Biosynthetic alternatives for corneal transplant surgery

Successful restoration of corneal surface integrity with a tissue-engineered allogeneic implant in severe keratitis patients

OBJECTIVES

Corneal diseases are among the main causes of blindness, with approximately 4.6 and 23 million patients worldwide suffering from bilateral and unilateral corneal blindness, respectively. The standard treatment for severe corneal diseases is corneal transplantation. However, relevant disadvantages, particularly in high-risk conditions, have focused the attention on the search for alternatives.

METHODS

We report interim findings of a phase I-II clinical study evaluating the safety and preliminary efficacy of a tissue-engineered corneal substitute composed of a nanostructured fibrin-agarose biocompatible scaffold combined with allogeneic corneal epithelial and stromal cells (NANOULCOR). 5 subjects (5 eyes) suffering from trophic corneal ulcers refractory to conventional treatments, who combined stromal degradation or fibrosis and limbal stem cell deficiency, were included and treated with this allogeneic anterior corneal substitute.

RESULTS

The implant completely covered the corneal surface, and ocular surface inflammation decreased following surgery. Only four adverse reactions were registered, and none of them were severe. No detachment, ulcer relapse nor surgical re-interventions were registered after 2 years of follow-up. No signs of graft rejection, local infection or corneal neovascularization were observed either. Efficacy was measured as a significant postoperative improvement in terms of the eye complication grading scales. Anterior segment optical coherence tomography images revealed a more homogeneous and stable ocular surface, with complete scaffold degradation occurring within 3-12 weeks after surgery.

CONCLUSIONS

Our findings suggest that the surgical application of this allogeneic anterior human corneal substitute is feasible and safe, showing partial efficacy in the restoration of the corneal surface.

Rational design of peptide-based implants for corneal bioengineering

Regeneration of damaged cornea can save vision for millions of patients. Most of these patients are waiting for transplantation of a donor cornea or suitable substitute to restore vision. Although donor cornea transplantation is the most clinically accepted treatment, shortage of donor cornea results in almost 69 out of every 70 patients untreated with the waiting list for transplantation drastically increasing every year according to a prepandemic estimation. Therefore, corneal replacements are coming up as a cutting-edge alternative strategy. In view of the peptides, especially collagen-like peptides and peptide amphiphiles with bioactive functional motifs demonstrate promising avenue for the corneal tissue engineering and promoting regeneration, by their hierarchical self-assembling propensity to acquire desired nano- to macroscale 3D architecture. Here, we analyze rational peptide designing, self-assembly, and strategies of peptide/peptide-based nanoscale building blocks to create the extracellular matrix mimetic implants for functional regeneration of the cornea.

Effect of Covid-19 on Eye Banks and Corneal Transplantations: Current Perspectives

The coronavirus disease 2019 (COVID-19) pandemic exerted a great impact on medical practice, which was reframed according to the actual needs. Ophthalmological services and procedures including corneal transplantation did not represent an exception. The adoption and implementation of new standard operating procedures as well as of new technologies for remote consultation and smart-working reshaped daily activities of both eye bankers, physicians, researchers, and patients. Regulatory restrictions were issued redefining corneal donor eligibility criteria, as well as handling and harvesting procedures of donor ocular tissues. Surgical schedules underwent an abrupt contraction with prioritization of urgent procedures. Local lockdowns and confinement strategies resulted in both a reduction and redirection of research activities. The evaluation of SARS-CoV-2 colonization of ocular tissues, long-term corneal storage techniques, new disinfection strategies, split corneal transplants and cell-based therapies for the treatment of corneal disease peaked in the pipeline. Aim of this article is to summarizes the overall impact of the pandemic on the corneal transplantation machinery, and the current and future perspectives for the corneal transplant community.

Electrical Signal Initiates Kinetic Assembly of Collagen to Construct Optically Transparent and Geometry Customized Artificial Cornea Substitutes

Corneal transplantation is an effective treatment for reconstructing injured corneas but is very limited due to insufficient donors, which has led to a growing demand for development of artificial corneal substitutes (ACSs). Collagen is a potential building block for ACS fabrication, whereas technically there are limited capabilities to control the collagen assembly for creating highly transparent collagen ACSs. Here, we report an electro-assembly technique to kinetically control collagen assembly on the nanoscale that allows the yielding collagen ACSs with structure determined superior optics. Structurally, the kinetically electro-assembled collagen (KEA-Col) is composed of partially aligned microfibrils (∼10 nm in diameter) with compacted lamellar organization. Optical analysis reveals that such microstructure is directly responsible for its optimal light transmittance by reducing light scattering. Moreover, this method allows the creation of complex three-dimensional geometries and thus is convenient to customize collagen ACSs with specific curvatures to meet refractive power requirements. Available properties (e.g., optics and mechanics) of cross-linked KEA-Cols were studied to meet the clinical requirement as ACSs, and in vitro tests further proved their beneficial characteristics of cell growth and migration. An in vivo study established a rabbit lamellar keratectomy corneal wound model and demonstrated the customized collagen ACSs can adapt to the defective cornea and support epithelial healing as well as stroma integration and reconstruction with lower immunoreaction compared with commercial xenografts, which suggests its promising application prospects. More broadly, this work illustrates the potential for enlisting electrical signals to mediate collagen's assembly and microstructure organization for specific structural functionalization for regenerative medicine.

Critical media attributes in E-beam sterilization of corneal tissue

When ionizing irradiation interacts with a media, it can form reactive species that can react with the constituents of the system, leading to eradication of bioburden and sterilization of the tissue. Understanding the media's properties such as polarity is important to control and direct those reactive species to perform desired reactions. Using ethanol as a polarity modifier of water, we herein generated a series of media with varying relative polarities for electron beam (E-beam) irradiation of cornea at 25 kGy and studied how the irradiation media's polarity impacts properties of the cornea. After irradiation of corneal tissues, mechanical (tensile strength and modulus, elongation at break, and compression modulus), chemical, optical, structural, degradation, and biological properties of the corneal tissues were evaluated. Our study showed that irradiation in lower relative polarity media improved structural properties of the tissues yet reduced optical transmission; higher relative polarity reduced structural and optical properties of the cornea; and intermediate relative polarity (ethanol concentrations = 20-30% (v/v)) improved the structural properties, without compromising optical characteristics. Regardless of media polarity, irradiation did not negatively impact the biocompatibility of the corneal tissue. Our data shows that the absorbed ethanol can be flushed from the irradiated cornea to levels that are nontoxic to corneal and retinal cells. These findings suggest that the relative polarity of the irradiation media can be tuned to generate sterilized tissues, including corneal grafts, with engineered properties that are required for specific biomedical applications. STATEMENT OF SIGNIFICANCE: Extending the shelf-life of corneal tissue can improve general accessibility of cornea grafts for transplantation. Irradiation of donor corneas with E-beam is an emerging technology to sterilize the corneal tissues and enable their long-term storage at room temperature. Despite recent applications in clinical medicine, little is known about the effect of irradiation and preservation media's characteristics, such as polarity on the properties of irradiated corneas. Here, we have showed that the polarity of the media can be a valuable tool to change and control the properties of the irradiated tissue for transplantation.

Artificial Cornea: Past, Current, and Future Directions

Corneal diseases are a leading cause of blindness with an estimated 10 million patients diagnosed with bilateral corneal blindness worldwide. Corneal transplantation is highly successful in low-risk patients with corneal blindness but often fails those with high-risk indications such as recurrent or chronic inflammatory disorders, history of glaucoma and herpetic infections, and those with neovascularisation of the host bed. Moreover, the need for donor corneas greatly exceeds the supply, especially in disadvantaged countries. Therefore, artificial and bio-mimetic corneas have been investigated for patients with indications that result in keratoplasty failure. Two long-lasting keratoprostheses with different indications, the Boston type-1 keratoprostheses and osteo-odonto-keratoprostheses have been adapted to minimise complications that have arisen over time. However, both utilise either autologous tissue or an allograft cornea to increase biointegration. To step away from the need for donor material, synthetic keratoprostheses with soft skirts have been introduced to increase biointegration between the device and native tissue. The AlphaCor™, a synthetic polymer (PHEMA) hydrogel, addressed certain complications of the previous versions of keratoprostheses but resulted in stromal melting and optic deposition. Efforts are being made towards creating synthetic keratoprostheses that emulate native corneas by the inclusion of biomolecules that support enhanced biointegration of the implant while reducing stromal melting and optic deposition. The field continues to shift towards more advanced bioengineering approaches to form replacement corneas. Certain biomolecules such as collagen are being investigated to create corneal substitutes, which can be used as the basis for bio-inks in 3D corneal bioprinting. Alternatively, decellularised corneas from mammalian sources have shown potential in replicating both the corneal composition and fibril architecture. This review will discuss the limitations of keratoplasty, milestones in the history of artificial corneal development, advancements in current artificial corneas, and future possibilities in this field.

In situ Tissue Regeneration in the Cornea from Bench to Bedside

Corneal blindness accounts for 5.1% of visual deficiency and is the fourth leading cause of blindness globally. An additional 1.5-2 million people develop corneal blindness each year, including many children born with or who later develop corneal infections. Over 90% of corneal blind people globally live in low- and middle-income regions (LMIRs), where corneal ulcers are approximately 10-fold higher compared to high-income countries. While corneal transplantation is an effective option for patients in high-income countries, there is a considerable global shortage of corneal graft tissue and limited corneal transplant programs in many LMIRs. In situ tissue regeneration aims to restore diseases or damaged tissues by inducing organ regeneration. This can be achieved in the cornea using biomaterials based on extracellular matrix (ECM) components like collagen, hyaluronic acid, and silk. Solid corneal implants based on recombinant human collagen type III were successfully implanted into patients resulting in regeneration of the corneal epithelium, stroma, and sub-basal nerve plexus. As ECM crosslinking and manufacturing methods improve, the focus of biomaterial development has shifted to injectable, in situ gelling formulations. Collagen, collagen-mimetic, and gelatin-based in situ gelling formulas have shown the ability to repair corneal wounds, surgical incisions, and perforations in in-vivo models. Biomaterial approaches may not be sufficient to treat inflammatory conditions, so other cell-free therapies such as treatment with tolerogenic exosomes and extracellular vesicles may improve treatment outcomes. Overall, many of the technologies described here show promise as future medical devices or combination products with cell or drug-based therapies. In situ tissue regeneration, particularly with liquid formulas, offers the ability to triage and treat corneal injuries and disease with a single regenerative solution, providing alternatives to organ transplantation and improving patient outcomes.

Covalently Crosslinked Hydrogels via Step-Growth Reactions: Crosslinking Chemistries, Polymers, and Clinical Impact

Hydrogels are an important class of biomaterials with the unique property of high-water content in a crosslinked polymer network. In particular, chemically crosslinked hydrogels have made a great clinical impact in past years because of their desirable mechanical properties and tunability of structural and chemical properties. Various polymers and step-growth crosslinking chemistries are harnessed for fabricating such covalently crosslinked hydrogels for translational research. However, selecting appropriate crosslinking chemistries and polymers for the intended clinical application is time-consuming and challenging. It requires the integration of polymer chemistry knowledge with thoughtful crosslinking reaction design. This task becomes even more challenging when other factors such as the biological mechanisms of the pathology, practical administration routes, and regulatory requirements add additional constraints. In this review, key features of crosslinking chemistries and polymers commonly used for preparing translatable hydrogels are outlined and their performance in biological systems is summarized. The examples of effective polymer/crosslinking chemistry combinations that have yielded clinically approved hydrogel products are specifically highlighted. These hydrogel design parameters in the context of the regulatory process and clinical translation barriers, providing a guideline for the rational selection of polymer/crosslinking chemistry combinations to construct hydrogels with high translational potential are further considered.

Cytocompatibility and Suitability of Protein-Based Biomaterials as Potential Candidates for Corneal Tissue Engineering

The vision impairments suffered by millions of people worldwide and the shortage of corneal donors show the need of substitutes that mimic native tissue to promote cell growth and subsequent tissue regeneration. The current study focused on the in vitro assessment of protein-based biomaterials that could be a potential source for corneal scaffolds. Collagen, soy protein isolate (SPI), and gelatin films cross-linked with lactose or citric acid were prepared and physicochemical, transmittance, and degradation measurements were carried out. In vitro cytotoxicity, cell adhesion, and migration studies were performed with human corneal epithelial (HCE) cells and 3T3 fibroblasts for the films’ cytocompatibility assessment. Transmittance values met the cornea’s needs, and the degradation profile revealed a progressive biomaterials’ decomposition in enzymatic and hydrolytic assays. Cell viability at 72 h was above 70% when exposed to SPI and gelatin films. Live/dead assays and scanning electron microscopy (SEM) analysis demonstrated the adhesion of both cell types to the films, with a similar arrangement to that observed in controls. Besides, both cell lines were able to proliferate and migrate over the films. Without ruling out any material, the appropriate optical and biological properties shown by lactose-crosslinked gelatin film highlight its potential for corneal bioengineering.