Rheological Properties and Age-Related Changes of the Human Vitreous Humor

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.

A Hydrogel Vitreous Substitute that Releases Antioxidant

“…Rheological experiments showed viscoelastic properties of the PEGDA and PEGDA‐ co ‐PEGMA hydrogels similar to the native tissue . The linear viscoelastic region for both hydrogels was determined to be below 10% strain ( Figure 3 A).…”

“…The storage modulus ( G ′) and loss modulus ( G ″) of both hydrogels were in the same order of magnitude as the natural human vitreous (Figure 3B). The storage modulus of the human vitreous ranges from 1 to 7 Pa, whereas its loss modulus ranges from 0.3 to 1 Pa . The storage and loss moduli of the PEGDA hydrogel were statistically larger than those of the natural human vitreous with the storage modulus ranging from 5 to 11 Pa and the loss modulus averaging 0.9 Pa.…”

“…The PEGDA and PEGDA‐ co ‐PEGMA hydrogels have the most similar viscoelastic properties to those of the human vitreous humor, compared to previous studies (Table ). While the PEGDA‐ co ‐PEGMA hydrogel appeared to match the modulus of the human vitreous better than the PEGDA hydrogel, it should be noted that the reported properties of the human vitreous were dissected from older donors . It was suggested that the modulus of the in vivo vitreous might be higher compared to the dissected vitreous .…”

“…Due to its viscoelastic behavior, the vitreous serves as a mechanical damper for the eye, absorbing impacts and protecting surrounding intraocular tissues. The vitreous phase separates with age, resulting in collagen fibril aggregation that dissociates from the hyaluronic acid of the collagen–hyaluronic acid network, creating pockets of liquid . The phase separation of the vitreous compromises its ability to function as a protective structure for the retina and increases the risk for vision‐threatening problems such as retinal detachments or tears.…”

Etiology of posterior subcapsular cataracts based on a review of risk factors including aging, diabetes, and ionizing radiation

“…exploratory analysis The vitreous humor arises in childhood and forms a slowturnover hydrogel that occupies $80% of the eye's volume. The vitreous both liquefies by syneresis and locally stiffens as people age, increasing the risk of nuclear cataracts (Harocopos et al 2004;Tram and Swindle-Reilly 2018). The vitreous is composed of $99% water, with small amounts of collagen and hyaluronic acid (or hyaluronan).…”