Liquid metal printed electronics are drawing more and more attention widely across the world, due to the excellent fluidity and the high electrical conductivity of the gallium-based liquid metal such as EGaIn (Ga 75.5 wt.%, In 24.5 wt.%) [1,2]. Compared with other conductive inks such as silver ink [3,4] and carbon ink [5,6], the electrical conductivity of gallium-based liquid metal ink is higher and stable, without worrying about the problem that the performance loss caused by long-time storage due to volatilization of the solvent. Besides, since it keeps liquid state at room temperature, the gallium-based liquid metal ink can respond in good quality with the bending, stretching, or twisting of the substrate [7,8]. Therefore, the liquid metal printed circuit is also a better choice for the fabrication of flexible electronics. What is more, the gallium-based liquid metal ink is easy to be collected and processed into fresh ink when separated from the circuit board. This means it can be potentially recycled to reduce waste and pollution to the environment [9]. Meanwhile, the biocompatibility of liquid metal ink is rather attractive, too. Kim et al. [10] studied the effect of EGaIn release on various human cells, and it was found that the naturally released Ga and In ions did not affect the tested cells. There has been a lot of research on the applications of gallium-based liquid metal in the field of biomedical engineering, such as drug carrier and neural junction, and so on [[11], [12], [13], [14], [15]].

In addition to the conductive ink, a “green” material as the substrate for liquid metal electronics is also worth considering. In recent years, more and more materials have been adopted as the substrates for liquid metal printed electronics. For example, PVC and PET films are the first two substrates used, which have good chemical stability, electrical insulation, and good adhesion to liquid metal ink [16,17]. Also, PET film has good transmittance and can endure temperatures as high as 300 ℃. In addition, PDMS and Ecoflex are two kinds of elastic substrates with excellent stretchability during the fabrication of the liquid metal stretchable electronics [[18], [19], [20]]. PDMS is a kind of polymer organic silicon compound with good biocompatibility and high transparency, widely used in microfluidic and other fields. Ecoflex is also a two-component adhesive before forming the film, which is softer and more elastic than PDMS. However, none of these substrates can be easily degraded.

Cellulose-based materials have been used more frequently in the synthesis of composite materials because of their biodegradability and renewability [[21], [22], [23], [24]]. Paper is a kind of low-cost and ubiquitous cellulose material. And more functional liquid metal electronics on paper have been realized [25,26]. Li et al. [27] developed a method for fabricating and recycling of paper-based circuit by depositing the liquid metal particle film on paper. In this way, the liquid metal particle film needed to cover the whole paper, and the recycling process was carried out in ethanol. Besides, the nanocellulose-based membrane is a new type and attractive substrate for flexible electronics [[28], [29], [30], [31]]. The advantages of nanocellulose include excellent mechanical properties, lower density, and higher chemical resistance [29]. Zhang et al. ^[30] fabricated an ultrathin Janus film by mixing of liquid metal nanoparticles with nanocellulose and polyvinyl alcohol suspension. Li et al. [31] prepared the conductive paper and free organic solvent ink by mixing liquid metal droplets and nanocellulose suspension, followed by deposition and drying. As a result, the conductive ink would cover the entire membrane, rather than form an electrically conductive selectively pattern according to the circuit diagram.

In this paper, reusable nanocellulose-based liquid metal (NC-LM) flexible electronics via the evaporation-induced transfer printing have been developed. The flexible liquid metal pattern is more stable since the conductive lines fabricated using transfer printing are embedded in the membrane. Compared with other printed substrates, the NC-LM circuit is ultrathin and degradable. Several examples of NC-LM flexible circuits have also been designed, and NC-LM electronics also provide a practical way to prepare a multilayer circuit. Besides, the mechanism of the evaporation-induced transfer printing technology has been revealed. More importantly, the NC-LM circuit can be dissolved in long-term immersion in water. Both the liquid metal ink and nanocellulose suspension can get recycled, which completely meets the requirements of environmental protection.

In the experiments, carboxylated nanocellulose liquid CNF-P with a concentration of 1.2 mmol/g was purchased directly from Guangxi, China. To prepare a uniform nanocellulose solution, 100 mL liquid nanocellulose and about 40~60 mL deionized water were first mixed and stirred together. Then the stirred liquid was put into the ultrasonic mixer instrument so that the stirring probe was at the bottom of the liquid as far as possible. After sonication, a uniform nanocellulose solution was obtained. The ultrasonic treatment used pulse mode of 100 W.

The electronic ink used in this paper is EGaIn alloy. Gallium (99.99 %, purchased from Shanxi Zhaofeng Gallium Co., Ltd) and indium (99.995 %, purchased from Zhuzhou Smelter Group Co., Ltd) were mixed at the ratio of 75.5:24.5 by weight. Then the mixture was heated to 60 °C and stirred for 30 min under the protection of nitrogen gas to obtain liquid EGaIn alloy.

Fig. 1 the major steps of the transfer printing method to fabricate the LM-NC circuit. The PVC film played the role of sacrifice substrate during the transfer printing process. Firstly, the designed circuit was printed on a 0.25 mm thick PVC film with the help of the liquid metal printer (Fig. 1(a)). The printer uses room temperature liquid metal materials as conductive ink and can realize the direct printing process of electronic circuits on PVC or PET substrates through the tapping-mode technique [32]. The circumstance inside the printing chamber of the machine was maintained constant at a temperature of about 25 °C and a relative humidity of about 40 %. Immediately after the printing, the treated nanocellulose solution was spread on the PVC substrate to completely cover the liquid metal circuit (Fig. 1(b)). After that, it was placed at room temperature with a relative humidity of about 30 %-40 % for about two days. Then the water in the nanocellulose suspension gradually evaporated until a thin membrane was completely formed (Fig. 1(c)). At this time, the nanocellulose membrane was partially separated from the PVC substrate and then can be easily peeled off (Fig. 1(d)). Meanwhile, the liquid metal line was partially embedded in the nanocellulose membrane to copy the pattern that was printed on the PVC substrate (Fig. 1(e)). There was still some EGaIn ink remaining on the PVC substrate, which could continue to be used for another transfer printing. After obtaining the NC-LM circuit, the components (such as LEDs) were mounted on the pads through the connection between the pins and the liquid metal ink (Fig. 1(f)). The pins of components required preprocessing to improve its wettability with EGaIn. To further enhance the bonding strength, the entire circuit and components could also be encapsulated with a liquid composed of nitrocellulose and acetone. After the packaging layer was cured, the electronic components were bonded to the flexible circuit without affecting the circuit flowability at the contact points.

The optical images of NC-LM circuits were acquired by a Nikon TS-100 microscope. An emission scanning electron microscope (SEM, FEI guangta 200) was used to observe the morphology of the printed lines.

During the accuracy evaluation test and thickness measurement of the transfer printing method, the designed width of the eight printed lines is 0.8 mm, 1.0 mm, 1.5 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, and 10.0 mm. The images of the printed lines were captured with the Nikon TS-100 microscope and the width was measured using the software ImageJ. The thickness values of the printed lines were acquired from the SEM images of the cross-section. Each sample was measured three times at different positions.

The conductivity of the liquid metal circuit was measured using the KEITHLEY 2450 by the four-wire method. Three liquid metal lines with 0.8 mm, 1.0 mm and 4.0 mm width were measured. The lengths of all the printed lines are 10 cm. The probe was connected to both ends of the liquid metal line to measure the resistance value between the two ends of the liquid metal line. The input current was 10 mA. During the bending test, the liquid metal lines were bent with 0°, 30°, 60°, 90°, 120°, and 150° successively. The bending radius is 5 cm. Afterward, the liquid metal lines were stored in an almost constant environment with a relative humidity of about 40 % and a temperature of 25 ℃ for 49 days, and the resistance changes were measured every 7 days.

Fig. 2(a-e) shows several samples of liquid metal patterns on the nanocellulose membrane, fabricated by the evaporation-enabled transfer printing method. Fig. 2(a) is a large-area circuit on a thin nanocellulose membrane. The conductive lines and pads are all printed out in good quality. Fig. 2(b) presents several interdigital electrodes. What printed in Fig. 2(c) are several flexible RFID antennas. These liquid metal patterns on the nanocellulose membrane are flexible, which are easy to be bent, rolled up or twisted with the deformation of the substrate, and can be attached to non-flat objects (Fig. 2(d)). Besides, as shown in Fig. 2(e), the present printing method also works well for printing characters.

Fig. 2(f) shows the SEM image of a printed line transferred to the nanocellulose membrane from the PVC substrate. The silver-white part in the SEM image is the EGaIn ink, whose boundary is very clear. The combination of the SEM image and EDS layered image in Fig. 2(g) depicts the profile of the transversal surface, in which the yellow area is the EGaIn ink. It can be found that the EGaIn ink is embedded in the nanocellulose membrane in a circular arc, and the liquid metal line, covered with nanocellulose membrane, also has a small arcuate hump. The thickness of this liquid metal line is 25 μm, and it is about 55 μm at its thickest point of the circuit. This embedded structure makes the LM-NC pattern more stable.

Besides, as shown in Fig. 2(h), the thickness of the liquid metal line with different width has been evaluated. The actual thickness values of 0.8 mm, 1.0 mm, 1.5 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm and 10 mm printed lines were measured as 25.3 ± 1.3 mm, 26.1 ± 0.6 mm, 31.6 ± 1.5 mm, 35.2 ± 1.1 mm, 43.3 ± 1.5 mm, 46.5 ± 0.6 mm, 34.8 ± 0.8 mm, and 43.1 ± 1.5 mm. Clearly, these values have no correlation with line width, but they are no more than 50 μm. Therefore, compared with the liquid metal circuits on other substrates (like PDMS, PVC), one of the prominent features of the LM-NC circuit fabricated by the transfer printing method is the ultrathin characteristic.

Fig. 2(i) also compares the width differences between the designed value and the actual liquid metal lines transferred into the nanocellulose membrane. The designed width of the eight printed lines shown in the inset is 0.8 mm, 1.0 mm, 1.5 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, and 10.0 mm. And the actual width values are 0.78 ± 0.12 mm, 0.97 ± 0.01 mm, 1.46 ± 0.4 5 mm, 1.91 ± 0.40 mm, 2.95 ± 1.04 mm, 3.98 ± 0.33 mm, 4.97 ± 0.28 mm and 9.91 ± 0.10 mm. It is found that the measured values of the actual lines are slightly less than the designed ones, and the maximum error is not more than 4.5 %. In other words, the evaporation-enabled transfer printing method is acceptable to ensure the accuracy of the printed liquid metal patterns.

Further, the electrical performance of liquid metal lines with different width has been evaluated. The resistance values are 2.80 ± 0.26 Ω (0.8 mm), 1.96 ± 0.15 Ω (1.0 mm), 0.55 ± 0.07 Ω (4.0 mm), which decrease gradually with the increase of the line width, as shown in Fig. 3(a). Fig. 3(b) displays the electrical resistances of the liquid metal lines (10 mm in length) after bending at different angles. The initial resistances of the three samples before bending are 2.70 Ω, 1.97 Ω, 0.48 Ω, respectively. Since the liquid metal ink keeps liquid state at room temperature and embedded in the substrate, it can respond in good quality when the substrate is bent. It is obvious that the resistance values of different lines are approximately stable and only have tiny fluctuation as the bending angle increases, which shows that transfer circuits on the nanocellulose membrane have good stability of conductivity under the bending condition.

Also, the resistance changes of the lines have been measured for 42 days. As shown in Fig. 3(c), it can be seen that the resistance of each printed line increases slightly after 7 days, and then it’s almost constant. This is because the liquid metal could spontaneously form trace amounts of the oxide layer (such as Ga₂O₃) on the surface once exposed to the air. Since the metallic oxide has poorer electrical conductivity than pure metal [33], the resistance of liquid metal lines after deposited on the substrates increased slightly at the second measurement 7 days later. Besides, the easily-formed oxide layer can prevent the liquid metal to be further oxidized, so the electrical conductivity was stable after the second measurement.

To demonstrate the performance of the NC-LM circuit, various samples mounted with LEDs have been fabricated (Fig. 4(a)). As shown in Fig. 4(b), the NC-LM circuit can also be rolled around the wrist as a wristband. Besides, since the NC-LM circuit is very thin and lightweight, it is easy to be attached to other surfaces. Fig. 4(c) provides an example, in which a tiny NC-LM circuit with a lighted LED was stuck to the fingernail compliantly. Besides, NC-LM electronics can also facilitate the fabrication of multilayer circuits because of ultrathin property. Fig. 4(d) reveals the major step in the preparation of the double-layer circuit. Two NC-LM circuits are glued together. The top layer gets holes at the electrical junction of the two layers, and then the liquid metal line at the bottom layer connects with the pins of the components. Fig. 4(e, f) present the double-layer circuit and its performance as a 2 × 2 LED array.

Since the liquid metal has high surface tension, many solid substrates are not wetted by liquid metal. Fig. 5(a) depicts the contact angle of the EGaIn ink on a formed nanocellulose membrane. The contact angle is about 135°. The surface wettability of the PVC substrate is poor as well, whose contact angle is about 145° [32]. The liquid metal printer used in Fig. 1 can direct print liquid metal pattern on the PVC substrate, because the contact angles decrease when applying external pressure. However, this method does not right for nanocellulose membrane owning to its thinness and lower compressive strength. In this case, the transfer printing strategy is developed for the NC-LM circuit.

Fig. 5(b) reveals the formation process of the nanocellulose membrane on the liquid metal ink during the evaporation-induced transfer printing method. Nanocellulose solution contains a large proportion of water, with some long strip of nano-sized cellulose fibrils floating in the solution (i in Fig. 5(b)). When placed in the air, water in the solution will gradually evaporate and then the cellulose will deposit down and gradually aggregate together (ii in Fig. 5(b)). As the water evaporates further, the cellulose is more tightly arranged. Shown as iii in Fig. 5(b), the cellulose fibrils tend to contact in the direction perpendicular to the surface of the PVC substrate, but aggregate around the surface of liquid metal in various directions. As a result, the cellulose fibrils wrap tightly around the liquid metal.

There are adsorption phenomena of reducing surface energy at any interface. However, the nanocellulose solution covers the liquid metal in the form of the liquid state, which can change the interface shape and then be easier to form an adsorption action with the liquid metal surface at the molecular level. Therefore, the nanocellulose membrane over the liquid metal will be more skin-friendly to the liquid metal than the interface between the liquid metal and the ready-made nanocellulose membrane or PVC substrate. In addition, the contact surface between liquid metal and PVC substrate is a plane on the macro level, while the contact surface between liquid metal and nanocellulose membrane is a camber surface that has a larger contact area, so the overall affinity will be stronger. Therefore, As shown in Fig. 5(c), when peeling the nanocellulose membrane off from the PVC substrate, the EGaIn ink which is tightly embedded into the membrane can successfully stay on the membrane.

Fig. 6 demonstrates the recycling process of the NC-LM circuit. First, a small piece of NC-LM circuit was put in the culture dish filled with deionized water. It could be seen that the NC-LM circuit was immersed in the water and became saturated at once (Fig. 6(a)). About 24 h later, the NC-LM circuit was no longer complete but split into pieces (Fig. 6(b)). After soaking for another 24 h, the liquid metal pattern became very blurry and the membrane was more saturated at the same time (Fig. 6(c)). During the process of nanocellulose degradation, the ultrasonic shaking was able to accelerate the degradation rate by pulverizing the broken membrane. As shown in Fig. 6(d), after the ultrasonic treatment, there was no trace of the membrane in the surface dish, and the obtained solution was very uniform and transparent. Besides, the liquid metal suspended in the solution aggregated gradually into many small droplets. Then after another 24 h, the nanocellulose membrane completely returned to the initial state of nanocellulose solution, while the liquid metal droplets agglomerated into larger ones and some of them precipitated to the bottom of the culture dish (Fig. 6(e)). Afterward, the nanocellulose solution and the liquid metal ink were separated through centrifugal treatment. The liquid metal ink was at the bottom of the centrifuge tube and could be reused as the metal ink to print patterns (Fig. 6(f)). As for the nanocellulose solution at the top, it was re-spread on the liquid mental circuits (Fig. 6(g)). Finally, the recycled circuit was obtained, which had the same morphology and properties as the original circuits (Fig. 6(h)). The nanocellulose film is degradable in water and the liquid metal ink can be easy to be separated from the substrate and reused. Therefore, the NC-LM circuit is reusable.

In summary, this paper introduces a kind of reusable and ultrathin liquid metal circuit creatively using nanocellulose as the substrate with the transfer printing method. Owing to the poor wettability of liquid metal on the nanocellulose membrane, liquid mental ink cannot be directly printed or brushed on the film in a good state. Therefore, this paper developed an evaporation-induced transfer printing technology to clear up the issues nicely. During the evaporation process, the nanosized cellulose fibrils aggregate and cling tightly around the liquid metal, leading to good adhesion. As a result, after peeling nanocellulose membrane off from the PVC substrate, parts of liquid metal are transferred to the nanocellulose membrane. The NC-LM circuit is ultrathin with just tens of microns. And since the liquid metal is almost completely embedded in the nanocellulose membrane, the NC-LM circuit is also very stable. Meanwhile, the NC-LM circuit can be degraded in water and turn to the original form including nanocellulose solution and liquid metal ink, which contributing to circuits’ excellent recyclability. Therefore, this study provides a facile way for the fabrication of renewable flexible electronics.

This work was financially supported by the National Natural Science Foundation of China (No. 51605472), the Beijing Municipal Science & Technology Commission research fund (No. Z171100000417004). We acknowledge the help from Beijing DREAM Ink Technologies Co., Ltd. and its engineers.