In vitro evaluation ofin situ gels as short term vitreous substitutes

In vivo evaluation of a Nano-enabled therapeutic vitreous substitute for the precise delivery of triamcinolone to the posterior segment of the eye

This study focused on the design of a thermoresponsive, nano-enabled vitreous substitute for the treatment of retinal diseases. Synthesis of a hydrogel composed of hyaluronic acid and a poloxamer blend was undertaken. Poly(D,L-lactide-co-glycolide) acid nanoparticles encapsulating triamcinolone acetonide (TA) were synthesised with a spherical morphology and mean diameter of ~ 153 nm. Hydrogel fabrication and nanoparticle loading within the hydrogel was confirmed via physicochemical analysis. Gelation studies indicated that hydrogels formed in nine minutes and 10 min for the unloaded and nanoparticle-loaded hydrogels, respectively. The hydrogels displayed in situ gel formation properties, and rheometric viscoelastic studies indicated the unloaded and loaded hydrogels to have modulus values similar to those of the natural vitreous at 37 °C. Administration of the hydrogels was possible via 26G needles allowing for clinical application and drug release of triamcinolone acetonide from the nanoparticle-loaded hydrogel, which provided sustained in vitro drug release over nine weeks. The hydrogels displayed minimal swelling, reaching equilibrium swelling within 12 h for the unloaded hydrogel, and eight hours for the nanoparticle-loaded hydrogel. Biodegradation in simulated vitreous humour with lysozyme showed < 20% degradation within nine weeks. Biocompatibility of both unloaded and loaded hydrogels was shown with mouse fibroblast and human retinal pigment epithelium cell lines. Lastly, a pilot in vivo study in a New Zealand White rabbit model displayed minimal toxicity with precise, localised drug release behaviour, and ocular TA levels maintained within the therapeutic window for the 28-day investigation period, which supports the potential applicability of the unloaded and nanoparticle-loaded hydrogels as vitreous substitutes that function as drug delivery systems following vitrectomy surgery.

Polysaccharide-based hydrogels for medical devices, implants and tissue engineering: A review

Hydrogels are highly biocompatible biomaterials composed of crosslinked three-dimensional networks of hydrophilic polymers. Owing to their natural origin, polysaccharide-based hydrogels (PBHs) possess low toxicity, high biocompatibility and demonstrate in vivo biodegradability, making them great candidates for use in various biomedical devices, implants, and tissue engineering. In addition, many polysaccharides also show additional biological activities such as antimicrobial, anticoagulant, antioxidant, immunomodulatory, hemostatic, and anti-inflammatory, which can provide additional therapeutic benefits. The porous nature of PBHs allows for the immobilization of antibodies, aptamers, enzymes and other molecules on their surface, or within their matrix, potentiating their use in biosensor devices. Specific polysaccharides can be used to produce transparent hydrogels, which have been used widely to fabricate ocular implants. The ability of PBHs to encapsulate drugs and other actives has been utilized for making neural implants and coatings for cardiovascular devices (stents, pacemakers and venous catheters) and urinary catheters. Their high water-absorption capacity has been exploited to make superabsorbent diapers and sanitary napkins. The barrier property and mechanical strength of PBHs has been used to develop gels and films as anti-adhesive formulations for the prevention of post-operative adhesion. Finally, by virtue of their ability to mimic various body tissues, they have been explored as scaffolds and bio-inks for tissue engineering of a wide variety of organs. These applications have been described in detail, in this review.

Ocular Delivery of Metformin for Sustained Release and in Vivo Efficacy

Metformin is known to lower inflammation, independent of its anti-diabetic action. Thus, topical metformin can be a therapeutic strategy for managing ocular inflammation associated with diabetes. To achieve this and address the issues of ocular retention and controlled release an in situ gel of metformin was developed. The formulations were prepared using sodium hyaluronate, hypromellose, and gellan gum. The composition was optimized by monitoring gelling time/capacity, viscosity, and mucoadhesion. MF5 was selected as the optimized formulation. It showed both chemical and physiological compatibility. It was found to be sterile and stable. MF5 exhibited sustained release of metformin for 8h that fitted best with zero-order kinetics. Further, the release mode was found to be close to the Korsmeyer-Peppas model. Supported by an ex vivo permeation study, it showed potential for prolonged action. It showed a significant reduction in ocular inflammation that was comparable to that of the standard drug. MF5 shows translational potential as a safe alternative to steroids for managing ocular inflammation.

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.

Vitreous Substitutes from Bench to the Operating Room in a Translational Approach: Review and Future Endeavors in Vitreoretinal Surgery

Vitreous substitutes are indispensable tools in vitreoretinal surgery. The two crucial functions of these substitutes are their ability to displace intravitreal fluid from the retinal surface and to allow the retina to adhere to the retinal pigment epithelium. Today, vitreoretinal surgeons can choose among a plethora of vitreous tamponades, and the tamponade of choice might be difficult to determine in the ever-expanding range of possibilities for a favorable outcome. The currently available vitreous substitutes have disadvantages that need to be addressed to improve the surgical outcome achievable today. Herein, the fundamental physical and chemical proprieties of all vitreous substitutes are reported, and their use and clinical applications are described alongside some surgical techniques of intra-operative manipulation. The major upcoming developments in vitreous substitutes are extensively discussed, keeping a translational perspective throughout. Conclusions on future perspectives are derived through an in-depth analysis of what is lacking today in terms of desired outcomes and biomaterials technology.

Vitreous Substitutes as Drug Release Systems

Vitreous substitutes are traditionally used to stabilize the retina after vitrectomy. In recent years, various approaches have been developed for using the vitreous substitute not only as a tamponade but also as a drug release system to tackle ocular diseases. This review provides an overview of the requirements for vitreous substitutes and discusses the current clinically applied as well as novel polymer-based vitreous substitutes as drug delivery systems, including their release mechanisms, efficiencies, challenges, and future perspectives.

Carboxymethyl chitosan modified lipid nanoformulations as a highly efficacious and biocompatible oral anti-leishmanial drug carrier system

Herein, carboxymethyl chitosan (CMC) grafted lipid nanoformulations were facilely prepared by thin-film hydration method as a highly efficient biocompatible anti-leishmanial carrier encapsulating amphotericin B (AmB). Nanoformulations were characterized for their physicochemical characteristics wherein TEM analysis confirmed the spherical structure, whereas FTIR analysis revealed the conjugation of CMC onto nanoformulations and confirmed the free state of AmB. Furthermore, the wettability study confirmed the presence of CMC on the surface of nanoformulations attributed to the enhanced hydrophilicity. Surface hydrophilicity additionally contributes towards consistent mucin retention ability for up to 6 h, superior mucoadhesiveness, and hence enhanced bioavailability. The proposed nanoformulations with high encapsulation and drug loading properties displayed controlled drug release in the physiological microenvironment. In vitro, antileishmanial results showed an astounding 97% inhibition in amastigote growth. Additionally, in vivo studies showed that treatment with nanoformulations significantly reduced the liver parasitic burden (93.5%) without causing any toxicity when given orally.

Long-Term Biocompatibility of a Highly Viscously Thiol-Modified Cross-Linked Hyaluronate as a Novel Vitreous Body Substitute

Purpose: In surgical ophthalmology, the treatment of complicated retinal and vitreous diseases is one of the central challenges. For this purpose, the vitreous body is removed as part of the standard therapy and replaced by a temporary tamponade to stabilize the position of the retina. Since the tamponading properties of previous materials such as silicone oils, gases, or semi-fluorinated alkanes are a combination of their surface tension and their buoyancy vector, they cannot completely fill the vitreous cavity. The aim of this work was to test in vivo a novel vitreous body substitute (ViBos strong) based on cross-linked hyaluronic acid for its compatibility.

Methods: A pars plana vitrectomy with posterior vitreous detachment was performed in the right eye of 18 pigmented rabbits, with subsequent injection of ViBos strong. Follow-up examination included slit-lamp examination, funduscopy, intraocular pressure measurements (IOP), optical coherence tomography (OCT), and electroretinogram (ERG) measurements. The rabbits were sacrificed at three different time points (1, 3, and 6 months; each 6 animals) and examined macroscopically and prepared for histological examination (HE staining) and immunohistochemistry (Brn3a and glial fibrillary acidic protein (GFAP)).

Results: ViBos strong demonstrated good intraoperative handling and remained stable for at least 1 month and degraded slowly over 6 months. IOP was within clinical acceptable values at all follow-up examinations. Retinal function was well preserved after instillation of the hydrogel and comparable to the untreated eye after 6 months in OCT, ERG, and histological examinations. An increase in the GFAP expression was found in the surgery eyes, with a peak in the 3-month group. The Brn3a expression was not significantly affected by vitrectomy with ViBos strong.

Conclusion: Highly viscously thiol-modified cross-linked hyaluronate showed a good biocompatibility in rabbit eyes over 6 months after vitrectomy, making it a promising potential as a vitreous substitute.

Injectable self-crosslinking hydrogels based on hyaluronic acid as vitreous substitutes

After vitrectomy, the ideal vitreous substitute should be implanted to maintain the normal function of the eye. However, the existing materials (such as silicone oil, air, perfluorocarbons, etc.) still have some shortcomings and cannot fully meet the clinical needs. In this study, thiolated hyaluronic acid (SH-HA) was prepared based on hyaluronic acid. The SH-HA hydrogel was formed by a simple transformation of the sulfhydryl group to the disulfide bond, which had high transparency, controllable swelling property, suitable mechanical strength, excellent biocompatibility and similar physical and chemical properties to natural vitreous. SH-HA hydrogel was filled into the eyes of experimental rabbits to replace their own vitreous after vitrectomy. During the 90 days follow-up period, SH-HA hydrogel showed excellent intraocular compatibility, maintained normal intraocular pressure (IOP), and no cataract, endophthalmitis, retinal detachment and other complications were observed. In general, SH-HA hydrogel has great potential as a vitreous substitute.

Non-invasive transferrin targeted nanovesicles sensitize resistant glioblastoma multiforme tumors and improve survival in orthotopic mouse models

The blood-brain barrier (BBB) and tumor heterogeneity have resulted in abysmally poor clinical outcomes in glioblastoma (GBM) with the standard therapeutic regimen. Despite several anti-glioma drug delivery strategies, the lack of adequate chemotherapeutic bioavailability in gliomas has led to a suboptimal therapeutic gain in terms of improvement in survival and increased systemic toxicities. This has paved the way for designing highly specific and non-invasive drug delivery approaches for treating GBM. The intranasal (IN) route is one such delivery strategy that has the potential to reach the brain parenchyma by circumventing the BBB. We recently showed that in situ hydrogel embedded with miltefosine (HePc, proapoptotic anti-tumor agent) and temozolomide (TMZ, DNA methylating agent) loaded targeted nanovesicles prevented tumor relapses in orthotopic GBM mouse models. In this study, we specifically investigated the potential of a non-invasive IN route of TMZ delivered from lipid nanovesicles (LNs) decorated with surface transferrin (Tf) and co-encapsulated with HePc to reach the brain by circumventing the BBB in glioma bearing mice. The targeted nanovesicles (228.3 ± 10 nm, -41.7 ± 4 mV) exhibited mucoadhesiveness with 2% w/v mucin suggesting their potential to increase brain drug bioavailability after IN administration. The optimized TLNs had controlled, tunable and significantly different release kinetics in simulated cerebrospinal fluid and simulated nasal fluid demonstrating efficient release of the payload upon reaching the brain. Drug synergy (combination index, 0.7) showed a 6.4-fold enhanced cytotoxicity against resistant U87MG cells compared to free drugs. In vivo gamma scintigraphy of ^99mTc labeled LNs showed 500- and 280-fold increased brain concentration post 18 h of treatment. The efficacy of the TLNs increased by 1.8-fold in terms of survival of tumor-bearing mice compared to free drugs. These findings suggested that targeted drug synergy has the potential to intranasally deliver a high therapeutic dose of the chemotherapy agent (TMZ) and could serve as a platform for future clinical application.

Methacrylated Gellan Gum/Poly-l-lysine Polyelectrolyte Complex Beads for Cell-Based Therapies

Cell encapsulation strategies using hydrogel beads have been considered as an alternative to immunosuppression in cell-based therapies. They rely on layer-by-layer (LbL) deposition of polymers to tune beads' permeability, creating a physical barrier to the host immune system. However, the LbL approach can also create diffusion barriers, hampering the flow of essential nutrients and therapeutic cell products. In this work, the polyelectrolyte complex (PEC) methodology was used to circumvent the drawbacks of the LbL strategy by inducing hydrogel bead formation through the interaction of anionic methacrylated gellan gum (GG-MA) with cationic poly-l-lysine (PLL). The interfacial complexation between both polymers resulted in beads with a cell-friendly GG-MA hydrogel core surrounded by a PEC semipermeable membrane. The beads showed great in vitro stability over time, a semi-permeable behavior, and supported human adipose-derived stem cell encapsulation. Additionally, and regarding immune recognition, the in vitro and in vivo studies pointed out that the hydrogel beads behave as an immunocompatible system. Overall, the engineered beads showed great potential for hydrogel-mediated cell therapies, when immunoprotection is required, as when treating different metabolic disorders.

An in situ-forming polyzwitterion hydrogel: Towards vitreous substitute application

Development of a biostable and biosafe vitreous substitute is highly desirable, but remains a grand challenge. Herein, we propose a novel strategy for constructing a readily administered vitreous substitute based on a thiol-acrylate clickable polyzwitterion macromonomer. A biocompatible multivinyl polycarboxybetaine (PCB-OAA) macromonomer is designed and synthesized, and mixed with dithiothreitol (DTT) via a Michael addition reaction to form a hydrogel in vitreous cavity. This resultant PCB-OAA hydrogel exhibits controllable gelation time, super anti-fouling ability against proteins and cells, excellent biocompatibility, and approximate key parameters to human vitreous body including equilibrium water content, density, optical properties, modulus. Remarkably, outperforming clinically used silicone oil in biocompatibility, this rapidly formed hydrogel in the vitreous cavity of rabbit eyes remains stable in vitreous cavity, showing an appealing ability to prevent significantly inflammatory response, fibrosis and complications such as raised intraocular pressure (IOP), and cataract formation. This zwitterionic polymer hydrogel holds great potential as a vitreous substitute.

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.

Current Situation and Challenges in Vitreous Substitutes

Vitreo-retinal disorders constitute a significant portion of treatable ocular diseases. These pathologies often require vitreo-retinal surgery and, as a consequence, the use of vitreous substitutes. Nowadays, the vitreous substitutes that are used in clinical practice are mainly divided into gases (air, SF₆ , C₂ F₆ , C₃ F₈ ) and liquids (perfluorocarbon liquids, silicone oils, and heavy silicone oils). There are specific advantages and drawbacks to each of these, which determine their clinical indications. However, developing the ideal biomaterial for vitreous substitution continues to be one of the most important challenges in ophthalmology, and a multidisciplinary approach is required. In this sense, recent research has focused on the development of biocompatible, biodegradable, and injectable hydrogels (natural, synthetic, and smart), which also act as medium and long-term internal tamponade agents. This comprehensive review aims to cover the main characteristics and indications for use of the extensive range of vitreous substitutes that are currently used in clinical practice, before going on to describe the hydrogels that have been developed recently and which have emerged as promising biomaterials for vitreous substitution.

Dopamine-Functionalized Gellan Gum Hydrogel as a Candidate Biomaterial for a Retinal Pigment Epithelium Cell Delivery System

In this study, dopamine-functionalized gellan gum (DFG) hydrogel was prepared as a carrier for retinal pigment epithelium (RPE) cell delivery via a carbodiimide reaction. The carboxylic acid of gellan gum (GG) was replaced with catechol in a 21.3% yield, which was confirmed by NMR. Sol fraction and weight loss measurements revealed that dopamine improved degradability in the GG hydrogel. Measurements of the viscosity, injection force, and compressibility also showed that dopamine-functionalized GG hydrogels had more desirable rheological/mechanical properties for improving injectability. These characteristics were confirmed to arise from the GG's helix structure loosened by the dopamine's bulky nature. Moreover, dopamine's hydrophilic characteristics were confirmed to create a more favorable microenvironment for cell growth by promoting swelling capability and cell attachment. This improved biocompatibility became more pronounced when the hydrophilicity of dopamine was combined with a larger specific surface area stemming from the less porous structure of the dopamine-grafted hydrogels. This effect was apparent from the live/dead staining images of the as-prepared hydrogels. Meanwhile, the nonionic cross-linked DFG (DG) hydrogel showed the lowest protein expression in the immunofluorescence staining images obtained after 28 days of culture, supporting that it had the highest degradability and associated cell-releasing ability. That tendency was also observed in the gene expression data acquired by real-time polymerase chain reaction (RT-PCR) analysis. RT-PCR analysis also revealed that the DG hydrogel carrier could upregulate the visual function-related gene of RPE. Overall, the DG hydrogel system demonstrated its feasibility as a carrier of RPE cells and its potential as a means of improving visual function.

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.

Alginate- and Hyaluronic Acid-Based Hydrogels as Vitreous Substitutes: An In Vitro Evaluation

Purpose

To study alginate- and hyaluronic acid–based hydrogels in vitro as vitreous substitutes.

Methods

Biopolymeric hydrogels based on high-molecular alginate (0.5% and 1.0%) and hyaluronic acid (1.0% and Healaflow) were compared with extracted human vitreous bodies and silicone oil (SIL-5000) regarding their optical properties (refractive index, transmission) and viscoelastic characteristics (storage modulus G′, loss modulus G″). The cytotoxic (metabolic activity, apoptosis) and antiproliferative profiles were determined using cultured human fibroblasts, ARPE-19, and photoreceptor cells. The hydrogel systems were applied to human fetal retinal pigment epithelial cells cultured for two months until maximum transepithelial electrical resistance (TEER) to investigate the effect of the gel matrices on tight junctions using TEER measurements and immunostainings against the tight junction protein ZO-1.

Results

Tested alginate- and hyaluronic acid–based hydrogels resembled the natural refractive index of human vitreous bodies (1.3356–1.3360) in contrast to SIL-5000 (1.4034) and showed high optical transparency (>90%) within the visible light region. The biopolymeric hydrogels exhibited viscoelastic properties similar to juvenile vitreous bodies with G′>G″ adjustable via different gelation times, contrary to SIL-5000 (G′<G″). The metabolic activity, apoptosis and tight junctions of all tested ocular cells were unaffected by the alginate- and hyaluronic acid–based vitreous substitutes.

Conclusions

The present in vitro study demonstrates good optical, viscoelastic, and biocompatible properties of alginate- and hyaluronic acid–based hydrogels required for their use as vitreous substitutes.

Translational Relevance

Biopolymer-based hydrogels represent a promising vitreous replacement strategy to treat vitreoretinal diseases.

A self-healing and injectable hydrogel based on water-soluble chitosan and hyaluronic acid for vitreous substitute

Vitreous, an essential dioptric medium for the human eyes, must be filled with artificial materials once damaged. Carboxymethyl chitosan (CMCTS) is one of the most important water-soluble chitosan derivatives with improved biocompatibility and biodegradability. In this study, oxidized hyaluronic acid (OHA) was prepared as crosslinking reagent. CMCTS and OHA were used to develop a biocompatible, self-repairing and in-situ injectable hydrogel for vitreous substitutes. Results showed the hydrogel with controllable swelling properties, high transparency, acceptable cytocompatibility on mouse fibroblast L929 and histocompatibility in vivo. Furthermore, hydrogel was injected in-situ into the vitreous cavity after vitrectomy on New Zealand Rabbits, no significant and persistent adverse effects were observed during the 90-day follow-up period. In addition, the hydrogel maintained intraocular pressure of the operated eyes and the inherent position of the retina. Collectively, this injectable, biodegradable, nontoxic hydrogel possessed enormous potential to become a vitreous substitute material.

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.

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.

Bioinspired Thermosensitive Hydrogel as a Vitreous Substitute: Synthesis, Properties, and Progress of Animal Studies

In many vitreal diseases, the surgeon removes the natural vitreous and replaces it with silicone oils, gases, or balanced salt solutions to fill the eyeball and hold the retina in position. However, these materials are often associated with complications and have properties that differ from natural vitreous. Herein, we report an extension of our previous work on the synthesis of a biomimetic hydrogel that is composed of thiolated gellan as an analogue of type II collagen and poly(methacrylamide-co-methacrylate-co-bis(methacryloyl)cystamine), a polyelectrolyte, as an analogue of hyaluronic acid. This thermosensitive hydrogel can be injected into the eye as a viscous solution at 45 °C. It then forms a physical gel in situ when it reaches body temperature, and later forms disulfide covalent crosslinks. In this article, we evaluated two different formulations of the biomimetic hydrogels for their physical, mechanical, and optical properties, and we determined their biocompatibility with several cell lines. Finally, we report on the progress of the four-month preclinical evaluation of our bio-inspired vitreous substitute in comparison to silicone oil or a balanced salt solution. We assessed the eyes with a slit-lamp examination, intraocular pressure measurements, electroretinography, and optical coherence tomography. Preliminary results are very encouraging for the continuing evaluation of our bio-inspired hydrogel in clinical trials.

Characterization of silk-hyaluronic acid composite hydrogels towards vitreous humor substitutes

Multiple ophthalmic pathologies, such as retinal detachment and diabetic retinopathy, require the removal and replacement of the vitreous humor. Clinical tamponades such as silicone oil and fluorinated gases are utilized but limited due to complications and toxicity. Therefore, there is a need for biocompatible, stable, vitreous humor substitutes. In this study, enzymatically crosslinked silk-hyaluronic acid (HA) hydrogels formed using horseradish peroxidase and H2O2 were characterized for use as vitreous humor substitutes. The composite network structure was characterized with dynamic light scattering. In addition, the rheological, optical, and swelling properties of hydrogels with varying silk to HA ratios and crosslinking densities controlled via H2O2 were determined over time. Hydrogels had refractive indexes of 1.336 and were clear with 75-91% light transmission. Hydrogel shear storage modulus ranged between ~6 and 240 Pa where increased H2O2 increased the modulus. After 1 month of aging, there were no changes in modulus for hydrogels with lower silk ratios, while those with higher silk ratios exhibited a significant increase in modulus. Decreasing H2O2 concentration in the reactions led to increased hydrogel volume during swelling, with higher silk ratios returning to their original size after 15 days. Dynamic light scattering results show three diffusive modes, revealing the possible structures of the hydrogel composite and are consistent with the mechanical properties and swelling results. The normalized intraocular pressure of ex vivo porcine eyes after injecting hydrogels were comparable with those treated with silicone oil showing the potential clinical utility of the hydrogels as vitreous substitutes. The versatility of the silk-HA hydrogel system, the tunable swelling properties, and the stability of hydrogels with lower silk ratios show the benefit of utilizing silk-HA hydrogels as vitreous substitutes.

Ex vivo biophysical characterization of a hydrogel-based artificial vitreous substitute

Purpose

To characterize the biophysical properties of an artificial vitreous body substitute (VBS), which consists of a biocompatible, cross-linked, hyaluronic acid (HA)-based hydrogel, by analysing the VBS’s influence on intraocular pressure (IOP) and retinal integrity in distinct ex vivo eye models in order to evaluate the its potential for in vivo biocompatibility testing.

Methods

Pig eyes were obtained immediately postmortem, and VBS was injected after core-vitrectomy. IOP was followed for 24 h (n = 5). VBS influence on retinal integrity was investigated using isolated bovine retinas superfused with an oxygen saturated nutrient solution. An electroretinogram (ERG) was recorded on explanted bovine retinae using silver/silver chloride electrodes; after application of VBS for 2 min, a washout period of 70 min was employed. The percentage of a-and b-wave reduction at the end of the washout phase was compared to baseline values (n = 5). Data were calculated throughout as the mean and the standard deviation. qRT-PCR (Bax/Bcl–2-ratio, GFAP- and PGP9.5-levels) or western blot analysis was used to test for toxicity of Princess Volume after 24 h (and β-3 tubulin with GAPDH as a control gene). Significance was estimated by Student´s t-test; p ≤0.05 was considered to be statistically significant.

Results

The IOP increased non-significantly by 10% after 24 h. Short-term biocompatibility testing using isolated superfused bovine retinas showed neither significant reductions of the b-wave nor the a-wave amplitudes (b-wave reduction 14.2%, p>0.05; a-wave reduction 23.9%, p>0.05). qRT-PCR and western blot analysis did not reveal significant toxicity after 24 h.

Conclusions

The manufactured HA-based hydrogel showed highly favourable biophysical characteristics in the explored ex vivo models, justifying in vivo studies enabling the assessment of biocompatibility.

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

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.

Antifouling Super Water Absorbent Supramolecular Polymer Hydrogel as an Artificial Vitreous Body

Recently, there has been a high expectation that high water absorbent hydrogels can be developed as an artificial vitreous body. However, the drawbacks associated with in vivo instability, biofouling, uncontrollable in situ reaction time, and injection-induced precrosslinked fragmentation preclude their genuine use as vitreous substitutes. Here, a supramolecular binary copolymer hydrogel termed as PNAGA-PCBAA by copolymerization of N-acryloyl glycinamide (NAGA) and carboxybetaine acrylamide (CBAA) is prepared. This PNAGA-PCBAA hydrogel physically crosslinked by dual amide hydrogen bonds of NAGA exhibits an ultralow solid content (1.6, 98.4 wt% water content), and shear-thinning behavior, body temperature extrudability/self-healability, rapid network recoverability, and very close key parameters (modulus, antifouling/antifibrosis, light transmittance, refractive index, ultrastability) to human vitreous body. It is demonstrated that the hydrogel can be readily injected by a 22G needle into the rabbits' eyes where the gelling network is rapidly recovered. After 16 weeks postoperation, the hydrogel acts as a very stable vitreous substitute without affecting the structure of soft tissues in eye, or eliciting adverse effects. This supramolecular binary copolymer hydrogel finds a broad application in ophthalmic fields as not only a self-recoverable permanent vitreous substitute, but also transient intraocular filling for prevention of inner tissues in postsurgical eyes.

Data on Manuka Honey/Gellan Gum composite hydrogels for cartilage repair

This work contains original data supporting our research paper “Antibacterial effectiveness meets improved mechanical properties: Manuka Honey/Gellan Gum composite hydrogels for cartilage repair”, Bonifacio et al., in press [1], in which innovative composite hydrogels, based on Gellan Gum/Manuka honey/Halloysite nanotubes were described as biomaterials for cartilage regeneration. Here the composites were further examined by means of Fourier Transform Infrared Spectroscopy, in Attenuated Total Reflectance mode (FT-IR/ATR). Materials devoted to cartilage replacement must possess adequate fluid permeability and lubricating capability, therefore, a deeper investigation on water uptake kinetics of freeze-dried specimens up to 21 days in PBS was carried out. Moreover, since the degradation rate of a biomaterial plays a pivotal role in tissue engineering, weight loss measurements of the prepared hydrogels were performed in simulated synovial fluid, in phosphate buffer solution (PBS) and in lysozyme. Scanning Electron Microscopy images provide insight into the morphology of the freeze-dried samples.

Finally, additional information on Staphylococcus aureus and Staphylococcus epidermidis ability to adhere onto the prepared hydrogel composites in short times were obtained, as well as the chondrogenic potential of the composites assessed by SDS-PAGE followed by Coomassie blue gel staining.

Chemically Modified Gellan Gum Hydrogels with Tunable Properties for Use as Tissue Engineering Scaffolds

Gellan gum is a naturally occurring polymer that can cross-link in the presence of divalent cations to form biocompatible hydrogels. However, physically cross-linked gellan gum hydrogels lose their stability under physiological conditions, thus restricting the applications of these hydrogels in vivo. To improve the mechanical strength of the gels, we incorporated methacrylate into the gellan gum and chemically cross-linked the hydrogel through three polymerization methods: step growth through thiol-ene photoclick chemistry, chain-growth via photopolymerization, and mixed model in which both mechanisms were employed. Methacrylation was confirmed and quantified by proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared spectroscopy. The mechanical properties and chemistry of the cross-linked gels were systematically altered by varying the reaction conditions. The compression moduli of the resulting hydrogels ranged between 6.4 and 17.2 kPa. The swelling ratios of the hydrogels were correlated with the compression moduli and affected by the addition of calcium. In vitro enzymatic degradation rate was found to depend on the degree of methacrylation. NIH/3T3 fibroblast cell proliferation and morphology were related to substrate stiffness, with a high stiffness leading generally to higher proliferation. The proliferation is further affected by the thiol-ene ratio. These results suggest that a hydrogel platform based on the gellan gum can offer versatile chemical modifications and tunable mechanical properties. The influence of these substrates on cell behavior suggests that the gellan gum hydrogels have the flexibility to be engineered for a variety of biomaterials applications.

Synthesis and characterization of in situ forming anionic hydrogel as vitreous substitutes

The natural vitreous is a biological hydrogel consisting primarily of a collagen and anionic hyaluronate. It is surgically removed in many ocular diseases and replaced with fluids, gases, or silicone oils. We have been interested in developing synthetic hydrogels as vitreous substitutes. In this study, we combined the stiffness and hydrophobicity of polymethacrylamide (PMAM) and the anionic nature of polymethacrylate (PMAA) to make copolymers that would mimic the natural vitreous. We used bis-methacryloyl cystamine (BMAC) to introduce thiol groups for reversible crosslink. The Mn of copolymers ranged from ∼100 k to ∼200 k Da (polydisperisty index of 1.47-2.63) and their composition as determined by titration, 1 H NMR and disulfide test were close to the feed ratio. The reactivities of monomers were as follows: MAM > MAA ∼ BMAC. Copolymers with higher MAA contents gelled faster, swelled more, and had higher storage modulus (1.5 to 100 Pa) comparable to that of the natural vitreous. We evaluated the biocompatibility of copolymers by electric cell-substrate impedance sensing (ECIS) using human retinal pigment epithelial cells, primary porcine retinal pigmented epithelial cells, human microvascular endothelial cells adult dermis, and a fibroblast line 3T3. The biocompatibility decreases as the content of BMAC increases. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 977-988, 2017.

[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.

Biomimetic hydrogel with tunable mechanical properties for vitreous substitutes

The vitreous humor of the eye is a biological hydrogel principally composed of collagen fibers interspersed with hyaluronic acid. Certain pathological conditions necessitate its removal and replacement. Current substitutes, like silicone oils and perfluorocarbons, are not biomimetic and have known complications. In this study, we have developed an in situ forming two-component biomimetic hydrogel with tunable mechanical and osmotic properties. The components are gellan, an analogue of collagen, and poly(methacrylamide-co-methacrylate), an analogue of hyaluronic acid; both endowed with thiol side groups. We used response surface methodology to consider seventeen possible hydrogels to determine how each component affects the optical, mechanical, sol-gel transition temperature and swelling properties. The optical and physical properties of the hydrogels were similar to vitreous. The shear storage moduli ranged from 3 to 358Pa at 1Hz and sol-gel transition temperatures from 35.5 to 43°C. The hydrogel had the ability to remain swollen without degradation for four weeks in vitro. Three hydrogels were tested for biocompatibility on primary porcine retinal pigment epithelial cells, human retinal pigment epithelial cells, and fibroblast (3T3/NIH) cells, by electric cell-substrate impedance sensing system. The two-component hydrogels allowed for the tuning and optimizing of mechanical, swelling, and transition temperature to obtain three biocompatible hydrogels with properties similar to the vitreous. Future studies include testing of the optimized hydrogels in animal models for use as a long-term substitute, whose preliminary results are mentioned.

STATEMENT OF SIGNIFICANCE

Although hydrogels are researched as long-term vitreous substitute, none have advanced sufficiently to reach clinical application. Our work focuses on the development of a novel two component in situ forming hydrogel that bio-mimic the natural vitreous. Our thiol-containing copolymers can be injected as an aqueous solution into the vitreous cavity wherein, at physiological temperature, the rigid component will instantaneously form a physical gel imbedding the random coil copolymer. Upon subsequent oxidation, the two components will form disulfide cross-links and a stable reversible hydrogel capable of providing osmotic pressure to reattach the retina. It may be left in the eye permanently or easily removed by injection of a simple reducing agent to cleave the disulfide bonds, rather than surgery. This contribution is significant because it is expected to provide patients with a much better quality of life by improving surgical outcomes, creating much less post-operative burden, and reducing the need for secondary surgeries.

Recent progress in gellan gum hydrogels provided by functionalization strategies

Gellan gum and its functionalized derivatives present a wide range of applications that open up new possibilities in tissue engineering and regenerative medicine.

Protein, cell and bacterial response to atmospheric pressure plasma grafted hyaluronic acid on poly(methylmethacrylate)

Hyaluronic acid (HA) has been immobilised on poly(methyl methacrylate) (PMMA) surfaces using a novel dielectric barrier discharge (DBD) plasma process for the purposes of repelling protein, cellular and bacterial adhesion in the context of improving the performance of ophthalmic devices. Grafting was achieved by the following steps: (1) treatment of the PMMA with a DBD plasma operating at atmospheric pressure, (2) amine functionalisation of the activated polymer surface by exposure to a 3-aminopropyltrimethoxysilane (APTMS) linker molecule and (3) reaction of HA with the surface bound amine. The mechanism and effectiveness of the grafting process was verified by surface analysis. XPS data indicates that the APTMS linker molecule binds to PMMA via the Si-O chemistry and has the required pendant amine moiety. The carboxylic acid moiety on HA then binds with this -NH2 group via standard carbodiimide chemistry. ToF-SIMS confirms the presence of a coherent HA layer the microstructure of which is verified by AFM. The plasma grafted HA coating surfaces showed a pronounced decrease in protein and cellular adhesion when tested with bovine serum albumin and human corneal epithelial cells, respectively. The ability of these coatings to resist bacterial adhesion was established using Staphylococcus aureus NTC8325. Interestingly, the coatings did not repel bacterial adhesion, indicating that the mechanism of adhesion of bacterial cells is different to that for the surface interactions of mammalian cells. It is proposed that this difference is a consequence of the specific HA conformation that occurs under the conditions employed here. Hence, it is apparent that the microstructure/architecture of the HA coatings is an important factor in fabricating surfaces intended to repel proteins, mammalian and bacterial cells.

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.

Stimulus-responsive hydrogels: Theory, modern advances, and applications

Over the past century, hydrogels have emerged as effective materials for an immense variety of applications. The unique network structure of hydrogels enables very high levels of hydrophilicity and biocompatibility, while at the same time exhibiting the soft physical properties associated with living tissue, making them ideal biomaterials. Stimulus-responsive hydrogels have been especially impactful, allowing for unprecedented levels of control over material properties in response to external cues. This enhanced control has enabled groundbreaking advances in healthcare, allowing for more effective treatment of a vast array of diseases and improved approaches for tissue engineering and wound healing. In this extensive review, we identify and discuss the multitude of response modalities that have been developed, including temperature, pH, chemical, light, electro, and shear-sensitive hydrogels. We discuss the theoretical analysis of hydrogel properties and the mechanisms used to create these responses, highlighting both the pioneering and most recent work in all of these fields. Finally, we review the many current and proposed applications of these hydrogels in medicine and industry.

An in situ-forming zwitterionic hydrogel as vitreous substitute

An in situ-forming zwitterionic gel as vitreous substitute.