Stability characterization of microfluidic lipid-stabilized double emulsions under physiologically-relevant conditions

Double emulsions (DEs) are water-in-oil-in-water (or oil-in-water-in-oil) droplets with the potential to deliver combinatory therapies due to their ability to co-localize hydrophilic and hydrophobic molecules in the same carrier. However, DEs are thermodynamically unstable and only kinetically trapped. Extending this transitory state, rendering DEs more stable, would widen the possibilities of real-world applications, yet characterization of their stability in physiologically-relevant conditions is lacking. In this work, we used microfluidics to produce lipid-stabilized DEs with reproducible monodispersity and high encapsulation efficiency. We investigated DE stability under a range of physico-chemical parameters such as temperature, pH and mechanical stimulus. Stability through time was inversely proportional to temperature. DEs were significantly stable up to 8 days at 4 oC, 5 days at RT and 2 days at 37 oC. When encapsulating a cargo, DE stability decreased significantly. When exposed to a pH change, unloaded DEs were only significantly unstable at the extremes (pH 1 and 13), largely outside physiological ranges. When exposed to flow, unloaded DEs behaved similarly regardless of the mechanical stimulus applied, with approximately 70% remaining after 100 flow cycles of 10s. These results indicate that lipid-stabilized DEs produced via microfluidics could be tailored to endure physiologically-relevant conditions and act as carriers for drug delivery. Special attention should be given to the composition of the solutions, e.g. osmolarity ratio between inner and outer solutions, and the interaction of the molecules, e.g. carrier and cargo, involved in the final formulation.双重乳液（Double emulsions, DEs）是指油包水包油（或水包油包油）液滴，因其可在同一载体中共定位亲水与疏水分子，具备递送联合疗法的潜力。然而，双重乳液在热力学上并不稳定，仅处于动力学捕获状态。若能延长这一暂态并提升其稳定性，将拓宽其实际应用场景，但目前学界尚缺乏其在生理相关条件下的稳定性表征研究。本研究采用微流控技术（microfluidics）制备了具备可重复单分散性（monodispersity）与高包封率（encapsulation efficiency）的脂质稳定型双重乳液。我们探究了其在温度、pH值、机械刺激（mechanical stimulus）等一系列理化参数（physico-chemical parameters）下的稳定性表现。双重乳液的稳定性留存率与温度呈负相关：脂质稳定型双重乳液在4℃下可稳定留存长达8天，室温（room temperature, RT）下为5天，37℃下则为2天。当包封载荷物（cargo）后，双重乳液的稳定性会显著下降。当遭遇pH变化时，未包封载荷物的双重乳液仅在极端pH值（pH 1与pH 13）下才会出现显著不稳定，而该范围基本超出了生理范围（physiological ranges）。当受到流动刺激时，未包封载荷物的双重乳液的稳定性表现与施加的机械刺激类型无关；在10秒/次的100次流动循环（flow cycles）后，仍有约70%的液滴保持完整。上述结果表明，通过微流控技术制备的脂质稳定型双重乳液可通过定制化调整以适配生理相关条件，可用作药物递送（drug delivery）载体。研究中需特别关注溶液的组成参数，例如内、外溶液间的渗透压比（osmolarity ratio），以及最终制剂中涉及的分子（如载体与载荷物）之间的相互作用。