Chemical and clinical observations on the fate of various vitreous substitutes

Desired properties of polymeric hydrogel vitreous substitute

Vitreous replacement is a commonly employed method for treating a range of ocular diseases, including posterior vitreous detachment, complex retinal detachment, diabetic retinopathy, macular hole, and ocular trauma. Various clinical substitutes for vitreous include air, expandable gas, silicone oil, heavy silicone oil, and balanced salt solution. However, these substitutes have drawbacks such as short retention time, cytotoxicity, high intraocular pressure, and the formation of cataracts, rendering them unsuitable for long-term treatment. Polymeric hydrogels possess the potential to serve as ideal vitreous substitutes due to their structure-mimicking to natural vitreous and adjustable mechanical properties. Replacement with hydrogels as the tamponade can help maintain the shape of the eyeball, apply pressure to the detached retina, and ensure the metabolic transport of substances without impairing vision. This literature review examines the required properties of artificial vitreous, including the optical properties, rheological properties, expansive force action, and physiological and biochemical functions of chemically and physically crosslinked hydrogels. The strategies for enhancing the biocompatibility and injectability of hydrogels are also summarized and discussed. From a clinical ophthalmology perspective, this paper presents the latest developments in vitreous replacement, providing clinicians with a comprehensive understanding of hydrogel clinical applications, which offers guidance for future design directions and methodologies for hydrogel development.

Substitutes and Colloidal System for Vitreous Replacement and Drug Delivery: Recent Progress and Future Prospective

Vitreoretinal surgeries for ocular diseases such as complicated retinal detachment, diabetic retinopathy, macular holes and ocular trauma has led to the development of various tamponades over the years in search for an ideal vitreous substitute. Current clinically used tamponade agents such as air, perfluorocarbons, silicone oil and expansile gases serve only as a short-term solution and harbors various disadvantages. However, an ideal long-term substitute is yet to be discovered and recent research emphasizes on the potential of polymeric hydrogels as an ideal vitreous substitute. This review highlights the recent progress in the field of vitreous substitution. Suitability and adverse effects of various tamponade agents in present day clinical use and biomaterials in the experimental phase have been outlined and discussed. In addition, we introduced the anatomy and functions of the native vitreous body and the pathological conditions which require vitreous replacement.

Vitreous substitutes: An overview of the properties, importance, and development

Vitreous or vitreous humor is a complex transparent gel that fills the space between the lens and retina of an eye and acts as a transparent medium that allows light to pass through it to reach the photoreceptor layer (retina) of the eye. The vitreous humor is removed in ocular surgery (vitrectomy) for pathologies like retinal detachment, macular hole, diabetes-related vitreous hemorrhage detachment, and ocular trauma. Since the vitreous is not actively regenerated or replenished, there is a need for a vitreous substitute to fill the vitreous cavity to provide a temporary or permanent tamponade to the retina following some vitreoretinal surgeries. An ideal vitreous substitute could probably be left inside the eye forever. The vitreous humor is transparent, biocompatible, viscoelastic and highly hydrophilic; polymeric hydrogels with these properties can be a potential candidate to be used as vitreous substitutes. To meet the tremendous demand for the vitreous substitute, many scientists all over the world have developed various kinds of vitreous substitutes or tamponade agent. Vitreous substitutes, whatsoever developed till date, are associated with several advantages and disadvantages, and there is no ideal vitreous substitute available till date. This review highlights the polymer-based vitreous substitutes developed so far, along with their advantages and limitations. The gas-based and oil-based substitutes have also been discussed but very briefly.

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

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.

Recent Progress in Using Biomaterials as Vitreous Substitutes

Vitreous substitutes are crucial adjuncts during vitreo-retinal surgery for retinal diseases such as complicated retinal detachment, macular holes, complications of diabetic retinopathy, and ocular trauma involving posterior segment. In retinal detachment surgery, an internal tamponade agent is required to provide internal pressure for reattachment of the detached neurosensory retina. Current available options serve only as a temporary surgical adduct or short-term solution and are associated with inherent problems. Despite many years of intensive research, an ideal vitreous substitute remains elusive. Indeed, the development of an ideal vitreous substitute requires the concerted efforts of synthetic chemists and biomaterial engineers, as well as ophthalmic surgeons. In this review, we propose that polymeric hydrogels present the future of artificial vitreous substitutes due to its high water composition, optical transparency, and rheological properties that closely mimic the natural vitreous. In particular, thermosensitive smart hydrogels, with reversible sol to gel change, have emerged as the material class with the most potential to succeed as ideal vitreous substitutes, facilitating easy implementation during surgery. Importantly, these smart hydrogels also display potential as efficacious drug delivery systems.

Towards an ideal biomaterial for vitreous replacement: Historical overview and future trends

Removal of the natural vitreous body from the eye and its substitution with a tamponade agent may be necessary in cases of complicated retinal detachment. Many materials have been variously proposed and tested over the years in an attempt to find an ideal vitreous substitute. This review highlights the evolution of research in the field of vitreous replacement and chronicles the main advances that have been made in such a context. The suitability and limitations of vitreous tamponade agents and substitutes in current clinical use are examined, and the future promise of experimentally tested biomaterials are described and discussed. Future trends in research are also considered and, specifically, the great potential of polymeric hydrogels is emphasized, as they seem to be very effective in closely mimicking the features of the natural vitreous and they could successfully act as long-term vitreous substitutes without inducing clinical complications in the patient's eye.

Vitrectomy and cystoid macular edema

Twenty-two patients who underwent vitrectomy surgery for chronic cystoid macular edema (CME) were evaluated. These patients all had uneventful intracapsular cataract extraction, but CME with vitreous adhesions to the cataract incision later developed. Vitrectomy was performed to remove the vitreous from the anterior segment structures in each case. Nineteen of the 22 patients showed improved visual acuities within six months, and all but two had complete resolutions of the CME by one year. These two patients did show marked improvements of visual acuity. The majority of patients younger than 65 years of age had final visual acuities of 20/50 or better, whereas the majority of those older than 65 years had less than 20/50. The results were not influenced by the duration or the pattern of the CME prior to vitrectomy. A prospective study to determine the value of vitrectomy in this type of patient is suggested.