Inkjet Printing of Silver Nanowires for Stretchable Heaters

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Cite This: ACS Appl. Nano Mater. 2018, 1, 4528−4536

Inkjet Printing of Silver Nanowires for Stretchable Heaters Qijin Huang,† Karam Nashwan Al-Milaji,† and Hong Zhao*,† †

Virginia Commonwealth University, Department of Mechanical and Nuclear Engineering, BioTech One, 800 East Leigh Street, Richmond, Virginia 23219, United States

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S Supporting Information *

ABSTRACT: Inkjet printing is a promising technique for large-scale printed flexible and stretchable electronics. However, inkjet printing of silver nanowires (AgNWs) still presents many challenges. In this study, inkjet printing of high-concentration AgNW ink on flexible substrates is demonstrated, and liquid polydimethylsiloxane (PDMS) is then spin-coated on top of the printed AgNWs patterns to form stretchable conductors. We analyzed the relationship between the surface microstructure and electrical property of the stretchable AgNW conductors during various stretching/releasing cycles. Three consecutive stages of the resistance change, including conditioning, equilibrium, and rising stages, can be observed as a result of the morphology change. We also have demonstrated that the inkjet-printed stretchable AgNW conductor can be used as a stretchable heater. All of these characteristics indicate the excellent potential of inkjet printing of AgNWs for developing large-area flexible and stretchable electronics. KEYWORDS: inkjet printing, silver nanowires, stretchable electronics, stretching/releasing cycles, stretchable heaters

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roll-to-roll manufacturing process.35 Among all conductive nanomaterials, random percolation network of metal nanowires, such as silver nanowires (AgNWs), have shown excellent potential as flexible and stretchable conductors due to their high electrical conductivity and mechanical ductility.11,36−39 However, direct deposition of AgNWs using inkjet printing technique still presents many challenges: high aspect ratio of AgNWs makes it extremely difficult to print without blocking of the jetting nozzle and the low volume fraction of AgNWs in the printing inks leads to multiple printing passes and challenges in solvent management.40−42 Therefore, studies on inkjet printing of AgNW inks with both high solid concentrations and stable jetting should be performed. A basic requirement for flexible and stretchable conductors is to maintain adequate conductivity under a large deformation, especially for stretchable electronics. When depositing AgNWs onto a substrate or embedding them into an elastic matrix, the electrical resistance of AgNW conductor has been found to increase with an increasing strain and often

INTRODUCTION Flexible and stretchable conductors, which functionalize in building connections between different components of the devices, are key constitutions in flexible and stretchable electronics.1−5 They have been the focus of extensive research for various applications, e.g., stretchable antennas, wearable sensors, energy devices, and displays, etc.6−10 Flexible and stretchable conductors are typically fabricated by depositing conductive nanomaterials onto a surface or embedding them into a plastic, fabric, or elastic substrate/matrix.11−14 Most of the existing methods to deposit conductive nanomaterials are based on solution coating and deposition, including drop casting, spin coating, spray coating, and Meyer rod coating.15−20 For the development of flexible and stretchable electronics, the low-cost and scalable manufacturing using printing techniques, such as inkjet printing,10,21−27 screen/ stencil printing,28−30 and electrohydrodynamic (EHD) jet printing,31,32 could provide a large degree of freedom in designing and patterning target devices with both high electrical and mechanical performance. Inkjet printing is particularly attractive in making functional electronic devices10 due to its direct writing process, excellent material compatibility,33,34 and its capability to integrate into a high throughput, © 2018 American Chemical Society

Received: May 17, 2018 Accepted: August 28, 2018 Published: August 28, 2018 4528

DOI: 10.1021/acsanm.8b00830 ACS Appl. Nano Mater. 2018, 1, 4528−4536

Article

ACS Applied Nano Materials

Figure 1. (a) Schematic illustration of the inkjet printing process. (b) Photographic image of the droplet during the printing process. (c) Inkjetprinted single pass of AgNW lines on the PET substrate with different line lengths of L = 5, 10, 15, 20, 30, and 40 mm, respectively. (d) Inkjetprinted 20 mm-length of AgNW lines on the photo paper substrate with different printing passes of N = 1, 2, 3 and 4, respectively. (e) SEM image of the printed AgNW line in the central area. (f) SEM image of the printed AgNW line in the edge area. (g) Photographic image of the inkjetprinted letters of “VCU”. (h) Photographic image of a 3 × 3 capacitor array defined by interdigitated electrodes. The flexible PET substrate is used in parts e−h.

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exhibit a hysteresis between stretching/releasing cycles.43,44 However, relationships between microstructure and each stretching/releasing cycle still need systematic investigations. Herein, we report inkjet printing of AgNW ink with a high solid concentration of 10 mg mL−1 on a variety of flexible substrates, including photo paper and polyethylene terephthalate (PET). By transferring the inkjet-printed AgNW patterns onto a PDMS substrate, stretchable AgNW conductors can be obtained. The strain-sensing characteristics of the stretchable AgNW conductors including stretching/ releasing response and hysteresis performance have been investigated. Three different stages of the resistance change, including conditioning, equilibrium, and rising stages, can be observed during the entire stretching/releasing cycles. Finally, the stretchable AgNW conductor is demonstrated as a stretchable heater with a thermochromic dye indicating its temperature.

RESULTS AND DISCUSSION

Employing inkjet printing technique to fabricate AgNW patterns requires the formulation of suitable ink. To formulate proper AgNW ink, several parameters, including the length of the AgNWs, the AgNW concentration, and the ink solvent, were carefully considered. These parameters are crucial for a stable printing process and fabrication of uniform AgNW patterns. The printhead used in our work has a nozzle diameter (a) of 80 μm. To eliminate nozzle clogging during printing, the longest dimension of the nanomaterials should be less than a/ 50.40,45 However, this requirement might not be as stringent for nanowires due to the possibility of shear flow-induced alignment as the 1-dimensional (1D) wires pass through the nozzles.40 The alignment phenomenon for the printed AgNWs has been observed in others’ work.31,46 Thus, the as-prepared AgNWs with ∼14.3 μm in length were reduced to ∼3.6 μm by sonication-driven scission process in an ultrasonic bath for 15 h (Figure S1). It is worth noting that increasing the ultrasonic bath time leads to further shorten the length of the AgNWs. 4529

DOI: 10.1021/acsanm.8b00830 ACS Appl. Nano Mater. 2018, 1, 4528−4536

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ACS Applied Nano Materials

Figure 2. (a) Schematic illustration of the fabrication of stretchable AgNW conductor. (b) Geometrical shape and dimensions of the dog boneshaped sample used for stretching/releasing test. The AgNW conductor shown in the figure is not to scale. (c) Measured resistance of the AgNW conductors with different line lengths. The red line shows the linear fitting of the first 5 data points. (d) Cross-sectional SEM image of the AgNW conductor (N = 4).

However, silver nanoparticles can be seen in the ink, which influences the electrical and mechanical properties of the final stretchable AgNW conductors. Pure ethanol is used as the solvent, since ethanol dries fast after the droplet is deposited on the substrate. Minimum coffee-ring formation is exhibited in the printed AgNW patterns due to fast removal of the solvent by imbibing into the substrates and evaporation. The solid concentration of the AgNW ink is 10 mg mL−1, which is much higher than those reported by other groups.40,42,47 Thus, only a few printing passes are needed to create effective AgNW percolation for conductive patterns, which can significantly reduce the printing time. The printing process is schematically illustrated in Figure 1a. Briefly, 1 mL of AgNW ink was loaded into the ink cartridge. By optimizing the jetting waveform, stable and consistent inkjet droplets can be generated without satellites (Figure 1b). The droplet diameter is ∼102 μm which is equivalent to ∼550 pL in volume. To obtain uniform patterns, the droplet spacing is fixed at 100 μm. By using the optimal inkjet printing process (i.e., ink formulation, substrate selection, and printing parameters), we were able to print uniform AgNW lines on flexible PET substrates (Figure 1c) and photo paper substrates (Figure 1d) with different printing passes (N) and line length (L). Figure S2 present the optical images of the printed lines on the PET substrate and the photo paper substrate with N = 1 and 4, respectively. The widths (w) of the printed lines on the PET substrate are about 240 μm (single pass) and 370 μm (4 passes). The printed lines spread less on the photo paper substrate than those printed on the PET substrate probably

due to capillary wicking and pinning effect. Figure 1e,f shows the top-view scanning electron microscope (SEM) images of the AgNWs at the center and the edge of the printed AgNW line with 4 printing passes. In the central area, the AgNWs are randomly oriented forming a percolation network. Moreover, the AgNW line has relatively clean and smooth edges, which is essential to the reproducibility and stability of circuit design. Figure 1g,h shows the letters of “VCU” and a 3 × 3 capacitor array defined by interdigitated electrodes printed with a single pass on the PET substrate. The printed patterns on photo paper substrates are shown in Figure S3. The enlargement optical images of the interdigitated electrodes are provided in Figure S4. Figure 2a illustrates the fabrication process of the stretchable AgNW conductor. Briefly, the printed AgNW lines on PET substrate were placed into an oven and dried at 80 °C for 1 h to remove the solvent and form a conductive film of AgNW network. Next, liquid PDMS was spin-coated onto the AgNW lines. The liquid PDMS dried at room temperature for 1 h, followed by curing at 80 °C for 30 min. After curing the PDMS, the AgNWs were buried into the PDMS surface without significant voids. After peeling off, the AgNWs were successfully transferred from the PET substrate to the PDMS substrate, and stretchable AgNW conductors were fabricated. To test the electrical resistance and stretchability, two large dots (diameter of ∼1.5 mm) as contact pads, were printed at both ends of the AgNW conductor during the inkjet printing process. Furthermore, the AgNW conductors were cut into dog bone-shaped samples, as schematically illustrated in Figure 4530

DOI: 10.1021/acsanm.8b00830 ACS Appl. Nano Mater. 2018, 1, 4528−4536

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ACS Applied Nano Materials

Figure 3. (a) Resistance of AgNW conductor during 50 stretching/releasing cycles between 0 and 50% strain. (b) Resistance of AgNW conductor during the first 15 stretching/releasing cycles between 0 and 50% strain. (c) Resistance change of the AgNW conductor as a function of strain in the conditioning stage. (d) Resistance change of the AgNW conductor as a function of strain in the equilibrium stage.

PDMS, which forms the conductive and stretchable layer, and the bottom layer is pure PDMS, which acts as the stretchable substrate. The thickness (h) of the top AgNW/PDMS composite is estimated ∼300−500 nm. The electrical conductivity (σ) is further calculated following the equation: σ =L/(Rwh), where L is the line length and w is the line width. The AgNW conductor with 4 printing passes has a typical electrical conductivity of ∼5300−8900 S cm−1 before stretching, which is better than or comparable with that of other stretchable AgNW conductors with similar embedding structures (Table S1). To investigate the electrical performance as a function of stretching/releasing cycles, the stretchable AgNW conductors were extended to 50% strain and relaxed back to zero strain condition at the same rate. The stretching/releasing speed in this work was 0.4 mm/s. Three AgNW conductors were tested for each stretching/releasing test. Figure 3a shows the resistance of AgNW conductor during 50 stretching/releasing cycles between 0 and 50% strain. The initial resistance (R0) of the AgNW conductor is 161.3 Ω. The resistance increases to 286.9 Ω during the first stretching to 50% strain, and after releasing, the resistance cannot go back to the initial value at 0% strain. The resistance after the first stretching/releasing cycle is 195.4 Ω. Namely, a large increase of 77.9% in resistance occurs during its first stretching and an increase over 21% in resistance resulted after the first stretching/releasing

2b. The dog bone-shaped samples were tested on a homemade stretcher with the stretching/releasing direction (X-direction) parallel to the longitudinal axis of the samples. With an increase in printing pass, the density of the AgNWs in the conductor increases; thus, the resistance of the conductor decreases. Figure S5 shows the measured resistance (R) of AgNW conductor as a function of printing pass. When the printing pass increases from 2 to 8, the resistance of the conductor is reduced from 1865 Ω to 147 Ω. It is worth noting that the printed lines become nonuniform with multiple printing passes (e.g., N = 8) due to excessive solvent and insufficient drying time between printing passes. Therefore, four printing passes are used for the following experiment. The measured resistance of AgNW conductors with different lengths is displayed in Figure 2c. For a line length no longer than 30 mm, the resistance of the conductors shows a clear linear correlation with a y-intercept close to zero, indicating very good uniformity of the printed lines. Moreover, it also indicates that the contact resistance at the interface between the AgNW lines and the contact pad is negligible relative to the AgNW line resistance. With an increase in the line length, the printed patterns show somewhat ragged line edges and the resistance per unit length is larger. Thus, lines with 20 mm length is used for the following tests. Figure 2d shows the cross-section image of the stretchable AgNW conductor. It can be seen that the top layer is a composite of AgNWs and 4531

DOI: 10.1021/acsanm.8b00830 ACS Appl. Nano Mater. 2018, 1, 4528−4536

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Figure 4. (a and b) SEM images of the AgNW conductor after the 1st stretching/releasing cycle between 0 and 50% strain. (c and d) SEM images of the AgNW conductor after the 50th stretching/releasing cycle between 0 and 50% strain.

base (∼200 μm). When the AgNW conductor is stretched, the AgNWs are more spatially separated with an attempt to align themselves along the stretching direction to accommodate the deformation, while the surface of the AgNWs/PDMS composite film may not show obvious changes. When the strain is released, macroscopically, the conductor with the PDMS base could completely recover to its initial length. However, the AgNWs can only slide back to a certain degree but cannot completely go back to their initial position due to the frictional forces between the AgNWs and the PDMS. The preferred orientation or alignment of 1D nanomaterials along the stretching direction has been found in copper nanowireand carbon nanotube-based stretchable conductors, respectively.48,49 The alignment of the AgNWs reduces the electrical resistance of the AgNW conductor. Thus, during the following second to fourth stretching/releasing cycles between 0 and 50% strain in the conditioning stage, the preferred orientation or alignment of the AgNWs causes a slight decrease in resistance. The resistance of the conductor after the fourth cycle is 179.8 Ω in comparison with the resistance of 195.4 Ω after the first cycle. During the fifth to eighth cycles, it is the equilibrium stage and the hysteresis curves of each cycle nearly overlap with each other (Figure 3d). From the ninth cycle, the rising stage begins. We also compared the resistance change of the 11th, 21st, 31st, and 41st stretching/releasing cycles (Figure S7). The resistance of the conductor increases gradually, indicating an irreversible increase in the number of disconnected AgNWs. Figure 4c,d shows the SEM images of

cycle. During the second to fourth stretching/releasing cycles, the resistances at 50% strain and at 0% strain both show moderate decreases. However, during the fifth to eighth cycles, the resistances at 50% strain and at 0% strain are pretty stable. From the ninth cycle and beyond, the resistance at 50% strain and at 0% strain both increase gradually. After 50 stretching/ releasing cycles, the resistance at 0% strain increases to 340.4 Ω, which is 2.09 times of the initial resistance R0. From the above observation, we hypothesize that the resistance change during stretching/releasing cycles experiences three consecutive stages, i.e., conditioning stage, equilibrium stage, and rising stage (Figure 3b). Different resistance changes of the AgNW conductor as a function of strain in both conditioning and equilibrium stages can also be seen in the hysteresis curves in Figure 3c,d, respectively. To probe the underlying mechanism of the electrical behavior under stretching/releasing cycles, the morphologies of the AgNW conductors were further analyzed by SEM. Figure 4a,b show the SEM images of the conductor after the first stretching/releasing cycle. It can be observed that surface buckling appears after the first cycle, which is perpendicular to the stretching/releasing direction. The periodic wrinkle pattern with ∼2 μm waves appears on the surface of the conductor. Schematic illustration of the formation mechanism of buckling structure during the first stretching/releasing cycle is shown in Figure S6. The stretchable AgNW conductor can be treated as two layers: the top layer is a thin AgNWs/PDMS composite film (less than 1 μm) and the bottom layer is a thick PDMS 4532

DOI: 10.1021/acsanm.8b00830 ACS Appl. Nano Mater. 2018, 1, 4528−4536

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Figure 5. (a) Resistance of AgNW conductor during 100 stretching/releasing cycles between 0 and 25% strain. Inset of part a is the first 30 cycles. (b) Resistance of AgNW conductor during 50 stretching/releasing cycles between 0 and 75% strain. (c) SEM image of the AgNW conductor after the 50th stretching/releasing cycle between 0 and 75% strain. (d) The electrical response curves of the 1st stretching/releasing cycle at the ultimate strains of 25%, 50%, and 75%.

PDMS composite film and the sandwich structure of PDMS/ AgNW/PDMS was formed. AgNWs are perfectly embedded at the center of the stretchable conductor. As shown in Figure S8a, the resistance shows almost no change in the 0% and 50% strain form the 13th to 50th, which is significantly improved from the previous one (Figure 3a). The designed sandwich structure results in a more uniform distribution of strain, a higher resistance to surface buckling, and less damage to the embedded AgNWs. The other method is to precondition the AgNW conductor at a larger strain and then use it at a smaller strain. The AgNW conductor was first stretched and released for 5 cycles between 0% and 75% strain and then tested for cyclic stretching/releasing at 25% strain. As shown in Figure S8b, the resistance shows almost no change in each of the following stretching/releasing cycles. Both of the two methods are expected to improve the electrical and mechanical performance of the stretchable AgNW conductor. Stretchable heaters are promising candidates for personal thermal management, thermochromic color indicators, and healthcare purposes.50−54 Our inkjet-printed stretchable AgNW conductor can be used as a stretchable heater, which provides spatially controlled heating. The monochrome image, which was used for printing the AgNW heater (VCU letters), is shown in Figure S9. In this study, a thermochromic dye is used

the AgNW conductor after the 50th stretching/releasing cycle. It can be seen that the deeper buckling and fracture of AgNWs/PDMS composite film lead to a permanent loss of AgNWs’ contact as well as detachment of some AgNWs from the PDMS surface due to the out-of-plane buckling of AgNWs. Figure 5a,b shows the resistance of printed stretchable conductor as a function of cyclic strains of 25% and 75%, respectively. Both of them show the typical three stages during the entire stretching/releasing cycles. As shown in Figure 5a, the equilibrium stage lasts longer for 25% strain, which means the stretchable conductor with buckling structure could keep stable for more cycles at a small strain. A larger resistance increase can be observed at 75% strain after 50 stretching cycles (Figure 5b) and cracks of PDMS are visible (Figure 5c). We also compared the resistance change of the first stretching/ releasing cycle at 25%, 50%, and 75% strain. As shown in Figure 5d, the stretching curves of the three different strains can almost overlap with each other, further suggesting the reproducibility of our inkjet-printed process. To effectively suppress the increase of resistance during the stretching/releasing cycles, two possible methods are proposed in our study. One method is to encapsulate the as-prepared AgNW conductor by employing another layer of PDMS on top of it. A layer of PDMS was drop-casted on top of the AgNWs/ 4533

DOI: 10.1021/acsanm.8b00830 ACS Appl. Nano Mater. 2018, 1, 4528−4536

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Figure 6. Optical images of the stretchable heater/thermochromic color indicators working at (a−d) 0% strain and (e−h) 25% strain. Scale bars are 10 mm.

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as the color indicator for temperature. The well-mixed thermochromic elastomer was drop-casted on top of the printed AgNW heater. After drying and peeling off, the stretchable heater laminated with the thermochromic color indicator was fabricated with thickness of ∼2 mm. The thermochromic dye has a switching temperature of 47 °C. The thermochromic color indicator appears black at room temperature and the VCU letters on the back of the indicator are invisible (Figure 6a). Under the operating voltage of 10 V, the black indicator gradually becomes transparent (Figure 6b,c) with the increase of heating time; and the VCU letters on the back of the indicator can clearly be seen after 240 s (Figure 6d). It is also noticed that due to the asymmetric letter “C” in the pattern, the right-hand side of it is not well heated leading to a black spot (lower temperature) on the heater. Inkjet printing has the advantage of designing on-demand spatial distribution of resistance for specific heating applications. At 25% strain, the black indicator can also become partially transparent (Figure 6e−h), but it takes longer time to heat up due to its higher resistance under stretching. The stretchable heater demonstrated in this study enables a wider application in wearable health care devices.

EXPERIMENTAL SECTION

AgNW Ink Preparation. AgNWs were synthesized using a modified polyol reduction method.55,56 The as-prepared AgNWs have an average length of 14.3 μm and an average diameter of 80 nm. Ultrasonication-driven scission of the AgNWs was carried out to shorten their length in an ultrasonic cleaning bath (Fisher Scientific, MH series) operating at a frequency of 40 kHz ± 6% and a rated power of 110 W. The ultrasonic time is 15 h. Then, the shorter AgNWs with an average length of 3.6 μm were dispersed in pure ethanol to form the AgNW ink with a concentration of 10 mg mL−1 for the inkjet printing process. Inkjet Printing and Fabrication of Stretchable AgNW Conductor. The printing of the AgNW ink was performed by an inkjet printing platform (Jetlab 4, MicroFab). The platform consists of four printing stations, and only one station was used in this study driven by a waveform generator (Jetdriver III, MicroFab). The diameter of the nozzle is 80 μm (MJ-ATP-01-80-8MX, MicroFab). The printing frequency was set at 500 Hz. A customized waveform was utilized, which had a maximum voltage of 160 V and a pulse width of 130 μs. A stable jetting was obtained without satellite droplets. The coated PET (Mitsubishi NB-TP-3GU100) and photo paper (HP Premium Plus Photo Paper, Glossy) were used as flexible substrates. For printing AgNW lines, the droplet spacing of 100 μm and lines with varied lengths and printing passes were employed. To facilitate testing the electrical and mechanical performance of the AgNW conductor, 1000 droplets were jetted at two ends of each line to form the contact pads. To print the letters of “VCU”, the 3 × 3 capacitor array, and the heater patterns, monochrome bitmap files were used and the spacing in two directions (X-step and Y-step) were both 100 μm. After printing, the AgNW lines on the PET substrate were dried in an oven at 80 °C for 1 h. Liquid PDMS (Sylgard 184, Dow Corning) base and curing agent in the proportion of 10:1 by weight were mixed in a planetary mixer (ARE-310, Thinky Mixer) for 30 s at 2000 rpm and then degassed for an additional 30 s at 2200 rpm. The well-mixed PDMS was spin-coated (500 rpm for 20 s) on top of the printed patterns. Subsequently, the PDMS was dried at room temperature for 1 h to ensure that it penetrated into the AgNW patterns and then dried at 80 °C for 30 min. After peeling off from the PET substrate, the stretchable AgNW conductors with thickness of ∼200 μm were fabricated. Fabrication of Stretchable Thermochromic Color Indicator. Leuco dye (black 47C, LCR Hallcrest) was first mixed with liquid PDMS (Sylgard 184, Dow Corning) at a weight ratio of 1:30 to prepare thermochromic elastomer. The well-mixed thermochromic elastomer was drop-casted on top of the inkjet-printed patterns. After drying and peeling off, the stretchable thermochromic color indicator was fabricated. Characterization. The morphologies and dimensions of the AgNW lines were obtained by using an optical microscope (Axio Scope.A1, Zeiss) and field-emission scanning electron microscopy (FE-SEM, HITACHI SU-70) operated at 5 kV. A Fluke 289 TrueRMS Industrial Logging Multimeter was used to measure the

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CONCLUSION AND FUTURE PERSPECTIVES In summary, a high concentration AgNW ink was formulated and inkjet printed on flexible substrates. Stretchable AgNW conductors were fabricated by inkjet printing technique based on AgNW/PDMS composite. We analyzed the relationship between the surface microstructure and electrical property of the stretchable conductor during various stretching/releasing cycles. Three consecutive stages, including conditioning, equilibrium, and rising stages, can be observed. Moreover, encapsulating the AgNW conductor by another layer of PDMS or preconditioning the AgNW conductor could improve the electrical and mechanical performance of the stretchable conductor. In addition, inkjet-printed stretchable AgNW conductor can be used as stretchable heaters, indicating that integration of AgNWs with inkjet printing holds promising potential for commercially relevant, highly scalable applications in flexible and stretchable electronics. Inkjet printing of AgNWs for flexible and stretchable electronics is very important, especially stretchable AgNW conductors that can be patterned in a single inkjet printing process. Future work will be conducted in the fabrication of embedded conductive AgNW lines by inkjet printing, perhaps by directly inkjet printing of AgNWs into liquid PDMS, where printing and packaging can be done in a single step. This will promote further growth of printable stretchable electronics. 4534

DOI: 10.1021/acsanm.8b00830 ACS Appl. Nano Mater. 2018, 1, 4528−4536

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ACS Applied Nano Materials resistance of the AgNW conductor. To test the electrical and mechanical performance, the dog bone-shaped samples were used for the measurements. Two ends of the samples were attached to a mechanical stretcher and then stretching/releasing cycles were applied to the samples. The effective length of the AgNW conductor is 20 mm, and the stretching/releasing speed is 0.4 mm/s.

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(4) Mohammed, M. G.; Kramer, R. All-Printed Flexible and Stretchable Electronics. Adv. Mater. 2017, 29, 1604965. (5) Kim, H. J.; Sim, K.; Thukral, A.; Yu, C. Rubbery Electronics and Sensors from Intrinsically Stretchable Elastomeric Composites of Semiconductors and Conductors. Sci. Adv. 2017, 3, e1701114. (6) Khan, S.; Lorenzelli, L.; Dahiya, R. S. Technologies for Printing Sensors and Electronics over Large Flexible Substrates: A Review. IEEE Sens. J. 2015, 15, 3164−3185. (7) Liu, Y.; Pharr, M.; Salvatore, G. A. Lab-on-Skin: A Review of Flexible and Stretchable Electronics for Wearable Health Monitoring. ACS Nano 2017, 11, 9614−9635. (8) Dickey, M. D. Stretchable and Soft Electronics Using Liquid Metals. Adv. Mater. 2017, 29, 1606425. (9) Li, D.; Lai, W. Y.; Zhang, Y. Z.; Huang, W. Printable Transparent Conductive Films for Flexible Electronics. Adv. Mater. 2018, 30, 1704738. (10) Gao, M.; Li, L.; Song, Y. Inkjet Printing Wearable Electronic Devices. J. Mater. Chem. C 2017, 5, 2971−2993. (11) Xu, F.; Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv. Mater. 2012, 24, 5117−5122. (12) Jin, H.; Matsuhisa, N.; Lee, S.; Abbas, M.; Yokota, T.; Someya, T. Enhancing the Performance of Stretchable Conductors for ETextiles by Controlled Ink Permeation. Adv. Mater. 2017, 29, 1605848. (13) Li, W.; Hu, D.; Li, L.; Li, C. F.; Jiu, J.; Chen, C.; Ishina, T.; Sugahara, T.; Suganuma, K. Printable and Flexible Copper-Silver Alloy Electrodes with High Conductivity and Ultrahigh Oxidation Resistance. ACS Appl. Mater. Interfaces 2017, 9, 24711−24721. (14) Yao, S.; Swetha, P.; Zhu, Y. Nanomaterial-Enabled Wearable Sensors for Healthcare. Adv. Healthcare Mater. 2018, 7, 1700889. (15) Rathmell, A. R.; Wiley, B. J. The Synthesis and Coating of Long, Thin Copper Nanowires to Make Flexible, Transparent Conducting Films on Plastic Substrates. Adv. Mater. 2011, 23, 4798−803. (16) Mirri, F.; Ma, A. W.; Hsu, T. T.; Behabtu, N.; Eichmann, S. L.; Young, C. C.; Tsentalovich, D. E.; Pasquali, M. High-Performance Carbon Nanotube Transparent Conductive Films by Scalable Dip Coating. ACS Nano 2012, 6, 9737−9744. (17) Wang, J.; Liang, M.; Fang, Y.; Qiu, T.; Zhang, J.; Zhi, L. RodCoating: Towards Large-Area Fabrication of Uniform Reduced Graphene Oxide Films for Flexible Touch Screens. Adv. Mater. 2012, 24, 2874−2878. (18) Hempel, M.; Nezich, D.; Kong, J.; Hofmann, M. A Novel Class of Strain Gauges Based on Layered Percolative Films of 2D Materials. Nano Lett. 2012, 12, 5714−5718. (19) Akter, T.; Kim, W. S. Reversibly Stretchable Transparent Conductive Coatings of Spray-Deposited Silver Nanowires. ACS Appl. Mater. Interfaces 2012, 4, 1855−1859. (20) Park, S.; Vosguerichian, M.; Bao, Z. A Review of Fabrication and Applications of Carbon Nanotube Film-Based Flexible Electronics. Nanoscale 2013, 5, 1727−1752. (21) Li, J.; Ye, F.; Vaziri, S.; Muhammed, M.; Lemme, M. C.; Ostling, M. Efficient Inkjet Printing of Graphene. Adv. Mater. 2013, 25, 3985−3992. (22) Shen, W.; Zhang, X.; Huang, Q.; Xu, Q.; Song, W. Preparation of Solid Silver Nanoparticles for Inkjet Printed Flexible Electronics with High Conductivity. Nanoscale 2014, 6, 1622−1628. (23) Shimoni, A.; Azoubel, S.; Magdassi, S. Inkjet Printing of Flexible High-Performance Carbon Nanotube Transparent Conductive Films by “Coffee Ring Effect. Nanoscale 2014, 6, 11084− 11089. (24) Jiang, J.; Bao, B.; Li, M.; Sun, J.; Zhang, C.; Li, Y.; Li, F.; Yao, X.; Song, Y. Fabrication of Transparent Multilayer Circuits by Inkjet Printing. Adv. Mater. 2016, 28, 1420−1426. (25) Cheng, T.; Zhang, Y.-Z.; Yi, J.-P.; Yang, L.; Zhang, J.-D.; Lai, W.-Y.; Huang, W. Inkjet-Printed Flexible, Transparent and Aesthetic Energy Storage Devices Based on PEDOT:PSS/Ag Grid Electrodes. J. Mater. Chem. A 2016, 4, 13754−13763.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00830. Optical images of the AgNWs and histograms of AgNWs length distribution before and after sonication-induced scission; optical images of the inkjet-printed AgNWs on the PET substrate and photo paper substrate with different printing passes; photographic images of inkjetprinted patterns on the photo paper substrate; optical images of the printed interdigitated electrodes on the PET substrate and photo paper substrate; resistance of the AgNW conductors with different printing passes; schematic illustration of the formation mechanism of buckling structure and the orientation change of the AgNWs during the 1st stretching/releasing cycle; resistance change of the 11th, 21st, 31st, and 41st stretching/releasing cycle between 0 and 50% strain; resistance of the sandwich structure of PDMS/AgNW/ PDMS conductor for the 50 stretching/releasing cycles between 0 and 50% strain; resistance of the stretchable AgNW conductor for 100 stretching/releasing cycles between 0 and 25% strain after the initial five stretching/ releasing cycles between 0 and 75% strain; monochrome image used for printing the AgNW heater; and comparison of embedded stretchable AgNW conductors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 804-827-7025. Fax: 804-827-7030. E-mail: hzhao2@ vcu.edu. ORCID

Qijin Huang: 0000-0002-7390-6976 Hong Zhao: 0000-0001-9148-0158 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from Jeffress Trust Awards Program in Interdisciplinary Research (The Thomas F. and Kate Miller Jeffress Memorial Trust, Bank of America, Trustee) and the startup fund at Virginia Commonwealth University.

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REFERENCES

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DOI: 10.1021/acsanm.8b00830 ACS Appl. Nano Mater. 2018, 1, 4528−4536