Glyceryl methacrylate hydrogel as a vitreous implant. An experimental study

Advances in Polysaccharide- and Synthetic Polymer-Based Vitreous Substitutes

The vitreous humour is a gel-like structure that composes the majority of each eye. It functions to provide passage of light, be a viscoelastic dampener, and hold the retina in place. Vitreous liquefaction causes retinal detachment and retinal tears requiring pars plana vitrectomy for vitreous substitution. An ideal vitreous substitute should display similar mechanical, chemical, and rheological properties to the natural vitreous. Currently used vitreous substitutes such as silicone oil, perfluorocarbon liquids, and gases cannot be used long-term due to adverse effects such as poor retention time, cytotoxicity, and cataract formation. Long-term, experimental vitreous substitutes composed of natural, modified and synthetic polymers are currently being studied. This review discusses current long- and short-term vitreous substitutes and the disadvantages of these that have highlighted the need for an ideal vitreous substitute. The review subsequently focuses specifically on currently used polysaccharide- and synthetic polymer-based vitreous substitutes, which may be modified or functionalised, or employed as the derivative, and discusses experimental vitreous substitutes in these classes. The advantages and challenges associated with the use of polymeric substitutes are discussed. Innovative approaches to vitreous substitution, namely a novel foldable capsular vitreous body, are presented, as well as future perspectives related to the advancement of this field.

Polymeric hydrogel as a vitreous substitute: current research, challenges, and future directions

Vitreoretinal surgery is an essential approach to treat proliferative diabetic vitreopathy, retinal detachment, retinal tear, ocular trauma, and macular holes. The removal of the natural vitreous and the replacement with substitutes are critical steps for retina reattachment. Vitreous substitutes including silicone oil (SiO), air, sulfur hexafluoride (SF₆), and perfluoropropane (C₃F₈), have been widely applied in clinical practice. However, these substitutes are reported to cause complications such as emulsification, high intraocular pressure, and lens opacification. Polymeric hydrogels are a kind of material with favorable physical, mechanical properties, and adaptable biocompatibility, thus being highly expected to be ideal vitreous substitutes. Despite years of research, very few polymeric hydrogels can be applied practically in the vitreous cavity. In this review, we focus on the development of polymeric natural-based hydrogels and synthetic hydrogels. Particularly, we pay attention to recent advances in the novel stimuli-response and self-assembly supramolecular hydrogels. Characterized by easy injectability and long residence time, this kind of hydrogel becomes the potentially promising candidates for ideal vitreous substitutes. Finally, we evaluate the current challenges and provide the future directions of vitreous substitutes.

Polymeric hydrogels as a vitreous replacement strategy in the eye

Vitreous endo-tamponades are commonly used in the treatment of retinal detachments and tears. They function by providing a tamponading force to support the retina after retina surgery. Current clinical vitreous endo-tamponades include expansile gases (such as sulfur hexafluoride (SF₆) and perfluoropropane (C₃F₈)) and also sislicone oil (SiO). They are effective in promoting recovery but are disadvantaged by their lower refractive indices and lower densities as compared to the native vitreous, resulting in immediate blurred vision after surgery and necessitating patients to assume prolonged face-down positioning respectively. While the gas implants diffuse out over time, the SiO implants are non-biodegradable and require surgical removal. Therefore, there is much demand to develop an ideal vitreous endo-tamponade that can combine therapeutic effectiveness with patient comfort. Polymeric hydrogels have since attracted much attention due to their favourable properties such as high water content, high clarity, suitable refractive indices, suitable density, tuneable rheological properties, injectability, and biocompatibility. Many design strategies have been employed to design polymeric hydrogel-based vitreous endo-tamponades and they can be classified into four main strategies. This review seeks to analyse these various strategies and evaluate their effectiveness and also propose the key criteria to design successful polymeric hydrogel vitreous endo-tamponades.

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.

[Artificial vitreous body: Strategies for vitreous body substitutes]

BACKGROUND

Although numerous advances have been made in technology and techniques of pars plana vitrectomy and tamponades, there are still unsolved issues, such as proliferative vitreoretinopathy (PVR), multiple retinal breaks and persistent hypotonia. All available internal tamponades (e.g, gases, oils and fluorocarbons) are hydrophobic, so they approximate the neurosensory retina to the retinal pigment epithelium due to buoyant force and surface tension. Even though these tamponade materials exhibit various beneficial attributes in the clinical application, the hydrophobic nature has clear disadvantages and compartmentalization and significant incidence of PVR development still occur.

RESULTS AND CONCLUSION

An ideal vitreous body substitute should mimic the native human vitreous body, in both form and function. Vitreous body substitutes, such as hydrogels fulfill the biophysical needs in a similar manner to the natural vitreous body by providing an internal tamponade effect through swelling pressure and viscosity. New approaches range from cross-linked semisynthetic to synthetic polymers. These hydrogels have a good biocompatibility, optical clarity, a refractive index and rheological properties that are similar to the natural human vitreous body and are able to act as anti-adhesive and anti-migrative agents and can therefore reduce PVR. Furthermore, hydrogels could also serve as controlled-release drug-delivery systems for anti-proliferative, neuroprotective or nutritive drugs.

Vitreous substitutes: the present and the future

Vitreoretinal surgery has advanced in numerous directions during recent years. The removal of the vitreous body is one of the main characteristics of this surgical procedure. Several molecules have been tested in the past to fill the vitreous cavity and to mimic its functions. We here review the currently available vitreous substitutes, focusing on their molecular properties and functions, together with their adverse effects. Afterwards we describe the characteristics of the ideal vitreous substitute. The challenges facing every ophthalmology researcher are to reach a long-term intraocular permanence of vitreous substitute with total inertness of the molecule injected and the control of inflammatory reactions. We report new polymers with gelification characteristics and smart hydrogels representing the future of vitreoretinal surgery. Finally, we describe the current studies on vitreous regeneration and cell cultures to create new intraocular gels with optimal biocompatibility and rheological properties.

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.

Poly (triethylenglycol monomethacrylate) and poly (glycerol monomethacrylate) cross-linked gel as potential viscoelastics for intraoperative use

BACKGROUND

The highly swelling poly (glycerol monomethacrylate) gel (polyGLYMA) and hydrophilic polymer poly (triethylenglycol monomethacrylate (polyTEGMA) were tested as potential viscoelastics for intraoperative use in anterior segment surgery.

METHODS

PolyGLYMA was implanted into the anterior chamber in 5 rabbits, and 40% polyTEGMA in 16 rabbits. The eyes were enucleated 1 week to 3 months after the operation. The corneal endothelium was examined with specular microscopy, and then the whole eye histopathologically.

RESULTS

In all eyes of the polyGLYMA group, the clinical findings were characterized by a marked ciliary injection and severe secondary glaucoma, and the histologic ones by a marked inflammatory infiltration and thickening of Descemet's membrane in the anterior chamber angle. Specular microscopy revealed a decrease in the endothelial cell density and polymorphism of the endothelial cells. In the polyTEGMA group, the anterior segment and the fundus were physiologic all the time, and specular microscopy and histologic findings showed no degenerative and/or inflammatory changes.

CONCLUSIONS

PolyGLYMA proved unsuitable for intracameral application in rabbits. The new polymer polyTEGMA is characterized by high biologic tolerance after its implantation into the anterior chamber of rabbits. PolyTEGMA 40% might be considered as a potential viscoelastic material in humans.

Synthetic polymers as materials for artificial vitreous body: review and recent advances

In spite of a century of recorded attempts to replace the vitreous body of the eye with a foreign material, there is no permanent substitute currently available, and the success of some materials as temporary substitutes is still very limited. Among the large number of materials proposed or tested as vitreous substitutes, synthetic polymers have been episodically considered during the past four decades. This article will review these attempts, their outcomes, and recent progress in this field. There were only seven synthetic polymers and a few semisynthetic polymers (modified biopolymers) so far proposed or tested as vitreous substitutes. It appears that probably a synthetic hydrogel with very high water content would eventually be the material of choice for a permanent vitreous substitute, but the ideal material has not yet been found.

Scleral implants: an historical perspective

Scleral implants are made of absorbable or nonabsorbable materials. One category of absorbable materials consists of donor tissue, either autogenous (fascia lata, plantaris gracilis tendon) or from cadavers (dura mater, sclera, fascia lata). A second category includes gelatin, reconstituted collagen, absorbable gut, fibrin, blood plasma, air, and sodium hyaluronate; of these, specially prepared gelatin seems to be the most useful. Nonabsorbable implant materials have proven more practical than absorbable implants. Solid silicone rubber is currently the most popular scleral implant material. It is soft, easy to handle, and well tolerated. Expandable implants, either temporary or permanent, are used in the form of a silicone balloon filled with liquid. Silicone sponge, also widely used, has the advantage of great softness. However, hydrogels seem to be the ideal scleral implants. The only one commercially available is Refojo's MAI implant. It is very soft and is not damaged by sutures, has a smooth surface, is molded in several sizes and shapes, and has small pores that microorganisms cannot penetrate. When saturated with a water-soluble antibiotic before implantation, this implant releases the antibiotic postoperatively for a longer time than any other implant material. Finally, surgical adhesives are useful when the sclera is too thin or weak to tolerate sutures. The best adhesive available seems to be isobutyl cyanoacrylate.