Reversibly Stretchable, Optically Transparent Radio-Frequency

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Reversibly Stretchable, Optically Transparent Radio-Frequency Antennas Based on Wavy Ag Nanowire Networks Byoung Soo Kim,†,‡,⊥ Keun-Young Shin,†,⊥ Jun Beom Pyo,†,⊥ Jonghwi Lee,‡ Jeong Gon Son,† Sang-Soo Lee,*,†,§ and Jong Hyuk Park*,† †

Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Department of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea § KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea ‡

S Supporting Information *

ABSTRACT: We report a facile approach for producing reversibly stretchable, optically transparent radio-frequency antennas based on wavy Ag nanowire (NW) networks. The wavy configuration of Ag NWs is obtained by floating the NW networks on the surface of water, followed by compression. Stretchable antennas are prepared by transferring the compressed NW networks onto elastomeric substrates. The resulting antennas show excellent performance under mechanical deformation due to the wavy configuration, which allows the release of stress applied to the NWs and an increase in the contact area between NWs. The antennas formed from the wavy NW networks exhibit a smaller return loss and a higher radiation efficiency when strained than the antennas formed from the straight NW networks, as well as an improved stability in cyclic deformation tests. Moreover, the wavy NW antennas require a relatively small quantity of NWs, which leads to low production costs and provides an optical transparency. These results demonstrate the potential of these wavy Ag NW antennas in applications of wireless communications for wearable systems. KEYWORDS: silver nanowire network, wavy configuration, stretchable transparent antenna, radio-frequency antenna, wearable antenna substrates can function as deformable antennas.22−24 In particular, a mechanically tunable, reversibly deformable AgNW-based antenna has recently been fabricated.25−27 Previous approaches, however, need to enhance the performance of the antennas under mechanical deformation; for example, the maximum strain for the Ag-NW-based antennas was insufficient and the stability in response to repeated cyclic deformation was not demonstrated.25−27 Manipulating the structure of NW networks is one approach used to improve the performance of NW-based antennas. Effective networking of NWs increases both the electrical conductivity and the mechanical deformability. It has been shown that wavy configurations of Ag NWs enable enhanced conductivity and deformability of the resulting networks.28 Moreover, the wavy networks require a smaller quantity of Ag NWs compared with the previous Ag-NW-based antennas.25−27 This is important for reducing production costs and improving an optical transparency. Herein, we describe a facile method of producing reversibly stretchable, optically transparent RF antennas using wavy Ag

1. INTRODUCTION Antennas are commonly used electrical devices that transmit and receive signals. Antennas for radio-frequency (RF) electromagnetic waves are important components for wireless communications.1−6 Antennas incorporated with sensors allow monitoring information on the human body in real-time via wireless communications. Thus, developing RF antennas that are suitable for wearable applications has significant potential in telemedicine.7−14 RF antennas for wearable systems should be electrically conductive and mechanically deformable. The electrical conductivity governs the radiating efficiency, and the mechanical deformability is critical to enable conformable contact with the moving body. However, conventional RF antennas have been fabricated by patterning metals on rigid substrates,15−17 which cannot easily be strained in response to mechanical deformations, such as bending, twisting, and stretching. To address this issue, mechanically pliable antennas with high electrical conductivity are required. Exploiting conductive one-dimensional nanomaterials is a promising method to provide deformable electrical devices in virtue of their electrical conductivity and mechanical flexibility.18−22 Indeed, it has been reported that conductive one-dimensional nanomaterials including carbon nanotubes and metal nanowires (NWs) deposited on elastomeric © XXXX American Chemical Society

Received: October 27, 2015 Accepted: January 13, 2016

A

DOI: 10.1021/acsami.5b10317 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic diagram showing the fabrication process used to form the wavy Ag-NW-based monopole patch RF antennas. (a) Deposition of Ag NWs on an AAO membrane via vacuum filtration. (b) Floating the Ag NW network on water by dipping the membrane into the water. (c) Compression of the floating Ag NW network to form the wavy configuration. (d) Transfer of the Ag NW network onto a PDMS substrate. (e) Photographs of a stretchable, transparent antenna formed using wavy Ag NWs. were plotted on the OM images (Figure S1 in the Supporting Information). NWs crossing two edges of the square grids were selected, and Lc and L0 were evaluated using the image processing software (ImageJ, National Institutes of Health). The average degree of waviness (WAvg) for each NW network was calculated from the W of 50 NWs from five OM images. The optical transmittance at visible wavelengths (i.e., 400−800 nm) was determined using an ultraviolet− visible spectrometer (V-600, Jasco). The sheet resistance of the Ag NW networks was measured using a four-point probe (Cresbox, Napson). At a given strain, the variation in the resistance of the Ag NW networks was analyzed using the in-house stretching equipment connected to a multimeter (6512, Keithley). The strain, applied uniaxially in the same direction as the compression during the formation of the wavy NW networks, was expressed as the ratio of the extended length to the initial length of the substrate (i.e., 20 mm). The stability of the resistance in response to cyclic deformation was investigated by applying cycles of 20% tensile strain, followed by zero strain, with a crosshead speed of 0.5 mm/s. 2.4. Characterization of Ag-NW-Based Antennas. The characteristics of the Ag-NW-based antennas at frequencies from 300 kHz to 8.5 GHz were examined using an RF network analyzer (E5071B, Agilent Technologies). In the range of operating frequencies (500 MHz to 3.0 GHz), the PDMS substrates had a relative permittivity of 2.5−3.0 and a loss tangent of 0.01−0.05. Impedance curves for each antenna were plotted on the Smith chart by normalizing to the characteristic impedance of the system (i.e., 50 Ω). The antennas were connected to SMA connectors, without using an external impedance matching network. A ground plane, which is a copper foil with dimensions of 100 mm × 80 mm, was directly connected to the power supply. Tensile stain was applied to the antennas using the in-house stretching equipment. The radiation pattern and efficiency of the antennas were measured in an anechoic chamber (Electromagnetic Wave Technology Institute, Korea). The measuring system contained a network analyzer (E8362B, Agilent Technologies), a base station simulator (E5515C, Agilent Technologies), a wibro/wimax communication tester (CMW270, Rohde & Schwarz), and a vector signal generator (E4438C, Agilent Technologies).

NW networks. The wavy configuration of the networks is achieved by floating the Ag NW networks on water and then compressing them. Stretchable antennas are obtained by transferring the NW networks onto elastomeric substrates. The wavy configuration of the NW networks provides excellent performance as antennas under mechanical deformation. Moreover, the resulting antennas exhibit a high optical transparency, which makes them suitable for many device applications.

2. EXPERIMENTAL SECTION 2.1. Materials. A 5 mg/mL dispersion of Ag NWs (Seashell Technology) was diluted with ethanol (∼2 μg/mL) prior to use. The average diameter of the Ag NWs was ∼115 nm, and they were ∼40 μm long. Anodized aluminum oxide (AAO) membranes with a pore size of ∼100 nm were acquired from Whatman. Polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning) substrates were prepared by mixing prepolymer and curing agent with a mass ratio of 10:1, and subsequent curing at 80 °C for 3 h. The PDMS substrates were approximately 200 μm thick. 2.2. Fabrication of Wavy Ag NW Networks. The Ag NW networks were formed by filtering the diluted NW solution using AAO membranes.19,28 Different concentrations of solution were used to obtain NW networks with areal densities of 0.066, 0.088, and 0.132 μg/mm2. The dimensions of the NW networks were 25 mm × 25 mm. By dipping the NW networks in water, they were detached from AAO membranes, and floated on the water surface. Wavy configurations of NWs were created via uniaxial compression. The difference of the length after the compression was 7.5 mm, which was ∼30% of the initial length (i.e., 25 mm). The resulting NW networks were transferred to PDMS substrates28,29 and then trimmed to have dimensions of ∼16 mm × 20 mm. Networks consisting of NWs with a straight configuration were also prepared by floating them on water and transferring to the substrates without compression. 2.3. Characterization of Wavy Ag NW Networks. Characterization was performed after transferring the Ag NW networks to PDMS substrates. The morphology of the Ag NW networks was observed using an optical microscope (OM) (DM 2500M, Leica) and a scanning electron microscope (SEM) (JEOL 6701F, JEOL). Using the OM images, the degree of waviness (W) of individual NWs in the network was determined as follows W = Lc /L0

3. RESULTS AND DISCUSSION Ag NWs have been used in a wide range of applications, including antennas, due to their high electrical conductivity. However, conventional Ag NW networks have shown limited stretchability due to the small yield strain of Ag NWs,30 making them inappropriate for wearable systems. The stretchability of

(1)

where Lc is the contour length and L0 is the end-to-end length of NWs. To obtain Lc and L0, square grids with dimensions of 25 μm × 25 μm B


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Figure 2. Optical micrographs of (a−c) straight Ag NW networks and (d−f) wavy Ag NW networks formed via compression of the straight Ag NWs with Δ = 30%. The initial areal densities of NW networks were (a) 0.066, (b) 0.088, and (c) 0.132 μg/mm2; following compression, the areal densities increased to (d) 0.094, (e) 0.126, and (f) 0.189 μg/mm2, respectively. The morphology was observed after transferring the NW networks to PDMS substrates.

Figure 3. Properties of the Ag NW networks before and after manipulating the configurations via compression with Δ = 30%. (a) The average degree of waviness, (b) the sheet resistance, and (c−d) the optical transmittance of the NW networks. These characterizations were carried out after transferring the networks to PDMS substrates.

that it floated on the water surface (Figure 1b). The surface tension of the water prevents the network from sinking. Third, the floating NW network was compressed uniaxially, which allows manipulation of the configuration of the NW network (Figure 1c). This water-assisted process reduces the friction between the network and the substrate and prevents the mechanical failure of the NW network. Fourth, the compressed NW network was transferred to an elastomeric substrate by contact from the air side of the water−air interface (Figure 1d). Finally, the NW network on the elastomeric substrate was dried, producing a wavy Ag NW antenna. Figure 1e shows photographs of a stretchable, transparent, monopole patch RF antenna. The radiating efficiency of antennas is largely affected by the electrical conductivity. To improve the performance of NW-

Ag NW networks has been increased by manipulating their structure.19,20,28,31,32 In particular, wavy Ag NW networks prepared by using a facile floating−compression method exhibited highly enhanced stretchability, making these wavy Ag NW networks suitable for deformable antennas.28 Figure 1 shows the floating−compression method used to form the wavy Ag NW networks. First, Ag NWs were deposited on an AAO membrane by filtering the NW dispersion, resulting in the formation of a Ag NW network (Figure 1a). It should be noted that vacuum filtration provides relatively strong contacts between the NWs at the junction points of the network compared to drop-casting. The contacts in the NW network are important because they improve mechanical stability and electrical conductivity. Second, the Ag NW network was detached from the membrane by dipping it into water, such C


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Figure 4. Electromechanical stability of the Ag NW networks with various areal densities and different configurations. (a) The resistance of the Ag NW networks as a function of strain. Some error bars are not shown as they are smaller than the size of the data points. (b) The resistance of the Ag NW networks as a function of the number of stretching cycles. Samples were repeatedly stretched to 20% and released back to 0%. (c) An illustration of the mechanism of the mechanical response to strain, as well as optical micrographs showing the straight and wavy NW networks with 0% strain and with 30% strain. The yellow circles in the image of the straight NW network indicate the breakage of the Ag NWs. The scale bars in the images are 20 μm.

NW networks exhibited different behavior in the structural transformation depending on areal density. Following compression, the NW networks with a lower areal density became wavier than those with a higher areal density. To understand the relationship between the structural transformation and the areal density, the waviness of NW networks needs to be quantified. For this, W of individual NWs was introduced, and WAvg of NW networks was calculated. Figure 3a shows WAvg as a function of the areal densities. Regardless of the areal density, the straight NW networks (i.e., with Δ = 0%) had almost identical Wavg values of approximately 1.03. However, Wavg of the compressed NW networks varied according to the initial areal density. For NW networks with initial areal densities of 0.066, 0.088, and 0.132 μg/mm2, following compression with Δ = 30%, Wavg values increased to 1.31, 1.20, and 1.09, respectively. The initial areal density of the NW networks was negatively correlated with Wavg following compression. This is due to the friction between the NWs at the junction points, which restricts the motion of NWs during compression and inhibits the development of the wavy configuration. In addition to the configuration of the NW networks, the areal density affects the sheet resistance, transparency, and stretchability. Figure 3b shows the sheet resistance of the NW networks as a function of the areal densities. The straight NW networks with areal densities of 0.066, 0.088, and 0.132 μg/mm2 had sheet resistances of 32.6, 12.4, and 3.8 Ω/sq, respectively.

based antennas, their resistance should be minimized. Under static conditions, using a large quantity of Ag NWs lowers the resistance of the films. However, because the presence of large quantities of Ag NWs leads to a large stiffness of the films, their mechanical deformability deteriorates as the quantity of NWs increases. Consequently, the quantity of Ag NWs in the network should be optimized to guarantee both electrical conductivity and mechanical deformability simultaneously. The quantity of Ag NWs in the network can be expressed as the areal density, i.e., the mass of Ag NWs divided by the area. In this study, Ag NW networks with areal densities of 0.066, 0.088, and 0.132 μg/mm2 were prepared. Then, a wavy configuration was formed in each network by using the floating−compression method. The properties of the Ag NW networks were characterized to find the optimal areal density for the fabrication of Ag NW antennas. Figure 2 shows the morphology of the NW networks with various areal densities before and after manipulating the configuration via the floating−compression method. Initially, most NWs in the networks appeared as straight lines. Following the compression process, wavy configurations developed in the NW networks. The compressive strain (Δ) that was applied to the NW networks was expressed as the ratio of the compressed length of the networks to the initial length. For all networks, the process resulted in Δ = 30%; correspondingly, the areal densities increased from 0.066, 0.088, and 0.132 μg/mm2 to 0.094, 0.126, and 0.189 μg/mm2, respectively. However, the D


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Figure 5. Return loss (RL) as a function of strain for the Ag-NW-based antennas. The antennas were formed with straight NW networks with areal densities of (a) 0.088 and (b) 0.132 μg/mm2, as well as (c) a wavy NW network with an areal density of 0.126 μg/mm2. Curves were only measured when the RL at the resonant frequency was less than −15 dB. (d) The transmitted power at the resonant frequency as a function of strain.

because smaller amounts of Ag are required compared with conventional Ag NW antennas.25−27 Moreover, such a high transparency of NW-based antennas has not been reported previously, and therefore, the antennas produced in this study have the potential to extend the range of applications of NWbased antennas. Considering the properties of NW networks including the configuration, it was determined that the optimal areal density after compression for Ag-NW-based antennas is 0.126 μg/mm2. Figure 4a shows the resistance of both straight and wavy NW networks as a function of strain. The wavy NW network with an areal density of 0.126 μg/mm2 exhibited a significantly lower resistance over the entire range of applied strain than the equivalent NW network that was formed without compression (i.e., the straight NW network with an areal density of 0.088 μg/mm2). In addition, the wavy network exhibited an equivalent resistance at zero strain to the straight network with an areal density of 0.132 μg/mm2. Furthermore, as the strain increased, the wavy network exhibited only a slight increase in resistance, whereas the resistance of the straight network increased rapidly; this implies that the wavy configuration enhanced the stretchability of the NW network. Moreover, as shown in Figure S3 in the Supporting Information, with strains of over 20%, the resistance of the wavy network with an areal density of 0.126 μg/mm2 and WAvg of 1.20 was smaller than that of a wavy NW network with a higher areal density (0.189 μg/mm2) but a smaller WAvg (1.09). These show that the wavy configuration of the NW network resulted in a significant improvement in the stretchability compared with the straight NWs. To investigate the stability of NW networks in response to repeated stretching, the resistance of NW networks was

Following compression, the sheet resistance decreased because of the increase in areal density and the structural transformation of NWs. Compression reduced the area of the NW networks without changing the number of NWs. This resulted in the increase of the electrically conductive parts in the networks. In addition, compression induced microscopic overlapping and bundling of the wavy NWs (Figure S2d in the Supporting Information), increasing the contact areas between NWs, thus expanding the electrically conductive paths along the networks. The Ag NW networks with an areal density of 0.132 μg/mm2 showed the smallest reduction in the sheet resistance after compression. It was because the highest areal density restricted the structural transformation and so as the resistance reduction by the overlapping and bundling effects was minimal. As shown in Figure 3a, the largest structural transformation (change in waviness) by compression was observed in the NW networks with an areal density of 0.066 μg/mm2. Nevertheless, the NW networks with an areal density of 0.088 μg/mm2 (i.e., the medium areal density) exhibited the largest reduction in sheet resistance of 64% after compression. It was presumably because the number of NWs in the networks with the lowest areal density was insufficient to form the effective conductive paths among NWs. Figure 3c shows the optical transmittance of the NW networks as a function of the areal densities. The transmittance decreased as the areal density increased. Nevertheless, all straight NW networks exhibited high transmittance of over 80% at 550 nm and, except for the densest network, the compressed NW networks maintained optical transmittances higher than 80% at 550 nm, as shown in Figure 3d. From the optical transmittance, the wavy configuration of the NW networks appears to be favorable for reducing manufacturing costs, E


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Figure 6. Return loss (RL) as a function of the number of stretching cycles for Ag-NW-based antennas. The cyclic deformation was applied to the antennas by stretching 20% tensile strain, followed by zero strain, repeatedly. The antennas were formed with straight NW networks with areal densities of (a) 0.088 and (b) 0.132 μg/mm2, and (c) a wavy NW network with an areal density of 0.126 μg/mm2. Curves were only measured when the RL at the resonant frequency was less than −15 dB. (d) The transmitted power at the resonant frequency as a function of the number of stretching cycles.

that the antennas can radiate or receive signals properly. At zero strain, all of the NW-based antennas exhibited a relatively small RL (less than −15 dB) with a wide bandwidth of at least 300 MHz, which is suitable for practical applications. The RL of the antennas is largely affected by the electrical conductivity of the NW network. Thus, initially, the antenna formed using the straight NW network with an areal density of 0.132 μg/mm2 exhibited a smaller RL than that formed using the wavy NW network with an areal density of 0.126 μg/mm2, which is consistent with the trend of the sheet resistance data in Figure 3b. However, the RL of the antennas formed using the straight NW network increased markedly as the strain increased, resulting in an RL of more than −15 dB at strains of over 20% (Figure 5a,b). This is due to the decrease in the electrical conductivity of the straight NW networks caused by mechanical failure of Ag NWs under large strains. On the contrary, the antenna formed with the wavy NW network exhibited excellent performance under large applied strains. The RL was approximately −21 dB at a strain of 20%, and remained at less than −15 dB at a strain of 40% (Figure 5c). Interestingly, the RL increased significantly as the strain increased beyond 30%, which corresponds to the compressive strain used during fabrication. This is presumably because Ag NWs practically straighten when the tensile strain reaches 30%. Figure 5d shows the transmitted power calculated from the RL of the NW antennas at the resonant frequency as a function of the strain. The antenna formed from the wavy NWs exhibited the largest transmitted power at strains of 20% or larger. Moreover, the stretchability of the antenna formed using the wavy NWs exceeded that of previous Ag-NW-based antennas.27 Although the previous antennas utilized a large quantity of Ag NWs

measured as a function of the number of stretching cycles. The resistance of the straight NW networks increased rapidly to over 200 Ω within 100 cycles, regardless of the areal density, as shown in Figure 4b; however, the resistance of the wavy NW network remained at approximately 50 Ω after 300 cycles. This implies that the wavy configuration was relatively stable in response to mechanical deformation, showing promising potential as deformable antennas with high reliability. Figure 4c shows optical micrographs of the morphology of the straight and wavy NW networks with zero strain and 30% strain. When the NW networks were strained, clear differences in the structure appeared. Many of the straight NWs broke, due to tensile failure as a result of the mechanical deformation (see the yellow circles in Figure 4c). In contrast, the wavy NWs were able to withstand the applied strain by straightening, preventing breakage. PDMS substrates incorporating the straight and wavy Ag NW networks were used to form monopole patch antennas. The performance of the resulting antennas was investigated at various strains, as well as following repeated cyclic deformation. The return loss (RL) is a parameter that can be used to indicate the amount of power lost in the input signal. The bandwidth refers to the frequency range where the RL is below −10 dB. The RL in decibels (dB) can be expressed as follows RL (dB) = 10 log10

Pr Pi

(2)

where Pr is the reflected power and Pi is the incident power. Figure 5 shows RL curves for the antennas formed using Ag NW networks with straight and wavy configurations at various strains. The baseline for evaluating the RL at the resonant frequency was −15 dB (i.e., 96.84% of the transmitted power) F


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Figure 7. Three-dimensional radiation patterns as a function of the number of stretching cycles for Ag-NW-based antennas. The cyclic deformation was applied to the antennas by stretching 20% tensile strain, followed by zero strain, repeatedly. The antennas were formed with straight NW networks with areal densities of (a) 0.088 and (b) 0.132 μg/mm2, and (c) a wavy NW network with an areal density of 0.126 μg/mm2.

Table 1. Radiation Properties of Ag-NW-Based Antennas as a Function of the Number of Stretching Cycles straight NW network (0.088 μg/mm2)

straight NW network (0.132 μg/mm2)

wavy NW network (0.126 μg/mm2)

cycle (n)

resonant frequency (GHz)

peak gain (dBi)

radiation efficiency (%)

cycle (n)


peak gain (dBi)


cycle (n)


peak gain (dBi)


0 40

2.40 2.40

1.10 −3.18

19.02 8.09

0 60

2.20 2.20

2.09 −2.64

37.33 10.46

0 100 200 300

2.30 2.30 2.30 2.30

3.55 0.32 −1.50 −2.35

31.67 21.01 14.79 10.97

Information). These significant improvements in the reversible stretchability are attributed to the wavy configuration of the NW networks, which has great potential for deformable antennas formed using wavy Ag NWs. The variations in the radiation properties of the Ag-NWbased antennas in response to mechanical deformations were investigated. The radiation pattern and efficiency of the antennas were monitored in an anechoic chamber before and after stretching cycles. Figure 7 shows the three-dimensional radiation patterns as a function of the number of stretching cycles for Ag-NW-based antennas. Further details of the radiation properties of the antennas, including the resonant frequency, peak gain, and radiation efficiency, are summarized in Table 1. Initially, the Ag NW networks on PDMS substrates exhibited proper values of the radiation efficiency, which can be applied to omnidirectional monopole antennas. However, the radiation efficiency of the antennas formed with straight NW networks decreased significantly after only 40 or 60 cycles (Figure 7a,b). On the contrary, the wavy NW antennas maintained the radiation efficiency over 10% after 300 cycles (Figure 7c). These results demonstrate that wavy Ag NW networks are beneficial for the deformable antennas compared to straight NW networks. The two-dimensional radiation patterns as a function of the number of stretching cycles for Ag-NW-based antennas are also presented in Figures S5 and S6 in the Supporting Information.

embedded in an elastomeric substrate, the maximum strain was limited to 15%. The variation in the resonant frequency of the Ag-NW-based antennas was also investigated. As shown in Figure 5, the resonant frequency of the antennas shifted to higher frequencies as the applied tensile strain increased. In particular, the resonant frequency of the wavy NW antennas shifted by more than 150 MHz at a strain of 40%. This mechanical tunability of the resonant frequency has been reported previously in deformable antennas.25−27 Theoretical calculations have been predicted that the resonant frequency increases with the tensile strain. The calculations are consistent with the experimental results reported here. The resonant frequencies of the prepared antennas at zero strain differed even though they had nominally identical dimensions. This is presumably due to the errors resulting from fabrication tolerances. Further details of the antennas, including the resonant frequencies and bandwidths, can be found in the Supporting Information (Table S1). To examine the stability of the NW-based antennas in response to repeated deformations, cyclic stretching tests were performed (Figure 6). Antennas formed with straight NW networks with areal densities of 0.088 and 0.132 μg/mm2 exhibited a maximum number of cycles of 40 and 60, respectively, as shown in Figure 6a,b. In contrast, the wavy NW antennas maintained high transmitted power (>97.6%) over 300 cycles, as shown in Figure 6d. Such high stability in response to cyclic deformation has not been achieved previously. Moreover, the resonant frequency changed by less than 1% during the tests (Table S2 in the Supporting

4. CONCLUSIONS We have shown that a wavy network of Ag NWs can be used to fabricate optically transparent deformable antennas, which are G


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(2) Mak, A. C. K.; Rowell, C. R.; Murch, R. D.; Mak, C.-L. Reconfigurable Multiband Antenna Designs for Wireless Communication Devices. IEEE Trans. Antennas Propag. 2007, 55, 1919−1928. (3) Winters, J. H.; Salz, J.; Gitlin, R. D. The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems. IEEE Trans. Commun. 1994, 42, 1740−1751. (4) Patwari, N.; Ash, J. N.; Kyperountas, S.; Hero, A. O.; Moses, R. L.; Correal, N. S. Locating the Nodes: Cooperative Localization in Wireless Sensor Networks. IEEE Signal Process. Mag. 2005, 22, 54−69. (5) Cheng, S.; Wu, Z. A Microfluidic, Reversibly Stretchable, LargeArea Wireless Strain Sensor. Adv. Funct. Mater. 2011, 21, 2282−2290. (6) Alamouti, S. M. A Simple Transmit Diversity Technique for Wireless Communications. IEEE J. Sel. Areas Commun. 1998, 16, 1451−1458. (7) Son, D.; Lee, J.; Qiao, S.; Ghaffari, R.; Kim, J.; Lee, J. E.; Song, C.; Kim, S. J.; Lee, D. J.; Jun, S. W.; Yang, S.; Park, M.; Shin, J.; Do, K.; Lee, M.; Kang, K.; Hwang, C. S.; Lu, N.; Hyeon, T.; Kim, D.-H. Multifunctional Wearable Devices for Diagnosis and Therapy of Movement Disorders. Nat. Nanotechnol. 2014, 9, 397−404. (8) Choi, M. K.; Park, O. K.; Choi, C.; Qiao, S.; Ghaffari, R.; Kim, J.; Lee, D. J.; Kim, M.; Hyun, W.; Kim, S. J.; Hwang, H. J.; Kwon, S.-H.; Hyeon, T.; Lu, N.; Kim, D.-H. Cephalopod-Inspired Miniaturized Suction Cups for Smart Medical Skin. Adv. Healthcare Mater. 2016, 5, 80−87. (9) Gatzoulis, L.; Iakovidis, I. Wearable and Portable eHealth Systems. IEEE Eng. Med. Biol. Mag. 2007, 26, 51−56. (10) Hertleer, C.; Rogier, H.; Vallozzi, L.; Van Langenhove, L. A. Textile Antenna for Off-Body Communication Integrated into Protective Clothing for Firefighters. IEEE Trans. Antennas Propag. 2009, 57, 919−925. (11) Cotton, S. L.; Scanlon, W. G. Channel Characterization for Single- and Multiple-Antenna Wearable Systems Used for Indoor Body-to-Body Communications. IEEE Trans. Antennas Propag. 2009, 57, 980−990. (12) Kaufmann, T.; Fumeaux, C. Wearable Textile Half-Mode Substrate-Integrated Cavity Antenna Using Embroidered Vias. IEEE Antennas Wirel. Propag. Lett. 2013, 12, 805−808. (13) Hussain, A. M.; Ghaffar, F. A.; Park, S. I.; Rogers, J. A.; Shamim, A.; Hussain, M. M. Metal/Polymer Based Stretchable Antenna for Constant Frequency Far-Field Communication in Wearable Electronics. Adv. Funct. Mater. 2015, 25, 6565−6575. (14) Zeng, W.; Shu, L.; Li, Q.; Chen, S.; Wang, F.; Tao, X.-M. FiberBased Wearable Electronics: A Review of Materials, Fabrication, Devices, and Applications. Adv. Mater. 2014, 26, 5310−5336. (15) Alù, A.; Engheta, N. Tuning the Scattering Response of Optical Nanoantennas with Nanocircuit Loads. Nat. Photonics 2008, 2, 307− 310. (16) Zuo, J.; Chen, X.; Han, G.; Li, L.; Zhang, W. An Integrated Approach to RF Antenna-Filter Co-Design. IEEE Antennas Wirel. Propag. Lett. 2009, 8, 141−144. (17) Adams, J. J.; Duoss, E. B.; Malkowski, T. F.; Motala, M. J.; Ahn, B. Y.; Nuzzo, R. G.; Bernhard, J. T.; Lewis, J. A. Conformal Printing of Electrically Small Antennas on Three-Dimensional Surfaces. Adv. Mater. 2011, 23, 1335−1340. (18) Hu, L.; Kim, H. S.; Lee, J.-Y.; Peumans, P.; Cui, Y. Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes. ACS Nano 2010, 4, 2955−2963. (19) Lee, P.; Lee, J.; Lee, H.; Yeo, J.; Hong, S.; Nam, K. H.; Lee, D.; Lee, S. S.; Ko, S. H. Highly Stretchable and Highly Conductive Metal Electrode by Very Long Metal Nanowire Percolation Network. Adv. Mater. 2012, 24, 3326−3332. (20) Xu, F.; Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv. Mater. 2012, 24, 5117−5122. (21) Han, S.; Hong, S.; Ham, J.; Yeo, J.; Lee, J.; Kang, B.; Lee, P.; Kwon, J.; Lee, S. S.; Yang, M.-Y.; Ko, S. H. Fast Plasmonic Laser Nanowelding for a Cu-Nanowire Percolation Network for Flexible Transparent Conductors and Stretchable Electronics. Adv. Mater. 2014, 26, 5808−5814.

stable in response to cyclic stretching tests. The wavy configuration of the Ag NWs was achieved using a facile fabrication process, whereby the NW networks were floated on water, and then compressed. RF antennas were formed by transferring the compressed NW networks onto elastomeric substrates. The structural advantages of the wavy configuration enabled these NW networks to exhibit low resistance and enhanced stretchability compared with straight NW networks. These properties resulted in excellent performance of the antennas in response to deformation, including a small RL and a high radiation efficiency at a given strain and reversible stretchability. Moreover, the wavy NW antennas required a relatively small quantity of NWs, which leads to low production costs and provides an optical transparency. These results will be of importance to realize deformable antennas for wearable systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10317. Figures: Description of measuring degree of waviness, SEM images of straight and wavy Ag NW networks, electromechanical stability of the Ag NW networks with various areal densities and different configurations, photographs of a Ag-NW-based antenna mounted on the in-house stretching equipment, two-dimensional radiation patterns as a function of the number of stretching cycles. Tables: Resonant frequencies and bandwidths as a function of strain, resonant frequencies and bandwidths as a function of the number of stretching cycles (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.H.P.). *E-mail: [email protected] (S.-S.L.). Author Contributions ⊥

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. B.S.K., K.-Y.S., and J.B.P. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant (code no. 2011-0032156) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT & Future Planning, Republic of Korea. We also acknowledge the financial support from the R&D Convergence Program of National Research Council of Science and Technology of Republic of Korea and a Korea Institute of Science and Technology internal project. S.-S.L. appreciates the research grant from the KU-KIST Graduate School.

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REFERENCES

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Reversibly Stretchable, Optically Transparent Radio-Frequency

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