(400h) Nanoemulsion-Laden Hydrogels for Tunable Release of Immiscible Active Ingredients

Emulsions and surfactants maintain the stability of cargo-laden hydrogels and are crucial in drug delivery, especially in cases where independently tunable transport of otherwise immiscible molecules is required. This study leverages a thermoresponsive oil-in-water nanoemulsion system to investigate how nanoemulsion microstructure affects the delivery kinetics of hydrophilic and hydrophobic molecules. The precursor consists of 20 vol% poly(dimethylsiloxane) droplets (diameter 2a = 46.3 nm ± 68%) dispersed in a continuous phase containing 33 vol% poly(ethylene glycol) diacrylate (PEGDA, molecular weight of 700 Da) and 200 mM sodium dodecyl sulfate. Hydrophilic methylene blue and hydrophobic coumarin-6 (concentrations c_MB = c_C6 = 0.1 mg/ml) are loaded into the nanoemulsions. Above the gelation point (T_gel = 35.5 ± 1.8°C), nanoemulsions self-assemble and form a bicontinuous network with tunable morphology. At different temperatures (ΔT = T - T_gel = 0, 2, 4, 5, 10, and 15°C), gelled nanoemulsions are photocrosslinked under ultraviolet light to form organohydrogels. Confocal laser scanning microscopy shows that the characteristic length scales of the oil domains peak at ΔT = 2°C before decreasing at higher temperatures. Diffusion studies conducted using UV-vis spectroscopy over a span of 6 days shows that D_eff is non-monotonic with ΔT for both C6 and MB where C6 decreases at 2°C before increasing and MB has a minimum at 5°C. The diffusion coefficient of C6 scales inversely with the characteristic length scale with a power law dependence while the diffusion coefficient of MB scales inversely with the characteristic length above a critical mesh size at ΔT = 5°C. We hypothesize that hydrophobic C6 traverses the network of hydrophobic domains where the smaller channels may be more connected, resulting in a higher diffusion coefficient at lower temperatures. On the other hand, hydrophilic MB diffuses through the aqueous network where the transport is analogous to cylindrical Hagen-Poiseuille type flow in which a smaller channel width results in faster diffusion. By adjusting the synthesis temperatures, these organohydrogels can be engineered to release either hydrophilic or hydrophobic drugs tailored to individual patient needs.