Stretchable polymeric-gel-based sponge with tunable wettability via segmented network design

Abstract

A polymeric-gel-based sponge with robust stretchability and tunable wettability is developed. Unlike conventional sponges, its wettability is permanently tailored during synthesis. This transparent, mechanically stable material is applicable in oil/water separation systems and stretchable electronics, offering a promising approach for next-generation functional sponges with adaptable and stable surface properties.

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New concepts

This work establishes a new concept of a single-step, polymeric-gel-based sponge platform that permanently embeds tunable wettability into the material's backbone, rather than relying on fragile surface treatments or continuous external stimuli. By copolymerizing hydrophilic and hydrophobic monomers within a crosslinked, stretchable matrix, we produce a sponge whose wettability is programmed and locked into the 3D network during synthesis. This is distinctly different from most existing approaches that add or remove coatings or invoke stimulus-responsive transitions that can degrade over time. The permanent, on-demand wettability in our sponge—ranging from superhydrophilic to highly hydrophobic—persists even under large deformations or repeated mechanical stresses. Consequently, our strategy enables robust performance in tasks requiring controlled fluid interactions, such as selective oil absorption in harsh environmental conditions and facile infiltration of high-surface-tension liquid metals for wearable electronics. By integrating a broad spectrum of surface free energies with inherent elasticity, this new concept points to a paradigm shift in materials design, opening avenues for multifunctional porous systems that simultaneously demand mechanical adaptability and stable interfacial control.

Introduction

Sponges have long been considered versatile tools owing to their high porosity, large surface area, and ability to interact with liquids.^1–4 Porous materials have evolved from traditional domestic uses to specialized engineered structures capable of performing complex tasks such as selective oil/water separation, advanced sensing, and controlled fluid transport because of their outstanding accommodability for liquids.^2,4–7 By tuning their intrinsic properties such as pore structure, surface chemistry, and mechanical behavior, researchers have developed sponge-based materials for use in environmental remediation, soft electronics, and biomedicine.^2,5,8,9

The sponge matrix is the most important sponge component that strongly influences the properties of sponges.^10,11 Various sponge matrices have been fabricated from materials ranging from synthetic polymers (e.g., polyurethane foams^3,12 and poly(dimethylsiloxane) (PDMS), melamine)^4,5,7,13,14 to natural cellulose⁸ and even 2D carbon materials.^14–16 Such sponges can realize unique functionalities owing to their porous architecture. For instance, sponges based on commercial melamine, polyurethane, and cellulose have inherently high absorption capacities. However, these sponges typically exhibit both hydrophilic/oleophilic properties and thus indiscriminately absorb both target and irrelevant liquids, rendering them suboptimal for selective separation tasks. To address this issue, researchers have coated sponges with low-surface-free-energy materials and engineered hierarchical micro/nanoscale roughness to achieve superhydrophobic and superoleophilic surfaces, enabling effective oil/water separation.^{4,5,13,14,17,18} Although these modifications have considerably enhanced sponge performance, they frequently involve complex chemical treatments or rely on fragile surface coatings prone to wear and environmental degradation.

In addition to the fluid separation functionality, stretchability has emerged as an important requirement for next-generation sponges. Stretchable sponges fabricated from elastomers such as PDMS and polyurethane acrylate possess dynamic shape adaptability, which is critical for their application in conformal soft machines, soft electronic conductors, and wearable devices.^9,19–21 Moreover, possible strategies for controlling the wettability of sponge surfaces through external stimuli have been explored. For example, sponges that can shift their wetting states upon UV irradiation (e.g., titanium oxide nanoparticles)¹⁷ or with pH changes (e.g., via copolymer chains containing ionizable groups)¹⁸ have been designed. Despite their stimulus-responsive behavior, such systems require continuous or repeated exposure to external stimuli, making it difficult to maintain a stable, pre-defined wettability profile. In addition, these external triggers can lead to unpredictable wetting transitions, reducing the reliability and long-term stability of the functional properties of sponges. Despite these advancements, a significant gap remains: no existing sponge matrix can simultaneously (1) achieve substantial mechanical stretchability and (2) permanently and systematically control surface wettability to implement a user-defined stable state without continuous external stimuli. The ability to integrate these features would open new avenues for robust and reliable applications ranging from advanced fluid management systems to integrated soft electronic conductors.^22,23

In this work, we address the abovementioned gap by introducing a novel sponge through polymeric-gel-based matrix platform with an inherently stretchable matrix and tunable wettability. In contrast to previous studies that achieved wettability control through surface coatings, our approach focuses on a designing the base polymer that can be chemically modified to incorporate hydrophilic or hydrophobic moieties. In addition, by carefully selecting and integrating these functionalities during the sponge synthesis process, a stable and precise predetermined surface free energy is maintained without external environmental triggers. Furthermore, the porous structure of the sponge was engineered, enabling on-demand tuning of pore density and establishing a clear correlation between porous structure and surface wetting properties. The resulting sponge is transparent and elastically compliant and can be engineered to realize on-demand and permanent wettability adjustments with surface free energies ranging from 72.6 to 22.1 mN m⁻¹. Additionally, the applicability of the sponge is demonstrated by utilizing it for oil/water separation and as a stretchable soft electronic conductor, highlighting the application scope of this new class of sponge materials. Specifically, our materials integrate tunable wettability, controlled porosity, and multifunctional capabilities, offering a versatile platform for both soft electronics and environmental applications, unlike previous ultra-stretchable LM–polymer composites focusing solely on mechanical performance.²⁴

Experimental section

Chemicals

EA, PEGDMA (M_n = 550), HEA, HFBA, gallium–indium eutectic (99.9%), and SYLGARD 184 were purchased from Sigma-Aldrich. Azobisisobutyronitrile (AIBN), sodium chloride (NaCl), ethyl alcohol, and tetrahydrofuran were sourced from DUKSAN. Diiodomethane, hexadecane, hexane, paraffin oil, pump oil, and silicone oil were also obtained from Sigma-Aldrich. All chemicals were used without further purification.

Preparation of polymeric gel-based matrix

A matrix solution composed of EA (monomer), PEGDMA (crosslinker), and AIBN (thermal initiator) mixed in a weight ratio of 20 : 1.5 : 1.5 was prepared under magnetic stirring for 1 h. Next, VHB tape (3 M, cut to 5 mm × 30 mm, thickness 0.5 mm) was attached to a thin glass substrate as a spacer and covered with another glass substrate. The as-prepared solution was injected into the gap between the glass substrates using a syringe via capillary force. The solution was cured in a vacuum oven at 60 °C for 24 h.

Preparation of PGM sponges

The same matrix solution with the polymeric gel was poured onto a Teflon plate containing the prepared sacrificial NaCl frame and cured in a vacuum oven at 60 °C for 24 h. As a result, a composite material consisting of PGM and the embedded salt frame was obtained. To remove the salt frame, the composite was immersed in a beaker with water and subjected to ultrasonic agitation for 12 h. Finally, the sponge was dried in a vacuum oven at 60 °C for 48 h.

Preparation of wettability-controlled PGM sponges

To fabricate the wettability-controlled PGM sponges, a procedure similar to that used for the PGM sponge was employed with modified monomer contents. For the PGM–HEA sponge, EA and HEA were used as the monomers. Specifically, EA, HEA, PEGDMA, and AIBN were mixed in a weight ratio of 14 : 6 : 1.5 : 1.5. For the PGM-HFBA sponge, EA and HFBA were utilized as the monomers; EA, HFBA, PEGDMA, and AIBN were mixed in a weight ratio of 14 : 6 : 1.5 : 1.5. After preparing the corresponding monomer solutions, the subsequent curing, salt removal, and drying processes were identical to those employed for the PGM sponge.

Characterization

The mechanical properties of the porous sponges were evaluated using a universal testing machine (UTM) combined with a MultiTest-2.5i instrument and Emperor TM Force software. Scanning electron microscopy (SEM) was conducted with a Tescan Vega2 microscope at a high voltage of 20 kV to observe NaCl particles and the porous sponge structure. Energy dispersive spectroscopy (EDS) was performed using a Bruker XFlash Detector 410 spectrometer to determine the elemental composition of the sponges. Mercury porosimetry was conducted using a Micromeritics AutoPore IV 9500 instrument to calculate the bulk density of the porous materials. Contact angle and surface tension measurements were performed with a Kruss drop-shape analyzer (DSA100) to evaluate the wettability of the sponges.

Results and discussion

Preparation and characterization of the polymeric-gel-based matrix (PGM)

A stretchable PGM was synthesized through a meticulously controlled polymerization and crosslinking process, resulting in a robust, flexible, and porous three-dimensional (3D) network with highly tunable surface properties. The chemical structure of the matrix (Fig. 1(a)) consists of ethyl acrylate (EA) as the main monomer and poly(ethylene glycol)dimethacrylate (PEGDMA) as the crosslinker. This acrylate-based design induces synergy between the physical crosslinks formed from PEGDMA ethylene oxide groups via hydrogen bonds and chemical crosslinks produced during polymerization. The interplay of these interactions creates a net-like architecture, which is not only stretchable and elastic but also mechanically durable.^21,25–28 The flexible acrylate monomer and ethylene glycol chains obtained from PEGDMA also increase flexibility. To achieve adjustable matrix wetting properties, we incorporated two acrylate-based monomers with opposite surface free energy characteristics. Hydrophilic 2-hydroxyethyl acrylate (HEA) with a terminal hydroxyl group was added to create a high-surface-energy superhydrophilic matrix with hydrogen bonds between terminal groups and water.^29,30 In contrast, hydrophobic 2,2,3,4,4,4-hexafluorobutyl acrylate (HFBA) was utilized to produce a low-surface-energy hydrophobic matrix with strong C–F bonds (Fig. 1(b)).^6,31,32 All chemicals formed homogeneous mixture with high miscibility, generating well-distributed crosslinked structure (Fig. S1, ESI†). This selective integration of functional monomers enables precise wettability control from 72.6 to 22.1 mN m⁻¹ of surface free energy, which is critical for various applications such as oil–water separation and anti-fouling systems.

Fig. 1 (a) Structure and network architecture of the PGM. (b) Chemical structures of the monomer, crosslinker, and wetting-adjustable moieties. (c) Representative strain–stress curves of the PGM films with various PEGDMA contents. (d) Young's modulus plotted as a function of the PEGDMA content to correlate the crosslinking density with mechanical stiffness. (e) Gel content and swelling rate plotted as functions of the PEGDMA content to illustrate the trade-off between mechanical stability and swelling behavior. (f) Photographs of the PGM film during mechanical stretching and relaxation, demonstrating its high elasticity and good shape recovery properties. (g) Schematic of the PGM structural response to mechanical deformation, emphasizing the roles of chemical and physical linkages in stress distribution. (h) Schematic of the mechanism underlying surface wettability modulation upon selective incorporation of hydrophilic (HEA) and hydrophobic (HFBA) moieties. (i) Contact angle and (j) surface free energy plotted as functions of the HEA and HFBA contents to demonstrate the tunability of PGM wettability with transition from relatively high to low surface free energy.

The mechanical properties of the matrix were systematically optimized by varying the PEGDMA content between 1 and 10 wt% (Fig. 1(c)). As the PEGDMA content increased, the Young's modulus increased, reflecting an increase in crosslinking density (Fig. 1(d)). However, excessive crosslinking (e.g., at 10 wt% PEGDMA) considerably reduced matrix stretchability and flexibility, because the high stiffness suppressed material deformability. Conversely, a lower PEGDMA content (e.g., 1 wt%) resulted in an increase in material elasticity; however, the matrix exhibited insufficient mechanical robustness for practical utilization.

The matrix mechanical properties were further characterized by gel content and swelling rate measurements (Fig. 1(e)). Although the addition of PEGDMA at a low content resulted in a high swelling rate, the PGM structural stability was compromised due to the low density of crosslinked parts, leading to high elongation and low robustness. Moreover, the formulations of PGM films with high PEGDMA contents demonstrated enhanced mechanical robustness but poorer swelling behavior. Based on these results, the formulation was optimized with 7 wt% PEGDMA, which provided a sufficiently high crosslinking density and maintained mechanical integrity and flexibility. This formulation possessed a stable gel content and exhibited moderate swelling behavior and robust elastic recovery, making it suitable for functional applications requiring both high durability and elasticity (calculation details of the gel contents and swelling rates are provided in ESI†). This optimized formulation ensures that PGM is versatile for a wide range of practical applications in which high mechanical performance and flexibility are critical.

The polymerization process in was confirmed via Fourier transform infrared (FT-IR) spectroscopy (Fig. S2, ESI†), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) (Fig. S3, ESI†), and X-ray photoelectron spectroscopy (XPS; Fig. S4, ESI†). The obtained results verified the successful incorporation of functional groups into the polymer matrix and formation of crosslinked networks, further demonstrating the structural integrity and functionality of the synthesized matrix. They also proved the reliability of the formulation and effectiveness of the fabrication process.

The mechanical stretchability of the PGM was confirmed through tensile testing, demonstrating excellent elongation and elastic recovery (Fig. 1(f) and (g) and Movie S1, ESI†). The network structure distributes stress uniformly, preventing local deformation and ensuring long-term mechanical integrity.

Furthermore, UV-Vis spectroscopy demonstrated high optical transparency across the visible spectrum (Fig. S5, ESI†), making the PGM a viable candidate for optical applications.^33,34

A key feature of PGM is its ability to adjust its surface wettability through the controlled incorporation of HEA and HFBA species (PGM–HEA and PGM–HFBA films, respectively) (Fig. 1(h)). By varying the content of these functional monomers, the surface free energy of the matrix can be finely tuned, as demonstrated in Fig. 1(i) and (j) (calculation details of the surface free energy is provided in ESI†). Increasing the HEA content of PGM–HEA lowered the contact angles for both water and hexadecane, indicating a high wettability, whereas at higher HFBA contents, the contact angles were significantly increased, achieving a relatively low wettability. This precise control over surface free energy highlights material versatility, allowing the seamless transition between the comparatively high to low surface free energy for specific applications. The hydrophilic surface facilitates water purification³⁵ and drug delivery,³⁶ whereas the hydrophobic surface can be leveraged for developing water-repellent coatings^33,37 and anti-fouling surfaces.³⁸ The ability to tune both mechanical and surface properties through simple monomer adjustments demonstrates the adaptability of this material for diverse applications. The synthesized PGM combines transparency, elasticity, and tunable wettability, making it suitable for advanced sensing technologies, environmental remediation, and adaptive material systems. This innovative design allows efficient customization to meet the demands of various real-world applications, ensuring both functionality and durability.

Preparation of PGM sponges with on-demand tunable wettability and stability

A highly versatile PGM sponge was created by utilizing the designed and synthesized polymeric-gel-based matrix material with tunable wetting properties described above. An eco-friendly salt-templating method was employed to fabricate a porous structure in a straightforward and sustainable manner as shown in Fig. 2(a) (details are provided in ESI†). The fabrication process involved infiltrating the gel-based matrix solution including monomers, a crosslinker, and an initiator into a pre-fabricated salt template followed by curing. Subsequently, the internal salt template was removed in water (the corresponding scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) data are provided in Fig. S6, ESI†), resulting in the formation of a porous structure. This template method is nontoxic and environmentally friendly because it does not require any solvents or water-insoluble salt frames, demonstrating not only its versatility and practicality but also environmental remediation properties.^4,13,39–42

Fig. 2 (a) Schematic of the salt-templating process for fabricating a porous PGM sponge through PGM penetration into the salt frame. (b) SEM images of NaCl particles (top) and pore structures (bottom) obtained at various ball milling times (scale bar: 500 μm). (c) Diameters of NaCl particle, sponge pore and bulk density plotted as functions of the ball milling time. (d) Photographs of the PGM sponge upon mechanical stretching (elongation: ∼104%) and after the subsequent shape recovery. (e) Stress–strain curves of the sponges according to the ball milling time. (f) Elongation at break (%) plotted against the pore size for the results obtained in the present work (highlighted in yellow) and those in previous works. (g) Wettability control (left) via the incorporation of hydrophilic (HEA, top) or hydrophobic (HFBA, bottom) moieties and optical images of water droplets on the corresponding sponge surfaces (right). The water droplets were false-colored in blue for clearer visualization. (h) Water and hexadecane contact angles measured as functions of the fractions of the hydrophilic and hydrophobic moieties, illustrating the tunable wetting behavior of the sponge.

By adjusting the particle size of the salt used in the template, the salt-templating method also allowed precise control over the sponge pore size, as shown in Fig. 2(b). The salt particle size was modified via ball milling for various durations (10 min, 30 min, 1 h, and 3 h). As the milling time increased, the salt particle size decreased, reducing the pore size of the final porous material (Fig. S7, ESI†). This relationship is presented in Fig. 2(c), where the pore size distribution narrows with increasing milling time, and the error bars were calculated based on measurements from more than 20 pores collected from the cross-sectional images of the sponge. For instance, the 30 min ball-milled sample exhibited a wide pore size range (247–83 μm), while the 1 h and 3 h samples demonstrated much smaller and more uniform pore sizes of 23 and 16 μm, respectively. The bulk density of the porous material also varied with the pore size. The 30 min sample exhibited the lowest bulk density owing to its larger and less compact pore network, while the 1 h and 3 h samples possessed higher densities because of their smaller and tightly packed pore structures. These results highlight the versatility of the salt-templating method for tailoring the physical properties of the PGM sponge by simply adjusting the salt particle size.

Despite its porosity (avg. 60–65%), the sponge maintained superior mechanical properties, as shown in Fig. 2(d) and Movie S2 (ESI†). The porous PGM sponge demonstrated higher flexibility and elasticity than its bulk film counterpart. Interestingly, the elongation at break of the porous structures significantly increased with increasing ball milling time (Fig. 2(e)). With a less stretchable polymer matrix, the elongation at break is decreased with decreasing pore size and increasing bulk density because of the high proportion of the less stretchable polymer matrix, as reported in previous studies (demarcated by the gray-colored oval in Fig. 2(f)).^19,43–48 In contrast, stretchable PGM promotes the elongation of porous structures because of the increased fraction of the stretchable polymer matrix despite the high bulk density. Among the various durations, 30 min was identified the optimal ball milling time, at which excellent elongation, low density, and adequate ball milling were achieved. Specifically, the 30 min sample possesses a tightly packed pore structure with a wide range of pore sizes and low bulk density. Consequently, the 30 min sample is considered the best among all samples, because it exhibits liquid absorbability, low density, and excellent elongation.

The tunable wettability of the porous PGM sponge was further investigated by incorporating HEA and HFBA to modulate the surface free energy. Fig. 2(g) shows the water and hexadecane (oil) contact angles, while Fig. 2(h) displays the relationships between the HEA and HFBA contents and contact angle. The superhydrophobic PGM–HFBA sponge exhibited a water contact angle close to 140°, reflecting a significantly reduced surface free energy in relation to that of the pristine material. In addition, the highly porous structure contributes to the water contact angle increment by changing hydrophobic to superhydrophobic surface, which might follow Cassie–Baxter model (detailed explanation in Fig. S8, ESI†). This confirms the suitability of the PGM–HFBA sponge for oil/water separation.^40,49 In contrast, the hydrophilic PGM–HEA sponge absorbed water completely, impeding direct contact angle measurement and this behavior might be defined by Wenzel model (deeply described in Fig. S8, ESI†).^50–54 This behavior indicates a substantial increase in the surface free energy and highlights the applicability of the PGM–HEA sponge as a hydrogel or strain sensor upon the absorption of high-surface free energy liquids.^1,55,56 Additionally, all samples easily absorbed oil, precluding contact angle measurements for this liquid and confirming the broad compatibility of this material with oil-like substances.

Overall, these results demonstrate that the salt-templated PGM sponge not only retains the mechanical and tunable surface properties of the bulk gel-based matrix but also surpasses its performance with high flexibility, good stretchability, and controllable wettability. This highly versatile and eco-friendly material has a great application potential in environmental remediation, selective filtration, and development of adaptive interfaces.

Application in oil/water separation

The performance of the PGM–HFBA sponge with tunable wettability for water/oil separation was evaluated. As shown in Fig. 3(a), the PGM–HFBA sponge with a low surface free energy was applied as a functional material for water/oil separation.⁵⁷ This sponge effectively repelled water with a minimum amount of HFBA, while selectively absorbing oils and organic solvents, thereby making it highly suitable for separating oil from water/oil mixtures.

Fig. 3 (a) Photographs showing selective oil absorption by the PGM-HFBA sponge (20 wt% of HFBA) from a water–oil mixture. (b) Releasing the absorbed oil under compression, enabling repeated sponge use. (c) Separation efficiency maintained almost 100% over 100 consecutive absorption–release cycles. (d) Stable absorption capacity (≈300%) throughout 100 cycles. (e) Absorption capacity for various representative oils, highlighting sponge versatility for multiple applications (error bars calculated through measurements from more than 10 data points each). (f) Photographs showing the scrubbing (top) and taping (bottom) durability tests. (g) Water contact angles measured over 200 scrubbing per taping cycles, confirming robust hydrophobicity of the sponge under mechanical stress.

Fig. 3(b) illustrates the applicability of PGM–HFBA for removing oil not only from water–oil mixtures but also from a single oil phase (details are provided in Movies S3 and S4, ESI†). In addition, the absorbed oil can be easily removed by applying pressure, allowing the sponge to be reused multiple times (Movies S5 and S6, ESI†). This reusability highlights the environmentally friendly and sustainable nature of the material. The separation performance of the sponge is presented in Fig. 3(c). The sponge maintained a separation efficiency of >98% across 100 consecutive cycles. In addition, its absorption capacity was repeatedly recorded over 100 cycles. Even after repeated use, the absorption capacity remained stable at approximately 300% up to 100 cycles, as shown in Fig. 3(d) (details regarding the evaluation and calculation of the separation efficiency and absorption capacity are provided in ESI,† and corresponding photographs are provided in Fig. S9). In addition, the shapes and porous structures of the sponges were well maintained during 100 cycles (Fig. S10, ESI†). These results emphasize the robustness and reliability of the material for long-term applications. Moreover, as presented in Fig. 3(e), the sponge exhibited a high absorption capacity for a wide range of oils, including diiodomethane, hexadecane, hexane, paraffin oil, pump oil, and silicone oil. All oils were absorbed at capacities of >200%, with pump oil absorbed by the PGM–HFBA sponge at a capacity of ∼600%. In addition, PGM-HFBA sponge exhibited high absorption capacity even for commercial organic solvents such as ethanol and acetone as shown in Fig. S11 (ESI†), and absorption capacity values expressed mL g⁻¹ (Tables S1 and S2, ESI†) and recovery rate (Fig. S12, ESI†) are provided. Furthermore, PGM-HFBA sponge showed superior absorption capacity compared to previous studies (Table S3, ESI†). Therefore, this versatility in handling diverse organic liquids underlines its adaptability for various industrial and environmental applications.

Durability and mechanical stability are crucial properties for the practical utilization of functional materials.⁵⁸ To evaluate these properties, we subjected the sponge to rigorous taping and scrubbing tests for over 200 cycles. As shown in Fig. 3(f), the sponge retained both its structural integrity and hydrophobicity despite the repeated mechanical stress. Unlike conventional coatings that lose their water-repellent characteristics once the surface layer is damaged,⁴⁹ the HFBA moieties in the sponge are integrated throughout the whole polymer matrix rather than merely attached to the surface. Consequently, even under the prolonged mechanical abrasion and handling, the sponge wetting characteristics remain essentially unchanged. This robustness is further corroborated by the water contact angle measurement results (Fig. 3(g)), which indicate negligible variations over 200 cycles. Thus, the sponge consistently exhibits reduced surface free energy and possesses hydrophobic properties under severe mechanical deformations.

The ability of this material to combine high selectivity, excellent reusability, and outstanding durability renders it a promising candidate for oil/water separation applications. Its compatibility with various organic solvents coupled with the long-term stability and environmentally friendly characteristics make it suitable for environmental remediation, including oil spill cleanup and wastewater treatment. Additionally, its robustness against mechanical wear promotes its application in industrial settings where frequent utilization and handling are required. These findings provide a strong foundation for advancing the development of tunable wetting materials in functional separation technologies.

Application as a stretchable conductor

The PGM–HEA sponge was employed as a strain-sensitive conductor by infiltrating it with a liquid metal (GaIn). The high surface free energy imparted by the HEA component allowed the liquid metal to effectively wet and penetrate the sponge, creating a robust and interconnected conductive network. Fig. 4(a) illustrates how easily the liquid metal penetrates the PGM–HEA sponge and remains there during repeated stretching and relaxation, forming continuous electrical pathways.⁵⁹ As shown in Fig. 4(b), the resistivity of the PGM–HEA sponge exhibits minimal variations in relation to that of the sponge without HEA upon mechanical deformation (photographs of the resistance measurement process are provided in Fig. S13, ESI†). This stability is attributed to the enhanced surface free energy provided by HEA, which promotes the formation of strongly interconnected liquid metal networks that maintain the high electrical conductivity even at an 80% tensile strain. The photographs presented in Fig. 4(c) show the sponge during stretching and relaxation (a detailed stretching process is shown in Movie S7, ESI†). To preserve its structural integrity, the sponge was encapsulated with the same gel-based matrix material (see the description in ESI†). Even upon continuous mechanical loading, the liquid–metal-filled sponge retains excellent elasticity and flexibility, thereby sustaining electrical conductivity. Fig. 4(d) displays a schematic of the sponge liquid metal penetration and distribution processes. The hydrophilic PGM–HEA promotes easy infiltration and strong adhesion of the liquid metal, producing a conductive network that remains stable under deformation. In detail, lower contact angle between liquid metal and PGM–HEA facilitates capillary force, enhancing the infiltration of liquid metal.⁶⁰ Unlike the previously developed coating-based approaches, our method uses a matrix entirely composed of HEA, ensuring that every internal channel of the sponge maintains the same wetting properties. Consequently, even high-surface-tension liquid metals can uniformly and deeply penetrate the sponge structure. The SEM image and EDS map presented in Fig. 4(e) confirm thorough and uniform metal infiltration, where Ga (green), In (blue), and C (red) atoms form well-distributed metallic pathways, which are essential for preserving conductivity under strain (detailed EDS images are provided in Fig. S14, ESI†). The results of compression and release tests presented in Fig. 4(f) highlight the sponge mechanical resilience. The material exhibited excellent elasticity and restoration, maintaining its structure and conductive pathways even after the application of repeated mechanical stress. The durability of the conductive network was further examined by performing multiple stretching and relaxation cycles (Fig. 4(g)). The resistivity ratio exhibited minimal changes even under high tensile strain, confirming the reliability of the material for extended use. Further tests involving stretching and twisting (Fig. 4(h) and (i)) revealed the material's exceptional flexibility and stability of its electrical properties. Even at a tensile strain of 100%, the sponge continued to conduct electricity, as indicated by the illumination of the light LEDs connected to the sponge. Twisting did not interrupt the conductive pathways, highlighting the sponge durability under different mechanical conditions (Movie S8, ESI†). Finally, Fig. 4(j) illustrates the applicability of this sponge as an e-skin conductor in wearable electronics.

Fig. 4 (a) Schematic of the PGM–HEA sponge employed as a strain conductor under repeated stretching and relaxation (the sponge dimensions are provided in the illustration). (b) and (c) Resistivity changes measured during stretching–relaxation cycles with the corresponding photographs. (d) Schematic of the liquid metal infiltration of the PGM–HEA sponge porous network. (e) SEM image (top; GaIn is false-colored in purple) and EDS map (bottom; Ga is colored in green, In is colored in blue and C is colored in red) of the liquid–metal-filled PGM–HEA sponge (scale bar: 300 μm). (f) Photographs of the liquid–metal-absorbed sponge during compression and subsequent release, exhibiting stable conduction after a compression–release cycle. (g) Resistivity changes recorded over 1000 stretching–relaxation cycles, demonstrating robust electrical stability. (h)–(j) Photographs of the sponge conductor obtained under (h) a 100% tensile strain, (i) twisting, and (j) attachment on a finger joint while powering a light-emitting diode, indicating reliable conductivity under diverse mechanical deformations.

When integrated into a finger hinge, the sponge maintained conductivity during repeated bending, keeping a connected light bulb illuminated even upon significant deformation (Movie S9, ESI†). These findings confirm that the PGM–HEA sponge with tunable wettability and robust mechanical flexibility is a promising material for next-generation flexible electronics and wearable devices. Moreover, the synergy between the hydrophilic matrix and liquid metal impregnation opens new avenues for improved scalability in manufacturing as well as enables the integration of other functional materials, allowing further customization of its properties to meet specific application requirements. Consequently, the elastic sponge based PGM with high mechanical stretchability and adjustable surface wettability developed and characterized in this study achieved user-defined stable wettability without continuous external stimuli indicating infinite potential for numerous applications require wettability control such as oil/water separation and liquid–metal-absorbing soft electronics.

Conclusions

This study presents a stretchable PGM sponge with on-demand adjustable wettability, addressing the limitations of conventional porous materials. The PGM sponge is highly elastic, mechanically durable, and structurally stable. Unlike the previous studies, the PGM sponge achieves permanent and tunable wettability without requiring continuous external stimuli or vulnerable surface coating by incorporating hydrophilic (HEA) and hydrophobic (HFBA) moieties, making it suitable for various functional applications. In contrast to conventional PDMS-based sponges, which hinder liquid metal infiltration due to their superhydrophobic surface, the PGM–HEA sponge enables seamless liquid metal absorption, forming a highly conductive, stretchable network ideal for soft electronics and strain sensors. Meanwhile, the PGM-HFBA sponge exhibits outstanding oil/water separation performance, maintaining efficiency and reusability over multiple cycles. This work establishes PGM sponges as a next-generation platform for adaptive porous materials, combining mechanical flexibility, tunable surface properties, and multifunctionality. These features position PGM sponges as outstanding materials for wearable electronics, environmental remediation, and advanced fluid management systems.

Author contributions

Conceptualization: H. Y., and H. S. K.; investigation: H. Y., E.-H. S., and H. S. K; writing – original draft: H. Y., and H. S. K.; writing – review & editing: H. Y., J. H. K., J. Q. K, J. W. B., E.-H. S., and H. S. K.; funding acquisition: E.-H. S.

Conflicts of interest

There are no conflicts to declare.

Data availability

All supporting data, including raw experimental results, processed datasets, and detailed analysis protocols, are provided in the electronic ESI† accompanying this article. If you have any questions regarding the data, please contact the corresponding author.

Acknowledgements

This work was supported by the Ministry of Science and ICT (KS2521-30, KRICT). This study was also supported by Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Science and ICT (No. RS-2025-00573139).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00359h

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