From improving eyesight to disease theranostics: The impact of ocular fluid mechanics research

The eye, serving as both an outward window to the world and an inward window into our health, represents a unique dichotomy. This is mirrored in the research in the field of ocular fluid mechanics,¹ which seeks to enhance both human vision, and ocular therapeutics and diagnostics (i.e., theranostics). Research into developing better contact lenses,^2–5 advancements in artificial corneas,^6,7 mathematical models for tear film dynamics,^8,9 and noninvasive tools for tear film visualization^10–13 have collectively enhanced vision for millions affected by ocular disorders. Similarly, the exploration of intraocular drug delivery,^14,15 retinal imaging,^16–18 and intraocular flows^19,20 has not only pushed the boundaries in treating ocular disorders but also opened a significant window for diagnosing and treating systemic disorders such as diabetes, autoimmune disorders, and neurological conditions. The articles in this special issue explore this dichotomy and are briefly summarized below.

The articles by Vega et al.²¹ and Taranchuk et al.²² provide a better understanding of the multicomponent human tear film. Vega et al. explored the viscoelastic behavior of both artificial and human tears using a microrheological approach—a scalable and translatable method for characterizing human tear films. Interestingly, they discovered that human tear films have an order of magnitude longer viscoelastic relaxation time compared to certain artificial tear films, offering substantial guidance for the development of superior artificial tear solutions. On the other hand, Taranchuk et al. propose an enhanced model for the lipid layer of the tear film. By acknowledging the crystalline characteristics of the lipid layer, they demonstrate the potential of using nematic liquid crystals as a more realistic model for the tear film lipid layer compared to Newtonian fluids. Their research suggests that the elasticity of nematic liquid crystals can induce complex patterns and behaviors during extension flow in the film, including rippling, thinning, and breaking, which echo in vivo behavior. Such behaviors are absent in Newtonian models, hinting at the promise of this new model in elucidating complex tear film dynamics more accurately.

The articles by Mandal et al.²³ and Wei et al.²⁴ concern the human cornea. Mandal et al. introduce a constitutive model that characterizes the mechanical behavior of the human cornea, utilizing a fractional calculus approach. This model considers the cornea as a viscoelastic material with a power-law relaxation function and uses fractional derivatives to describe its time-dependent response to mechanical loading. Models of this nature could serve as predictive tools for corneal biomechanics, guide surgical procedures, and inspire the development of advanced artificial corneas. Motivated by the significance of artificial corneas, Wei et al. describe a novel method to fabricate biomimetic corneas with variable focus utilizing profile-followed coating (PFC) of UV-curable liquid onto an array of glycerol droplets. The resulting artificial cornea system has a large focus range under a modest applied voltage of 5 V. This innovation not only holds promise for replacing diseased human corneas but also opens possibilities for tunable optical systems.

In some instances, visual impairments can be addressed without corneal replacement, utilizing phakic intraocular lenses (PIL). In their study, Yifan et al.²⁵ delved into the effects of implanting phakic intraocular lenses (PIL) on the flow of aqueous humor, using a numerical approach. The study contrasts a novel sine wave phakic refractive lens with two established PILs, namely, the implantable contact lens and the posterior chamber phakic refractive lens. Results from the computational fluid dynamics model reveal that the sine wave phakic refractive lens outperforms the existing options in improving aqueous humor flows, consequently reducing the intraocular pressure, increasing vaulting distance, decreasing shear stress, and enhancing oxygen transport. Investigations of this nature can contribute to the enhancement of phakic lens design, leading to minimal disruption of natural aqueous humor dynamics.

Understanding aqueous humor dynamics is equally critical for therapeutic strategies. Sacco et al.,²⁶ employing a hydraulic network model for aqueous humor circulation and intraocular pressure, investigated the role of conventional and unconventional adaptive pathways in intraocular pressure reduction. Such models can be used as a quantitative tool to complement in vivo studies and optimize medications for neuro-ocular diseases such as glaucoma. Motivated by the importance of the flows in the eye for disease diagnostics, Julien et al.²⁷ studied microvascular hemodynamics in the retina. The microvascular hemodynamics is captured by solving the Navier–Stokes equations on a 1D network model constructed using morphometric and velocimetric data obtained through clinical multimodal imaging. With further work, these models could offer invaluable insights into the onset and progression of vascular retinal diseases, including diabetic retinopathy, age-related macular degeneration, and retinal vein occlusion.

Collectively, the articles in this special issue underscore the intricate interplay between fluid mechanics and ocular health and enhance our scientific understanding of numerous issues related to human vision and ocular theranostics. We hope that the scientific community will draw and build upon these articles to pave the way for a clearer and healthier future.

The guest editors would like to thank the editorial board of Physics of Fluids, especially the Editor-in-Chief Professor Alan Jeffrey Giacomin, the Journal Manager Dr. Mark Paglia, and the Editorial Assistant Jaimee-Ian Rodriguez, for their kind help and efforts.