Direct Pen Writing of Adhesive Particle-Free Ultrahigh Silver Salt-Loaded Composite Ink for Stretchable Circuits Mingjun Hu, Xiaobing Cai, Qiuquan Guo, Bin Bian, Tengyuan Zhang, and Jun Yang* Department of Mechanical and Materials Engineering, The University of Western Ontario, London, Ontario N6A 5B9, Canada S Supporting Information *
ABSTRACT: In this article, we describe a writable particle-free ink for fast fabrication of highly conductive stretchable circuits. The composite ink mainly consists of soluble silver salt and adhesive rubber. Low toxic ketone was employed as the main solvent. Attributed to ultrahigh solubility of silver salt in short-chain ketone and salt-assisted dissolution of rubber, the ink can be prepared into particle-free transparent solution. As-prepared ink has a good chemical stability and can be directly filled into ballpoint pens and use to write on different substrates to form well adhesive silver salt-based composite written traces as needed. As a result of high silver salt loading, the trace can be converted into highly conductive silver nanoparticle-based composites after in situ reduction. Because of the introduction of adhesive elastomeric rubber, the as-formed conductive composite written trace can not only maintain good adhesion to various substrates but also show good conductivity under various deformations. The conductivity of written traces can be enhanced by repeated writing-reduction cycles. Different patterns can be fabricated by either direct handwriting or hand-copying. As proof-of-concept demonstrations, a typical handwriting heart-like circuit was fabricated to show its capability to work under different deformations, and a pressure-sensitive switch was also manufactured to present pressure-dependent change of resistance. KEYWORDS: flexible and stretchable electronics, silver nanoparticles, elastomeric conductor, writable and printable ink, printed electronics
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fast facile fabrication of stretchable circuits and patterns has been the recent research focus in stretchable electronics. In the past years, many methods, like photolithography,15 screen printing,16 inkjet printing,17 mask-assisted spraying,18,19 direct nozzle writing,20 and vacuum filtration,21 have been employed for the fabrication of a variety of stretchable circuits. However, these methods are either largely limited by the substrates required to have complicated pretreatment or post-treatment,18,19,22,23 or have high requirements on the ink formulation.16,17 Furthermore, the preparation of highly conductive stretchable patterns still remain challenging. In this article, we proposed a more flexible handwriting method for the fast manufacture of highly conductive stretchable circuits. Recently, handwriting has been developed into an important method for the fabrication of various circuits and functional patterns, such as direct writing of metal nanoparticles and nanowire inks on paper,24−26 pen writing
ttributed to the unique capability to work under deformation, stretchable electronics have recently gained great attention and are evolving into an important branch of emerging electronics technology that enable a wide range of new applications, especially in human− machine interfaces that usually require the attached electronics lightweight, comfortable, implantable, and conformable.1−6 As an irreplaceable component of stretchable electronics, conductive elastomers have recently been a highly desirable material and realized by various methods. Generally, the emerging conductive elastomers can be divided into two categories: structure-dependent conductors, such as waveshaped, arc-shaped, spring-like, serpentine, and network shaped conductors which can withstand deformation by structure change,7−10 and composition-dependent conductors, such as liquid alloy, elastomeric matrix-based conductive composites, which mainly take advantage of the deformability of conductorloaded elastomeric matrix or conductive fluid.11−14 Nevertheless, the functionalities of an elastomeric conductor and the extension of the practical applications still more rely on the design of stretchable circuits and the integration of other functional components into stretchable circuits. Therefore, the © XXXX American Chemical Society
Received: August 14, 2015 Accepted: December 1, 2015
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Figure 1. (a1) Typical adhesive silver-based ink; (a2) direct writing of functional circuits by ballpoint pen; (b) UV−vis spectra of as-prepared solution and ink; (c) the relationship between shear rate and shear stress for the inks with different silver salt concentration, and the concentration of SIS is 20 mg/mL; (d) the viscosity of different inks as a function of shear rate; (e) UV−vis spectra of 1 g/mL silver salt ink after different storage periods; (insert) digital photos of fresh ink and the ink after one month under ambient conditions; (f) FT-IR spectra of different composite solutions after drying; (g1) written trace of a ballpoint pen filled by adhesive silver salt ink; (g2) silver lines converted from g1 after in situ reduction; (h) XRD patterns of a silver-based written trace before and after reduction; and (i) TGA curves of written traces before and after reduction.
of carbon nanotubes and liquid metal inks on flexible or rigid substrates,27,28 and direct pen writing of soluble salts inks on metal thin films followed by heat-treatment for the preparation of superconductors,29 but this method is rarely applied to the preparation of stretchable circuits and patterns, especially for the fabrication of highly conductive stretchable circuits under all-room temperature. Herein, we proposed an all-room temperature process for the production of stretchable circuits. In the whole process, no harsh experimental condition was required. As-prepared particle-free adhesive ink is low toxic and can be filled into a pen and well written on the different substrates with stable flow rate to form uniform written traces, which can then be converted into conductive lines rapidly by in situ reduction. Because of the introduction of a highly adhesive water-insoluble elastomeric polymer in the ink, the dry written traces can withstand the washing of aqueous reducing solution and well stick on the substrate. In addition, owing to the addition of elastomeric matters, the reduced written traces can be stretched together with the elastomeric substrates without obvious fracture. High solubility of metal salt in short-chain ketones can guarantee high metal loading in the ink and further result in the high conductivity of written traces after reduction. The conductivity can also be further improved by repeated
writing−reduction cycles. As-prepared stretchable circuits can well maintain the running of different electronic devices under various deformations. In addition, as a proof-of-concept demonstration, a handwriting pressure-sensitive switch was designed and installed in series circuit showing good control on the “on” and “off” states of circuits.
RESULTS AND DISCUSSIONS In comparison to other silver salts, silver trifluoroacetate has ultrahigh solubility in many low toxic organic solvents, such as short-chain alcohol and ketone. High solubility can facilitate high loading of solid metal salt in the same flow amount of ink, and further result in high-content metal particles in written traces after reduction for enhanced conductivity. A small amount of SIS, a common rubber adhesive,30 was added into the ink to act as an adhesive and elastomeric support matrix to impart the ink with good adhesion to the substrate and stretchability after drying. In fact, pure SIS cannot be well dissolved into butanone and tends to form a milky suspension solution, but with the help of silver trifluoroacetate, SIS rubber becomes highly soluble in butanone and can generate a transparent solution. As we know, silver ions can interact with B
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solvent (Figure S2). Figure 1d shows the viscosity change of ink as the function of shear rate. In this figure, all of the inks have the same concentration of SIS rubber (20 mg/mL), but they present significantly distinct viscosity, which indicates that the ink viscosity is related to the concentration of soluble silver salt. Higher silver salt concentration results in enhanced interaction between rubber molecules and silver complexes, and thus bigger ink viscosity. The initial ink viscosity for 1 g/mL silver salt ink is about 35 times of water, far less than most silver nanoparticle-based ink,25,36 which will benefit the outflow of ink through the ultrafine nozzle. Figure 1e presents the UV−vis spectra of as-prepared ink after different storage times. Under ambient conditions, the ink turned yellow gradually and the absorption in the range from 320 to 450 nm increased, but no sediment could be observed after one month, which indicates that the ink is comparatively stable for long-time application as common ink. Figure 1f shows the FT-IR spectra of dry composite written traces. Although in the written traces the silver complex is dominant, we can still observe a weak absorption peak of SIS in the 2800−3000 cm−1 range. SIS will help to stick silver salts together and further bond them with an elastomeric substrate. In our experiments, elastomeric SIS films were employed as the substrate. When the SIS-containing ink was written on SIS film, the solvent butanone can partly etch and infiltrate into the SIS substrate before evaporation and fuse written traces and the substrate together. After drying, the written trace was reduced by the mixed solution of formaldehyde and sodium hydroxide. Three white composite written traces and the corresponding black straight lines after reduction were shown in Figure 1g. The three black straight lines have different widths, and they were obtained using ballpoint pens with different penpoint diameters, 0.38 mm, 0.5 mm, and 0.7 mm. It indicates that the width of the written trace can be controlled by adjusting the size of penpoint. The substances and crystal phase of written traces were detected by XRD, and the patterns were presented in Figure 1h. The XRD patterns before and after reduction are totally different, and the peaks after reduction well correspond to the fcc phase of metal silver according to the standard XRD spectrum. The composition of written traces before and after reduction was investigated by EDX and TGA analysis. According to the composition of ink, the ratio of silver in the written trace after reduction should be about 96 wt % and 67 v %, and from TGA curves, it was found that the content of silver in the reduced sample is about 94 wt %, which indicates that there is a high conversion rate from silver salt to metal silver after chemical reduction. In addition, in contrast to the written trace before reduction, the written trace after reduction does not degrade evidently at 200 °C, which corresponds to the decomposition temperature of silver salt complex, and it means that there is almost no residual silver salt in the reduced written traces and that silver and SIS rubber composites have been the dominant components, which was also verified by EDX elemental mapping (Figure S1). Isolated silver salt-based hybrid written traces were turned into high-content silver nanoparticle-loaded conductive lines by in situ reduction. The employed typical reducing agent is formaldehyde solution, a cheap and strong reducing agent under alkali conditions. Sodium hydroxide was an important assistant agent during reduction. On the one hand, sodium hydroxide can react with soluble silver salt quickly to get insoluble oxides and avoid the diffusion of silver ions into the solution; on the other hand, sodium hydroxide can adjust the
double bonds of SIS molecules and help the dissolution of SIS rubber in the ketone.31,32 In our experiments, the concentration of SIS was set at 20 mg/mL, an appropriate value to prevent the excessive diffusion into the solution of silver salt in written traces during the reducing process, which often happens for the written trace with a low SIS concentration, as well as to avoid high resistance as a result of the introduction of excessive isolated rubber at a high SIS concentration. When the concentration of SIS was increased to above 40 mg/mL, the viscosity of ink will become too high to write fluently. A typical ink is shown in Figure 1a1 and presents good transparency and fluidity although with a 1 g/mL solid silver salt loading. Figure 1b shows the UV−vis spectra of the as-prepared solution and ink. For the SIS butanone solution, it can be seen that there is obvious absorption in the whole visible light area, which indicates that the SIS block copolymer cannot completely be dissolved by butanone to obtain 20 mg/mL of transparent solution. Nevertheless, after adding silver salt, no obvious absorption peak can be observed in the range from 400 to 450 nm typically associated with the absorption of silver particles.33,34 It indicates the ink is silver particle-free. In addition, in the 400 nm−800 nm range of visible light, the absorbance is almost close to zero, which implies high transparency and complete dissolution of related solutes. Apparently, silver salt can help the dissolution of SIS copolymer in butanone to turn a milky suspension into transparent solution. As is known, a double bond can act as an electron donor, and silver ions can well accept electrons because of their unoccupied orbital, and thus there is an interaction derived from charge transfer between silver ions and rubber molecules.18,23,35 When butanone partly infiltrates into the rubber matrix, it will also carry a lot of silver ions into the matrix. The interaction of silver ions and rubber molecules will facilitate the disentanglement of polymer chains and thus enhance the dissolution to SIS rubber in butanone. It is crucial for the preparation of particle-free adhesive ink and enables the codissolution of two important components, metal sources and adhesive rubber, to endow the ink with double functions of conduction and adhesion. In order to further improve the writability of as-prepared ink, some other additives were also required. We employ butanone as the main solvent owing to its special advantages, such as low toxicity, weak reducing ability, and outstanding dissolubility to the used metal salt and adhesive rubber, which endow the ink with good chemical stability and other multiple functions, but butanone is a volatile solvent with low boiling point and surface tension, which may affect the handwriting quality of ink. Although a low boiling point can make the ink dry fast after writing for better writing efficiency, fast solvent evaporation easily causes the precipitation of solutes at the penpoint and blocks the ink tunnel. Low surface tension may render the spreading of the ink on the substrate and result in lower resolution. Therefore, there is a need to modify the ink by adding a small amount of additives for better writing quality, such as assistant solvents with higher boiling point, like 1,2-propanediol, benzyl alcohol, and dimethyl acetylamide, and surfactants, etc. These high boiling solvents can act as humectant and lubricant agents to prevent the clogging of the penpoint and enhance the stability of handwriting. Also, high boiling solvents usually have bigger surface tension, which can help to limit the spreading of ink and enhance the resolution of writing, which has actually been proved by the experiments that the ink with high boiling point solvent can generate thinner written lines than pure butanone C
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Figure 2. Different magnified SEM images of the surface morphology of silver-based written traces after reduction with different writingreduction modes: (a,b,c) 1W1R mode; (d,e,f) 5W1R mode; and (g,h,i) 2-5W1R mode.
PH value of the solution and enhance the reducing ability of formaldehyde. The conductivity of the written trace is not good enough when the ink was only written and reduced once due to the as-formed thin silver nanoparticles layer. In order to improve the conductivity, the thickness of the written traces was increased by repeated writing−reduction cycles. Several typical writing modes were adopted in the experiments to investigate the change of the conductivity with repeated writing−reduction cycles. The different writing modes were named as c-mWnR, in which c represents the number of times of the cycles, m is the number of times of writing, and n is the number of times of reduction. For example, 2-5W1R means that the written trace was obtained by repeating the action “writing 5 times followed by reduction once” twice. Figure 2a shows a typical straight line drawn by a ballpoint pen with the penpoint diameter of 0.7 mm. The width of the straight line is about 680 μm measured from SEM images, which is very close to the penpoint diameter of 0.7 mm. Figure 2d shows the written trace with 1-5W1R mode, and the width of the written trace did not change a lot, although repeated writing was done. Even under the 2-5W1R mode shown in Figure 2f, we cannot observe obvious extension of the width of the written trace. In addition, the edge of the written trace is clear, and no obvious satellite dots are present. Thus, the written trace can maintain relatively high resolution even under repeated writing− reduction cycles. Attributed to the high loading of silver salt in composite ink, silver nanoparticles are dominant in written traces after reduction. These silver nanoparticles were buried in the SIS matrix and built up the percolation network. The elastomeric SIS matrix can help the written trace to withstand the deformation and prevent breakage under deformation. With repeated writing−reduction cycles, the thickness of silver nanoparticle layers increased, which is presented in Figure S3, and the surface layer of written traces gradually becomes porous maybe due to volume shrinkage and the loss of AgTFA during reduction. In order to further investigate the distribution of silver nanoparticles in the SIS matrix, we wrote directly with the ink on a copper grid for TEM observation, and TEM
images of a written trace after reduction are shown in Figure S4. It can be seen that silver nanoparticles can form some selfassembled aggregates in written traces, and with increasing the thickness, a lot of one-dimensional silver assembled structures can be observed, which will benefit in improving the conductivity of written traces. From Figure S3, we can see that the thickness of the silver layer increased from a nanoscale level to several microns after repeated writing−reduction cycles. Under the 1W1R mode, the silver layer is uneven, and after repeated writing−reduction cycles, the silver layer became thick and uniform gradually, which will help to decrease the defects of written traces and thus improve conduction. The conductivity of silver-based written traces under the 1W1R mode greatly depends on the fluid rate and the composition of the ink. The fluid rate of ink is related to the ink viscosity and the size of the nozzle. As is above-mentioned, the ink viscosity was determined by the concentration of silver salt and SIS rubber. Herein, the concentration of silver salt has been set at 1 g/mL, and thus the effect of SIS concentration on the conduction was investigated (Figure 3a). It was found that in low SIS concentration, the conduction of the written trace was not affected significantly by the SIS concentration, but in high SIS concentration, the conduction will become remarkably poor as a result of high viscosity-induced low fluid rate of ink. In this article, we set the concentration of SIS rubber in the ink at 20 mg/mL except as noted, an optimized value for good combination performance of written traces, including adhesion, resolution, conduction, stretchability, etc. The fluid rate of ink was also affected by the size of the penpoint. The written traces drawn by a ballpoint pen with a larger diameter of penpoint have better conductivity after reduction. For instance, the resistance of the written trace from a pen of 0.38 mm diameter after one-time writing and reduction is about 650 Ω·cm−1, and the resistance from a pen of 0.7 mm decreases to about 400Ω· cm−1. In general, the conductivity is not good enough when one-time writing was done due to the small thickness of the silver layer. Also, the conductivity of the written trace greatly depends on the substrate. (Figure S5) When the ink was D
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Figure 3. (a) Effects of SIS concentration on the resistance of written traces under different writing modes; (b) the resistance of written traces with multiple writing and one-time reduction; (c) the resistance of different written traces with different writing−reduction cycles after reduction; (d) the ratio of the resistance of different written conductive lines to average resistance Ra showing the deviation of individual resistance to average resistance; (e) the relationship between the resistance and strain of written traces under different writing−reduction modes; (f) the change of the ratio of the resistance of written trace after release Rr to the initial resistance R0 with increasing the number of stretching−release cycles under the 2-5W1R writing mode; (g1,g2) digital photos to show the resistance of silver lines of 5 cm under the 25W1R mode and 5-5W1R mode; and (h1,h2) digital photos to show the resistance of a typical written trace of 1 cm under the 5-5W1R writing mode with different strains; (h1) strain = 0%; (h2) strain = 100%.
applied on a smooth substrate, such as PET and PI film, the written trace has much better conductivity than that on rough paper and soft rubber. In this article, because we focused on the direct writing of stretchable electronics, the employed substrate is SIS rubber in all of the measurements except noted otherwise. Repeated writing can decrease the defects of written traces and increase the thickness and will thus boost the conduction quickly. The resistance of a 0.7 mm-wide written line under different writing modes was shown in Figure 3c. In the first several writing−reduction cycles, the resistance decreases significantly, and more writing times result in the continuous decrease of resistance. Under 5-3W1R and 5-5W1R modes, the resistance can both be less than 1 Ω·cm−1, which is desirable in stretchable circuits. However, for the direct writing of stretchable circuits, the resolution of written traces and writing efficiency were also of concern. Too many writing− reduction cycles are time-consuming and also sacrifice the resolution. It is a must to optimize the writing mode. As we know, in the process of direct writing, the reduction process is a more time-consuming step and should be avoided as much as
possible. Thus, we investigated the conductivity of written traces under repeated writing and one-time reduction, as shown in Figure 3b, and found that the resistance declined dramatically with increasing writing times in the initial stage and then became steady after 5 times of writing−reduction cycles. Generally, the second cycle decreases the resistance the quickest. For instance, under the 2-5W1R mode, the resistance of written traces can drop to about 2Ω·cm−1 abruptly from about 9Ω·cm−1 of the 1-5W1R mode. The resistance can be further decreased to 0.8 Ω·cm−1 under the 5-5W1R mode but with the loss of resolution and writing efficiency. For instance, the width of the written trace will be extended by 20−30% under the 5-5W1R mode, and the writing speed will be slowed down to about 40% of that under the 2-5W1R mode. In addition, we also investigate the resistance deviation under 25W1R mode, and it was found that the resistance of almost all of the conductive lines stayed within (1 ± 20%) × Ra, where Ra is the average resistance (Figure 3d). Thus, 2-5W1R is a good writing mode to obtain a good combination performance of conductance and resolution in an efficient way. E
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Figure 4. (a) Heart-like circuit drawn by a ballpoint pen; (b) 14 LED lights were lit to show a regular heart shape; LED lights can still work under different deformations; (c) outward bending; (d) inward bending; (e) stretching; and (f) flash photograph under stretching.
Figure 5. (a1) Schematic diagram of a pressure-sensitive switch, in which each layer is depicted by different colors; (a2) interdigitated electrode; (a3) silver parallel line pattern; (a4) an assembled pressure-sensitive switch; (b) three-layered structure of a pressure-sensitive switch; (c) the relationship between the resistance and external pressure; (d,e,f) digital photos to show that LED lights can be well controlled by an as-designed pressure-sensitive switch.
of resistance and strain for the written traces under different writing modes. The thicker the written trace is, the better the conductance is, and the more slowly the conductance degrades during stretching. For the written traces under 2-5W1R mode and 5-5W1R mode, they can endure the strain much larger than 200% while keeping normal function. However, due to the
In general, due to the introduction of SIS rubber, the written traces can maintain conductivity under deformation. Even for the written traces under the 1W1R mode, they can still work at the strain of 60%. When repeated writing−reduction cycles were applied, the written traces become more durable and can withstand larger deformation. Figure 3e shows the relationship F
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polymer in ink, the written traces have not only good adhesion onto the substrate but also good stretchability after drying. Because of the extremely high solubility of inorganic silver salt in short-chain ketone, the loading of more than 1 g/mL silver salt can be achieved in as-prepared ink, and thus result in high metal content in written traces after reduction, which guarantee good conductivity of written circuits under normal and stretching states. The conductivity of written traces can be further enhanced by repeated writing−reduction cycles, and make the circuits able to work under a strain above 200%. An optimal 2-5W1R writing mode was proposed for good circuit performance and writing efficiency. As-prepared stretchable circuits and patterns can show many applications, such as in stretchable conductive connectors, in pressure-sensitive switch as presented above, and so on. Significantly, direct writing of stretchable circuits can greatly simplify the fabrication of stretchable circuits and patterns, and broaden the application of stretchable electronics technology in daily life.
small width of the written trace and the mismatching of elasticity between the substrate and written traces, the recovery of the conductivity of the written trace is not very synchronous with the shape recovery of the substrate after stretching. Figure 3f showed the change of the ratio of the resistance after release Rr to the initial resistance R0 with increasing stretching−release cycles. In the first several cycles, the conductance encountered a relatively large loss maybe due partly to the fracture of written traces and subsequently was kept constant basically after tens of stretching−releasing cycles. Although the conductivity cannot be recovered completely, as a stretchable interconnector, a small amount of loss of conductivity is insignificant because Rr/ R0 can always stay within 2 even for the strain of 100%. Figure 3h shows a typical highly conductive written trace with the resistance of about 0.8 Ω·cm−1, and at ε = 100%, the resistance was changed into 20.8 Ω (10.4 Ω·cm−1). Although there is some degradation in the conduction of circuits after stretching, the conductivity is still good enough to enable the function of common electronic devices. A simple heart-like circuit was designed to describe the function of hand-written stretchable circuits, and the circuit is shown in Figure 4a. Fourteen LED lights were installed on an elastomeric SIS substrate and were connected into a parallel circuit. These LED lights can be lit at the same time at normal state to show a regular heart shape, and also, the circuit shows its ability to endure various deformations, such as bending, stretching, and twisting, and obviously, we can observe that the lights can work well under deformation. Furthermore, a pressure-sensitive switch was designed to show a variety of applications of our handwriting patterns (Figure 5). The switch was constructed by three layered patterns: an interdigitated silver electrode, an isolated parallel rubber line array, and a conductive parallel silver line array. The three layers were assembled according to Figure 5a1 and b, two conductive silver patterns separated by an isolated elastomeric rubber line array, which was obtained by handwriting SIS rubber solution on conductive silver lines array. The thickness of the rubber line was controlled by writing times. The as-prepared pressuresensitive switch is shown in Figure 5a4. Under external pressure, the elastomeric silver patterns and isolated rubber pattern deform and enable the contact of two silver patterns and lead to the connection of the interdigitated electrode. When the pressing force was increased, the contact area was enlarged and resulted in the decrease of the contact resistance. The change of resistance with external pressure is shown in Figure 5c. It can be seen that the resistance is sensitive to pressing and that under an external pressure of 200 Pa, the switch starts to become conductive. With a further increment of pressure, the resistance decreases dramatically and finally achieves the saturated value Rs at about 1000 Pa. Figure 5d,e,f shows a series circuit controlled by a pressure-sensitive switch. LED light was installed in this circuit and can show a rapid switch of “on” and “off” states before and after pressing (SI mov1), indicating good controllability of “turn on” and “turn off” due to fast pressure-sensitive deformation of elastomeric conductive patterns.
EXPERIMENTAL PROCEDURES Chemicals. Polystyrene-block-polyisoprene-block-polystyrene (SIS, styrene 22 wt %), silver trifluoroacetate (98%, AgTFA), silver nitrate (≥99.0%), butanone (≥99.0%), ethyl acetate (≥99.8%), dimethylformamide (≥99.8%, DMF), dimethylacetamide (≥99.5% DMAc), chloroform (≥99.9%), dichloromethane (≥99.8%), formaldehyde solution (36.5−38% in H2O), hydrazine hydrate (50−60%), and sodium hydroxide (≥97%) were purchased from Sigma Aldrich. Acetone and tetrahydrofuran (THF) were purchased from Caledon Corp. All chemicals were used as received without further purification. Preparation of Adhesive Silver-Based Inks. In a typical experiment, 1 g of silver trifluoroacetate was dissolved into butanone, and then 20 mg of SIS was added and dissolved in the solution to form 1 mL of transparent ink. It was noted that the SIS elastomer cannot be well dissolved in pure butanone but can be completely dissolved by silver trifluoroacetate butanone solution to get particle-free adhesive silver-based ink. A small amount of high boiling point solvents, such as dimethylacetamide (DMAc), can be added into the ink to improve the fluidity of ink and prevent the clogging of the penpoint, and the typical volume ratio of butanone and DMAc is 9:1. Direct Writing of Silver-Based Ink for Stretchable Circuits. The ink was filled into the refill of a ballpoint pen. The ink traces were obtained by writing on different substrates. To prepare stretchable circuits, a stretchable SIS substrate was prepared by depositing the SIS chloroform solution on an aluminum dish and then peeling off the film after drying. As-obtained SIS films were cut into required shapes for subsequent use. For the writing of complicated patterns, the patterns were first printed on paper, and then, transparent SIS films were used to cover the printed patterns. Thereafter, the patterns were copied onto the SIS films by handwriting using a ballpoint pen filled with asprepared silver ink. Subsequently, the written traces were reduced by the mixed solution of formaldehyde (50 mL/L) and sodium hydroxide (0.25 M) for 5 min. Repeated writing−reduction cycles can be executed to enhance the conductivity. Preparation of the Pressure-Sensitive Switch. The interdigitated electrode (28.9 mm line length, 0.7 mm line width, and 0.5 mm spacing) and parallel line pattern (32 mm line length, 0.7 mm line width, and 1 mm spacing) were printed on plain A4 paper. Transparent SIS films with the appropriate size were used to cover the pattern. Then, the patterns were copied onto the SIS films by handwriting. Five repeated writing operations were executed, and then silver salt patterns were in situ reduced by the mixed solution of HCHO and NaOH. Then, the patterns were dried, and another five repeated writings were done on original metal traces. The written traces were reduced again by the same reducing agent. After drying, a ballpoint pen filled with 40 mg/mL SIS rubber ethyl acetate solution was employed to write the parallel rubber lines (30 mm line length, 0.7 mm line width, and 3 mm spacing) over the parallel line pattern along
CONCLUSIONS In conclusion, we have developed a kind of low-cost and lowtoxicity adhesive particle-free composite silver-based ink for direct writing of stretchable circuits. Various circuits can be easily fabricated on the elastomeric substrate by handwriting. Attributed to the introduction of an adhesive elastomeric G
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ACS Nano the direction perpendicular to silver lines. The writing was repeated 10 times. Thereafter, the parallel line pattern was carefully stuck on the interdigitated electrode to construct the pressure-sensitive switch. Characterization. SEM images and an energy-dispersive X-ray (EDX) spectrum and elemental mapping were taken by a Hitachi S4500 field-emission scanning electron microscope (FE-SEM) at a 5 kV accelerating voltage and Stereoscan 440 scanning electron microscope (SEM). TEM images were obtained by TEM Phillips 420 at an accelerating voltage of 200 kV. Silver ink viscosity is acquired using an MCR 302 rheometer with 20 mL ink loading at room temperature. UV−vis spectra were obtained through a Cary 100 Bio UV−vis spectrophotometer. Thermogravimetric analysis (TGA) was executed to detect the content of silver and silver salt in SIS rubber (Q500, TA Instruments). FT-IR analysis was performed using FT-IR NICOLET 6700 (Thermo Scientific Co.). XRD patterns were obtained on a Rigaku Ultima IV X-ray Diffraction Generator with Cu Kα radiation (λ = 1.54178 Å). Sheet resistances were measured by a four-point probe method modified from Keithley 2750. The conductivity of the silverbased written trace under normal and stretching states was measured by a common multimeter and Keithley 2750.
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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/acsnano.5b05082. Additional EDX analysis, SEM images, TEM images, and digital photos (PDF) LED light circuit showing a rapid switch of “on” and “off” states (AVI)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acsnano.5b05082 ACS Nano XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsnano.5b05082 ACS Nano XXXX, XXX, XXX−XXX
