Biphasic liquid metal composites as soldering systems for robust soft–rigid interfacing in stretchable hybrid electronics

Abstract

Stretchable hybrid electronics are integral in numerous domains such as healthcare, soft robotics, and human–machine interfaces. However, their development encounters significant challenges under mechanical deformation, primarily due to stress concentration at soft–rigid interfaces. Structural engineering or rigid-filler composites, as conventional soldering solutions, face critical limitations including restricted strain tolerance and inherent trade-offs between conductivity and stretchability. Intriguingly, liquid metals (LMs) can offer fluidic conductivity and extreme stretchability. By further hybridizing with polymers or particulates, biphasic LM composites have emerged as advanced soldering systems to realize robust soft–rigid connections, thus moving toward fabrication of reliable stretchable hybrid electronics. This article reviews recent biphasic LM composites serving as soldering systems for hybrid electronic integration. Key design considerations in fabrication of competent solders are firstly discussed. Next, various material combinations in the biphasic LM composites, as well as methods used to connect and weld dissimilar functional components, are discussed. Finally, the current challenges and future perspectives of these LM-based soldering systems are proposed.

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Jie Li

Jie Li is currently pursuing a BS degree at the Suzhou University of Science and Technology. His research interests focus on the design and application of biphasic LM-based soldering systems.

Kai Zhao

Kai Zhao is a lecturer in the School of Materials Science and Engineering at the Suzhou University of Science and Technology. He received his PhD degree in 2022 from the Dalian University of Technology. His scientific interests include liquid metal-based flexible electronics and devices for wearable sensing applications.

Changqing Ye

Changqing Ye is currently a professor at the Suzhou University of Science and Technology. He received his PhD degree in materials science and engineering from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2012. His present research is focused on flexible and wearable materials and devices, bioinspired photonic crystals and applications of highly efficient low-power upconversion.

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1. Introduction

Stretchable hybrid devices have subversively changed the field of traditional solid electronics by providing improved flexibility, comfort and deformability.^1–4 We have now witnessed their great potential in numerous domains, such as medical healthcare,⁵ soft robotics⁶ and human–machine interfaces.⁷ Typically, they are obtained via assembling commercially available rigid electronic devices (e.g., batteries, microcontrollers, and amplifiers) and stretchable electrical interconnects onto flexible substrates for data acquisition and communication. However, most of these hybrid electronics have not satisfactorily performed under deformation and suffer from inferior stretchability limited by the prominent discrepancy in the modulus between soft conductors and rigid electronics.^8–10 In this context, the soft–rigid connections tend to experience severe stress concentration upon stretching due to the interface mismatch, which lead to irreversible electrical degradation and permanent debonding failures.¹¹ Therefore, it is highly desirable yet challenging to develop soldering systems that reliably interface with both rigid electronics and soft conductors/substrates under large deformations.

At present, much effort has been devoted to realizing robust soft–rigid connections via either structural designs or material innovations. On one hand, deterministic structures including serpentine,¹² wavy,¹³ buckling,¹⁴ and rigid-island,¹⁵ are being combined with rigid components to construct stretchable hybrid devices. Upon stretching, the strain energy is mainly consumed by structural evolutions, rather than affecting the functional components. However, the moderate critical strain (<200%) and component areal density of these structure-based strategies restrict their development. Material innovations, on the other hand, can cope with these issues by exploiting intrinsically stretchable materials or composites as soldering systems.¹⁶ Successful implementations are embodied by conductive composites, which are prepared by dispersing rigid conductive fillers (e.g., Au nanoparticles, Ag flakes, and carbon materials) into a polymer matrix.^17–19 However, such composites often exhibit undesirable electromechanical properties for soldering applications,^20,21 particularly resistance fluctuations under strain, which can be ascribed to the: (1) stress concentration at filler/matrix interfaces, causing interfacial debonding or microcracks; (2) disruption of percolation networks during deformation, as rigid fillers cannot accommodate large strains; (3) inherent trade-offs between filler loading (required for high conductivity) and composite flexibility, since a high filler content exacerbates interfacial stress and stiffens the matrix.

Compared to rigid filler-based composites, liquid metals (LMs), especially Ga-based alloys, stand out as promising candidates for the development of high-performance soldering systems due to their unique combination of fluidity and conductivity.^22–25 As a special class of fluids, they have exceptionally high electrical conductivity (∼10⁶ S m⁻¹) and extreme stretchability owing to their liquid states, avoiding the disintegration issues as observed in rigid filler-based solders.^26,27 Moreover, LMs can easily deform with the displacement of the welding position, ensuring electrical stabilities even under large deformation. Despite these merits, the high surface tension and low viscosity of LMs may lead to leakage and limited adhesion to most surfaces.^28,29 This creates a potential failure of the pure LM-based solders. Intriguingly, biphasic material architectures are recently proposed as a strategy to tackle these issues.³⁰ Such biphasic systems combine superiorities of solid-phase materials in terms of stability and scalability, with deformability and self-healing of liquids. Guided from this, a series of biphasic LM composites have been explored by researchers, i.e., solid–liquid mixtures of Ga-based LMs, with various solid materials that comprise particles, polymers or their combinations.^31–33 These composites have been proven effective in establishing robust soft–rigid connections via their hybrid characteristics, thus moving toward reliable soldering of stretchable hybrid electronics.

Unlike conventional interface materials, biphasic LM soldering systems are expected to unify electrical conduction, mechanical compliance, dynamic adaptability and recyclability into a single multifunctional platform. This transforms passive junctions into active, strain-invariant interconnects essential for stretchable hybrid electronics. Considering the importance of LM hybrids in soldering systems, systematic discussion and summary regarding this field are highly valuable. In this review, we present recent advancements in biphasic LM composites serving as solders for stretchable hybrid electronics (Fig. 1). The key attributes of high-performance soldering systems are first introduced. Then, different kinds of biphasic LM composites for electrical interfacing in stretchable electronics are separately discussed, with their design and soldering principles highlighted. Finally, we elucidate the future perspectives of biphasic LM systems, along with some challenges that need to be overcome especially in welding stretchable hybrid devices. We believe that their rapid development is expected to revolutionize the field of advanced electronic integration.

Fig. 1 Schematic illustration depicting the structure and versatile applications of a biphasic LM-based soldering system.

2. Design considerations of advanced soldering systems

For welding applications in flexible scenarios, the design of competent soldering systems needs to consider both functionality and mechanical compliance concurrently. As the vital component of stretchable hybrid electronics, advanced solders have several key attributes that need to be taken into consideration during preparation, i.e., electrical characteristics, flexibility/stretchability, high adhesion, recyclability and patterning processability (Fig. 2), which will be elaborated on below.

Fig. 2 Overview of the key design considerations of an advanced soldering system. Image for electrical property. Reproduced with permission.³⁴ Copyright 2025, Wiley-VCH. Image for flexibility and stretchability. Reproduced with permission.³⁵ Copyright 2024, Springer Nature. Image for high adhesion. Reproduced with permission.³⁶ Copyright 2022, Wiley-VCH. Image for recyclability. Reproduced with permission.³⁷ Copyright 2024, Wiley-VCH. Image for patterning processability. Reproduced with permission.³⁸ Copyright 2024, Wiley-VCH.

2.1. Electrical properties

Electrical properties are a key parameter for constructing competent solders. While they are not directly responsible for signal acquisition and communication, their electrical characteristics are crucial in determining the sensitivity, stability, and overall fidelity of the entire hybrid system.³⁹ In general, conductivity and electromechanical performance are its two aspects. A conductivity >1000 S cm⁻¹ is essential to minimize resistive power losses (<5%) and high-frequency signal distortion (<3 dB attenuation).^40–42 This threshold ensures compatibility with commercial rigid components (e.g., μLEDs and IC chips) while preventing Joule heating-induced degradation. In terms of electromechanical coupling, it is equally important for solders to maintain their electrical conductance during deformations.⁴³ On this basis, inconsistent and unreliable performance of systems can be efficiently avoided, enabling excellent stability and durability.

2.2. Flexibility and stretchability

The mechanical properties of solder materials, particularly their flexibility and stretchability, are critical for flexible hybrid electronics. These devices, which find applications in wearable technologies, biomedical devices, and stretchable displays, require materials that can endure repetitive bending, cyclic stretching, and mechanical deformation without compromising their functional integrity.^44,45 To ensure robust performance, the solder materials must exhibit exceptional strain capacity (>100%), thereby effectively dissipating the strain energy during large deformation while safeguarding the functionality of the embedded rigid components responsible for data processing and conversion.

2.3. High adhesion

High adhesion properties are critical for soldering systems, as they ensure robust electrical and mechanical integration between stretchable interconnects and rigid chips.⁴⁶ In flexible hybrid electronics, strong adhesion is essential to prevent delamination and debonding failures at soft/rigid interfaces, even when components are subjected to repeated bending, stretching and distortion. Furthermore, effective adhesion enhances the reliability of the system by enabling efficient dissipation of the strain energy, thereby protecting delicate electronic components from mechanical damage. Thus, achieving high adhesion strength while maintaining flexibility is vital for ensuring the long-term stability, durability, and performance of flexible electronic devices.

2.4. Recyclability

Under the guidance of green economic policies, the recycling of flexible hybrid electronics has become a focal point of attention. As a crucial enabler, the development of recyclable soldering systems holds significant importance. This appealing attribute not only mitigates environmental pollution caused by the widespread disposal of electronic waste (e-waste), but also facilitates the efficient recovery of valuable resources contained within e-waste.^47–49 This approach aligns with global initiatives aimed at advancing sustainable development. Consequently, the integration of degradable and recyclable soldering systems represents a critical step toward the realization of environmentally friendly and sustainable electronic technologies.

2.5. Patterning processability

The patterning processability of soldering systems is of paramount importance, particularly in high-precision, multi-pin chip welding applications. In such scenarios, achieving reliable electrical connections while avoiding unintended short circuits between adjacent pins presents a significant challenge.³⁵ To address this hurdle, advanced soldering systems with superior patterning processability offer a promising solution. These systems not only enhance the reliability and performance of electronic devices but also meet the growing demand for miniaturization and integration in modern electronics. Therefore, the patterning processability of soldering systems is essential for high-performance, high-density electronic packaging technologies.

3. Biphasic LM composites for advanced soldering systems

To overcome the challenges inherent in pure LM-based solders, significant advancements have been made through the engineering of biphasic LM composites. These hybrid systems innovatively integrate LMs with solid components including polymers, metals and particle fillers. To date, three predominant configurations have emerged as effective solutions for establishing robust electrical interfaces: (1) LM–solid filler composites, (2) LM–polymer composites, and (3) ternary LM–solid filler–polymer composites. These are introduced in this section and highlighted in terms of structure–property relationships that govern soldering performances.

3.1. LM–solid filler composites

LM–solid filler composites represent a distinct class of biphasic systems where solid fillers (particles, flakes or networks) are dispersed within LMs or vice versa, fundamentally differing from surface modification techniques (e.g., Ga₂O₃ oxide removal by HCl vapor)⁵⁰ or interface designs reliant on wettability tuning.⁵¹ In these composites, fillers directly modify bulk material properties through: (1) electrical percolation (filler–LM contacts bypass oxide barriers); (2) mechanical reinforcement (filler networks resist leakage); (3) interfacial adhesion (fillers enhance bonding to substrates). Therefore, this integrated filler–LM synergy overcomes the limitations of pure LMs (e.g., leakage and poor adhesion) by establishing percolation networks that ensure structural stability and metallic conductivity.

In general, metals such as silver (Ag) and copper (Cu) are widely adopted as solid phases due to their high conductivity and robustness. In 2023, Kim et al. engineered a stretchable soldering sticker through the laser-assisted fabrication of LM–AgNW thin films.⁵² This approach enabled exceptional electromechanical synergy by anchoring fragmented LM nanoparticles (LMNPs) at AgNW junctions via laser irradiation, establishing continuous conductive networks with nanoscale-interpenetrating architectures. As shown in Fig. 3a, the welded interfaces preserved stable conductivity while bridging flexible conductors and rigid components, maintaining both electrical continuity and structural integrity under extreme deformation. By incorporating a stiffness-gradient hydrogel substrate, they further demonstrated a flexible circuit board with high stretchability and durability, capable of mounting various functional chips.⁵³ Thanks to the gradient stiffness structure, a balance was successfully achieved between the substantial stiffness contrast and robust interfaces of rigid chips and LM–AgNW interconnects. Consequently, this substrate ensures the stable performance of mounted sensor chips under various stretching conditions (Fig. 3b).

Fig. 3 (a) LM–AgNW composite-based stretchable soldering sticker for stretchable electronics integration. Reproduced with permission.⁵² Copyright 2023, Wiley-VCH. (b) LM–AgNW composites integrated with a stiffness-programmed substrate for stretchable electronics integration. Reproduced with permission.⁵³ Copyright 2024, Wiley-VCH. (c) Biphasic LM composites containing a mixture of liquid and crystalline solids for hybrid electronics integration. Reproduced with permission.⁵⁶ Copyright 2021, Springer Nature. (d) Self-mixed biphasic LM composites for reliable soldering in neuromorphic circuits. Reproduced with permission.⁵⁸ Copyright 2024, Wiley-VCH.

Beyond metallic fillers, thermally transformed LM-derived solids can also serve as functional fillers. For example, thermal depletion of LMNP cores yields crystalline semiconductors (e.g., γ-Ga₂O₃).^54,55 Following this strategy, Liu et al. harnessed these solids in biphasic composites to achieve reliable interfaces.⁵⁶ As shown in Fig. 3c, the crystalline solids acted as wettability-enhancing fillers, forming mechanical interlocks at soft–rigid interfaces when penetrated under pressure. Meanwhile, they bridged the interparticle gaps of LMNPs, enabling stable electrical connections even at 500% strain. Besides, engineered alloys (e.g., bismuth indium tin (BiInSn)) can also serve as functional solid phases. For instance, Chen et al. designed a biphasic LM–BiInSn system embedded in micropillar channels, achieving 38.2 MPa compression tolerance while maintaining stretchability.⁵⁷ More recently, Lee et al. designed LM–solid composites through a self-mixed biphasic LMNP system using reduced graphene oxide (rGO)-encapsulated LMNPs as solid anchors.⁵⁸ Unlike composites requiring sintering or surface treatments, the self-mixed architecture leveraged rGO-solidified fillers to intrinsically bridge LM domains. As depicted in Fig. 3d, this enables reliable soldering in neuromorphic circuits with robust strain-insensitive signaling.

3.2. LM–polymer composites

Alternatively, liquid metals can be integrated with polymers to construct LM–polymer composites. Compared with solid counterparts, the inclusion of polymers not only enhances the colloidal stability of LMNPs and rheological tunability, but also facilitates robust interfacial adhesion between LM components and substrates^59,60— a critical feature for constructing reliable soldering systems. For example, Tang et al. developed an intrinsically adhesive solder through embedding LMNPs within acrylate polymer matrices.³⁶ The peeling-induced phase separation created a Janus architecture, where the LMNPs enriched on the surface were responsible for forming electrical connections, while the polymer matrix served to adhere to different surfaces (Fig. 4a). On this basis, such a solder enabled in situ welding of rigid components onto the skin with exceptional interfacial strength, which could remain functional even under large deformations. Besides, Song et al. presented stretchable bilayer interfaces featuring a nanoscale styrene–ethylene–butylene–styrene (SEBS) top layer and a SEBS–LM bottom composite (Fig. 4b).⁶¹ Owing to its self-adhesiveness, the top SEBS layer could form instant and strong connections with different surfaces through gentle pressure. Meanwhile, continuous electrical pathways were also realized via synergistic electron tunneling effects in the ultrathin SEBS layer, which effectively coupled with the LMNP networks in the underlying composite. Thus, such bilayer composites showed good weldability with reliable electrical interfaces.

Fig. 4 (a) Intrinsically sticky solder capable of in situ welding of electronics. Reproduced with permission.³⁶ Copyright 2022, Wiley-VCH. (b) Bilayer LM–SEBS composites for electrical interfacing. Reproduced with permission.⁶¹ Copyright 2024, Royal Society of Chemistry. (c) LM-based self-solder with excellent weldability for stretchable electronics. Reproduced with permission.⁶⁶ Copyright 2023, Springer Nature. (d) Stretchable interconnects enabled by LMNPs confined within a supramolecular polymer. Reproduced with permission.³⁴ Copyright 2025, Wiley-VCH. (e) LM-UPy modified polymer composites with high adhesion. Reproduced with permission.³⁷ Copyright 2024, Wiley-VCH. (f) LM–dynamic IL composites with healing welding for integrated printed circuit boards. Reproduced with permission.⁶⁷ Copyright 2024, Wiley-VCH.

Intriguingly, LM–polymer composites also enable substrate-level self-healing. Chen et al. demonstrated an imprintable dynamic covalent elastomer with embedded LM patterns, achieving 94.3% mechanical self-healing efficiency and stable conductivity recovery after damage.⁶² Notably, recent studies have revealed that supramolecular polymers serve as versatile host matrices for LMs, facilitating the development of high-performance soldering systems.^63–65 The dynamic nature of their noncovalent interactions allows reversible dissociation and reorganization under external stimuli, thereby providing many advantages such as enhanced processability, intrinsic self-healing and efficient recyclability. Following this strategy, Ai et al. designed LM-based supramolecular polymeric composites as self-solders for hybrid electronic integration.⁶⁶ These composites were synthesized through the co-assembly of LMNPs with linear supramolecular polymers and ureidopyrimidinone (UPy)-functionalized small-molecule modulators. Remarkably, dynamic interactions within the solders enabled a series of unique merits, such as facile integration, circuit recyclability and chip recovery via mild heating. Consequently, the integrated circuit demonstrated superior stability and robustness, capable of enduring significant shear stress (Fig. 4c). On this basis, they designed supramolecular polymers with exceptional stretchability and toughness through hydrogen bond density optimization.³⁴ Further confinement within LMNPs not only stabilized conductive pathways but also enhanced surface adhesion, achieving a remarkable 2800% electrical stretchability. Therefore, the resulting LM composite-interconnected circuits demonstrated extraordinary stability under repeated mechanical stress (Fig. 4d), showing superior robustness and durability. In 2024, Cao et al. developed LM-UPy-modified polymer composites with broadly tunable adhesion.³⁷ As shown in Fig. 4e, these composites demonstrated superior adhesion from UPy-derived hydrogen bonds and enhanced cohesion through LMNPs. Such robust adhesion enabled reliable electrical connections with rigid chips, as evidenced by minimal LED brightness variation under strain.

More recently, dynamic ionic liquids (ILs) are being considered as promising polymer carriers for developing next-generation soldering systems owing to their molecularly designable and dynamic structures.^68–73 A notable example is the work by Chen et al., who engineered novel LM–dynamic IL composites using [EMIm] thioctic acid (TA) for integrated electronics applications.⁶⁷ As illustrated in Fig. 4f, the composite demonstrated exceptional interfacial adhesion through multiple interfacial interactions with substrates, with the incorporation of LMNPs providing additional reinforcement. The dynamic nature of the composites, enabled by disulfide bond exchange reactions, allowed successful integration of 42 μLEDs in parallel through healing welding and thermal-assisted debonding processes. In summary, LM–polymer composites intrinsically resolve soft–rigid mismatch through dynamic material responses, whereas hard–soft structures rely on extrinsic geometric design. This enables robust soldering in high-strain applications while offering additional functionalities (e.g., self-healing and recyclability) critical for next-generation electronics.

3.3. Ternary LM–solid filler–polymer composites

By blending LMs with polymers, researchers have achieved enhanced patterning ability and substrate adhesion in soldering systems. However, such LM–polymer composites usually necessitate post-processing through mechanical or thermal sintering to restore electrical conductivity.⁵⁸ To this end, ternary biphasic composites comprising LMs, polymers and functional particulate fillers have been proposed. Unlike LM–polymer binaries requiring sintering to restore conductivity, ternary composites eliminate this step through solid-filler-mediated percolation. Fillers (e.g., Ag flakes and CNTs) penetrate LM oxide layers during mixing, establishing direct electrical contacts between LM droplets, while polymer matrices stabilize these hybrid networks via phase separation. Therefore, such innovative combinations eliminate the sintering while maintaining electrical robustness through optimized phase-separated architectures, promoting the emergence of advanced LM-based solders. In 2020, Dou et al. developed a stretchable conductive adhesive by incorporating PDMS, LMs, and CNTs, in which CNTs served as co-fillers to build conductive networks with LMNPs. (Fig. 5a).⁷⁴ The adhesives demonstrated remarkable stretchability, high electrical conductivity and strong adhesion to polymer substrates, effectively preventing circuit disruption during mechanical deformation. These superior properties significantly enhance the operational performance and reliability of integrated devices.

Fig. 5 (a) Stretchable conductive adhesive based on ternary LM–CNT–PDMS composites. Reproduced with permission.⁷⁴ Copyright 2020, Elsevier. (b) Matrix of LEDs under strain enabled by trinary composites that include LMs, Ag flakes and a SIS block copolymer. Reproduced with permission.⁷⁵ Copyright 2021, American Chemical Society. (c) Reversible polymer–gel transition of SIS–Ag flake–LM composites for ultrastretchable chip-integrated circuits. Reproduced with permission.⁷⁶ Copyright 2021, Springer Nature. (d) Electrically conductive adhesive based on an LM–Ag-epoxy matrix for hybrid electronics integration. Reproduced with permission.³⁸ Copyright 2024, Wiley-VCH. (e) Universal assembly of nano/micro-LMNPs in polymers enables elastic printed circuit boards. Reproduced with permission.⁷⁷ Copyright 2022, AAAS. (f) Bilayer LM composite for robust soft–rigid soldering. Reproduced with permission.⁷⁸ Copyright 2023, Wiley-VCH.

In addition to carbon materials, Ag flakes and nanowires have also been used as conductive co-fillers for establishing electrical connections between LMNPs. In 2021, Lopes et al. reported a trinary composite that consisted of LMNPs, Ag flakes uniformly dispersed within the matrix of a styrene–isoprene–styrene (SIS) block copolymer.⁷⁵ This unique formulation yielded a biphasic paste exhibiting exceptional smear resistance, strong adhesion and remarkable elasticity, making it suitable for electronics integration. As shown in Fig. 5b, a 9 × 3 matrix of LEDs fabricated with this biphasic paste remained functional even being subject to repetitive 30% strain cycles, suggesting excellent welding performance. Building upon this research, they further designed an ink by planetary mixing of the SIS polymer, Ag flakes and LMNPs.⁷⁶ The resulting SIS–Ag–LMNP composite exhibited a dynamic pol–gel transition triggered by solvent vapor exposure after the precise placement of rigid chips, enabling robust mechanical and electrical integration of the components (Fig. 5c). The optimized system achieved unprecedented mechanical compliance, with chip-integrated circuits maintaining functionality beyond 500% elongation—a critical advancement for soft, deformation-resistant electronics. In 2024, Pozarycki et al. introduced an electrically conductive adhesive comprising LMNPs interconnected by Ag flakes within an epoxy matrix.³⁸ It achieved high flexibility, electrical conductivity and robust adhesion to flexible circuit substrates, showing great potential for durable flexible electronics integration. The adhesive's reliability under dynamic mechanical loads was validated through a suspended-load test, where a vertically bonded LED remained fully operational while sustaining a 20 g weight (Fig. 5d).

In addition, Lee et al. engineered a ternary LM composite system by acoustically assembling a percolating network of LMNPs—comprising large particles (2–3 μm) and smaller interconnector particles (100 nm, served as solid fillers)—within polymer matrices.⁷⁷ This architecture achieved high conductivity (∼2.1 × 10⁶ S m⁻¹), negligible resistance fluctuations under strain, and exceptional interfacial adhesion (>596 J m⁻²). As illustrated in Fig. 5e, the ternary system enabled the robust soldering of rigid components (e.g., LEDs and sensors) onto stretchable circuits, showing great potential for use in elastic printed circuit boards. By incorporating bulk LMs, LMNPs within thermoplastic polyurethane, Chen et al. developed a bilayer LM composite for robust soft–rigid soldering.⁷⁸ It was prepared via sonication assembly and peeling steps, realizing ultrahigh conductivity (∼22 532 S cm⁻¹), stretchability (2260% strain), and self-soldering capability. During welding, the bulk LM-rich top layer enabled low-resistance bonding with rigid components, while the LMNP–polymer bottom layer ensured substrate adhesion and strain tolerance. On this basis, the soft–rigid interface was highly robust upon stretching and could withstand an elongation of ≈400% (Fig. 5f).

4. Conclusions and perspectives

As discussed in the work, biphasic LM-based soldering systems have emerged as transformative solutions for robust soft–rigid integration in stretchable hybrid electronics. These systems are primarily categorized into three configurations: (1) LM–solid filler composites, where conductive fillers (e.g., Ag flakes and carbon nanotubes) synergize with LMs to enhance conductivity and mitigate leakage; (2) LM–polymer composites, leveraging dynamic polymers (e.g., supramolecular networks and ionic liquids) to achieve self-healing and strong interfacial adhesion; (3) ternary LM–solid filler–polymer composites, such as LM–Ag–polymer hybrids, which integrate high stretchability with strain-insensitive conductivity. To summarize the technical distinctions, Table 1 compares key performance metrics across the three composite types. Critically, LM composites surpass conventional solders in signal stability during repeated deformation. As quantified in Table 2, they achieve 1–2 orders of magnitude higher cycling durability than silver pastes or geometric stress-relief structures, while maintaining a minimal resistance drift (<8%). This stems from their innate ability to: (i) redistribute strain via liquid-phase flow, (ii) dynamically repair damage, and (iii) prevent interfacial delamination through tailored adhesion chemistry. Collectively, these hybrid systems address critical challenges like interfacial stress concentration, LM leakage and electromechanical coupling, enabling functional stretchable circuits, wearables, and bio-integrated devices.

Table 1 Comparison of three types of biphasic LM soldering systems

Composite type	Electrical conductivity	Strain tolerance	Adhesion strength	Pros	Cons
LM–solid filler	Ultra-high: ∼10^6 S m^−1 (e.g., LM–AgNW)	Up to 1500%	Moderate (requires surface treatment)	– Highest conductivity	– Risk of leakage
				– Minimal resistance fluctuation under strain	– Poor intrinsic adhesion
				– Simple fabrication
LM–polymer	Moderate: 10^2–10^4 S m^−1 (e.g., supramolecular LM)	Extreme: Up to 2800%	High (intrinsic adhesiveness)	– Extreme stretchability	– Lower conductivity
				– Self-healing/recyclability	– May require sintering
				– Tunable adhesion	– Limited to specific polymers
				– Colloidal stability
LM–solid–polymer	High: 10^4–10^5 S m^−1 (e.g., LM–Ag-epoxy)	High: >500%	Very high (robust bonding)	– No sintering needed	– Complex formulation
				– Balanced conductivity/stretchability	– Potential phase separation
				– Superior adhesion

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Table 2 Comparative performance of different soldering systems under cyclic mechanical loads

Soldering type	Cyclic strain	Cycle life	Resistance drift (ΔR/R0)	Key limitations overcome by LM composites
Silver paste	<10%	<50 cycles	>200%	• Brittle fracture at filler/matrix interfaces
				• Irreversible crack propagation
Geometric stress-relief structure	∼100%	∼1000 cycles	∼15%	• Delamination at rigidity transitions
				• Geometric constraints limit density
LM composite	50–200%	∼1000–10 000 cycles	<8%	Not applicable

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Despite remarkable achievements in this burgeoning field, there are still some challenges that remain to be solved (Fig. 6). One significant challenge is the long-term stability of these soldering systems, particularly the underexplored degradation under cyclic mechanical loads, such as LM oxidation and polymer fatigue. To tackle this, future research should focus on multifunctional material engineering and dynamic stabilization strategies. Integrating antioxidant additives into LMs, utilizing self-healing polymers, or employing advanced encapsulation techniques can optimize composite architectures, thereby enhancing the overall durability and reliability. Another key issue lies in achieving reliable interfacial compatibility with heterogeneous materials. Current solutions still fall short in establishing universal bonding mechanisms. To overcome this limitation, one promising strategy is to develop adaptive LM composites with stimuli-responsive interfaces. These smart materials, capable of pH- or temperature-triggered adhesion modulation, offer a promising pathway for achieving dynamic and versatile bonding across diverse substrate materials while maintaining electrical and mechanical performance. Additionally, future work must bridge LM solder recyclability to functional device reuse. This demands: (i) stimuli-responsive interfaces for non-destructive disassembly, (ii) closed-loop LM reclamation systems, and (iii) standardized reuse protocols. Success in these areas will position LM solders as enablers of circular electronics economies. Lastly, the manufacturing processes of LM solders face challenges in high-resolution, large-area patterning and roll-to-roll compatibility. To this end, advance 3D/4D printing techniques are employed to pattern LM solders, thereby enabling multilayer, high-density circuits with sub-50 μm resolution. These approaches, combined with AI-driven process control, can bridge lab-scale innovations to the industrial-scale production of various flexible circuits.

Fig. 6 Current challenges and future directions for biphasic LM soldering systems.

By addressing these challenges, biphasic LM soldering systems could redefine standards for next-generation stretchable electronics, enabling seamless integration in healthcare, soft robotics, and human–machine interfaces. We anticipate that the ongoing endeavors to realize such soldering systems will accelerate the development of cutting-edge applications regarding stretchable hybrid electronics.

Author contributions

Jie Li: investigation, data curation, conceptualization, and writing – original draft. Kai Zhao: investigation, methodology, and writing – review & editing. Changqing Ye: investigation, methodology, and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

This review does not include any original research results, software, or code, nor were any new data generated or analysed. Additional information about this review is available upon request from the authors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51873145) and the Natural Science Foundation of Jiangsu Province-Youth Foundation (No. BK20230655).

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