The Injectable Oxidated Hyaluronic Acid/Adipic Acid Dihydrazide Hydrogel as a Vitreous Substitute

The vitreous body is a clear, transparent gelatinous substance in the vitreous cavity of the eye that is posterior to the lens and anterior to the retina. It occupies two-thirds of the ocular volume, with a weight of approx. 4 g and a volume of about 4 ml. The main components of vitreous body include water (98%), collagen fibrils, glycosaminoglycans, hyaluronic acid (HA) and other rest solutes. Specific diseases, age-related degeneration or trauma can lead to pathological changes in the vitreous body, including HA degeneration and collagen precipitation, which result in liquefaction of the matrix [5]. A degenerated or liquefied vitreous body will lead to floater formation and eventually result in posterior vitreous detachment and possible retinal detachment.
Among clinical treatments, pars plana vitrectomy (PPV) is one of the most important surgeries for treating a number of ocular-related diseases, including diabetic retinopathy, complex retinal detachment (for example, due to trauma) and macular hole, during PPV, the vitreous body is cut and aspirated, and then is typically replaced with a vitreous substitute, such as gas (air, perfluropropane or sulfur hexafluoride) or silicone oil. Vitreous substitutes are used to fill vitreous cavity and help reattach the retina after vitrectomy surgery. Postoperatively, a vitreous substitute can keep the retina in position while the adhesion between the retina and the retinal pigment epithelium (RPE) cells are formed. Gases, which are lighter than water, are useful for flattening a detached retina and keeping it attached while healing occurs. However, it is frequently necessary to maintain a face-down position following surgery for a week or more when gas is used. Since the 1960s silicone oil is sometimes used instead of gases to keep retina attached postoperatively in complicated retinal detachments, or in patients unable to position postoperatively (e.g., children), but long-term complications can occur if the silicone oil is not removed later. Besides, silicone oil also seems to be cytotoxic to ocular tissues, such as corneal endothelial cells
In the present study, we developed a colorless, transparent and injectable hydrogel with appropriate refractive index as a vitreous substitute. The hydrogel is formed by oxidated hyaluronic acid (oxi-HA) cross-linked with adipic acid dihydrazide (ADH). Hyaluronic acid (HA) was oxidized by sodium periodate to create aldehyde functional groups, which could be cross-linked by ADH. The refractive index of this hydrogel ranged between 1.3420 and 1.3442, which is quite similar to human vitreous humor (1.3345). Rheological properties were measured to evaluate the working ability of the hydrogel for further clinical application. The oxi-HA/ADH in situ forming hydrogel can transform from liquid form into a gel-like matrix within 3–8 min, depending on the operational temperature. The degradation tests demonstrated that the hydrogel could maintain the gel matrix over 35 days, depending on the ADH concentration. In addition, the cytotoxicity was evaluated on retina pigmented epithelium (RPE) cells cultivated following the ISO standard (tests for in vitro cytotoxicity), and the hydrogel was found to be non-toxic. In the animal study, the oxi-HA/ADH hydrogel was injected into the vitreous cavity of rabbit eyes. The evaluations of slit-lamp observation, intraocular pressure, cornea thickness, electroretinography (ERG) and histological examination showed no significant abnormal biological reactions for 4 weeks. This study suggests that the injectable oxi-HA/ADH hydrogel should be a potential vitreous substitute.