Abstract
The vitreous humor is a fragile, transparent hydrogel situated between the lens and the retina, occupying 80% of the eye's volume. Due to its viscoelastic behavior, the vitreous serves as a mechanical damper for the eye, absorbing impacts, and protecting the lens and retina. The vitreous liquefies with age, which compromises its function as a shock absorber and causes complications including retinal detachment, macular holes, and vitreous hemorrhage. Studies on the viscoelastic properties of the vitreous have been limited. Rheological testing of the vitreous has commonly been done on non-primate mammalian species. Human vitreous rheological properties have been previously reported; however, various measurement techniques were used, resulting in data that differed by orders of magnitude. Shear rheometry is commonly used to characterize soft tissues and hydrogels such as the vitreous humor. However, no human vitreous rheological data have been reported using this technique, preventing direct comparison to other published work. Additionally, no age-related changes in the mechanical properties of the human vitreous humor have been reported. Human vitreous samples (n = 39, aged 62 ± 15 years) were tested using a shear rheometer. Small amplitude oscillatory shear and creep experiments were performed. The linear viscoelastic region of the human vitreous was found to be below 1% strain. The solid phase of the old human vitreous was found to be stiffer than the young human vitreous and the porcine vitreous. The stiffness of the human vitreous gel also appeared to be positively correlated with age. Vitreous dehydration due to a decrease in hyaluronic acid concentration with age was proposed to cause the stiffening of the solid phase of the vitreous gel. Vitreous liquefaction, therefore, might be characterized as a simultaneous increase in liquid volume and localized stiffening of the vitreous gel. The phase separation of the vitreous humor with age has been hypothesized as the cause of many vitreous-related complications. This study provides viscoelastic properties and age-related changes of the human vitreous humor, which will aid in the design of biomimetic vitreous substitutes, enhancement in analyzing intravitreal transport of therapeutics, and understanding the pathological conditions of the vitreous humor.
Highlights
The vitreous humor is a fragile, transparent hydrogel situated between the lens and the retina, occupying 80% of the eye’s volume
It is important to study the mechanical properties of the vitreous to gain insight into diseases that might be caused by vitreous liquefaction, to study the release and transport profile of intraocular drugs injected into the vitreous humor, and to guide the design of vitreous substitutes that can replace both the form and function of the natural vitreous humor
The linear viscoelastic region for human vitreous was determined to be below 1% strain
Summary
The vitreous humor is a fragile, transparent hydrogel situated between the lens and the retina, occupying 80% of the eye’s volume. The vitreous is a viscoelastic gel composed of ∼99% water with a framework of type II collagen and hyaluronic acid. The vitreous humor serves as a mechanical damper for the eye, absorbing impacts and protecting the lens and retina. The vitreous liquefies with age due to a variety of factors such as oxidative damage, digestion by enzymes, and collagen mutations (Okada et al, 1989; Brown et al, 1996), resulting in phase-separation and gel network collapse. It is important to study the mechanical properties of the vitreous to gain insight into diseases that might be caused by vitreous liquefaction, to study the release and transport profile of intraocular drugs injected into the vitreous humor, and to guide the design of vitreous substitutes that can replace both the form and function of the natural vitreous humor. Quantitative measurements of human vitreous humor, especially its rheological properties, are scarce in the literature
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