Development of hydrogel-based keratoprostheses: a materials perspective

Mechanical, optical, chemical, and biological evaluations of fish scale-derived scaffold for corneal replacements: A systematic review

Corneal blindness is commonly treated through corneal replacement with allogeneic corneal donors, which may face shortage. Regarding this issue, xenogeneic alternatives are explored. Fish scale-derived scaffolds (FSSs) are among the alternatives due to the lower risk of infection and abundant sources of raw materials. Unfortunately, the information about mechanical, optical, chemical, and biological performances of FSSs for corneal replacements is still scattered, as well as about the fabrication techniques. This study aims to gather scattered pieces of information about the mentioned performances and fabrication techniques of FSSs for corneal replacements. Sorted from four scientific databases and using the PRISMA checklist, eleven relevant articles are collected. FSSs are commonly fabricated using decellularization and decalcification processes, generating FSSs with parallel multilayers or crossed fibers with topographic microchannels. In the collected studies, similar mechanical properties of FSSs to native tissues are discovered, as well as good transparency, light remittance, but poorer refractive indexes than native tissues. Biological evaluations mostly discuss histology, cell proliferations, and immune responses on FSSs, while only a few studies examine the vascularization. No studies completed comprehensive evaluations on the four properties. The current progress of FSS developments demonstrates the potential of FSS use for corneal replacements.

Embedded 3D Bioprinting of Collagen Inks into Microgel Baths to Control Hydrogel Microstructure and Cell Spreading

Microextrusion-based 3D bioprinting into support baths has emerged as a promising technique to pattern soft biomaterials into complex, macroscopic structures. It is hypothesized that interactions between inks and support baths, which are often composed of granular microgels, can be modulated to control the microscopic structure within these macroscopic-printed constructs. Using printed collagen bioinks crosslinked either through physical self-assembly or bioorthogonal covalent chemistry, it is demonstrated that microscopic porosity is introduced into collagen inks printed into microgel support baths but not bulk gel support baths. The overall porosity is governed by the ratio between the ink's shear viscosity and the microgel support bath's zero-shear viscosity. By adjusting the flow rate during extrusion, the ink's shear viscosity is modulated, thus controlling the extent of microscopic porosity independent of the ink composition. For covalently crosslinked collagen, printing into support baths comprised of gelatin microgels (15-50 µm) results in large pores (≈40 µm) that allow human corneal mesenchymal stromal cells (MSCs) to readily spread, while control samples of cast collagen or collagen printed in non-granular support baths do not allow cell spreading. Taken together, these data demonstrate a new method to impart controlled microscale porosity into 3D printed hydrogels using granular microgel support baths.

Advances in 3D bioprinting technology for functional corneal reconstruction and regeneration

Corneal transplantation constitutes one of the major treatments in severe cases of corneal diseases. The lack of cornea donors as well as other limitations of corneal transplantation necessitate the development of artificial corneal substitutes. Biosynthetic cornea model using 3D printing technique is promising to generate artificial corneal structure that can resemble the structure of the native human cornea and is applicable for regenerative medicine. Research on bioprinting artificial cornea has raised interest into the wide range of materials and cells that can be utilized as bioinks for optimal clarity, biocompatibility, and tectonic strength. With continued advances in biomaterials science and printing technology, it is believed that bioprinted cornea will eventually achieve a level of clinical functionality and practicality as to replace donated corneal tissues, with their associated limitations such as limited or unsteady supply, and possible infectious disease transmission. Here, we review the literature on bioprinting strategies, 3D corneal modelling, material options, and cellularization strategies in relation to keratoprosthesis design. The progress, limitations and expectations of recent cases of 3D bioprinting of artifial cornea are discussed. An outlook on the rise of 3D bioprinting in corneal reconstruction and regeneration is provided.

Recent advancements in polymeric nanofibers for ophthalmic drug delivery and ophthalmic tissue engineering

Nanofibers due to their unique properties such as high surface-to-volume ratio, porous structure, mechanical strength, flexibility and their resemblance to the extracellular matrix, have been researched extensively in the field of ocular drug delivery and tissue engineering. Further, different modifications considering the formulation and process parameters have been carried out to alter the drug release profile and its interaction with the surrounding biological environment. Electrospinning is the most commonly used technique for preparing nanofibers with industrial scalability. Advanced techniques such as co-axial electrospinning and combined system such as embedding nanoparticles in nanofiber provide an alternative approach to enhance the performance of the scaffold. Electrospun nanofibers offers a matrix like structure for cell regeneration. Nanofibers have been used for ocular delivery of various drugs like antibiotics, anti-inflammatory and various proteins. In addition, lens-coated medical devices provide new insights into the clinical use of nanofibers. Through fabricating the nanofibers researchers have overcome the issues of low bioavailability and compatibility with ocular tissue. Therefore, nanofibers have great potential in ocular drug delivery and tissue engineering and have the capacity to revolutionize these therapeutic areas in the field of ophthalmology. This review is mainly focused on the recent advances in the preparation of nanofibers and their applications in ocular drug delivery and tissue engineering. The authors have attempted to emphasize the processing challenges and future perspectives along with an overview of the safety and toxicity aspects of nanofibers.

Interpenetrating polymeric network (IPNs) in ophthalmic drug delivery: Breaking the barriers

To maintain the therapeutic drug concentration for a prolonged period of time in aqueous and vitreous humor is primary challenge for ophthalmic drug delivery. Majority of the locally administered drug into the eye is lost as to natural reflexes like blinking and lacrimation resulting in the short span of drug residence. Consequently, less than 5% of the applied drug penetrate through the cornea and reaches the intraocular tissues. The major targets for optimal ophthalmic drug delivery are increasing drug residence time in cul-de-sac of the eye, prolonging intraocular exposure, modulating drug release from the delivery system, and minimizing pre-corneal drug loss. Development of in situ gel, contact lens, intraocular lens, inserts, artificial cornea, scaffold, etc., for ophthalmic drug delivery are few approaches to achieve these major targeted objectives for delivering the drug optimally. Interpenetrating polymeric network (IPN) or smart hydrogels or stimuli sensitive hydrogels are the class of polymers that can help to achieve the targets in ophthalmic drug delivery due to their versatility, biocompatibility and biodegradability. These novel ''smart" materials can alter their molecular configuration and result in volume phase transition in response to environmental stimuli, such as temperature, pH, ionic strength, electric and magnetic field. Hydrogel and tissue interaction, mechanical/tensile properties, pore size and surface chemistry of IPNs can also be modulated for tuning the drug release kinetics. Stimuli sensitive IPNs has been widely exploited to prepare in situ gelling formulations for ophthalmic drug delivery. Low refractive index hydrogel biomaterials with high water content, soft tissue-like physical properties, wettability, oxygen, glucose permeability and desired biocompatibility makes IPNs versatile candidate for contact lenses and corneal implants. This review article focuses on the exploration of these smart polymeric networks/IPNs for therapeutically improved ophthalmic drug delivery that has unfastened novel arenas in ophthalmic drug delivery.

Preliminary Study on Fish Scale Collagen Lamellar Matrix as Artificial Cornea

To construct a novel artificial cornea biomaterial, a method to prepare collagen lamellar matrix was developed in this study using grass carp scales as raw materials. The relationship between the structure of fish scale collagen lamellar matrix and the optical and mechanical properties was analyzed, and co-culture of it and rat bone marrow mesenchymal stem cells (BMSCs) was performed to preliminarily analyze the cellular compatibility of fish scale collagen lamellar matrix. The results show that the grass carp scales could be divided into base region, lateral region and parietal region according to the surface morphology. The inorganic calcium in the surface layer could be effectively removed by decalcification, and the decalcification rate could reach 99%. After etching treatment, homogeneous collagen lamellar matrix could be obtained. With the decalcification and etching treatment, the water content of the sample increased gradually, but the cross-linking treatment had no obvious effect on the water content of fish scale collagen lamellar matrix. Fish scale collagen lamellar matrix has good transparency, refractive index, mechanical properties and cellular compatibility, which may represent a prospect for the construction of cornea tissue engineering products.

Characterization of Tunable Poly-ε-Lysine-Based Hydrogels for Corneal Tissue Engineering

A family of poly-ε-lysine hydrogels can be synthesized by crosslinking with bis-carboxylic acids using carbodiimide chemistry. In addition to creating hydrogels using a simple cast method, a fragmented method is used to introduce increased porosity within the hydrogel structure. Both methods have created tunable characteristics ranging in their mechanical properties, transparency, and water content, which is of interest to corneal tissue engineering and can be tailored to specific cellular needs and applications. With a worldwide shortage of cornea donor tissue available for transplant and limitations including rejection and potential infection, a synthetic material that can be used as a graft, or a partial thickness corneal replacement, would be an advantageous treatment method. These hydrogels can be tuned to have similar mechanical and transparency properties to the human cornea. They also support the attachment and growth of corneal epithelial cells and the integration of corneal stromal cells.

Electrospun GelMA fibers and p(HEMA) matrix composite for corneal tissue engineering

The development of biocompatible and transparent three-dimensional materials is desirable for corneal tissue engineering. Inspired from the cornea structure, gelatin methacryloyl-poly(2-hydroxymethyl methacrylate) (GelMA-p(HEMA)) composite hydrogel was fabricated. GelMA fibers were produced via electrospinning and covered with a thin layer of p(HEMA) in the presence of N,N'-methylenebisacrylamide (MBA) as cross-linker by drop-casting. The structure of resulting GelMA-p(HEMA) composite was characterized by spectrophotometry, microscopy, and swelling studies. Biocompatibility and biological properties of the both p(HEMA) and GelMA-p(HEMA) composite have been investigated by 3D cell culture, red blood cell hemolysis, and protein adsorption studies (i.e., human serum albumin, human immunoglobulin and egg white lysozyme). The optical transmittance of the GelMA-p(HEMA) composite was found to be approximately 70% at 550 nm. The GelMA-p(HEMA) composite was biocompatible with tear fluid proteins and convenient for cell adhesion and growth. Thus, as prepared hydrogel composite may find extensive applications in future for the development of corneal tissue engineering as well as preparation of stroma of the corneal material.

Stem cell-based therapeutic strategies for corneal epithelium regeneration

Any significant loss of vision or blindness caused by corneal damages is referred to as corneal blindness. Corneal blindness is the fourth most common cause of blindness worldwide, representing more than 5% of the total blind population. Currently, corneal transplantation is used to treat many corneal diseases. In some cases, implantation of artificial cornea (keratoprosthesis) is suggested after a patient has had a donor corneal transplant failure. The shortage of donors and the side effects of keratoprosthesis are limiting these approaches. Recently, researchers have been actively pursuing new approaches for corneal regeneration because of these limitations. Nowadays, tissue engineering of different corneal layers (epithelium, stroma, endothelium, or full thickness tissue) is a promising approach that has attracted a great deal of interest from researchers and focuses on regenerative strategies using different cell sources and biomaterials. Various sources of corneal and non-corneal stem cells have shown significant advantages for corneal epithelium regeneration applications. Pluripotent stem cells (embryonic stem cells and iPS cells), epithelial stem cells (derived from oral mucus, amniotic membrane, epidermis and hair follicle), mesenchymal stem cells (bone marrow, adipose-derived, amniotic membrane, placenta, umbilical cord), and neural crest origin stem cells (dental pulp stem cells) are the most promising sources in this regard. These cells could also be used in combination with natural or synthetic scaffolds to improve the efficacy of the therapeutic approach. As the ocular surface is exposed to external damage, the number of studies on regeneration of the corneal epithelium is rising. In this paper, we reviewed the stem cell-based strategies for corneal epithelium regeneration.

Corneal epithelium tissue engineering: recent advances in regeneration and replacement of corneal surface

Currently, many corneal diseases are treated by corneal transplantation, artificial corneal implantation or, in severe cases, keratoprosthesis. Owing to the shortage of cornea donors and the risks involved with artificial corneal implants, such as infection transmission, researchers continually seek new approaches for corneal regeneration. Corneal tissue engineering is a promising approach that has attracted much attention from researchers and is focused on regenerative strategies using various biomaterials in combination with different cell types. These constructs should have the ability to mimic the native tissue microenvironment and present suitable optical, mechanical and biological properties. In this article, we review studies that have focused on the current clinical techniques for corneal replacement. We also describe tissue-engineering and cell-based approaches for corneal regeneration.

Sutureless Femtosecond Laser-Assisted Anterior Lamellar Keratoplasty Using a Bioengineered Cornea as a Viable Alternative to Human Donor Transplantation for Superficial Corneal Opacities

PURPOSE

To evaluate the safety and efficacy of a bioengineered corneal implant using femtosecond laser-assisted anterior lamellar keratoplasty for superficial corneal opacities.

METHODS

Six eyes of 6 consecutive patients with superficial corneal stromal opacities involving <220 μm owing to various pathologies were included in the study. Preoperatively, all patients underwent anterior segment optical coherence tomography (Visante; Carl Zeiss Meditec AG) to evaluate the depth of the corneal opacity. All patients underwent sutureless femtosecond laser-assisted anterior lamellar keratoplasty using a bioengineered collagen corneal implant (linkcor). Visual indices, refraction, and keratometry were evaluated preoperatively and 12 months postoperatively.

RESULTS

Corrected distance visual acuity improved significantly in all patients (P = 0.02). A significant decrease was seen in refractive astigmatism postoperatively (P = 0.04). Flat keratometry reduced significantly 12 months after the intervention (P = 0.04). No intraoperative or early postoperative complications were noticed. All implants were fully covered by healthy epithelium within a month after the surgery and remained clear at follow-up visits. The results of this procedure remained stable throughout the follow-up period. In 1 patient, mild inferior collagen melting and epithelial defect formation occurred at 1-year follow-up. Despite frequent topical corticosteroid therapy the melting progressed, the collagen tissue was removed and the patient was treated accordingly with good visual outcomes.

CONCLUSIONS

Femtosecond laser-assisted anterior lamellar keratoplasty with bioengineered corneal (linkcor) implantation is an effective treatment for improving vision quality in anterior stromal opacities. This procedure reduces the need for human donor tissue and avoids human donor-related and suturing complications.

Preparation of PVA Fluorescent Gel and Luminescence of Europium Sensitized by Terbium (III)

Polyvinyl alcohol (PVA) gel has a very wide range of applications in agriculture, military, industry, and other fields. As a widely used water-soluble polymer, PVA has good mechanical properties, excellent spinnability, good hydrophilicity, remarkable physical and chemical stability, good film formation, is non-polluting, and exhibits good natural degradation and biocompatibility. It is an ideal gel preparation material. Incorporation of rare-earth elements into PVA polymers can be used to prepare rare-earth luminescent gel materials. Results show that the luminescent efficiency of complexes is mainly related to their structure, ligand substituents, synergists, and the electronic configuration of doped rare-earth ions. Fluorescent gel films were prepared by adding europium, terbium, and europium/terbium co-doped into PVA, and their fluorescence properties were compared and analyzed. It was found that, in addition to the above factors, the sensitization of terbium to europium, and the fluorescence-quenching effect of hydroxyl groups, will influence the fluorescence properties. This has opened a new route for the application of rare-earth materials and may have value in the field of new materials.

Polyolefin-Supported Hydrogels for Selective Cleaning Treatments of Paintings

Surface decontamination is of general concern in many technical fields including optics, electronics, medical environments, as well as art conservation. In this respect, we developed thin copolymer networks covalently bonded to flexible polyethylene (PE) sheets for hydrogel-based cleaning of varnished paintings. The syntheses of acrylates and methacrylates of the surfactants Triton X-100, Brij 35, and Ecosurf EH-3 or EH-9 and their incorporation into copolymers with acrylamide (PAM) and N-(4-benzoylphenyl)acrylamide are reported. Photocrosslinked polymer networks were prepared from these copolymers on corona-treated PE sheets, which can be swollen with aqueous solution to form hydrogel layers. The cleaning efficacy of these PE-PAM hydrogel systems, when swollen with appropriate cleaning solutions, was evaluated on painting surfaces in dependence of the PAM copolymer composition and degree of crosslinking. Specifically, soil and varnish removal and varnish surface solubilization were assessed on mock-ups as well as on paintings, indicating that even surfactant-free cleaning solutions were effective.

Confocal and Histological Features After Poly(Ethylene Glycol) Diacrylate Corneal Inlay Implantation

Purpose

To evaluate the in vivo biocompatibility of photopolymerized poly(ethylene glycol) diacrylate (PEGDA) intrastromal inlays in rabbit corneas.

Methods

Sixty-three eyes of 42 New Zealand rabbits were included. Manual intrastromal pockets were dissected in 42 eyes. PEGDA inlays were obtained using a specifically designed photomask and were inserted in the intrastromal pocket of 21 eyes (inlay group); the remaining 21 right eyes did not receive any implant (pocket-only group). Twenty-one eyes with no intervention were used as controls. In vivo confocal microscopy (IVCM) was performed at every visit. After 2 months, rabbits were sacrificed and corneas removed for histological analysis.

Results

Corneas remained clear in all but two animals, and five cases of corneal neovascularization were seen (P = 0.2). Inlays remained stable without evidence of lateral or anterior migration, and no other complications were observed. No changes in anterior and posterior keratocyte density (P = 0.3 and P = 0.1, respectively) or endothelial cell density (P = 0.23) were observed between groups during the study time by IVCM. On pathology samples, thinning of the epithelium over the inlay area and epithelial hyperplasia over the edges were observed. A polygonal empty space with no evidence of PEGDA hydrogel within the midstroma was seen in the inlay group. Keratocytes were normal in shape and number in the vicinity of the PEGDA implant area.

Conclusions

Photopolymerized PEGDA intrastromal inlays have shown relatively good safety and stability in rabbit corneas. Inlays were biostable in the corneal environment and remained transparent during follow up.

Translational Relevance

The investigated PEGDA is promising for the development of biocompatible intrastromal implants.

Biocompatibility of Nanocellulose-Reinforced PVA Hydrogel with Human Corneal Epithelial Cells for Ophthalmic Applications

Transparent composite hydrogel in the form of a contact lens made from poly(vinyl alcohol) (PVA) and cellulose nanocrystals (CNCs) was subjected to in vitro biocompatibility evaluation with human corneal epithelial cells (HCE-2 cells). The cell response to direct contact with the hydrogels was investigated by placing the samples on top of confluent cell layers and evaluating cell viability, morphology, and cell layer integrity subsequent to 24 h culture and removal of the hydrogels. To further characterize the lens-cell interactions, HCE-2 cells were seeded on the hydrogels, with and without simulated tear fluid (STF) pre-conditioning, and cell viability and morphology were evaluated. Furthermore, protein adsorption on the hydrogel surface was investigated by incubating the materials with STF, followed by protein elution and quantification. The hydrogel material was found to have affinity towards protein adsorption, most probably due to the interactions between the positively charged lysozyme and the negatively charged CNCs embedded in the PVA matrix. The direct contact experiment demonstrated that the physical presence of the lenses did not affect corneal epithelial cell monolayers in terms of integrity nor cell metabolic activity. Moreover, it was found that viable corneal cells adhered to the hydrogel, showing the typical morphology of epithelial cells and that such response was not influenced by the STF pre-conditioning of the hydrogel surface. The results of the study confirm that PVA-CNC hydrogel is a promising ophthalmic biomaterial, motivating future in vitro and in vivo biocompatibility studies.

Providing Antibacterial Activity to Poly(2-Hydroxy Ethyl Methacrylate) by Copolymerization with a Methacrylic Thiazolium Derivative

Antimicrobial polymers and coatings are potent types of materials for fighting microbial infections, and as such, they have attracted increased attention in many fields. Here, a series of antimicrobial copolymers were prepared by radical copolymerization of 2-hydroxyethyl methacrylate (HEMA), which is widely employed in the manufacturing of biomedical devices, and the monomer 2-(4-methylthiazol-5-yl)ethyl methacrylate (MTA), which bears thiazole side groups susceptible to quaternization, to provide a positive charge. The copolymers were further quantitatively quaternized with either methyl or butyl iodide, as demonstrated by nuclear magnetic resonance (NMR) and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). Then, the polycations were characterized by zeta potential measurements to evaluate their effective charge and by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to evaluate their thermal properties. The ζ-potential study revealed that the quaternized copolymers with intermediate compositions present higher charges than the corresponding homopolymers. The cationic copolymers showed greater glass transition temperatures than poly(2-hydroxyethyl methacrylate) (PHEMA), with values higher than 100 °C, in particular those quaternized with methyl iodide. The TGA studies showed that the thermal stability of polycations varies with the composition, improving as the content of HEMA in the copolymer increases. Microbial assays targeting Gram-positive and Gram-negative bacteria confirmed that the incorporation of a low number of cationic units into PHEMA provides antimicrobial character with a minimum inhibitory concentration (MIC) of 128 µg mL-1. Remarkably, copolymers with MTA molar fractions higher than 0.50 exhibited MIC values as low as 8 µg mL-1.

High modulus hydrogels for ophthalmic and related biomedical applications

This paper presents three families of semi-interpenetrating polymer network (SIPN) hydrogels based on an ester-based polyurethane (EBPU) and hydrophilic monomers: N,N-dimethylacrylamide (NNDMA), N-vinyl pyrrolidone (NVP) and acryloylmorpholine (AMO) as potential materials for keratoprosthesis, orthokeratology and mini-scleral lens application. Hydrogels sheets were synthesized via free-radical polymerization with methods developed in-house. SIPN hydrogels were characterized for their equilibrium water content, mechanical and surface properties. Three families of optically clear SIPN-based hydrogels have been synthesized in the presence of water with >10% of composition attributable to EBPU. Water contents of SIPN materials ranged from 30% to 70%. SIPNs with ≤15% EBPU of total composition showed little influence to mechanical properties, whereas >15% EBPU contributed significantly to an increase in material stiffness. In the hydrated state, SIPNs with ≤15% EBPU of total composition show little difference in polar component (γ^p ) of surface free energy, whereas for >15% EBPU there is a decrease in γ^p . The EBPU SIPN hydrogels display complementary material properties for keratoprosthesis, orthokeratology, and mini-scleral applications. © 2018 The Authors. Journal of Biomedical Materials Research Part B: Applied Biomaterials published by Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1645-1653, 2019.

Polymerizable-group capped ZnS nanoparticle for high refractive index inorganic-organic hydrogel contact lens

Refractive index (RI) is an important parameter for contact lens biomaterials. In this paper, a novel polymerizable-group capped ZnS nanoparticle (NP) was synthesized by chemical link between hydroxyl group on the surface of ZnS (ME-capped) and isocyanate group of polymerizable molecule of 2-isocyanatoethyl methacrylate. Then the ZnS NP copolymerized with monomer of 2-hydroxyethyl methacrylate (HEMA) and N,N-dimethylacrylamide (DMA) to prepare high refractive index hydrogel contact lens with high content of inorganic ZnS NP. Increasing polymerizable-group capped ZnS content in the hydrogels improved its RI value and mechanical properties, however decreased slightly its transmittance, equilibrium (ESR) and lysozyme deposition on the hydrogel surface. The ZnS-containing hydrogels possessed good cytocompatibility and in vivo biocompatibility in rabbit eyes, demonstrating a potential application as high RI ocular refractive correction biomaterial.

Corneal regeneration: A review of stromal replacements

Corneal blindness is traditionally treated by transplantation of a donor cornea, or in severe cases by implantation of an artificial cornea or keratoprosthesis. Due to severe donor shortages and the risks of complications that come with artificial corneas, tissue engineering in ophthalmology has become more focused on regenerative strategies using biocompatible materials either with or without cells. The stroma makes up the bulk of the corneal thickness and mainly consists of a tightly interwoven network of collagen type I, making it notoriously difficult to recreate in a laboratory setting. Despite the challenges that come with corneal stromal tissue engineering, there has recently been enormous progress in this field. A large number of research groups are working towards developing the ideal biomimetic, cytocompatible and transplantable stromal replacement. Here we provide an overview of the approaches directed towards tissue engineering the corneal stroma, from classical collagen gels, films and sponges to less traditional components such as silk, fish scales, gelatin and polymers. The perfect stromal replacement has yet to be identified and future research should be directed at combined approaches, in order to not only host native stromal cells but also restore functionality.

STATEMENT OF SIGNIFICANCE

In the field of tissue engineering and regenerative medicine in ophthalmology the focus has shifted towards a common goal: to restore the corneal stroma and thereby provide a new treatment option for patients who are currently blind due to corneal opacification. Currently the waiting lists for corneal transplantation include more than 10 million patients, due to severe donor shortages. Alternatives to the transplantation of a donor cornea include the use of artificial cornea, but these are by no means biomimetic and therefore do not provide good outcomes. In recent years a lot of work has gone into the development of tissue engineered scaffolds and other biomaterials suitable to replace the native stromal tissue. Looking at all the different approaches separately is a daunting task and up until now there was no review article in which every approach is discussed. This review does include all approaches, from classical tissue engineering with collagen to the use of various alternative biomaterials and even fish scales. Therefore, this review can serve as a reference work for those starting in the field and but also to stimulate collaborative efforts in the future.

Synthesis and Characterisation of Photocrosslinked poly(ethylene glycol) diacrylate Implants for Sustained Ocular Drug Delivery

Purpose

To investigate the sustained ocular delivery of small and large drug molecules from photocrosslinked poly(ethylene glycol) diacrylate (PEGDA) implants with varying pore forming agents.

Methods

Triamcinolone acetonide and ovalbumin loaded photocrosslinked PEGDA implants, with or without pore-forming agents, were fabricated and characterised for chemical, mechanical, swelling, network parameters, as well as drug release and biocompatibility. HPLC-based analytical methods were employed for analysis of two molecules; ELISA was used to demonstrate bioactivity of ovalbumin.

Results

Regardless of PEGDA molecular weight or pore former composition all implants loaded with triamcinolone acetonide released significantly faster than those loaded with ovalbumin. Higher molecular weight PEGDA systems (700 Da) resulted in faster drug release of triamcinolone acetonide than their 250 Da counterpart. All ovalbumin released over the 56-day time period was found to be bioactive. Increasing PEGDA molecular weight resulted in increased system swelling, decreased crosslink density (Ve), increased polymer-water interaction parameter (χ), increased average molecular weight between crosslinks (Mc) and increased mesh size (ε). SEM studies showed the porosity of implants increased with increasing PEGDA molecular weight. Biocompatibility showed both PEGDA molecular weight implants were non-toxic when exposed to retinal epithelial cells over a 7-day period.

Conclusion

Photocrosslinked PEGDA implant based systems are capable of controlled drug release of both small and large drug molecules through adaptations in the polymer system network. We are currently continuing evaluation of these systems as potential sustained drug delivery devices.

Evaluation of stability and biocompatibility of PHEMA-PMMA keratoprosthesis by penetrating keratoplasty in rabbits

Artificial corneas have been developed as an alternative to natural donor tissue to replace damaged or diseased corneas. This study was conducted to evaluate the stability and biocompatibility of PHEMA-PMMA [poly (2-hydroxyl methacrylate)-poly (methyl methacrylate)] keratoprostheses in rabbits following penetrating keratoplasty. Sixteen male New Zealand White rabbits aged 16 weeks were divided into three groups. Group I and group II contained six rabbits each, while the control group had four rabbits. Experimental surgery was conducted under general anesthesia. The cornea was penetrated using an 8 mm diameter biopsy punch. In group I (core 5 mm & skirt 3 mm) and group II (core 6 mm & skirt 2 mm), the keratoprosthesis was placed into the recipient full thickness bed and sutured into position with double-layer continuous. In the control group, corneal transplantation using normal allogenic corneal tissue was performed with the same suture method. After four and eight weeks, keratoprosthesis devices were evaluated by histopathological analysis of gross lesions. Post-operative complications were observed, such as extrusion and infection in experimental groups. Most corneas were maintained in the defect site by double-layer continuous suture materials for 4 weeks and kept good light transmission. However, most artificial cornea were extruded before 8 weeks. Overall, combined PHEMA and PMMA appears to have sufficient advantages for production of artificial corneas because of its optical transparency, flexibility and other mechanical features. However, the stability and biocompatibility were not sufficient to enable application in humans and animals at the present time using penetrating keratoplasty. Further studies are essential to improve the stability and biocompatibility with or without other types of keratoplasty.

New strategy for design and fabrication of polymer hydrogel with tunable porosity as artificial corneal skirt

In order to obtain an ideal material using for artificial corneal skirt, a porous polymer hydrogel containing 2-hydroxyethyl methacrylate (HEMA), trimethylolpropane triacrylate (TMPTA) and butyl acrylate was prepared through one-step radical polymerization method and the usage of CaCO₃ whisker as porogen. The physical-chemical properties of the fabricated polymer hydrogel can be adjusted by CaCO₃ whisker content, such as pore size, porosity, water content of materials and surface topography. Then a series of cell biology experiments of human corneal fibroblasts (HCFs) were carried out to evaluate its properties as an artificial corneal skirt, such as the adhesion of cells on the materials with different pore size and porosity, the apoptosis on materials with different characteristics, the distribution of the cells on the material surface. The results revealed that high porosity not only could improve water content of hydrogel, but also strengthen the adhesion of HCFs on hydrogel. In addition, high porosity hydrogel with the whisker shape of pores showed much elongate spindle-like morphology than those low porosity hydrogels. MTT assay certified that the resulted polymer hydrogel material possessed excellent biocompatibility and was suitable for HCFs growing, making it promising for being developed as artificial corneal skirt.

Biocompatibility of a fish scale-derived artificial cornea: Cytotoxicity, cellular adhesion and phenotype, and in vivo immunogenicity

PURPOSE

To determine whether a fish scale-derived collagen matrix (FSCM) meets the basic criteria to serve as an artificial cornea, as determined with in vitro and in vivo tests.

METHODS

Primary corneal epithelial and stromal cells were obtained from human donor corneas and used to examine the (in)direct cytotoxicity effects of the scaffold. Cytotoxicity was assessed by an MTT assay, while cellular proliferation, corneal cell phenotype and adhesion markers were assessed using an EdU-assay and immunofluorescence. For in vivo-testing, FSCMs were implanted subcutaneously in rats. Ologen(®) Collagen Matrices were used as controls. A second implant was implanted as an immunological challenge. The FSCM was implanted in a corneal pocket of seven New Zealand White rabbits, and compared to sham surgery.

RESULTS

The FSCM was used as a scaffold to grow corneal epithelial and stromal cells, and displayed no cytotoxicity to these cells. Corneal epithelial cells displayed their normal phenotypical markers (CK3/12 and E-cadherin), as well as cell-matrix adhesion molecules: integrin-α6 and β4, laminin 332, and hemi-desmosomes. Corneal stromal cells similarly expressed adhesion molecules (integrin-α6 and β1). A subcutaneous implant of the FSCM in rats did not induce inflammation or sensitization; the response was comparable to the response against the Ologen(®) Collagen Matrix. Implantation of the FSCM in a corneal stromal pocket in rabbits led to a transparent cornea, healthy epithelium, and, on histology, hardly any infiltrating immune cells.

CONCLUSION

The FSCM allows excellent cell growth, is not immunogenic and is well-tolerated in the cornea, and thus meets the basic criteria to serve as a scaffold to reconstitute the cornea.

Corneal Stroma Regeneration with Acellular Corneal Stroma Sheets and Keratocytes in a Rabbit Model

Acellular corneal stroma matrix has been used for corneal stroma engineering. However, because of its compact tissue structure, regrowth of keratocytes into the scaffold is difficult. Previously, we developed a sandwich model for cartilage engineering using acellular cartilage sheets. In the present study, we tested this model for corneal stroma regeneration using acellular porcine corneal stroma (APCS) sheets and keratocytes. Porcine corneas were decellularized by NaCl treatment, and the APCS was cut into 20-μm-thick sheets. A rabbit corneal stroma defect model was created by lamellar keratoplasty and repaired by transplantation of five pieces of APCS sheets with keratocytes. Six months after transplantation, transparent corneas were present in the experimental group, which were confirmed by anterior segment optical coherence tomography examination and transmittance examination. The biomechanical properties in the experimental group were similar to those of normal cornea. Histological analyses showed an even distribution of keratocytes and well-oriented matrix in the stroma layer in the experimental group. Together, these results demonstrated that the sandwich model using acellular corneal stroma sheets and keratocytes could be potentially useful for corneal stroma regeneration.

Biocompatibility of poly(ethylene glycol) and poly(acrylic acid) interpenetrating network hydrogel by intrastromal implantation in rabbit cornea

We evaluated the biocompatibility of a poly(ethylene glycol) and poly(acrylic acid) (PEG/PAA) interpenetrating network hydrogel designed for artificial cornea in a rabbit model. PEG/PAA hydrogel measuring 6 mm in diameter was implanted in the corneal stroma of twelve rabbits. Stromal flaps were created with a microkeratome. Randomly, six rabbits were assigned to bear the implant for 2 months, two rabbits for 6 months, two rabbits for 9 months, one rabbit for 12 months, and one rabbit for 16 months. Rabbits were evaluated monthly. After the assigned period, eyes were enucleated, and corneas were processed for histology and immunohistochemistry. There were clear corneas in three of six rabbits that had implantation of hydrogel for 2 months. In the six rabbits with implant for 6 months or longer, the corneas remained clear in four. There was a high rate of epithelial defect and corneal thinning in these six rabbits. One planned 9-month rabbit developed extrusion of implant at 4 months. The cornea remained clear in the 16-month rabbit but histology revealed epithelial in-growth. Intrastromal implantation of PEG/PAA resulted in a high rate of long-term complications.

Keratoconus: tissue engineering and biomaterials

Keratoconus (KC) is a bilateral, asymmetric, corneal disorder that is characterized by progressive thinning, steepening, and potential scarring. The prevalence of KC is stated to be 1 in 2000 persons worldwide; however, numbers vary depending on size of the study and regions. KC appears more often in South Asian, Eastern Mediterranean, and North African populations. The cause remains unknown, although a variety of factors have been considered. Genetics, cellular, and mechanical changes have all been reported; however, most of these studies have proven inconclusive. Clearly, the major problem here, like with any other ocular disease, is quality of life and the threat of vision loss. While most KC cases progress until the third or fourth decade, it varies between individuals. Patients may experience periods of several months with significant changes followed by months or years of no change, followed by another period of rapid changes. Despite the major advancements, it is still uncertain how to treat KC at early stages and prevent vision impairment. There are currently limited tissue engineering techniques and/or "smart" biomaterials that can help arrest the progression of KC. This review will focus on current treatments and how biomaterials may hold promise for the future.

Amine functional hydrogels as selective substrates for corneal epithelialization

The aim of this study was to develop a synthetic hydrogel to act as a corneal substitute capable of selectively supporting the adhesion and proliferation of limbal epithelial cells (LECs) while inhibiting growth of limbal fibroblasts. Deficiency of LECs causes conjunctival epithelial cells to move over the cornea, producing a thick scar pannus. Unilateral defects can be treated using LEC cultured from the unaffected eye, transplanting them to the affected cornea after scar tissue is removed. The underlying wound bed is often damaged, however, hence the need to develop a corneal inlay to aid in corneal re-epithelialization. Transparent epoxy-functional polymethacrylate networks were synthesized using a combination of glycerol monomethacrylate, ethylene glycol dimethacrylate, lauryl methacrylate and glycidyl methacrylate that produced two different bulk hydrogel compositions with different equilibrium water contents (EWCs): Base 1 and Base 2, EWC=55% and 35%, respectively. Two sets of amine-functional hydrogels were produced following reaction of the epoxide groups with excesses of either ammonia, 1,2-diamino ethane, 1,3-diamino propane, 1,4-diamino butane or 1,6-diamino hexane. Neither series of hydrogels supported the proliferation of limbal fibroblasts irrespective of amine functionalization but they both supported the adhesion and proliferation of limbal epithelial cells, particularly when functionalized with 1,4-diamino butane. With Base 1 hydrogels (less so with Base 2) a vigorous epithelial outgrowth was seen from small limbal explants and a confluent epithelial layer was achieved in vitro within 6days. The data support the development of hydrogels specific for epithelial formation.

Gelatin nanofiber-reinforced alginate gel scaffolds for corneal tissue engineering

A severe shortage of donor cornea is now an international crisis in public health. Substitutes for donor tissue need to be developed to meet the increasing demand for corneal transplantation. Current attempts in designing scaffolds for corneal tissue regeneration involve utilization of expensive materials. Yet, these corneal scaffolds still lack the highly-organized fibrous structure that functions as a load-bearing component in the native tissue. This work shows that transparent nanofiber-reinforced hydrogels could be developed from cheap, non-immunogenic and readily available natural polymers to mimic the cornea's microstructure. Electrospinning was employed to produce gelatin nanofibers, which were then infiltrated with alginate hydrogels. Introducing electrospun nanofibers into hydrogels improved their mechanical properties by nearly one order of magnitude, yielding mechanically robust composites. Such nanofiber-reinforced hydrogels could serve as alternatives to donor tissue for corneal transplantation.

Recent advances in the design of artificial corneas

PURPOSE OF REVIEW

Artificial corneas are being developed to meet a shortage of donor corneas and to address cases in which allografting is contraindicated. A range of artificial corneas has been developed. Here we review several newer designs and especially those inspired by naturally occurring biomaterials found with the human body and elsewhere.

RECENT FINDINGS

Recent trends in the development of artificial corneas indicate a move towards the use of materials derived from native sources including decellularized corneal tissue and tissue substitutes synthesized by corneal cells in vitro when grown either on their own or in conjunction with novel protein-based scaffolds. Biologically inspired materials are also being considered for implantation on their own with the view to promoting endogenous corneal tissue.

SUMMARY

More recent attempts at making artificial corneas have taken a more nature-based or nature-inspired approach. Several will in the near future be likely to be available clinically.

Development of high refractive ZnS/PVP/PDMAA hydrogel nanocomposites for artificial cornea implants

A series of high refractive index (RI) ZnS/PVP/PDMAA hydrogel nanocomposites containing ZnS nanoparticles (NPs) were successfully synthesized via a simple ultraviolet-light-initiated free radical co-polymerization method. The average diameter of the ZnS NPs is ∼ 3 nm and the NPs are well dispersed and stabilized in the PVP/PDMAA hydrogel matrix up to a high content of 60 wt.% in the hydrogel nanocomposites. The equilibrium water content of ZnS/PVP/PDMAA hydrogel nanocomposites varied from 82.0 to 66.8 wt.%, while the content of mercaptoethanol-capped ZnS NPs correspondingly varied from 30 to 60 wt.%. The resulting nanocomposites are clear and transparent and their RIs were measured to be as high as 1.58-1.70 and 1.38-1.46 in the dry and hydrated states, respectively, which can be tuned by varying the ZnS NPs content. In vitro cytotoxicity assays suggested that the introduction of ZnS NPs added little cytotoxicity to the PVP/PDMAA hydrogel and all the hydrogel nanocomposites exhibited minimal cytotoxicity towards common cells. The hydrogel nanocomposites implanted in rabbit eyes can be well tolerated over 3 weeks. Hence, the high RI ZnS/PVP/PDMAA hydrogel nanocomposites with adjustable RIs developed in this work might potentially be a candidate material for artificial corneal implants.

Property-based design: optimization and characterization of polyvinyl alcohol (PVA) hydrogel and PVA-matrix composite for artificial cornea

Each approach for artificial cornea design is toward the same goal: to develop a material that best mimics the important properties of natural cornea. Accordingly, the selection and optimization of corneal substitute should be based on their physicochemical properties. In this study, three types of polyvinyl alcohol (PVA) hydrogels with different polymerization degree (PVA1799, PVA2499 and PVA2699) were prepared by freeze-thawing techniques. After characterization in terms of transparency, water content, water contact angle, mechanical property, root-mean-square roughness and protein adsorption behavior, the optimized PVA2499 hydrogel with similar properties of natural cornea was selected as a matrix material for artificial cornea. Based on this, a biomimetic artificial cornea was fabricated with core-and-skirt structure: a transparent PVA hydrogel core, surrounding by a ringed PVA-matrix composite skirt that composed of graphite, Fe-doped nano hydroxyapatite (n-Fe-HA) and PVA hydrogel. Different ratio of graphite/n-Fe-HA can tune the skirt color from dark brown to light brown, which well simulates the iris color of Oriental eyes. Moreover, morphologic and mechanical examination showed that an integrated core-and-skirt artificial cornea was formed from an interpenetrating polymer network, no phase separation appeared on the interface between the core and the skirt.

Time-dependent fracture toughness of cornea

The fracture and time-dependent properties of cornea are very important for the development of corneal scaffolds and prostheses. However, there has been no systematic study of cornea fracture; time-dependent behavior of cornea has never been investigated in a fracture context. In this work, fracture toughness of cornea was characterized by trouser tear tests, and time-dependent properties of cornea were examined by stress-relaxation and uniaxial tensile tests. Control experiments were performed on a photoelastic rubber sheet. Corneal fracture resistance was found to be strain-rate dependent, with values ranging from 3.39±0.57 to 5.40±0.48kJm(-2) over strain rates from 3 to 300mmmin(-1). Results from stress-relaxation tests confirmed that cornea is a nonlinear viscoelastic material. The cornea behaved closer to a viscous fluid at small strain but became relatively more elastic at larger strain. Although cornea properties are greatly dependent on time, the stress-strain responses of cornea were found to be insensitive to the strain rate when subjected to tensile loading.

Evaluation of corneal cell growth on tissue engineering materials as artificial cornea scaffolds

The keratoprosthesis (KPro; artificial cornea) is a special refractive device to replace human cornea by using heterogeneous forming materials for the implantation into the damaged eyes in order to obtain a certain vision. The main problems of artificial cornea are the biocompatibility and stability of the tissue particularly in penetrating keratoplasty. The current studies of tissue-engineered scaffold materials through comprising composites of natural and synthetic biopolymers together have developed a new way to artificial cornea. Although a wide agreement that the long-term stability of these devices would be greatly improved by the presence of cornea cells, modification of keratoprosthesis to support cornea cells remains elusive. Most of the studies on corneal substrate materials and surface modification of composites have tried to improve the growth and biocompatibility of cornea cells which can not only reduce the stimulus of heterogeneous materials, but also more importantly continuous and stable cornea cells can prevent the destruction of collagenase. The necrosis of stroma and spontaneous extrusion of the device, allow for maintenance of a precorneal tear layer, and play the role of ensuring a good optical surface and resisting bacterial infection. As a result, improvement in corneal cells has been the main aim of several recent investigations; some effort has focused on biomaterial for its well biological properties such as promoting the growth of cornea cells. The purpose of this review is to summary the growth status of the corneal cells after the implantation of several artificial corneas.

The influence of substrate topography on the migration of corneal epithelial wound borders

Currently available artificial corneas can develop post-implant complications including epithelial downgrowth, infection, and stromal melting. The likelihood of developing these disastrous complications could be minimized through improved formation and maintenance of a healthy epithelium covering the implant. We hypothesize that this epithelial formation may be enhanced through the incorporation of native corneal basement membrane biomimetic chemical and physical cues onto the surface of the keratoprosthesis. We fabricated hydrogel substrates molded with topographic features containing specific bio-ligands and developed an in vitro wound healing assay. In our experiments, the rate of corneal epithelial wound healing was significantly increased by 50% in hydrogel surfaces containing topographic features, compared to flat surfaces with the same chemical attributes. We determined that this increased healing is not due to enhanced proliferation or increased spreading of the epithelial cells, but to an increased active migration of the epithelial cells. These results show the potential benefit of restructuring and improving the surface of artificial corneas to enhance epithelial coverage and more rapidly restore the formation of a functional epithelium.

Tissue engineering of the corneal endothelium: a review of carrier materials

Functional impairment of the human corneal endothelium can lead to corneal blindness. In order to meet the high demand for transplants with an appropriate human corneal endothelial cell density as a prerequisite for corneal function, several tissue engineering techniques have been developed to generate transplantable endothelial cell sheets. These approaches range from the use of natural membranes, biological polymers and biosynthetic material compositions, to completely synthetic materials as matrices for corneal endothelial cell sheet generation. This review gives an overview about currently used materials for the generation of transplantable corneal endothelial cell sheets with a special focus on thermo-responsive polymer coatings.

In vivo biocompatibility of two PEG/PAA interpenetrating polymer networks as corneal inlays following deep stromal pocket implantation

This study compared the effects of implanting two interpenetrating polymer networks (IPNs) into rabbit corneas. The first (Implant 1) was based on PEG-diacrylate, the second (Implant 2) was based on PEG-diacrylamide. There were inserted into deep stromal pockets created using a manual surgical technique for either 3 or 6 months. The implanted corneas were compared with normal and sham-operated corneas through slit lamp observation, anterior segment optical coherence tomography, in vivo confocal scanning and histological examination. Corneas with Implant 1 (based on PEG-diacrylate) developed diffuse haze, ulcers and opacities within 3 months, while corneas with Implant 2 (based on PEG-diacrylamide) remained clear at 6 months. They also exhibited normal numbers of epithelial cell layers, without any immune cell infiltration, inflammation, oedema or neovascularisation at post-operative 6 month. Morphological studies showed transient epithelial layer thinning over the hydrogel inserted area and elevated keratocyte activity at 3 months; however, the epithelium thickness and keratocyte morphology were improved at 6 months. Implant 2 exhibited superior in vivo biocompatibility and higher optical clarity than Implant 1. PEG-diacrylamide-based IPN hydrogel is therefore a potential candidate for corneal inlays to correct refractive error.

Composite electrospun gelatin fiber-alginate gel scaffolds for mechanically robust tissue engineered cornea

A severe shortage of good quality donor cornea is now an international crisis in public health. Alternatives for donor tissue need to be urgently developed to meet the increasing demand for corneal transplantation. Hydrogels have been widely used as scaffolds for corneal tissue regeneration due to their large water content, similar to that of native tissue. However, these hydrogel scaffolds lack the fibrous structure that functions as a load-bearing component in the native tissue, resulting in poor mechanical performance. This work shows that mechanical properties of compliant hydrogels can be substantially enhanced with electrospun nanofiber reinforcement. Electrospun gelatin nanofibers were infiltrated with alginate hydrogels, yielding transparent fiber-reinforced hydrogels. Without prior crosslinking, electrospun gelatin nanofibers improved the tensile elastic modulus of the hydrogels from 78±19 kPa to 450±100 kPa. Stiffer hydrogels, with elastic modulus of 820±210 kPa, were obtained by crosslinking the gelatin fibers with carbodiimide hydrochloride in ethanol before the infiltration process, but at the expense of transparency. The developed fiber-reinforced hydrogels show great promise as mechanically robust scaffolds for corneal tissue engineering applications.