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Highly Stretchable and Transparent Metal Nanowire Heater for Wearable Electronics Applications Sukjoon Hong, Habeom Lee, Jinhwan Lee, Jinhyeong Kwon, Seungyong Han, Young D. Suh, Hyunmin Cho, Jaeho Shin, Junyeob Yeo,* and Seung Hwan Ko* The electrically driven resistive heater finds its applications in broad areas including temperature maintenance in industrial processes,[1] heating of microchannel chips,[2] sensors[3,4] or displays,[5,6] defogging in cold climates[7–10] and even painting conservation.[11] Recently, great attention has been devoted to flexible and transparent heaters in particular due to the emergence of next-generation devices having irregular shapes on nonrigid and nonplanar substrate together with large growing market for smart windows in automobiles or residences that require high optical clarity.[12] The most commonly used conducting material for conventional transparent heaters has been conductive metal-oxide represented by indium tin oxide (ITO), but its usage in flexible or stretchable substrates is highly limited due to its fragile ceramic nature[13] and harsh processing conditions.[14] As a consequence, newly developed percolative nanostructured networks with low-dimensional carbon materials,[5,7,15,16] metal nanowires,[6,8,9,17] and hybrid nanocomposites[4,10,18] are the main focus of current research for pursuing simultaneous flexibility and transparency in such heaters. Based on those candidates, recent meaningful outcomes are reported in flexible transparent heaters obtaining high temperatures,[8] large-area uniform heating[9] and fast thermal response.[5,18] However, the latest progress on the development of next generation future electronic devices[19–21] in both industry and academia supports that the ultimate form of electrical heater should not only be flexible and transparent, but also highly stretchable to be ultimately applicable for wearable electronics. Stretchable and transparent heaters are expected to be particularly invaluable for personal thermal management[22] and healthcare purposes,[23,24] but their implementation brings additional requirements that are very challenging to satisfy all at once. While low sheet resistivity and high optical transmittance are prerequisites for any low-voltage transparent Dr. S. Hong, H. Lee, Dr. J. Lee, Dr. J. Kwon, Dr. S. Han, Y. D. Suh, H. Cho, J. Shin, Prof. S. H. Ko Applied Nano and Thermal Science Lab Department of Mechanical Engineering Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea E-mail: [email protected] Dr. J. Yeo Laser Thermal Lab Department of Mechanical Engineering University of California Berkeley, CA 94720, USA E-mail: [email protected]
DOI: 10.1002/adma.201500917
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heater, stretchability accompanied by mechanical, electrical, and thermal stability under dynamic strain is a new vital requirement for a wearable heater.[25] Additionally, it is preferable to have constituents of widely studied, lightweight, and nontoxic materials so that the resultant wearable heater is scalable and directly applicable to other relevant research.[26,27] Here, we demonstrate a highly stretchable and transparent Ag nanowire (STAN) heater for wearable electronics applications for the first time based on partially embedded Ag NW percolation network on polydimethylsiloxane (PDMS) film that operates under strain over 60% and other real-time deformations commonly associated with human motion such as bending and twisting. Ag NW network, based on its superior electrical conductivity at high aspect ratios,[28] has largely been studied as a stretchable or transparent conductor[29] to date, but no attempts have been made toward a highly transparent and stretchable heater for wearable electronics application. The unique structure of Ag NW network/PDMS reported in this study generates Joule heating with fast thermal response at enhanced stability to endure repeated large mechanical strain with small variance in resistance. It is confirmed that the proposed wearable STAN heater operates consistently at an elevated temperature under various mechanical deformation, whereas the spatial temperature distribution is further controlled by manipulating the spatial current density via patterning of the NW percolation network through direct laser ablation for the applications that require nonuniform or site specific heating. The examples of the STAN heaters attached to the glass vial and human wrist further prove its effectiveness in heat transfer to the adjacent material and the direct compatibility with human skin for versatile wearable applications. The schematic structure of the transparent and stretchable heater proposed in this Communication is illustrated in Figure 1a, which is composed of uniformly distributed Ag NW at random directions forming a percolation network on a PDMS film. For the preparation of the Ag NW percolation network, vacuum filtration transfer[29] is directly applied on the elastic PDMS film. The detailed information on the Ag NW synthesis and preparation of Ag NW percolation network can be found in the Experimental Section and Figure SI 1 of the Supporting Information. For the actual use of the prepared electrode as a stretchable heater, the PDMS film immediately after the vacuum filtration transfer of Ag NW is trimmed and fixed between two glass substrates. Detailed information for preparing electrical contacts is included in Figure SI 2, Supporting Information. The Ag NW network/PDMS electrode does not require any post thermal treatment,[30] since the Ag NW network/PDMS electrode immediately after the transfer already exhibits excellent
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 4744–4751
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COMMUNICATION Figure 1. Highly stretchable and transparent heater. a) Schematic illustration of the stretchable and transparent heater composed of Ag NW percolation network on PDMS film. b,c) Pseudocolor image at room temperature (left) and infrared camera thermal image (right) of a Ag NW/PDMS stretchable and transparent heater operating at 60 °C with (b) no strain and (c) at 60% strain condition.
electrical conductivity (Figure SI 3, Supporting Information), probably due to the mechanical pressing of the NW junction upon the transfer.[31] A constant DC bias voltage is applied at two ends of the Ag NW network electrode to induce electrically driven resistive Joule heating. The corresponding heater can be stretched altogether during its heating operation since the underlying substrate (PDMS) as well as the conducting medium (Ag NW network) are both stretchable while maintaining their electrical conductance and optical transmittance. Figure 1b,c is the representative demonstrations of the proposed transparent and stretchable heater captured by infrared (IR) camera during heating operation at stretched conditions of either 0% or 60%. Since the temperature change can be verified instantaneously through the IR camera, the applied voltage is controlled in situ to reach 60 °C in both cases. It is noticeable that the heater operates stably at both conditions, which is satisfactory for general wearable applications.[25,32] The photograph of the as-prepared STAN heater is shown in Figure 2a where the area subject to the Ag NW network transfer is denoted with blue dotted circle. The PDMS film exhibits superior optical transmittance even with the Ag NW percolation network. Besides, no apparent sign of degradation is found on either the PDMS film or the transferred Ag NW percolation network upon bending or twisting as shown in Figure 2b. The individual Ag NWs at 100–150 nm diameter with high aspect ratios are observable in the scanning electron microscope (SEM) image in Figure 2c, consisting of mutually connected conductive paths throughout the surface as reported elsewhere.[9] Its high magnification SEM image in the inset, however, reveals that these NWs are partially embedded in the PDMS substrate, which is a unique characteristics of the Ag NW network/PDMS electrode in this study. To be a competent stretchable and transparent heater, the resultant Ag NW network electrode is required to possess both high electrical conductivity and optical transparency with substantial stretchability. As these parameters are closely related to the amount of Ag NW on PDMS substrate, the spectral transmittance (Figure 3a) and strain-dependent resistance of Ag NW network (Figure 3b) are thoroughly investigated at different
Adv. Mater. 2015, 27, 4744–4751
Ag NW areal density in the range of 132 to 528 mg m−2. It is confirmed that the total transmittance at 550 nm reaches 93% for 132 mg m−2 when an empty PDMS at equivalent thickness is used as a reference, while the electrical resistance as well as the optical transmittance naturally decline as the density of Ag NW increases. The strain-dependent electrical characteristics of Ag NW network electrodes are measured on a motorized stages at every 5% strain for 5 cycles upon stretching and releasing. In every case, the resistance increases along with the applied strain, while the initial conductance is not fully recovered after a stretching/releasing cycle as analogous to the electrodes with similar configurations.[33] The strain dependent resistance
Figure 2. Macroscopic and microscopic image of stretchable and transparent heater. a) Digital image of as-prepared transparent and stretchable Ag NW/PDMS electrode. The area with Ag NW percolation network is denoted with a blue dotted line. b) Image of bent STAN heater. c) SEM image of Ag NW/PDMS electrode showing individual Ag NWs. Inset is a magnified image at NW junction.
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Figure 3. Optical, electrical, and mechanical characterization of stretchable and transparent heater. a) Optical transmittance of STAN heater for various areal NW density. b) Strain-dependent electrical resistance of STAN heater under repeated stretching test.
stabilizes in a few stretching/releasing cycles, yet this aging process is much more rapidly completed for the electrode with higher Ag NW density. It is also notable that the increase in resistance compared to its initial resistance (R/R0) is suppressed for the electrode with more Ag NW as confirmed from the slope of the strain–resistance curve in Figure 3b. Such small variation in the resistance at an applied strain, conceivably correlated to an increase in fused interconnections between Ag NWs,[17] is favorable for the realization of stretchable heaters operating at low voltage with minimal change in performance. The Ag NW/PDMS electrode after the aging process still exhibits excellent optical transmittance and electrical conductivity (>85% at 30 Ω sq−1) which enable low voltage operation of the STAN heater at high optical transparency. As shown in Figure 4, the Joule heating characteristics of the STAN heater are examined for 132 mg m−2 Ag NW density. Constant DC bias voltage is applied between two side ends of the target electrode, while the voltage is increased by 1 V every 1 min until the heater fails. Time dependent temperatures at various applied voltage are presented in Figure 4a together with the corresponding temperature distribution captured by IR camera as insets. The average temperature at the heating region is always lower than the maximum temperature with a smoother curve, and the increase in temperature is roughly proportional to the square of applied voltage as predicted. At 10 V bias voltage, we conjecture that the local temperature exceeds 200 °C at the moment of failure according to extrapolation from preceding data. The lower and upper operating temperature limits for PDMS are not well defined, but it is widely accepted that PDMS can withstand temperatures above 200 °C due to high thermal stability resulting from the strong siloxane bond. Since the temperature range required in general wearable applications is much lower,[32] the thermal stability of the STAN heater is estimated to be appropriate for our purpose. Besides, the distorted temperature profiles at the high voltages in Figure 4a insets are not from substrate damage, but due to the highly nonuniform surface from sagging of the underlying PDMS layer as it expands at high temperatures and deviates from the focal plane of the IR camera. The performance of the STAN heater is confirmed by applying stepwise tensile strain to the heater in real-time under a constant bias voltage. For the stretchable heater test,
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the Ag NW network electrode with higher Ag NW density at 396 mg m−2 has been employed to enable efficient heating at low voltage. The temperature changes according to the applied strain at 3, 4, and 5 V DC bias voltage are recorded in Figure 4b while the strain is increased up to 30% with 10% increments every 3 min. The STAN heaters exhibit rapid response in temperature upon the application of voltage and converge to a plateau in relation to the applied voltage. When a tensile strain is applied, the STAN heater works stably at a reduced temperature because heat generated by Joule heating at a fixed bias voltage is inversely proportional to the resistance of the target electrode and strained Ag NW network increase resistance. The repeatability of voltage dependent Joule heating is also confirmed in either 0% or 30% strain condition (see Figure SI 4, Supporting Information). A real-time movie clip of a STAN heater operating at 5 V DC is captured by IR camera and included as Supplementary Movie 1, Supporting Information, to validate the prominent thermal stability of the STAN heater. The change in temperature, along with the strain, is unavoidable even at constant resistance as the increase in the surface area also affects the maximum temperature of the heater. As a result, the temperature can be held constant at an arbitrary strain only by adjusting the applied voltage through a feedback module. In Figure 4c, the temperature of STAN heater is maintained constant at 50 °C while stretching by applying a different voltage at each strain. For instantaneous feedback in actual applications, the temporal response of the heater is an important parameter, and the response time is dependent on the thickness of the underlying substrate once the configuration and the amount of conducting material are fixed. By reducing the thickness of the PDMS film down to
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Highly Stretchable and Transparent Metal Nanowire Heater for Wearable Electronics Applications Sukjoon Hong, Habeom Lee, Jinhwan Lee, Jinhyeong Kwon, Seungyong Han, Young D. Suh, Hyunmin Cho, Jaeho Shin, Junyeob Yeo,* and Seung Hwan Ko* The electrically driven resistive heater finds its applications in broad areas including temperature maintenance in industrial processes,[1] heating of microchannel chips,[2] sensors[3,4] or displays,[5,6] defogging in cold climates[7–10] and even painting conservation.[11] Recently, great attention has been devoted to flexible and transparent heaters in particular due to the emergence of next-generation devices having irregular shapes on nonrigid and nonplanar substrate together with large growing market for smart windows in automobiles or residences that require high optical clarity.[12] The most commonly used conducting material for conventional transparent heaters has been conductive metal-oxide represented by indium tin oxide (ITO), but its usage in flexible or stretchable substrates is highly limited due to its fragile ceramic nature[13] and harsh processing conditions.[14] As a consequence, newly developed percolative nanostructured networks with low-dimensional carbon materials,[5,7,15,16] metal nanowires,[6,8,9,17] and hybrid nanocomposites[4,10,18] are the main focus of current research for pursuing simultaneous flexibility and transparency in such heaters. Based on those candidates, recent meaningful outcomes are reported in flexible transparent heaters obtaining high temperatures,[8] large-area uniform heating[9] and fast thermal response.[5,18] However, the latest progress on the development of next generation future electronic devices[19–21] in both industry and academia supports that the ultimate form of electrical heater should not only be flexible and transparent, but also highly stretchable to be ultimately applicable for wearable electronics. Stretchable and transparent heaters are expected to be particularly invaluable for personal thermal management[22] and healthcare purposes,[23,24] but their implementation brings additional requirements that are very challenging to satisfy all at once. While low sheet resistivity and high optical transmittance are prerequisites for any low-voltage transparent Dr. S. Hong, H. Lee, Dr. J. Lee, Dr. J. Kwon, Dr. S. Han, Y. D. Suh, H. Cho, J. Shin, Prof. S. H. Ko Applied Nano and Thermal Science Lab Department of Mechanical Engineering Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea E-mail: [email protected] Dr. J. Yeo Laser Thermal Lab Department of Mechanical Engineering University of California Berkeley, CA 94720, USA E-mail: [email protected]
DOI: 10.1002/adma.201500917
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wileyonlinelibrary.com
heater, stretchability accompanied by mechanical, electrical, and thermal stability under dynamic strain is a new vital requirement for a wearable heater.[25] Additionally, it is preferable to have constituents of widely studied, lightweight, and nontoxic materials so that the resultant wearable heater is scalable and directly applicable to other relevant research.[26,27] Here, we demonstrate a highly stretchable and transparent Ag nanowire (STAN) heater for wearable electronics applications for the first time based on partially embedded Ag NW percolation network on polydimethylsiloxane (PDMS) film that operates under strain over 60% and other real-time deformations commonly associated with human motion such as bending and twisting. Ag NW network, based on its superior electrical conductivity at high aspect ratios,[28] has largely been studied as a stretchable or transparent conductor[29] to date, but no attempts have been made toward a highly transparent and stretchable heater for wearable electronics application. The unique structure of Ag NW network/PDMS reported in this study generates Joule heating with fast thermal response at enhanced stability to endure repeated large mechanical strain with small variance in resistance. It is confirmed that the proposed wearable STAN heater operates consistently at an elevated temperature under various mechanical deformation, whereas the spatial temperature distribution is further controlled by manipulating the spatial current density via patterning of the NW percolation network through direct laser ablation for the applications that require nonuniform or site specific heating. The examples of the STAN heaters attached to the glass vial and human wrist further prove its effectiveness in heat transfer to the adjacent material and the direct compatibility with human skin for versatile wearable applications. The schematic structure of the transparent and stretchable heater proposed in this Communication is illustrated in Figure 1a, which is composed of uniformly distributed Ag NW at random directions forming a percolation network on a PDMS film. For the preparation of the Ag NW percolation network, vacuum filtration transfer[29] is directly applied on the elastic PDMS film. The detailed information on the Ag NW synthesis and preparation of Ag NW percolation network can be found in the Experimental Section and Figure SI 1 of the Supporting Information. For the actual use of the prepared electrode as a stretchable heater, the PDMS film immediately after the vacuum filtration transfer of Ag NW is trimmed and fixed between two glass substrates. Detailed information for preparing electrical contacts is included in Figure SI 2, Supporting Information. The Ag NW network/PDMS electrode does not require any post thermal treatment,[30] since the Ag NW network/PDMS electrode immediately after the transfer already exhibits excellent
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 4744–4751
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 1. Highly stretchable and transparent heater. a) Schematic illustration of the stretchable and transparent heater composed of Ag NW percolation network on PDMS film. b,c) Pseudocolor image at room temperature (left) and infrared camera thermal image (right) of a Ag NW/PDMS stretchable and transparent heater operating at 60 °C with (b) no strain and (c) at 60% strain condition.
electrical conductivity (Figure SI 3, Supporting Information), probably due to the mechanical pressing of the NW junction upon the transfer.[31] A constant DC bias voltage is applied at two ends of the Ag NW network electrode to induce electrically driven resistive Joule heating. The corresponding heater can be stretched altogether during its heating operation since the underlying substrate (PDMS) as well as the conducting medium (Ag NW network) are both stretchable while maintaining their electrical conductance and optical transmittance. Figure 1b,c is the representative demonstrations of the proposed transparent and stretchable heater captured by infrared (IR) camera during heating operation at stretched conditions of either 0% or 60%. Since the temperature change can be verified instantaneously through the IR camera, the applied voltage is controlled in situ to reach 60 °C in both cases. It is noticeable that the heater operates stably at both conditions, which is satisfactory for general wearable applications.[25,32] The photograph of the as-prepared STAN heater is shown in Figure 2a where the area subject to the Ag NW network transfer is denoted with blue dotted circle. The PDMS film exhibits superior optical transmittance even with the Ag NW percolation network. Besides, no apparent sign of degradation is found on either the PDMS film or the transferred Ag NW percolation network upon bending or twisting as shown in Figure 2b. The individual Ag NWs at 100–150 nm diameter with high aspect ratios are observable in the scanning electron microscope (SEM) image in Figure 2c, consisting of mutually connected conductive paths throughout the surface as reported elsewhere.[9] Its high magnification SEM image in the inset, however, reveals that these NWs are partially embedded in the PDMS substrate, which is a unique characteristics of the Ag NW network/PDMS electrode in this study. To be a competent stretchable and transparent heater, the resultant Ag NW network electrode is required to possess both high electrical conductivity and optical transparency with substantial stretchability. As these parameters are closely related to the amount of Ag NW on PDMS substrate, the spectral transmittance (Figure 3a) and strain-dependent resistance of Ag NW network (Figure 3b) are thoroughly investigated at different
Adv. Mater. 2015, 27, 4744–4751
Ag NW areal density in the range of 132 to 528 mg m−2. It is confirmed that the total transmittance at 550 nm reaches 93% for 132 mg m−2 when an empty PDMS at equivalent thickness is used as a reference, while the electrical resistance as well as the optical transmittance naturally decline as the density of Ag NW increases. The strain-dependent electrical characteristics of Ag NW network electrodes are measured on a motorized stages at every 5% strain for 5 cycles upon stretching and releasing. In every case, the resistance increases along with the applied strain, while the initial conductance is not fully recovered after a stretching/releasing cycle as analogous to the electrodes with similar configurations.[33] The strain dependent resistance
Figure 2. Macroscopic and microscopic image of stretchable and transparent heater. a) Digital image of as-prepared transparent and stretchable Ag NW/PDMS electrode. The area with Ag NW percolation network is denoted with a blue dotted line. b) Image of bent STAN heater. c) SEM image of Ag NW/PDMS electrode showing individual Ag NWs. Inset is a magnified image at NW junction.
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Figure 3. Optical, electrical, and mechanical characterization of stretchable and transparent heater. a) Optical transmittance of STAN heater for various areal NW density. b) Strain-dependent electrical resistance of STAN heater under repeated stretching test.
stabilizes in a few stretching/releasing cycles, yet this aging process is much more rapidly completed for the electrode with higher Ag NW density. It is also notable that the increase in resistance compared to its initial resistance (R/R0) is suppressed for the electrode with more Ag NW as confirmed from the slope of the strain–resistance curve in Figure 3b. Such small variation in the resistance at an applied strain, conceivably correlated to an increase in fused interconnections between Ag NWs,[17] is favorable for the realization of stretchable heaters operating at low voltage with minimal change in performance. The Ag NW/PDMS electrode after the aging process still exhibits excellent optical transmittance and electrical conductivity (>85% at 30 Ω sq−1) which enable low voltage operation of the STAN heater at high optical transparency. As shown in Figure 4, the Joule heating characteristics of the STAN heater are examined for 132 mg m−2 Ag NW density. Constant DC bias voltage is applied between two side ends of the target electrode, while the voltage is increased by 1 V every 1 min until the heater fails. Time dependent temperatures at various applied voltage are presented in Figure 4a together with the corresponding temperature distribution captured by IR camera as insets. The average temperature at the heating region is always lower than the maximum temperature with a smoother curve, and the increase in temperature is roughly proportional to the square of applied voltage as predicted. At 10 V bias voltage, we conjecture that the local temperature exceeds 200 °C at the moment of failure according to extrapolation from preceding data. The lower and upper operating temperature limits for PDMS are not well defined, but it is widely accepted that PDMS can withstand temperatures above 200 °C due to high thermal stability resulting from the strong siloxane bond. Since the temperature range required in general wearable applications is much lower,[32] the thermal stability of the STAN heater is estimated to be appropriate for our purpose. Besides, the distorted temperature profiles at the high voltages in Figure 4a insets are not from substrate damage, but due to the highly nonuniform surface from sagging of the underlying PDMS layer as it expands at high temperatures and deviates from the focal plane of the IR camera. The performance of the STAN heater is confirmed by applying stepwise tensile strain to the heater in real-time under a constant bias voltage. For the stretchable heater test,
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the Ag NW network electrode with higher Ag NW density at 396 mg m−2 has been employed to enable efficient heating at low voltage. The temperature changes according to the applied strain at 3, 4, and 5 V DC bias voltage are recorded in Figure 4b while the strain is increased up to 30% with 10% increments every 3 min. The STAN heaters exhibit rapid response in temperature upon the application of voltage and converge to a plateau in relation to the applied voltage. When a tensile strain is applied, the STAN heater works stably at a reduced temperature because heat generated by Joule heating at a fixed bias voltage is inversely proportional to the resistance of the target electrode and strained Ag NW network increase resistance. The repeatability of voltage dependent Joule heating is also confirmed in either 0% or 30% strain condition (see Figure SI 4, Supporting Information). A real-time movie clip of a STAN heater operating at 5 V DC is captured by IR camera and included as Supplementary Movie 1, Supporting Information, to validate the prominent thermal stability of the STAN heater. The change in temperature, along with the strain, is unavoidable even at constant resistance as the increase in the surface area also affects the maximum temperature of the heater. As a result, the temperature can be held constant at an arbitrary strain only by adjusting the applied voltage through a feedback module. In Figure 4c, the temperature of STAN heater is maintained constant at 50 °C while stretching by applying a different voltage at each strain. For instantaneous feedback in actual applications, the temporal response of the heater is an important parameter, and the response time is dependent on the thickness of the underlying substrate once the configuration and the amount of conducting material are fixed. By reducing the thickness of the PDMS film down to
