When Shear and Interfaces Matter: In Vivo Water-in-Silicone Oil Droplet Formation during Long-Term Vitreous Tamponade

Silicone oil (SO) is widely used as a long-term intraocular tamponade, yet its multiphase behavior in vivo remains incompletely understood. While oil-in-water (O/W) emulsification has been extensively studied, the potential formation of water-in-oil (W/O) droplets within the oil phase has received little attention. Here, we provide the first systematic in vivo evidence for W/O droplet formation during long-term vitreous tamponade using a controlled porcine model complemented by analysis of explanted human SO samples. W/O droplets were detected directly within the vitreous cavity, demonstrating that intraocular SO functions as a dynamic multiphase soft material rather than a strictly single continuous phase. A pronounced dependence of droplet size distributions on sampling conditions was observed. Aspiration through narrow-gauge needles induced shear-driven droplet fragmentation, whereas coaspiration of aqueous fluid during infusion generated large artifactual droplets, highlighting that explantation procedures can strongly bias ex vivo observations. Oil viscosity modulated the sensitivity of the system to shear but did not determine droplet occurrence. Complementary in vitro experiments confirmed that needle passage fragments pre-existing droplets rather than inducing de novo droplet formation. By integrating in vivo observations with in vitro phase behavior and interfacial viscoelasticity data, we establish a mechanistic framework in which shear-driven emulsification and protein-mediated interfacial stabilization govern W/O droplet formation and persistence in oil-dominated biological environments. These findings demonstrate that intraocular SO should be regarded as a confined, protein-active multiphase material system and that clinical sampling itself represents a microcapillary shear process influencing observed microstructures. Beyond ophthalmology, this work highlights the critical role of interfacial phenomena and sampling-induced artifacts in the characterization of oil-based biomaterials and soft multiphase systems in complex biological settings.