Recruitment of multiple cell lines by collagen-synthetic copolymer matrices in corneal regeneration

3D Bioprinting of Acellular Corneal Stromal Scaffolds with a Low Cost Modified 3D Printer: A Feasibility Study

PURPOSE

Loss of corneal transparency is one of the major causes of visual loss, generating a considerable health and economic burden globally. Corneal transplantation is the leading treatment procedure, where the diseased cornea is replaced by donated corneal tissue. Despite the rise of cornea donations in the past decade, there is still a huge gap between cornea supply and demand worldwide. 3D bioprinting is an emerging technology that can be used to fabricate tissue equivalents that resemble the native tissue, which holds great potential for corneal tissue engineering application. This study evaluates the manufacturability of 3D bioprinted acellular corneal grafts using low-cost equipment and software, not necessarily designed for bioprinting applications. This approach allows access to 3D printed structures where commercial 3D bioprinters are cost prohibitive and not readily accessible to researchers and clinicians.

METHODS

Two extrusion-based methods were used to 3D print acellular corneal stromal scaffolds with collagen, alginate, and alginate-gelatin composite bioinks from a digital corneal model. Compression testing was used to determine moduli.

RESULTS

The printed model was visually transparent with tunable mechanical properties. The model had central radius of curvature of 7.4 mm, diameter of 13.2 mm, and central thickness of 0.4 mm. The compressive secant modulus of the material was 23.7 ± 1.7 kPa at 20% strain. 3D printing into a concave mold had reliability advantages over printing into a convex mold.

CONCLUSIONS

The printed corneal models exhibited visible transparency and a dome shape, demonstrating the potential of this process for the preparation of acellular partial thickness corneal replacements. The modified printing process presented a low-cost option for corneal bioprinting.

Electro-compacted collagen for corneal epithelial tissue engineering

Bioengineered corneal substitutes offer a solution to the shortage of donor corneal tissue worldwide. As one of the major structural components of the cornea, collagen has shown great potential for tissue-engineered cornea substitutes. Herein, free-standing collagen membranes fabricated using electro-compaction were assessed in corneal bioengineering application by comparing them with nonelectro-compacted collagen (NECC). The well-organized and biomimetic fibril structure resulted in a significant improvement in mechanical properties. A 10-fold increase in tensile and compressive modulus was recorded when compared with NECC membranes. In addition to comparable transparency in the visible light range, the glucose permeability of the electro-compacted collagen (ECC) membrane is higher than that of the native human cornea. Human corneal epithelial cells adhere and proliferate well on the ECC membrane, with a large cell contact area observed. The as-described ECC has appropriate structural, topographic, mechanical, optical, glucose permeable, and cell support properties to provide a platform for a bioengineered cornea; including the outer corneal epithelium and potentially deeper corneal tissues.

Crosslinker-free collagen gelation for corneal regeneration

Development of an artificial cornea can potentially fulfil the demand of donor corneas for transplantation as the number of donors is far less than needed to treat corneal blindness. Collagen-based artificial corneas stand out as a regenerative option, having promising clinical outcomes. Collagen crosslinked with chemical crosslinkers which modify the parent functional groups of collagen. However, crosslinkers are usually cytotoxic, so crosslinkers need to be removed from implants completely before application in humans. In addition, crosslinked products are mechanically weak and susceptible to enzymatic degradation. We developed a crosslinker free supramolecular gelation strategy using pyrene conjugated dipeptide amphiphile (PyKC) consisting of lysine and cysteine; in which collagen molecules are intertwined inside the PyKC network without any functional group modification of the collagen. The newly developed collagen implants (Coll-PyKC) are optically transparent and can effectively block UV light, are mechanically and enzymatically stable, and can be sutured. The Coll-PyKC implants support the growth and function of all corneal cells, trigger anti-inflammatory differentiation while suppressing the pro-inflammatory differentiation of human monocytes. Coll-PyKC implants can restrict human adenovirus propagation. Therefore, this crosslinker-free strategy can be used for the repair, healing, and regeneration of the cornea, and potentially other damaged organs of the body.

A comparison between dehydrated and cryopreserved heterologous corneal grafts for penetrating keratoplasty in dogs and cats

OBJECTIVE

To compare the efficacy of dehydrated and cryopreserved heterologous corneal grafts in the management of full-thickness corneal defects in cats and dogs.

ANIMALS STUDIED

Sixty-five cats (81 eyes) and 42 dogs (51 eyes) with full-thickness corneal defects of different origin.

MATERIALS AND METHODS

This prospective randomized trial included 132 animal eyes (81 feline and 51 canine) with different full-thickness corneal defects. Penetrating keratoplasty with cryopreserved and dehydrated corneal grafts was performed in 57 and 75 eyes, respectively. Follow-up lasted for 6 months.

RESULTS

Optically clear cornea with mild scarring was achieved in 40.4% of cases (23/57 eyes) after cryopreserved and in 42.7% of cases (32/75 eyes) after dehydrated corneal grafting. Moderate scarring after cryopreserved and dehydrated corneal grafting was observed in 35.1% of cases (20/57 eyes) and 37.3% of cases (28/75 eyes), respectively. Severe scarring and corneal opacities with severe vision loss after cryopreserved and dehydrated corneal grafting developed in 22.8% of cases (13/57 eyes) and 18.7% of cases (14/75 eyes), respectively. In two groups of animals combined, the affected eyes were salvaged and visual function improvement of varying degree was achieved in 78.0% of cases (103/132 eyes). Transplant rejection warranted enucleation in 1.75% of cases (1/57 eyes) and 1.3% of cases (1/75 eyes) after cryopreserved and dehydrated corneal grafting, respectively.

CONCLUSIONS

No statistically significant differences in clinical outcomes of penetrating keratoplasty with cryopreserved and dehydrated corneal grafts were observed. Dehydrated cornea may be considered a viable alternative to cryopreserved grafts for the management of full-thickness corneal defects.

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.

Safety of grafting acellular human corneal lenticule seeded with Wharton's Jelly-Derived Mesenchymal Stem Cells in an experimental animal model

The present study was conducted to evaluate safety of grafting acellular human corneal lenticule seeded with Wharton's Jelly-derived Mesenchymal Stem Cells (WJSC) in an experimental animal model. Human corneal lenticules were decellularized with a rate of about 97% with an acceptable lack of cytotoxicity and relatively intact ultrastructure of the lenticules. 12 rabbits underwent unilateral stromal pocketing with implantation of decellularized lenticules. Implantation was performed for 6 rabbits along with graft recellularization with WJSCs. Rabbits were euthanized after 1 month (n = 6) and 3 months (n = 6) to evaluate progression of graft bio-integration. No clinical rejection sign was detected during the study. Histopathological analysis showed that, grafts were integrated well with the least distortion of surrounding collagen bundles. After 3 months, labeled WJCS was detected representing viability of stem cells in the host. Increased expression of keratocyte-specific markers showed the potential of recruiting WJSCs as keratocyte progenitor cells to reinforce corneal ultrastructure.

A hybrid scaffold of gelatin glycosaminoglycan matrix and fibrin as a carrier of human corneal fibroblast cells

A hybrid scaffold of gelatin-glycosaminoglycan matrix and fibrin (FGG) has been synthesized to improve the mechanical properties, degradation time and cell response of fibrin-like scaffolds. The FGG scaffold was fabricated by optimizing some properties of fibrin-only gel and gelatin-glycosaminoglycan (GG) scaffolds. Mechanical analysis of optimized fibrin-only gel showed the Young module and tensile strength of up to 72 and 121 KPa, respectively. Significantly, the nine-fold increase in the Young modulus and a seven-fold increase in tensile strength was observed when fibrin reinforced with GG scaffold. Additionally, the results demonstrated that the degradation time of fibrin was enhanced successfully up to 7 days which was much longer time compared to fibrin-only gel with 38 h of degradation time. More than 45% of FGG initial mass was preserved on day 7 in the presence of aprotinin. Human corneal fibroblast cells (HCFCs) were seeded on the FGG, fibrin-only gel and GG scaffolds for 5 days. The FGG scaffold showed excellent cell viability over 5 days, and the proliferation of HCFCs also increased significantly in comparison with fibrin-only gel and GG scaffolds. The FGG scaffold illustrates the great potential to use in which appropriate stability and mechanical properties are essential to tissue functionality.

Development, structure, and bioengineering of the human corneal stroma: A review of collagen-based implants

Bio-engineering technologies are currently used to produce biomimetic artificial corneas that should present structural, chemical, optical, and biomechanical properties close to the native tissue. These properties are mainly supported by the corneal stroma which accounts for 90% of corneal thickness and is mainly made of collagen type I. The stromal collagen fibrils are arranged in lamellae that have a plywood-like organization. The fibril diameter is between 25 and 35 nm and the interfibrillar space about 57 nm. The number of lamellae in the central stroma is estimated to be 300. In the anterior part, their size is 10-40 μm. They appear to be larger in the posterior part of the stroma with a size of 60-120 μm. Their thicknesses also vary from 0.2 to 2.5 μm. During development, the acellular corneal stroma, which features a complex pattern of organization, serves as a scaffold for mesenchymal cells that invade and further produce the cellular stroma. Several pathways including Bmp4, Wnt/β-catenin, Notch, retinoic acid, and TGF-β, in addition to EFTFs including the mastering gene Pax-6, are involved in corneal development. Besides, retinoic acid and TGF- β seem to have a crucial role in the neural crest cell migration in the stroma. Several technologies can be used to produce artificial stroma. Taking advantage of the liquid-crystal properties of acid-soluble collagen, it is possible to produce transparent stroma-like matrices with native-like collagen I fibrils and plywood-like organization, where epithelial cells can adhere and proliferate. Other approaches include the use of recombinant collagen, cross-linkers, vitrification, plastically compressed collagen or magnetically aligned collagen, providing interesting optical and mechanical properties. These technologies can be classified according to collagen type and origin, presence of telopeptides and native-like fibrils, structure, and transparency. Collagen matrices feature transparency >80% for the appropriate 500-μm thickness. Non-collagenous matrices made of biopolymers including gelatin, silk, or fish scale have been developed which feature interesting properties but are less biomimetic. These bioengineered matrices still need to be colonized by stromal cells to fully reproduce the native stroma.

New Developments in Medical Applications of Hybrid Hydrogels Containing Natural Polymers

New trends in biomedical applications of the hybrid polymeric hydrogels, obtained by combining natural polymers with synthetic ones, have been reviewed. Homopolysaccharides, heteropolysaccharides, as well as polypeptides, proteins and nucleic acids, are presented from the point of view of their ability to form hydrogels with synthetic polymers, the preparation procedures for polymeric organic hybrid hydrogels, general physico-chemical properties and main biomedical applications (i.e., tissue engineering, wound dressing, drug delivery, etc.).

Effect of Cross-Linking Density on the Structures and Properties of Carbodiimide-Treated Gelatin Matrices as Limbal Stem Cell Niches

Given that human amniotic membrane is a valuable biological material not readily available for corneal epithelial tissue engineering, gelatin is considered as a potential alternative to construct a cellular microenvironment. This study investigates, for the first time, the influence of cross-linking density of carbodiimide-treated gelatin matrices on the structures and properties of artificial limbal stem cell niches. Our results showed that an increase in the carbodiimide concentration from 1.5 to 15 mM leads to an upward trend in the structural and suture strength of biopolymers. Furthermore, increasing number of cross-linking bridges capable of linking protein molecules together may reduce their crystallinity. For the samples treated with 50 mM of cross-linker (i.e., the presence of excess N-substituted carbodiimide), abundant N-acylurea was detected, which was detrimental to the in vitro and in vivo ocular biocompatibility of gelatin matrices. Surface roughness and stiffness of biopolymer substrates were found to be positively correlated with carbodiimide-induced cross-link formation. Significant increases of integrin β1 expression, metabolic activity, and ABCG2 expression were noted as the cross-linker concentration increased, suggesting that the bulk crystalline structure and surface roughness/stiffness of niche attributed to the number of cross-linking bridges may have profound effects on a variety of limbal epithelial cell behaviors, including adhesion, proliferation, and stemness maintenance. In summary, taking the advantages of carbodiimide cross-linking-mediated development of gelatin matrices, new niches with tunable cross-linking densities can provide a significant boost to maintain the limbal stem cells during ex vivo expansion.

Biomaterials for corneal bioengineering

Corneal transplantation is an important surgical treatment for many common corneal diseases. However, a worldwide shortage of tissue from suitable corneal donors has meant that many people are not able to receive sight-restoring operations. In addition, rejection is a major cause of corneal transplant failure. Bioengineering corneal tissue has recently gained widespread attention. In order to facilitate corneal regeneration, a range of materials is currently being investigated. The ideal substrate requires sufficient tectonic durability, biocompatibility with cultured cellular elements, transparency, and perhaps biodegradability and clinical compliance. This review considers the anatomy and function of the native cornea as a precursor to evaluating a variety of biomaterials for corneal regeneration including key characteristics for optimal material form and function. The integration of appropriate cells with the most appropriate biomaterials is also discussed. Taken together, the information provided offers insight into the requirements for fabricating synthetic and semisynthetic corneas for in vitro modeling of tissue development and disease, pharmaceutical screening, and in vivo application for regenerative medicine.

Bioactive Hydrogel-Filament Scaffolds for Nerve Repair and Regeneration

The design of novel biomaterials is crucial for the advancement of tissue engineering in nerve regeneration. In this study we developed and evaluated novel biosynthetic scaffolds comprising collagen crosslinked with a terpolymer of poly(N-isopropylacrylamide) (PNiPAAm) as conduits for nerve growth. These collagen-terpolymer (collagen-TERP) scaffolds grafted with the laminin pentapeptide YIGSR were previously used as corneal substitutes in pigs and demonstrated enhanced nerve regeneration compared to allografts. The purpose of this project was to enhance neuronal growth on the collagen-TERP scaffolds through the incorporation of supporting fibers. Neuronal growth on these matrices was assessed in vitro using isolated dorsal root ganglia as a nerve source. Statistical significance was assessed using a one-way ANOVA. The incorporation of fibers into the collagen-TERP scaffolds produced a significant increase in neurite extension (p<0.05). The growth habit of the nerves varied with the type of fiber and included directional growth of the neurites along the surface of certain fiber types. Furthermore, the presence of fibers in the collagen-TERP scaffolds appeared to influence neurite morphology and function; neurites grown on fibers-incorporated collagen-TERP scaffolds expressed higher levels of Na channels compared to the scaffolds without fiber. Overall, our results suggest that incorporation of supporting fibers enhanced neurite outgrowth and that surface properties of the scaffold play an important role in promoting and guiding nerve regeneration. More importantly, this study demonstrates the potential value of tissue engineered collagen-TERP hybrid scaffolds as conduits in peripheral nerve repair.

Asymmetrical Polymer Vesicles for Drug delivery and Other Applications

Scientists have been attracted by polymersomes as versatile drug delivery systems since the last two decades. Polymersomes have the potential to be versatile drug delivery systems because of their tunable membrane formulations, stabilities in vivo, various physicochemical properties, controlled release mechanisms, targeting abilities, and capacities to encapsulate a wide range of drugs and other molecules. Asymmetrical polymersomes are nano- to micro-sized polymeric capsules with asymmetrical membranes, which means, they have different outer and inner coronas so that they can exhibit better endocytosis rate and endosomal escape ability than other polymeric systems with symmetrical membranes. Hence, asymmetrical polymersomes are highly promising as self-assembled nano-delivery systems in the future for in vivo therapeutics delivery and diagnostic imaging applications. In this review, we prepared a summary about recent research progresses of asymmetrical polymersomes in the following aspects: synthesis, preparation, applications in drug delivery and others.

Controlling the 3D architecture of Self-Lifting Auto-generated Tissue Equivalents (SLATEs) for optimized corneal graft composition and stability

Ideally, biomaterials designed to play specific physical and physiological roles in vivo should comprise components and microarchitectures analogous to those of the native tissues they intend to replace. For that, implantable biomaterials need to be carefully designed to have the correct structural and compositional properties, which consequently impart their bio-function. In this study, we showed that the control of such properties can be defined from the bottom-up, using smart surface templates to modulate the structure, composition, and bio-mechanics of human transplantable tissues. Using multi-functional peptide amphiphile-coated surfaces with different anisotropies, we were able to control the phenotype of corneal stromal cells and instruct them to fabricate self-lifting tissues that closely emulated the native stromal lamellae of the human cornea. The type and arrangement of the extracellular matrix comprising these corneal stromal Self-Lifting Analogous Tissue Equivalents (SLATEs) were then evaluated in detail, and was shown to correlate with tissue function. Specifically, SLATEs comprising aligned collagen fibrils were shown to be significantly thicker, denser, and more resistant to proteolytic degradation compared to SLATEs formed with randomly-oriented constituents. In addition, SLATEs were highly transparent while providing increased absorption to near-UV radiation. Importantly, corneal stromal SLATEs were capable of constituting tissues with a higher-order complexity, either by creating thicker tissues through stacking or by serving as substrate to support a fully-differentiated, stratified corneal epithelium. SLATEs were also deemed safe as implants in a rabbit corneal model, being capable of integrating with the surrounding host tissue without provoking inflammation, neo-vascularization, or any other signs of rejection after a 9-months follow-up. This work thus paves the way for the de novo bio-fabrication of easy-retrievable, scaffold-free human tissues with controlled structural, compositional, and functional properties to replace corneal, as well as other, tissues.

Degradation of silk films in multipocket corneal stromal rabbit models

INTRODUCTION

The need for human cornea tissues continues to grow as an alternative option to donor tissues. Silk protein has been successfully used as a substrate to engineer corneal epithelium and stroma in vitro. Herein, we investigated the in vivo response and the effect of silk crystalline structure (beta sheet) on degradation rate of silk films in rabbit multipocket corneal models.

METHODS

Three different surgical techniques (peripheral-median P-M, central-superficial C-S, central-deep C-D) were used to assess the in vivo response as well as the degradation profile of silk films with low, medium and high beta sheet (crystalline) content at 2 and 3 months after surgery.

RESULTS

Approach C-D showed signs of sample degradation without inflammation, with one single incision and a pocket created by flushing air two thirds deep in the corneal stroma. In comparison, approaches P-M and C-S with multiple incisions presented manually dissected surgical pockets resulted in inflammation and possible extrusion of the samples, respectively. Low beta sheet samples lost structural integrity at 2 months after surgery C-D, while medium and high beta sheet content films showed initial evidence of degradation.

CONCLUSIONS

The in vivo response to the silk films was dependent on the location of the implant and pocket depth. Crystallinity content in silk films played a significant role in the timing of material degradation, without signs of inflammation and vascularization or changes in stromal organization.

Progress in Corneal Stromal Repair: From Tissue Grafts and Biomaterials to Modular Supramolecular Tissue-Like Assemblies

Corneal injuries and degenerative conditions have major socioeconomic consequences, given that in most cases, they result in blindness. In the quest of the ideal therapy, tissue grafts, biomaterials, and modular engineering approaches are under intense investigation. Herein, advancements and shortfalls are reviewed and future perspectives for these therapeutic strategies discussed.

Corneal grafting for the treatment of full-thickness corneal defects in dogs: a review of 50 cases

OBJECTIVE

To describe corneal grafting for the treatment of full-thickness corneal defects in dogs and to determine its effectiveness in preserving vision.

METHODS

A review of the medical records of dogs that underwent corneal grafting following corneal perforations (≥3 mm) at the VTH-UAB from 2002 to 2012 was carried out.

RESULTS

Fifty dogs of different breed, age and gender were included. Brachycephalic breeds were overrepresented (37/50;74%). All cases were unilateral, with euryblepharon being the most common concurrent ocular abnormality (20/50;40%). Full-thickness penetrating keratoplasties (FTPK) were performed in 21/50 eyes (42%) and lamellar keratoplasties (LK) in 29/50 eyes (58%). Frozen grafts (FroG) were used in 43/50 eyes (86%) and fresh homologous grafts (FreHoG) in 7/50 (14%). Of the former group, 26 were homologous (FroHoG:60%) and 17 heterologous (FroHeG:40%). A combination of topical medication (antibiotics, corticosteroids, cycloplegics, and 0.2% cyclosporine A) and systemic mycophenolate mofetil was administered. Median follow-up time was 200 days. Postsurgical complications included wound dehiscence (6/50;12%) and glaucoma (4/50;8%). Clinical signs of graft rejection were diagnosed as follows: FroHoG (13/26;50%), FroHeG (11/17;65%), FreHoG (4/7;57%), FTPK (12/21;57%), and LK (16/29;55%). Medical treatment successfully controlled graft rejection in 11/28 eyes (39%). Good anatomical outcome was achieved in 86% (43/50), of which 95% (41/43) were visual at last examination, with moderate opacification to complete transparency of the graft present in 48.2%.

CONCLUSIONS

Corneal grafting is an effective surgical treatment for full-thickness corneal defects in dogs. If graft rejection is present, additional medical or surgical therapy may be necessary, achieving a highly satisfactory visual outcome.

Macroporous thin membranes for cell transplant in regenerative medicine

The aim of this paper is to present a method to produce macroporous thin membranes made of poly (ethyl acrylate-co-hydroxyethyl acrylate) copolymer network with varying cross-linking density for cell transplantation and prosthesis fabrication. The manufacture process is based on template techniques and anisotropic pore collapse. Pore collapse was produced by swelling the membrane in acetone and subsequently drying and changing the solvent by water to produce 100 microns thick porous membranes. These very thin membranes are porous enough to hold cells to be transplanted to the organism or to be colonized by ingrowth from neighboring tissues in the organism, and they present sufficient tearing stress to be sutured with surgical thread. The obtained pore morphology was observed by Scanning Electron Microscope, and confocal laser microscopy. Mechanical properties were characterized by stress-strain experiments in tension and tearing strength measurements. Morphology and mechanical properties were related to the different initial thickness of the scaffold and the cross-linking density of the polymer network. Seeding efficiency and proliferation of mesenchymal stem cells inside the pore structure were determined at 2 h, 1, 7, 14 and 21 days from seeding.

Corneal tissue engineering: recent advances and future perspectives

To address the growing need for corneal transplants two main approaches are being pursued: allogenic and synthetic materials. Allogenic tissue from human donors is currently the preferred choice; however, there is a worldwide shortage in donated corneal tissue. In addition, tissue rejection often limits the long-term success of this approach. Alternatively, synthetic homologs to donor corneal grafts are primarily considered temporary replacements until suitable donor tissue becomes available, as they result in a high incidence of graft failure. Tissue engineered cornea analogs would provide effective cornea tissue substitutes and alternatives to address the need to reduce animal testing of commercial products. Recent progress toward these needs is reviewed here, along with future perspectives.

Regeneration of corneal epithelium utilizing a collagen vitrigel membrane in rabbit models for corneal stromal wound and limbal stem cell deficiency

PURPOSE

This study was performed to evaluate the potential of a collagen-based membrane, collagen vitrigel (CV), for reconstructing corneal epithelium in the stromal wound and limbal stem cell deficiency (LSCD) models.

METHODS

Three groups of rabbits were used in the stromal wound model: CV affixed using fibrin glue (CV + FG group, n = 9), fibrin glue only (FG group, n = 3) and an untreated control group (n = 3). In the LSCD model, one group received CV containing human limbal epithelial cells (CV + hLEC group, n = 2) and the other was an untreated control (n = 1). Gross observation, including fluorescent staining, pathological examination, immunohistochemistry and electron microscopy, was used to evaluate the effect of CV on the corneal epithelium.

RESULTS

In the stromal wound model, fluorescent staining showed that epithelial reconstruction occurred as rapidly in the CV + FG group as it did in the control group. The pathological examination proved that the CV supported a healthy corneal epithelium in the CV + FG group, whereas FG led to hypertrophy and inappropriate differentiation of corneal epithelium in the FG group. In the LSCD model, the corneas in the CV + hLEC group showed sustained tissue transparency with good epithelialization, low inflammatory response and reduced neovascularization. However, the control cornea was translucent and showed high amounts of inflammation and neovascularization.

CONCLUSION

We have demonstrated that CV supports corneal epithelial differentiation and prevents epithelial hypertrophy, in addition to serving as a scaffold for hLEC transplantation, without complications.

Application of retinoic acid improves form and function of tissue engineered corneal construct

Retinoic acid has recently been shown to control the phenotype and extracellular matrix composition of corneal stromal cells cultured in vitro as monolayers. This study set out to investigate the effects of retinoic acid on human corneal keratocytes within a 3D environment. Human corneal keratocytes were encapsulated in collagen gels, which were subsequently compressed under load, and cultured in serum-free media supplemented with 10 µM retinoic acid or DMSO vehicle for 30 days. Cell proliferation was quantified on selected days, while the expression of several important keratocytes markers was evaluated at day 30 using RT-PCR and immunoblotting. The weight and size of the collagen constructs were measured before and after hydration and contraction analyses. Retinoic acid enhanced keratocyte proliferation until day 30, whereas cells in control culture conditions showed reduced numbers after day 21. Both gene and protein expressions of keratocyte-characteristic proteoglycans (keratocan, lumican and decorin), corneal crystallins and collagen type I and V were significantly increased following retinoic acid supplementation. Retinoic acid also significantly reduced the expression of matrix metalloproteases 1, 3 and 9 while not increasing α-smooth muscle actin and fibronectin expression. Furthermore, these effects were also correlated with the ability of retinoic acid to significantly inhibit the contractility of keratocytes while allowing the build-up of corneal stromal extracellular matrix within the 3D constructs. Thus, retinoic acid supplementation represents a promising strategy to improve the phenotype of 3D-cultured keratocytes, and their usefulness as a model of corneal stroma for corneal biology and regenerative medicine applications.

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.

Progress in the development of a corneal replacement: keratoprostheses and tissue-engineered corneas

Rapid progress has been made in the past 5 years in the development of corneal replacements. Traditionally they are divided into two categories, keratoprostheses and tissue-engineered corneal equivalents, as replacement tissues are increasingly in demand worldwide. There are currently several different keratoprosthesis models in clinical use around the world. The most popular and most widely publicized is the AlphaCor model, which has enjoyed significant clinical success. However, improvements remain to be made, and the aim of most of the current research is to better understand the interactions between a synthetic material and the surrounding biology on a more fundamental level. This improved understanding will no doubt lead to improvements in current models and to the development of new models in the near future. While tissue-engineered corneal equivalents have been under investigation for considerably less time, there is growing evidence to suggest that a tissue-engineered corneal equivalent comprised of primarily natural materials will exist in the not too distant future. Research groups have reported strong in vitro and in vivo results. The strength of the collagen matrix and its ability to support cell infiltration have been the primary avenues of research. Various collagen crosslinking techniques have been used. Infiltration of three major cells of the cornea has been observed. Most importantly, the ability of these materials to support nerve ingrowth has been demonstrated. While challenges remain with both types of corneal replacements, the considerable progress in the recent past suggests that reliable implants for the treatment of a variety of corneal diseases will be available. This review will provide an overview of recent results, and will provide insight into the future of research on corneal replacements.

A review of polypeptide-based polymersomes

Self-assembled systems from biodegradable amphiphilic polymers at the nanometer scale, such as nanotubes, nanoparticles, polymer micelles, nanogels, and polymersomes, have attracted much attention especially in biomedical fields. Among these nano-aggregates, polymersomes have attracted tremendous interests as versatile carriers due to their colloidal stability, tunable membrane properties and ability of encapsulating or integrating a broad range of drugs and molecules. Biodegradable block polymers, especially aliphatic polyesters such as polylactide, polyglycolide and poly (ε-caprolactone) have been widely used as biomedical materials for a long time to well fit the requirement of biomedical drug carriers. To have a precise control of the aggregation behavior of nano-aggregates, the more ordered polypeptide has been used to self-assemble into the drug carriers. In this review we focus on the study of polymersomes which also named pepsomes formed by polypeptide-based copolymers and attempt to clarify the polypeptide-based polymersomes from following aspects: synthesis and characterization of the polypeptide-based copolymers, preparation, multifunction and application of polypeptide-based polymersomes.

Epoxy cross-linked collagen and collagen-laminin Peptide hydrogels as corneal substitutes

A bi-functional epoxy-based cross-linker, 1,4-Butanediol diglycidyl ether (BDDGE), was investigated in the fabrication of collagen based corneal substitutes. Two synthetic strategies were explored in the preparation of the cross-linked collagen scaffolds. The lysine residues of Type 1 porcine collagen were directly cross-linked using l,4-Butanediol diglycidyl ether (BDDGE) under basic conditions at pH 11. Alternatively, under conventional methodology, using both BDDGE and 1-Ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC)/N-hydroxysuccinimide (NHS) as cross-linkers, hydrogels were fabricated under acidic conditions. In this latter strategy, Cu(BF₄)₂·XH₂O was used to catalyze the formation of secondary amine bonds. To date, we have demonstrated that both methods of chemical cross-linking improved the elasticity and tensile strength of the collagen implants. Differential scanning calorimetry and biocompatibility studies indicate comparable, and in some cases, enhanced properties compared to that of the EDC/NHS controls. In vitro studies showed that human corneal epithelial cells and neuronal progenitor cell lines proliferated on these hydrogels. In addition, improvement of cell proliferation on the surfaces of the materials was observed when neurite promoting laminin epitope, IKVAV, and adhesion peptide, YIGSR, were incorporated. However, the elasticity decreased with peptide incorporation and will require further optimization. Nevertheless, we have shown that epoxy cross-linkers should be further explored in the fabrication of collagen-based hydrogels, as alternatives to or in conjunction with carbodiimide cross-linkers.

The formation of a tissue-engineered cornea using plastically compressed collagen scaffolds and limbal stem cells

Collagen has excellent biocompatibility, is biodegradable, and possesses low immunogenicity. Therefore, this protein is a very suitable substrate for the formation of a corneal scaffold for therapeutic use. The highly hydrated nature of conventional collagen gels, however, results in a gel that is structurally weak and difficult to manipulate. In this chapter, we describe a novel method to cultivate limbal epithelial cells (LEC) on a compressed collagen scaffold. The compressed collagen scaffold can be rapidly constructed using a cell-independent process, which produces dense and mechanically strong collagen constructs with controllable microscale features.We embedded corneal keratocytes in a collagen gel, which we subsequently compressed and coated with laminin. The resulting construct supported the physiological morphology and stratification of LEC. The expression of a specific marker for differentiated LEC, cytokeratin 3 (CK3), and a marker for undifferentiated LEC, cytokeratin 14 (CK14), were similar in LEC expanded on both the compressed collagen construct and the leading conventional scaffold, denuded amniotic membrane (AM). We therefore demonstrate that a laminin-coated, compressed collagen gel containing keratocytes can support LEC expansion, stratification, and differentiation to a degree that is comparable to denuded AM. Our novel compressed collagen/keratocyte construct has potential for use as a tissue-engineered artificial cornea.

Strategies for developing decellularized corneal scaffolds

The main obstacle to successfully engineering corneal tissue has been the replication of the structural and biochemical composition of native cornea in a scaffold. In recent years decellularized corneas have been under investigation as an alternative scaffold source for use in engineering cornea. Several strategies for lysing cells and removing cellular material from corneas are discussed. The removal of such cellular components and antigen molecules whilst maintaining the corneal extracellular matrix components and architecture is required to generate scaffolds capable of generating functional tissue grafts suitable for transplantation. Different techniques to ascertain the degree of decellularization and the change in structural, mechanical and biological characteristics of the corneas after treatment are examined. In addition several in vitro and in vivo studies have been performed to ascertain the suitability of decellularized corneas as a scaffold for restoring vision.

A peptide talk between JC virus and the human host: from silent infection to autoimmunity

Analysis of JC virus (JCV) polyprotein for peptide sharing with the human proteome reveals that the virus has hundreds of pentapeptide sequences in common with the human proteins. The datum is interesting in light of the fundamental role exerted by short amino acid sequences in protein-protein interactions and, consequently, in biochemical reactions and immune recognition. Searching for new approaches to understand the JCV infection scenarios, from the immunoevasion phenomenon underlying the viral asymptomatic stay in the human host to the (re)activation phase and associated pathogenic sequelae, the present study describes the diffuse pentapeptide communication network between JCV and the human host.

Regenerative approaches as alternatives to donor allografting for restoration of corneal function

A range of alternatives to human donor tissue for corneal transplantation are being developed to address the shortfall of good quality tissues as well as the clinical conditions for which allografting is contraindicated. Classical keratoprostheses, commonly referred to as artificial corneas, are being used clinically to replace minimal corneal function. However, they are used only as last resorts, as they are associated with significant complications, such as extrusion/rejection, glaucoma, and retinal detachment. The past few years have seen significant developments in technologies designed to replace part or the full thickness of damaged or diseased corneas with materials that encourage regeneration to different extents. This review describes selected examples of these corneal substitutes, which range from cell-based regenerative strategies to keratoprostheses with regenerative capabilities via tissue-engineered scaffolds pre-seeded with stem cells. It is unlikely that one corneal substitute will be best for all indications, but taken together, the various approaches may soon be able to supplement the supply of human donor corneas for transplantation or allow restoration of diseased or damaged corneas that cannot be treated by currently available techniques.

Disorganized collagen scaffold interferes with fibroblast mediated deposition of organized extracellular matrix in vitro

Many tissue engineering applications require the remodeling of a degradable scaffold either in vitro or in situ. Although inefficient remodeling or failure to fully remodel the temporary matrix can result in a poor clinical outcome, very few investigations have examined in detail, the interaction of regenerative cells with temporary scaffoldings. In a recent series of investigations, randomly oriented collagen gels were directly implanted into human corneal pockets and followed for 24 months. The resulting remodeling response exhibited a high degree of variability which likely reflects differing regenerative/synthetic capacity across patients. Given this variability, we hypothesize that a disorganized, degradable provisional scaffold could be disruptive to a uniform, organized reconstruction of stromal matrix. In this investigation, two established corneal stroma tissue engineering culture systems (collagen scaffold-based and scaffold-free) were compared to determine if the presence of the disorganized collagen gel influenced matrix production and organizational control exerted by primary human corneal fibroblast cells (PHCFCs). PHCFCs were cultured on thin disorganized reconstituted collagen substrate (RCS--five donors: average age 34.4) or on a bare polycarbonate membrane (five donors: average age 32.4 controls). The organization and morphology of the two culture systems were compared over the long-term at 4, 8, and 11/12 weeks. Construct thickness and extracellular matrix organization/alignment was tracked optically with bright field and differential interference contrast (DIC) microscopy. The details of cell/matrix morphology and cell/matrix interaction were examined with standard transmission, cuprolinic blue and quick-freeze/deep-etch electron microscopy. Both the scaffold-free and the collagen-based scaffold cultures produced organized arrays of collagen fibrils. However, at all time points, the amount of organized cell-derived matrix in the scaffold-based constructs was significantly lower than that produced by scaffold-free constructs (controls). We also observed significant variability in the remodeling of RCS scaffold by PHCFCs. PHCFCs which penetrated the RCS scaffold did exert robust local control over secreted collagen but did not appear to globally reorganize the scaffold effectively in the time period of the study. Consistent with our hypothesis, the results demonstrate that the presence of the scaffold appears to interfere with the global organization of the cell-derived matrix. The production of highly organized local matrix by fibroblasts which penetrated the scaffold suggests that there is a mechanism which operates close to the cell membrane capable of controlling fibril organization. Nonetheless, the local control of the collagen alignment produced by cells within the scaffold was not continuous and did not result in overall global organization of the construct. Using a disorganized scaffold as a guide to produce highly organized tissue has the potential to delay the production of useful matrix or prevent uniform remodeling. The results of this study may shed light on the recent attempts to use disorganized collagenous matrix as a temporary corneal replacement in vivo which led to a variable remodeling response.

Design properties of hydrogel tissue-engineering scaffolds

This article summarizes the recent progress in the design and synthesis of hydrogels as tissue-engineering scaffolds. Hydrogels are attractive scaffolding materials owing to their highly swollen network structure, ability to encapsulate cells and bioactive molecules, and efficient mass transfer. Various polymers, including natural, synthetic and natural/synthetic hybrid polymers, have been used to make hydrogels via chemical or physical crosslinking. Recently, bioactive synthetic hydrogels have emerged as promising scaffolds because they can provide molecularly tailored biofunctions and adjustable mechanical properties, as well as an extracellular matrix-like microenvironment for cell growth and tissue formation. This article addresses various strategies that have been explored to design synthetic hydrogels with extracellular matrix-mimetic bioactive properties, such as cell adhesion, proteolytic degradation and growth factor-binding.

Degradation, cytotoxicity, and biocompatibility of NIPAAm-based thermosensitive, injectable, and bioresorbable polymer hydrogels

A thermosensitive, injectable, and bioresorbable polymer hydrogel, poly(N-isopropylacrylamide-co-dimethyl-γ-butyrolactone acrylate-co-acrylic acid) [poly(NDBA)], was synthesized by radical copolymerization with 7.00 mol % dimethyl-γ-butyrolactone acrylate in tetrahydrofuran. The chemical composition was determined by acid titration in conjunction with (1) H NMR quantification. The molecular weight and polydispersity were determined by gel permeation chromatography in conjunction with static light scattering. The degradation properties of the polymer hydrogel were characterized using differential scanning calorimetry, percentage mass loss, cloud point test, and swelling ratio over time. It was found that the initial lower critical solution temperature (LCST) of the polymer is between room temperature and body temperature and that it takes about 2 weeks for the LCST to surpass body temperature under physiological conditions. An indirect cytotoxicity test indicated that this copolymer has relatively low cytotoxicity as seen with 3T3 fibroblast cells. The in vivo-gelation and degradation study showed good agreement with in vitro-degradation findings, and no detrimental effects to adjacent tissues were observed after the complete dissolution of the polymer. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part A:, 2011.

Examining the suitability of riboflavin/UVA treatment for strengthening the stromal bioequivalent of a human cornea construct

PURPOSE

To improve the mechanical stability of a tissue-engineered human cornea construct, which is used as an in vitro model for drug absorption studies, the collagen matrix of this construct is to be strengthened by collagen cross-linking. A suitable method to induce photooxidative cross-linking of collagen fibrils is UVA irradiation combined with riboflavin as a photosensitizer.

MATERIALS AND METHODS

After riboflavin/UVA treatment, the viscoelastic properties of the collagen matrix and the molecular weight of its proteins, as well as cell viability of the human corneal keratocytes (HCK) incorporated in the stromal matrix, were analyzed depending on the dose of irradiation. In addition, the cell damage to the HCKs after riboflavin/UVA treatment was also analyzed in monolayer cultures. Various luminescent cell assays were performed to clarify whether the decrease of cell viability was a consequence of apoptosis or necrosis. Furthermore, fluorescent double staining was carried out using an apoptotic/necrotic cells detection kit.

RESULTS

The improvement of mechanical properties was low, whereas resultant cell damage was considerable and enduring. When lower doses of irradiation were used, the reduction of cell viability was triggered by apoptosis while necrosis supervened for increased doses of irradiation.

CONCLUSION

We conclude that in contrast to clinical applications, the riboflavin/UVA treatment does not seem to be a suitable method to obtain a sufficiently firm stromal matrix including vital keratocytes to build a tissue-engineered human cornea construct to be used as an in vitro model for drug absorption studies.

Plastic compression of a collagen gel forms a much improved scaffold for ocular surface tissue engineering over conventional collagen gels

We compare the use of plastically compressed collagen gels to conventional collagen gels as scaffolds onto which corneal limbal epithelial cells (LECs) are seeded to construct an artificial corneal epithelium. LECs were isolated from bovine corneas (limbus) and seeded onto either conventional uncompressed or novel compressed collagen gels and grown in culture. Scanning electron microscopy (SEM) results showed that fibers within the uncompressed gel were loose and irregularly ordered, whereas the fibers within the compressed gel were densely packed and more evenly arranged. Quantitative analysis of LECs expansion across the surface of the two gels showed similar growth rates (p > 0.05). Under SEM, the LECs, expanded on uncompressed gels, showed a rough and heterogeneous morphology, whereas on the compressed gel, the cells displayed a smooth and homogeneous morphology. Transmission electron microscopy (TEM) results showed the compressed scaffold to contain collagen fibers of regular diameter and similar orientation resembling collagen fibers within the normal cornea. TEM and light microscopy also showed that cell-cell and cell-matrix attachment, stratification, and cell density were superior in LECs expanded upon compressed collagen gels. This study demonstrated that the compressed collagen gel was an excellent biomaterial scaffold highly suited to the construction of an artificial corneal epithelium and a significant improvement upon conventional collagen gels.

Biosynthetic corneal substitute implantation in dogs

PURPOSE

To assess integration of a biosynthetic corneal implant in dogs.

METHODS

Three normal adult laboratory Beagles underwent ophthalmic examinations, including slit-lamp biomicroscopy, indirect ophthalmoscopy, applanation tonometry, and Cochet-Bonnet aesthesiometry. Biosynthetic corneas fabricated from glutaraldehyde crosslinked collagen and copolymers of collagen and poly(N-isopropylacrylamide-co-acrylic acid-co-acryloxysuccinimide, denoted as TERP) were implanted into dogs by a modified epikeratoplasty technique. Ophthalmic examinations and aesthesiometry were performed daily for 5 days and then weekly thereafter for 16 weeks. Corneal samples underwent histopathological and transmission electron microscopy examination at 16 weeks.

RESULTS

Implants were epithelialized by 7 days. Intraocular pressure was within normal range throughout the study. Aesthesiometry values dropped from an average of 3.67 cm preoperatively to less than 1 mm for all dogs for the first postoperative weeks. By week 16, the average Cochet-Bonnet value was 1.67 cm, demonstrating partial recovery of functional innervation of the implant. No inflammation or rejection of the implant occurred, and minimal haze formation was noted. Light microscopy revealed thickened but normal epithelium over the implant with fibroblast migration into the scaffold. On transmission electron microscopy, the basement membrane was irregular but present and adhesion complexes were noted.

CONCLUSION

Biosynthetic corneal implantation is well tolerated in dogs, and the collagen-polymer hybrid construct holds promise for clinical application in animals and humans.

Protein information content resides in rare peptide segments

Discovering the informational rule(s) underlying structure-function relationships in the protein language is at the core of biology. Current theories have proven inadequate to explain the origins of biological information such as that found in nucleotide and amino acid sequences. Here, we demonstrate that the information content of an amino acid motif correlates with the motif rarity. A structured analysis of the scientific literature supports the theory that rare pentapeptide words have higher significance than more common pentapeptides in biological cell 'talk'. This study expands on our previous research showing that the immunological information contained in an amino acid sequence is inversely related to the sequence frequency in the host proteome.

Collagen and glycopolymer based hydrogel for potential corneal application

6-Methacryloyl-alpha-D-galactopyranose (MG) was synthesized, and characterized by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectrometry, and single-crystal X-ray diffraction. A series of interpenetrating polymer network (IPN) hydrogels was fabricated by simultaneously photocuring MG crosslinked by poly(ethylene glycol) diacrylate and chemically crosslinking type I collagen with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide. The successful incorporation of the glycopolymer, polymer MG, into collagen hydrogel was confirmed by FTIR and solid-state (13)C NMR. The optical characteristics of the IPN hydrogels are comparable to those of human corneas. The tensile strength and modulus of the hydrogels are enhanced by incorporation of polymer MG in comparison to that of the control collagen hydrogel. Biodegradation results indicated that polymer MG enhanced the stability of the composite hydrogels against collagenase. In vitro results demonstrated that the IPN hydrogel supported the adhesion and proliferation of human corneal epithelial cells and outperformed human cornea in blocking bacteria adhesion. Taken together, the IPN hydrogel might be a promising material for use in corneal lamellar keratoplasty.

Collagen-phosphorylcholine interpenetrating network hydrogels as corneal substitutes

A biointeractive collagen-phospholipid corneal substitute was fabricated from interpenetrating polymeric networks comprising 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide and N-hydroxysuccinimide crosslinked porcine atelocollagen, and poly(ethylene glycol) diacrylate crosslinked 2-methacryloyloxyethyl phosphorylcholine (MPC). The resulting hydrogels showed an overall increase in mechanical strength beyond that of either original component and enhanced stability against enzymatic digestion (by collagenase) or UV degradation. More strikingly, these hydrogels retained the full biointeractive, cell friendly properties of collagen in promoting corneal cell and nerve in-growth and regeneration (despite MPC's known anti-adhesive properties). Measurements of refractive indices, white light transmission and backscatter showed the optical properties of collagen-MPC are comparable or superior to those of the human cornea. In addition, the glucose and albumin permeability were comparable to those of human corneas. Twelve-month post-implantation results of collagen-MPC hydrogels into mini-pigs showed regeneration of corneal tissue (epithelium, stroma) as well as the tear film and sensory nerves. We also show that porcine collagen can be substituted with recombinant human collagen, resulting in a fully-synthetic implant that is free from the potential risks of disease transmission (e.g. prions) present in animal source materials.