An Implantable Transparent Conductive Film with Water Resistance

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An Implantable Transparent Conductive Film with Water Resistance and Ultra-bendability for Electronic Devices Youngjun Song, Sejung Kim, and Michael J. Heller ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11801 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

An Implantable Transparent Conductive Film with Water Resistance and Ultrabendability for Electronic Devices Youngjun Song1,2,3*, Sejung Kim4 and Michael J. Heller4,5* 1

Department of Electrical and Computer Engineering, University of California San Diego, La

Jolla, CA, USA 92093-0448 2

StandardBioelectronics. Co., Dosan-ro 341beon-gil, Seo-gu, Daejeon, Republic of Korea 35320

3

Environment & Energy Research Team, Hyundai Motor Co., 37, Cheoldobangmulgwan-ro,

Uiwang-si, Gyeonggi-do, Korea 16082 4

Department of Nanoengineering, University of California San Diego, La Jolla, CA, USA

92093-0448 5

Department of Bioengineering, University of California San Diego, La Jolla, CA, USA 92093-

0448

Author Information Corresponding author * E-mail: [email protected] (Y. Song), [email protected] (M.J. Heller)

ACS Paragon Plus Environment


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Abstract Recently, instead of indium tin oxide (ITO), the random mesh pattern of metallic nanowires for flexible transparent conducting electrodes (FTCEs) has received a great amount of interest due to its flexibility, low resistance, reasonable price and compliant processes. Mostly, nanowires for FTCEs are fabricated by spray or mayer coating methods. However, the metallic nanowires layer of FTCEs, which is fabricated by these methods, has spiked surface roughness and low junction contact between the nanowires that leads to their high sheet resistance value. Also, the nanowires on FTCEs are easy to peel-off through exterior forces such as bending, twisting or contact. To solve these problems, we demonstrate novel methods through which silver nanowires (AgNW) are deposited onto nano-size porous nitrocellulose (NC) substrate by electrophoretic deposition (EPD) and an opaque and porous substrate. Respectively, through dimethylsulfoxide (DMSO) treatment, AgNW onto NC (AgNW/NC) is changed to the transparent and nonporous FTCEs. It enhances the junction contact of AgNW by EPD and also allows a permanent attachment of AgNW onto substrate. To show the mechanical strength of AgNW onto transparent nitrocellulose (AgNW/TNC), it is tested by applying diverse mechanical stress, such as a binding test (3M peel-off), compressing, bending, twisting and folding. Next, we demonstrate that AgNW/TNC can be effectively implanted onto the normal newspapers and papers. As paper electronics, light-emitting diodes (LED), which is laminated onto paper, is successfully operated through basic AgNW/TNC strip circuit. Finally, it is demonstrated that AgNW/TNC and AgNW/TNC onto paper are water resistant for 15 minutes, due to the insulation properties of the nonporous substrate.

Keywords Implantable electronics, Silver nanowire, Transparent, Flexibility, Nitrocellulose

1. Introduction The attention of ultra-bendable transparent conductive films (TCFs) is being focused these days on light1, cheap2 and disposable3 electronics due to the needs for various applications such as paper electronics4,5 and wearable devices6-8. The main key characteristics of these application are


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bendability and implantable property with invisibility. For ultra-bendable TCFs, highly conductive nanomaterials including AgNW9, carbon nanotubes10, graphene11 and PEDOT:PSS12 (Tab. 1) have been introduced diverse sheet resistances13 (Fig. S1) , instead of layering brittle transparent conductive oxides (TCOs)14-17. Also, the bendable and flexible substrates have been introduced diverse materials such as polydimethylsiloxane (PDMS)18-21 and polyethylene terephthalate (PET)22-24. However, these substrates for paper electronics and wearable devices are lack of adhesiveness25-28, due to the hydrophobic surface29. The conductive materials are easy to detach from the surface by weak mechanical forces30. Also, the substrates, which are nonbiodegradable materials have a limitation of disposability.

Recently, studies of bendable bio-substrates using the groups of cellulose materials such as nanofilber cellulose (NFC)32-35 have been broadly investigated as a biodegradable and disposable possibility for flexible TCFs 36,37. This is due to cheap prices ($4/kg)38, high transmittance and environmental friendly properties40,46. Cellulose materials are easy to obtain from wood pulp like that used for paper fabrication39,40. In addition, cellulose substrate has a low thermal expansion (CTE)41,42 and the fabrication process is both simple and well-known manufacture. In spite of the numerous advantages of cellulose substrate, however, the NFC substrate has a low adhesive performance with conductive nanomaterials43 as well as NFC itself 44. Thus, NFC substrate can deform during a long time period and has a low water resistance due to its porous structure45,46.

In order to overcome these limitations and improve further performance of flexible TCF, herein, we demonstrate the flexible AgNW/TNC fabricated by the our EPD method and solvent evaporation casting process. Commonly, EPD method is that materials are directly deposited onto electrodes. For fabrication transparent conductive electrode, normal EPD method is need to extra-process to fabricate transparent electrodes such as transferring47 or etching48. However, we demonstrate that the AgNW, which has high conductivity than others materials (Tab. 1) are deposited uniformly onto opaque and nano-size porous NC substrate which is layered onto electrode by EPD. Respectively, the opaque and porous AgNW/NC is changed to transparent and nonporous AgNW/TNC by the solvent evaporation casting process. For this process, NC is exposed to diverse solvents. Due to the solubility49,51, NC has a diverse transmittances following the surface roughness and porosity.50,51 During the process of becoming transparent, the AgNW



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network is embedded into surface of TNC layer. It allows an enhancement of mechanical strength of AgNW/TNC.

Our method with NC substrate has several benefits compared with previous methods and materials. First, the conductive materials are directly deposited onto the substrate, while most methods are need to transfer process47. Second, this EPD system is reusable because the materials are directly deposited onto target. Third, diverse conductive materials can be fabricated by our EPD method. Moreover, the sheet resistances are controlled by the concentration of solution and deposition time. Fifth, our bendable TCF has the mechanical strength because the conductive materials are well embedded onto substrate through solvent evaporation casting process. Sixth, due to the reconstruction into a structure without pores51, our product has water resistance. And our product can be implanted invisible due to transparency. Finally, our product is easy to consist of a circuit through scissoring, because the property of our substrate is similar to paper52.

For electrical and mechanical characterization, we demonstrate that the AgNW/TNC is analyzed several methods: compressing strain, twisting, bending and folding. We demonstrate a 17 Ω/sq transparent conductive electrode consisting of a network of flexible AgNW that has been successfully implanted onto newspaper and paper without losing conductivity. Also, the LED circuit device, which AgNW/TNC implant onto papers is operated under the folding condition. The AgNW/TNC shows the maintain of electrical property under the various environmental condition such as overheating, humidity condition (85 % at 85 oC), water contact.

2. Experimental Section 2.1. Preparation of conductive materials

The 0.5 % w/v AgNW (116 nm diameter and 20-55 µm lengths) suspension in IPA was purchased from Sigma-Aldrich, Co. LLC. The suspension was diluted from 0.05 % w/v to 0.25 % w/v with DI water. The AgNW in IPA and DI water suspension was sonicated for 1hour. Before the use of the AgNW suspension, the vial of AgNW in IPA and DI water was sonicated


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for 5 minutes. Multi-wall carbon nanotubes and CVD single wall carbon nanotubes were purchased from Cheap Tube, Inc. and arc-discharged single wall carbon nanotubes were purchased from Sigma-Aldrich Corp. and carboxymethyl cellulose (CMC, 9,000 MW) was purchased from Sigma-Aldrich Corp. All carbon nanotubes were mixed with 10:1 ratio of CMC: DI water and sonicated for 2 hours. The solution was used only from the top 75 % of the tube.

2.2. AgNW/NC Fabrication by EPD system

For the EPD process, a metal electrode composed of Ti/Au (50 nm/ 200 nm) was sputtered onto a cleaned glass slide (3 inch x 2 inch) via a directly current (DC) magnetron sputtering (Denton Vacuum Discovery 18 system). Then, a plastic wall with 0.5 mm of thickness as a spacer was attached to the bottom electrode used as an anode using double-sided pressure sensor tape (PST) on the metal electrodes in order to contain the conductive materials suspension. The porous NC with a 0.2 µm pore size (Bio-Rad, Inc.) activated with 0.1 % w/v poly-L-lysine (PLL, SigmaAldrich, Co. LLC) was applied to the top electrode used as a cathode and then the conductive materials solution was poured into the fabricated cathode. The electrodes were mounted in a parallel-plate configuration with a gap of 0.5 mm. The DC bias was applied by a DC power supply (3006B Protek Inc.) across the electrodes for 30 seconds, and the current was monitored for the deposition.

2.3. Solvent evaporation casting process

The conductive materials onto NC, which are directly lifted off after fabrication by EPD were dried using an oven at 85 oC for 3 minutes in order to evaporation of water residue. After DMSO was dropped on a flat blank glass slide, the conductive materials onto NC was laid onto the slide. The samples were dried in an oven at 85 oC for 30 minutes. After DMSO was totally dried, the TCFs are lifted off on flat glass side. To see diverse transparency, NC substrate was tested through solvent evaporation casting process using diverse solutions such as DMSO (Sigma Aldrich, Co. LLC), 99 % grade EtOH (Sigma Aldrich, Co. LLC), 95 % grade EtOH (Sigma Aldrich, Co. LLC) and water. After the solutions were dropped on a flat blank glass slide, NC were laid onto the slide. The samples were dried in an oven at 85 oC for 30 minutes. After



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solvents were totally dried, the substrates are lifted off on flat glass side. For laminating of NC onto papers, DMSO was sufficiently dropped onto newspapers and papers to ensure they fully wet. Afterwards, AgNW/NC was laid onto the wet newspapers and papers, the sample was dried in an oven at 85 oC for 3 hours.

2.4. Characterization and measurement

To measure the transmittance, a UV-vis spectrometer (PerkinElmer, Inc.) was used from 240 to 850 nm with an incident angle of 90º. Total transmittance and haze were measured by NDH7000SP haze meter (Nippon Denshoku Co.). The sheet resistance of the conductive materials onto NC film were measured by the Jandel four probes with RM 3000 test (Jandel, Inc.) at multiple points and averaged to get the final sheet resistance value. The morphology of the films was recorded on Phillips XL30 ESEM operating at 10 kV. The surface roughness of films was measured in tapping mode with a Si tip (resonance frequency =320 kHz; spring constance =42 N/m) by Atomic force microscopy (AFM, Dimension 3100 Veeco, Inc.) X-ray photoelectron spectroscopy (XPS) measurement were performed on AXIS Supra photoelectron spectrometer (Kratos Ltd.). For the NCs coated by PLL and pristine, the wide-scan spectrum and peaks of N1s, C1s and O1s were analyzed. The zeta potentials of AgNW was measured by zeta sizer (Photal Otsuka Electronics, Inc.) Electric properties were characterized using Fluke 289 digital multimeters (Fluke Co.) Keithley Model 2400 source meter (Keithley Instruments, Inc.). To exposure humidity condition, the sample was remained for 2 hours under 85% relative humidity at 85 oC (85RH @ 85 oC) in humidity chamber (Daesung E &T Ltd.). For electrical test, the sample was dried for 1 hour outside the chamber at room temperature of 20 oC.

2.5. Fabrication of LED circuit Blue and orange color LED components are purchased from Dievcesmart Co.. The silver paste (Elcoat P-100) was purchased from Cans. Co.. For LED circuit, AgNW/NCs were cut by scissor. The AgNW/NCs strips were consisted of circuits onto the wet paper. The LEDs were mount onto AgNW/TNC onto paper. by silver paste. The silver paste was cured 2 hours at room temperature.

3. Results and discussions


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3.1. Fabrication of AgNW/NC by EPD

A schematic diagram of our EPD method is shown in Fig. 1a. The top gold electrode serves as a deposition site and the bottom electrode has 0.5 mm thickness chamber (For detail, see Fig 1, Fig S2 and Experimental section). First, the NC membrane surface, which has 0.1 to 2 µm pores, is treated with 1 % PLL solution for enhanced binding performance with conductive materials The resulting of NC membrane coated by with PLL has a positively charged surface without change of morphology (bottom left image of Fig. 1b), due to the presence of NH3+ molecules (The XPS spectra of pristine and PLL-coated NC are shown in Fig. S3). The NC substrate is attached onto the top electrode (step 2 in Fig. 1a) and the AgNW suspensions (0.05 %w/v to 0.25 %w/v concentration in IPA and DI water; see the Experimental section for details) are poured in the other electrode chamber (step 3 in Fig. 1a). The electrode chamber is closed and applied by a 2.5 V. The AgNW in suspension, which has the negative zeta potential (-17.94 mV shown in Fig. S4) is moved toward the NC substrate onto the positive top electrode. (Fig. 1c) The PLL-coated NC, which has a positive charge to make a easy to bind negative charged materials, help to bind AgNW due to a electrostatic binding. (step 4 in Fig. 1a) The randomly spiked AgNW network is rearranged to the flat network surface by the electrical filed. Also, the use of top electrode for deposition site is made to minimize nonspecific binding and detaching AgNWs. Finally, the flat AgNW network is layered onto the porous nitrocellulose substrate (bottom left of image in Fig. 1d) after being dried for 5 minutes at 85 oC. (step 6 in Fig. 1a)

In order to optimize our EPD method for fabrication of AgNW/NC, our EPD method is controlled by the deposition time and AgNW concentration. The results of sheet resistances are measured as shown in bottom left graph in Fig. 1d and Fig. S5. The deposition time is optimized to 20 seconds for our EPD system, which has short distance (0.5 mm) chamber thickness. The electrode chamber, which has designed to be sensitive to electrical fields (V/mm) is controlled by limited voltages (2.5 V) and deposition times, in order to avoid electrical chemistry attack53. At 0.05 % w/v in the AgNW suspension, AgNW/NC has 80 Ω/sq sheet resistance value for 10 seconds deposition times. And the AgNW/NC has 17.2 Ω/sq sheet resistance value for 20 seconds deposition times. However, the sheet resistance values are not highly increased for 30



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and 60 seconds. We believe that the most AgNW is deposited for 20 seconds and the AgNWs is embedded into the NC pores to be broken network junction for long deposition time. Also, the concentration of AgNW has dominant effect for the sheet resistance. The sheet resistance (bottom left graph in Fig. 1d and Fig. S5) show that the increased concentration has leads to decrease in the value of sheet resistance. Furthermore, AgNW network has a relatively low sheet resistance, which is similar results like other fabrication methods shown in Fig. S113,54. For direct comparing with other conductive materials (Tab. 1), we provide the current-voltage (IV) curve of carbon nanotube network onto NC, which are fabricated by our EPD method as shown in Fig. S6

3.2. Transparent process by solvent evaporation For transparency, AgNW/NC is treated with DMSO at 85 oC for 30 minutes. Fig. 2a and b show the SEM images (left) and optical images (right) of AgNW/NC and AgNW/TNC. Through the DMSO evaporation process, the porous and opaque NC substrate is transformed into pore-less and transparent substrate with embedment of the AgNW percolating network into the surface. For the process, the sheet resistance value (17.2 Ω/sq) is unchanged (Fig. S7) and the transmittance of AgNW/TNC is 82 %. The IV curves of AgNW/NC and AgNW/TNC are provided at Fig. 3a. The main reason for a transparency is that the porous NC structure as shown in Fig. 4d, h has been reconstructed during the evaporation of DMSO, followed by forming a non-porous fully packed structure (Fig. 4a, e). The process is need to heat at least 30 minutes at 85 oC for the evaporation of all DMSO. If the process is stopped in less than around 30 minutes, the substrate is remained the molten states like a gel. If it is overheating for more than an hour, the sheet resistance of AgNW/TNC is unchanged. The IV curve of AgNW/TNC is provided at Fig. 3b. Also, the AgNW/TNC (Fig. 3a) is compared with carbon nanotube network onto TNC (Fig. S8 and S9) in Fig. S6.

The key factor of transmittance for TNC is kinds of solvents. We demonstrate the fabrication of TNCs using diverse solvents and drying conditions. The results of TNCs are provided as shown in Fig. 4 and Fig. S10. The opaque NC (Fig. 4d, Fig. 4n; blue line) has a 92 % transmittance at 550 nm (Fig. 4a, Fig. 4n; purple line), by drying (evaporation of DMSO in an oven at 85 oC). Also, TNC dried by 99.9 % EtOH at 85 oC has a 76 % transmittance at 550 nm (Fig. 4b, Fig. 4n;


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green line) and TNC dried by 99.9 % EtOH at 25 oC has a 32.5 % transmittance at 550 nm (Fig. 4c, Fig. 4n; red line). However, in the case of EtOH containing water (95 % ethanol, 5 % water), NC substrate reverts to its original opaque status during the drying process. (Fig. S11) We believe that the water disturbs the nitrocellulose melting caused by the solvents. The NC substrate treated by DMSO has a more uniform morphology that a high surface roughness (Rrms is 451.67 nm as shown in Fig. 4l,m) of pristine NC is changed to a low surface roughness of 1.2843 nm as shown in Fig. 4i,m. However, the roughness of the substrate is increased when the transmittances are reduced: 76 % (Rrms is 60.01 nm, as shown in Fig. 4j,m) and the 32.5 % (Rrms is 177.23 nm as shown in Fig. 4k,m). According to SEM and AFM results, these surfaces roughness effects of transmittance; a more uniform surface shows higher transmittances. Furthermore, we monitored the transmittance for thermal stability of optical transparency at Fig. S 12, according to the previous report of S. Nogi et al.42 The total transmittance and haze are not change for the for 2 hours at 85oC. The total transmittance is constant at 95 % and the haze is constant at 6.6 % for the heating process.

As other evidence of the process of filling in the pores to become the TNC, the thickness is reduced from 100 µm to 50 µm, after the DMSO solvent process. The porosity is calculated by density according to the previous report of H. Sehaqui et. al.36 Before measuring the weight of the NC and TNC, the substrate is oven dried to evaporate contained moisture After measuring the weight and volume of NC and TNC, porosity is calculated following 1460 kgm-3 as density of cellulose using the formula.

Porosity= 1- ρsample/ρcellulose

(1)

The densities of samples (ρsample) are calculated after the samples are measured for weights and volumes. Using the formula (1), AgNW/NC, which has 50 % of porosity, is transited to AgNW/TNC, which has 98 % of porosity. Also, the NC and TNC are exactly same weights per unit area (146 g). This shows that the cellulose material, which is treated by DMSO is reconstructed by filling in the pores of NC and fully packed substrate makes it transparent. Furthermore, the TNC, which is fully packed and low surface roughness has low haze (Fig. S13).



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3.3. Mechanical test of AgNW/TNC

For solvent evaporation casting process, AgNW network, which is embedded into surface of TNC has an increased mechanical strength including bending performance (Fig. 2d, e), according to ref 26. For binding, AgNW/NC and AgNW/TNC are carried out by a 3M tape peeloff test26 in Fig. 2 c. The resistance of AgNW/NC and AgNW/TNC are measured, while 3M tape is attached and peeled off onto the surfaces. The original resistance values of AgNW/NC (16.5 Ω) are increased to 94.6 Ω after peeling-off the 3M tape for the first time. (R1/R0 is 5.73.) The values are increased after peeling-off the 3M tape for the multiple times: R4/R0 is 27.13, R8/R0 is 123.63 and R9/R0 is 166.67 (Gray in Fig. 2c). However, the ratio for resistance values of AgNW/TNC are no significant variation for repeated peeling-off test: R1/R0 is 1.17, R4/R0 is 1.64, R8/R0 is 1.71, and R9/R0 is 1.76 (Blue in Fig. 2c). The resistance changing ratio of AgNW/TNC is shown to provide better performance than the resistance changing ratio of AgNW/NC, which is increased by 90 times. The SEM surface images of the AgNW/NC and AgNW/TNC after peeling-off the 3M tape for the multiple times are provided at Fig. 5. The most AgNWs of AgNW/TNC are remained and keep the network under the peel-off condition, while the AgNWs of AgNW/NC are detached onto surfaces (Fig. S14).

Also, we provide 3M peel-off test of AgNWs coated onto PDMS and PET by spray coating methods. For direct comparison with AgNW/TNC (1.76 after 9 times peel-off), we demonstrate 3M tape peel-off tests of AgNW networks onto PDMS (AgNW/PDMS) and AgNW networks onto PET (AgNW/PET), which are fabricated by spray coating method. The AgNWs are deposited onto amine activated substrates by 3-aminopropyltriethoxysilane (APTES) 25,55. Fig. S15 shows the results of the resistances for AgNW/PDMS and AgNW/PET, while 3M tape is attached and peeled off onto the surfaces. The change of resistance for AgNW/PDMS is maximized after 2 times. The change of resistance for AgNW/PET is 25 (R5/R0) times after 5 times peel-off.

We also performed diverse bending and folding tests for the mechanical property of AgNW/TNC according to refs 55-56. For electrical characterization of AgNW/TNC, the IV curves of unfolding and folding statues are provided at Fig. 6. The curves are constant under the unfolding


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and folding states, Also, the curves are constant under the bending condition. First, Fig. 7a shows the results of the test for compressive strain. The resistance of AgNW/TNC is reduced when the film is compressed on the AgNW side. The change ratio of resistance (R/R0) is reduced from 0.93 at 20 % of the compressive strain to 0.82 at 70 %. This is because the network of AgNW is increased during the compression process. Next, we monitor the change ratio of the resistance depending on the angles of bending as shown Fig. 7b. When subject to a rectangular bending ratio (-90o, 90o), the resistances are slightly reduced (R/R0 is 0.92-0.97). When subject to full inward bending of the AgNW network face (-180o), the resistance is reduced (R/R0 is 0.85). However, when subject to full outward bending of the AgNW network face (180o), the resistance is increased (R/R0 is 1.32), due to the disconnection of junction of AgNWs. However, the value of resistance is recovered when the AgNW/TNC reverts to the original flat state. Third, a twist test for the AgNW/TNC strip (2 cm x 8 cm) is performed from -90o to 90o. During twisting, the values of the ratio of the resistance are slightly changed (Fig. 7c). Additionally, for the extreme endurance test, the AgNW/NC and AgNW/TNC are folded outside in Fig. 7d. Due to the different binding properties between the two flexible electrodes, the resistance of AgNW/NC is easier to increase than the resistance of AgNW/TNC, which maintains the original value. After being folded eleven times, the change ratio of resistance of AgNW/NC is 60 and the ratio of resistance of AgNW/TNC still stays under 2. Fifth, we monitor the change ratio of the multiple inward and outward bending tests. The outward folding stress test shows that R11/R0 is 2.2, while the inward folding stress test shows that R11/R0 is1.3 (Fig. 7e). The results of AgNW/TNC are similar property compared with previous reports35,56,57 including the results of carbon nanotube/TNCs (Fig. S16-S19).

3.4. Lamination onto paper

Using the binding ability by DMSO evaporation process, we demonstrate that an opposite side of AgNW/TNC can be laminated on to paper after deposition of AgNWs by EPD. Fig. 8a shows the diagram of the lamentation process onto another substrate such as paper. The AgNW/NC, which the sheet resistance is 17 Ω/sq, is layered on to paper, which is wetted by DMSO. The paper with AgNW/TNC is the baked at 85oC in an oven for 30 minutes. Once the DMSO evaporates, the AgNW/TNC with the sheet resistance is 17 Ω/sq is laminated onto the paper substrate. We



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demonstrate this laminating process on a school newspaper. (Fig. 8b) The AgNW network is connected securely to the surface. The AgNW/TNC on the newspaper is then subjected to an LED light at 2.5 V as shown in Fig. 8c.

Fig. 8d-f shows the SEM surface views of AgNW/TNC. The AgNW/TNC on the newspaper has a thickness of around 50 µm. The surface has a random order network which is connected AgNWs. For more detail, Fig. 8g,h show SEM cross-sectional views of the AgNW/TNC attached to the newspapers. While the paper area has a rough surface condition and pores, the TNC area has no pores and is fully packed. Additionally, we demonstrate LED lamentation onto the paper substrate using two AgNW/TNC strips as shown in Fig. 9a. The AgNW/NC strips, which are cut using scissors, are layered first on paper, which is wetted by DMSO. Next, the LED is attached using Ag paste. The LED laminated paper is turned on through AgNW/TNC strips, which have a 2.5V current applied to them as shown in. To ensure a robust test of the AgNW/TNC paper, the ratio of resistance is measured by bending ratio (Fig. 9b) and folding /unfolding (Fig. 9c). The resistance is 6.9 Ω for 2 mm radius during bending, while the resistance value is 6.8 Ω at flat status. The maximum change of resistance is 2.21, for 11 cycles of folding and unfolding. Fig. 9d shows the operation of the simple LED circuit under the folding and unfolding condition. The LED is operated under the one folding, double folding and unfolding condition, respectively.

3.5. Water resistance test

Because TNC is pore-less and a fully packed structure after the DMSO process, TNC has water resistance. Fig. 10 shows the water resistance of the AgNW/TNC and AgNW/TNC on paper. The AgNW/TNC is connected with copper tape and flipped. The water is dropped onto the TNC. For AgNW/TNC on paper, the water is dropped onto the papers, which is laminated with TNC. During the water droplet evaporation, we monitored the changing of resistance of AgNW/TNC and AgNW/TNC on paper. The resistance of the AgNW/TNC on paper changes during the 4 minutes. (Fig. 10c) This is due to the fact that water dispersed in paper by a capillaries effect temporally meets the AgNWs electrodes area. After water evaporation from the paper, the resistance value returns back to the original resistance value. After 15 minutes, the water droplet


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is completely evaporated and the resistance is not changed due to the droplet. Finally, we demonstrate the IV curve under 85RH @ 85 oC. The curve of AgNW/TNC shows the constant value after 2 hours in the humidity chamber and 1 hour in the room temperature (Fig. 11).

4. Conclusion In conclusion, we developed a unique EPD process with biocompatible nitrocellulose substrate to demonstrate a highly transparent and mechanical strength electrode by combining a solvent evaporation process with permanent binding. The flexible conductive substrate is fabricated directly onto the nitrocellulose substrate membrane, which has 0.2 µm particle filter pores through our EPD method for AgNWs, while a normal EPD method for nanoparticles59,60 is needed for additional processes such as transfer process61, lamination62 and electrode etching48. In order to make the substrate transparent and to laminate it onto paper, a solvent evaporation process is performed. The process can be made pore-less with solvent by melting and re-forming it. The usages of this EPD method and solvent evaporation process can obtain AgNW/TNC with diverse benefits; i) smooth surface, ii) increased adhesion between AgNWs and substrate, iii) enhanced junction conductivity, iv) mechanical strength, v) water-resistance and vi) relatively easy and thin process of lamination onto paper. To understand the properties of nitrocellulose substrate treated by DMSO solvent, we investigated the transmittance of substrate created by different solvent processes, the binding affinity of AgNWs, and carried out diverse mechanical tests of the AgNW/TNC and water-resistance functions of the nitrocellulose substrate after the solvent process.

Since this flexible film has a high transmittance value without losing electrical properties, the film can be used for diverse applications such as gas sensors63,64, touch screens65, solar cells66, and organic light emitting diodes67. Also, the biocompatibility of nitrocellulose is high due to its extraction from wood 40,41. The unique properties of lightweight, flexible, transparent, and thin substrates have played a key role in daily life such as paper. We believe that our AgNW/TNC, fabricated by combining them with EPD and a solvent process, can prove useful for future daily electronic devices such as disposable paper electronics68, water proof electronics69 and wearable electronics 6-8.



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Author Information Corresponding author * E-mail: [email protected] (Y. Song), [email protected] (M.J. Heller)

Acknowledgements This research was supported by Samsung Advanced Institute of Technology Global Research Outreach (SAIT-GRO) program.

Associated Content Supporting Information Sheet resistance of transparent electrodes fabricated by diverse nanomaterials (Fig. S1), optical images of EPD chamber (Fig. S2), XPS data of nitrocellulose with and without coating PLL (Fig. S3), zeta potential of AgNW suspension (Fig. S4), sheet resistances of AgNW/NC by EPD times and concentration (Fig. S5), IV curve of AgNW/NC, carbon nanotubes/NC, carbon nanotubes/TNC (Fig. S6), the change of sheet resistance (R/R0) for AgNW/NC and AgNW/TNC (Fig. S7), sheet resistance of carbon nanotubes/TNCs (Fig. S8), SEM images of the surfaces of carbon nanotubes/NC and TNC (Fig. S9), AFM images and surfaces roughness of TNCs and NC (Fig. S10), optical images of 95 % EtOH experiment process (Fig. S11), total transmittance and haze at 85oC for 2 hours (Fig. S12), total transmittance and haze of TNC fabricated by diverse solvents (Fig S.13), optical images of tapes for 3M tapes peel-off tests (Fig. S14), the change of resistances by 3M tape peel-off test of AgNW/PDMS and PET (Fig. S15), bending and folding test (red arrow: folded line) a) SEM image before bending (Fig. S16), the ratio of change for resistance(R/R0) for bending cycles of single wall carbon nanotubes/TNC (Fig. S17), SEM images and optical image of single wall carbon nanotubes/TNC under folding (Fig. S18), the change of sheet resistance before/after folding of single wall carbon nanotubes/TNC (Fig. S19). These materials are available free of charge via the Internet at http://pubs.acs.org/.

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Figures legends Table 1. Comparison of relative conductivity of diverse conductive nanoparticles materials Materials

Conductivity (S/cm)

Ref. no.

Carbon nanotubes

596

18

Graphene

262

19

PEDOT:PSS

900

20

AgNW

8130

17



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Figure 1. The procedure and results of AgNW/NC fabricated by EPD: (a) the process of EPD, which is directly deposited onto NC using EPD chamber system, (b) diagram and SEM image for NC, which is coated by poly L-lysine (XPS data as shown in Figure S3), (c) the procedure and diagram of AgNW/NC (zeta potential of AgNW as shown in Figure S4), (d) diagram of structure and SEM image for AgNW/NC, which has diverse sheet resistances (left bottom) by times of EPD and concentrations of AgNW suspension.


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Figure 2. The transparent process and the results of mechanical stability tests according the ref 26. (a), (b) SEM images (left), diagram (center), optical images (right) of AgNW/NC and AgNW/TNC, (c) the change of resistances by 3M tape peel-off test and (d), (e) bending test of AgNW/NC and AgNW/TNC. (The resistance are measured through copper tapes (0.5 cm width and 1 cm length), which are connected with gaps of 1 cm by 1 cm onto AgNW/NC and AgNW/TNC).



(a)

Current (µA)

10

5

0

-5 AgNW/NC AgNW/TNC

-10 -2

-1

0

1

2

Voltage (mV)

(b) 10

Current (µA)

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5

0

-5 Drying (30 min @ 85oC) Heating (2hours @ 85oC)

-10 -2

-1

0

1

2

Voltage (mV)

Figure 3. The I-V curves of AgNW/TNC for DMSO evaporation casting process and overheating: (a) the linear I-V curves of AgNW/NC and AgNW/TNC and (b) the linear I-V curves of AgNW/TNC for normal drying times (30 minutes at 85 oC) and overheating times (2 hours at 85 oC).


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Figure 4. Morphology and transmittance by diverse condition of solvent evaporation casting process: (a)-(d) optical images of TNC by drying (a) DMSO (85oC), (b) EtOH (85oC), (b) EtOH (25oC) and (d) pristine, (e)-(h) SEM images of TNC by drying (e) DMSO (85oC), (f) EtOH (85oC), (g) EtOH (25oC) and (h) pristine, (i)-(l) AFM images of TNC by drying (i) DMSO (85oC), (j) EtOH (85oC), (k) EtOH (25oC) and (l) pristine, (m) surface roughness verse transmittance (The averages of Rrms are measured 3 times as shown in Figure S6) and (n) transmittances. Scale bars of (a)-(d) are 2 cm and scale bars of (e)-(l) are 5 µm.



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Figure 5. SEM images of 3M peel-off tests (a) AgNW/NC (b) AgNW/TNC after 3M tape peel-off test.


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Figure 6. IV curves for AgNW/TNC under (a) folding and (b) bending condition.



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Figure 7. Mechanical stability of AgNW/TNC evaluated using various methods according to the methods of refs 56-57: (a)-(e) ratio of change resistance (R/R0) of (a) compressive strain, (b) bending angle, (c) twist angle, (d) folding cycles and (e) bending cycles. Reproduced with permission from reference 56. Copyright 2010 John Wiley and Sons. Reproduced with permission from reference 57. Copyright 2014 John Wiley and Sons.


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Figure 8. Lamination of AgNW/TNC onto paper (a) Diagram of lamination, (b) Optical images of AgNW/TNCs onto UCSD newspaper, which can be see through the newspaper, (c) LED operation through AgNW/TNC onto newspaper which is applied by 2.5 V and resistance measurement of surface, (d)-(h) SEM images of AgNW/TNC onto newspaper (The red arrow is AgNW/TNC).



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Figure 9. LED circuit with AgNW/TNC onto papers (a) diagram of structure and optical images of operation, (b) the resistances for bending radius and (c) the change of folding cycles test (red dot is unfolding and black dot is folding). (d) The folding operation of LED circuit with AgNW/TNC onto papers, according to the method of ref 58. Reproduced with permission from reference 58. Copyright 2016 John Wiley and Sons.


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Figure 10. water resistance test for (a) AgNW/TNC and (b) AgNW/TNC onto paper and (c) results.



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Figure 11. The I-V curves of AgNW/TNC for humidity (The blue dot is pristine AgNW/TNC and the green dot is 85 % humidity at 85 oC).


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TOC


An Implantable Transparent Conductive Film with Water Resistance

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