Nathan Ravi

Vitreous Substitutes: A Comprehensive Review

Vitreoretinal disorders constitute a significant portion of treatable ocular disease. Advances in vitreoretinal surgery have included the development and characterization of suitable substitutes for the vitreous. Air, balanced salt solutions, perfluorocarbons, expansile gases, and silicone oil serve integral roles in modern vitreoretinal surgery. Vitreous substitutes vary widely in their properties, serve different clinical functions, and present different shortcomings. Permanent vitreous replacement has been attempted with collagen, hyaluronic acid, hydroxypropylmethylcellulose, and natural hydrogel polymers. None, however, have proven to be clinically viable. A long-term vitreous substitute remains to be found, and recent research suggests promise in the area of synthetic polymers. Here we review the currently available vitreous substitutes, as well those in the experimental phase. We classify these compounds based on their functionality, composition, and properties. We also discuss the clinical use, advantages, and shortcomings of the various substitutes. In addition we define the ideal vitreous substitute and highlight the need for a permanent substitute with long-term viability and compatibility. Finally, we attempt to define the future role of biomaterials research and the various functions they may serve in the area of vitreous substitutes.

In situ formation of hydrogels as vitreous substitutes: Viscoelastic comparison to porcine vitreous†

The human vitreous is a gelatinous substance predominantly composed of water (97-99%). Vitreous substitutes are needed for treatment of retinal detachments by reapproximating the retina to the back of the eye, or during vitrectomies for maintenance of ocular volume. None of the current substitutes can be used long-term due to their short retention time, toxicity, or complications such as glaucoma or cataracts. In addition, all of the current compounds have a specific gravity less than water and so are not appropriate for inferior retinal detachments. The viscoelastic properties of the porcine vitreous were analyzed to develop a model for ideal substitutes. Synthetic polymers that form hydrogels in situ were researched for the development of artificial vitreous. In this study, the physical, mechanical, and optical properties of four self-gelling polyacrylamide copolymeric hydrogels were determined and compared with those of the porcine vitreous. The 2% formulation of high crosslink density, hydrophobically modified hydrogel most closely mimicked the porcine vitreous. The viscoelastic properties of hydrogels capable of being formed in situ were compared to those of the porcine vitreous to determine their efficacy as vitreous substitutes.

Rabbit study of an in situ forming hydrogel vitreous substitute.

PURPOSE An in situ forming hydrogel was evaluated as a potential vitreous substitute in rabbits. METHODS The hydrogel used a disulfide cross-linker that was then reduced to produce an injectable thiol-containing polymer solution. The disulfide cross-links reformed by air oxidation of the thiols and produced a stable hydrogel once inside the eye. The polymer was clear, autoclavable, and could be stored easily in the presence of nitrogen gas. Capillary rheometry was used to measure the viscoelastic properties of the hydrogels and the porcine vitreous. Fourteen black rabbits underwent a pars plana, 25-gauge, three-port vitrectomy by a single surgeon with injection of a vitreous substitute. RESULTS The refractive indices of the hydrogels were measured by refractometry and were shown to be close to 1.33, and the 2% hydrogel matched the mechanical properties of the natural vitreous humor. The reduced polymeric hydrogel was easily injectable through a small-gauge needle into the vitreous cavity and did not show any fragmentation. The material underwent gelation within the eye, remained optically clear, and appeared well tolerated clinically. Slit lamp examination, dilated fundus examination, and electroretinograms showed no evidence of vitritis, uveitis, or endophthalmitis after 1 week. Histopathologic evaluation did not reveal any overt toxicity or gross morphologic changes in the retina. CONCLUSIONS The fact that this process of in situ gelation gives rise to hydrogels that are biocompatible and physically and optically similar to the natural vitreous suggests its suitability as a permanent vitreous substitute. Hydrogel candidates will be further studied to evaluate long-term biocompatibility and degradation in vivo.

Recent advances in polymeric vitreous substitutes

The vitreous humor occupies two thirds of the volume of the eye and is the major component behind the lens. The human vitreous is a gelatinous substance predominantly composed of water (98–99%). Its functions include holding the retina in place and circulating metabolites throughout the eye. The vitreous liquifies with age, facilitating posterior vitreous detachment, which can lead to retinal tears, intravitreal hemorrhage or retinal detachment. Vitreous substitutes are needed to tamponade the retina or during vitrectomies for treatment of retinal detachments. Gases, perfluorocarbon liquids and silicone or fluorosilicone oils are currently used as vitreous substitutes; however, none of these substitutes can be used long term due to the short retention time of the gaseous substitutes, cell toxicity or other complications, such as glaucoma or cataracts. Vitreous substitutes, both experimental and clinical, will be reviewed, along with promising experimental artificial vitreous; polymeric hydrogels.

A Knock-In Mouse Model for the R120G Mutation of αB-Crystallin Recapitulates Human Hereditary Myopathy and Cataracts

An autosomal dominant missense mutation in αB-crystallin (αB-R120G) causes cataracts and desmin-related myopathy, but the underlying mechanisms are unknown. Here, we report the development of an αB-R120G crystallin knock-in mouse model of these disorders. Knock-in αB-R120G mice were generated and analyzed with slit lamp imaging, gel permeation chromatography, immunofluorescence, immunoprecipitation, histology, and muscle strength assays. Wild-type, age-matched mice were used as controls for all studies. Both heterozygous and homozygous mutant mice developed myopathy. Moreover, homozygous mutant mice were significantly weaker than wild-type control littermates at 6 months of age. Cataract severity increased with age and mutant gene dosage. The total mass, precipitation, and interaction with the intermediate filament protein vimentin, as well as light scattering of αB-crystallin, also increased in mutant lenses. In skeletal muscle, αB-R120G co-aggregated with desmin, became detergent insoluble, and was ubiquitinated in heterozygous and homozygous mutant mice. These data suggest that the cataract and myopathy pathologies in αB-R120G knock-in mice share common mechanisms, including increased insolubility of αB-crystallin and co-aggregation of αB-crystallin with intermediate filament proteins. These knock-in αB-R120G mice are a valuable model of the developmental and molecular biological mechanisms that underlie the pathophysiology of human hereditary cataracts and myopathy.

A facile synthesis of highly water-soluble, core–shell organo-silica nanoparticles with controllable size via sol–gel process

A series of highly water-soluble organo-silica nanoparticles, ranging from 2 to 10nm in diameter, were synthesized by the cohydrolysis and copolycondensation reactions. omega-methoxy(polyethyleneoxy)propyltrimethoxysilane (PEG6-9) and hydroxymethyltriethoxysilane (HMTEOS) mixtures were catalyzed by sodium hydroxide in the presence of surfactant benzethonium chloride (BTC) with various ratios of PEG6-9/HMTEOS at room temperature. The synthesized organo-silica nanoparticles possess a core-shell structure with a core of organo-silica resulting from HMTEOS and a monolayer shell of PEG6-9. The chemo-physical characteristics of the particles were studied by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, (29)Si nuclear magnetic resonance (NMR), dynamic light scattering (DLS), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The molecular weight and particle size of the particles increased with increasing HMTEOS molar ratios. The richest HMTEOS composition for the water-soluble particles was found to be HMTEOS:PEG6-9=80:20, where the particles had a 6nm diameter core and a 0.8nm thick shell. We propose that these water-soluble organo-silica nanoparticles will be suitable for biomedical applications.

Injectable in situ Physically and Chemically Crosslinkable Gellan Hydrogel

An injectable, in situ physically and chemically crosslinkable gellan hydrogel is synthesized via gellan thiolation. The thiolation does not alter the gellans unique 3-D conformation, but leads to a lower phase transition temperature under physiological conditions and stable chemical crosslinking. The synthesis and hydrogels are characterized by (1)H NMR, FT-IR, CD, or rheology measurements. The injectability and the tissue culture cell viability is also tested. The thiolated gellan hydrogel exhibits merits, such as ease for injection, quick gelation, lower gelling temperature, stable structure, and nontoxicity, which make it promising in biomedicine and bioengineering as an injectable hydrogel.

Mechanism of insolubilization by a single-point mutation in alphaA-crystallin linked with hereditary human cataracts.

AlphaA-crystallin is a small heat shock protein that functions as a molecular chaperone and a lens structural protein. The R49C single-point mutation in alphaA-crystallin causes hereditary human cataracts. We have previously investigated the in vivo properties of this mutant in a gene knock-in mouse model. Remarkably, homozygous mice carrying the alphaA-R49C mutant exhibit nearly complete lens opacity concurrent with small lenses and small eyes. Here we have investigated the 90 degrees light scattering, viscosity, refractive index, and bis-ANS fluorescence of lens proteins isolated from the alphaA-R49C mouse lenses and found that the concentration of total water-soluble proteins showed a pronounced decrease in alphaA-R49C homozygous lenses. Light scattering measurements on proteins separated by gel permeation chromatography showed a small amount of high-molecular mass aggregated material in the void volume which still remains soluble in alphaA-R49C homozygous lens homogenates. An increased level of binding of beta- and gamma-crystallin to the alpha-crystallin fraction was observed in alphaA-R49C heterozygous and homozygous lenses but not in wild-type lenses. Quantitative analysis with the hydrophobic fluorescence probe bis-ANS showed a pronounced increase in fluorescence yield upon binding to alpha-crystallin from mutant as compared with the wild-type lenses. These results suggest that the decrease in the solubility of the alphaA-R49C mutant protein was due to an increase in its hydrophobicity and supra-aggregation of alphaA-crystallin that leads to cataract formation. Our study further shows that analysis of mutant proteins from the mouse model is an effective way to understand the mechanism of protein insolubilization in hereditary cataracts.

Comparison of the behavior of natural and refilled porcine lenses in a robotic lens stretcher

The mechanism by which the eye dynamically changes focal distance (accommodation), and the mechanism by which this ability is lost with age (presbyopia), are still contested. Due to inherent confounding factors in vivo, in vitro measurements have been undertaken using a robotic lens stretcher to examine these mechanisms as well as the efficacy of lens refilling - a proposed treatment for presbyopia. Dynamic forces, anterior and posterior curvatures, and lens thickness are all correlated for young natural and refilled porcine lenses. Comparisons are made to lenses refilled with a homogeneous polymer system. The amplitude of accommodation of the young porcine lens is very small such that it may be a suitable model for presbyopia. The behavior of refilled lenses was highly dependent on the refill volume. The volume could be tuned to maximize accommodative amplitude in the refilled lens.

Internal Osmotic Pressure as a Mechanism of Retinal Attachment in a Vitreous Substitute

In this study, the possibility of using the internal osmotic pressure of intraocular polymeric hydrogel materials to attach the retina in the repair of a retinal tear or hole was investigated. This is in contrast to the conventional methods of retinal detachment repair (intraocular gas, polydimethylsiloxane, or n-perfluorooctane), which rely on surface tension and have recognized limits. The system selected for implementation of this scheme was based on an acrylamide copolymer that was crosslinked in an aqueous solution to provide a transparent hydrogel which allowed control of the swelling pressure. Synthetic hydrogels, such as those selected here, provide an alternative to materials currently used as vitreous prostheses.