EGaIn-based liquid-metal electrodes for electronic interfaces and contact engineering

Gyeong Seop Kim; Ho Yun Lee; Mileu Bang; Sungchul Mun; Young Tack Lee; Bo-In Park; Ji-Hoon Kang

doi:10.1016/j.isci.2026.117626

. 2026 Sep 24;29(10):117626. doi: 10.1016/j.isci.2026.117626

EGaIn-based liquid-metal electrodes for electronic interfaces and contact engineering

Gyeong Seop Kim ^1,², Ho Yun Lee ¹, Mileu Bang ³, Sungchul Mun ⁴, Young Tack Lee ^1,^∗, Bo-In Park ^3,^5,^∗∗, Ji-Hoon Kang ^1,^2,^∗∗∗

PMCID: PMC13634404 PMID: 42830940

Summary

Liquid metals offer a unique combination of metallic conductivity and fluid-like deformability, but their practical implementation as electrodes requires precise control over dynamic and chemically active interfaces. This review examines eutectic gallium-indium (EGaIn) as a representative gallium-based liquid metal and analyzes how its native oxide skin, wetting behavior, reactive alloying, and contact mechanics govern electrical contact formation, interfacial stability, and device integration. Across electronic interfaces, bioelectrode systems, and semiconductor contacts, recent studies show that oxide engineering, interfacial chemistry, mechanical confinement, controlled activation, self-packaging, and low-damage metallization can transform liquid metals from soft conductors into reliable interface materials. These strategies enable conformal printed electrodes, stretchable and leakage-resistant interconnects, implantable and dry bioelectrodes, and low-thermal-budget contacts for fragile electronic materials. By organizing these advances within a unified interface-engineering framework, this review highlights the potential of liquid-metal electrodes as adaptive, programmable, and quantitatively controllable interfacial platforms for next-generation electronic and bioelectronic systems.

Keywords: liquid-metal electrodes, eutectic gallium-indium, EGaIn, contact engineering, electronic interfaces, wetting, printed electrodes, bioelectrodes, biocompatibility, semiconductor contacts

Graphical abstract

Bioelectronics; Electronic materials; Devices

Introduction

Liquid metals have attracted considerable attention across diverse fields, including flexible electronics, bio-interfaces, soft robotics, thermal management, and printed devices, owing to their unique combination of metallic conductivity and fluid-like deformability. Historically, mercury (Hg) served as the prototypical liquid metal and was extensively used in studies of electrical contact and fluidic conductors.¹^,² However, its high toxicity and environmental risks have severely restricted its practical deployment and large-scale adoption.³ In contrast, gallium (Ga), with a melting point of ∼29.8 °C, has emerged as an attractive alternative that can readily transition to a liquid state near ambient and physiological temperatures. In addition to elemental Ga, several low-melting liquid-metal systems, including eutectic gallium-indium (EGaIn), gallium-indium-tin (Galinstan), and sodium-potassium alloys (NaK), have been actively investigated.⁴^,⁵^,⁶ Owing to their relatively low toxicity, high conductivity, mechanical compliance, and process compatibility, these liquid-metal systems enable functionalities that are difficult to realize using conventional rigid electrodes.⁷

Among these materials, EGaIn has emerged as one of the most extensively studied liquid metal systems because it remains stably liquid at room temperature while providing high electrical conductivity, excellent processability, and a self-limiting native oxide skin.⁴^,⁸^,⁹ Rather than acting as a passive surface byproduct, this oxide layer serves as a critical interfacial component that governs wetting, adhesion, patternability, transferability, interfacial reactions, alloying behavior, and ultimately electrical contact formation.⁸^,⁹^,¹⁰^,¹¹^,¹² Accordingly, EGaIn should be viewed not merely as a deformable conductor, but as an interfacial engineering platform capable of enabling conformal contact formation, low-thermal-budget integration, dynamic contact-state modulation, and tunable electrode energetics.¹³^,¹⁴^,¹⁵^,¹⁶ These characteristics are particularly advantageous in emerging electronic and bioelectronic systems, where conventional metal electrodes often suffer from mechanical mismatch, incomplete interfacial coupling, damage to delicate active materials, and reliance on thermally intensive fabrication processes.

Despite these advantages, EGaIn-based interfaces continue to face critical challenges, including leakage arising from intrinsic fluidity, oxide-mediated variability in interfacial properties, Ga-induced corrosion and embrittlement of structural metals, and the need for robust encapsulation and long-term operational stability.¹⁷^,¹⁸^,¹⁹ These considerations highlight that EGaIn should be regarded not merely as a deformable metallic conductor, but as a complex interfacial system whose physicochemical properties must be carefully engineered for practical device integration. In this review, we focus specifically on EGaIn as a representative Ga-based liquid metal and examine how its interfacial characteristics govern electrical contact formation, interface stability, and device integration across diverse electronic systems. We begin with the fundamental properties and oxide-enabled interfacial characteristics of EGaIn, and then examine its applications in electrical interfaces, bioelectrode and soft-contact systems, and semiconductor contact engineering, followed by a discussion of the remaining challenges and future opportunities. Through this perspective, we aim to establish a unified framework for understanding how EGaIn can evolve from a deformable metallic conductor into a quantitatively engineered and predictable interface platform for next-generation electronic and bioelectronic systems.

Theoretical and interfacial foundations of EGAIN

Formation and mechanical-thermodynamic effects of the native oxide skin

A nanometer-thick native oxide skin (Ga₂O₃) forms instantaneously on EGaIn upon air exposure and governs the mechanical behavior of the liquid metal. Because the standard Gibbs-free energy of formation favors Ga₂O₃ over In₂O₃, indium is largely excluded from the surface oxidation reaction, resulting in the formation of a Ga₂O₃-dominated oxide layer. Using X-ray photoelectron spectroscopy (XPS), Farrell et al. showed that a self-limiting skin of ∼0.7–3.0 nm forms via the Cabrera-Mott mechanism within milliseconds, even at ppm-level oxygen, as shown in Figure 1A.⁹^,²⁰^,²¹ The skin stops growing not because it reaches thermodynamic equilibrium, but because the diminishing Mott potential kinetically passivates further ion diffusion and arrests the oxide at a metastable thickness.

Physical Characteristics and Interfacial Dynamics of EGaIn

(A) Native oxide of liquid metal and its interfaces (γ_o: outward surface energy, γ_ol: inward interfacial energy, σ: mechanical tension).

(B) Effect of asymmetric native oxides on the adhesion of liquid metal to SiO₂. Bare liquid metal shows weak adhesion without wetting the substrate (left), whereas the presence of native oxide lowers the interfacial energy, resulting in strong adhesion (right). Reprinted with permission under a Creative Commons CC BY License from ref.²¹ Copyright (2024) John Wiley & Sons.

(C) Schematic of the fabrication flow for liquid metal circuits. The process includes 1) Cu deposition on polydimethylsiloxane (PDMS), 2) laser patterning, 3) liquid metal application, 4) component placement, 5) HCl vapor soldering, and 6) final elastomer encapsulation. Reprinted with permission from ref.²² Copyright (2018) John Wiley & Sons.

(D) Schematic of the Marangoni-induced spreading of EGaIn alloy under non-wetting conditions with the Cu surface connected to the anode. Reprinted from Applied Surface Science, Vol 490, Ni Zhang, Ping Shen, Yue Cao, Rui-Fen Guo, Qi-Chuan Jiang, Electrically induced spreading of eutectic gallium-indium (EGaIn) on Cu substrate in an alkali solution under wetting and non-wetting conditions, Pages 598–603, Copyright (2019), with permission from Elsevier.²³

(E) Separation behavior of an EGaIn droplet from a solid surface. While typical retraction leaves metal residue, slower retraction rates enable successful, residue-free separation, provided the contact angle is sufficiently large. Reprinted with permission under a Creative Commons CC BY License from ref.²⁴ Copyright (2025) Springer Nature.

The oxide skin most prominently affects EGaIn by imparting a viscoelastic mechanical response. In contrast to bulk EGaIn, which behaves as a low-viscosity Newtonian liquid (∼2 mPa s), the oxide layer gives rise to yield-stress-dominated interfacial mechanics. As a result, EGaIn exhibits an apparently solid-like response at small strains. Jacob et al. used interfacial rheology with a Du Noüy ring geometry to decouple the oxide skin from the liquid bulk.²⁰ They showed that the interface behaves as a viscoelastic solid with a surface yield stress of approximately 0.5 N/m. This interfacial yield stress allows EGaIn to suppress capillary-driven relaxation and maintain non-spherical, three-dimensional structures against gravity until the applied stress exceeds a critical threshold.

Furthermore, the oxide sharply lowers the apparent effective interfacial energy. Applying Johnson-Kendall-Roberts (JKR) contact mechanics, Jung et al. found that the skin leads to an apparent reduction in effective interfacial tension from 724 mN/m to a plateau of ∼10 mN/m, a decrease of nearly two orders of magnitude.²¹ The intrinsic surface tension of the bulk liquid remains unchanged; rather, the solid skin mechanically bears the interfacial stress and shields the high-energy liquid core, mitigating the energetic asymmetry without requiring any surfactant. As illustrated in Figure 1B, this apparent reduction enables high wettability and strong adhesion even on low-surface-energy polymers and solid electrodes, underpinning conformal electrode control.

Environment-dependent wetting, dewetting, and adhesion mechanics

The oxide continuously interacts with its environment, driving dramatic wettability transitions rather than maintaining fixed behavior. By investigating EGaIn tunneling junctions comprising self-assembled monolayers (SAMs) of varied chemistries, Carlotti et al. demonstrated that the injection current can be modulated by up to two orders of magnitude by controlling the measurement environment, including oxygen and humidity levels, and the interfacial wetting behavior. This environmental and chemical sensitivity highlights that the wetting properties of the native oxide strongly govern the electrical contact quality, providing a critical basis for actively optimizing charge transport in molecular and soft electronics.²⁵

Moisture acts as a powerful dewetting factor. Khan et al. demonstrated that injecting EGaIn into water-filled microchannels establishes an interfacial slip layer, where the water-induced chemical and structural modification of the native oxide reduces its surface yield stress by a factor of five and effectively suppresses wall adhesion. Despite water being ∼55 × more viscous than air at room temperature, this lubricating slip lowered the critical injection pressure by 40% compared to dry conditions,²⁶ establishing aqueous environments as an effective strategy for controlling oxide adhesion and encapsulation.

Impact of surface topography on interfacial behavior

Interfacial behavior also depends strongly on substrate roughness and topography. As roughness increases, EGaIn adopts a metastable Cassie-Baxter-like state due to trapped air, inducing super-repulsion.²⁷ Using femtosecond-laser processing to form hierarchical micro/nanostructures on polydimethylsiloxane (PDMS), Zhang et al. achieved near-complete dewetting and ultra-low adhesion in roughened regions while retaining strong oxide-mediated adhesion on flat regions, demonstrating high-resolution patterning via physical modulation alone.²⁸

Conversely, surface texture can act as an anchor for the liquid metal. Analyzing the direct-write printing of EGaIn on stretchable polymers, Boley et al. showed that applied pressure forces the liquid into micro-roughness, inducing a Cassie-to-Wenzel transition. In this fully wetted state, the oxide mechanically interlocks with surface asperities, producing strong contact-angle hysteresis and pinning that exceeds the retraction force. As a result, printed traces retain free-standing microscale interconnects without beading or collapsing. As illustrated in Figure 1C, a strain sensor based on this mechanism exhibited linear and stable performance with a gauge factor of ∼1.5 up to 50% strain.²⁹

Oxide skin elimination and dynamic adjustment

Because the insulating oxide raises contact resistance, eliminating or controlling it is essential. Recent strategies achieve this via three distinct routes: permanent chemical etching (for static junctions), reversible electrochemical modulation (for dynamic control), and chemical-free physical peeling (for dry environments).

For permanent interconnections, Ozutemiz et al. utilized HCl vapor to locally etch the interfacial oxide between EGaIn pads and copper. This irreversible decomposition enables pristine metal-to-metal contact, acting as a solder-like interface that significantly enhances conductivity and durability.²²

Conversely, for real-time adaptation, Zhang et al. demonstrated reversible electrochemical modulation in NaOH. A cathodic copper substrate reductively removes the oxide for complete wetting, while reversing the polarity drives Marangoni flows that suppress it (Figure 1D), thereby making the oxide state programmable by voltage alone.²³

Finally, when corrosive etchants or electrolytes are prohibitive, physical peeling offers a dry alternative. Jung et al. elucidated a ‘residue-free peeling' mechanism: while fast retraction (>0.1 mm/s) or small contact angles fracture the skin and leave residue, kinematic peeling below this threshold at a high contact angle cleanly delaminates the entire oxide without mechanical fracture (Figure 1E).²⁴

Solid-liquid electrical contact resistance

System-level electrical efficiency is ultimately dictated by solid-liquid contact resistance, where compliant, reactive contact of EGaIn overcomes the inherent roughness limits of solid-solid junctions. Ozutemiz et al. successfully lowered the total contact resistance below 0.05 Ω by HCl-vapor etching the EGaIn-copper interface.²² Furthermore, by using a Transmission Line Model (TLM) to isolate the pure interfacial resistance for EGaIn on various electrodes, Zu et al. demonstrated that spontaneous reactive wetting shifts the interface from discrete van der Waals contacts to a stable, metallic-bond-mediated wetting state that seamlessly bridges microscale interfacial asperities.¹⁹

This structural transformation directly enhances macroscopic electrical performance. Contact resistance falls as wettability and the effective contact area grow, reaching a quasi-steady state after ∼20 days. Pure copper (T2) achieves a specific contact resistivity of ∼1.36 × 10⁻⁵ Ω cm², which is superior to that of conventional direct compression connections. Material selection plays a critical role: while brass (H62) initially wets better via rapid Zn diffusion, it promotes liquid metal embrittlement (LME) as Ga penetrates grain boundaries, threatening long-term structural integrity. In contrast, pure copper (T2) resists LME at room temperature, preserving ductile fracture characteristics and ensuring a stable electrical interface.

Reactive wetting and intermetallic micro-mechanisms

The macroscopic electrical improvements discussed above are fundamentally driven by reactive wetting and intermetallic compound (IMC) formation at the microscale. For the model Ga-Cu system, Cui et al. revealed via first-principles calculations and experiments that wetting is fundamentally driven by metallic bonding, as illustrated in Figure 2A. In 1 mol/L NaOH, a Ga droplet on pristine Cu does not wet (contact angle 133.58°); however, on a pre-formed CuGa₂ layer, it spreads completely (15.65°) (Figure 2B). Based on the calculated electronic structure (Figure 2C), density functional theory (DFT) analysis revealed that the work function of the CuGa₂ (010) surface (4.47 eV) is remarkably close to the effective work function of bulk liquid Ga (4.32 eV). This close matching reduces the interfacial energy barrier for electron transport, while the high adsorption energy of a single Ga atom on CuGa₂ (3.29 eV, well above those of In and Sn) drives strong orbital hybridization and metallic bonding.³⁰

Reactive Wetting, Intermetallic Compound (IMC) Formation, and Electromechanical Stability of Liquid Metal (LM) Contacts

(A) Interfacial properties and wetting mechanism at the Ga/CuGa₂ interface. The figure illustrates the density of states, the atom adsorption model, and a schematic of the metallic bond-enabled wetting of gallium-based liquid metals (LM) on the CuGa₂ surface.

(B) Static contact angles of Ga droplets at 310 K, comparing the wetting behavior on Cu versus CuGa₂ substrates in NaOH and HCl solutions. The CuGa₂ layer significantly enhances wettability regardless of the chemical environment.

(C) Electronic structure analysis of CuGa₂ (010) (charge density distribution, electron localization function (ELF) map, and electrostatic potential profile along the z axis). Adapted with permission from ref.³⁰ Copyright (2018) American Chemical Society.

(D) Long-term contact stability of liquid metal wires with different electrode materials. Cu electrodes exhibit excellent stability for 1000 h at both room temperature (RT) and 100 °C, whereas Sn and Ag electrodes dissolve within 100 h.

(E) Intermetallic compound (IMC) formation and microstructural changes at the LM/Cu interface over time and temperature (transmission electron microscopy (TEM)/scanning electron microscopy (SEM) images and compositional mapping).

(F) Cyclic strain-resistance test results of liquid metal wires at various strains (100%, 300%, and 500%) and comparison with the theoretical prediction model (Pouillet’s law). Adapted with permission from ref.³¹ Copyright (2025) American Chemical Society.

The chemical identity of the electrode dictates long-term reliability by controlling this IMC layer. Tracking EGaIn interfaces with Cu, Sn, and Ag at room temperature (RT) and 100 °C for up to 1000 h (Figures 2D and 2E), Lee et al. found that Sn fully dissolved within 100 h, while Ag developed severe cracking and surging resistance. Copper, however, formed a self-limiting dual-alloy barrier, consisting of a Cu₉Ga₄ layer adjacent to bulk Cu and a CuGa₂ layer adjacent to the liquid metal, which kept Ga’s effective diffusivity extremely low under ambient conditions.³¹

However, reactive wetting at the EGaIn-Cu interface should be regarded as a time- and condition-dependent process. Initial Cu-Ga interfacial alloying promotes reactive wetting, intimate metallic contact, and the formation of stable electrical pathways. In contrast, continued Cu/Ga interdiffusion under elevated temperatures may lead to excessive intermetallic growth, Kirkendall void formation, increased resistance, and interfacial embrittlement. For the Cu-based interfaces discussed here, stable contact behavior was demonstrated under the specific thermal-aging and electromechanical testing conditions reported in the previous study, including aging at room temperature and 100 °C for up to 1,000 h and cyclic deformation of elastomer-encapsulated liquid-metal wires, which retained electrical continuity over more than 15,000 cycles at strains up to 300% (Figure 2F).³¹ These results, however, should not be interpreted as demonstrating long-term stability under sustained electrical loading or elevated current densities. Therefore, metallic-bond-mediated interfacial alloying should be considered a condition-dependent contributor to the stability of EGaIn-Cu contacts within the tested operating window, rather than a universal fundamental mechanism governing the long-term stability of stretchable electronics.

Electrical interfaces

EGaIn combines metallic conductivity with liquid-state deformability and room temperature processing, which makes it attractive for electrodes that must follow soft, curved, or repeatedly deformed substrates. Table 1 places these features beside representative solid metals, conducting polymers, carbon composites, ionic hydrogels, and MXene. The comparison highlights distinctive advantage of EGaIn: instead of accommodating strain only through cracks, serpentine patterns, or percolating fillers, the conductive phase can rearrange by flow. At the same time, the native Ga₂O₃ skin, high surface tension, poor wetting, and lack of intrinsic shape retention mean that useful EGaIn electrodes are created through interfacial and structural design rather than the liquid metal alone.⁴²

Table 1.

Comparison of unmodified electrode materials for flexible/stretchable electronics

Material	Mechanical tensile stability	Process temp (°C)	Drawbacks	Electrical conductivity (S/cm)	Reference
Au	up to 15% tensile strain	25	• low stretchability • crack formation • vacuum deposition	3.45 × 10⁵	Putz et al.³²; Faurie et al.³³; Kornbluth et al.³⁴
PEDOT:PSS	elongation at break = 10% tensile strength = 57 MPa	160 (vacuum annealing)	• low conductivity • limited stretchability • thermal annealing	3	Okuzaki et al.³⁵
Carbon composite (pristine CCB/SR)	elongation at break = 90.3% tensile stress = 0.767 MPa	25	• low conductivity • percolation dependence • curing inhibition	0.102	Dou et al.³⁶
Ionic hydrogel (PAAm/LiCl)	rupture stretch λ = 23 (=2200% engineering strain)	50 (thermal gelation)	• dehydration • ionic conduction • electrochemical instability	0.08	Chen et al.³⁷; Fang et al.³⁸
MXene (pure Ti₃C₂T_x film)	strain to failure = 1.0 ± 0.2% tensile strength = 22 ± 2 MPa	60 (film drying)	• oxidation • low stretchability • interflake brittleness	2.40 × 10³	Ling et al.³⁹; Liu et al.⁴⁰; Zhang et al.⁴¹
EGaIn	no conventional fracture strain (liquid)	25	• oxide skin • poor wettability • shape instability	3.40 × 10⁴	Dickey et al.⁴²

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EGaIn conformal contact

EGaIn is particularly useful for conformal contacts because its conductive phase can redistribute as the underlying surface bends or stretches. However, liquid-like deformability does not automatically produce intimate adhesion to hydrogels, textiles, or skin-facing substrates.⁴³ Bulk EGaIn tends to bead and aggregate because of its high surface tension, while the oxide skin that helps it adhere can also impede electrical contact. The two studies discussed later in discussion solve this problem at different length scales: one modifies the EGaIn interface for direct printing on hydrogel, and the other programs the mechanical properties of a textile so that liquid-metal wiring and rigid chips remain integrated during motion.

Jiang et al. introduced highly oxidized EGaIn (hoEGaIn) for direct ink writing on acrylamide hydrogel.⁴⁴ Increasing the oxide content transformed the highly fluid metal into an ink with improved shape retention and interfacial affinity. As shown in Figure 3A, hoEGaIn exhibits reduced droplet aggregation and improved wetting behavior compared with conventional EGaIn. Relative to conventional EGaIn, hoEGaIn showed an approximately 28.5% reduction in apparent surface tension, an approximately 24.4% increase in adhesion, and lower contact angles on the hydrogel. These changes suppressed droplet aggregation and enabled continuous patterns with a minimum line width of 65 μm. The printed traces reached a conductivity of 2.22 × 10⁶ S m⁻¹ and could also be deposited on polyethylene terephthalate (PET), polyvinyl chloride (PVC) tape, Teflon tape, iron, poly methyl methacrylate (PMMA), PDMS, glass, and label substrates, indicating that the oxidation strategy was not limited to one hydrogel chemistry. To demonstrate device function, the authors printed a cantilever-beam strain sensor in which the acrylamide (AAm) hydrogel served as both substrate and encapsulating layer. The sensor exhibited response and recovery times of 62.1 and 63.9 ms, respectively, and maintained repeatable resistance changes during 4,000 loading cycles. The work therefore shows how controlled oxidation can simultaneously improve wetting, print fidelity, and mechanical coupling, although excessive oxide would still increase electrical resistance and complicate later activation.

Conformal contact and printed EGaIn electrodes

(A) Comparison of droplet aggregation and wetting behavior between EGaIn and highly oxidized EGaIn (hoEGaIn). Adapted with permission from ref.⁴⁴ Copyright (2026) American Chemical Society.

(B and C) (B) Comparison between laser-programmed textile hybrid electronics (LPTHE) fabricated directly on textile substrates, and conventional flexible printed circuit board (FPCB)-based patches. Reprinted with permission under a Creative Commons CC BY License from ref.⁴⁵ Copyright (2026) Springer Nature. BLE (Bluetooth low energy) (C) Hydrogel-assisted stabilization of EGaIn droplets for structurally robust printed electrodes.

(D) Three-dimensional EGaIn architectures fabricated by direct ink writing. Reprinted with permission under a Creative Commons CC BY License from ref.⁴⁶ Copyright (2024) Springer Nature.

(E) Water-assisted rupture of oxide shells and coalescence of EGaIn particles during PVP-mediated sintering.

(F) Fabrication process of stretchable printed conductors based on PVP-assisted water-sintering. Reprinted with permission under a Creative Commons CC BY License from ref.⁴⁷ Copyright (2025) Springer Nature.

Luo et al. approached conformal contact from the substrate side through laser-programmed textile hybrid electronics (LPTHE).⁴⁵ As shown in Figure 3B, the LPTHE platform is fabricated directly on the textile substrate, in contrast to conventional flexible printed circuit board (FPCB)-based patches. Selective laser-polymer interaction locally changed the textile surface and increased the modulus of treated regions by 2.7- to 14.9-fold while preserving the porous structure. Rigid chips were mounted on these locally reinforced regions, whereas the surrounding untreated textile retained softness, stretchability, and skin conformity. Liquid metal served as both a deformable interconnect and a solder-like interface to electronic components. The programmed stiffness gradient isolated strain around chip terminals, reduced wire disconnection, and prevented cross-layer short circuits that otherwise occur when textile yarns move independently. The authors demonstrated a 5 × 5 light-emitting diode (LED) display that remained functional under deformation and an untethered patch that monitored electrocardiogram (ECG), acceleration, temperature, and humidity during exercise. Continuous signals were collected under stretching and twisting up to 50% strain. After 10 min of exercise, the skin beneath the permeable LPTHE remained comparatively dry, whereas an FPCB patch of the same layout accumulated sweat and eventually lost reliable ECG waveforms through delamination. This study illustrates that EGaIn conformality is most effective when the surrounding substrate actively manages the mechanical mismatch between soft wiring and rigid components.

Together, these studies define two complementary routes to EGaIn conformal contact. The hoEGaIn approach modifies the liquid-metal surface so that a stable conductive pattern can be formed directly on a hydrated substrate, whereas LPTHE modifies the substrate so that the liquid-metal interconnect is protected within a mechanically graded and breathable system. In both cases, conformality results from coordinated wetting, adhesion, and strain distribution rather than from fluidity alone.

Printed electrodes

Printed EGaIn electrodes must reconcile two opposing requirements.⁴⁸^,⁴⁹ During deposition, the ink must flow through a nozzle or screen and then rapidly recover enough elasticity or yield stress to retain the printed geometry.⁵⁰ After deposition, the isolated droplets must become electrically connected.⁵¹^,⁵²^,⁵³^,⁵⁴ Native oxide shells are central to both processes: they stabilize droplets and support viscoelastic behavior before printing, but they also electrically separate the particles. The following two studies illustrate how the continuous phase and the post-print activation method can be designed together for three-dimensional direct ink writing and scalable screen printing. For this reason, printability is better described by the combined evolution of shear thinning, storage modulus, yield behavior, droplet loading, shape recovery, and activation than by a single viscosity value.⁵⁵^,⁵⁶^,⁵⁷ An ink may extrude smoothly yet collapse after deposition, or it may retain its form but remain electrically insulating. The most practical formulations therefore separate structural stabilization during printing from controlled coalescence after the pattern has been formed.

Lin et al. developed a liquid-metal high-internal-phase-emulsion gel (LM-HIPEG) for direct ink writing.⁴⁶ Carbopol hydrogel formed the continuous phase, while gallium-indium droplets occupied up to 82.5 vol % of the ink. As illustrated in Figure 3C, the hydrogel phase stabilizes the densely packed EGaIn droplets and supports structural retention before electrical activation. At this high loading, interactions among the densely packed droplets supplied elasticity and a high storage modulus, whereas the hydrated Carbopol phase lubricated droplet motion under nozzle shear. This combination allowed the material to flow continuously during extrusion and recover a shape-stable gel state immediately after printing. Importantly, the hydrogel also reduced destructive contact between droplets and protected the thin oxide shells from uncontrolled rupture inside the nozzle, which would otherwise cause local coalescence and clogging. The resulting LM-HIPEG could be printed into self-supporting three-dimensional architectures, as shown in Figure 3D, as well as patterns on flexible and non-planar substrates and alternating structures with other printable materials. Electrical activation was achieved through an electrocapillary phenomenon: an applied electric field deformed the droplets, ruptured their oxide shells, and induced demulsification and connection between neighboring liquid-metal domains. The study therefore decoupled printability from conductivity in time, preserving a stable emulsion during fabrication and generating metallic pathways only after the desired three-dimensional geometry had been established.

Shang et al. developed a complementary screen-printing route for large-area stretchable multilayer circuits.⁴⁷ EGaIn droplets were dispersed in the green solvent propylene glycol using polyvinylpyrrolidone (PVP) as a non-hazardous dispersion agent. After printing and drying, the traces were initially insulating because each particle remained surrounded by its oxide shell and a PVP-rich matrix. As shown in Figure 3E, during water-spray sintering, the PVP absorbs water and swells, while subsequent drying generates shrinkage forces that locally fracture the oxide shells and promote coalescence of adjacent EGaIn droplets into conductive domains. The overall PVP-assisted screen-printing and water-sintering process is summarized in Figure 3F. Optimizing the PVP concentration and spray conditions produced a conductivity of 7.32 ± 0.30 × 10⁵ S m⁻¹. The method generated line widths from approximately 0.2 to 1 mm, remained effective in multilayer stacks, and retained stable ink performance after storage for up to eight months. With a printed thermoplastic polyurethane (TPU) encapsulation layer, the conductors remained highly conductive to large strain and showed less than a 10% resistance increase during 500 cycles at 50% strain. The process was finally used to fabricate fully printed multilayer near-field-communication tags, demonstrating registration, vertical interconnection, and circuit operation at a manufacturing-relevant scale.

The two printing strategies solve different bottlenecks. LM-HIPEG prioritizes three-dimensional shape stability and electrically triggered connection, whereas the PVP ink prioritizes large-area screen compatibility and water-assisted activation. In both systems, the oxide shell is retained long enough to stabilize the ink and is ruptured only after printing. Long-term reliability consequently depends on how well the activated liquid domains remain confined by the hydrogel, polymer matrix, or encapsulation layer during repeated deformation and environmental exposure.

Flexible and stretchable interconnects

Flexible and stretchable interconnects exploit ability of EGaIn to preserve a conducting pathway while a device elongates, folds, twists, or conforms to a three-dimensional surface.⁵⁸^,⁵⁹^,⁶⁰ The liquid phase avoids the irreversible crack propagation characteristic of continuous solid-metal films, but it must be patterned and confined to prevent spreading, leakage, or short circuits. The following studies use two different architectures: a hybrid particle/continuous-liquid bilayer designed as an active organic light-emitting diode (OLED) cathode, and an ultrathin micromesh that distributes strain while physically restricting liquid-metal displacement.

Lee et al. developed a hybrid liquid-metal (Hyb-LM) cathode for intrinsically stretchable OLEDs.⁶¹ A liquid-metal-particle layer was first formed and then selectively ruptured at the surface, producing a bilayer composed of a continuous LM top layer supported by an underlying particle network. The continuous surface lowered lateral resistance and provided a uniform electron-injection interface, while the particle-rich lower region improved mechanical integration with the stretchable device stack. The cathode exhibited a work function of approximately 4.1 eV, reflectance of about 90%, and a sheet resistance of 2.70 × 10⁻² Ω sq⁻¹. Its normalized resistance was only 1.03 at 150% strain, and patterned features of approximately 200 μm were demonstrated. OLEDs incorporating the cathode showed a turn-on voltage of 3.0 V, a maximum luminance of 17,670 cd m⁻², and a current efficiency of 10.35 cd A⁻¹. As shown in Figure 4A, OLED pixels employing the Hyb-LM cathode exhibit enhanced electroluminescence compared with those using the LMP cathode at 4 V. The devices continued to emit under 30% and 60% tensile strain, as shown in Figure 4B. Rather than functioning only as a compliant wire, EGaIn therefore acted as a low-work-function, reflective, and mechanically stable optoelectronic contact. The result depended on controlled surface rupture: an inactivated particle cathode retained insulating oxide barriers and produced inferior electroluminescence.

Flexible and stretchable EGaIn interconnects and active optoelectronic electrodes

(A) Photographs (top) and optical microscope images (bottom) of organic light-emitting diode (OLED) pixels employing hybrid liquid-metal (Hyb-LM) and liquid-metal-particle (LMP) cathodes at 4 V, highlighting enhanced electroluminescence in the Hyb-LM devices.

(B) Mechanical deformation of a Hyb-LM-based intrinsically stretchable OLED under 0%, 30%, and 60% tensile strain. Reprinted with permission from ref.⁶¹ Copyright (2026) John Wiley & Sons.

(C) Conformal transfer of liquid-metal microparticle films transferred onto a fingertip.

(D) Encapsulated liquid-metal micromesh electrodes in pristine and uniaxially stretched states, demonstrating extreme mechanical deformability.

(E) Normalized resistance evolution of encapsulated liquid-metal micromesh electrodes during 2,000 stretch-relaxation cycles at 300% strain.

(F) Normalized resistance evolution of encapsulated liquid-metal micromesh electrodes during 10,000 folding-relaxation cycles at a bending radius of 70 μm. Error bars represent the standard deviation of three parallel samples. Reprinted with permission under a Creative Commons CC BY License from ref.⁶² Copyright (2026) Springer Nature.

Yang et al. focused on extreme deformation and leakage control using ultrathin liquid-metal micromesh electrodes.⁶² Liquid-metal microparticles were assembled at an interface, transferred to target substrates, and locally sintered with a picosecond laser. Laser-induced rupture and coalescence did not generate a fully dense film; surface tension drove partial dewetting and produced interconnected liquid-metal ligaments separated by microscale voids. As shown in Figure 4C, the liquid-metal microparticle film can be conformally transferred onto a highly curved surface such as a fingertip. Figure 4D further shows the encapsulated micromesh electrodes in the pristine and stretched states, illustrating their extreme mechanical deformability. The resulting mesh reached a minimum thickness of 317 nm, could be stretched to 1,200%, and maintained R/R₀ = 1.66 at 300% strain. Encapsulated electrodes remained electrically stable during 2,000 stretch-relaxation cycles at 300% strain and 10,000 folding-relaxation cycles at a bending radius of 70 μm, as shown in Figures 4E and 4F, respectively. The defined mesh pathways confined the liquid metal and produced a leakage resistance of 968.75 kPa, while the void-rich architecture reduced stress concentration at folds. The electrodes were demonstrated in LED arrays, wireless power-transfer structures, and angular sensors. This study shows that fluidity of EGaIn can be retained at the ligament scale while macroscopic flow is suppressed through an ultrathin, patterned network.

These two examples extend EGaIn interconnects in different directions. The Hyb-LM architecture emphasizes electronic function and interface quality within an intrinsically stretchable optoelectronic stack, whereas the micromesh emphasizes foldability, leakage resistance, and geometric confinement. Both rely on spatially controlled particle activation rather than unrestricted bulk liquid metal, demonstrating that reliable EGaIn interconnects arise from a designed relationship among oxide rupture, liquid redistribution, and surrounding structure.

Bioelectrode and soft-contact interfaces

In addition to the electrical and mechanical considerations described above, bioelectrodes should also meet specific biological and interfacial requirements. EGaIn can reduce mechanical mismatch and preserve conductive routing during body motion, but the complete device must also maintain a stable tissue or skin interface, prevent leakage, and limit chemical exposure. Table 2 compares representative bioelectrode materials and provides context for the distinctive role of EGaIn: its primary advantage is not a universally lowest impedance, but the ability to combine metallic conduction with large deformation when the liquid phase is appropriately isolated from the biological environment.

Table 2.

Comparison of unmodified electrode materials for bioelectrode interfaces

Material	Purpose	Mechanical organization suitability	Biological risk	Reference
Au	• electrical stimulation • neural/skin reference electrode	• Young’s modulus: 55–67 GPa • fracture strain: 0.9–2.0%	• stiffness • mismatch • tissue damage • inflammation	Kim et al.⁶³; Kwon et al.⁶⁴
PEDOT:PSS	• low-impedance recording/stimulation • neural interfaces	• Young’s modulus: 337 MPa • tensile strength: 20.8 MPa • elongation: 10%	• delamination debris • PSS leaching • inflammation	He et al.⁶⁵; Gerwig et al.⁶⁶
Ag/AgCl	• ECG/EEG/EMG recording reference electrode • nonpolarizable interface	• Young’s modulus: 63/20 GPa • elastic limit: 26 MPa	• Ag⁺ leaching • tissue accumulation • heavy-metal toxicity	Huang et al.⁶⁷; Combes et al.⁶⁸; Wang et al.⁶⁹
Ionic hydrogel	• ion-conductive soft interface/electrode • signal transmission	• Young’s modulus: 100 ± 16 kPa • stretchability: 660 ± 72%; ΔR <15% (50 cycles)	• dehydration Swelling • crosslinker residues	Chowdhury et al.⁷⁰; Kang et al.⁷¹
EGaIn	• stretchable bioelectrode and interconnect • neural/skin recording	• critical surface stress: 0.5 N m⁻¹ • liquid/flowable at RT	• Ga/In-ion leaching • agitation cytotoxicity	Kim et al.⁷; Dickey et al.⁴²

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Implantable electrodes

Choi et al. developed a strain-adaptive fiber-interlocked electronic (SAFIE) patch for sutureless epicardial interfacing.⁷² The patch consisted of a catechol-conjugated alginate (Alg-CA) ionic tissue adhesive, an electrospun fiber-interlocked self-healing elastomer, and a fatigue-resistant EGaIn/polymer composite containing the recording electrodes and interconnects. The porous interlocked layer allowed Alg-CA to penetrate the network and dissipate shear energy, producing an adhesion strength of approximately 7.2 kPa and conformal attachment to a beating heart in less than 0.5 s without heat, light, or prolonged compression. As shown in Figure 5A, the tissue-bonding behavior of SAFIE patches is compared with and without Alg-CA before and after attachment to tissue. The adhesive structure remained attached during 500 stretch-release cycles, accommodated stretching up to 660% on porcine heart tissue, and resisted washing by physiological buffer and blood flow. In the conductive layer, pressure-induced rupture of oxide-coated EGaIn particles generated percolating liquid pathways. The optimized composite showed less than 0.2% resistance change at 50% strain after 1,000 cycles and dissipated residual stress more effectively than solid-filler composites. The patch enabled multichannel ECG recording and electrical stimulation in awake, freely moving rats. Representative ECG signals recorded using EGaIn composite electrodes with and without Alg-CA are shown in Figure 5B. Signals remained detectable for four weeks with a signal-to-noise ratio (SNR) of approximately 20–30 dB, and histology at three days, one week, and four weeks showed only mild inflammation and immune response. Thus, the study primarily demonstrates how rapid adhesion, mechanical stress relaxation, and a confined EGaIn composite can work together to stabilize a dynamic epicardial interface.

EGaIn-based implantable bioelectronic interfaces

(A) Photographs of strain-adaptive fiber-interlocked electronic (SAFIE) patches with and without catechol-conjugated alginate (Alg-CA) in the pristine state and 60 s after tissue bonding.

(B) ECG signals recorded during rat heart stimulation using EGaIn composite electrodes without Alg-CA (top) and with Alg-CA (bottom). Reprinted with permission from ref.⁷² Copyright (2023) Springer Nature.

(C) Schematic illustration of a wireless soft cuff-type sensor incorporating soft ferromagnetic and self-healing layers for adaptive implantation and wearable signal readout.

(D) Wireless soft cuff-type sensor wrapped around a rat artery.

(E) Resonant-frequency response of the wireless sensor measured under through-skin conditions. Adapted with permission from Kim et al. Copyright (2026) PNAS.⁷³

Kim et al. integrated EGaIn into wireless soft pressure sensors for large-area mapping and adaptive implantation.⁷³ EGaIn formed both the deformable electrodes of microfabricated capacitive sensors and the low-resistance traces of inductive coils in a thin, battery-free inductive-capacitive (LC) platform. Arrays of the wireless units provided spatial pressure mapping on artificial tissue models, while miniaturized wearable readout electronics incorporated into a medical glove enabled real-time interrogation of resonant frequency. For adaptive implantation, the authors combined the sensor with soft ferromagnetic and self-healing layers. The resulting adaptive wireless cuff architecture is schematically illustrated in Figure 5C. An external magnetic field folded the cuff around a rat femoral artery, eliminating sutures and allowing the device to follow the pulsating vessel. The conformally wrapped cuff configuration around the artery is shown in Figure 5D. The sensor detected arterial pressure changes through skin and retained wireless operation for more than 30 h in phosphate-buffered saline. The corresponding through-skin resonant-frequency response is shown in Figure 5E, demonstrating wireless interrogation through overlying tissue. The self-healing layer also restored mechanical continuity after damage, supporting repeated handling and implantation. The study is therefore notable for system-level integration: EGaIn simultaneously enabled a soft sensor electrode, a compact wireless inductor, and a deformable interconnection network. The in vivo demonstration focused on acute arterial monitoring, whereas large-area mapping was primarily validated using engineered tissue platforms.

Both implantable systems use EGaIn as a mechanically adaptive conductor inside a multilayer device rather than as freely exposed bulk metal. In SAFIE, the polymer composite and tissue-adhesive hydrogel separate the liquid pathways from the epicardium; in the wireless cuff, dielectric and self-healing layers define the sensor geometry and contact with the artery. This architecture preserves the principal mechanical advantage of EGaIn while making adhesion, sealing, and biological compatibility properties of the complete device rather than of the liquid metal alone.⁷⁴^,⁷⁵^,⁷⁶^,⁷⁷^,⁷⁸^,⁷⁹^,⁸⁰

Dry bioelectrodes

Niu et al. developed a pressure-activated breathable liquid-metal electrode (S-GIPE) for long-term sleep electroencephalogram (EEG) monitoring.⁸¹ As shown in Figure 6A, the S-GIPE consists of a porous PET fiber matrix incorporating EGaIn and poly(3-mercaptopropylsilsesquioxane). EGaIn was first converted into particles by ultrasonication and modified with (3-mercaptopropyl)triethoxysilane before patterned deposition on a porous PET fiber matrix. During ambient drying, silane hydrolysis and condensation formed poly(3-mercaptopropylsilsesquioxane), which entangled with the textile fibers and encapsulated the liquid-metal droplets. The printed structure was electrically insulating before activation. Local pressure ruptured selected droplets and exposed fresh EGaIn, creating a conductive surface only where contact was required; patterned activation of 100 sites produced a spatially uniform low-resistance array. The activated interface retained its electrical properties after 500 tape-peel cycles. In 24 h human umbilical vein endothelial cell (HUVEC) culture, the complete electrode showed 97.09% cell viability. Its water-vapor transmission rate (WVTR) exceeded 1,500 g m⁻² day⁻¹, substantially higher than the approximately 490 g m⁻² day⁻¹ of commercial gel electrodes. After continuous 6 h wear, the S-GIPE left no obvious skin marks, whereas the commercial electrodes produced visible imprints and adhesive residue. As shown in Figure 6B, the S-GIPE maintained substantially greater breathability than commercial gel electrodes during continuous 6 h wear. The electrode also acquired high-fidelity EEG after seven-day exposure to air, water, or artificial sweat and recorded a continuous 6 h sleep period with distinguishable time-frequency features across sleep stages. The corresponding time-frequency EEG spectrum acquired during sleep monitoring is presented in Figure 6C.

Liu et al. designed semi-liquid-metal (SLM) fiber electrodes for rapid ECG acquisition in emergency and contaminated-skin environments.⁸² To reduce the excessive fluidity of EGaIn, silver-coated copper microparticles were incorporated at an optimized content of 15%, producing an SLM with a conductivity up to 9 × 10⁶ S m⁻¹ while retaining deformability. A poly(methacrylic acid) adhesive layer promoted bonding between the oxide-covered SLM and a pre-stretched polyurethane fiber. As shown in Figure 6D, the resulting SLM-coated PU fiber was configured into different skin-contact geometries, including single-strand, parallel, and double-helical structures. The authors compared single-strand, parallel, braided, and double-helical cores; the double-helical structure carried a larger SLM loading, reached an elongation at break of approximately 570%, and distributed strain more uniformly along the coating. Instead of using a broad adhesive patch, the SLM fiber was wrapped around a finger or limb so that elastic constriction supplied stable contact, as shown in Figure 6E. The fiber remained conductive during stretching, pressure, and repeated touching, and showed stable resistance during 14-day monitoring. Simultaneous ECG measurements produced waveforms comparable to those obtained using conventional Ag/AgCl electrodes, as shown in Figure 6F. Figure 6G further compares the peeling force of the SLM fiber electrode with that of conventional Ag/AgCl patch electrodes under simulated challenging skin conditions. Under hair, water, oil, mud, and dust contamination, conventional Ag/AgCl patches lost adhesion, whereas the wrapped SLM electrode maintained a peeling force of approximately 7.3–9.1 N cm⁻¹. Because the contact occupied only narrow fiber lines, the surrounding skin remained open to air, supporting rapid deployment and comfortable postoperative monitoring.

The S-GIPE and SLM-fiber electrodes address different limitations of conventional dry electrodes.⁸⁴ S-GIPE uses a breathable textile and pressure-selective activation to combine low-impedance contact with a stable particle reservoir, whereas the SLM fiber uses mechanical wrapping to maintain contact when adhesive patches fail. Their reported 6-h EEG, environmental aging, cytotoxicity, and contaminated-skin tests provide evidence for the intended wearable scenarios, while longer repeated-wear studies remain necessary to determine how friction, sweat, oxide regeneration, and occasional liquid-metal exposure affect the interface over extended use.

System-level optimization strategies

For biological applications, optimization of EGaIn should be understood primarily as control of risk and long-term interface stability.⁸⁵^,⁸⁶ The fluid conductor can accommodate deformation, but an unconfined or poorly designed epidermal interface may occlude the skin, trap sweat, and destabilize signal quality during prolonged wear.⁸⁷^,⁸⁸^,⁸⁹^,⁹⁰

Li et al. addressed these issues through an intelligent breathing electronic skin inspired by Nepenthes.⁸³ As illustrated in Figure 6H, the device was built on an electrospun Janus SBS/PT (SPT) membrane comprising a skin-facing hydrophobic styrene-butadiene-styrene (SBS) layer and an outer hydrophilic polyacrylonitrile (PAN)/TPU (PT) layer, with screen-printed EGaIn-based conductive patterns forming the complete SPTL e-skin. As shown in Figure 6I, digital photographs obtained after 2 days of continuous coverage on a volunteer’s forearm showed that the SPTL-covered region remained comparatively close to its initial skin condition, whereas cloth tape, PVC tape, and a commercial Ag/AgCl electrode produced more evident occlusion- or adhesive-related skin changes. The asymmetric wettability of the fibrous membrane produced directional sweat transport from the skin side toward the hydrophilic outer layer while maintaining interconnected pathways for gas and water-vapor exchange. Figure 6J further shows that the air permeability of SPT and SPTL reached 25.16 and 20.02 mm s⁻¹, respectively, compared with only 0.82 mm s⁻¹ for PVC tape and 0.77 mm s⁻¹ for cloth tape. As shown in Figure 6K, their water-vapor transmission rates were also high, reaching 865.02 g m⁻² day⁻¹ for SPT and 890.46 g m⁻² day⁻¹ for SPTL. Thus, rather than relying only on geometric porosity of the conductor, this strategy integrates a wettability gradient, a permeable fibrous scaffold, and patterned EGaIn routing to actively manage perspiration while preserving an electrically functional epidermal interface.

The SPTL study further linked this breathing architecture with short-term skin compatibility and multimodal electrophysiological operation. Together with the measured air permeability and moisture permeability (WVTR), these observations support the role of directional sweat removal and vapor transport in reducing moisture accumulation at the skin-device interface. The platform was also configured for electromyography (EMG)/ECG and EEG/electrooculography (EOG) acquisition, demonstrating that the breathable membrane could serve as a functional bioelectronic substrate rather than only as a passive permeable film. However, the 2-day photographic comparison is a short-term visual assessment and does not provide histological evidence, inflammatory-marker quantification, or chronic sensitization data. Accordingly, the study supports reduced occlusion-related irritation and improved wearing comfort, but it should not be interpreted as establishing the absence of inflammation during long-term repeated use.

Seo et al. addressed the complementary risk of leakage through ligand-bound liquid-metal particles (LB-LMPs) and self-packaging phase separation.¹⁷ The fabrication of LB-LMP-based stretchable multilayer circuits and their large-scale integration are illustrated in Figure 6L. Amine-based ligands bound to the particle surface during ink preparation and suppressed further oxide growth, improving storage stability. During solvent evaporation, ligand desorption promoted in situ contact and sintering between particles. At the same time, vertical phase separation formed an LM-rich conductive region beneath a polymer-rich insulating surface, automatically creating a packaging layer without a separate encapsulation step as illustrated in Figure 6M. The resulting conductors exceeded 8.75 × 10⁶ S m⁻¹. As shown in Figure 6N, the LB-LMP films maintained stable resistance as a function of ink aging time, supporting their long-term ink-storage stability. The conductors also preserved their initial resistance after 10,000 cycles at 100% strain. As shown in Figure 6O, tape-peel testing demonstrated strong interfacial adhesion and no visible leakage from the LB-LMP/polymer film after peeling. A four-layer printed circuit containing 26 chips and 24 vertical interconnects remained leakage-free at 215% strain. The process was compatible with multiple polymer matrices and enabled stretchable multilayer circuits and large-area interconnects.

For bio-interfaces, the authors further combined the self-packaged LB-LMP conductor with a PEDOT:PSS-containing surface electrode. This division of function is important for safety: the liquid-metal-rich region supplies highly deformable routing, while the PEDOT:PSS/polymer layer provides a more controlled biological and electrochemical contact. The hybrid electrode improved fibroblast viability compared with bare LMP electrodes, produced only minimal inflammatory response after two-week dorsal subcutaneous implantation in mice, and stimulated the sciatic nerve at voltages down to approximately 60 mV. These results indicate that packaging and material-role separation can reduce direct liquid-metal exposure. Nevertheless, short subcutaneous implantation, tape peeling, and tensile cycling do not reproduce chronic organ motion, sterilization, fatigue cracks, or quantitative Ga/In release after barrier damage. Because the conductive layer is intentionally concealed beneath an insulating surface, defects at vias, cut edges, electrode openings, and component joints become the most probable paths for leakage.

Taken together, the SPTL and LB-LMP studies show that no single optimization strategy resolves every biological risk. The SPTL architecture prioritizes air and water-vapor transport together with directional sweat management for epidermal interfaces, whereas the LB-LMP approach prioritizes conductor containment, oxide control, and self-packaged leakage resistance. Optimization must therefore match the intended interface. Skin electrodes require sufficient permeability, controlled exposed area, stable adhesion or conformal contact, abrasion resistance, and repeated-wear evaluation under sweat. Implants require continuous barriers, sterilization compatibility, cyclic soaking under mechanical strain, quantitative Ga/In release measurements, and post-explant inspection. In both cases, EGaIn is most effectively used as a deformable conductive element while the surrounding scaffold, polymer package, hydrogel, PEDOT:PSS layer, or established metal contact is engineered to control the biological interface.

Semiconductor contact engineering

Low-damage contact formation

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and organic self-assembled monolayers (SAMs) are extremely sensitive to interfacial states. Conventional physical vapor deposition (PVD) techniques involve high-energy adatoms that physically penetrate SAMs, causing electrical shorts, and create deep-level defects in 2D materials. These defects severely exacerbate Fermi-level pinning, typically resulting in a low pinning factor (|S| ∼ 0.1–0.3). While intermediate strategies such as transfer-printing or buffer layers were introduced to mitigate this damage, they often suffer from residue contamination or increased tunneling resistance.

To achieve truly non-destructive metallization, EGaIn has been adopted. Due to its spontaneously formed, self-limiting native oxide skin (∼0.7 nm thick), EGaIn maintains a moldable shape, providing a soft yet conformal electrical contact. Chiechi et al. first demonstrated this by employing EGaIn tips as top electrodes for SAM-based molecular junctions. This soft-contact approach avoided the kinetic penetration of metal atoms, dramatically increasing the yield of working junctions to 70–85% (compared to ∼25% for conventional mercury and nearly 0% for directly evaporated metals) without perturbing the underlying molecules.⁹¹

Extending this paradigm from organic molecules to 2D materials, Xiong et al. vacuum-injected liquid metal into the microchannels of MoS₂ transistors. By entirely avoiding energetic deposition, they achieved an atomically smooth interface and significantly suppressed defect-induced Fermi-level pinning (Figure 7A). Consequently, they demonstrated a significantly mitigated Fermi-level pinning effect, achieving a factor of |S| = 0.7, which approaches the ideal Schottky-Mott limit and confirms that liquid metals can reliably preserve the intrinsic properties of fragile interfaces.⁹²

EGaIn-Enabled Semiconductor Contact Engineering: Low-Damage Formation, Conformal Interfaces, and Oxide-Mediated Barrier Effects

(A) Flexible MoS₂ transistors based on solid-liquid contacts. LM and ionic liquid are encapsulated in microfluidic channels as the electrode and dielectric, respectively, to form pristine interfaces that avoid Fermi-level pinning and charge trapping. This structure achieves a near-ideal subthreshold swing of 60.7 mV/dec, outperforming conventional electrolyte-gated MoS₂ devices. Reprinted with permission from ref.⁹² Copyright (2025) John Wiley & Sons.

(B) Schematic illustrations of an ideal molecular junction without defects and a defective junction with a disordered SAM layer on a rough bottom electrode, alongside the equivalent circuit model of the junction. Adapted from ref.⁹³ Fig.1 with permission from the Royal Society of Chemistry.

(C) Schematic illustration of the fabrication process for EGaIn source/drain (S/D) electrodes by the casting method. The process involves Pt deposition on a glass substrate, followed by EGaIn casting and the subsequent formation of a Pt-EGaIn alloy, where Pt serves as a buffer layer during EGaIn diffusion. The corresponding work function information for Pt, EGaIn, and the resulting Pt-EGaIn alloy is also presented.

(D) Energy band diagrams of Pt, h-BN, and MoS₂, along with the band-bended energy band diagrams of the Pt/h-BN/MoS₂ and Pt-EGaIn/h-BN/MoS₂ metal insulator semiconductor (MIS) structures under a thermal equilibrium state. Adapted with permission from ref.⁹⁴ Copyright (2026) American Chemical Society.

Conformal metal/semiconductor interface

When solid metal electrodes are mechanically attached to a semiconductor, nano- and microscale surface roughness inevitably produces microscopic air gaps that degrade contact characteristics. In contrast, EGaIn flows seamlessly along complex surface topographies to establish a void-free, conformal metal/semiconductor interface.

Zhao et al. utilized EGaIn as an “adaptive-deformable electrode” for microscale perovskite single crystals, which exhibit considerable surface roughness and are highly vulnerable to physical pressure.⁹⁵ The liquid electrode achieved complete conformal coverage over the surface curvature without damaging the crystal, thereby preventing charge trapping caused by geometric non-uniformity. Furthermore, the freely repositionable nature of the contact enabled the unambiguous identification of contact-geometry- and pathway-dependent charge transport within a single crystal.

Oxide-mediated barrier effects

The nanometer-thick native oxide (Ga₂O₃) that forms instantaneously on EGaIn functions as a tunneling barrier governing charge transport at semiconductor junctions, giving rise to unique oxide-mediated barrier effects.

As depicted in Figure 7B, in molecular tunnel junctions (Ag^TS/SAM//Ga₂O₃/EGaIn), Suchand Sangeeth et al. decoupled the individual circuit components and revealed that bottom-electrode-induced defects affect only the SAM resistance, leaving the contact resistance and SAM capacitance unaffected. This established the Ga₂O₃-mediated contact as a well-defined and reproducible circuit element.⁹³

Beyond acting as a passive resistive element, the oxide can also serve as an active medium for device control. As depicted in the electrode fabrication and energy band structures in Figures 7C and 7D, Guo et al. demonstrated that in 2D semiconductor vertical diodes, the direction of bipolar current rectification mediated by the ultrathin Ga₂O₃ layer can be actively switched in situ by modulating the contact area through mechanical pressure.⁹⁶ The native oxide thus functions as a key enabler for reconfigurable switching devices.

Alloying-assisted contact modification

The chemical reactivity and room temperature diffusivity of EGaIn enable alloying-assisted contact modification at low temperatures. This serves as an alternative to conventional high-temperature contact formation processes, which severely degrade flexible devices and emerging channel materials possessing limited thermal budgets.

Kim et al. demonstrated this by depositing a thin Pt buffer layer above the MoS₂ channel of a top-gated metal-insulator-semiconductor (MIS) transistor and subsequently casting EGaIn on top. This induced spontaneous “dynamic metal alloying”, wherein EGaIn penetrates the Pt electrode to form a Pt-EGaIn alloy entirely at room temperature.⁹⁴ By physically rearranging the electrode composition without imposing thermal or mechanical stress on the underlying channel, this approach provides a robust pathway toward stable electrode-semiconductor integration without requiring high-temperature annealing.

Effective-Contact-State Control, Work Function Tuning, and quantum interference

To achieve reliable control over charge transport and energy-level alignment at the single-molecule level, establishing a stable electrode interface is crucial. To evaluate the charge-transport characteristics of self-assembled monolayer (SAM)-based tunneling junctions in molecular electronics, Nijhuis et al. introduced the liquid metal EGaIn, which spontaneously forms a ∼0.7 nm thick Ga₂O₃ surface film, as a top electrode, and schematically compared the performance of junctions based on microchannels (cross-bar arrays) and cone-shaped tips.⁹⁷ Even though, owing to surface roughness, the actual area of electrical contact is only ∼20–50% of the geometric footprint, this system showed very high yields of working junctions of 70–85%, compared with ∼25% for conventional mercury electrodes. Measurements on SAMs of n-alkanethiolates consistently yielded a tunneling decay constant β ∼ 1.0 nC⁻¹ (or ∼0.80 Å⁻¹) and a pre-exponential factor J₀ ∼ 3.0 × 10² A/cm², demonstrating that charge transport is dominated by the intrinsic structure of the SAM rather than by defects in the electrodes (Figure 8A).

Effective-Contact-State Control via Alloying-Assisted Modification, Work Function Tuning, and Quantum Interference

(A) Figure 1. Plots of vs. V, and histograms of log(|J|)at−0.50 and −0.20 V (with Gaussian fits) for junctions incorporating SAMs of SC₁₁CH₃ (1–3), SC₁₅CH₃ (4–6), SC₁₄CH₃ (7–9), and SC₁₇CH₃ (10–11). N_|J| indicates the total number of measured values of current density plotted in the histogram. The data for junctions with SAMs of SC₁₁CH₃ or SC₁₇CH₃ SAM have been reproduced by two researchers. Adapted with permission from ref.⁹⁷ Copyright (2012) American Chemical Society.

(B) Schematic illustration of EGaIn tunneling junctions incorporating pyridyl- and phenyl-terminated self-assembled monolayers (SAMs) on Au. The single-atom substitution (N versus C-H) at the terminal ring significantly alters the molecular dipole, which in turn induces distinct work function shifts (ΔΦ) and different transition voltages (V_trans). Adapted with permission from ref.⁹⁸ Copyright (2013) American Chemical Society.

(C) Schematic of the tunneling junctions (not to scale) of gold on-mica supporting SAMs of thiolated arylethynylenes with cores of anthracene (AC; linear-conjugation), 9,10-anthraquinone (AQ; cross conjugation), or 9,10-dihydroanthracene (AH; broken-conjugation) connected at the 2,6 positions (indicated with gray circles). The thiolate groups at the Ga-In/Ga₂O₃ interface comprise a random mixture of free thiols and thioacetates.

(D) Optimized geometries of the AC, AQ, and AH molecules employed in the transport calculations. To account for thermal flexibility at ambient temperature, two distinct conformations of AH are presented (bottom). In all configurations, the molecules are chemisorbed onto the face centered cubic (FCC) hollow sites of the gold electrodes.

(E) Left: plots of the geometric mean (lines) and Gaussian (μlog) mean (symbols) of log|J| versus V for AC (solid line; 9), AH (dotted line; 1), and AQ(dashed line; b). Right: plots of the normalized histograms of log|J| at 0.4 V and the Gaussian fits for AC (top; solid line), AH (center; dotted line), and AQ (bottom; dashed line). The value of the geometric mean of log|J| is indicated with a solid arrow, and n is the total number of traces. These data reveal no appreciable difference between the geometric and Gaussian means for AH and AQ and clearly show that we cannot make a meaningful distinction between AH and AQ. The data for AC, however, form a truncated Gaussian distribution such that the Gaussian mean is more than an order of magnitude higher than the geometric mean. In either case, AC is clearly more conductive than either AH or AQ. Adapted with permission from ref.⁹⁹ Copyright (2011) American Chemical Society.

Building on this stable EGaIn junction system, Fracasso et al. compared the energy-level alignment and the work function shift of pyridine-based SAMs (Py-SAMs), in which a single carbon-hydrogen (C-H) bond of the phenyl ring of oligo(phenylene) molecules bearing alkanethiol tails is replaced by nitrogen (N), against the parent phenyl-based structure (Ph-SAM).⁹⁸ The two series gave β values of 0.42 ± 0.08 Å⁻¹ and 0.44 ± 0.04 Å⁻¹, respectively, which are not statistically different; therefore, the conductance data (J/V, β, J₀) alone could not distinguish the two series. However, because of the difference in molecular dipole direction, the work function shift (ΔΦ) of the metal surface was induced in opposite senses, ranging from +0.05 to +0.34 eV for the Ph-SAMs and from −0.11 to −0.39 eV for the Py-SAMs. As a result, the transition voltage (V_trans) of the Ph-SAMs was ∼0.3 V larger, confirming that even a single-atom substitution modulates the offset between the Fermi energy (E_f) and HOMO level (the highest occupied molecular orbital, E_HOMO), as reflected in V_trans, so that the two series can be differentiated without any prior knowledge (Figure 8B).

Furthermore, Fracasso et al. probed quantum interference effects within the monolayer by holding the physical thickness of the molecules fixed at ∼2.4 nm and modulating only the conjugation pattern of the anthracene core, comparing three structures: linear-conjugation anthracene (AC), cross-conjugation anthraquinone (AQ), and broken-conjugation 9,10-dihydroanthracene (AH) (Figure 8C).⁹⁹ AQ, in which destructive quantum interference occurs, and AH, in which the conjugated pathway is broken, exhibited similar current densities of J ≈ 10^–1 A/cm² at 0.4 V. In contrast, AC, which shows no interference feature, recorded a significantly higher current density. Depending on the statistical analysis method used, the current density of AC was evaluated to be one order of magnitude higher when derived from the log-geometric average, and up to two orders of magnitude higher based on Gaussian fits (Figure 8E). This is in qualitative agreement with Green’s-function-based density functional tight-binding (gDFTB) transport calculations (Figure 8D), clearly showing that charge transport is dominated by the through-bond pathway within the molecule rather than by molecule-molecule interactions. Overall, Table 3 summarizes representative EGaIn-enabled contact-engineering strategies across molecular and semiconductor devices, highlighting how soft-contact formation, conformal interfacing, oxide-mediated transport, dynamic alloying, and molecular-level electronic effects collectively govern charge transport and interfacial performance.

Table 3.

Summary of EGaIn-enabled contact engineering strategies in molecular and semiconductor devices

Target material and device	EGaIn contact strategy	Underlying mechanism	Key findings	Reference
SAM-based molecular junctions	EGaIn tips as top electrodes	soft-contact avoiding kinetic penetration	yield of working junctions increased to 70–85%	Chiechi et al.⁹¹
MoS₂ transistors	vacuum-injected into microchannels	atomically smooth interface avoiding energetic deposition	suppressed defect-induced Fermi-level pinning (‖S‖ = 0.7)	Xiong et al.⁹²
microscale perovskite single crystals	adaptive-deformable electrode	conformal coverage preventing charge trapping	contact-geometry- and pathway-dependent charge transport	Zhao et al.⁹⁵
molecular tunnel junctions	Ga₂O₃ mediated contact	decoupled individual circuit components	defects affect only the SAM resistance	Sangeeth et al.⁹³
2D semiconductor vertical diodes	modulating contact area through mechanical pressure	mediation by the ultrathin Ga₂O₃ layer	actively switched in situ bipolar current rectification	Guo et al.⁹⁶
top-gated MIS transistors (MoS₂)	EGaIn cast on a thin Pt buffer layer	dynamic metal alloying at room temperature	stable integration without high-temperature annealing	Kim et al.⁹⁴
SAM-based tunneling junctions	microchannels and cone-shaped tips	transport dominated by intrinsic SAM structure	consistent tunneling decay constant ( $β \sim 1.0 n_{C}^{- 1}$ )	Nijhuis et al.⁹⁷
pyridine- and phenyl-based SAMs	single-atom substitution	dipole-induced work function shift (ΔΦ)	differentiation enabled by transition voltage (V_trans)	Fracasso et al.⁹⁸
arylethynylene thiolate SAMs	modulating the conjugation pattern	through-bond pathway and quantum interference	one to two orders of magnitude higher current density	Fracasso et al.⁹⁹

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Outlook

EGaIn-based liquid metals are increasingly being developed beyond passive soft conductors toward functional interfacial platforms. From this perspective, liquid metals should be regarded not merely as deformable conductors, but as interfacial design platforms whose behavior can be tailored through wetting, oxide chemistry, and interfacial reactions.⁸^,⁹^,¹⁰ As summarized in Figure 9, two promising directions emerge from this perspective, including wetting-mediated contact reconfiguration and low-thermal-budget semiconductor integration.

Interfacial design strategies for future liquid-metal electrodes

(A) Surface-chemistry-dependent wetting states illustrating the tunability of liquid-metal contacts from partial wetting to complete wetting or non-wetting. Adapted with permission from ref.¹⁰ Copyright (2025) American Chemical Society, (B) Liquid-metal-derived ultrathin oxide layers enabling low-temperature integration and transfer of dielectric materials onto two-dimensional semiconductors. Adapted with permission from ref.¹⁶ Copyright (2024) American Chemical Society.

One important opportunity is the development of wetting-mediated reconfigurable contacts. As shown in Figure 9A, Zhang et al. demonstrated that the interfacial state of a liquid metal can vary from partial wetting to complete wetting or non-wetting depending on surface chemistry, indicating that contact formation in liquid-metal systems is not necessarily fixed but can instead be programmed through interfacial control.¹⁰ Looking ahead, such wetting tunability may enable more than passive adhesion or geometric conformity; it may provide a route toward active regulation of effective contact area, air-gap suppression, and mechanically persistent electrical coupling across soft, irregular, and dynamic surfaces. A key next step will therefore be to establish quantitative relationships between wetting-state transitions and their resulting electrical characteristics.

A second promising direction is low-thermal-budget semiconductor integration enabled by liquid-metal-derived interfacial layers. As highlighted in Figure 9B, Venkatakrishnarao et al. demonstrated that ultrathin liquid-metal oxides can provide a transferable platform for integrating functional dielectric layers with two-dimensional semiconductors, thereby offering a potential route toward low-damage and back-end-compatible processing.¹⁶ More broadly, such approaches suggest that liquid-metal interfaces may facilitate the heterogeneous integration of not only dielectric layers but also contacts, interconnects, and multifunctional interfacial stacks in emerging semiconductor systems, where direct deposition or high-temperature processing can readily degrade interfacial quality.

Despite these opportunities, the practical deployment of EGaIn-based liquid-metal interfaces still faces several unresolved challenges. Owing to its intrinsic fluidity and high surface tension, EGaIn is vulnerable to leakage, interfacial drift, and short-circuit risks unless robust encapsulation or self-packaging strategies are employed. In addition, Ga-induced corrosion or embrittlement of neighboring metals remains a critical materials-compatibility issue, particularly for long-term device operation and heterogeneous integration. The native oxide layer of EGaIn further presents a dual character: while it can support adhesion and interfacial stabilization, it can also introduce variability in wetting, contact resistance, and electrical reproducibility. Another important challenge is the lack of standardized reliability assessment protocols. Although many studies report conductivity, stretchability, or short-term stability, direct comparison between different liquid-metal systems remains difficult because testing conditions, encapsulation methods, and failure criteria are often inconsistent. As a preliminary framework, EGaIn based future studies are encouraged for the following research categories. 1) Electrical stability (initial resistance, contact resistance, resistance drift, and bias-dependent degradation) 2) Mechanical durability (strain range, bending radius, cyclic loading conditions, and the number of deformation cycles) 3) Interfacial and chemical stability (leakage threshold, oxide-state variation, corrosion, and embrittlement of adjacent metals for temperature, humidity, immersion) 4) Biocompatibility (long-term stability, inflammation, cytotoxicity). In addition, specimen dimensions, electrode composition, encapsulation architecture, test atmosphere, sample number, statistical variation, and explicit failure criteria should be consistently documented. Where possible, normalized metrics such as ΔR/R₀, specific contact resistivity, leakage pressure, failure cycles, and resistance drift per unit time should be used to enable quantitative comparison across different material systems and device platforms. These limitations suggest that future progress will depend not only on demonstrating new device functionalities, but also on establishing reliable encapsulation strategies, chemically stable barrier layers, oxide-state control methods, and standardized reliability metrics for leakage, corrosion, interfacial drift, and long-term operational stability.¹⁷^,¹⁸^,¹⁹

Ultimately, further progress in liquid-metal electrode research will depend on the ability to translate their concepts into predictive, quantitatively controlled, and application-relevant interface engineering strategies. Reconfigurable wetting and low-temperature integration should therefore be viewed not as isolated phenomena, but as complementary elements of a broader framework for transforming static electrical contacts into adaptive and programmable interfaces.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIT) (grant nos. RS-2026-25487234 and RS-2025-16071033). It was also partly supported by research funds for newly appointed professors of Jeonbuk National University in 2025.

Author contributions

G.S.K., H.Y.L., M.B., and S.M. contributed equally to this work. G.S.K., H.Y.L., Y.T.L., and J.-H.K. were responsible for the conceptualization of the paper, data curation, illustrations, and writing of the sections “electrical interfaces” and “bioelectrode and soft-contact interfaces”. They contributed to the review and editing process, with Y.T.L. and J.-H.K. additionally providing supervision during the development of these sections. M.B. and B.-I.P. were responsible for data curation, illustrations, and writing of the sections “theoretical and interfacial foundations of EGaIn” and “semiconductor contact engineering”. They contributed to the review and editing process, with B.-I.P. additionally providing supervision during the development of these sections. S.M. contributed to the conceptualization, data curation, and writing of the content related to wearable electronic devices and physiological signal monitoring, as well as the critical review and editing of the manuscript. All authors have read and approved the final manuscript.

Declaration of interests

The authors have no conflicts of interest to declare.

Contributor Information

Young Tack Lee, Email: ytlee@inha.ac.kr.

Bo-In Park, Email: bipark@jbnu.ac.kr.

Ji-Hoon Kang, Email: jhkang57@inha.ac.kr.

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PERMALINK

EGaIn-based liquid-metal electrodes for electronic interfaces and contact engineering

Gyeong Seop Kim

Ho Yun Lee

Mileu Bang

Sungchul Mun

Young Tack Lee

Bo-In Park

Ji-Hoon Kang

Summary

Graphical abstract

Introduction

Theoretical and interfacial foundations of EGAIN

Formation and mechanical-thermodynamic effects of the native oxide skin

Figure 1.

Environment-dependent wetting, dewetting, and adhesion mechanics

Impact of surface topography on interfacial behavior

Oxide skin elimination and dynamic adjustment

Solid-liquid electrical contact resistance

Reactive wetting and intermetallic micro-mechanisms

Figure 2.

Electrical interfaces

Table 1.

EGaIn conformal contact

Figure 3.

Printed electrodes

Flexible and stretchable interconnects

Figure 4.

Bioelectrode and soft-contact interfaces

Table 2.

Implantable electrodes

Figure 5.

Dry bioelectrodes

Figure 6.

System-level optimization strategies

Semiconductor contact engineering

Low-damage contact formation

Figure 7.

Conformal metal/semiconductor interface

Oxide-mediated barrier effects

Alloying-assisted contact modification

Effective-Contact-State Control, Work Function Tuning, and quantum interference

Figure 8.

Table 3.

Outlook

Figure 9.

Acknowledgments

Author contributions

Declaration of interests

Contributor Information

References

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