Biomaterials-enabled cornea regeneration in patients at high risk for rejection of donor tissue transplantation

Prevention of Corneal Myofibroblastic Differentiation In Vitro Using a Biomimetic ECM Hydrogel for Corneal Tissue Regeneration

Corneal scarring is one of the major causes of blindness, affecting millions worldwide. Despite recent advancements in surgical strategies, there is an unmet need for a clinically feasible material and methods to prevent scarring following corneal injury. In this study, we report the potential utility of a hydrogel derived from cadaveric animal corneas, using a decellularized corneal matrix hydrogel (abbreviated as dCMH), which is prepared by a simple method. This hydrogel is easily injectable, biocompatible, and has the ability to maintain good shape-retention properties at 37 °C, which make it suitable for in vivo applications. Furthermore, our gene expression studies and immunofluorescence studies indicate that dCMH maintains the morphology and function of keratocytes in vitro and prevents their transdifferentiation to myofibroblasts. From the above results, it is evident that dCMH maintains the keratocytes with the ability to regenerate the corneal defect without scar. We thus suggest a simple yet effective approach for corneal tissue decellularization and that dCMH can be a promising material for prophylaxis against blinding scar formation in an injured cornea.

Bio-orthogonally crosslinked hyaluronate-collagen hydrogel for suture-free corneal defect repair

Biomaterials that mimic corneal stroma could decrease the need for donor corneal tissue and could decrease the prevalence of complications associated with corneal transplantation, including infection and rejection. We developed a bio-orthogonally crosslinked hyaluronate-collagen hydrogel which can fill corneal defects in situ without the need for any sutures, initiators, or catalysts. We studied the effects of biorthogonal crosslinking on the light transmittance of the hydrogel, which was greater than 97% water. The transmittance of the optimized hydrogel in the visible light range was over 94%. We also investigated the mechanical properties, refractive index, morphology, biocompatibility, and corneal re-epithelialization capacity of the hyaluronate-collagen hydrogel. Our in vitro, in vivo, and ex vivo results demonstrated that this bio-orthogonally crosslinked hyaluronate-collagen hydrogel has excellent potential as a biomaterial for cornea repair and regeneration.

Bioengineering Approaches for Corneal Regenerative Medicine

BACKGROUND

Since the cornea is responsible for transmitting and focusing light into the eye, injury or pathology affecting any layer of the cornea can cause a detrimental effect on visual acuity. Aging is also a reason for corneal degeneration. Depending on the level of the injury, conservative therapies and donor tissue transplantation are the most common treatments for corneal diseases. Not only is there a lack of donor tissue and risk of infection/rejection, but the inherent ability of corneal cells and layers to regenerate has led to research in regenerative approaches and treatments.

METHODS

In this review, we first discussed the anatomy of the cornea and the required properties for reconstructing layers of the cornea. Regenerative approaches are divided into two main categories; using direct cell/growth factor delivery or using scaffold-based cell delivery. It is expected delivered cells migrate and integrate into the host tissue and restore its structure and function to restore vision. Growth factor delivery also has shown promising results for corneal surface regeneration. Scaffold-based approaches are categorized based on the type of scaffold, since it has a significant impact on the efficiency of regeneration, into the hydrogel and non-hydrogel based scaffolds. Various types of cells, biomaterials, and techniques are well covered.

RESULTS

The most important characteristics to be considered for biomaterials in corneal regeneration are suitable mechanical properties, biocompatibility, biodegradability, and transparency. Moreover, a curved shape structure and spatial arrangement of the fibrils have been shown to mimic the corneal extracellular matrix for cells and enhance cell differentiation.

CONCLUSION

Tissue engineering and regenerative medicine approaches showed to have promising outcomes for corneal regeneration. However, besides proper mechanical and optical properties, other factors such as appropriate sterilization method, storage, shelf life and etc. should be taken into account in order to develop an engineered cornea for clinical trials.

Investigation of Cancer Cell Migration and Proliferation on Synthetic Extracellular Matrix Peptide Hydrogels

Chemical and mechanical properties of a tumor microenvironment are essential players in cancer progression, and it is important to precisely control the extracellular conditions while designing cancer in vitro models. The study investigates synthetic hydrogel matrices from multi-arm polyethylene glycol (PEG) functionalized with collagen-like peptide (CLP) CG(PKG)₄(POG)₄(DOG)₄ alone and conjugated with either cell adhesion peptide RGD (mimicking fibronectin) or IKVAV (mimicking laminin). Human glioblastoma HROG36, rat C6 glioma cells, and A375 human melanoma cells were grown on the hydrogels and monitored for migration, proliferation, projected cell area, cell shape index, size and number, distribution of focal contacts in individual cells, and focal adhesion number. PEG-CLP-RGD induced migration of both glioma cell lines and also stimulated proliferation (assessed as metabolic activity) of HROG36 cells. Migration of C6 cells were also stimulated by PEG-CLP-IKVAV. These responses strongly correlated with the changes in adhesion and morphology parameters of individual cells – projected cell area, cell shape index, and focal contact number. Melanoma A375 cell proliferation was increased by PEG-CLP-RGD, and this was accompanied by a decrease in cell shape index. However, neither RGD nor IKVAV conjugated to PEG-CLP stimulated migratory capacity of A375 cells. Taken together, the study presents synthetic scaffolds with extracellular matrix (ECM)-mimicking peptides that allow for the exploration of the effect of ECM signaling to cancer cells.

Intrastromal implantation of chicken corneal grafts into the cornea of rabbits for corneal thickening: an experimental study

PURPOSE

To evaluate the feasibility and effects of the intrastromal implantation of chemically modified corneal stroma obtained from chicken into the corneas of rabbits for corneal thickening.

METHODS

Chicken corneas were cut, debrided, treated with cross-linking and implanted in an intrastromal pouch created in the cornea of 10 white New Zealand rabbits with femtosecond laser. Slit-lamp biomicroscopy and optical coherence tomography were performed immediately, 7, 30 and 90 days postoperatively. Corneas were removed at 90 days and cut in two halves. One half was sent to histological analysis for the presence of necrosis, polymorphonuclear inflammatory cells, blood vessels and fibrosis, while the other half was evaluated with transmission electron microscopy to verify tissue organization and the presence of keratocytes and inflammatory cells. Corneal thicknesses were comparatively analyzed over time with Wilcoxon test (p ≤ 0.05).

RESULTS

The chicken grafts were incorporated into the cornea of all animals over time. Mean rabbit cornea thickness increased from 338 µm preoperatively to 538 µm (p < 0.0077) at 90 days, while mean chicken graft thickness decreased from 350 to 215 µm (p < 0.0077). No clear signs of rejection attributable to the xenograft were observed in any of the implanted eyes. However, some macroscopic and histological events were observed in some of the eyes, probably due to procedural issues during implantation.

CONCLUSION

The intrastromal implantation of chicken grafts was shown to be feasible and predictable to thicken the recipient rabbit cornea without apparent rejection. However, before being considered in humans, further meticulous clinical trials are required to establish the clinical utility, safety and efficacy of xenografts for the treatment of patients with advanced keratoconus.

Corneal Stem Cells as a Source of Regenerative Cell-Based Therapy

In the past few years, intensive research has focused on corneal stem cells as an unlimited source for cell-based therapy in regenerative ophthalmology. Today, it is known that the cornea has at least two types of stem cells: limbal epithelial stem cells (LESCs) and corneal stromal stem cells (CSSCs). LESCs are used for regeneration of corneal surface, while CSSCs are used for regeneration of corneal stroma. Until now, various approaches and methods for isolation of LESCs and CSSCs and their successful transplantation have been described and tested in several preclinical studies and clinical trials. This review describes in detail phenotypic characteristics of LESCs and CSSCs and discusses their therapeutic potential in corneal regeneration. Since efficient and safe corneal stem cell-based therapy is still a challenging issue that requires continuous cooperation between researchers, clinicians, and patients, this review addresses the important limitations and suggests possible strategies for improvement of corneal stem cell-based therapy.

New Technologies in Clinical Trials in Corneal Diseases and Limbal Stem Cell Deficiency: Review from the European Vision Institute Special Interest Focus Group Meeting

To discuss and evaluate new technologies for a better diagnosis of corneal diseases and limbal stem cell deficiency, the outcomes of a consensus process within the European Vision Institute (and of a workshop at the University of Cologne) are outlined. Various technologies are presented and analyzed for their potential clinical use also in defining new end points in clinical trials. The disease areas which are discussed comprise dry eye and ocular surface inflammation, imaging, and corneal neovascularization and corneal grafting/stem cell and cell transplantation. The unmet needs in the abovementioned disease areas are discussed, and realistically achievable new technologies for better diagnosis and use in clinical trials are outlined. To sum up, it can be said that there are several new technologies that can improve current diagnostics in the field of ophthalmology in the near future and will have impact on clinical trial end point design.

Prospects and Challenges of Translational Corneal Bioprinting

Corneal transplantation remains the ultimate treatment option for advanced stromal and endothelial disorders. Corneal tissue engineering has gained increasing interest in recent years, as it can bypass many complications of conventional corneal transplantation. The human cornea is an ideal organ for tissue engineering, as it is avascular and immune-privileged. Mimicking the complex mechanical properties, the surface curvature, and stromal cytoarchitecure of the in vivo corneal tissue remains a great challenge for tissue engineering approaches. For this reason, automated biofabrication strategies, such as bioprinting, may offer additional spatial control during the manufacturing process to generate full-thickness cell-laden 3D corneal constructs. In this review, we discuss recent advances in bioprinting and biomaterials used for in vitro and ex vivo corneal tissue engineering, corneal cell-biomaterial interactions after bioprinting, and future directions of corneal bioprinting aiming at engineering a full-thickness human cornea in the lab.

Simultaneous Interpenetrating Polymer Network of Collagen and Hyaluronic Acid as an In Situ-Forming Corneal Defect Filler

Timely treatment of corneal injuries injury can help to prevent corneal scarring, blindness, and the need for corneal transplantation. This work describes a novel hydrogel that can fill corneal defects and assist in corneal regeneration. This hydrogel is a simultaneous interpenetrating polymer network (IPN) composed of collagen cross-linked via strain-promoted azide-alkyne cycloaddition reaction and hyaluronic acid cross-linked via thiol-ene Michael click reaction. The formation of the IPN gel was confirmed via FTIR spectra, UV-vis spectra, and morphological changes. We compared the gelation time, mechanical properties, transmittance, and refractive index of the IPN gel to the collagen gel, hyaluronic acid gel, and semi-IPN gel. The IPN combined the advantages of collagen and hyaluronic acid gels and supported corneal epithelial cell growth on its surface. When applied to corneal stromal defects in vivo, the IPN avoided epithelial hyperplasia, decreased stromal myofibroblast formation, and increased tight junction formation in the regenerated epithelium.

Multifunctional synthetic Bowman's membrane-stromal biomimetic for corneal reconstruction

As the outermost layer of the eye, the cornea is vulnerable to physical and chemical trauma, which can result in loss of transparency and lead to corneal blindness. Given the global corneal donor shortage, there is an unmet need for biocompatible corneal substitutes that have high transparency, mechanical integrity and regenerative potentials. Herein we engineered a dual-layered collagen vitrigel containing biomimetic synthetic Bowman's membrane (sBM) and stromal layer (sSL). The sBM supported rapid epithelial cell migration, maturation and multilayer formation, and the sSL containing tissue-derived extracellular matrix (ECM) microparticles presented a biomimetic lamellar ultrastructure mimicking the native corneal stroma. The incorporation of tissue-derived microparticles in sSL layer significantly enhanced the mechanical properties and suturability of the implant without compromising the transparency after vitrification. In vivo performance of the vitrigel in a rabbit anterior lamellar keratoplasty model showed full re-epithelialization within 14 days and integration of the vitrigel with the host tissue stroma by day 30. The migrated epithelial cells formed functional multilayer with limbal stem cell marker p63 K14 expressed in the lower layer, epithelial marker K3 and K12 expressed through the layers and tight junction protein ZO-1 expressed by the multilayers. Corneal fibroblasts migrated into the implants to facilitate host/implant integration and corneal stromal regeneration. In summary, these results suggest that the multi-functional layers of this novel collagen vitrigel exhibited significantly improved biological performance as corneal substitute by harnessing a fast re-epithelialization and stromal regeneration potential.

Bacterial nanocellulose as a corneal bandage material: a comparison with amniotic membrane

Corneal trauma and ulcerations are leading causes of corneal blindness around the world. These lesions require attentive medical monitoring since improper healing or infection has serious consequences in vision and quality of life. Amniotic membrane grafts represent the common solution to treat severe corneal wounds. However, amniotic membrane's availability remains limited by the dependency on donor tissues, its high price and short shelf life. Consequently, there is an active quest for biomaterials to treat injured corneal tissues. Nanocellulose synthetized by bacteria (BNC) is an emergent biopolymer with vast clinical potential for skin tissue regeneration. BNC also exhibits appealing characteristics to act as an alternative corneal bandage such as; high liquid holding capacity, biocompatibility, flexibility, natural - but animal free-origin and a myriad of functionalization opportunities. Here, we present an initial study aiming at testing the suitability of BNC as corneal bandage regarding preclinical requirements and using amniotic membrane as a benchmark. Bacterial nanocellulose exhibits higher mechanical resistance to sutures and slightly longer stability under in vitro and ex vivo simulated physiological conditions than amniotic membrane. Additionally, bacterial nanocellulose offers good conformability to the shape of the eye globe and easy manipulation in medical settings. These excellent attributes accompanied by the facts that bacterial nanocellulose is stable at room temperature for long periods, can be heat-sterilized and is easy to produce, reinforce the potential of bacterial nanocellulose as a more accessible ocular surface bandage.

Specialty Tough Hydrogels and Their Biomedical Applications

Hydrogels have long been explored as attractive materials for biomedical applications given their outstanding biocompatibility, high water content, and versatile fabrication platforms into materials with different physiochemical properties and geometries. Nonetheless, conventional hydrogels suffer from weak mechanical properties, restricting their use in persistent load-bearing applications often required of materials used in medical settings. Thus, the fabrication of mechanically robust hydrogels that can prolong the lifetime of clinically suitable materials under uncompromising in vivo conditions is of great interest. This review focuses on design considerations and strategies to construct such tough hydrogels. Several promising advances in the proposed use of specialty tough hydrogels for soft actuators, drug delivery vehicles, adhesives, coatings, and in tissue engineering settings are highlighted. While challenges remain before these specialty tough hydrogels will be deemed translationally acceptable for clinical applications, promising preliminary results undoubtedly spur great hope in the potential impact this embryonic research field can have on the biomedical community.

Biomaterials for topical and transdermal drug delivery in reconstructive transplantation

Lifelong systemic immunosuppression remains the biggest challenge in vascularized composite allotransplantation (VCA) due to the adverse effects it causes. Since VCA is a life-enhancing procedure as compared with solid organ transplant which is life-saving; one needs to weigh the benefits and risks carefully. Thus, there is a huge unmet clinical need to design biomaterial-based vehicles that can deliver drugs more efficiently, topically and locally to eliminate adverse effects of systemic immune suppression. This review discusses several biomaterial-based systems that have been carefully designed, conceived and attempted to make VCA a more patient compliant approach. Variety of promising preclinical studies has shown the feasibility of the approaches, and clinical trials are required to bridge the gap. Several challenges for the future and new approaches have been discussed.

Activation of dendritic cells by crosslinked collagen hydrogels (artificial corneas) varies with their composition

Activated T cells are known to promote fibrosis, a major complication limiting the range of polymeric hydrogels as artificial corneal implants. As T cells are activated by dendritic cells (DC), minimally activating hydrogels would be optimal. In this study, we evaluated the ability of a series of engineered (manufactured/fabricated) and natural collagen matrices to either activate DC or conversely induce DC apoptosis in vitro. Bone marrow DC were cultured on a series of singly and doubly crosslinked hydrogels (made from recombinant human collagen III [RHCIII] or collagen mimetic peptide [CMP]) or on natural collagen-containing matrices, Matrigel^TM and de-cellularised mouse corneal stroma. DC surface expression of major histocompatibility complex Class II and CD86 as well as apoptosis markers were examined. Natural matrices induced low levels of DC activation and maintained a "tolerogenic" phenotype. The same applied to singly crosslinked CMP-PEG gels. RHCIII gels singly crosslinked using either N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide with the coinitiator N-hydroxy succinimide (EDC-NHS) or N-cyclohexyl-N-(2-morpholinoethyl)carbodiimide metho-p-toulenesulfonate with NHS (CMC-NHS) induced varying levels of DC activation. In contrast, however, RHCIII hydrogels incorporating an additional polymeric network of 2-methacryloyloxyethyl phosphorylcholine did not activate DC but instead induced DC apoptosis, a phenomenon observed in natural matrices. This correlated with increased DC expression of leukocyte-associated immunoglobulin-like receptor-1. Despite low immunogenic potential, viable tolerogenic DC migrated into and through both natural and manufactured RHCIII gels. These data show that the immunogenic potential of RHCIII gels varies with the nature and composition of the gel. Preclinical evaluation of hydrogel immunogenic/fibrogenic potential is recommended.

Corneal injury: Clinical and molecular aspects

Currently, over 10 million people worldwide are affected by corneal blindness. Corneal trauma and disease can cause irreversible distortions to the normal structure and physiology of the cornea often leading to corneal transplantation. However, donors are in short supply and risk of rejection is an ever-present concern. Although significant progress has been made in recent years, the wound healing cascade remains complex and not fully understood. Tissue engineering and regenerative medicine are currently at the apex of investigation in the pursuit of novel corneal therapeutics. This review uniquely integrates the clinical and cellular aspects of both corneal trauma and disease and provides a comprehensive view of the most recent findings and potential therapeutics aimed at restoring corneal homeostasis.

Corneal Repair and Regeneration: Current Concepts and Future Directions

The cornea is a unique tissue and the most powerful focusing element of the eye, known as a window to the eye. Infectious or non-infectious diseases might cause severe visual impairments that need medical intervention to restore patients' vision. The most prominent characteristics of the cornea are its mechanical strength and transparency, which are indeed the most important criteria considerations when reconstructing the injured cornea. Corneal strength comes from about 200 collagen lamellae which criss-cross the cornea in different directions and comprise nearly 90% of the thickness of the cornea. Regarding corneal transparency, the specific characteristics of the cornea include its immune and angiogenic privilege besides its limbus zone. On the other hand, angiogenic privilege involves several active cascades in which anti-angiogenic factors are produced to compensate for the enhanced production of proangiogenic factors after wound healing. Limbus of the cornea forms a border between the corneal and conjunctival epithelium, and its limbal stem cells (LSCs) are essential in maintenance and repair of the adult cornea through its support of corneal epithelial tissue repair and regeneration. As a result, the main factors which threaten the corneal clarity are inflammatory reactions, neovascularization, and limbal deficiency. In fact, the influx of inflammatory cells causes scar formation and destruction of the limbus zone. Current studies about wound healing treatment focus on corneal characteristics such as the immune response, angiogenesis, and cell signaling. In this review, studied topics related to wound healing and new approaches in cornea regeneration, which are mostly related to the criteria mentioned above, will be discussed.

Decellularization and recellularization of cornea: Progress towards a donor alternative

The global shortage of donor corneas for transplantation has led to corneal bioengineering being investigated as a method to generate transplantable tissues. Decellularized corneas are among the most promising materials for engineering corneal tissue since they replicate the complex structure and composition of real corneas. Decellularization is a process that aims to remove cells from organs or tissues resulting in a cell-free scaffold consisting of the tissues extracellular matrix. Here different decellularization techniques are described, including physical, chemical and biological methods. Analytical techniques to confirm decellularization efficiency are also discussed. Different cell sources for the recellularization of the three layers of the cornea, recellularization methods used in the literature and techniques used to assess the outcome of the implantation of such scaffolds are examined. Studies involving the application of decellularized corneas in animal models and human clinical studies are discussed. Finally, challenges for this technology are explored involving scalability, automatization and regulatory affairs.

In Vitro Cultivation of Limbal Epithelial Stem Cells on Surface-Modified Crosslinked Collagen Scaffolds

PURPOSE

To investigate the efficacy of recombinant human collagen type I (RHC I) and collagen-like peptide (CLP) hydrogels as alternative carrier substrates for the cultivation of limbal epithelial stem cells (LESC) under xeno-free culture conditions.

METHODS

Human LESC were cultivated on seven different collagen-derived hydrogels: (1) unmodified RHC I, (2) fibronectin-patterned RHC I, (3) carbodiimide-crosslinked CLP (CLP-12 EDC), (4) DMTMM- (4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methyl-morpholinium-) crosslinked CLP (CLP-12), (5) fibronectin-patterned CLP-12, (6) "3D limbal niche-mimicking" CLP-12, and (7) DMTMM-crosslinked CLP made from higher CLP concentration solution. Cell proliferation, cell morphology, and expression of LESC markers were analyzed. All data were compared to cultures on human amniotic membrane (HAM).

RESULTS

Human LESC were successfully cultivated on six out of seven hydrogel formulations, with primary cell cultures on CLP-12 EDC being deemed unsuccessful since the area of outgrowth did not meet quality standards (i.e., inconsistence in outgrowth and confluence) after 14 days of culture. Upon confluence, primary LESC showed high expression of the stem cell marker ΔNp63, proliferation marker cytokeratin (KRT) 14, adhesion markers integrin-β4 and E-cadherin, and LESC-specific extracellular matrix proteins laminin-α1, and collagen type IV. Cells showed low expression of differentiation markers KRT3 and desmoglein 3 (DSG3). Significantly higher gene expression of KRT3 was observed for cells cultured on CLP hydrogels compared to RHC I and HAM. Surface patterning of hydrogels influenced the pattern of proliferation but had no significant effect on the phenotype or genotype of cultures. Overall, the performance of RHC I and DMTMM-crosslinked CLP hydrogels was equivalent to that of HAM.

CONCLUSION

RHC I and DMTMM-crosslinked CLP hydrogels, irrespective of surface modification, support successful cultivation of primary human LESC using a xeno-free cultivation protocol. The regenerated epithelium maintained similar characteristics to HAM-based cultures.

Sutureless repair of corneal injuries using naturally derived bioadhesive hydrogels

Corneal injuries are common causes of visual impairment worldwide. Accordingly, there is an unmet need for transparent biomaterials that have high adhesion, cohesion, and regenerative properties. Herein, we engineer a highly biocompatible and transparent bioadhesive for corneal reconstruction using a visible light cross-linkable, naturally derived polymer, GelCORE (gel for corneal regeneration). The physical properties of GelCORE could be finely tuned by changing prepolymer concentration and photocrosslinking time. GelCORE revealed higher tissue adhesion compared to commercial adhesives. Furthermore, in situ photopolymerization of GelCORE facilitated easy delivery to the cornea, allowing for bioadhesive curing precisely according to the required geometry of the defect. In vivo experiments, using a rabbit stromal defect model, showed that bioadhesive could effectively seal corneal defects and induce stromal regeneration and re-epithelialization. Overall, GelCORE has many advantages including low cost and ease of production and use. This makes GelCORE a promising bioadhesive for corneal repair.

Finding an Optimal Corneal Xenograft Using Comparative Analysis of Corneal Matrix Proteins Across Species

Numerous animal species have been proposed as sources of corneal tissue for obtaining decellularized xenografts. The selection of an appropriate animal model must take into consideration the differences in the composition and structure of corneal proteins between humans and other animal species in order to minimize immune response and improve outcome of the xenotransplant. Here, we compared the amino-acid sequences of 16 proteins present in the corneal stromal matrix of 14 different animal species using Basic Local Alignment Search Tool, and calculated a similarity score compared to the respective human sequence. Primary amino acid structures, isoelectric point and grand average of hydropathy (GRAVY) values of the 7 most abundant proteins (i.e. collagen α-1 (I), α-1 (VI), α-2 (I) and α-3 (VI), as well as decorin, lumican, and keratocan) were also extracted and compared to those of human. The pig had the highest similarity score (91.8%). All species showed a lower proline content compared to human. Isoelectric point of pig (7.1) was the closest to the human. Most species have higher GRAVY values compared to human except horse. Our results suggest that porcine cornea has a higher relative suitability for corneal transplantation into humans compared to other studied species.