In-Vitro Simulations of Drug Delivery in the Vitreous Chamber of the Eye

The vitreous chamber has an approximately spherical shape, and is filled with vitreous humor, a transparent material with viscoelastic properties. The vitreous humor has the mechanical roles of supporting the eye shape, promoting the adherence between the retina and the choroid, and acting as a barrier between the anterior and posterior segments of the eye for transport of heat and molecules [1]. Often, in elderly people, the vitreous humor has almost Newtonian properties as a consequence of a liquefaction process consisting of degradation of the collagenous framework of the gel. Moreover, after vitrectomy the vitreous humor is sometimes replaced with viscous tamponade fluids (typically silicone oils). Since intra-vitreal drug injection is increasingly used to treat retinal diseases, and the efficacy of this procedure depends on molecular transport processes after the injection, much of the biomechanical research on the vitreous humor has focused on understanding transport processes in the vitreous chamber. Many authors have considered purely diffusive transport or alternatively diffusion with advective transport due to creeping bulk flow only [2,3]. However, when the vitreous is liquefied, rotational motion of the eye has been shown to induce significant fluid flow, which plays a fundamental role for drug delivery in the vitreous chamber [4–7]. The most significant contribution to mass transport is due to the generation of a steady streaming flow, i.e. a steady flow resulting from the non-linearity in the equations. This flow component gives rise to a slow particle drift that becomes dominant over long times.