Ink Spraying Based Liquid Metal Printed Electronics for Directly Making Smart Home Appliances

The quickly emerging smart home is heavily involved with various information technologies where electronic devices play perhaps the most important roles. However, there is currently a strong lack of an efﬁcient tool for the direct manufacture of electronics as desired. Meanwhile, although printed electronics has aroused much attention in making printed circuit board, radio frequency identiﬁcation card, sensor and so on, few existing approaches can fulﬁll the above needs. Among various technologies, liquid metal spraying method owns rather unique merits in printing electronically conductive patterns on nearly all kinds of substrates, either hard or soft, rough or smooth. For this reason, we are dedicated here to extend such method into a generalized strategy of printing electronics for the direct construction of smart home appliances which would generate profound inﬂuence on future daily lives. Through adopting the room temperature liquid metal GaIn 24.5 as the electronic ink and with various designed masks, a group of typical conductive patterns were directly printed on the surfaces of organic glass, marble windowsill, paper, wood plank of cabinet, building wall and so on, which consist of the most popular materials that could serve as circuit substrates in a home. It was shown that this energy-saving printing method brings about big convenience for equipping future smart home residents. Several new conceptual homemade printed patterns or circuits in providing services such as decoration and entertainment were illustrated. Further, the basic electronic structures of the smart home and the generalized concept of adopting any substrate surface as printed circuit board were explained. The liquid metal spray printing suggests a powerful tool for making consumer electronics and constructing future smart home. It also raised important scientiﬁc and technical issues worth of pursuing in the coming time.

The term smart home refers to a residence equipped with technology which allows network communication between all the devices and appliances in the home with the goal to remote control, monitor and access the services that respond to the needs of the inhabitants. [1][2][3][4] The research and development on smart home to fulfill varying needs such as health care for the elderly have in fact gathered much interest both in academia and industry. [5][6][7] Diverse sensing systems including image, sound, temperature and light sensors are deployed in a smart home and the generated data stream is transmitted via radio frequency identification (RFID) networks, global positioning system (GPS) and so on. [7][8][9][10][11] Clearly, along this trend, we would anticipate that, in such a smart grid and if needed, electronic circuits should be able to stand everywhere such as ceilings, walls, beds, chairs, windows and so on. For this purpose, we could depict such a typical schematic drawing for the smart home equipped with various printed electronics as Figure 1. To fulfill such coming needs, we are dedicated here to illustrate a generalized printed electronics strategy for the rapid fabrication of electronic circuits or metal patterns with the specific purpose of directly making functional devices or connection wires to compose home appliances.
Printed electronics 12 has attracted great attention over a number of areas such as transistors, [13][14] RFID tags, 15 photovoltaics arrays 16 and so on. Various kinds of printing techniques including screen printing, 17 micro-contact printing, 18 drop-on-demand (DOD) inkjet printing 19 have been developed. Among these techniques, spray printing emerged as an effective method to fabricate flexible electronics. Siegel et al. 20 made printed circuit boards on paper substrates via spray deposition method with acrylic-based metal flakes. Akhavan et al. 21 fabricated photovoltaic devices on glass and plastic substrates by spray-coating the copper indium diselenide nanocrystals. Kim et al. 22 utilized the spray coating method for patterning carbon nanotube (CNT) films on polydimethylsiloxane (PDMS) substrate. Recently, Zhang et al. 23 proposed an atomized spraying method for directly patterning electronic features with room temperature liquid z E-mail: jliu@mail.ipc.ac.cn metal ink on polyvinylchloride (PVC), rubber, typing paper, cotton cloth and even tree leaf substrates. This liquid metal spray printing method suggests big potential for making electronics as freely as possible and is thus extended here as a generalized printed electronics way to directly deposit conductive patterns and functional circuits on the substrates for making smart home appliances. For illustration purpose, only most typical electronic devices such as lighting system decorated on pervasive surfaces will be selectively printed and investigated as an example for brief.

Experimental
Here, GaIn 24.5 and 705 silicone rubber are adopted as the representative liquid metal ink and the encapsulated material, respectively. GaIn 24.5 is a kind of nontoxic room temperature liquid metal and its melting point is about 15.7 • C. 705 silicone rubber is a kind of neutral transparent single component room temperature vulcanizing (RTV) silicone rubber which was often used for encapsulating and sealing the electrical components to resist dampness and shock. Figure 2A-2E illustrates the fabrication process of the liquid metal pattern. Liquid metal atomized microdrops are generated in an airbrush whose principle can be found in the work of Zhang et al. 23 These microdrops are sprayed uniformly on the mask and the target substrate which can be but not restricted to glass, wood plank, floor tile or any other building materials. The sprayed microdrops merge together due to their surface tensions and a liquid metal pattern is formed after removing the mask. Then 705 silicone rubber is dripped from a syringe nozzle to encapsulate the liquid metal pattern. Such self-leveling material which owns good flow characteristic before being cured will fill the gap between the pattern and the substrate and spread over the pattern to create a flat surface. To prevent it from flowing around, a barrier plate is placed on the substrate. As 705 silicone rubber becomes transparent elastomer after being cured through absorbing the water vapor in air (the surface curing time is 3-30 min and the curing depth is 2-3 mm at room temperature), the liquid metal pattern will thus be encapsulated   to resist destroying. After removing the barrier plate, an encapsulated liquid metal pattern is produced. A fabricated comb capacitor pattern on slide glass substrate is presented in Figure 2F, as well as the used stainless steel mask.

Results and Discussion
The adhesion characteristics of the atomized liquid metal microdrops.-As the liquid metal ink GaIn 24.5 is sprayed from the airbrush nozzle, large number of microdrops are generated and their sizes are within the range of 700 nm to 50 μm when the air supply pressure is set as 350 kPa. 23 A gallium-based liquid metal sphere is coated by a layer of oxide (Ga 2 O 3 /Ga 2 O) when it is in air environment. [24][25] Assuming that R and d respectively represent the radius of the liquid metal sphere and the depth of the gallium oxide layer shown in Figure 3A. Theoretically, the volume fraction of the oxide content in the sphere can be expressed as Given that d equals to 1 and R is in the range of 0 and 20, the F-d curve relationship is depicted in Figure 3B. It can be seen that the volume fraction of the gallium oxide F increases with the decrease of the sphere radius R. As there is a positive correlation between the surface tension of the droplet and the gallium oxide content, 23 the smaller the liquid metal droplet, the larger the surface tension and thus better adhesion between the ink and the substrate. Besides, the more oxide content of a liquid metal droplet, the smaller contact angle between the droplet and the solid substrate. 26 According to the Young-Dupre equation, 27 one has where W SL represents the work of adhesion which refers to the work required to separate a droplet from the solid substrate, γ L represents the surface tension of the droplet, and θ is the contact angle between the droplet and the solid substrate. Overall, a small liquid metal droplet has better adhesion property compared to a larger one due to its larger γ L and smaller θ. When a liquid metal droplet with coating layer impacts the solid surface at a certain speed, splashing or bouncing phenomenon will occur. 28 Small microdrops will be splashed into the gap between the mask and the substrate as shown in Figure 3A. The roughness of the substrate surface has an effect on the contact angle between the liquid metal droplet and the substrate according to Wenzel's theory. 29 And the adhesion work of the liquid metal droplet to the substrate will thus be influenced by the substrate roughness referring to equation 2. When the contact angle θ >90 • , the finer the substrate surface, the smaller the contact angle θ. And when θ <90 • , the finer the substrate surface, the larger the contact angle θ.
The transmission electron microscope (TEM) image of the liquid metal GaIn 24.5 particles prepared by ultrasonic disruption method in deionized water with an ultrasonic crushing apparatus is shown in Figure 4A. It can be seen that the particles mainly exhibit sphere-like shape due to the large surface tension (0.624 N · m −1 ) of GaIn 24.5 . Energy dispersive X-ray (EDX) was performed to analyze the surface element composition for the liquid metal GaIn 24.5 sprayed on the PVC film substrate and the result is shown in Figure 4B. There are three elements gallium, indium and oxide on the liquid metal surface and the measured mass fractions are respectively 74.93%, 24.41% and 0.66%. X-ray photoelectron spectroscopy (XPS) measurements were carried out by using Mg K α X-ray radiation to investigate the chemical compositions of the oxide layer. And the result is presented in Figure 4C. The main photoelectron component centered at a binding energy (BE) of 18.2 eV is due to Ga in metallic state, 30 while the two others at 20.8 eV and 19.8 eV are the characteristics for Ga 2 O 3 and Ga 2 O. [30][31] In addition, the peak located at 16.3 eV is reasonable assigned to In 4d (In 0 ) spectrum. 25,32 Characterization of the printed patterns.- Figure 5A shows the measured width values of a line printed by using the liquid metal spray printing method on PVC substrate. The average width of such line is 2.035 mm, and the calculated standard deviation is 0.02 mm. The subtle variation in the line width indicates a high printing accuracy. The scanning electron microscope (SEM) images of the surface topography and the thickness of the printed line are respectively shown in Figure 5B and 5C. It can be seen that there are micro-size liquid metal particles scattered on the line surface. The thickness of the printed line is only 20.94 μm, which is much smaller than that of the track made by liquid metal direct writing method. 33 The atomic force microscope (AFM) image of the surface for the printed line is presented in Figure 5D and the measured root mean square roughness (Rq) and the roughness average (Ra) are 115 nm and 90.2 nm, respectively. The uneven surface of the printed pattern is mainly due to the oxide layer on the surface which reduces the flowability of liquid metal. The influence factors of the surface roughness of the printed objects include the size of the generalized microdrops and the spray uniformity of the liquid metal ink. To ensure the printing quality, the mask should be pressed tightly against the substrate. Figure 5E and 5F show the photos and SEM images of the printing pattern boundary when the stainless steel mask is under 4.44 kPa and 1.34 kPa, respectively. It can be seen that the boundary line when the mask under 4.44 kPa appears much clearer than the other one. This is due to that the microdrops cannot easily edge in the narrow gap between the mask and the substrate.
The electrical stability of the printed circuit is a research focus when performing the liquid metal spray printing method for smart home. The measured resistance-temperature relationship of a printed line resistor and the capacitance-temperature relationship of a printed comb capacitor are shown in Figure 6A. It can be seen that the resistance of the resistor rises slightly (from 3.800 to 3.844 ) with the increase of the temperature in the range of 26.5 • C to 45.5 • C, while the variation of the capacitance of the capacitor presents a de- scending trend (from 2.408 nF to 2.344 nF). It can be interpreted as follows: The irregular motion of the liquid metal atoms intensifies as the temprature increases, which hinders the motion of the free electrons. This leads to the decrease of the electrical conductivity of the liquid metal which measures its ability to conduct an electric current. In order to measure the electrical stability of the liquid metal pattern, the resistance-time ralationship of a printed line resistor was measured (as shown in Figure 6B) in the indoor environment for 18 h. The mean resistance and the standard deviation are respectively 5.123 and 0.013 , which indicates a stable electrical performance of the line resistor. The small fluctuation in the measured resistance value is manifested in the inset of Figure 6B. The opposite changing trends in the resistance-time and the temperature-time relationships confirm the interpretation mentioned above.
In order to investigate the feasibility of applying the liquid metal atomized spraying method for making the smart home appliances, various indoor objects surfaces are introduced as the target substrates to perform the printing. Some fabricated masks made of plastic are shown in Figure 7A. Figure 7B and 7C show the logos of the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences and the Bioheat Transfer Lab which are the authors' institute and lab, respectively. These patterns are printed on the organic glass and can serve as a kind of common decorative material. Figure 7D illustrates a printed RFID pattern on the marble windowsill. Figure 7E and 7F present a printed Christmas electronic card pattern on a red paper substrate hung on the door and a printed RFID pattern on the wood plank of a small cabinet, respectively. Figure 8 shows a Christmas electronic card pattern printed on the PVC substrate and illuminated by a yellow LED light. These figures suggest that various appliance shapes can be directly printed on the surfaces in a smart home which represents a printed circuit board in a broad sense. In addition, the printed patterns can provide services of decoration and entertainment for the inhabitants according to need.
Strategy of constructing a smart home with printed electronics.-A smart home is typically equipped with a large number of networked sensors, electrical appliances and biomedical monitors. We would draft that in the future in principle, any surfaces of walls, floors, ceilings, beds, cabinets, electrical equipment, tables and chairs etc. can all serve as the substrates to implement the liquid metal spray printing technology, if desired. There, patterns, electronic circuits and a group of different functional devices such as RFID, connecting wires for TV display, recorder etc. can be directly printed with high convenience. Although only typical examples were illustrated, the presently developed self-service printing technology significantly simplifies the circuit manufacturing process and thus has extensive practical values for the coming use. In order to promote this technology, efforts should be made in several aspects including finding more metal inks ranging from low-melting point to high-melting point materials, rapid manufacturing of masks and substrates for printing purpose, safety test and recycling of the inks and so on. Compared to solid metal materials, room temperature liquid metal has unique advantages in printing flexible electronics due to its flowability. However, the electrical conductivity of this kind of ink is relatively low and patterns thus printed still need packaging otherwise they can be easily damaged. 34 To solve this problem, higher melting-point metal inks such as aluminum-based alloys, as well as the matched substrate materials should be developed. The patterns can be printed in an ambient whose temperature is higher than the melting point of the ink and then they would be cooled to solid after printing. Liquid metal ink should be sprayed uniformly and the mask should be pressed tightly against the substrate to ensure the printing quality. In addition, the metal inks should be nontoxic and   recyclable to ensure that it will not affect the health of the residents and the living environment.

Conclusions
In summary, a general strategy of applying liquid metal spray printing method for directly making electronic appliances of future smart home is established. The fabrication processes of liquid metal patterns and packaging layer were demonstrated. Patterns and circuits were directly printed with a mask on a variety of home purpose material surfaces such as organic glass, marble windowsill, paper, wood plank of cabinet, building wall and so on by using a room temperature liquid metal GaIn 24.5 ink. Several influencing factors to ensure the printing quality were clarified. Overall, this straightforward manufacturing method is time, energy and cost saving, which allows the residents to flexibly print out their own electronic patterns according to personal needs. To promote the extensive application of this self-service printing technology, more nontoxic metal inks with wide melting point range should still be investigated. In addition, with the increasing use of the method, recycling of the inks also needs to be paid with enough attention so as to protect the living environment in the long run.