Marangoni Flow-Driven Self-Assembly of Biomimetic Jellyfish-like Hydrogels for Spatially Controlled Enzyme Catalysis

Abstract

Enzymatic catalysis has gained significant attention in green chemistry due to its high specificity and efficiency under mild conditions. However, challenges related to enzyme immobilization and spatial control often limit its practical applications. In this work, we report a Marangoni flow-driven strategy to fabricate a biomimetic jellyfish-like hydrogel with tunable tentacle-like structures. The formation process occurs entirely in an aqueous system without organic solvents or post-treatment, enabling the construction of ultra-thin, free-standing hydrogels through spontaneous interfacial self-assembly. The resulting structure exhibits high surface-area geometry and excellent biocompatibility, providing a versatile platform for localized enzyme loading. This method offers a simple and scalable route for engineering soft materials with complex morphologies, and expands the design space for bioinspired hydrogel systems.

1. Introduction

Enzymatic catalysis has become an important part of green chemistry [1], valued for its high specificity, efficiency, and sustainability under mild reaction conditions [2,3]. However, the practical application of free enzymes faces significant challenges. When dispersed directly in aqueous solutions, enzymes lack spatial control over their catalytic activity, leading to unregulated reaction zones and potential limitations in substrate diffusion. This spatial ambiguity not only reduces catalytic efficiency but also complicates process optimization, especially in industrial settings. Moreover, traditional enzyme immobilization strategies typically rely on rigid carriers (nanoparticles or metal–organic frameworks [4]) to stabilize enzymes and facilitate recycling. For example, Wei et al. reported a solid-state mechanochemical strategy to encapsulate enzymes within metal–organic frameworks (MOFs), effectively protecting the enzyme activity under biologically unfavorable conditions and enabling more stable catalytic performance [5]. While these materials improve enzyme stability, their inherent lack of biocompatibility and their non-degradable nature raise environmental concerns, particularly in sensitive ecosystems or biomedical applications. For instance, micro- or nano-sized substrates, despite enhancing mass transfer efficiency, are notoriously difficult to recover, resulting in secondary pollution and increased operational costs [6].

To overcome these dual challenges of spatial control and environmental compatibility, bioinspired architectures have emerged as innovative enzyme immobilization platforms [7,8]. Li et al., inspired by the adhesive capability of mussel foot proteins, developed a bio-adhesive hydrogel with strong wet adhesion properties, offering an adaptable and robust interface for biological applications [9]. Drawing inspiration from natural systems, such as the predation mechanism of jellyfish [10,11,12], offers a promising approach to reconciling efficient catalysis with sustainable design. Jellyfish utilize flexible tentacles and dynamic fluid interactions to capture prey efficiently [13,14]—a process analogous to the need for enzymes to interact with substrates in a controlled spatial domain. Mimicking this strategy, we propose a biomimetic jellyfish-like hydrogel system formed by Marangoni flow at liquid–liquid interfaces [15,16,17]. The Marangoni effect refers to the mass transfer of fluid along an interface caused by a gradient in surface tension. This flow occurs when there is a difference in surface tension between two adjacent regions of a liquid [18,19]. This design leverages hydrodynamic forces to self-assemble a soft, biocompatible hydrogel matrix at the interface, which not only benefits the immobilization of enzymes (such as catalase) but also dynamically regulates their spatial distribution. The Marangoni effect, driven by interfacial tension gradients, enables the spontaneous formation of structured hydrogels without external energy input, making this design inherently aligned with the principles of green chemistry [20,21].

The resulting hydrogel mimics a jellyfish-like tentacle structure, providing an expanded surface area for enzyme attachment. The loose, porous structure of these tentacles enhances stable enzyme binding [22,23,24], while the use of green materials ensures biocompatibility [25,26,27]. Furthermore, the macroscopic size of the hydrogel facilitates easy recovery, overcoming the trade-off between the mass transfer efficiency and recyclability seen in micro-/nanosystems [28,29,30]. In this study, we demonstrate that the integration of Marangoni flow-driven self-assembly and biomimetic hydrogel design offers a novel pathway to engineering enzyme-loaded systems with tunable catalytic performance, environmental compatibility, and scalability. This approach not only overcomes the limitations of conventional enzyme immobilization but also establishes a sustainable framework with broad applications, from industrial biocatalysis to environmental remediation.

2. Materials and Methods

2.1. Materials

Sodium Alginate (Alg, AR, Macklin Biochemical Technology Co., Ltd., Shanghai, China), ε-Poly-L-lysine (ε-PL, MW < 5000, Macklin Biochemical Technology Co., Ltd., Shanghai, China), Fe₃O₄ (magnetite) nanoparticle dispersion (25% in H₂O, Macklin Biochemical Technology Co., Ltd., Shanghai, China), Anhydrous Calcium Chloride (CaCl₂, 99.99% (metals basis), Macklin Biochemical Technology Co., Ltd., Shanghai, China). The Catalase (CAT, 3500 μ/mg, Macklin Biochemical Technology Co., Ltd., Shanghai, China) and Hydrogen peroxide (H₂O₂, 3%, LIRCON Medical Technology Co., Ltd., Dezhou, China) were used without further purification. The deionized water was made in our laboratory.

2.2. Methods

A simple experimental setup is illustrated below. The sodium alginate solution was extruded at a rate of 3 μL/s, controlled by a microinjector pump (Figure 1). A needle with an inner diameter of 2 mm was chosen to control the droplet volume, approximately 0.05 mL. The needle was positioned 1 cm above the surface of the underlying solution to ensure that the initial kinetic energy of the droplet was minimal upon impact with the solution, thereby preventing droplet sinking or breakage.

2.3. Characterization

Field Emission Scanning Electron Microscopy (FESEM): A field emission scanning electron microscope (SU8010, Hitachi High-Tech, Tokyo, Japan) was used to analyze the surface morphology of the samples. During standard scanning, an accelerating voltage of 5 kV was applied. For elemental composition analysis using energy-dispersive X-ray spectroscopy (EDS), the accelerating voltage was increased to 15 kV.

Fourier-Transform Infrared Spectroscopy (FT-IR): A VERTEX70 FT-IR spectrometer (Bruker, Ettlingen, Germany), with a resolution of 2 cm⁻¹, was used for the infrared spectral analysis.

Surface Tension Measurement: A KRÜSS contact angle instrument (Hamburg, Germany) was used to measure the surface tension of each liquid sample by the pendant drop method at 28 °C. Each measurement was repeated three times, and the average value was taken. The surface tension at the liquid–air interface creates an internal pressure difference within the droplet, causing an increase in internal pressure. This relationship between the pressure difference (Δp), the surface curvature radii (r₁ and r₂), and the surface tension (σ) can be described by the Young–Laplace equation:

$\Delta p=\sigma ·(1/r_1+1/r_2)$

By analyzing the droplet morphology to obtain r₁ and r₂, the surface tension of the sample can be calculated [31,32]. Further details on these procedures can be found in Supporting Information Figure S1.

Image Analysis: The areas and perimeters of the jellyfish-like droplets were determined from images using Python’s OpenCV library. Briefly, each image was first converted to grayscale to eliminate background noise. Unwanted artifacts were removed, and the resulting image was filtered to reduce noise. Finally, the number of boundary pixels was recorded as the perimeter, and the total number of pixels identified as the droplet served as the area. For images with different scale bars, the conversion from pixels to actual dimensions was based on the number of pixels corresponding to the scale bar length. Further details on these procedures can be found in Supporting Information Figure S2.

MTT Assay: A cell suspension was prepared at a density of 1000–10,000 HIMECs (Human Intestinal Microvascular Endothelial Cells) per well in 100 µL, and the cells were allowed to adhere overnight. The next day, the cells were treated with varying concentrations of the test solutions, prepared by diluting a stock solution as follows:

Alg/SDS Solutions: Solutions of sodium alginate (Alg) containing 0.6 wt%, 0.5 wt%, 0.4 wt%, 0.3 wt%, 0.2 wt%, 0.1 wt%, 0.05 wt%, 0.01 wt%, 0.005 wt%, or 1 wt% SDS were configured.

Biomimetic Jellyfish-like Hydrogels: For samples containing 0.1–0.6 wt% SDS, the solid gel was ground into small particles and dispersed in PBS at 5 wt% for testing. For samples with SDS below 0.1 wt%, an equal volume of 1 wt% ε-polylysine (ε-pl) solution was mixed and stirred together with the corresponding Alg-SDS solution before testing.

After 48 h of incubation at 37 °C with 5% CO₂, the culture medium was removed. Then, 10 µL of 0.5% MTT solution was added to each well, and the plates were incubated for an additional 4 h at 37 °C. Subsequently, the MTT solution was discarded, and 100 µL of dimethyl sulfoxide (DMSO) was added to dissolve the formazan crystals formed by metabolically active cells. The optical density (OD) of each well was measured at 570 nm and 650 nm using a microplate reader. Corrected OD values were obtained by subtracting the OD at 650 nm from the OD at 570 nm.

The Bradford technique is used to calculate enzyme load: A 100 μL CAT/PBS solution with a concentration gradient of 0.05–0.12 mg/mL was placed in a 2 mL centrifuge tube, and 990 μL of Komas Brilliant Blue staining solution was added to stain the free CAT. The standard curve of concentration versus absorbance was plotted. Take 100 μL of the remaining enzyme solution of the biomimetic jellyfish-like hydrogel system in a 2 mL centrifuge tube and repeat the above operation. Standard curves were used to quantify enzyme concentration and the quantity of leftover enzymatic solution, and enzymatic loading was obtained by difference.

The relative enzyme activity calculation: A standard curve of H₂O₂ concentration versus absorbance at 240 nm was established using a UV–vis spectrophotometer with gradient concentrations of H₂O₂ ranging from 0.05% to 1.5%. A biomimetic jellyfish-like hydrogel was immersed in 15 mL of H₂O₂ solution at 25 °C and shaken at 200 rpm. The absorbance of the solution at 240 nm was measured, and the H₂O₂ concentration was calculated based on the standard curve. To rule out the effect of spontaneous H₂O₂ decomposition, a blank control experiment without enzyme addition was conducted. The actual amount of H₂O₂ decomposition catalyzed by the enzyme was determined by subtracting the decomposition observed in the blank control from that in the experimental group. The relative catalytic performance of catalase (CAT) under different conditions was evaluated using relative activity, defined as follows Equation (1):

(1)$\mathrm{Relative} \mathrm{activity} \left(\%\right)={\displaystyle \frac{{\mathrm{H}}_2{\mathrm{O}}_2 \mathrm{decomposed} \mathrm{under} \mathrm{each} \mathrm{condition}}{{\mathrm{H}}_2{\mathrm{O}}_2 \mathrm{decomposed} \mathrm{with} \mathrm{free} \mathrm{enzyme}}}\times 100\%$

The amount of free enzyme used in the calculation was determined based on the enzyme loading content of the immobilized system.

The catalytic activity of immobilized α-amylase and β-amylase was evaluated using a starch–iodine colorimetric method.

α-Amylase and β-amylase were immobilized onto the biomimetic jellyfish-like hydrogel using the same method, and the enzyme loading was determined accordingly. Either the immobilized enzymes or their free counterparts were immersed in a 0.5 wt% starch solution. After 30 min, 2 mL of the reaction mixture was withdrawn and mixed with 1 mL of 0.005 M iodine solution. After reacting for 20 min, the absorbance at 580 nm was measured using a UV–vis spectrophotometer. The relative enzymatic activity was calculated using the following equation:

(2)$\mathrm{R}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e} \mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\left(\%\right)={\displaystyle \frac{\mathrm{S}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{h} \mathrm{d}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{e}\mathrm{d} \mathrm{u}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{r} \mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{\mathrm{S}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{h} \mathrm{d}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{e}\mathrm{d} \mathrm{b}\mathrm{y} \mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e} \mathrm{e}\mathrm{n}\mathrm{z}\mathrm{y}\mathrm{m}\mathrm{e}}}\times 100\%$

Each catalytic cycle was repeated 12 times to evaluate the reusability of the immobilized enzymes.

3. Results and Discussion

As shown in Figure 2a, when sodium alginate droplets (0.2 wt% SDS, 2 wt% Alg) come into contact with the surface of a ε-PL solution (1 wt%), they rapidly form droplets with tentacle-like structures around the perimeter within approximately 10 s (the case without surfactants is in Supporting Information Figure S3). These tentacles grow exclusively in the two-dimensional plane at the liquid surface (Figure 2b) and do not appear at any other spatial locations. The carboxyl groups of Alg and the amino groups of ε-PL can form a complex through electrostatic interactions, creating a cohesive barrier that prevents further diffusion of solutes. As a result, no tentacles are formed at locations above or below the liquid surface (Figure 2c).

By measuring the surface tension of the Alg droplets containing SDS and the ε-PL solution (Table 1), we hypothesize that the formation of tentacles is induced by Marangoni flow at the liquid–liquid interface. The surface tension gradient between the two solutions drives the Marangoni flow, causing fluid to move horizontally from the low surface tension interior of the droplet (35.72 mN/m) toward the high surface tension exterior solution (71.11 mN/m). This outward fluid movement creates favorable conditions for tentacle growth. In contrast, in the vertical direction, the presence of the coacervate layer prevents fluid from escaping, thereby inhibiting tentacle formation.

By adjusting the surfactant concentration, we modulate the intensity of the Marangoni flow [33] and thus alter the tentacle growth rate. Meanwhile, changing the Alg concentration (which means adjusting the viscosity) regulates the speed of droplet deformation [34]. These two factors act antagonistically to jointly determine the growth process of the tentacles (Figure S4). Intuitively, with increasing surfactant concentration, the tentacles become longer and more densely distributed around the droplet (Figure 3a–c). In contrast, as the solution concentration decreases, the droplet’s overall spreading area increases significantly, yet the tentacle morphology does not vary uniformly (Figure 3d). At certain concentrations, the tentacles appear much more elongated and denser than under other conditions (Figure 3e), whereas at lower concentrations, they look swollen (Figure 3f).

In order to investigate these changes more precisely, we analyzed the area and perimeter of the jellyfish-like droplets through image processing and then calculated the perimeter-to-area ratio (P/A) to quantify tentacle growth. When the surfactant concentration reached 0.2 wt%, the morphological variations in the tentacles became relatively limited (Figure 3g). Although higher surfactant concentrations can accelerate tentacle growth, the increased solution viscosity limits fluid mobility, thereby constraining the growth rate. As the solution viscosity decreases, such constraints are alleviated, resulting in denser, longer tentacles. However, when the viscosity is too low, the droplet spreads so quickly that many tentacles are covered by the expanding liquid before they can fully form. This behavior manifests as an initial rise, followed by a subsequent decline, in the P/A value (Figure 3h).

Figure 4a shows the tentacle growth of the biomimetic jellyfish-like droplet as captured by a camera. Once the droplet contacts the solution surface, the carboxyl groups in Alg rapidly bind to the amino groups in ε-PL, forming a coacervate barrier through electrostatic interactions. This coacervate layer between the two aqueous phases prevents the further diffusion of solutes from the Alg droplet into the solution beneath [35]. Meanwhile, a strong Marangoni flow drives the droplet to spread outward across the horizontal liquid surface. Since the rate of droplet spread is slower than the rate of interfacial complexation, some peripheral regions form a dense coacervate layer earlier, leading to spatially nonuniform diffusion confinement. In contrast, regions with weaker complexation allow for continued solute transport, giving rise to protrusions and the eventual formation of “tentacle”—like structures. This process occurs within approximately six seconds and the tentacle morphology remains stable thereafter, as shown in the time-lapse images (Figure 4a). To further support the proposed formation mechanism, microscopic imaging of the edge regions (Figure 4b) revealed spatial variations in thickness across the tentacle structure. From left to right, the images represent the tentacle structure extending from its base toward the outer edge. A dense shadow adjacent to the root of the tentacle indicates the formation of a thick coacervate layer at the interface. As the tentacle extends outward, the microscopic images become progressively lighter, suggesting a gradual reduction in material content and concentration. Additionally, the accompanying video (Supporting Information Video S1) dynamically captures the formation and stabilization of the tentacle structures within seconds, further corroborating the proposed mechanism.

The SEM images of the central, transition, and peripheral regions of the jellyfish-like droplets (Figure 5a) reveal that the central region of the droplet is relatively dense (Figure 5b). As the structure extends towards the tentacle edges, it becomes increasingly porous and loose. This porous architecture facilitates the attachment of enzymes and other nanomaterials [36]. In the transition zone at the periphery of the central circular region, which corresponds to the base of the tentacles, wrinkle-like structures are observed (Figure 5c). These wrinkles are likely a result of the antagonistic forces between viscous forces and Marangoni flow. At positions closer to the tentacle edges, a sponge-like porous structure becomes evident (Figure 5d,e), and the tentacle itself becomes extremely thin, with a thickness of approximately 3.46 μm.

Further elemental composition analysis was performed on the three regions using EDS point scanning (Figure 5f–h, Table S1). The low sodium content indicates that the counterions (Na⁺) from the Alg and ε-PL complex dissociate into the surrounding solution, providing the coacervate predominantly composed of polymer chains that have lost their coordinating counterions. A significant change in sulfur content was observed as the tentacles developed—the proportion of sulfur, originating from SDS, increased from 2.2% in the center to 5.3% at the edge. Additionally, the oxygen content from the sodium alginate decreased from 28.3% in the center to 11.1% at the periphery, indicating that alginate concentration is higher in the central region and gradually decreases towards the outer tentacles. Another notable change was in the nitrogen content from ε-PL, which increased from 11.8% in the center to 17.5% at the tentacle edge, supporting the idea that ε-PL interacts with SDS in the outer regions. Since the complexation between ε-PL and SDS is weaker than that with alginate, this likely contributes to the looser, more porous sponge-like structure observed at the tentacle periphery (Figure 5e). The coacervate forming the tentacles contains abundant hydroxyl, carboxyl, and amino groups, which facilitate enzyme attachment [37,38]. These findings confirm that as the position shifts from the central region to the tentacle edges, ε-PL/SDS complexes progressively replace ε-PL/Alg complexes. Due to the lower molecular weight and weaker complexation strength of ε-PL/SDS compared to ε-PL/Alg, the central region remains dense and compact, while the tentacle edges represent a looser, more porous structure.

The FT-IR analysis shows peaks at 1407 cm⁻¹ and 1630 cm⁻¹ (carboxylate –COO⁻ stretching from sodium alginate), 1036 cm⁻¹ (S=O stretching from sulfate ester), and 1540 cm⁻¹ (amide II band from N–H bending and C–N stretching). The 1540 cm⁻¹ peak is notably stronger in the peripheral region than in the central region, consistent with EDS results, confirming the increased presence of ε-PL/SDS complexes at the periphery (Figure 6).

As all of the materials used are biocompatible, and the porous tentacle structure facilitates enzyme binding, the biomimetic jellyfish-like hydrogel is well-suited for enzyme immobilization in catalytic applications. The porous morphology increases the surface area and provides physical confinement for enzyme molecules (Figure S5). Meanwhile, potential weak interactions—such as hydrogen bonding, electrostatic attraction, or hydrophobic interactions—may also contribute to the immobilization. Given its high perimeter-to-area ratio, we selected 1 wt% Alg with 0.2 wt% SDS for hydrogel fabrication. As shown in Figure 7a, a droplet was first deposited onto a 1 wt% ε-PL solution to form a biomimetic jellyfish-like droplet. After stabilization for 10 min, 0.1 mL of 25 wt% Fe₃O₄ nanoparticle dispersion was added to the center, allowing the nanoparticles to uniformly sediment at the bottom, and they were then left undisturbed for another 10 min. The droplet was then transferred to 0.1 M CaCl₂, inducing Alg crosslinking to encapsulate the Fe₃O₄ nanoparticles (10 min). After washing with deionized water, the hydrogel was immersed in 0.25 mg/mL catalase solution, gently stirred (5 h), and incubated (10 min) to allow enzyme attachment. The hydrogel was then washed and stored in PBS (Figure 7b). The immobilization likely involves a combination of physical entrapment facilitated by the hydrogel’s porous tentacle structure, as well as possible non-covalent interactions (hydrogen bonding, electrostatic or hydrophobic interactions), similar to mechanisms reported in previous studies [39,40]. Zeta potential measurements further support this interpretation, showing that catalase carries a net negative charge while ε-polylysine (ε-PL) is positively charged under experimental conditions, indicating that electrostatic attraction plays a key role in the enzyme attachment process [41,42]. As the tentacle regions of the hydrogel are enriched in ε-PL, these regions exhibit higher local loading as a result of enhanced electrostatic interaction. MTT assays confirmed the biocompatibility of the system. PBS solutions containing 1 wt% Alg with varying SDS concentrations (Figure 7c), as well as the biomimetic jellyfish-like hydrogels formed from them (Figure 7d), showed no significant changes in corrected OD values compared to the blank group, indicating no cytotoxicity [43,44].

In practical biocatalytic applications, enzymes are often subjected to environmental fluctuations, including extreme pH or temperatures conditions. Therefore, the chemical and catalytic robustness of the immobilized enzyme system is a key factor for its reusability and practical implementation. Compared with the hydrogel without tentacles, the jellyfish-like hydrogel exhibited higher catalytic efficiency (Figure 8a), which can be attributed to its enhanced enzyme loading capacity enabled by the tentacle structure (Figure S6). As shown in Figure 8b,c, the relative activity of catalase (CAT) immobilized within the biomimetic jellyfish-like hydrogel declined significantly more slowly than that of free CAT under both acidic (pH = 5) and alkaline (pH = 10) conditions, indicating better pH tolerance. This improved pH stability may be attributed to the porous tentacle network of the hydrogel, which physically confines the enzyme and provides a microenvironment that buffers against extreme pH-induced denaturation. In addition, the long-term thermal and storage stability of the immobilized enzyme system was investigated by incubating the jellyfish-like hydrogel-immobilized CAT at different temperatures (40 °C, 25 °C, and 5 °C) for 7 days (Figure 8d–f). Compared with the free CAT, the immobilized CAT retained a significantly higher proportion of its initial activity over time. At 5 °C, a common storage temperature, immobilized CAT maintained its activity better than the free enzyme, indicating that the hydrogel provides structural stabilization across diverse thermal conditions. Moreover, we tested the reusability of the hydrogel platform using other enzymes, including α-amylase and β-amylase. As illustrated in Figure 8g–i, all three enzymes retained over 50% of their initial activity after 12 catalytic cycles, highlighting the practical reusability of the jellyfish-like hydrogel framework for different enzyme systems. These results demonstrate that the jellyfish-like hydrogel not only enables effective enzyme immobilization but also enhances enzyme durability under multiple conditions, thereby confirming its potential for robust and reusable biocatalysis.

The biomimetic jellyfish-like hydrogel was used to simulate a spatially controlled enzymatic reaction in 0.2 wt% H₂O₂ solution (Supporting Information Video S2). Figure 9a–c demonstrate its controlled movement under non-contact magnetic actuation, forming a rectangular trajectory. In contrast, Figure 9d–f show a linear back-and-forth motion of a non-jellyfish-like hydrogel under identical conditions, suggesting the maneuverability of the biomimetic jellyfish-like hydrogel and the effective functionality of the magnetically loaded nanoparticles introduced via simple injection. The distinct motion trail left by the hydrogel is attributed to oxygen microbubbles adhering to the container bottom, whereas the non-jellyfish-like hydrogel leaves no visible trace. This suggests a significantly higher catalase loading capacity in the biomimetic jellyfish-like hydrogel, which is directly related to its high perimeter-to-area ratio and porous tentacle structure, providing an expanded enzyme immobilization space.

4. Conclusions

In this study, we developed a biomimetic jellyfish-like hydrogel with a highly tunable structure and enhanced enzymatic catalytic performance by leveraging Marangoni flow-driven self-assembly at the liquid–liquid interface. The formation mechanism was systematically investigated, revealing that SDS-induced Marangoni flow drives tentacle formation, while Alg viscosity restricts outward extension, resulting in a high perimeter-to-area ratio and a tentacle-rich structure. SEM and EDS analyses confirmed the gradual transition from a dense central region to a porous tentacle periphery, which is attributed to the progressive replacement of ε-PL/Alg complexes by ε-PL/SDS complexes. FT-IR spectroscopy further validated the composition distribution, supporting the structural findings.

By incorporating Fe₃O₄ nanoparticles and catalase, the hydrogel demonstrated excellent spatial control and catalytic efficiency, as evidenced by its precise magnetically guided motion and distinct oxygen bubble trails in H₂O₂ solution. The high catalase loading capacity was directly correlated with the porous tentacle structure, which enhances enzyme immobilization. Furthermore, comparative studies of relative enzymatic activity under different conditions—including pH extremes, temperature variation, and repeated reuse—confirmed that the immobilized enzymes retained significantly higher activity than their free counterparts, demonstrating the hydrogel’s practical potential in biocatalysis. Biocompatibility assessments via MTT assays confirmed that the biomimetic hydrogel exhibits no cytotoxicity, ensuring its suitability for biocatalytic and biomedical applications.

Overall, this study presents a simple and scalable strategy for fabricating biomimetic hydrogels with tunable morphology and high enzymatic catalytic efficiency. This approach not only advances the design of bioinspired catalytic platforms but also offers new opportunities for applications in biocatalysis, controlled drug delivery, and bioinspired material engineering.

Author Contributions

Investigation, A.Z. and Y.G.; data curation, A.Z., H.Z. and Y.G.; writing—original draft, A.Z.; validation, H.Z.; supervision, Y.F.; funding acquisition, Y.F.; project administration, Y.F.; writing—review and editing, Y.F. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data is contained within the article or Supplementary Materials.

Acknowledgments

Thanks to the Equipment Support Team from the Analytical and Testing Center of Northeastern University.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:

Alg	Sodium Alginate
ε-PL	ε-Poly-L-lysine

Footnotes

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Figures and Table

Figure 1 Schematic diagram of the experimental setup.

Figure 2 (a) A schematic diagram of the formation process of biomimetic jellyfish-like droplets; (b) a top-view photograph of the biomimetic jellyfish-like droplet; (c) a side-view photograph of the biomimetic jellyfish-like droplet.

Figure 3 Photographs of biomimetic jellyfish-like droplets formed under different solution conditions: (a) 0.1 wt% SDS, 2 wt% Alg; (b) 0.2 wt% SDS, 2 wt% Alg; (c) 0.3 wt% SDS, 2 wt% Alg; (d) 0.2 wt% SDS, 1.5 wt% Alg; (e) 0.2 wt% SDS, 1.0 wt% Alg; (f) 0.2 wt% SDS, 0.5 wt% Alg. (g,h) Present the relative perimeter and area, as well as the perimeter-to-area ratio (P/A), calculated via image recognition for the droplets in (a–c) and (d–f), respectively.

Figure 4 The tentacle formation process of the biomimetic jellyfish-like hydrogel. (a) Time-lapse images showing tentacle growth within 6 s after droplet deposition. (b) Microscopic images from tentacle root to tip (from left to right).

Figure 5 (a) An optical image of a biomimetic jellyfish-like droplet formed by depositing 0.2 wt% SDS and 1 wt% Alg into a 1 wt% ε-PL solution, with three distinct regions marked. (b–d) SEM images of the jellyfish-like droplet at different regions: (b) the central region, (c) the transition region, and (d) the peripheral region. (e) A cross-sectional SEM image of the tentacle edge. EDS point scanning results for elemental composition analysis of (f) the central region, (g) the transition region, and (h) the peripheral region, showing the distribution of C, N, O, Na, and S.

Figure 6 FT-IR spectra of the central and edge regions of a biomimetic jellyfish-like droplet formed by dropping 1 wt% Alg (with 0.2 wt% SDS) onto 1 wt% ε-PL solution.

Figure 7 (a) A schematic illustration of the biomimetic jellyfish-like hydrogel fabrication process. (b) A photo of the fabricated biomimetic jellyfish-like hydrogel. (c) The zeta potential measurements of individual components used in the hydrogel system under the experimental conditions, CAT (−11.67 ± 0.42 mV), ε-PL (18.87 ± 1.84 mV), and Alg (−47.36 ± 2.72 mV). (d) The cytotoxicity assay results of Alg solutions with varying SDS concentrations. (e) The cytotoxicity assay results of biomimetic jellyfish-like hydrogels formed from Alg solutions with varying SDS concentrations.

Figure 8 (a) Decomposition of H₂O₂ catalyzed by free CAT, CAT immobilized in hydrogel without tentacles, and CAT immobilized in jellyfish-like hydrogel over time. (b,c) Relative activity of free CAT and jellyfish-like hydrogel-immobilized CAT after incubation at pH = 5 (b) and pH = 10 (c) in PBS buffer for up to 60 min. (d–f) Relative activity of immobilized and free CAT incubated at 40 °C (d), 25 °C (e), and 5 °C (f) for 7 days, respectively. (g–i) Relative activity of catalase (g), α-amylase (h), and β-amylase (i) immobilized in jellyfish-like hydrogel over 12 catalytic cycles.

Figure 9 The motion states of the biomimetic jellyfish-like hydrogel at (a) 0 s, (b) 6 s, and (c) 12 s, respectively. (d–f) The motion states of the hydrogel without surfactant at (d) 0 s, (e) 3 s, and (f) 6 s, respectively.

Table 1

Surface Tension Measurement.

Solution	Surface Tension (mN/m)
Alg	64.69 ± 1.74
Alg with SDS	35.72 ± 1.52
ε-PL	71.11 ± 1.48

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/surfaces8020028/s1, Figure S1: Images of surface tension measurement results; Figure S2: Image processing of biomimetic jellyfish-like droplets; Figure S3: Photographs of Alg droplets without surfactants on the ε-PL solution surface; Figure S4: Biomimetic jellyfish-like droplets formed under different ε-PL concentrations; Table S1: Elemental weight percentages of C, N, O, Na, and S; Figure S5: Images of magnetic nanoparticle-loaded jellyfish-like and tentacle-free hydrogels; Figure S6: CAT loading capacity of the jellyfish-like hydrogel and tentacle-free hydrogel; Video S1: Biomimetic jellyfish-like hydrogel; Video S2: The time-lapse microscopic recording of tentacle formation in a jellyfish-like droplet.

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