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Encapsulated, High-performance, Stretchable Array of Stacked Planar Micro-supercapacitors as Waterproof Wearable Energy Storage Devices Hyoungjun Kim, Jangyeol Yoon, Geumbee Lee, Seung-ho Paik, Gukgwon Choi, Daeil Kim, BeopMin Kim, Goangseup Zi, and Jeong Sook Ha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03504 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 8, 2016

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ACS Applied Materials & Interfaces

Encapsulated, High-performance, Stretchable Array of Stacked Planar Micro-supercapacitors as Waterproof Wearable Energy Storage Devices Hyoungjun KimƗ, Jangyeol YoonƗ, Geumbee Leeǂ, Seung-ho Paik§, Gukgwon Choi§, Daeil KimƗ, Beop-Min Kim§, Goangseup Ziǁ, and Jeong Sook Ha*Ɨǂ

ƗDepartment

of Chemical and Biological Engineering, Korea University, 5-1 Anam-dong, Seoul

13l-701, Korea ǂKU-KIST

Graduate School of Converging Science and Technology, 5-1 Anam-dong, Seoul 13l-

701, Korea §

Department of Bio-convergence Engineering, Korea University, Seoul 136-703, korea

ǁDepartment

of Civil, Environmental and Architectural Engineering, Korea University, Seoul

136-701, Republic of Korea Keywords : Micro-supercapacitor; Stretchable; Encapsulation; Liquid metal; Waterproof; Oximeter

1 Environment ACS Paragon Plus


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ABSTRACT

We report the fabrication of an encapsulated, high-performance, stretchable array of stacked planar micro-supercapacitors (MSCs) as a wearable energy storage device for waterproof applications. A pair of planar all-solid-state MSCs with spray-coated multi-walled carbon nanotube electrodes and a drop-cast UV-patternable ion-gel electrolyte was fabricated on a polyethylene terephthalate film using serial connection to increase the operation voltage of the MSC. Additionally, multiple MSCs could be vertically stacked with parallel connections to increase both the total capacitance and the areal capacitance owing to the use of a solid-state patterned electrolyte. The overall device of five parallel-connected stacked MSCs, a μ-LED, and a switch was encapsulated in thin Ecoflex film so that the capacitance remained at 82% of its initial value even after four days in water; the μ-LED was lit without noticeable decrease in brightness under deformation including bending and stretching. Furthermore, an Ecoflex encapsulated oximeter wound around a finger was operated using the stored energy of the MSC array attached to the hand (even in water) to give information on arterial pulse rate and oxygen saturation in the blood. This study suggests potential applications of our encapsulated MSC array in wearable energy storage devices especially in water.


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1. Introduction Recently, extensive research on flexible/stretchable energy storage devices has been conducted, along with a dramatically increased interest in wearable,1,2 skin-attachable,3,4 and bioimplantable5,6 electronic devices. Particularly in the application7,8 of energy storage devices for the operation of small devices attached to non-coplanar surfaces such as human skin, stable performance and stretchability under humid conditions are desirable. As energy storage devices for operation of body-attachable and/or bio-implantable devices, electrical double layer capacitors (EDLCs) known as basic type of supercapacitors have various advantages

over

batteries

such

as

high

power

density,

long-term

stability,

fast

charging/discharging speeds, a simple structure, and a wide range of operation temperature; although, they have a relatively lower energy density.9-11 EDLCs store electrical energy between electric double layers generated by adsorption of charged ions on the electrodes of opposite polarity in the electrolyte. For effective adsorption of ions, carbon-based materials such as carbon nanotubes,12 graphene,13 activated carbon,14 or onion-like carbon15 have been used as electrode materials because of their high conductivities and high surface areas. Up-to now, extensive research on supercapacitors has mostly focused on obtaining high electrochemical performance16-19 via adopting various electrode and electrolyte materials. However, the importance on the stable performance under mechanical deformation such as bending, twisting and stretching has recently accelerated the development of flexible/stretchable supercapacitors. 19,20

To utilize supercapacitors as integrated energy storage devices on wearable electronics,

planar-type all-solid-state supercapacitors have recently been extensively investigated.21-24 In conventional supercapacitors using liquid electrolyte, it is necessary to have packaging for preventing leakage of the electrolyte. However, planar all-solid-state supercapacitors do not need



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packaging and a separator between electrodes so that the total thickness can be reduced and the fabrication process can be simplified. Furthermore, planar solid-state supercapacitors can be easily integrated into a circuit with various functional devices. Among planar solid-state spercapacitors, micro-supercapacitor (MSC) is considered to be promising because of its short channel length, resulting in a shorter ion path in the electrolyte.25,26 Also, MSC has a simple structure to be miniaturized, enabling the facile fabrication of electrically connected MSC array with desired performance. Recently, there have been many efforts to fabricate stretchable MSCs in a form of array.27,28 The application of external strain to integrated electronics frequently degrades device performance, mostly because of cracks in metallic electrodes or interconnections between active devices.29 Therefore, numerous efforts have been made to minimize the strain on active devices by adopting novel materials for electrical interconnections and substrates and by designing strain-minimized substrates. For example, polymer encapsulated serpentine thin metal films30,31 have been successfully demonstrated as stable electrical interconnections for stretchable electronic devices. In addition, the method of maintaining the device performance by applying pre-strain on a substrate and reproduction of a wavy shaped substrate30 have been successfully reported. However, these methods require very complex fabrication processes and protruding interconnections are liable to be damaged. As an alternative, liquid metal Galinstan,32,33 an eutectic alloy of Ga, In, and Sn, with high tensile strength, small volume change, and a high conductivity of 3.46 × 10 S/m was shown to have high stretchability when embedded in a flexible polymer substrate. Although stable performance of embedded Galinstan interconnections in stretchable devices has been reported,34 their fabrication was time-consuming, mainly because the interconnections


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were made by filling microchannels one-by-one via an injection method using a micro-syringe. Therefore, multiple electrical contacts between the Galinstan channel and the electrode of each device were made, increasing the chance of electrical disconnection and increasing the electrical contact resistance. Furthermore, it is desirable to encapsulate the entire integrated device so as to protect the human body from being contaminated by the chemicals used in the fabrication of the devices and protect the devices from environmental impacts such as scratching or humidity.35 In this work, for the practical application of such flexible/stretchable supercapacitors as wearable energy storage devices, we report on the fabrication of an encapsulated, stretchable, high-

performance, stacked planar MSC array and demonstrate its stable electrochemical performance both in air and water under severe deformations of bending, twisting, and stretching. Furthermore, the oximeter was operated successfully both in air and in water using stored energy from our fabricated MSCs. The planar all-solid-state MSC consists of electrodes of spray-coated multi-walled carbon nanotubes (MWNTs) on Au film and a UV-patternable ionogel electrolyte of poly(ethylene glycol)

diacrylate/1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide

(PEGDA/[EMIM][TFSI]).36 In order to increase the operation voltage, a specially designed MSC is fabricated on a PET film where a pair of MSCs is serially connected through interdigitated electrode patterns without conventionally used extra wire connections. As a result, the stable output voltage of 2.2 V is obtained although it is not doubled; in the case of a single MSC, the operation voltage is 1.5 V. Furthermore, multiple MSCs can be vertically stacked with parallel connections to increase both the total and areal capacitances, owing to the use of a solid-state patterned electrolyte. For the double-stacked MSCs, the areal capacitance of 0.51 mF/cm2 at a



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current density of 0.006 mA/cm2, an areal energy density of 0.34 µWh/cm2 at an areal power density of 0.006 mW/cm2, and a power density of 2.4 mW/cm2 at an energy density of 0.034 µWh/cm2 are obtained. Five such stacked MSCs are arranged with parallel connections on a specially designed Ecoflex substrate. Several long Galinstan interconnections are designed for the electrical connections between MSCs and integrated devices using a much simpler injection process compared to our previous works using embedded liquid metal interconnections.27 The fabricated MSC array is encapsulated by a thin Ecoflex film. The device’s mechanical and electrochemical stability is confirmed under deformation (including bending, twisting, and stretching), even in water. Repetitive uniaxial stretching (2,000 times) by 30% results in a retention of 88% of the initial capacitance. Furthermore, an oximeter is operated using the energy stored in our MSC array to measure the arterial pulse rate and O2 saturation in the blood both in air and in water. This study clearly demonstrates the potential of our stretchable MSC array as a wearable energy storage device in applications to operate small wearable devices, even in water.

2. Experimental section 2.1 Synthesis of PEGDA/[EMIM][TFSI] ionogel electrolyte Commercially

available

PEGDA

(Mw

=

575;

Sigma

Aldrich),

2-hydroxy-2-

methylpropiophenone (HOMPP; Sigma Aldrich), and the ionic liquid [EMIM][TFSI] (Sigma Aldrich) are used. [EMIM][TFSI], the PEGDA, and HOMPP [ultraviolet (UV) cross-linking initiator] are mixed at a mixing ratio of 88:8:4 (w/w), and the mixed solution is stirred using a Voltex-2 Genie (Scientific Industries) stirrer for 30 min.37

2.2. Fabrication of planar micro-supercapacitor


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On 100-µm-thick polyethylene terephthalate (PET) film, interdigitated electrodes of Ti(5nm)/Au(50 nm) were fabricated via photolithography and e-beam evaporation techniques. To functionalize the substrate with amine (-NH2) group, the fabricated PET is immersed in (3aminopropyl)-triethoxysilane (APTES, Sigma-Aldrich) solution for one day. After rinsing the substrate with DI water, MWNTs fuctionalized with a carboxyl acid group (MWNT-COOH) in DI water (1.0 mg/mL) is spray-coated on the Ti/Au current collector.21 To turn on the 2.1-V LED, two MSCs are designed to be serially connected, which can double the output voltage of the MSC. More detailed information regarding the dimension of the final MSC is shown in Figure S1. Finally, PEGDA/[EMIM][TFSI] electrolyte is drop-cast on the fabricated MSC. To cover only the interdigitated electrodes with the electrolyte, the coated electrolyte is patterned via exposure to UV light (365 nm, 80 mW/cm2) for 40 s with a shadow mask and then dipped in chloroform to remove the excess unexposed electrolyte. After rinsing with DI water and drying with N2 gas, the all-solid-state planar MSC is obtained.

2.3 Stacking of three MSCs For vertical stacking of MSCs, wet Ecoflex is applied as an adhesive to the four corners of the first MSC. Then, the second MSC with punched holes of 1.5 mm in diameter on the Au pad is positioned on top of the first MSC. After drying the Ecoflex, silver paste is put inside the holes to form electrical connections between the two MSCs. The same process is performed for stacking the third MSC, with punched holes of 2 mm in diameter, on the top of the second MSC. Then, the three MSCs are vertically stacked with parallel connection, thereby tripling the capacitance.



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2.4 Fabrication of stretchable substrate with an embedded stacked MSC array, µ-LED, and a switch An Ecoflex thin film with microchannels is prepared using a molding method. First, the Ecoflex pre-polymer and the curing agent (Ecoflex 0030, Smooth-on) are mixed at a mixing ratio of 1:1. The uncured Ecoflex is poured into an iron mold (214 mm × 80 mm × 1 mm). After curing in a 69°C oven for 1 h, the Ecoflex substrate is removed from the mold. After covering the air-exposed microchannels with PET film, a hole is made at the end of the PET film with a syringe needle. Then the liquid metal is injected into the channel via a syringe. To achieve effective contact with the fabricated stacked MSCs and liquid metal interconnections, uncured Ecoflex is applied to the front side of the stacked MSC as a glue. Then, the MSCs are attached to the Ecoflex substrate along the Galinstan-filled interconnections. Five parallel-connected arrays of stacked MSCs are integrated on a deformable Ecoflex substrate with a separation of 0.5 cm. First, five stacked MSCs, a 2.1-V µ-LED, and a switch are fabricated on the PET substrate; they are then attached to the Ecoflex substrate to minimize the strain exerted on them under deformation. The electrical connection between the µ-LED and the Galinstan interconnection is made via the Cu wire at the end of the LED. A piece of 150-µm-thick Ecoflex is added between the two pieces of Cu foil (8 × 18 mm2) to be used as a switch, which is glued on the PET film. The PET-Cu foil is also connected to the Cu wire and fixed to the liquid/metal interconnection using silver paste. After all of the devices are integrated on the same stretchable substrate, enameled wires are connected to the ends of the liquid metal interconnections to allow charging of the MSC array, even in water. Finally, by pouring uncured Ecoflex onto the stretchable substrates, a fully


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encapsulated, stretchable, stacked MSC array is fabricated. More detailed dimensions and optical images are shown in Figure S2.

2.5 Fabrication of encapsulated oximeter with two LEDs and photodetectors, and connection with MSC array The LEDs (L10922, HAMAMATSU Co.) and two photodetectors (S1787-04 and S8729-04, HAMAMATSU Co.) are integrated on an Ecoflex thin film (7 × 2.5 cm) at a spacing of 2 cm. LEDs are connected to the MSC array using external enamel wires. We use a voltage regulator (ADM7160AUJZ-1.8-R2) to keep the output voltage of the MSC array constant. Detectors are connected with both enamel wires and Agilent B1500A (Keysight Co.) using alligator connectors. Then, Ecoflex was poured on the LEDs and detector array to prohibit the penetration of water. To attach the MSC array to the back of the hand, Silbione (Silbione RT Gel 4717 A&B, Bluestar Silicones) was used as an adhesive silicon elastomer.38 More detailed information regarding the dimensions of the Ecoflex encapsulated oximeter is shown in Figure S3.

3. Results and discussion Figure 1 shows a schematic illustration of the fabrication of the planar MSC, vertical stacking of the MSCs, and the fabrication of the stretchable MSC array with injecting the liquid metal into the microchannels.



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Figure 1. Schematics of (a) fabricating the planar-type patterned MSC on a PET film, (b) vertical stacking of three MSCs with parallel connection, and (c) fabricating the stretchable MSC array on Ecoflex substrate with embedded liquid metal interconnections.

Prior to the spin-coating of 100-µm-thick PET film as a substrate for the MSC, uncured PDMS was spin-coated onto a Si substrate and lightly cured to achieve good adhesion. After spincoating of LOR and PR on the PET film, interdigitated electrodes were patterned using a photolithography process. Then, the Ti (5 nm)/Au (50 nm) film was deposited as a current collector using an e-beam evaporator. Here, a pair of MSCs with serial connection was designed for increasing the operation voltage of the MSC to the level required for turning on µ-LEDs. Instead of using a serial connection of two separate MSCs via interconnects, this paired design is


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expected to reduce the contact resistance and to increase the fill factor because it reduces the space required for interconnects. Then, functionalized MWNTs with a carboxylic acid group (COOH) were spray-coated on the patterned Ti/Au current collectors. Next, ionogel electrolyte of PEGDA/[EMIM][TFSI] was drop cast. After exposure to UV light using a shadow mask and rinsing with chloroform and DI water, the MSC with a patterned electrolyte was obtained [Figure 1a]. More detailed information regarding the dimensions of a single MSC and cross-sectional SEM images of the electrode are shown in Figure S1. Figure 1b shows the vertical stacking of the three MSCs for increasing both total and areal capacitances. The increased capacitance of the three stacked MSCs is shown in Figure S4. This vertical stacking can easily be implemented because of the use of a solid-state patterned electrolyte with flat surfaces. The final thickness of the three stacked MSCs is estimated to be ~ 1 mm. In Figure 1c, the fabrication process for a stretchable MSC array is shown. The Ecoflex substrate with microchannels is prepared using a molding method: After curing the Ecoflex film on an iron mold, it was peeled off and the microchannels were filled with liquid metal, via injection using a micro-syringe. On this fabricated substrate, stacked MSCs, a µ-LED, and a switch were attached using uncured Ecoflex as a glue along the Galinstan interconnections and Ag paste was deposited to create contacts between the devices and the Galinstan interconnections. Finally, uncured Ecoflex was poured onto the entire device array for encapsulation. The electrochemical performance of five parallel-connected double-stacked MSCs was investigated by measuring the cyclic voltammetry curves, galvanostatic charge-discharge curves, and electrochemical impedance spectra (EIS), as shown in Figure 2.



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Figure 2. Electrochemical performance of double-stacked MSC array integrated on the stretchable Ecoflex substrate. (a) Cyclic voltammetry curves at various scan rates from 0.1 V/s to 3 V/s. (b) Galvanostatic charge/discharge curves at current density of 0.02, 0.04, 0.06, 0.08, and 0.1 mA/cm2, respectively. (c) Nyquist impedance plot for a frequency range from 1 MHz to 100 mHz. The ESR value is estimated to be 23 Ω. (d) Areal capacitance at various current densities 12 Environment ACS Paragon Plus

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from 0.006 mA/cm2 to 1 mA/cm2. (e) Capacitance retention after 10,000 charge/discharge cycles at a current density of 0.2 mA/cm2. The inset shows the charge/discharge curves observed for repetitions between 9,990 and 10,000. (f) Ragone plots of areal energy density vs. power density.

By stacking two all-solid-state planar MSCs, the performance improved by a factor of two, compared to that of a single MSC (Figure S5). Here, five double-stacked MSCs with the same capacitance were connected in parallel. The reproducible performance of each double-stacked MSCs is confirmed in Figure S6. Additionally, we confirmed that the encapsulation with Ecoflex film did not deteriorate the performance of the stacked MSC (Figure S7). Figure 2a shows CV curves at different scan rates from 0.1 V/s to 3 V/s. Even at a high scan rate of 3 V/s, the CV curve retained its rectangular shape, suggesting stable operation of the stacked MSC array embedded in the deformable substrate. Figure 2b shows the galvanostatic charge-discharge curves of the current density from 0.02 mA/cm2 to 0.1 mA/cm2. Nearly symmetric triangular curves (regardless of the current density) indicate a high Coulombic efficiency, which corresponds to the efficiency of charging and discharging. The Coulombic efficiency was estimated to be 90%, based on the following equation.

η% =

× 100 1

Here, t and t are the discharging and charging times, respectively. The Nyquist plot obtained over a frequency range from 1 MHz to 100 mHz is shown in Figure 2c. The slope at low frequencies is very steep and the graph is nearly parallel to the imaginary part (-Z '' axis), indicating excellent capacitive behavior of the fabricated MSC array. An enlarged graph at 13 Environment ACS Paragon Plus


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higher frequencies is shown in the inset of Figure 2c; the equivalent series resistance (ESR) is estimated to be 23 Ω. For comparison, the electrochemical properties of a single MSC are shown in Figure S8. The total capacitance (Ccell) of the stacked MSC array was calculated using the galvanostatic

charge-discharge C =

curves

and

following

equation.39

2 ∆/∆

where ∆V and ∆t are discharge voltage range except for IR drop and discharge time, respectively. The areal capacitance (CA) of the MSC was obtained according to the equation, CA = Ccell / Acell, where Acell corresponds to the total area of the device. For calculation of the total area, the active area of the MSC is defined as the whole projected surface area including the interspaces.40 The active area of the MSC is 1.9cm2 as shown in the top scheme of Figure S1a, including interfaces of 150 µm and an electrode width of 500 µm. Therefore total area of 5 double-stacked MSC array is evaluated to be 9.5cm2. Figure 2d shows the areal capacitance for different current densities from 0.006 mA/cm2 to 1 mA/cm2. The highest areal capacitance of the double-stacked MSC array embedded in a stretchable substrate is calculated to be 0.51 mF/cm2 at a current density of 0.006 mA/cm2. Repeated cycles of charging and discharging may deteriorate the performance of the MSC; therefore, the long cyclic stability is considered to be an important parameter to assess the device quality in terms of energy storage. Our encapsulated MSC array maintained roughly 88% of its initial capacity after 10,000 repeated charging and discharging cycles [Figure 2e]. The inset of Figure 2e shows a stable and symmetrical shape of the charge-discharge curves up to 10,000 cycles. Figure 2f shows the Ragone plot, which exhibits the energy and power density of our MSC array. Areal energy density (EA) and power density (PA) are calculated to be 0.34 µWh/cm2


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at a power density of 0.006 mW/cm2 and 2.4 mW/cm2 at an energy density of 0.034 µWh/cm2, respectively using the following equations.39

E" =

C" × ∆V # 3 7,200

P" =

E" × 3,600 4 ∆t

where CA, ∆V, and ∆t are the areal capacitance, discharge voltage range, and discharge time, respectively. The calculated EA and PA values are larger than those reported in previous studies of MG/MWNT MSC24 (EA = 0.24 µWh/cm2 at PA = 0.01 mW/cm2 and PA = 0.1 mW/cm2 at EA = 0.026 µWh/cm2 ) and coaxial wet-spun yarn supercapacitors41 (EA =0.16 µWh/cm2 at PA = 0.0007 mW/cm2 and PA = 0.11 mW/cm2 at EA = 0.05 µWh/cm2). In addition, CV curves, galvanostatic charge-discharge curves, volumetric capacitance at different current densities, and the Ragone plot, which shows volumetric energy and power density of our MSC compared to those of previously reported works including commercial AC-SC, Li thin film battery and Al electrolytic capacitor, are shown in Figure S9. The strain distribution in our stretchable substrate with embedded devices under uniaxial stretching was obtained using finite element method (FEM) analysis and compared with the experimental measurements in Figure 3.



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Figure 3. (a) Definition of applied strain for our stretchable MSC array. Here, l is 12.5 cm. (b) Optical image (left) and corresponding strain distribution estimated using FEM analysis, (middle) zoomed near MSC array and µ-LED under 30% uniaxial strain. Strain distribution of the entire device including MSC, µ-LED, and switch with a color scale for strain (right). The applied strain (εapplied) is defined as εapplied = ∆l /l. ∆l = l’ - l, where l and l’ are the lengths of the entire substrate before and after stretching, respectively, as shown in Figure 3a. The photographic image in Figure 3b shows the distribution of the overall strain applied to the builtin device on the stretchable substrate under a uniaxial strain of 30%. The strain applied to the MSCs is approximately 0%, whereas 63% strain is applied to the thin Ecoflex film between the MSCs. This difference in applied strain between the MSCs and the thin film is attributed to the difference in the Young’s modulus between the rigid PET film used as the platform underneath the devices and the soft Ecoflex. PET has a Young’s modulus that is 29,000 times higher than that of Ecoflex.42,43 Therefore, strain induced by mechanical deformation is concentrated on the


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Ecoflex, whereas it is minimized in rigid devices so as to avoid deteriorating their performance. The corresponding strain distribution calculated using FEM analysis shows close agreement with the optical image of our fabricated stretchable device. On all of the PET films for devices, the applied strain is estimated to be 0%, which guarantees the mechanical stability of the device performance when the maximum strain of 94% was applied to the region near the edges of the stiff PET film under a uniaxial strain of 30%. These results imply that our encapsulated MSC array can be used as a stretchable energy storage device under external strains. Strain distributions for uniaxial stretching up to 70% and 100% are also shown in Figure S10, where the applied strain on the devices was also nearly 0% because the strain was concentrated on the Ecoflex substrate. In addition, we evaluated the electrochemical performance of our encapsulated MSC array under applied uniaxial stretching. In Figure 4a, CV curves taken with an increase in applied strain up to 80% did not result in any noticeable change in size or shape.



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Figure 4. The electrochemical performance of encapsulated MSC array upon uniaxial stretching. (a) CV curves obtained at a scan rate of 1 V/s under uniaxial stretching up to 80%. (b) Normalized capacitance (C/C0) with uniaxial strain up to 80%. Here, C0 and C are the capacitance before and after stretching, respectively. (c) CV curves obtained at a scan rate of 1 V/s with repetitive cycles of uniaxial stretching by 30%. (d) Normalized capacitance (C/C0) with stretching cycles for uniaxial stretching by 30%. Capacitance remained at 96% of the initial value under uniaxial stretching by 80%, as seen in the normalized capacitance (C/C0) with an increase in the applied strain, as shown in Figure 4b, where C0 and C are the capacitances before and after stretching, respectively. Repetitive cycles of stretching and releasing (2,000 times) by 30% resulted in a slight reduction of capacitance


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[Figure 4c] with a 12% decrease in C/C0 values [Figure 4d]. Our results confirm that our fabricated MSC array can be used as stable, stretchable energy storage devices. It is important to protect the devices from environmental changes such as sweat or rain, especially in applications to body-attached electronics. Figure 5a shows photographs of our encapsulated MSC array, µ-LEDs, and a switch on the stretchable Ecoflex substrate in water under various deformations.

Figure 5. (a) Optical images of the encapsulated MSC array integrated with a µ-LED, which was powered by the MSC array, under various deformations in water: without deformation, bending,



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winding, stretching by 30%, and twisting by 180° (in sequence). The scale bar corresponds to 2 cm. (b) CV curves measured under different types of deformation and (c) normalized capacitance (C/C0) measured in water for 4 days. Here, C0 and C are the capacitances before and after immersion in water, respectively. After immersing the device in water, the MSC array was charged with a battery via the enamel wire. By connecting the enamel wire of 100 µm diam, which had been made by coating insulating poly(amide-imides) resin solution onto a Cu wire, to the fabricated device, the stable electrochemical performance of the supercapacitor array in water was confirmed. Then, the 2.1-V µ-LED was turned

on by pressing the switch while under water. Upon bending with a bending diameter of 2 cm, winding with a diameter of 2.5 cm, uniaxial stretching by 30%, and twisting by 180°, the brightness of the LED did not exhibit any noticeable degradation. Measurements of CV curves under these deformations exhibited very stable rectangular shapes, without a noticeable change in capacitance [Figure 5b]. This high mechanical stability confirms the stable conductance of the embedded liquid metal interconnections, regardless of the change in shape of the device. Changes in capacitance with an increase in the elapsed time in water are shown in Figure 5c. After one day in water, the initial capacitance was maintained; however, it slowly decreased thereafter. After four days in water, roughly 82% of the initial capacitance was retained, implying permeation of water through the thin Ecoflex film to induce slight degradation of the MSC performance [Figure 5c]. We believe that our strategy for stretchable microsupercapacitor array, which is stable even in water, can be applied to other energy storage devices including high performance Li-ion batteries.44

Next, our fabricated stretchable MSC array was used as a body-attached energy storage device for the operation of a small bio-monitoring device, oximeter. Pulse oximetry is a representative


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non-invasive method for monitoring arterial pulse rate and O2 saturation in the blood.45 In transmission mode pulse oximetry, light from LEDs at two different wavelengths passes through a thin section of a human body, such as a fingertip or earlobe, to photodetectors.46 Then, the photodetectors measure the changes in the absorbance of light at each wavelength. Values of absorbance at each wavelength and SpO2 are calculated using equations including modified Beer-Lambert law (Methods in the supporting information). Our encapsulated MSC array was attached to the back of the hand. The oximeter encapsulated with Ecolex was wrapped around a finger. Figure 6a shows the operation of the oximeter with 5 sets of 3 stacked MSCs connected in parallel.



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Figure 6. (a) Schematic illustration of encapsulated MSC array, consisting of 5 sets of 3 stacked MSC in parallel, attached to the back of the hand with an oximeter wrapped around a finger in

water, where enameled wires are used for connection between the MSC array and the oximeter. (b) Schematic illustration of electrical circuit of MSCs and oximeter. Pulse waves and estimated SpO2 obtained by (c) 3 V battery and (d) MSC-array, respectively, in air (left) and water (right). The incoming signal to the photodetector was measured using precision measurement equipment (Agilent B1500A, Keysight Co.). An electrical circuit diagram of the system configuration is shown in Figure 6b. For the lightening of the LEDs, our three stacked MSC arrays supplied power via the enameled wires. With a single stacked MSC (Vout = 2.2 V), a µLED (1.8 V) can be lit for a short time. However, in order to operate the LED and oximeter for a reasonably long time, 5 paralled connected 3 stacked MSCs, making 15 MSCs in parallel, were used with the voltage regulator. Using the voltage regulator, we can maintain the operation voltage at 1.8 V and turn the LEDs on. Without the voltage regulator, the output voltage of the MSCs drops continuously, therefore degrading the light intensity during the measurements. Light from LEDs encapsulated with Ecoflex passes through a fingertip, and photodetectors sense the change of light intensity as a signal. This signal is transferred to the Agilent B1500A, which can analyze the pulse data. The time-dependent change in the absorbance of light for 670 nm and 870 nm was measured using our system. Figure 6c shows the pulse data and their corresponding SpO2 values (96~97%) both in air and in water, as obtained using a constant voltage source (a battery at 3 V). The detailed shape and size of the pulses are slightly different in air and in water; however, similar behaviors are observed. To show the application of our MSC array as bodyattached waterproof energy storage devices, the same measurements were performed, as shown in Figure 6d. This figure indicates that the oximeter produces similar pulse data (both in air and



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in water) as those obtained using exterior battery. Arterial pulse rate and SpO2 values were estimated to be 60-64 beats/min and 94~97%, respectively. Note that measurement of SpO2 in water will be very useful for detecting possible problems such as decompression sickness for divers. This study demonstrates the successful application of our fabricated MSC array as a wearable energy storage device, even in water.

4. Conclusion We demonstrated the fabrication of an encapsulated, high-capacitance, stretchable array of stacked planar MSCs that can power an oximeter both in air and in water. The total capacitance of the MSC array could be increased by vertically stacking planar MSCs. The design of a stretchable Ecoflex substrate with embedded long Galinstan microchannel interconnections exhibited high mechanical stability over various deformations of bending, twisting, and stretching. Furthermore, encapsulation of the entire device using Ecoflex film made it useful as a wearable energy storage device, even in water. As a result, the oximeter could be powered by the fabricated MSC array attached on the hand in water without any deterioration, relative to the performance in air, providing information on SpO2. This work clearly shows the potential application of our encapsulated stretchable MSC array as wearable energy storage devices, regardless of the environmental changes.


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Dimensions, optic image, CV, CD curves of stacked MSCs, Areal performance of single microsupercapacitor and volumetric performance of double-stacked MSC array and FEM analysis.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the financial support from the National Research Foundation of Korea(NRF) grant funded by the Korean Government (MEST) (Grant No. NRF-2016R1A2A1A05004935).



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