test for 12 days,which allows us to analyze the structural changes of the TEA. Duringthe accelerated stress test, the FT-IR spectrum of TEA retained its ori-ginal chemical structure, and showed only negligible peak formations ata wavelength of 3280 cm−1 (–OH stretching vibrations, marked bygreen region) and 1630 cm−1 (H-O-H bending vibrations, marked bypurple). These peaks were attributed to moisture absorption of the TEAdue to the high humidity conditions of the 85/85 test, resulting in shapedeformation and performance degradation of the WμLEDs after six days[56,57]. Fig. 4e displays the luminance-current-voltage (L-I-V) char-acteristics of the WμLED before and after a Q-SUN test for 72 h. The Q-SUN test was conducted in an isolated test chamber with artificialsunlight generated by a xenon (Xe) arc lamp with an irradiance of

0.34Wm−2 at 64 °C, as depicted in the inset of Fig. 4e. After the arti-ficial sunlight irradiation, the WμLEDs presented an increased Vf of0.3 V and a decreased irradiance of 2mWmm−2. As shown in Fig. 4f, aFT-IR analysis was carried out to confirm the photostability of theWμLED, based on the changes in the chemical structure of the TEA. Ingeneral, acrylic TEA polymers exhibit good photooxidation resistancebecause they do not absorb UV light in the range of 295–400 nm due tothe absence of inherent chromophores [58]. Therefore, after the arti-ficial sunlight exposure for 72 h, the TEA retained its inherent chemicalstructure. Notably, there are negligible peak generations in the –OHstretching (centered at 3160 cm−1) and C˭O stretching (centered at1710 cm−1) vibrational modes in the FT-IR spectrum, indicating pho-tooxidiation of the measured materials [58]. Our wireless-poweredμLED offers considerable potential for wearable outdoor display appli-cations, due to its excellent environmental stabilities.

2.5. Wearable μLED display

To confirm the practical applications of a wearable μLED display, a10× 10 WμLED array was stitched onto the pocket of a lab coat, asshown in Fig. 5a. Fig. 5b depicts the circuit diagram of the passive-matrix (PM) WμLEDs using a common cathode configuration [59]. Theoperation circuit modulated the voltage of one cathode row and allanode columns for expressing a word. As shown in Fig. 5c-g, WμLEDssuccessfully emitted brilliant red light on ordinary clothes, presentingthe letters “K”, “A”, “I”, “S” and “T” sequentially. These results de-monstrate that wireless-powered WμLEDs have significant potential fornext-generation wearable and self-powered μLED displays.

3. Conclusions

In summary, we have developed a wireless powered wearable μLED

Fig. 3. (a) Tensile test results from the WμLED and fabric. i) A photograph presenting the configuration of the tensile test. ii) Engineering stress-strain curves for theWμLED and fabric. Note that the inset panel summarized the average elastic modulus and tensile strength values for the WμLED and fabric. (b) Peel-off test results ofthe thin film-based WμLED on fabric. The upper inset is a photograph of the peel-off test. The lower inset is the peel strength comparison of the WμLED on glass, PET,and the fabric substrate. (c) i) Photograph of a 3D-DIC deformation measurement setup using a stereo-vision system. DIC analysis of the WμLED on fabric withuniaxial strain in ii) x- and iii) y-direction. (d) The forward voltage and irradiance of the WμLED during a durability test with 100,000 bending motions (bendingradius of 2.5 mm).

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display using a simple and rapid TEA-based transfer/bonding process. Awireless energy transfer system was successfully realized by high-effi-ciency resonant power transfer with 13.56MHz frequency, stably op-erating a 30× 30 WμLED array on a fabric substrate. Theoretical FEAsimulations and the development of customized transfer equipmentenabled TEA-based transfer of the WμLED and a wearable powertransfer system on clothing. The bonding stability of the WμLEDs wasexperimentally measured by a tensile test and a peel-off test, showinghigh tensile strength of ~ 58MPa and adhesion strength of ~ 3.3 N/mm. The TEA-bonding effect in WμLEDs was investigated by a 10%stretching test and a 100,000 bending durability test, showing

sustainability in harsh conditions of daily use (e.g., wrinkling, twisting,and stretching). In addition, WμLEDs exhibited outstanding devicestability under 85 °C/85% RH and harsh artificial sunlight conditionswith 340mWmm−2 irradiance, and even emitted the stable red light ina detergent solution, indicating their suitability for wearable andwashable electronics. Finally, a passive-matrix WμLED display wasdemonstrated and stably illuminated on the general clothing, dis-playing the clear letters of K, A, I, S, and T, clearly. We are currentlyinvestigating the integration of full-color WμLED displays with wear-able piezoelectric and triboelectric nanogenerators for self-poweredwearable system.

Fig. 4. (a) I-V curves of thin film-based WμLEDs with various device size (chip size of 50×50, 100× 100, 150× 150, 200×200, 250×250, and 300× 300 µm2).The inset image presents the 50× 50 µm2–sized WμLED on the fabric substrate, emitting brilliant red light. (b) Photographs of 30× 30 WμLED on textile floating ondetergent solution/bubbles. (c) 85/85 test results of irradiance and forward voltage during 12 consecutive test days. The inset shows a schematic illustration of the85/85 test. (d) Stacked FT-IR spectra of the TEA throughout the 85/85 test for 12 consecutive days. Note that the regions attributed to the –OH stretching and H-O-Hbending vibrations are depicted as a green and purple shaded region, respectively. (e) L-I-V characteristic of a WμLED before and after Q-SUN test. The inset shows aschematic illustration of the Q-SUN test. (f) FT-IR spectra of the TEA before and after the Q-SUN test. Note that yellow and blue shaded regions are used to indicatethe characteristic regions of the –OH and C=O stretching vibrations, respectively.

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4. Experimental section

4.1. Fabrication of WμLEDs on 100% cotton fabric

i) High-performance AlGaInP LED layers were formed on a motherGaAs wafer using metalorganic chemical vapor deposition (MOCVD).The μLED chips with metal-ohmic contacts (Cr/Au) were patterned byinductively coupled plasma reactive-ion etching (ICP-RIE), and sepa-rated by biocompatible polymer epoxy (thickness of 10 µm). Bottomelectrodes (Au) were deposited on the top surface of the polymer epoxyresin. The flipped μLED device was aligned on a TEA-laminated fabricsubstrate. The TEA is 3MTM Optically Clear Adhesive 8146-X (3MCompany, United States). The μLED arrays were directly bonded with atextile by a compression pressure of 90 kPa exerted by the customizedtransfer apparatus. The mother wafer was selectively removed by GaAswet etchant, and then a p-electrode (Cr/Au) was formed on the topsurface of the revealed μLED chips. Finally, the wearable vertical-structured μLEDs (VLEDs) were passivated by epoxy-based resin (seeFig. S1 in the Supporting information for the detailed fabrication pro-cedure of the WμLEDs).

4.2. Fabrication of flexible antennas

Cr/Au thin-films were deposited on a polyimide film as seed layersusing radio frequency (RF) sputtering. A Cu layer with a thickness of10 µm was uniformly electroplated on the Au layer, and patterned. Cuantennas with eight turns have a 400 µm width and 300 µm gap forresonant frequency of 13.56MHz, which is generally used in industrial,scientific and medical (ISM) applications.

4.3. Finite element analysis (FEM) simulation

High-pressure bonding was simulated using the material propertiesaccording to the real experimental conditions, including Young's mod-ulus, Poisson's ratio, and the dimensions of each component (a carriersubstrate, μLED chips, the TEA, and fabric), as shown in Table S1. Whenthe compression process with 90 kPa was performed to attach both theTEA and μLEDs on a fabric, the applied stress on the bottom edge of theμLEDs was evaluated to be a minimum of 0.2 GPa at the center and amaximum of 0.7 GPa at the left/right ends, as shown in Fig. S2. Thedeformation of the fabric substrate enabled concentration of the stressat both ends.

4.4. Digital image correlation (DIC) analysis

The 3D-DIC measurement setup was composed of two 6-megapixelcharge-coupled device (CCD) cameras, a LED lighting system, and a DICalgorithm program (AMARIS, GOM, Germany). The DIC system wascalibrated with a small measuring volume of 35× 30×15mm foraccurate deformation measurement. DIC samples of WμLEDs wereprepared by the TEA compression transfer/bonding process of a μLED(pressure of 90 kPa) on general fabric. Black spray was applied onto thespecimen surface to form fine speckle marking for the DIC patterntracking algorithm. Two images of the specimen were taken before andafter the TEA-based compression process to conduct the DIC analysis.

4.5. 85/85 and Q-SUN tests

For the 85/85 stability test, the WμLEDs were placed in a humidchamber under conditions of 85 °C/85% RH for 12 days. In order toconduct the Q-SUN test, the WμLEDs were aged in a Q-SUN Xe-1-B Xetest chamber (Q-LAB, United States); the specimen was exposed to ar-tificial sunlight produced by the 1800 W ozone-free Xe arc lamp fil-teded by a Dalight-Q filter (Q-LAB, United States). The acceleratedaging conditions were maintained at the irradiance of 0.34Wm−2 (at awavelength of 340 nm) and a temperature of 64 °C for 72 h.

4.6. Characterizations

The UV–Vis spectrum of WμLED was recorded using an UV 3101PC(Shimadzu, Japan) in the entire visible spectrum and the UVA region.The TGA curve was obtained with a TGA Q50 (TA Instruments, UnitedStates) with a ramping rate of 5 °Cmin−1 under a N2 atmosphere. Inorder to measure engineering stress-strain curves, a tensile test wasconducted according to the ASTM-D882-12 standard; rectangularshaped samples (6× 50mm) were tested under uniaxial tensile loadingwith a rate of 0.1mmmin−1. FT-IR spectra were collected using a FT-IR460 plus (JASCO, Japan) equipped with an attenuated total reflection(ATR) accessory.

Acknowledgements

H.E.L. and D.L. contributed equally to this work. This study wassupported by Wearable Platform Materials Technology Center (WMC)(NRF-2016R1A5A1009926), and Nano·Material Technology

Fig. 5. (a) Thin film-based WμLED arrays stitched on a lab coat. (b) Circuit diagram of the passive-matrix WμLED and its operation scheme. (c-g) Sequentialphotographs of the WμLED display showing the letters “K”, “A”, “I”, “S” and “T”.

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Development Program (NRF-2016M3A7B4905621) through theNational Research Foundation of Korea (NRF). D.L.’s present affiliationis Carbon Resources Institute, Korea Research Institute of ChemicalTechnology (KRICT).

Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2018.11.017.

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Han Eol Lee received his B.S. and Ph.D. degrees inMaterials Science and Engineering (MSE) from KoreaAdvanced Institute of Science and Technology (KAIST) in2013 and 2018, respectively. Currently, he is a BK21 Pluspostdoctoral research associate in the Department ofMaterials Sciences and Engineering at KAIST. His researchtopics are thin-film microLEDs, flexible electronics andlaser material interaction for flexible microLED displays.

Daewon Lee received his B.S. and M.S. degrees in MaterialsScience and Engineering (MSE) at KAIST under the super-vision of Prof. Byeong-Soo Bae. After graduation fromKAIST, he is currently working as a research scientist inCarbon Resources Institute at Korea Research Institute ofChemical Technology (KRICT). His research interests in-clude both fabrication of transparent substrates for flexibleoptoelectronics and development of electrocatalysts for co-production of hydrogen and value-added chemicals.

Byeong-Soo Bae received his B.S. degree in InorganicMaterials Engineering at Seoul National University, his M.S.degree in Materials Engineering at Drexel University, andhis Ph.D. degree in Materials Science and Engineering at theUniversity of Arizona. Since becoming a Professor inMaterials Science and Engineering (MSE) at KAIST in 1994,he has been working on the development of sol-gel siloxanehybrid materials (Hybrimers) for optical and display ap-plications. Currently, he is a Director of Wearable PlatformMaterials Technology Center that is an EngineeringResearch Center (ERC) supported by National ResearchFoundation (NRF) of Korea. In addition, he is a founder anda CEO of a spin-off company, Solip Tech. Co., Ltd., to

commercialize his achievements in Hybrimer technologies.

Keon Jae Lee received his Ph.D. in Materials Science andEngineering (MSE) at University of Illinois, UrbanaChampaign (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.

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