Macro- and Microscale Properties of the Vitreous Humor to Inform Substitute Design and Intravitreal Biotransport

Abstract

Research on the vitreous humor and development of hydrogel vitreous substitutes have gained a rapid increase in interest within the past two decades. However, the properties of the vitreous humor and vitreous substitutes have yet to be consolidated. In this paper, the mechanical properties of the vitreous humor and hydrogel vitreous substitutes were systematically reviewed. The number of publications on the vitreous humor and vitreous substitutes over the years, as well as their respective testing conditions and testing techniques were analyzed. The mechanical properties of the human vitreous were found to be most similar to the vitreous of pigs and rabbits. The storage and loss moduli of the hydrogel vitreous substitutes developed were found to be orders of magnitude higher in comparison to the native human vitreous. However, the reported modulus for human vitreous, which was most commonly tested in vitro, has been hypothesized to be different in vivo. Future studies should focus on testing the mechanical properties of the vitreous in situ or in vivo. In addition to its mechanical properties, the vitreous humor has other biotransport mechanisms and biochemical functions that establish a redox balance and maintain an oxygen gradient inside the vitreous chamber to protect intraocular tissues from oxidative damage. Biomimetic hydrogel vitreous substitutes have the potential to provide ophthalmologists with additional avenues for treating and controlling vitreoretinal diseases while preventing complications after vitrectomy. Due to the proximity and interconnectedness of the vitreous humor to other ocular tissues, particularly the lens and the retina, more interest has been placed on understanding the properties of the vitreous humor in recent years. A better understanding of the properties of the vitreous humor will aid in improving the design of biomimetic vitreous substitutes and enhancing intravitreal biotransport.

Crystalline Lens Vitamin C Levels in Cataract Surgical Patients

“…Other Authors reported that the physiological concentration of Vitamin C in the lens is 1 -2 mM (0.174 -0.348 g) [28], The fact that there was no significant difference in the levels of Vitamin C found in the two groups underscores the need for supplementation in both elderly and diabetic Nigerian populations. A study which analysed antioxidant markers such as catalase, Cu-Zn superoxide dismutase in these two groups found diabetic patients having significantly lower levels than their non-diabetic counterparts and proposed that elevated glucose levels might be involved in the insufficient production of antioxidant enzymatic activ-ity [29].…”

Novel vitreous substitutes: the next frontier in vitreoretinal surgery

“…An ideal vitreous substitute should be based on the viscoelastic properties of the natural, juvenile and healthy human vitreous (10 Pa). In contrast, a majority of the previously proposed vitreous substitutes have stiffnesses that are orders of magnitude higher than those of the reported human vitreous body, reviewed by Tram et al [13], and may thus cause mechanical damage to the surrounding tissue.…”

“…Recent tamponade strategies therefore aim at hydrophilic, hydrogelbased systems because of their favourable properties such as high water content, optical transparency, suitable refractive indices and densities, adjustable rheological properties, injectability, biocompatibility and their ability to tamponade the retina via viscosity and swelling pressure. Recent reviews affirm the relevance and need for new vitreous replacement strategies [8][9][10][11][12][13]. The following review will provide an update on new vitreous substitutes with a focus on polymer-based systems (for systematic literature review see Supplementary Information, http://links.lww.com/COOP/A39).…”

Lab-on-a-chip technologies for minimally invasive molecular sensing of diabetic retinopathy

“…The VH is important for the structural integrity of the eye as well as in nutrient and oxygen trafficking to other avascular structures of the eye. 26,27 Therefore, the VH coordinates eye growth and provides support to the retina. 28 Due to the VH's proximity to the retina and avascular nature, pathological changes of the retina are often reflected in the VH.…”