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Rewritable, Printable Conducting Liquid Metal Hydrogel Jung-Eun Park, Han Sol Kang, Jonghyek Baek, Tae Hyun Park, Seunghee Oh, Hyungsuk Lee, Min Koo, and Cheolmin Park ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b03405 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019
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Rewritable, Printable Conducting Liquid Metal Hydrogel Jung-Eun Park1†, Han Sol Kang1†, Jonghyek Baek2, Tae Hyun Park1, Seunghee Oh2, Hyungsuk Lee2, Min Koo1*, Cheolmin Park1* 1Department
2School
of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea
of Mechanical Engineering, Yonsei University, Seoul, 03722, Korea
*Corresponding authors:
[email protected] (M. Koo) and
[email protected] (C. Park) †These
authors contributed equally to this work.
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ABSTRACT
The development of high-performance printable electrical circuits, particularly based on liquid metals, is fundamental for device interconnection in flexible electronics, motivating numerous attempts to develop a variety of alloys and their composites. Despite their great potential, rewritable and printable electronic circuits based on liquid metals are still manufactured on demand. In this study, we demonstrate liquid metal-based hydrogels suitable for rewritable, printable electrical circuits. Our liquid-metal hydrogels are based on sedimentation-induced composites of eutectic gallium-indium (EGaIn) particles in poly(ethylene glycol) diacrylate (PEGDA). The EGaIn particles are vertically phase-segregated in the PEGDA. When a composite surface with high EGaIn content is gently scratched, the surface covering PEGDA is removed, followed by the rupture of the native oxide layers of the particles, and the exposed EGaIn becomes conductive. The subsequent water-driven swelling of PEGDA on the scratched surface completely erases the conductive circuit, causing the system to reset. Our friction-responsive liquid-metal hydrogel exhibits writing–erasing endurance for 20 cycles, with a dramatic change in the electrical resistance from metal (~1 Ω) to insulator (~107 Ω). By employing surface friction pen printing, we demonstrate mechanically flexible, rewritable, printable electrical conductors suitable for displays.
KEYWORDS: liquid metal hydrogels, conducting EGaIn/polymer, rewritable electric circuits, printable circuits, electrical interconnections, friction-induced circuit writing, water-assisted erasing
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The development of electrical interconnections with mechanical pliability and deformability is critical for flexible electronics, and liquid metals have been of great interest owing to their extraordinary properties originating from their metallic cores and liquid-like interface with their surroundings at room temperature.1-4 In particular, the recent emergence of less hazardous liquid metals, such as gallium-based eutectic alloys, has triggered renewed interest in the scientific community.5-7 The several properties of eutectic gallium-indium (EGaIn) liquid metal, including the high density,7 high surface tension,8 low viscosity,5,7 and low vapor pressure,9,10 as well as the highest conductivity among liquids,7,11 make it attractive for various applications in soft electronics, such as stretchable matrices,12-17 self-healable interconnects,16,18 reconfigurable wires,19-21 and designable electrodes.22-25 The thin oxide layer of EGaIn (thickness of ~3 nm, Ga2O3) readily forms on the surface under ambient conditions via native oxidation, which imparts good adhesion to a variety of surfaces, including low-surface energy nonpolar surfaces and polar surfaces.8 Owing to these characteristics, EGaIn particles have been successfully employed in various soft-electronic fabrication techniques, such as direct writing,13,26 spray deposition,22,27 designed circuit transfer,28,29 on a embedding single layer,30,31 and shear mixing with matrix polymers.1,4,6 In previous studies, by utilizing EGaIn particles with passivated Ga2O3 layers, which can be easily fractured by a pressure of only a few kilopascals, stress-responsive electrical circuitries were demonstrated, where percolated liquidmetal circuits were developed upon mechanical deformation16,18,19,22,32-35 The examples include pressure-sensitive circuits and stretchable electrodes. Owing to the difficulty of self-recovery to the original spherical droplets with the native oxide layers, most of the previously demonstrated liquid metals were, however, unsuitable for rewritable applications. Herein, we present liquid metal hydrogels (LMHs) suitable for mechanically flexible, printable,
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rewritable electrical conductors. Our LMHs are based on simple one-pot ultraviolet (UV) irradiation synthesis of a poly(ethylene glycol) diacrylate (PEGDA) hydrogel with vertically phase-segregated EGaIn particles. Controlled friction on a high-EGaIn population surface allows the removal of surface covering PEGDA, followed by the rupture of the native oxide layers of the particles in the scratched regions, resulting in a conductive circuit. The written circuit is readily erased via surface reconstruction into droplet EGaIn particles with the native oxide layers when the LMH is hydrated and swollen by water. Another electrical circuit is developed via subsequent friction-induced writing on the reset LMH, making the system rewritable. Our UV irradiationbased LMH is compatible with conventional photolithography, allowing the fabrication of microstructured, rewritable electrical conductors. Furthermore, by employing surface friction pen printing, we can demonstrate printable, rewritable electrical circuits suitable for displays.
RESULTS AND DISCUSSION
Liquid Metal Hydrogel (LMH). The fabrication of LMHs started with the preparation of microparticles of the EGaIn alloy (75.5 wt% Ga and 24.5 wt% In, Sigma–Aldrich) approximately 1 m in diameter in EtOH via sonication, followed by the mixing of the microparticles with a PEGDA (Mn: 700 g/mol, Sigma–Aldrich):EtOH = 1:1 solution. After the addition of a photoinitiator (2-hydroxy-2-methylpropiophenone, HOMPP), the mixture of EGaIn with a volume fraction (Φ) of 0.1 in the PEGDA solution was gently vortexed and carefully poured onto a mold with a bank of 100 m in height. The subsequent UV irradiation (wavelength of 375 nm) of the molded solution crosslinked the PEGDA, making the molded gel a solid-like film, as shown in Figure 1a (Figure S1). The surface scratching properties of an LMH are controlled by the degree
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of crosslinking and are critical for developing rewritable electrical conductors, as shown later. In this study, a lower crosslinking density yielded greater surface scratching. To minimize the undesired surface scratching of the insulating hydrogel, which may deteriorate the conductive liquid-metal pathway developed upon friction-induced writing, we employed low-molecular weight PEGDA, which yielded a high degree of crosslinking.36 The fabricated LMH was mechanically robust and flexible and optically opaque, with a light gray color, owing to the liquidmetal particles embedded in the hydrogel, as shown in Figure 1b. Our LMH had vertically phase-separated microstructure in which EGaIn microparticles (density of ρ = 6.25 g/cm3) preferentially settled at the bottom of the composite because of the gravitational sedimentation of the particles during the crosslinking of PEGDA (ρ = 1.12 g/cm3), as shown in Figure 1c. The schematic of Figure 1c shows an LMH bottom-side-up, as frictioninduced writing occurred on the high-EGaIn population surface in our system. The cross-sectional scanning electron microscopy (SEM) image of an LMH in Figure 1c indicates the vertically segregated microstructure with EGaIn particles closely packed from the top with a thickness of approximately 40 m (Figure S2). The high-EGaIn population surface of the LMH is observed, and the surface roughness (approximately 1 m) and the particle-size distribution are shown in Figures 1d and e, respectively. According to Stoke’s law, which defines the terminal velocity of a spherical particle due to the difference between its weight and buoyancy, the sedimentation velocity of an EGaIn particle is proportional to the square of the particle size, as given by the equation 𝑣 =
2(𝜌𝑝 ― 𝜌𝑓) 9
𝜇
𝑅2𝑔, where
g is the gravitational acceleration (m/s2), R is the radius of a spherical particle (m), ρp is the mass density of the particles (kg/m3), ρf is the mass density of the fluid (kg/m3), μ is the dynamic
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viscosity (kg/m*s), and v is the velocity (m/s).37-39 To develop a high-EGaIn population surface with the particles efficiently encapsulated with insulating PEGDA, which allowed for an initially non-conductive surface before friction-induced writing, we found that the size distribution of the EGaIn particles was critical. Thus, careful adjustment of the experimental conditions for the granulation of EGaIn was required. We found that the particles were optimal with their size of approximately a few micrometers, giving rise to the non-conductive, high-EGaIn population surface of the LMH with EGaIn particles completely encapsulated by the thin, networked PEGDA. EGaIn particles smaller than 1 μm in diameter were hardly settled down during composite fabrication. On the other hand, the particles larger than approximately 1 μm were rarely covered with PEGDA, making it difficult to form an initial insulating surface subject to friction induced writing (Figure S2). The roughened surface with the passivated PEGDA is also beneficial upon friction-driven electrical-circuit writing, providing appropriate surface friction.40,41 The UVinduced crosslinking of PEGDA was confirmed by Fourier transform infrared (FT-IR) spectroscopy, and the results are shown in Figure 1f. The intensities of the absorbance peaks at 1636, 1621, and 1410 cm-1 corresponding to the acrylate end group decreased owing to the radical attack of the UV photoinitiator, indicating the conversion of -C=C- bonds into -C-C- bonds with active radicals.42-45 The results show that a chemically networked hydrogel was successfully developed even with the EGaIn particles and that the LMH could be swollen and de-swollen by water, which is critical for electrical-circuit erasing. PEGDA is hard and brittle due to the glassy properties of PEGDA with its glass transition temperature of approximately 53C.46 When EGaIn was added in PEGDA for rewritable, printable conductors, the composites became softened due to the liquid-like properties of EGaIn. The composites became soft with EGaIn, resulting in the decrease of Young’s modulus as well as the
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increase of the elongation-at-break with EGaIn. As shown in Figure 1g and 1h, the Young’s modulus and elongation-at-break values were varied from 255 to 21 kPa and from approximately 7 to 28% as a function of the volume fraction of EGaIn, respectively. When a composite contained EGaIn greater than 15 %, free standing hydrogel network is hardly formed. In this composition, hydrogel is very sticky rather than a free standing gel, making it difficult to handle the hydrogel for the measurement of its mechanical properties. Additionally, the composites swollen with water were hardly examined of their mechanical properties due to the softness of the composites. Friction Induced Printing of Electric Circuit on LMH. An electrically conductive circuit was easily developed on the initially non-conductive high-EGaIn population surface of an LMH when the surface was gently scratched with a hard object, as schematically shown in Figure 2a. For the demonstration, we employed a pencil with a cylindrical metallic bar approximately 1 mm in diameter. When the surface was linearly scratched, a fluidic liquid-metal line with a width corresponding to the diameter of the metal cylinder was developed, and its electrical resistance was approximately 1.0 , as shown in Figure 2b. Apparently, conductive electrical circuit lines with a variety of line widths were easily fabricated by either controlling the scratching friction force or changing the size of the metal cylinder. To elucidate the mechanism of the electrical circuit lines under surface friction, we systematically examined the electrical resistance of an LMH with respect to the friction force, as shown in Figure 2c. Our homemade surface scratch equipment allowed us to vary the friction force on the LMH, and the results are shown in Figure 2d. In our system, when the surface friction force was greater than approximately 320 N, the electrical resistance of the LMH abruptly decreased in the log scale, yielding conductivity, indicating the successful
development
of
macroscopically
percolated
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liquid-metal
pathways.
The
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microstructures of the LMH with respect to the surface friction force indicated the development of smooth conductive circuit lines of liquid metal, as shown in Figure 2e. The roughened surface consisting of PEGDA-encapsulated EGaIn particles before scratching turned partially into dark gray flat patches when gentle friction was applied, resulting in the significant decrease of the electrical resistance, as shown in the second SEM micrograph of Figure 2e. The dark gray regions became more apparent and continuous with the increase of the frictional force, and eventually, a clearly defined liquid-metal line with a width of approximately 50 m and a resistance of
