Liquid Metal Patterns Research Articles

Facile Formation of Metallic Surface with Microroughness via Spray-Coating of Copper Nanoparticles for Enhanced Liquid Metal Wetting

This paper presents a simple, fast, and cost-effective method for creating metallic microstructured surfaces by spray-coating a dispersion of copper nanoparticles (CuNPs) onto polymethyl methacrylate (PMMA) substrates, enabling the imbibition-induced wetting of liquid metal. The formation of these microstructured patterns is crucial for the spontaneous wetting of gallium-based liquid metals. Traditional techniques for producing such microstructures often involve complex and costly lithography and vacuum deposition methods. In contrast, this study demonstrates that liquid metal wetting can occur with metal microstructures formed through a straightforward spray-coating process. To immobilize the CuNPs on the polymer substrate, an organic solvent that dissolves the polymer surface was employed as the dispersion medium. The effects of various spray-coating parameters, including distance and time, on the uniformity and immobilization of CuNP films were systematically investigated. Under optimal conditions (120 s of spray time and 10 cm spray distance), CuNPs dispersed in dichloromethane (DCM) yielded uniform and stable microstructured surfaces. The spontaneous wetting of gallium-based liquid metal was observed on the fabricated CuNP film. Additionally, liquid metal selectively wet the CuNP patterns formed by stencil techniques, establishing electrical connections between electrodes. These findings underscore the potential of spray-coating for fabricating metallic surfaces to drive the formation of liquid metal patterns in flexible electronics applications.

A Subtractive Method to Chemically Pattern Liquid Metal for Stretchable Circuits

AbstractAdvancements in biomedical research have spurred the development of stretchable electronic devices. While soft insulators are readily available, soft conductors with metal‐like electrical conductivity are rare. Gallium and its alloys, being nontoxic and intrinsically stretchable, are potentially ideal solutions. However, current additive liquid metal (LM) patterning methods face limitations in achieving high‐throughput, high‐resolution, and high‐density LM wiring. Here, a subtractive LM patterning method is developed to meet all these requirements simultaneously. The innovative method involves parallel filling a single continuous microfluidic mesh network with LM that short‐circuits all the pins and pads of a circuit, followed by parallel cutting of the unwanted short‐circuited interconnections using hydrochloric acid (HCl) vapor. Cutting locations are pre‐defined by designing narrower intersecting channels, leveraging capillary force for precise filling and cutting. The process is characterized using a multidimensional parametric study with varying LM line widths and HCl concentrations, and in situ impedance measurements to assess insulation performance. To showcase its high‐throughput capabilities, a mock circuit is used to successfully generate complex LM interconnects that connected hundreds of electrical pads. Finally, a stretchable LM circuit with a micro‐LED array is fabricated to demonstrate the practical application of this technology for massively parallel wiring in stretchable electronics.

93‐2: Highly Stretchable Liquid Metal‐based Deformable Micro‐LED Displays

Stretchable displays represent a cutting‐edge technology for creating novel freeform deformable displays. In this study, we developed a highly stretchable liquid metal‐based stretchable backplane technology for deformable micro‐LED displays. We developed a fine liquid metal patterning process and highly stretchable backplane fabrication technology to implement micro‐LED stretchable displays utilizing liquid metal electrodes. The developed stretchable display demonstrated exceptional flexibility allowing it to transition from planar to hemispherical shapes with up to 50% stretchability without disconnection of lines. Even when subjected to dome‐shaped deformation under 50% strain, the developed stretchable display utilizing liquid metal electrodes exhibited stable electrical characteristics, showcasing its practical applicability.

Liquid Metal‐Based High‐Density Interconnect Technology for Stretchable Printed Circuits

AbstractHigh‐density interconnect (HDI) technology for liquid metal (LM)‐based stretchable printed circuit boards is crucial for expanding their applicability. HDI technology provides high‐resolution multilayer circuits with a high density of components, as required for next generation of neuroprobes and ultrasonic and sensor arrays. This study presents a HDI technique utilizing laser engraved micro grooves in silicone with a protective lift‐off mask of polyvinyl alcohol (PVA) and subsequent microscale LM particle spray deposition. This approach achieves high‐resolution LM patterns and enables multilayer connectivity and high‐density integration of components simultaneously, that is, realizing HDI technology. It is demonstrated using a stretchable 0201 LED display with a density of six leads per mm2 and a cochlear implant (CI) electrode array. The demonstrated manufacturing of the CI has the potential to enable fully automated printed circuit board manufacturing of such implants with increased precision and throughput. Implants in guinea pigs show that the CI is capable of activating the auditory neuron with electrical auditory brainstem response (eABR) and electrical compound action potential (eCAP) of high quality. Moreover, the U‐shaped cross section of the LM interconnects provides a higher mechanical shock resistance of the circuit than that of normal rectangular cross sections.

(Invited) Electrodeposition of In and Ga from Non-Aqueous Electrolytes

Indium and gallium are metals whose applications have increased during the last decades. Both metals are primarily used in the form of binary, tertiary or quaternary (semi)conductor compounds in integrated circuits, photovoltaics, liquid crystal display devices, lasers and light-emitting diodes.In the past, the electrodeposition of indium and gallium has been investigated from both aqueous and non-aqueous electrolytes. Due to the negative standard reduction potentials of the two metals (E°In(III)/In(0) = −0.34 V and E°Ga(III)/Ga(0) = −0.53 V), their electrodeposition from aqueous electrolytes is hampered. This is especially true for gallium. In this talk, the electrodeposition of indium and gallium was investigated from several organic solvents. The investigated electrolytes are 1,2-dimethoxyethane (DME), polyethylene glycol (PEG), dimethyl sulfoxide (DMSO), and acetonitrile, containing chloride, bistriflimide, or methanesulfonate-based indium or gallium precursors.The electrochemistry and the electrodeposition of indium and gallium was studied using various electrochemical methods in combination with quartz crystal microbalance (QCM) and rotating (ring) disk electrodes. DMSO-based electrolytes enabled the electrodeposition of indium at room temperature and above its melting point (159.6 °C), while DME electrolytes gave rise to gallium electrodeposits composed of small liquid gallium spheres (Fig 1). In all the investigated electrolytes, the formation of intermediate monovalent indium(I) or gallium(I) species during reduction and oxidation was demonstrated. These species are unstable and react away via disproportionation reactions. Yet, it was found that the extent of their formation and their stability depends on the nature of the electrolyte. In addition, the electrodeposition of indium antimonide (InSb) films and eutectic gallium indium (EGaIn) liquid metal lines and patterns will be discussed, as well as the electrochemical synthesis of spherical indium nanoparticles.

Flexible Finely and Directly Patternable Liquid Metal Electrodes via Selective Surface Wetting Technique

Eutectic gallium–indium (EGaIn) is an ideal material for preparing flexible electrodes, but its high surface tension poses a challenge during deposition and patterning. Herein, we propose a laser-induced selective surface wetting technique (SSWT) to enable the facile and straightforward fabrication of flexible finely and directly patternable EGaIn liquid metal electrodes. Our proposed technique selectively controls the wettability of EGaIn by establishing a perfluorinated self-assembled monolayer on a zinc oxide nanorod array to impart superhydrophobicity and then inducing specific sites on the hydrophilized surface by ultraviolet (UV) pulsed laser ablation, thereby enabling fine patterning (linewidth, ~50 μm). Surface analysis of the effect of laser ablation was also performed to elucidate the mechanism of SSWT. The patterned EGaIn liquid metal electrode fabricated by SSWT exhibited superior flexibility, with a resistance change (ΔR/R0) of only 18.6% compared with a Ag thin film electrode, which showed a dramatic increase in ΔR/R0 to nearly 500% after 50,000 folding cycles at a peak strain of 2.5%. The simple and easily implementable liquid metal patterning technique proposed in this study may potentially be applied in the field of wearable and stretchable electronics, which requires extreme flexibility.

Numerical analysis of the behavior of molten pool and the suppression mechanism of undercut defect in TIG-MIG hybrid welding

In order to investigate the multi-coupling transport phenomena in TIG-MIG hybrid arc welding, a three-dimensional model including droplet and molten pool was established. An innovative distribution pattern of arc heat and force that adapted to the evolution of molten pool surface was proposed, achieving a unified distribution of "arc current density-arc pressure-electromagnetic force-arc heat". The effect of TIG arc on molten pool behavior and undercut defect was quantitatively analyzed by comparing TIG-MIG hybrid welding with traditional MIG welding. The results showed that the electromagnetic repulsion between the two arcs with opposite currents increased the aspect ratio of the hybrid arc heat and force distribution. It led to a long and narrow gouging region, where the promoting effect of inertial force on the undercut was weakened by arc pressure and hydrostatic pressure. The liquid metal with a lower cooling rate had sufficient time to fully fill the weld toe and suppressed undercut. Sensitivity and dimensional analysis illustrated that the TIG-MIG hybrid arc eliminated undercut by reducing the inertia force, adjusting the arc pressure distribution, and enhancing the hydrostatic pressure. The groove sizes was predicated based on molten pool characteristics, including the stress state of liquid metal, morphology, and fluid flow patterns. It revealed the quantitative relationships among welding parameters, behavior of molten pool, and weld bead formation, promoting the implementation of digital twinning technology in the manufacturing industries.

Ultrasonic-Enabled Nondestructive and Substrate-Independent Liquid Metal Ink Sintering.

Printing or patterning particle-based liquid metal (LM) ink is a good strategy to overcome poor wettability of LM for its circuits' preparation in flexible and printed electronics. Subsequently, a crucial step is to recover conductivity of LM circuits consisting of insulating LM micro/nano-particles. However, most widely used mechanical sintering methods based on hard contact such as pressing, may not be able to contact the LM patterns' whole surface conformally, leading to insufficient sintering in some areas. Hard contact may also break delicate shapes of the printed patterns. Hereby, an ultrasonic-assisted sintering strategy that can not only preserve original morphology of the LM circuits but also sinter circuits on various substrates of complex surface topography is proposed. The influencing factors of the ultrasonic sintering are investigated empirically and interpreted with theoretical understanding by simulation. LM circuits encapsulated inside soft elastomer are successfully sintered, proving feasibility in constructing stretchable or flexible electronics. By using water as energy transmission medium, remote sintering without any direct contact with substrate is achieved, which greatly protect LM circuits from mechanical damage. In virtue of such remote and non-contact manipulation manner, the ultrasonic sintering strategy would greatly advance the fabrication and application scenarios of LM electronics.

Maskless Fabrication of Highly Conductive and Ultrastretchable Liquid Metal Features through Selective Laser Activation.

In the rising field of stretchable electronics, liquid metals are ideal candidate conductors with metallic conductivity and intrinsic deformability. The complex patterning methods of liquid metal features have limited their widespread applications. In this study, we report a maskless fabrication approach for the facile and scalable patterning of liquid metal conductors on an elastomer substrate. Laser-activated patterns are employed as versatile templates to define arbitrary liquid metal patterns. The as-prepared liquid metal features show an excellent conductivity of 3.72 × 104 S/cm, a high resolution of 70 μm, ultrahigh stretchability of up to 1000% strain, and electromechanical durability. The practical suitability of liquid metal conductors is demonstrated by fabricating a stretchable light-emitting diode (LED) matrix and a smart sensing glove. The maskless fabrication technique introduced here allows versatile patterning of liquid metal conductors with affordable costs, which may stimulate a broad range of applications in stretchable electronic devices and systems.

Pressure Driven Rapid Reconfigurable Liquid Metal Patterning.

This paper proposes a method for pressure driven rapid reconfigurable liquid metal patterning. A sandwich structure of "pattern-film-cavity" is designed to complete this function. Both sides of the highly elastic polymer film are bonded with two PDMS slabs. One PDMS slab has microchannels patterned on the surface. The other PDMS slab has a large cavity on its surface for liquid metal storage. These two PDMS slabs are bonded together, face to face, with the polymer film in the middle. In order to control the distribution of the liquid metal in the microfluidic chip, the elastic film will deform under the high pressure of the working medium in the microchannels and then extrude the liquid metal into different patterns in the cavity. This paper studies the factors of liquid metal patterning in detail, including external control conditions, such as the type and pressure of the working medium and the critical dimensions of the chip structure. Moreover, both a single-pattern and a double-pattern chip are fabricated in this paper, which can form or reconfigure the liquid metal pattern within 800 ms. Based on the above methods, reconfigurable antennas of two frequencies are designed and fabricated. Meanwhile, their performance is simulated and tested by simulation and vector network tests. The operating frequencies of the two antennas are respectively significantly switching between 4.66 GHz and 9.97 GHz.

Frequency tunable liquid metal on paper microstrip patch antenna

We report a liquid metal microstrip antenna on a printing paper whose resonant frequency was tunable by folding the paper. The gallium-based alloy was used as liquid metal, which is instantly oxidized in ambient air. The oxidized liquid metal is easily wettable on printing paper surfaces, while it is non-wettable on laser-printed toner surfaces. An antenna with a 3 mm wide and 39 mm long copper-liquid metal overlapped-feedline linked to the liquid metal microstrip rectangle-shaped patch (40 × 31 mm2) was designed. The antenna pattern was printed on a printing paper using a laser printer. A liquid metal soaked roller was rolled on the antenna pattern printed paper, resulting in a liquid metal pattern where there was no toner. The resonant frequency of the liquid metal on the paper microstrip patch antenna was 2.88 GHz with −18.5 dB S11. The measured characteristics of the antenna were in good agreement with the simulation results. When the antenna was folded, the resonant frequency was changed to 2.68 GHz (x-axis folding) or 2.72 GHz (y-axis folding). Liquid metal on paper fabrication is simple but effective for creating custom-designed antennas that are easily re-designed and fabricated in the field.

레이저 유도 전사를 이용한 유연기판의 액체금속 미세 패터닝

We have developed a direct conductive patterning method with micro-scale line widths using the laser-induced-forward transfer (LIFT) and liquid metal. As this method does not need post-thermal processing, there is no thermal damage even on heat-sensitive polymer substrates by low-power laser irradiation on the dynamic release layer (DRL). Unlike other liquid metal patterning processes, this procedure can easily achieve fine line widths of a few tens of micrometers corresponding to laser spot size. The solid-state UV pulse laser with 266 nm wavelength and 20 ns pulse duration was used to transfer Eutectic Gallium Indium (EGaIn) liquid metal and the results for the single and multi-pulse laser irradiation were investigated to determine the effective process conditions. The applicability of flexible circuit fabrication and selective circuit repair was successfully tested on Polyimide (PI) substrate. After the LIFT process, the electrical properties of liquid metal on the pattern were measured to be approximately 5~8 x 10SUP-3/SUP Ω/m of resistance.

Liquid Metal Patterning and Unique Properties for Next-Generation Soft Electronics.

Room-temperature liquid metal (LM)-based electronics is expected to bring advancements in future soft electronics owing to its conductivity, conformability, stretchability, and biocompatibility. However, various difficulties arise when patterning LM because of its rheological features such as fluidity and surface tension. Numerous attempts are made to overcome these difficulties, resulting in various LM-patterning methods. An appropriate choice of patterning method based on comprehensive understanding is necessary to fully utilize the unique properties. Therefore, the authors aim to provide thorough knowledge about patterning methods and unique properties for LM-based future soft electronics. First, essential considerations for LM-patterning are investigated. Then, LM-patterning methods-serial-patterning, parallel-patterning, intermetallic bond-assisted patterning, and molding/microfluidic injection-are categorized and investigated. Finally, perspectives on LM-based soft electronics with unique properties are provided. They include outstanding features of LM such as conformability, biocompatibility, permeability, restorability, and recyclability. Also, they include perspectives on future LM-based soft electronics in various areas such as radio frequency electronics, soft robots, and heterogeneous catalyst. LM-based soft devices are expected to permeate the daily lives if patterning methods and the aforementioned features are analyzed and utilized.

Texture-structure-based liquid metal filling for blind-end microchannels and its application on multi-layer chips.

This research work reports a novel method to achieve fast liquid metal (LM) injection in blind-end microchannels which is especially suitable for multi-layer microfluidic chips. This method is based on a texture-like surface bonding technology. The texture-like surface is fabricated on a polydimethylsiloxane (PDMS) slab with standard soft-lithography technology and bonded with another PDMS slab with microelectrode patterns on it. When injected with LM, the texture-like structure can prevent the LM from entering but allows the air inside to be released during the injection to achieve perfect blind-end complex LM electrodes. The experimental results show that it can achieve fast and perfect LM injection in the blind-end pattern and can also prevent the large area of the flat chamber from collapsing during bonding. We also parametrically studied the texture structure's size for bonding strength between the texture structure and the blank PDMS surface. In addition, we integrate three layers of blind-end complex liquid metal patterns into one multi-layer chip using this technology and later use this structure to realize series connection of two LM-based electroosmotic micropumps (EOP). Compared with the conventional LM-based EOP, the structure of the EOP chip was greatly simplified and resulted in a higher level of integration.

Conformally Adhesive, Large-Area, Solidlike, yet Transient Liquid Metal Thin Films and Patterns via Gelatin-Regulated Droplet Deposition and Sintering.

Adhesion and spreading of liquid metals (LMs) on substrates are essential steps for the generation of flexible electronics and thermal management devices. However, the controlled deposition is limited by the high surface tension and peculiar wetting and adhesion behavior of LMs. Herein, we introduce gelatin-regulated LM droplet deposition and sintering (GLMDDS), for the upscalable production of conformally adhesive, solidlike, yet transient LM thin films and patterns on diverse substrates. This method involves four steps: homogeneous deposition of LM microdroplets, gelation of the LM-gelatin solution, toughening of the gelatin hydrogel by solvent displacement, and peeling-induced sintering of LM microdroplets. The LM thin film exhibits a three-layer structure, comprising an LM microdroplet-embedded tough organohydrogel adhesion layer, a continuous LM layer, and an oxide skin. The composite exhibits high stretchability and mechanical robustness, conformal adhesion to various substrates, high conductivity (4.35 × 105 S·m-1), and transience (86% LM recycled). Large-scale deposition (i.e., 5.6 dm2) and the potential for patterns on diverse substrates demonstrate its upscalability and broad suitability. Finally, the LM thin films and patterns are applied for flexible and wearable devices, i.e., pressure sensors, heaters, human motion tracking devices, and thermal management devices, illustrating the broad applicability of this strategy.

Controlling the oxidation and wettability of liquid metal via femtosecond laser for high-resolution flexible electronics.

Liquid metal-based electronic devices are attracting increasing attention owing to their excellent flexibility and high conductivity. However, a simple way to realize liquid metal electronics on a microscale without photolithography is still challenging. Herein, the wettability and adhesion of liquid metal are controlled by combining the stirring method, femtosecond laser microfabrication, and sacrificial layer assistant. The adhesive force of liquid metal is dramatically enhanced by adjusting its oxidation. The wetting area is limited to a micro-pattern by a femtosecond laser and sacrificial layer. On this basis, a high-resolution liquid metal printing method is proposed. The printing resolution can be controlled even less than 50 μm. The resultant liquid metal pattern is applied to electronic skin, which shows uniformity, flexibility, and stability. It is anticipated that this liquid metal printing method will hold great promise in the fields of flexible electronics.

Fabrication and integration of functional stretchable circuit based on liquid metal

A liquid metal patterning method and a fabrication process for liquid metal stretchable circuits are described. The initial resistance of the wire-electrode unit is only about 1.4 Ω, and the maximum applied strain is up to 260%. Static stretching and dynamic fatigue tests both reflect the excellent resistance stability. Two types of integrated packaging processes for liquid metal stretchable circuits, surface mount and embedded, are presented. The LED-wire unit manufactured by the embedded method can work normally under a strain of 260%. Through the integration process of surface mounting, the electronic devices are integrated onto the flexible substrate, and the flexible circuit system demonstration of a flash LED is realized. The circuit can work normally under 100% strain. The integration process for liquid metal based flexible electronics provides potential applications in flexible micro-systems.

Self-packaged high-resolution liquid metal nano-patterns

•Laser interference lithography is invented for nano-patterning liquid metal (LM) •The resolution in LM patterns breaks the optical limit of laser beams •Pulsed-laser-induced compression enables uniform ∼500-nm LM nanolayers •The robust oxide shell on LM boosts the mechanical properties and reliability High-resolution self-packaged conductive patterns are important in integrated electronics used in harsh environments. One of the most promising candidates is gallium-based liquid due to its unique properties. Here, we introduce an advanced liquid metal nano-patterning technique based on pulsed laser lithography (PLL) to create self-packaged, high-resolution liquid metal patterns. The method described here, for the first time, can directly generate liquid metal nano-patterns with ∼500-nm line width without being limited by laser beam size. Line-scanning pulsed-laser-induced shock and thermal effects could generate compression on the liquid metal to extrude ∼200-nm particles to an ∼30-nm layer covered by an ∼20-nm oxide shell with boosted mechanical properties. When subjected to external damage, the electrical functionality of the nano-patterns is well maintained due to the protective self-packaged shell and its 3D structure. The electrically self-packaged material with high resolution is a promising candidate to serve in demanding applications with high integration densities. High-resolution self-packaged conductive patterns are important in integrated electronics used in harsh environments. One of the most promising candidates is gallium-based liquid due to its unique properties. Here, we introduce an advanced liquid metal nano-patterning technique based on pulsed laser lithography (PLL) to create self-packaged, high-resolution liquid metal patterns. The method described here, for the first time, can directly generate liquid metal nano-patterns with ∼500-nm line width without being limited by laser beam size. Line-scanning pulsed-laser-induced shock and thermal effects could generate compression on the liquid metal to extrude ∼200-nm particles to an ∼30-nm layer covered by an ∼20-nm oxide shell with boosted mechanical properties. When subjected to external damage, the electrical functionality of the nano-patterns is well maintained due to the protective self-packaged shell and its 3D structure. The electrically self-packaged material with high resolution is a promising candidate to serve in demanding applications with high integration densities.

Self-healing and stretchable conductor based on embedded liquid metal patterns within imprintable dynamic covalent elastomer

A self-healing, robustly conductive and stretchable conductor based on embedded liquid metal patterns was fabricated via a structure-confined filling strategy.

Intermetallic wetting enabled high resolution liquid metal patterning for 3D and flexible electronics

The intermetallic wetting between metallic liquids and solid surfaces enables a high-resolution liquid metal patterning strategy which is widely applicable for fabricating functional patterns on versatile substrates and planar/3D geometries.