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Direct Desktop Printed-Circuits-on-Paper Flexible Electronics

Direct Desktop Printed-Circuits-on-Paper Flexible Electronics Direct Desktop Printed-Circuits-on-Paper Flexible Electronics SUBJECT AREAS: 1 1 1 1,2 Yi Zheng , Zhizhu He , Yunxia Gao & Jing Liu ELECTRICAL AND ELECTRONIC ENGINEERING Key Lab of Cryogenics and Beijing Key Lab of CryoBiomedical Engineering, Technical Institute of Physics and Chemistry, Chinese MECHANICAL ENGINEERING Academy of Sciences, Beijing 100190, P. R. China, Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, P. R. China. SURFACE PATTERNING APPLIED PHYSICS There currently lacks of a way to directly write out electronics, just like printing pictures on paper by an office printer. Here we show a desktop printing of flexible circuits on paper via developing liquid metal ink Received and related working mechanisms. Through modifying adhesion of the ink, overcoming its high surface 14 February 2013 tension by dispensing machine and designing a brush like porous pinhead for printing alloy and identifying matched substrate materials among different papers, the slightly oxidized alloy ink was demonstrated to be Accepted flexibly printed on coated paper, which could compose various functional electronics and the concept of 18 April 2013 Printed-Circuits-on-Paper was thus presented. Further, RTV silicone rubber was adopted as isolating inks and packaging material to guarantee the functional stability of the circuit, which suggests an approach for Published printing 3D hybrid electro-mechanical device. The present work paved the way for a low cost and easygoing 9 May 2013 method in directly printing paper electronics. t has been widely known that the invention of printed circuits on boards (PCB) has brought about tremendous Correspondence and advances to modern electronics. Recently, with more concerns raised in the complicated and energy- requests for materials I consuming fabrication processes of conventional electronics, plentiful attentions have been made to find an should be addressed to additive way to make electronic components on any desired substrates, especially those flexible ones. Among the 1,2 3,4 5–7 J.L. ([email protected]. many efforts ever made, functional inks based on organic substances , polymers , nanoparticles , thin-film 8,9 cn) semiconductors etc. have been developed for manufacturing flexible electronics, which significantly contrib- 10,11 12,13 14,15 16 uted to the development of antennas , transistors , solar cells , radio frequency identification (RFID) , 17–19 20 21,22 flexible displays , electronic clothes , sensors and so forth. For developing various promising flexible 23–26 substrate materials, paper is specially being paid with considerable attentions in printed electronics owing to its excellent properties such as easy accessibility, low-cost, foldability, disposability, retrievability and non- 27–32 pollution. Meanwhile, enormous efforts have also been made to find highly conductive inks so as to perform direct writing of flexible circuit. So far, many currently available electrical inks are still not conductive enough. To further improve their electrical conductivity, loading nanoparticle to the base material is an important approach. However, some basic issues still remain. For instance, such composite inks’ conductivity is heavily dependent on the particles’ loading ratio and bonding mechanism after solidification. However, a difficulty lies in is that the loading ratio of particles in solution is generally low. In addition, preparation of such conductive inks is still somewhat complicated. For some ink materials, it has to adopt relatively high post-processing temperature (for example 400uC) to improve their conductivity, while most plastic and paper substrates require processing temperature below 150uC. So far, there still lacks of a reliable way which is capable of directly writing out electronic circuits, just like printing a picture on paper via a desktop printer in office. Recently, the room temperature liquid metal or its alloys was found to own very interesting properties as printing inks especially in flexible electronics. Along this way, a direct approach to immediately write out 33,34 electronics through such inks is incubating . This affordable processing appears rather succinct and efficient. However, there is currently lacking of an automatic printing method for the increasing use of such promising technology which is a key towards pervasive consumer electronics. Clearly, the manual writing approach as demonstrated before would restrict the practical value of the liquid metal printed electronics after all. Therefore, an automatic and direct printing of electronics by developing a desktop printer with intelligent-controlling ability and high-precision is urgently needed to fulfill such requirement. Here we show an entire solution for automatic and direct printing of liquid metal ink on paper, through resolving a series of important issues, such as modifying the adhesion of the alloy inks, overcoming the high surface tension of the ink via the dispensing machine (Fig. 1) and designing a brush like porous pinhead for printing liquid alloy inks and identifying the matched substrate materials among different types of papers. The SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 1 www.nature.com/scientificreports Figure 1 | Schematics for experimental printing pinhead and desktop printer setup. prototyping desktop printer can be manipulated with ease to print Newton second law and uniformly accelerated linear motion law out various patterns on the selected papers as desired under control are suitable to describe such motion which are presented as follows: of the computer software. Particularly, paper with advantages of 1 s~ at ð1Þ flexibility, low-cost and recyclability was investigated as a highly suitable substrate material for printing electronics. This may lead to the concept of Printed-Circuits-on-Paper (PCP). Further, the G {f~ma ð2Þ room temperature vulcanized silicone rubber which has been widely where, s is the distance along paper, t is the detection time of gliding, utilized in electronics packaging was adopted as isolating inks so as to G is the gravity component which is parallel to the inclined paper print them out as encapsulated structure for packaging the liquid substrate; f is the resistance force containing the friction and metal circuits and making a multistory hybrid electro-mechanical adhesion force; m and a are the quality and acceleration of droplet, structure. Through these efforts, various typical circuits and compo- respectively. It can be seen from the curves that the relationship of s nents have been successfully printed on coated paper, including con- and t fits well with the equation (1). And according to equation (2), ductive wires, 3D conductor structure, inductance coil and flexible the acceleration can be described as: antenna etc. Results Printing papers. To evaluate the compatibility between the liquid metal ink and the paper substrate, a high-speed camera was utilized to record the gliding process of the liquid alloy droplet on various paper substrates with a uniform slope angle of 30u. To ensure the fluid feature, high-purity liquid alloy GaIn which was not 24.5 deliberately oxidized was used. For comparison purpose, droplets with same volume should pass through the same distance starting from a standstill state. Therefore, an injector pump with a speed of 10 ml/h was used to generate droplets so as to ensure same droplet volume and timing starting from the moment of gliding. Figure 2 shows the detailed gliding process of liquid alloy ink droplet on coated paper. Here, droplets on all papers have run the same distance as 60 mm. Figure 3 presents the distance (s) as a function of detection time and the internal schematic diagram depicts the force analysis of droplet on paper over the process. It is known that there are mainly three constant forces acting on the droplet, namely gravity (G), supporting force (N) and resistance (f). Figure 2 | Sequential optical images of liquid metal droplet gliding along the coated paper with a slope angle of 306 which were recorded by high- Droplets are considered to run uniformly accelerated linear motion along papers under the effects of forces. The classical speed camera. SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 2 www.nature.com/scientificreports Therefore, coated paper possesses the maximal adhesive force, indicating that coated paper has the best compatibility with liquid GaIn based alloy ink among all applied papers. 24.5 Coated paper with high smoothness and brightness is an advanced paper substrate which has been commonly used in printing book cover, elegant advertisements, commodity packaging, and labels, etc. It is mainly manufactured by coating a pigment compounds layer on base-paper. Conventional filler pigments mainly contain ground calcium carbonate (GCC), precipitated calcium carbonate aragonite (PCC) and Kaolin (mainly Al O ?2SiO ?H O), as well as chemical 2 3 2 2 additives such as dispersants, resins, sodium polyacrylate (NaPA), and poly (styrenebutadiene) (SB) latex binder etc. Investigations have been conducted to confirm that addition of the the above-men- 35–37 tioned fillers has a positive effect on the adhesion of paper .In order to further disclose the mechanism for the surface adhesive, an energy dispersive spectroscopy (EDS) analysis was also conducted to survey the surface elements of the tested coated paper. Figure 5 exhi- bits the weight ratio of various elements on the surface of the coated paper. It can be seen that oxygen and silicon elements occupy a large Figure 3 | Distance (s) of droplets as a function of measured time (t) on proportion among all elements. As a result, combined with the for- coated paper, office paper, wove paper, respectively. The inset shows the mer efforts, a better compatibility between paper and ink can be sketch map of force analysis of the droplet in the gliding process. obtained through properly oxidizing GaIn alloy ink. 24.5 Automatic printing of circuits on coated paper. Automatic G {f mg sin h{f f printing of GaIn based inks on coated paper was implemented a~ ~ ~g sin h{ ð3Þ 24.5 m m m utilizing the developed system, and the direct printing process was illustrated in Fig. 6. Through the evaluation as performed above, here where, h is the slope angle between paper substrates and horizontal the commercially available coated paper (with a weight specification line. Additionally, for all paper types, parameter h, g and m are all of 200 g/m ) was adopted as the printing substrate. Apparatus constant values. Therefore, there are only two variables in the equipped with single printing syringe A or B were utilized and the equation (3), a and f. From Fig. 3, it can be found that acceleration structure materials of matched printing needle were porous brush on coated paper is the minimum of all, which indicates that it has and orifice metal, respectively. In addition, syringe A and B were maximum resistance force on the surface of coated paper. In order to respectively loaded with GaIn ink and RTV silicone rubber. 24.5 clarify the surface roughness of three papers, we have performed an During the printing process, syringe A was firstly installed to print atomic force microscope (AFM) analysis. Figure 4A, B and C present required patterns on coated paper according to the digitalized the AFM images of surface morphology of wove paper, office paper program, and then overprinted by RTV silicone rubber from and coated paper, respectively. The measured average Rq of wove syringe B. When necessary, multilayer circuit structures can be paper, office paper and coated paper is 120.8 nm, 70.7 nm and fabricated by sequentially printing liquid metal circuitries 12.9 nm, respectively. It has been widely accepted that the rougher bypassing the upper surface of the cured silicone rubber. In this the surface, the larger the friction, which means coated paper has the way, a three dimensional (3D) hybrid electro-mechanical device on minimal surface friction. Conclusions regarding the numerical paper can be quickly printed out. relations can be obtained as follows: Several typical directly printed circuits containing bending circuit Total resistance : f wf wf ; ð4Þ wove coated office with regularity (1), packaged straight wires (2), circuit nodes (3), stereoscopic flexible circuits (4) and annulus wire with LED (5) on Surface friction : f wf f ; ð5Þ office coated wove w coated paper are presented in Fig. 7(A), which illustrate the excellent adhesion between GaIn based ink and coated paper, as well 24.5 Adhesive force : f wf wf : ð6Þ wove coated office as outstanding elastic characteristics of this kind of RTV silicone Figure 4 | Surface morphologies of three typical papers. AFM images (23 2 mm ) of surfaces of Wove paper (A), Office paper (B) and Coated paper (C), respectively. SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 3 www.nature.com/scientificreports again, indicating that the compatibility between the ink and coated paper is functionally favorable. Apparently, processing should be further advanced to fabricate thinner even nanoscale circuit to apply in highly required integrated electronics. This needs new efforts in the near future. Electrical reliability of flexible circuit as a function of bending angles. As an innovative ink with tremendous potentials to be further applied in future flexible electronics manufacturing, the resistance stability of the liquid metal before and after bending is vital. Here, a digital bridge with an equipment deviation of 0.003 V was employed to measure the resistance variation of liquid metal circuit when folding to five angles. Liquid metal circuit whose length and width is about 18 cm and 1 mm respectively was printed. Two copper wires with the same resistance of 0.0275 V Figure 5 | Energy dispersive spectroscopy (EDS) picture of elements on were applied to serve as measuring electrodes. The printed circuits coated paper surface. were well packaged by RTV silicone rubber and the operating frequency and voltage of digital bridge was set to 1.0 kHz and 1 V, respectively. Figure 9 shows the resistance results according to the measurements. It is necessary to point out that all the measured rubber. More printed functional electronics including inductance resistances are the total values that contain the resistances of both coil (1) and RFID (2) antenna are given in Fig. 7(B), and picture 3 circuit and copper electrodes, and in order to reduce the deviation, all reveals the excellent flexibility of the fabricated antenna after pack- resulting values in picture are an average of 3 measured values. It can aging. Experimental results confirm that the cured silicone rubber be evidently observed that resistances vary just in a limited range, can commendably prevent the liquid ink from flowing along the which indicates that the well-encapsulated printed liquid metal wire wires, which to a large extent will contribute to the functional on paper could meet well the requirement of the flexible circuit. stability of the printed circuits or electrical components. Meanwhile, the liquid silicone rubber scarcely affects the electrical It is well known that conductive inks of conventional printed performance of the printed liquid circuit. This is rather beneficial for electronics usually function after solidifying. This indicates that the wide adaptability of PCP. the fabricated electronics have mechanically flexible restriction. However, the present ink remains liquid under functional conditions after being encapsulated by RTV silicone rubber, while the substrate Discussion and the elastic rubber form a channel to maintain the electrical In this article, an innovative approach to automatically fabricate structure. Therefore, the fabricated circuits cannot be broken off electronics by directly printing liquid alloy based inks on coated easily even under frequent bending, showing an attractive and dis- paper is described. Compared with the conventional ways of making tinguished mechanical flexibility which is a critical advantage in flexible electronics using existing conductive inks, the present high- fabricating flexible electronics. Moreover, the well-encapsulated efficiency direct printing process has relatively lower-cost, simpler liquid alloy ink on paper can be conveniently and responsibly stored process, well acceptable conductivity and is more environmentally and carried. Overall, the present approach constitutes a vital step friendly which guarantees the really ‘‘green’’ fabrication of electron- towards truly directly printed electronics. ics. Moreover, the whole printing process is completed under room Figure 8A presented the physical images of the printed circuit with temperature, which hardly brings about detrimental influence to the a width of 1.5 mm on coated paper, and Fig. 8B and C gave the SEM paper substrate and is capable of ensuring the printed circuits’ own graphs of surface and cross-sectional topography of the printed excellent electrical performance and mechanical flexibility. Further, liquid wires respectively. It can be discovered from the images that the recycling of metal inks is easier to be completed just by properly the liquid metal wire is adhered closely to the surface of the paper destroying the external elastic silicone rubber and then efficiently which has a multi-groove structure. The directly printed liquid cir- reclaiming the noble material by 30% NaOH solution. Finally, liquid cuit on coated paper is relatively uniform and the thickness is about metal ink runs in liquid state which indicates that liquid metal cir- 120 mm. Simultaneously, the SEM graphs prove that GaIn based cuits printed on flexible substrates will not be easily fractured owing 24.5 ink possesses wonderful adhesion with coated paper substrate once to its unique liquidus property. Figure 6 | Schematic of liquid alloy printing of electrical circuitry on coated paper. Liquid metal (A) is firstly printed on paper (Step 1); Then RTV silicone rubber (B) is overprinted sequentially as encapsulated material (Step 2); When necessary, a multilayer circuit structure is fabricated by printing liquid metal (A) circuitry bypassing the upper surface of cured silicone rubber (Step 3). SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 4 www.nature.com/scientificreports Figure 8 | Characterization of the printed conductive objects. (A) Optical image of printed circuit on coated paper with a width of 1.5 mm. (B) and (C) are SEM images of surface and cross-section of printed liquid metal ink on coated paper. as conductive polymers, metal nanoparticles or semiconductors, fur- ther performance modification of such inks is likely to be realized to meet future high profile needs in a wide variety of flexible electronics. It is expected that such unique approach will be applied in a sufficient number of electrical areas including rapid-prototyping circuits (e.g., E-Card), integrated circuit on paper, sensors, 3D mechanical switches and antennas, electronic solar cell arrays, functional RFID tags on books, even daily life personal electronics and educational training. However, as a newly emerging technology, the present method also raised a series of practical issues waiting to be further better solved such as guaranteeing an ever higher resolution circuital uni- formity as well as more rapid printing speed of the process. In this side, considerable researches as implemented in our work disclosed that, through resolving the printing challenge caused by the large surface tension of GaIn24.5 ink (via oxidation and searching com- patible paper substrate), the commonly encountered troubles such as Figure 7 | Desktop printed electronic components. (A) Physical diagram frequent occurrence of the intermittent structure and droplet forma- of GaIn based liquid alloy directly printed on coated paper by a 24.5 tion of conductive line were successfully overcome. In addition, dispenser printer and optical images for various conductive wires on coated paper: (1) Manufacturing process of printed electronics. The inset shows the regular bending circuit; (2) Metal conductor lines covered with silicone rubber; (3) Multi-layer structure or electrical nodes; (4) Three- dimensional structure of printed conductor on paper; (5) Galvanical annulus wire attached with LED. (B) Optical images for printed functional components on coated paper: (1) Inductance coil; (2) RFID antenna; (3) Presentation of prominent flexibility of printed electronics. As disclosed by former research , the content of the oxides in liquid alloy can efficiently alter the viscosity of the ink which greatly affects the final performance of the circuits and components printed directly on papers. The adhesion of alloy ink can be upgraded through the process of oxidizing, as well as altering the weight ratio of the chemical composition. Also, the surface adhesion of the coated paper materials can be modified by ultraviolet-C ray radiation (UVC Irradiation) . Furthermore, the coated paper substrate can also be 36,37 simply manufactured on one’s own as needed . And reinforced RTV silicone rubbers or other alternative adhesives can be applied to substitute this basic encapsulated material for extreme working condition. In the near future, more liquid metals or alloys can be developed as suitable inks to be directly printed on various compat- Figure 9 | Resistance values of liquid circuit at five different bending ible papers. Clearly, combined with more functional materials such angles. SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 5 www.nature.com/scientificreports Printable isolation ink for packaging material. Considering the liquidus properties better circuital uniformity and resolution can still be possible in the of the current metal ink, when exposed in air, the fabricated patterns are apt to be coming time subject to further endeavor. Along this direction, several changed. For the purpose of simplifying, protecting, insulating and stabilizing the potential strategies such as improving hardness of the fur brush, printed liquid circuits, appropriate packaging on such PCP is very necessary. To date, altering the length of fur, identifying ideal distance between pinhead liquid silicone rubber (LSR) is increasingly utilized in electronics packaging because of its excellent weatherability and thermal stability, oxidation resistance, low- and paper substrate, adjusting the pressure of driving gas via the temperature flexibility, and good dielectric properties. Along this way, a kind of controller or developing more advanced pinhead can be applied to commercially available and low-cost single component room temperature fabricate smoother and narrower circuits. In addition, more paper vulcanizing (RTV) silicone rubber (Type 705, NAN DA, Liyang Kangda Chemical materials should also be tried as substrate materials so as to vigor- Co. Ltd, Jiangsu Provence, China) was introduced as printable packaging ink and demonstrated as an excellent basic packaging material herein. Such kind of rubbers ously advance the PCP technology. can be applied (liquid state) and cured (elastic state) at room temperature through In summary, the present method offers a vital opportunity to automatically absorbing moisture from the air without any heat treatment, and no realize rapid fabrication of inexpensive, disposable, conveniently environmentally harmful gas is discharged. Clearly, through combination of the portable circuits and components, which paved a way towards desk- flowable electrical and insulating inks, even a three-dimensional (3D) electronics device can be printed out using the present method, such that by printing inks on the top direct printed electronics. With designed software, required surface of the cured elastomer under control of digitized program. functional electronic circuits can be automatically printed on paper just by clicking the corresponding specific code or pictures as people Prototyping desktop printer and printing pen. A commercially available rubber usually do in office. This will extensively stimulate tremendous dispenser (TYSR-200D, Tinyo Electronics Corporation, Beijing, China) was modified efforts in either fundamental research or practical applications of to realize the present automatic printing. For the printing pinhead, a brush like porous needle pen was developed to contain liquid metal ink inside and then print it on the PCP in the coming time. paper. Both structures of the desktop printing apparatus and cross-section of the pinhead (inner diameter 0.51 mm) whose cusp is fur material are shown in Fig. 1. It is worth noting herein that according to various application requirements, the printing Methods speed is adjustable by inputting speed setting program via the teach pendant which Preparation of ink materials. The flowable and highly conductive room can be set in a range of 0.1,400 mm/s . In view of facilitating the printing quality, the temperature liquid metals or alloys at normal pressure and temperature are a kind printing speed and pressure herein were set at 10 mm/s and 10–30 psi, respectively. of newly emerging functional materials which were significantly neglected before. Such performance could serve well the need of PCP technology. Further improvement Several easily available metals or alloys such as gallium, indium, GaIn ,and of the printing speed can be made in the near future. In this research, the well- GaIn etc. all show very promising features in flexible electronics. However, a 24.5 prepared GaIn ink in syringe was pushed into the porous brush needle through 24.5 critical issue to impede their extensive applications in printed electronics is the nitrogen gas with prescribed pressure which can be regulated via the controller. Then large surface tension of the liquid metal ink and the poor adhesion thus caused, as requisite liquid metal patterns were printed automatically on paper via the fur brush well as lacking of an appropriate desktop printing machine. In our former efforts, under control of corresponding digitized program embedded in the teach pendant. oxidation on the metal ink was identified as an efficient approach to significantly Afterwards, RTV silicone rubber loaded in another printing syringe is controlled to modify the adhesion of liquid metals on various printing substrates. Therefore, overprint the liquid metal wires for the purpose of insulation and encapsulation. 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Enhanced surface wetting of pigment coated paper by UVC Printed-Circuits-on-Paper Flexible Electronics. Sci. Rep. 3, 1786; DOI:10.1038/srep01786 irradiation. Ind. Eng. Chem. Res. 49, 11351–11356 (2010). (2013). SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 7 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Scientific Reports Springer Journals

Direct Desktop Printed-Circuits-on-Paper Flexible Electronics

Zheng, Yi; He, Zhizhu; Gao, Yunxia; Liu, Jing
Scientific Reports , Volume 3 (1) – May 9, 2013
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Springer Journals
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Copyright © 2013 by The Author(s)
Subject
Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
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2045-2322
DOI
10.1038/srep01786
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Abstract

Direct Desktop Printed-Circuits-on-Paper Flexible Electronics SUBJECT AREAS: 1 1 1 1,2 Yi Zheng , Zhizhu He , Yunxia Gao & Jing Liu ELECTRICAL AND ELECTRONIC ENGINEERING Key Lab of Cryogenics and Beijing Key Lab of CryoBiomedical Engineering, Technical Institute of Physics and Chemistry, Chinese MECHANICAL ENGINEERING Academy of Sciences, Beijing 100190, P. R. China, Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, P. R. China. SURFACE PATTERNING APPLIED PHYSICS There currently lacks of a way to directly write out electronics, just like printing pictures on paper by an office printer. Here we show a desktop printing of flexible circuits on paper via developing liquid metal ink Received and related working mechanisms. Through modifying adhesion of the ink, overcoming its high surface 14 February 2013 tension by dispensing machine and designing a brush like porous pinhead for printing alloy and identifying matched substrate materials among different papers, the slightly oxidized alloy ink was demonstrated to be Accepted flexibly printed on coated paper, which could compose various functional electronics and the concept of 18 April 2013 Printed-Circuits-on-Paper was thus presented. Further, RTV silicone rubber was adopted as isolating inks and packaging material to guarantee the functional stability of the circuit, which suggests an approach for Published printing 3D hybrid electro-mechanical device. The present work paved the way for a low cost and easygoing 9 May 2013 method in directly printing paper electronics. t has been widely known that the invention of printed circuits on boards (PCB) has brought about tremendous Correspondence and advances to modern electronics. Recently, with more concerns raised in the complicated and energy- requests for materials I consuming fabrication processes of conventional electronics, plentiful attentions have been made to find an should be addressed to additive way to make electronic components on any desired substrates, especially those flexible ones. Among the 1,2 3,4 5–7 J.L. ([email protected]. many efforts ever made, functional inks based on organic substances , polymers , nanoparticles , thin-film 8,9 cn) semiconductors etc. have been developed for manufacturing flexible electronics, which significantly contrib- 10,11 12,13 14,15 16 uted to the development of antennas , transistors , solar cells , radio frequency identification (RFID) , 17–19 20 21,22 flexible displays , electronic clothes , sensors and so forth. For developing various promising flexible 23–26 substrate materials, paper is specially being paid with considerable attentions in printed electronics owing to its excellent properties such as easy accessibility, low-cost, foldability, disposability, retrievability and non- 27–32 pollution. Meanwhile, enormous efforts have also been made to find highly conductive inks so as to perform direct writing of flexible circuit. So far, many currently available electrical inks are still not conductive enough. To further improve their electrical conductivity, loading nanoparticle to the base material is an important approach. However, some basic issues still remain. For instance, such composite inks’ conductivity is heavily dependent on the particles’ loading ratio and bonding mechanism after solidification. However, a difficulty lies in is that the loading ratio of particles in solution is generally low. In addition, preparation of such conductive inks is still somewhat complicated. For some ink materials, it has to adopt relatively high post-processing temperature (for example 400uC) to improve their conductivity, while most plastic and paper substrates require processing temperature below 150uC. So far, there still lacks of a reliable way which is capable of directly writing out electronic circuits, just like printing a picture on paper via a desktop printer in office. Recently, the room temperature liquid metal or its alloys was found to own very interesting properties as printing inks especially in flexible electronics. Along this way, a direct approach to immediately write out 33,34 electronics through such inks is incubating . This affordable processing appears rather succinct and efficient. However, there is currently lacking of an automatic printing method for the increasing use of such promising technology which is a key towards pervasive consumer electronics. Clearly, the manual writing approach as demonstrated before would restrict the practical value of the liquid metal printed electronics after all. Therefore, an automatic and direct printing of electronics by developing a desktop printer with intelligent-controlling ability and high-precision is urgently needed to fulfill such requirement. Here we show an entire solution for automatic and direct printing of liquid metal ink on paper, through resolving a series of important issues, such as modifying the adhesion of the alloy inks, overcoming the high surface tension of the ink via the dispensing machine (Fig. 1) and designing a brush like porous pinhead for printing liquid alloy inks and identifying the matched substrate materials among different types of papers. The SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 1 www.nature.com/scientificreports Figure 1 | Schematics for experimental printing pinhead and desktop printer setup. prototyping desktop printer can be manipulated with ease to print Newton second law and uniformly accelerated linear motion law out various patterns on the selected papers as desired under control are suitable to describe such motion which are presented as follows: of the computer software. Particularly, paper with advantages of 1 s~ at ð1Þ flexibility, low-cost and recyclability was investigated as a highly suitable substrate material for printing electronics. This may lead to the concept of Printed-Circuits-on-Paper (PCP). Further, the G {f~ma ð2Þ room temperature vulcanized silicone rubber which has been widely where, s is the distance along paper, t is the detection time of gliding, utilized in electronics packaging was adopted as isolating inks so as to G is the gravity component which is parallel to the inclined paper print them out as encapsulated structure for packaging the liquid substrate; f is the resistance force containing the friction and metal circuits and making a multistory hybrid electro-mechanical adhesion force; m and a are the quality and acceleration of droplet, structure. Through these efforts, various typical circuits and compo- respectively. It can be seen from the curves that the relationship of s nents have been successfully printed on coated paper, including con- and t fits well with the equation (1). And according to equation (2), ductive wires, 3D conductor structure, inductance coil and flexible the acceleration can be described as: antenna etc. Results Printing papers. To evaluate the compatibility between the liquid metal ink and the paper substrate, a high-speed camera was utilized to record the gliding process of the liquid alloy droplet on various paper substrates with a uniform slope angle of 30u. To ensure the fluid feature, high-purity liquid alloy GaIn which was not 24.5 deliberately oxidized was used. For comparison purpose, droplets with same volume should pass through the same distance starting from a standstill state. Therefore, an injector pump with a speed of 10 ml/h was used to generate droplets so as to ensure same droplet volume and timing starting from the moment of gliding. Figure 2 shows the detailed gliding process of liquid alloy ink droplet on coated paper. Here, droplets on all papers have run the same distance as 60 mm. Figure 3 presents the distance (s) as a function of detection time and the internal schematic diagram depicts the force analysis of droplet on paper over the process. It is known that there are mainly three constant forces acting on the droplet, namely gravity (G), supporting force (N) and resistance (f). Figure 2 | Sequential optical images of liquid metal droplet gliding along the coated paper with a slope angle of 306 which were recorded by high- Droplets are considered to run uniformly accelerated linear motion along papers under the effects of forces. The classical speed camera. SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 2 www.nature.com/scientificreports Therefore, coated paper possesses the maximal adhesive force, indicating that coated paper has the best compatibility with liquid GaIn based alloy ink among all applied papers. 24.5 Coated paper with high smoothness and brightness is an advanced paper substrate which has been commonly used in printing book cover, elegant advertisements, commodity packaging, and labels, etc. It is mainly manufactured by coating a pigment compounds layer on base-paper. Conventional filler pigments mainly contain ground calcium carbonate (GCC), precipitated calcium carbonate aragonite (PCC) and Kaolin (mainly Al O ?2SiO ?H O), as well as chemical 2 3 2 2 additives such as dispersants, resins, sodium polyacrylate (NaPA), and poly (styrenebutadiene) (SB) latex binder etc. Investigations have been conducted to confirm that addition of the the above-men- 35–37 tioned fillers has a positive effect on the adhesion of paper .In order to further disclose the mechanism for the surface adhesive, an energy dispersive spectroscopy (EDS) analysis was also conducted to survey the surface elements of the tested coated paper. Figure 5 exhi- bits the weight ratio of various elements on the surface of the coated paper. It can be seen that oxygen and silicon elements occupy a large Figure 3 | Distance (s) of droplets as a function of measured time (t) on proportion among all elements. As a result, combined with the for- coated paper, office paper, wove paper, respectively. The inset shows the mer efforts, a better compatibility between paper and ink can be sketch map of force analysis of the droplet in the gliding process. obtained through properly oxidizing GaIn alloy ink. 24.5 Automatic printing of circuits on coated paper. Automatic G {f mg sin h{f f printing of GaIn based inks on coated paper was implemented a~ ~ ~g sin h{ ð3Þ 24.5 m m m utilizing the developed system, and the direct printing process was illustrated in Fig. 6. Through the evaluation as performed above, here where, h is the slope angle between paper substrates and horizontal the commercially available coated paper (with a weight specification line. Additionally, for all paper types, parameter h, g and m are all of 200 g/m ) was adopted as the printing substrate. Apparatus constant values. Therefore, there are only two variables in the equipped with single printing syringe A or B were utilized and the equation (3), a and f. From Fig. 3, it can be found that acceleration structure materials of matched printing needle were porous brush on coated paper is the minimum of all, which indicates that it has and orifice metal, respectively. In addition, syringe A and B were maximum resistance force on the surface of coated paper. In order to respectively loaded with GaIn ink and RTV silicone rubber. 24.5 clarify the surface roughness of three papers, we have performed an During the printing process, syringe A was firstly installed to print atomic force microscope (AFM) analysis. Figure 4A, B and C present required patterns on coated paper according to the digitalized the AFM images of surface morphology of wove paper, office paper program, and then overprinted by RTV silicone rubber from and coated paper, respectively. The measured average Rq of wove syringe B. When necessary, multilayer circuit structures can be paper, office paper and coated paper is 120.8 nm, 70.7 nm and fabricated by sequentially printing liquid metal circuitries 12.9 nm, respectively. It has been widely accepted that the rougher bypassing the upper surface of the cured silicone rubber. In this the surface, the larger the friction, which means coated paper has the way, a three dimensional (3D) hybrid electro-mechanical device on minimal surface friction. Conclusions regarding the numerical paper can be quickly printed out. relations can be obtained as follows: Several typical directly printed circuits containing bending circuit Total resistance : f wf wf ; ð4Þ wove coated office with regularity (1), packaged straight wires (2), circuit nodes (3), stereoscopic flexible circuits (4) and annulus wire with LED (5) on Surface friction : f wf f ; ð5Þ office coated wove w coated paper are presented in Fig. 7(A), which illustrate the excellent adhesion between GaIn based ink and coated paper, as well 24.5 Adhesive force : f wf wf : ð6Þ wove coated office as outstanding elastic characteristics of this kind of RTV silicone Figure 4 | Surface morphologies of three typical papers. AFM images (23 2 mm ) of surfaces of Wove paper (A), Office paper (B) and Coated paper (C), respectively. SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 3 www.nature.com/scientificreports again, indicating that the compatibility between the ink and coated paper is functionally favorable. Apparently, processing should be further advanced to fabricate thinner even nanoscale circuit to apply in highly required integrated electronics. This needs new efforts in the near future. Electrical reliability of flexible circuit as a function of bending angles. As an innovative ink with tremendous potentials to be further applied in future flexible electronics manufacturing, the resistance stability of the liquid metal before and after bending is vital. Here, a digital bridge with an equipment deviation of 0.003 V was employed to measure the resistance variation of liquid metal circuit when folding to five angles. Liquid metal circuit whose length and width is about 18 cm and 1 mm respectively was printed. Two copper wires with the same resistance of 0.0275 V Figure 5 | Energy dispersive spectroscopy (EDS) picture of elements on were applied to serve as measuring electrodes. The printed circuits coated paper surface. were well packaged by RTV silicone rubber and the operating frequency and voltage of digital bridge was set to 1.0 kHz and 1 V, respectively. Figure 9 shows the resistance results according to the measurements. It is necessary to point out that all the measured rubber. More printed functional electronics including inductance resistances are the total values that contain the resistances of both coil (1) and RFID (2) antenna are given in Fig. 7(B), and picture 3 circuit and copper electrodes, and in order to reduce the deviation, all reveals the excellent flexibility of the fabricated antenna after pack- resulting values in picture are an average of 3 measured values. It can aging. Experimental results confirm that the cured silicone rubber be evidently observed that resistances vary just in a limited range, can commendably prevent the liquid ink from flowing along the which indicates that the well-encapsulated printed liquid metal wire wires, which to a large extent will contribute to the functional on paper could meet well the requirement of the flexible circuit. stability of the printed circuits or electrical components. Meanwhile, the liquid silicone rubber scarcely affects the electrical It is well known that conductive inks of conventional printed performance of the printed liquid circuit. This is rather beneficial for electronics usually function after solidifying. This indicates that the wide adaptability of PCP. the fabricated electronics have mechanically flexible restriction. However, the present ink remains liquid under functional conditions after being encapsulated by RTV silicone rubber, while the substrate Discussion and the elastic rubber form a channel to maintain the electrical In this article, an innovative approach to automatically fabricate structure. Therefore, the fabricated circuits cannot be broken off electronics by directly printing liquid alloy based inks on coated easily even under frequent bending, showing an attractive and dis- paper is described. Compared with the conventional ways of making tinguished mechanical flexibility which is a critical advantage in flexible electronics using existing conductive inks, the present high- fabricating flexible electronics. Moreover, the well-encapsulated efficiency direct printing process has relatively lower-cost, simpler liquid alloy ink on paper can be conveniently and responsibly stored process, well acceptable conductivity and is more environmentally and carried. Overall, the present approach constitutes a vital step friendly which guarantees the really ‘‘green’’ fabrication of electron- towards truly directly printed electronics. ics. Moreover, the whole printing process is completed under room Figure 8A presented the physical images of the printed circuit with temperature, which hardly brings about detrimental influence to the a width of 1.5 mm on coated paper, and Fig. 8B and C gave the SEM paper substrate and is capable of ensuring the printed circuits’ own graphs of surface and cross-sectional topography of the printed excellent electrical performance and mechanical flexibility. Further, liquid wires respectively. It can be discovered from the images that the recycling of metal inks is easier to be completed just by properly the liquid metal wire is adhered closely to the surface of the paper destroying the external elastic silicone rubber and then efficiently which has a multi-groove structure. The directly printed liquid cir- reclaiming the noble material by 30% NaOH solution. Finally, liquid cuit on coated paper is relatively uniform and the thickness is about metal ink runs in liquid state which indicates that liquid metal cir- 120 mm. Simultaneously, the SEM graphs prove that GaIn based cuits printed on flexible substrates will not be easily fractured owing 24.5 ink possesses wonderful adhesion with coated paper substrate once to its unique liquidus property. Figure 6 | Schematic of liquid alloy printing of electrical circuitry on coated paper. Liquid metal (A) is firstly printed on paper (Step 1); Then RTV silicone rubber (B) is overprinted sequentially as encapsulated material (Step 2); When necessary, a multilayer circuit structure is fabricated by printing liquid metal (A) circuitry bypassing the upper surface of cured silicone rubber (Step 3). SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 4 www.nature.com/scientificreports Figure 8 | Characterization of the printed conductive objects. (A) Optical image of printed circuit on coated paper with a width of 1.5 mm. (B) and (C) are SEM images of surface and cross-section of printed liquid metal ink on coated paper. as conductive polymers, metal nanoparticles or semiconductors, fur- ther performance modification of such inks is likely to be realized to meet future high profile needs in a wide variety of flexible electronics. It is expected that such unique approach will be applied in a sufficient number of electrical areas including rapid-prototyping circuits (e.g., E-Card), integrated circuit on paper, sensors, 3D mechanical switches and antennas, electronic solar cell arrays, functional RFID tags on books, even daily life personal electronics and educational training. However, as a newly emerging technology, the present method also raised a series of practical issues waiting to be further better solved such as guaranteeing an ever higher resolution circuital uni- formity as well as more rapid printing speed of the process. In this side, considerable researches as implemented in our work disclosed that, through resolving the printing challenge caused by the large surface tension of GaIn24.5 ink (via oxidation and searching com- patible paper substrate), the commonly encountered troubles such as Figure 7 | Desktop printed electronic components. (A) Physical diagram frequent occurrence of the intermittent structure and droplet forma- of GaIn based liquid alloy directly printed on coated paper by a 24.5 tion of conductive line were successfully overcome. In addition, dispenser printer and optical images for various conductive wires on coated paper: (1) Manufacturing process of printed electronics. The inset shows the regular bending circuit; (2) Metal conductor lines covered with silicone rubber; (3) Multi-layer structure or electrical nodes; (4) Three- dimensional structure of printed conductor on paper; (5) Galvanical annulus wire attached with LED. (B) Optical images for printed functional components on coated paper: (1) Inductance coil; (2) RFID antenna; (3) Presentation of prominent flexibility of printed electronics. As disclosed by former research , the content of the oxides in liquid alloy can efficiently alter the viscosity of the ink which greatly affects the final performance of the circuits and components printed directly on papers. The adhesion of alloy ink can be upgraded through the process of oxidizing, as well as altering the weight ratio of the chemical composition. Also, the surface adhesion of the coated paper materials can be modified by ultraviolet-C ray radiation (UVC Irradiation) . Furthermore, the coated paper substrate can also be 36,37 simply manufactured on one’s own as needed . And reinforced RTV silicone rubbers or other alternative adhesives can be applied to substitute this basic encapsulated material for extreme working condition. In the near future, more liquid metals or alloys can be developed as suitable inks to be directly printed on various compat- Figure 9 | Resistance values of liquid circuit at five different bending ible papers. Clearly, combined with more functional materials such angles. SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 5 www.nature.com/scientificreports Printable isolation ink for packaging material. Considering the liquidus properties better circuital uniformity and resolution can still be possible in the of the current metal ink, when exposed in air, the fabricated patterns are apt to be coming time subject to further endeavor. Along this direction, several changed. For the purpose of simplifying, protecting, insulating and stabilizing the potential strategies such as improving hardness of the fur brush, printed liquid circuits, appropriate packaging on such PCP is very necessary. To date, altering the length of fur, identifying ideal distance between pinhead liquid silicone rubber (LSR) is increasingly utilized in electronics packaging because of its excellent weatherability and thermal stability, oxidation resistance, low- and paper substrate, adjusting the pressure of driving gas via the temperature flexibility, and good dielectric properties. Along this way, a kind of controller or developing more advanced pinhead can be applied to commercially available and low-cost single component room temperature fabricate smoother and narrower circuits. In addition, more paper vulcanizing (RTV) silicone rubber (Type 705, NAN DA, Liyang Kangda Chemical materials should also be tried as substrate materials so as to vigor- Co. Ltd, Jiangsu Provence, China) was introduced as printable packaging ink and demonstrated as an excellent basic packaging material herein. Such kind of rubbers ously advance the PCP technology. can be applied (liquid state) and cured (elastic state) at room temperature through In summary, the present method offers a vital opportunity to automatically absorbing moisture from the air without any heat treatment, and no realize rapid fabrication of inexpensive, disposable, conveniently environmentally harmful gas is discharged. Clearly, through combination of the portable circuits and components, which paved a way towards desk- flowable electrical and insulating inks, even a three-dimensional (3D) electronics device can be printed out using the present method, such that by printing inks on the top direct printed electronics. With designed software, required surface of the cured elastomer under control of digitized program. functional electronic circuits can be automatically printed on paper just by clicking the corresponding specific code or pictures as people Prototyping desktop printer and printing pen. A commercially available rubber usually do in office. This will extensively stimulate tremendous dispenser (TYSR-200D, Tinyo Electronics Corporation, Beijing, China) was modified efforts in either fundamental research or practical applications of to realize the present automatic printing. For the printing pinhead, a brush like porous needle pen was developed to contain liquid metal ink inside and then print it on the PCP in the coming time. paper. Both structures of the desktop printing apparatus and cross-section of the pinhead (inner diameter 0.51 mm) whose cusp is fur material are shown in Fig. 1. It is worth noting herein that according to various application requirements, the printing Methods speed is adjustable by inputting speed setting program via the teach pendant which Preparation of ink materials. The flowable and highly conductive room can be set in a range of 0.1,400 mm/s . In view of facilitating the printing quality, the temperature liquid metals or alloys at normal pressure and temperature are a kind printing speed and pressure herein were set at 10 mm/s and 10–30 psi, respectively. of newly emerging functional materials which were significantly neglected before. Such performance could serve well the need of PCP technology. Further improvement Several easily available metals or alloys such as gallium, indium, GaIn ,and of the printing speed can be made in the near future. In this research, the well- GaIn etc. all show very promising features in flexible electronics. However, a 24.5 prepared GaIn ink in syringe was pushed into the porous brush needle through 24.5 critical issue to impede their extensive applications in printed electronics is the nitrogen gas with prescribed pressure which can be regulated via the controller. Then large surface tension of the liquid metal ink and the poor adhesion thus caused, as requisite liquid metal patterns were printed automatically on paper via the fur brush well as lacking of an appropriate desktop printing machine. In our former efforts, under control of corresponding digitized program embedded in the teach pendant. oxidation on the metal ink was identified as an efficient approach to significantly Afterwards, RTV silicone rubber loaded in another printing syringe is controlled to modify the adhesion of liquid metals on various printing substrates. Therefore, overprint the liquid metal wires for the purpose of insulation and encapsulation. GaIn alloy with a melting point of 16uC was selected here as the testing ink. 24.5 For this purpose, high-purity gallium and indium (with purity of 99.99 percent) metals as source materials were weighted with a ratio of 75.5524.5 in line with the 1. Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances on chemical compositions of GaIn alloy. The total processes were generalized as 24.5 plastic. Nature 428, 911–918 (2004). follows: Firstly, configuring the eutectic GaIn alloy. The weighted gallium and 24.5 2. Parashkov, R., Becker, E., Riedl, T., Johannes, H. H. & Kowalsky, W. Large area indium metals according to the required percentage are mixed in the beaker which electronics using printing methods. Proc. IEEE 93, 1321–1329 (2005). was syringed by deionized water in advance and heated to about 50uC until metals 3. Yoshioka, Y. & Jabbour, G. E. Desktop inkjet printer as a tool to print conducting were fused completely, then stirred slightly. 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Enhanced surface wetting of pigment coated paper by UVC Printed-Circuits-on-Paper Flexible Electronics. Sci. Rep. 3, 1786; DOI:10.1038/srep01786 irradiation. Ind. Eng. Chem. Res. 49, 11351–11356 (2010). (2013). SCIENTIFIC REPORTS | 3 : 1786 | DOI: 10.1038/srep01786 7

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Scientific Reports – Springer Journals

Published: May 9, 2013

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