Droplet-Encapsulated Gels: Fabrication & Mechanics

• Droplet-encapsulated gels are multicomponent soft materials confined in liquid droplets that enable precise control of capillarity, elasticity, and interfacial composition.
• Fabrication techniques like microfluidics, liquid–liquid phase separation, and UV-crosslinking produce uniform core–shell structures with tunable properties.
• These gels exhibit coupled elastocapillary dynamics, responsive behavior, and broad applications in tissue engineering, drug delivery, and soft robotics.

Droplet-encapsulated gels are multicomponent soft materials in which the mechanics, structure, and function of gel phases are dictated by their confinement within liquid droplets. These systems enable precise control of microenvironmental parameters—capillarity, elasticity, interfacial composition, and rheology—and underpin current advances in biomaterials, soft robotics, mechanobiology, food science, and solution-phase gel physics. Technologies for droplet encapsulation span microfluidics, liquid–liquid phase separation, interfacial polymerization, and composite colloidal network assembly. The resulting gels exhibit tunable deformation, highly controllable organization, and stimuli-responsiveness. Droplet-encapsulated gels are systematically analyzed in the context of fabrication protocols, elasticity-capillarity interplay, internal microstructure, mechanical properties, and emerging applications.

1. Fabrication Strategies for Droplet-Encapsulated Gels

Microfluidic generation is the dominant platform for producing uniform droplet-encapsulated gels, offering high control over size, composition, and throughput. In co-flow geometries, millimeter-scale hydrogel droplets (0.5–2 mm diameter) are formed using rapid prototyping in thermoplastic and laser-cut molds, or durable PDMS casts. For example, Matrigel droplets are generated in channels with widths ranging 1.5–4.0 mm and heights 1.6–3.18 mm; the use of off-the-shelf systems and tape-based hydrophobic surfaces facilitates fabrication within 30 min for rapid-proto chips or 1 day for PDMS molds (Arnold et al., 2023).

Liquid–liquid phase separation (LLPS) enables the templating of core–shell microcapsules: gelatin-rich shells are formed around PEG-rich cores by controlled demixing in flow-focusing microfluidic chips, with shell thickness modulated by flow-rate ratios and crosslinking controlled through physical (triple-helix formation) and enzymatic (mTGase-mediated) processes (Xu et al., 2020). Emulsion-templated interfacial complexation utilizes thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAM) grafted to poly(L-lysine) to build robust gel shells at the droplet–oil interface. Heating above PNIPAM’s LCST triggers irreversible polymer enrichment at the interface and sequestration of cargo (proteins/nanoparticles) into the shell (Sixdenier et al., 2021).

UV-crosslinkable droplets, generated by flow-focusing microfluidics, employ polymer precursors (e.g., PEGDA) and photoinitiators, and achieve gelation via spatially resolved UV exposure. Incomplete curing produces core–shell structures, with gel formation initiated from the droplet center outward due to oxygen inhibition at the periphery (Marnoto et al., 23 Aug 2025).

2. Capillarity–Elasticity Coupling and Deformation Scaling

The mechanical behavior of droplet-encapsulated gels is fundamentally governed by the elastocapillary number $E_c = \gamma/(G\,R)$, where $\gamma$ is interfacial tension, $G$ is shear modulus, and $R$ is bead radius. Single gel beads pressed against the droplet interface flatten to produce contact discs whose radius scales as $a/R \propto E_c^{1/3}$, with experimental prefactor $C \simeq 1.4$ (Saita et al., 22 Nov 2025). Capillarity dominates ($E_c \gg 1$) for soft or small beads, inducing greater deformation; elasticity dominates ($E_c \ll 1$), suppressing interface-induced structural changes.

Theoretical frameworks employing mean-field free energy minimization in polymer–gel mixtures undergoing elastic phase separation yield the scaling law for stable microdroplet size: $R^* \approx 2\gamma/(\alpha G)$ ($\alpha \approx 2.5$), linking interfacial tension and gel modulus (Biswas et al., 2021).

Droplet breakup and size control in microfluidic channels operate in a geometry-controlled regime—verified by low capillary and Weber numbers (e.g., $\mathrm{Ca}_\text{oil} \approx 3.96 \times 10^{-4}$, $$\mathrm{We}_\text{disp} \approx 1.23 \times 10^{-7} – 4.93 \times 10^{-6}$$ for millimeter-scale Matrigel droplets). Empirical relations for diameter as a function of flow-rate ratio ($\phi = Q_\text{oil}/Q_\text{disp}$) follow $$D_\text{MG}(\phi) \approx 2.10\,\text{mm}\cdot\phi^{-0.28}$$ (Arnold et al., 2023).

3. Internal Gel Microstructure and Synthesis Confinement

Polymer network topology in nanogels synthesized within droplets is strongly determined by the degree of confinement at the time of crosslinking. Strongly confined systems (droplet radius comparable to chain contour length) yield shell-like architectures with central depletion in density. Moderately confined systems form centrally dense, core-rich networks. These differences manifest in mesh-size distributions, shortest path lengths, swelling/collapse behavior, and form factor signatures in scattering measurements (Minina et al., 2019).

Crosslinking density ($\phi_\text{links}$) tunes both network connectivity and mesh-size polydispersity. Ionic gels undergoing Debye–Hückel-mediated swelling demonstrate pronounced responsiveness to ionic strength, modifying radial density profiles and asphericity post-synthesis.

In composite protein–oil emulsion gels, network microstructure is fractal on micron scales (fractal dimension $D_f \approx 1.7$), with indistinguishable mesh sizes (5–20 $\mu$m) for droplet and protein strands at matched effective volume fraction (Roullet et al., 2020). This supports the composite-network model treating droplets and proteins as co-participating colloidal nodes.

4. Organization, Rheology, and Composite-Gel Mechanics

Encapsulation of multiple gel beads in single droplets leads to organized structures, dictated by the total capillary–elastic energy. Low bead fraction or large $E_c$ produces linear chains; higher bead loadings or small $E_c$ favor close-packed, three-dimensional clusters. Energetic analysis—including bead–interface contacts and angular capillary attraction—predicts final aggregate geometry (Saita et al., 22 Nov 2025).

Composite emulsion gels prepared with variable protein:oil-droplet ratios exhibit plateau storage moduli ($G'$) and loss moduli ($G''$) scaling as power laws in total solids content: $$G'(\phi_{\text{eff}}) = G'_0 \phi_{\text{eff}}^{\alpha'}$$ ($G'_0 = 4.78$ kPa, $\alpha' = 3.10$ droplets; $G'_0 = 2.42$ kPa, $\alpha' = 2.70$ proteins). Frequency-dependent moduli scale as $G'(\omega) = G'_0(\omega/1\,\text{Hz})^\beta$ ($\beta_\text{drop} \approx 0.10$, $\beta_\text{protein} \approx 0.22$) (Roullet et al., 2020, Roullet et al., 2020). Composite modulus interpolates linearly between pure gel limits according to composition ratio $r$: $$G'_\text{mix}(r, \phi_\text{tot}) = r\,G'_\text{droplet} + (1-r)G'_\text{protein}$$; dynamic exponents ($\beta$) also interpolate linearly.

Nonlinear rheology reveals strain-stiffening in both droplet and protein gels at low volume fraction ($\phi_{\text{eff}} < 0.3$), with droplet gels yielding more sharply at higher solids content, indicative of brittle network behavior; protein gels display gradual softening (Roullet et al., 2020).

5. Interfacial Complexation, Responsive Behaviors, and Cargo Handling

Interfacial polymer complexation enhances droplet-encapsulated gel shell formation, especially with thermoresponsive copolymers such as PLL-g-PNIPAM. Upon heating above the LCST ($T_c \approx 34^\circ$C), coil–globule transitions of PNIPAM drive enrichment of the polymer shell and irreversible nanoparticle segregation to the interface, forming gel-like mixed shells with thickness $\sim$1.5 $\mu$m over droplet diameters of 5–50 $\mu$m (Sixdenier et al., 2021). Gel shells produced via this method offer protection and controlled release for biomacromolecule cargo, with tunable kinetics via polymer amount, NP loading, and thermal profile.

Core–shell gelatin microcapsules engineered by LLPS and microfluidic manipulation feature adjustable shell thickness per mass-conservation and surface energy minimization: $h = R - [R^3 - 3V_\text{gel}/(4\pi)]^{1/3}$, modulated by the flow rates of gelatin and PEG inlets (Xu et al., 2020). Mechanical robustness is governed by shell thickness ($E_\text{eff} \propto (h/R)^2$); shells withstand osmotic buckling, mechanical compression, and permit stepwise release of spatially segregated cargos.

Stimulus-responsive systems utilize pNIPAM microgels in colloid–polymer mixtures where temperature, salt, and pH trigger phase transitions. Shell gelation occurs rapidly above $T_c \approx 33^\circ$C, solidification by salt addition yields mechanically robust shells, and permeability can be controlled by ionic conditions (Dang et al., 2021).

6. Fundamental Physics: Active Droplet-Gel Dynamics and Self-Propulsion

Droplet–encapsulated gels modelled via the Brinkman–Stokes hydrodynamic framework demonstrate the possibility of active self-propulsion under body forces residing in permeable rigid gels (Kree et al., 2018). Key parameters include the Brinkman length $\ell = \sqrt{\kappa}$ (gel permeability), specifying the range over which internal forces generate flow. Optimal gel fractions $\phi^*$ maximize both linear (translation) and rotational velocities, with vanishing propulsion at extreme diluteness ($\phi \to 0$) or at space-filling limits ($\phi \to 1$).

Lighthill efficiency for active propulsion ($\varepsilon = P_\text{ext}(v_g)/P_\text{act}$) monotonically decreases with increasing gel fraction; only full Brinkman hydrodynamics (not the singular Darcy limit) support nonzero self-propelled velocity and rotational rates under standard boundary conditions. Analytical expressions permit explicit calculation of flow fields, traction matching, and swimmer dynamics, situating droplet–gel models in the broader context of autonomous soft matter and cytosol biophysics.

7. Applications, Design Regimes, and Future Directions

Droplet-encapsulated gels find utility in tissue engineering (immortalized fibroblast viability in Matrigel droplets exceeding 90% over 7 days (Arnold et al., 2023)), drug delivery (stable, stimuli-responsive release profiles (Xu et al., 2020, Sixdenier et al., 2021, Dang et al., 2021)), programmable biomacromolecule carriers, synthetic cell mimics, and soft robotics (mechanically tunable, bucklable shells (Xu et al., 2020)). Culinary and consumer-product domains leverage elastocapillary scaling for mouthfeel and controlled rupture.

Design of next-generation systems incorporates microfluidic scalability ($\leq$30 min rapid-proto throughput), compatibility with alternative hydrogels (alginate, GelMA, HA) (Arnold et al., 2023), and tunability across length scales ($<$0.5 mm droplets via channel width reduction and surfactant increase, $>$2 mm beads via flow-rate adjustment). UV-crosslinking protocols explicitly control core–shell gelation profiles, accounting for intensity and oxygen-inhibition (Marnoto et al., 23 Aug 2025).

Thermodynamic and mechanical theory provides predictive scaling for droplet radius and shell strength: $R^* \approx 2\gamma/(\alpha G)$, $h/R$ scaling for stiffness, and energy landscapes for bead arrangement. Multi-stimuli systems integrating thermal, ionic, mechanical, and compositional control will continue to expand the functional scope of droplet-encapsulated gels in research and application.