Silk fibroin as a biomaterial substrate for corneal epithelial cell sheet generation

Cultivated autologous limbal epithelial cell (CALEC) transplantation: Development of manufacturing process and clinical evaluation of feasibility and safety

To treat unilateral limbal stem cell (LSC) deficiency, we developed cultivated autologous limbal epithelial cells (CALEC) using an innovative xenobiotic-free, serum-free, antibiotic-free, two-step manufacturing process for LSC isolation and expansion onto human amniotic membrane with rigorous quality control in a good manufacturing practices facility. Limbal biopsies were used to generate CALEC constructs, and final grafts were evaluated by noninvasive scanning microscopy and tested for viability and sterility. Cultivated cells maintained epithelial cell phenotype with colony-forming and proliferative capacities. Analysis of LSC biomarkers showed preservation of "stemness." After preclinical development, a phase 1 clinical trial enrolled five patients with unilateral LSC deficiency. Four of these patients received CALEC transplants, establishing preliminary feasibility. Clinical case histories are reported, with no primary safety events. On the basis of these results, a second recruitment phase of the trial was opened to provide longer term safety and efficacy data on more patients.

Evaluation and Application of Silk Fibroin Based Biomaterials to Promote Cartilage Regeneration in Osteoarthritis Therapy

Osteoarthritis (OA) is a common joint disease characterized by cartilage damage and degeneration. Traditional treatments such as NSAIDs and joint replacement surgery only relieve pain and do not achieve complete cartilage regeneration. Silk fibroin (SF) biomaterials are novel materials that have been widely studied and applied to cartilage regeneration. By mimicking the fibrous structure and biological activity of collagen, SF biomaterials can promote the proliferation and differentiation of chondrocytes and contribute to the formation of new cartilage tissue. In addition, SF biomaterials have good biocompatibility and biodegradability and can be gradually absorbed and metabolized by the human body. Studies in recent years have shown that SF biomaterials have great potential in treating OA and show good clinical efficacy. Therefore, SF biomaterials are expected to be an effective treatment option for promoting cartilage regeneration and repair in patients with OA. This article provides an overview of the biological characteristics of SF, its role in bone and cartilage injuries, and its prospects in clinical applications to provide new perspectives and references for the field of bone and cartilage repair.

Silk-Based Biomaterials for Designing Bioinspired Microarchitecture for Various Biomedical Applications

Biomaterial research has led to revolutionary healthcare advances. Natural biological macromolecules can impact high-performance, multipurpose materials. This has prompted the quest for affordable healthcare solutions, with a focus on renewable biomaterials with a wide variety of applications and ecologically friendly techniques. Imitating their chemical compositions and hierarchical structures, bioinspired based materials have elevated rapidly over the past few decades. Bio-inspired strategies entail extracting fundamental components and reassembling them into programmable biomaterials. This method may improve its processability and modifiability, allowing it to meet the biological application criteria. Silk is a desirable biosourced raw material due to its high mechanical properties, flexibility, bioactive component sequestration, controlled biodegradability, remarkable biocompatibility, and inexpensiveness. Silk regulates temporo-spatial, biochemical and biophysical reactions. Extracellular biophysical factors regulate cellular destiny dynamically. This review examines the bioinspired structural and functional properties of silk material based scaffolds. We explored silk types, chemical composition, architecture, mechanical properties, topography, and 3D geometry to unlock the body's innate regenerative potential, keeping in mind the novel biophysical properties of silk in film, fiber, and other potential forms, coupled with facile chemical changes, and its ability to match functional requirements for specific tissues.

Corneal Epithelial Regeneration: Old and New Perspectives

Corneal blindness is the fifth leading cause of blindness worldwide, and therapeutic options are still often limited to corneal transplantation. The corneal epithelium has a strong barrier function, and regeneration is highly dependent on limbal stem cell proliferation and basement membrane remodeling. As a result of the lack of corneal donor tissues, regenerative medicine for corneal diseases affecting the epithelium is an area with quite advanced basic and clinical research. Surgery still plays a prominent role in the treatment of epithelial diseases; indeed, innovative surgical techniques have been developed to transplant corneal and non-corneal stem cells onto diseased corneas for epithelial regeneration applications. The main goal of applying regenerative medicine to clinical practice is to restore function by providing viable cells based on the use of a novel therapeutic approach to generate biological substitutes and improve tissue functions. Interest in corneal epithelium rehabilitation medicine is rapidly growing, given the exposure of the corneal outer layers to external insults. Here, we performed a review of basic, clinical and surgical research reports on regenerative medicine for corneal epithelial disorders, classifying therapeutic approaches according to their macro- or microscopic target, i.e., into cellular or subcellular therapies, respectively.

Construction and properties of the silk fibroin and polypropylene composite biological mesh for abdominal incisional hernia repair

Background: In this study, a new composite biological mesh named SFP was prepared by combining silk fibroin with polypropylene mesh. The mechanism and clinical application value of the SFP composite mesh were explored.

Methods: The fibrous membrane was prepared by electrospinning of silk fibroin. The silk fibrous membrane was adhered to the polypropylene mesh by fibrin hydrogel to make a new composite mesh. The characterizations were verified by structural analysis and in vitro cell experiments. A total of 40 Sprague–Dawley rats were randomly divided into two groups, and 20 rats in each group were implanted with the SFP mesh and pure polypropylene mesh, respectively. The rats were sacrificed in batches on the 3rd, 7th, 14th, and 90th days after surgery. The adhesion degree and adhesion area on the mesh surface were compared, and a histopathological examination was carried out.

Results:In vitro cell function experiments confirmed that the SFP mesh had good cell viability. The control group had different degrees of adhesion on the 3rd, 7th, 14th, and 90th days after surgery. However, there was almost no intraperitoneal adhesions on the 3rd and 7th days after surgery, and some rats only had mild adhesions on the 14th and 90th days after surgery in the SFP group. There were statistically significant differences in the postoperative intraperitoneal adhesion area and adhesion degree between the two groups (p < 0.05). Histopathological examination confirmed that the mesenchymal cells were well arranged and continuous, and there were more new capillaries and adipocyte proliferation under the mesenchymal cells in the SFP group.

Conclusion: The SFP mesh shows good biocompatibility and biofunction in vitro and in vivo. It can promote the growth of peritoneal mesenchymal cells. The formation of a new mesenchymal cell layer can effectively reduce the extent and scope of adhesion between the mesh and abdominal organs. The SFP mesh will have a good application prospect in the field of abdominal wall hernia repair.

Use of Bombyx mori silk fibroin in tissue engineering: From cocoons to medical devices, challenges, and future perspectives

Silk fibroin has become a prominent material in tissue engineering (TE) over the last 20 years with almost 10,000 published works spanning in all the TE applications, from skeleton to neuronal regeneration. Fibroin is an extremely versatile biopolymer that, due to its ease of processing, has enabled the development of an entire plethora of materials whose properties and architectures can be tailored to suit target applications. Although the research and development of fibroin TE materials and devices is mature, apart from sutures, only a few medical products made of fibroin are used in the clinical routines. <40 clinical trials of Bombyx mori silk-related products have been reported by the FDA and few of them resulted in a commercialized device. In this review, after explaining the structure and properties of silk fibroin, we provide an overview of both fibroin constructs existing in the literature and fibroin devices used in clinic. Through the comparison of these two categories, we identified the burning issues faced by fibroin products during their translation to the market. Two main aspects will be considered. The first is the standardization of production processes, which leads both to the standardization of the characteristics of the issued device and the correct assessment of its failure. The second is the FDA regulations, which allow new devices to be marketed through the 510(k) clearance by demonstrating their equivalence to a commercialized medical product. The history of some fibroin medical devices will be taken as a case study. Finally, we will outline a roadmap outlining what actions we believe are needed to bring fibroin products to the market.

Material Engineering in Gut Microbiome and Human Health

Tremendous progress has been made in the past decade regarding our understanding of the gut microbiome's role in human health. Currently, however, a comprehensive and focused review marrying the two distinct fields of gut microbiome and material research is lacking. To bridge the gap, the current paper discusses critical aspects of the rapidly emerging research topic of "material engineering in the gut microbiome and human health." By engaging scientists with diverse backgrounds in biomaterials, gut-microbiome axis, neuroscience, synthetic biology, tissue engineering, and biosensing in a dialogue, our goal is to accelerate the development of research tools for gut microbiome research and the development of therapeutics that target the gut microbiome. For this purpose, state-of-the-art knowledge is presented here on biomaterial technologies that facilitate the study, analysis, and manipulation of the gut microbiome, including intestinal organoids, gut-on-chip models, hydrogels for spatial mapping of gut microbiome compositions, microbiome biosensors, and oral bacteria delivery systems. In addition, a discussion is provided regarding the microbiome-gut-brain axis and the critical roles that biomaterials can play to investigate and regulate the axis. Lastly, perspectives are provided regarding future directions on how to develop and use novel biomaterials in gut microbiome research, as well as essential regulatory rules in clinical translation. In this way, we hope to inspire research into future biomaterial technologies to advance gut microbiome research and gut microbiome-based theragnostics.

The Contribution of Silk Fibroin in Biomedical Engineering

Silk fibroin (SF) is a natural protein (biopolymer) extracted from the cocoons of Bombyx mori L. (silkworm). It has many properties of interest in the field of biotechnology, the most important being biodegradability, biocompatibility and robust mechanical strength with high tensile strength. SF is usually dissolved in water-based solvents and can be easily reconstructed into a variety of material formats, including films, mats, hydrogels, and sponges, by various fabrication techniques (spin coating, electrospinning, freeze-drying, and physical or chemical crosslinking). Furthermore, SF is a feasible material used in many biomedical applications, including tissue engineering (3D scaffolds, wounds dressing), cancer therapy (mimicking the tumor microenvironment), controlled drug delivery (SF-based complexes), and bone, eye and skin regeneration. In this review, we describe the structure, composition, general properties, and structure-properties relationship of SF. In addition, the main methods used for ecological extraction and processing of SF that make it a green material are discussed. Lastly, technological advances in the use of SF-based materials are addressed, especially in healthcare applications such as tissue engineering and cancer therapeutics.

An Insulin-like Growth Factor-1 Conjugated Bombyx mori Silk Fibroin Film for Diabetic Wound Healing: Fabrication, Physicochemical Property Characterization, and Dosage Optimization In Vitro and In Vivo

This study aimed to develop a silk fibroin (SF)-film for the treatment of chronic diabetic wounds. Silk fibroin was purified through a newly developed heating degumming (HD) process and casted on a hydrophobic surface to form SF-films. The process allowed the fabricated film to achieve a 42% increase in transparency and a 32% higher proliferation rate for BALB/3T3 fibroblasts compared to that obtained by conventional alkaline degumming treatment. Fourier transform infrared analysis demonstrated that secondary structure was retained in both HD- and alkaline degumming-derived SF preparations, although the crystallinity of beta-sheet in SF-film after the HD processing was slightly increased. This study also investigated whether conjugating insulin-like growth factor-1 (IGF-1) would promote diabetic wound healing and what the optimal dosage is. Using BALB/3T3 cells grown in hyperglycemic medium as a model, it was demonstrated that the optimal IGF-1 dosage to promote the cell growth was approximately 0.65 pmol. Further analysis of wound healing in a diabetic mouse model indicated that SF-film loaded with 3.25 pmol of IGF-1 showed significantly superior wound closure, a 13% increase at the 13th day after treatment relative to treatment with 65 pmol of free IGF-1. Improvement in diabetic wound healing was exerted synergistically by SF-film and IGF-1, as reflected by parameters including levels of re-epithelialization, epithelial tissue area, and angiogenesis. Finally, IGF-1 increased the epithelial tissue area and micro-vessel formation in a dose-dependent manner in a low dosage range (3.25 pmol) when loaded to SF-films. Together, these results strongly suggest that SF-film produced using HD and loaded with a low dosage of IGF-1 is a promising dressing for diabetic wound therapy.

Corneal Epithelial Stem Cells-Physiology, Pathophysiology and Therapeutic Options

In the human cornea, regeneration of the epithelium is regulated by the stem cell reservoir of the limbus, which is the marginal region of the cornea representing the anatomical and functional border between the corneal and conjunctival epithelium. In support of this concept, extensive limbal damage, e.g., by chemical or thermal injury, inflammation, or surgery, may induce limbal stem cell deficiency (LSCD) leading to vascularization and opacification of the cornea and eventually vision loss. These acquired forms of limbal stem cell deficiency may occur uni- or bilaterally, which is important for the choice of treatment. Moreover, a variety of inherited diseases, such as congenital aniridia or dyskeratosis congenita, are characterized by LSCD typically occurring bilaterally. Several techniques of autologous and allogenic stem cell transplantation have been established. The limbus can be restored by transplantation of whole limbal grafts, small limbal biopsies or by ex vivo-expanded limbal cells. In this review, the physiology of the corneal epithelium, the pathophysiology of LSCD, and the therapeutic options will be presented.

The effect of biodegradable silk fibroin-based scaffolds containing glial cell line-derived neurotrophic factor (GDNF) on the corneal regeneration process

Corneal injury due to ocular trauma or infection is one of the most challenging vision impairing pathologies. The aim of the work was to study the effect of biodegradable silk fibroin-based scaffolds containing GDNF on the corneal regeneration process. During cultivate the highest keratocytes proliferative activity was registered with scaffolds containing 250 ng/ml and 500 ng/ml GDNF. In mice with an experimental model of epithelial-stromal damage to the cornea, silk fibroin-based scaffolds containing GDNF in various concentrations were used (in groups 1, 2 and 3 silk fibroin-based scaffolds containing GDNF in a concentration of 50 ng/ml, 250 ng/ml and 500 ng/ml, respectively; in group 4 - silk fibroin-based scaffolds without GDNF; in group 5 - a solution of GDNF with concentration of 500 ng/ml; group 6- control). The area of the corneal epithelial defect in groups 2, 3, and 5 was less than in the other groups. The most pronounced positive immunohistochemical reaction with antibodies to Bcl2, Bax, phosphoERK1/2 and phospho-JNK1/2, Ki67, Gap43 was observed in groups 2 and 3. Thus, silk fibroin-based scaffolds with GDNF stimulate the epithelialization process, proliferative activity of epithelial cells and keratocytes, accelerate the formation of the stromal nerve plexus and exhibit anti-apoptotic activity.

Sustained Release of Insulin-Like Growth Factor-1 from Bombyx mori L. Silk Fibroin Delivery for Diabetic Wound Therapy

The goals of this study are to develop a high purity patented silk fibroin (SF) film and test its suitability to be used as a slow-release delivery for insulin-like growth factor-1 (IGF-1). The release rate of the SF film delivering IGF-1 followed zero-order kinetics as determined via the Ritger and Peppas equation. The release rate constant was identified as 0.11, 0.23, and 0.09% h^-1 at 37 °C for SF films loaded with 0.65, 6.5, and 65 pmol IGF-1, respectively. More importantly, the IGF-1 activity was preserved for more than 30 days when complexed with the SF film. We show that the IGF-1-loaded SF films significantly accelerated wound healing in vitro (BALB/3T3) and in vivo (diabetic mice), compared with wounds treated with free IGF-1 and an IGF-1-loaded hydrocolloid dressing. This was evidenced by a six-fold increase in the granulation tissue area in the IGF-1-loaded SF film treatment group compared to that of the PBS control group. Western blotting analysis also demonstrated that IGF-1 receptor (IGF1R) phosphorylation in diabetic wounds increased more significantly in the IGF-1-loaded SF films group than in other experimental groups. Our results suggest that IGF-1 sustained release from SF films promotes wound healing through continuously activating the IGF1R pathway, leading to the enhancement of both wound re-epithelialization and granulation tissue formation in diabetic mice. Collectively, these data indicate that SF films have considerable potential to be used as a wound dressing material for long-term IGF-1 delivery for diabetic wound therapy.

Oxygen Permeability of Silk Fibroin Hydrogels and Their Use as Materials for Contact Lenses: A Purposeful Analysis

Fibroin is a fibrous protein that can be conveniently isolated from the silk cocoons produced by the larvae of Bombyx mori silk moth. In its form as a hydrogel, Bombyx mori silk fibroin (BMSF) has been employed in a variety of biomedical applications. When used as substrates for biomaterial-cells constructs in tissue engineering, the oxygen transport characteristics of the BMSF membranes have proved so far to be adequate. However, over the past three decades the BMSF hydrogels have been proposed episodically as materials for the manufacture of contact lenses, an application that depends on substantially elevated oxygen permeability. This review will show that the literature published on the oxygen permeability of BMSF is both limited and controversial. Additionally, there is no evidence that contact lenses made from BMSF have ever reached commercialization. The existing literature is discussed critically, leading to the conclusion that BMSF hydrogels are unsuitable as materials for contact lenses, while also attempting to explain the scarcity of data regarding the oxygen permeability of BMSF. To the author’s knowledge, this review covers all publications related to the topic.

Silk films with nanotopography and extracellular proteins enhance corneal epithelial wound healing

Corneal wound healing depends on extracellular matrix (ECM) and topographical cues that modulate migration and proliferation of regenerating cells. In our study, silk films with either flat or nanotopography patterned parallel ridge widths of 2000, 1000, 800 nm surfaces were combined with ECMs which include collagen type I (collagen I), fibronectin, laminin, and Poly-D-Lysine to accelerate corneal wound healing. Silk films with 800 nm ridge width provided better cell spreading and wound recovery than other size topographies. Coating 800 nm patterned silk films with collagen I proves to optimally further increased mouse and rabbit corneal epithelial cells growth and wound recovery. This enhanced cellular response correlated with redistribution and increase in size and total amount of focal adhesion. Transcriptomics and signaling pathway analysis suggested that silk topography regulates cell behaviors via actin nucleation ARP-WASP complex pathway, which regulate filopodia formation. This mechanism was further explored and inhibition of Cdc42, a key protein in this pathway, delayed wound healing and decreased the length, density, and alignment of filopodia. Inhibition of Cdc42 in vivo resulted in delayed re-epithelization of injured corneas. We conclude that silk film nanotopography in combination with collagen I constitutes a better substrate for corneal wound repair than either nanotopography or ECM alone.

Application of Bombyx mori Silk Fibroin Films for Liquid-Crystal Devices

Biocompatible and biodegradable silk fibroin films show promise as an eco-friendly biomaterial with excellent mechanical, thermal, and optical transparency properties. In contrast, polyimide (PI) films adopted in the liquid-crystal display (LCD) industry for aligning LC molecules are synthesized using toxic chemicals, which are nonrecyclable and nonbiodegradable. In this work, Bombyx mori silk fibroin films are fabricated from the aqueous solution and applied as alignment films for LCDs. The thermal properties of the prepared regenerated silk fibroin materials under different heat treatment temperatures are investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The crystallinity of different heat-treated silk fibroin materials is determined by infrared spectroscopy. The silk fibroin film treated at a higher temperature exhibits better thermal stability due to the higher crystallinity of the β-form structure. The LCDs using silk fibroin alignment films show a low pretilt angle of 0.5° and an anchoring energy of ∼10^-3 J/m² similar to those of the conventional polyimide films.

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.

Mussel-inspired polydopamine-coated silk fibroin as a promising biomaterial

Silk fibroin (SF) is one of the natural biomaterials with promising and growing potential in different clinical applications such as corneal transplantation, donor site skin substitute and tympanic membrane. Some of the SFs that are extracted from mulberry silkworm do not have the arginyl–glycyl–aspartic acid (RGD) sequence for properly supporting cell adhesion and proliferation. Therefore, in the current study, polydopamine (PDA)-coated SFs were prepared to provide an RGD sequence, and the effect of PDA coating on different properties of SF was investigated. The results are also compared with those of an amniotic membrane (AM) that is a commercially available natural biomaterial for the mentioned applications. The Raman spectra showed characteristic peaks at 1581 and 1370 cm−1, which demonstrate the formation of the coating layer on the surface of the films. The results showed that coating led to no significant difference in surface hydrophilicity; a smoother surface; and improved cell attachment and distribution; and a little decrease in membrane transparency, but the membrane still being transparent enough to provide vivid vision through it.

In Vitro Culture of Human Corneal Endothelium on Non-Mulberry Silk Fibroin Films for Tissue Regeneration

Purpose

The purpose of this study was to determine if non-mulberry varieties of silk are suitable for the culture of corneal endothelium (CE).

Methods

Aqueous silk fibroin derived from Philosamia ricini (PR), Antheraea assamensis (AA), and Bombyx mori (BM) were cast as approximately 15 µm films with and without pores on which human CE cells were cultured. Tensile strength, elasticity, transmittance in visible range, and degradation properties of the films were characterised. Adhesion of CE to the silk films was quantified using MTT assay in addition to quantifying the number and area of focal adhesions using paxillin. Expression of CE markers was determined at the gene and protein levels using PCR and immunostaining, respectively. Barrier integrity of the cultured cells was measured as permeability to FITC dextran (10 kDa) in the presence or absence of thrombin.

Results

The films exhibited robust tensile strength, >95% transmittance and a refractive index comparable to the native cornea. BM degraded significantly faster when compared to PR and AA. A comparison between the three varieties of silk showed that significantly more cells were adhered to PR and AA than to BM. This was also reflected in the expression of stable focal adhesions on PR and AA, thus enabling the formation of intact monolayers of cells on these varieties unlike on BM. Treatment with thrombin significantly increased cellular permeability to dextran.

Conclusions

Our data shows that PR and AA varieties sufficiently support the growth and function of CE cells. This could be attributed to the presence of natural cell binding motifs (RGD) in these varieties.

Translational Relevance

Development of a suitable carrier for engineering the CE to address a major clinical requirement of healthy donor tissues for transplantation.

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.

A Review of the Emerging Role of Silk for the Treatment of the Eye

Silk is a remarkable biopolymer with a long history of medical use. Silk fabrications have a robust track record for load-bearing applications, including surgical threads and meshes, which are clinically approved for use in humans. The progression of top-down and bottom-up engineering approaches using silk as the basis of a drug delivery or cell-loaded matrix helped to re-ignite interest in this ancient material. This review comprehensively summarises the current applications of silk for tissue engineering and drug delivery, with specific reference to the eye. Additionally, the review also covers emerging trends for the use of silk as a biologically active biopolymer for the treatment of eye disorders. The review concludes with future capabilities of silk to contribute to advanced, electronically-enhanced ocular drug delivery concepts.

Angiogenesis and lymphangiogenesis in corneal transplantation-A review

Corneal transplantation has been proven effective for returning the gift of sight to those affected by corneal disorders such as opacity, injury, and infections that are a leading cause of blindness. Immune privilege plays an important role in the success of corneal transplantation procedures; however, immune rejection reactions do occur, and they, in conjunction with a shortage of corneal donor tissue, continue to pose major challenges. Corneal immune privilege is important to the success of corneal transplantation and closely related to the avascular nature of the cornea. Corneal avascularity may be disrupted by the processes of angiogenesis and lymphangiogenesis, and for this reason, these phenomena have been a focus of research in recent years. Through this research, therapies addressing certain rejection reactions related to angiogenesis have been developed and implemented. Corneal donor tissue shortages also have been addressed by the development of new materials to replace the human donor cornea. These advancements, along with other improvements in the corneal transplantation procedure, have contributed to an improved success rate for corneal transplantation. We summarize recent developments and improvements in corneal transplantation, including the current understanding of angiogenesis mechanisms, the anti-angiogenic and anti-lymphangiogenic factors identified to date, and the new materials being used. Additionally, we discuss future directions for research in corneal transplantation.

Treatment with solubilized Silk-Derived Protein (SDP) enhances rabbit corneal epithelial wound healing

There is a significant clinical need to improve current therapeutic approaches to treat ocular surface injuries and disease, which affect hundreds of millions of people annually worldwide. The work presented here demonstrates that the presence of Silk-Derived Protein (SDP) on the healing rabbit corneal surface, administered in an eye drop formulation, corresponds with an enhanced epithelial wound healing profile. Rabbit corneas were denuded of their epithelial surface, and then treated for 72-hours with either PBS or PBS containing 5 or 20 mg/mL SDP in solution four times per day. Post-injury treatment with SDP formulations was found to accelerate the acute healing phase of the injured rabbit corneal epithelium. In addition, the use of SDP corresponded with an enhanced tissue healing profile through the formation of a multi-layered epithelial surface with increased tight junction formation. Additional biological effects were also revealed that included increased epithelial proliferation, and increased focal adhesion formation with a corresponding reduction in the presence of MMP-9 enzyme. These in vivo findings demonstrate for the first time that the presence of SDP on the injured ocular surface may aid to improve various steps of rabbit corneal wound healing, and provides evidence that SDP may have applicability as an ingredient in therapeutic ophthalmic formulations.

Multi-layered silk film coculture system for human corneal epithelial and stromal stem cells

With insufficient options to meet the clinical demand for cornea transplants, one emerging area of emphasis is on cornea tissue engineering. In the present study, the goal was to combine the corneal stroma and epithelium into one coculture system, to monitor both human corneal stromal stem cell (hCSSC) and human corneal epithelial cell (hCE) growth and differentiation into keratocytes and differentiated epithelium in these three-dimensional tissue systems in vitro. Coculture conditions were first optimized, including the medium, air-liquid interface culture, and surface topography and chemistry of biomaterial scaffold films based on silk protein. The silk was used as scaffolding for both stromal and epithelial tissue layers because it is cell compatible, can be surface patterned, and is optically clear. Next, the effects of proliferating and differentiating hCEs and hCSSCs were studied in this in vitro system, including the effects on cell proliferation, matrix formation by immunochemistry, and gene expression by quantitative reverse transcription-polymerase chain reaction. The incorporation of both cell types into the coculture system demonstrated more complete differentiation and growth for both cell types compared to the corneal stromal cells and corneal epithelial cells alone. Silk films for corneal epithelial culture were optimized to combine a 4.0-μm-scale surface pattern with bulk-loaded collagen type IV. Differentiation of each cell type was in evidence based on increased expression of corneal stroma and epithelial proteins and transcript levels after 6 weeks in coculture on the optimized silk scaffolds.

Silk-Derived Protein Enhances Corneal Epithelial Migration, Adhesion, and Proliferation

Purpose

The corneal surface is vulnerable to a myriad of traumatic insults including mechanical, chemical, and thermal injuries. The resulting trauma may render the naturally occurring regenerative properties of the cornea incapable of restoring a healthy epithelial surface, and may result in the loss of corneal transparency and vision. Healing of the corneal epithelium requires a complex cascade of biological processes that work to restore the tissue after injury. New therapeutic agents that act on the multiple steps of the corneal wound-healing process would offer a potential for improving patient outcomes. Here, a novel silk fibroin–derived protein (SDP) was studied for potential impacts on wound healing through studying an in vitro model.

Methods

Solubilized SDP, produced from the Bombyx mori silkworm cocoon, was added to human corneal limbal-epithelial (hCLE) cultures to evaluate the material's effects on epithelial cell migration, proliferation, and adhesion through the use of various scratch wound assays and flow chamber studies.

Results

Results indicated that the addition of SDP to culture increased hCLE migration rate by over 50%, and produced an approximate 60% increase in cell proliferation. This resulted in a nearly 30% enhancement of in vitro scratch wound closure time. In addition, cultures treated with SDP experienced increased cell-matrix focal adhesion formation by over 95% when compared to controls.

Conclusions

The addition of SDP to culture media significantly enhanced hCLE cell sheet migration, proliferation, and attachment when compared to untreated controls, and indicates SDP's potential utility as an ophthalmic therapeutic agent.

Safety of Cultivated Limbal Epithelial Stem Cell Transplantation for Human Corneal Regeneration

Ex vivo cultivated limbal stem cell transplantation is a promising technique for the treatment of limbal stem cell deficiency. While the results of the clinical trials have been extensively reported since the introduction of the technique in 1997, little has been reported regarding the potential health risks associated with production processes and transplantation techniques. Culture procedures require the use of animal and/or human-derived products, which carry the potential of introducing toxic or infectious agents through contamination with known or unknown additives. Protocols vary widely, and the risks depend on the local institutional methods. Good manufacturing practice and xeno-free culture protocols could reduce potential health risks but are not yet a common practice worldwide. In this review, we focus on the safety of both autologous- and allogeneic-cultivated limbal stem cell transplantation, with respect to culture processes, surgical approaches, and postoperative strategies.

3D Functional Corneal Stromal Tissue Equivalent Based on Corneal Stromal Stem Cells and Multi-Layered Silk Film Architecture

The worldwide need for human cornea equivalents continues to grow. Few clinical options are limited to allogenic and synthetic material replacements. We hypothesized that tissue engineered human cornea systems based on mechanically robust, patterned, porous, thin, optically clear silk protein films, in combination with human corneal stromal stem cells (hCSSCs), would generate 3D functional corneal stroma tissue equivalents, in comparison to previously developed 2D approaches. Silk film contact guidance was used to control the alignment and distribution of hCSSCs on RGD-treated single porous silk films, which were then stacked in an orthogonally, multi-layered architecture and cultured for 9 weeks. These systems were compared similar systems generated with human corneal fibroblasts (hCFs). Both cell types were viable and preferentially aligned along the biomaterial patterns for up to 9 weeks in culture. H&E histological sections showed that the systems seeded with the hCSSCs displayed ECM production throughout the entire thickness of the constructs. In addition, the ECM proteins tested positive for keratocyte-specific tissue markers, including keratan sulfate, lumican, and keratocan. The quantification of hCSSC gene expression of keratocyte-tissue markers, including keratocan, lumican, human aldehyde dehydrogenase 3A1 (ALDH3A1), prostaglandin D2 synthase (PTDGS), and pyruvate dehydrogenase kinase, isozyme 4 (PDK4), within the 3D tissue systems demonstrated upregulation when compared to 2D single silk films and to the systems generated with the hCFs. Furthermore, the production of ECM from the hCSSC seeded systems and subsequent remodeling of the initial matrix significantly improved cohesiveness and mechanical performance of the constructs, while maintaining transparency after 9 weeks.

In vitro characterization of electrochemically compacted collagen matrices for corneal applications

Loss of vision due to corneal disease is a significant problem worldwide. Transplantation of donor corneas is a viable treatment option but limitations such as short supply and immune-related complications call for alternative options for the treatment of corneal disease. A tissue engineering-based approach using a collagen scaffold is a promising alternative to develop a bioengineered cornea that mimics the functionality of native cornea. In this study, an electrochemical compaction method was employed to synthesize highly dense and transparent collagen matrices. We hypothesized that chemical crosslinking of electrochemically compacted collagen (ECC) matrices will maintain transparency, improve stability, and enhance the mechanical properties of the matrices to the level of native cornea. Further, we hypothesized that keratocyte cell viability and proliferation will be maintained on crosslinked ECC matrices. The results indicated that uncrosslinked and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide-N-hydroxysuccinimide (EDC-NHS) crosslinked ECC matrices were highly transparent with light transmission measurements comparable to native cornea. Stability tests showed that while the uncrosslinked ECC matrices degraded within 6 h when treated with collagenase, EDC-NHS or genipin crosslinking significantly improved the stability of ECC matrices (192 h for EDC-NHS and 256 h for genipin). Results from the mechanical tests showed that both EDC-NHS and genipin crosslinking significantly improved the strength and modulus of ECC matrices. Cell culture studies showed that keratocyte cell viability and proliferation are maintained on EDC-NHS crosslinked ECC matrices. Overall, results from this study suggest that ECC matrices have the potential to be developed as a functional biomaterial for corneal repair and regeneration.

Optimization of Corneal Epithelial Progenitor Cell Growth on Bombyx mori Silk Fibroin Membranes

Scaffolds prepared from silk fibroin derived from cocoons of the domesticated silkworm moth Bombyx mori have demonstrated potential to support the attachment and growth of human limbal epithelial (HLE) cells in vitro. In this study, we attempted to further optimize protocols to promote the expansion of HLE cells on B. mori silk fibroin- (BMSF-) based scaffolds. BMSF films were initially coated with different extracellular matrix proteins and then analysed for their impact on corneal epithelial cell adhesion, cell morphology, and culture confluency. Results showed that collagen I, collagen III, and collagen IV consistently improved HCE-T cell adherence, promoted an elongated cell morphology, and increased culture confluency. By contrast, ECM coating had no significant effect on the performance of primary HLE cells cultured on BMSF films. In the second part of this study, primary HLE cells were grown on BMSF films in the presence of medium (SHEM) supplemented with keratinocyte growth factor (KGF) and the Rho kinase inhibitor, Y-27632. The results demonstrated that SHEM medium supplemented with KGF and Y-27632 dramatically increased expression of corneal differentiation markers, keratin 3 and keratin 12, whereas expression of the progenitor marker, p63, did not appear to be significantly influenced by the choice of culture medium.

Contact Lenses: A Delivery Device for Stem Cells to Treat Corneal Blindness

: Worldwide, 45 million people are blind. Corneal blindness is a major cause of visual loss, estimated to affect 10 million. For the most difficult to treat patients, including those with a disease called limbal stem cell deficiency, a donor corneal graft is not a viable option; thus, patients are treated with specialized stem cell grafts, which fail in a significant proportion (30 to 50%) of subjects. This unacceptable failure rate means there is a pressing need to develop minimally invasive, long-lasting, cost-effective therapies to improve patient quality of life and lessen the economic burden. Restoring vision in patients with severe corneal disease is the main focus of our research program; however, to achieve our goals and deliver the best quality stem cell therapy, we must first understand the basic biology of these cells, including their residence, the factors that support their long-term existence, markers to identify and isolate them, and carriers that facilitate expansion, delivery, and protection during engraftment. We recently achieved some of these goals through the discovery of stem cell markers and the development of a novel and innovative contact lens-based cell transfer technique that has been successfully trialed on patients with corneal blindness. Although several popular methodologies are currently available to nurture and transfer stem cells to the patients' ocular surface, contact lenses provide many advantages that will be discussed in this review article. The job for clinician-researchers will be to map precisely how these cells contribute to restoring ocular health and whether improvements in the quality of cells and the cell delivery system can be developed to reduce disease burden.

Tissue Regeneration: A Silk Road

Silk proteins are natural biopolymers that have extensive structural possibilities for chemical and mechanical modifications to facilitate novel properties, functions, and applications in the biomedical field. The versatile processability of silk fibroins (SF) into different forms such as gels, films, foams, membranes, scaffolds, and nanofibers makes it appealing in a variety of applications that require mechanically superior, biocompatible, biodegradable, and functionalizable biomaterials. There is no doubt that nature is the world's best biological engineer, with simple, exquisite but powerful designs that have inspired novel technologies. By understanding the surface interaction of silk materials with living cells, unique characteristics can be implemented through structural modifications, such as controllable wettability, high-strength adhesiveness, and reflectivity properties, suggesting its potential suitability for surgical, optical, and other biomedical applications. All of the interesting features of SF, such as tunable biodegradation, anti-bacterial properties, and mechanical properties combined with potential self-healing modifications, make it ideal for future tissue engineering applications. In this review, we first demonstrate the current understanding of the structures and mechanical properties of SF and the various functionalizations of SF matrices through chemical and physical manipulations. Then the diverse applications of SF architectures and scaffolds for different regenerative medicine will be discussed in detail, including their current applications in bone, eye, nerve, skin, tendon, ligament, and cartilage regeneration.

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.

From discovery to approval of an advanced therapy medicinal product-containing stem cells, in the EU

In 1997, the human corneal epithelium was reconstructed in vitro and transplanted on patients. Later, it became a routine treatment, before regulations considered advanced therapy medicinal products and drugs on the same lines. Manufacturing, before and after good manufacturing practice setting, was established in different facilities and the clinical application in several hospitals. Advanced therapy medicinal products, including stem cells, are unique products with different challenges than other drugs: some uncertainties, in addition to benefit, cannot be avoided. This review will focus on all recent developments in the stem cell-based corneal therapy.

Acellular porcine corneal matrix as a carrier scaffold for cultivating human corneal epithelial cells and fibroblasts in vitro

AIM

To investigate the feasibility of corneal anterior lamellar reconstruction with human corneal epithelial cells and fibroblasts, and an acellular porcine cornea matrix (APCM) in vitro.

METHODS

The scaffold was prepared from fresh porcine corneas which were treated with 0.5% sodium dodecyl sulfate (SDS) solution and the complete removal of corneal cells was confirmed by hematoxylin-eosin (HE) staining and 4', 6-diamidino-2-phenylindole (DAPI) staining. Human corneal fibroblasts and epithelial cells were cultured with leaching liquid extracted from APCM, and then cell proliferative ability was evaluated by MTT assay. To construct a human corneal anterior lamellar replacement, corneal fibroblasts were injected into the APCM and cultured for 3d, followed by culturing corneal epithelial cells on the stroma construction surface for another 10d. The corneal replacement was analyzed by HE staining, and immunofluorescence staining.

RESULTS

Histological examination indicated that there were no cells in the APCM by HE staining, and DAPI staining did not detect any residual DNA. The leaching liquid from APCM had little influence on the proliferation ability of human corneal fibroblasts and epithelial cells. At 10d, a continuous 3 to 5 layers of human corneal epithelial cells covering the surface of the APCM was observed, and the injected corneal fibroblasts distributed within the scaffold. The phenotype of the construction was similar to normal human corneas, with high expression of cytokeratin 12 in the epithelial cell layer and high expression of vimentin in the stroma.

CONCLUSION

Corneal anterior lamellar replacement can be reconstructed in vitro by cultivating human corneal epithelial cells and fibroblasts with an acellular porcine cornea matrix. This laid the foundation for the further transplantation in vivo.

Non-mulberry Silk Fibroin Biomaterial for Corneal Regeneration

PURPOSE

Successful repair of a damaged corneal surface is a great challenge and may require the use of a scaffold that supports cell growth and differentiation. Amniotic membrane is currently used for this purpose, in spite of its limitations. A thin transparent silk fibroin film from non-mulberry Antheraea mylitta (Am) has been developed which offers to be a promising alternative. The silk scaffolds provide sufficient rigidity for easy handling, the scaffolds support the sprouting, migration, attachment and growth of epithelial cells and keratocytes from rat corneal explants; the cells form a cell sheet, preserve their phenotypes, express cytokeratin3 and vimentin respectively. The films also support growth of limbal stem cell evidenced by expression of ABCG2. The cell growth on the silk film and the amniotic membrane is comparable. The implanted film within the rabbit cornea remains transparent, stable. The clinical examination as well as histology shows absence of any inflammatory response or neovascularization. The corneal surface integrity is maintained; tear formation, intraocular pressure and electroretinography of implanted eyes show no adverse changes. The silk fibroin film from non-mulberry silk worms may be a worthy candidate for use as a corneal scaffold.

Limbal Stem Cell Deficiency: Current Treatment Options and Emerging Therapies

Severe ocular surface disease can result in limbal stem cell deficiency (LSCD), a condition leading to decreased visual acuity, photophobia, and ocular pain. To restore the ocular surface in advanced stem cell deficient corneas, an autologous or allogenic limbal stem cell transplantation is performed. In recent years, the risk of secondary LSCD due to removal of large limbal grafts has been significantly reduced by the optimization of cultivated limbal epithelial transplantation (CLET). Despite the great successes of CLET, there still is room for improvement as overall success rate is 70% and visual acuity often remains suboptimal after successful transplantation. Simple limbal epithelial transplantation reports higher success rates but has not been performed in as many patients yet. This review focuses on limbal epithelial stem cells and the pathophysiology of LSCD. State-of-the-art therapeutic management of LSCD is described, and new and evolving techniques in ocular surface regeneration are being discussed, in particular, advantages and disadvantages of alternative cell scaffolds and cell sources for cell based ocular surface reconstruction.

Human pluripotent stem cell-derived limbal epithelial stem cells on bioengineered matrices for corneal reconstruction

Corneal epithelium is renewed by limbal epithelial stem cells (LESCs), a type of tissue-specific stem cells located in the limbal palisades of Vogt at the corneo-scleral junction. Acute trauma or inflammatory disorders of the ocular surface can destroy these stem cells, leading to limbal stem cell deficiency (LSCD) - a painful and vision-threatening condition. Treating these disorders is often challenging and complex, especially in bilateral cases with extensive damage. Human pluripotent stem cells (hPSCs) provide new opportunities for corneal reconstruction using cell-based therapy. Here, we investigated the use of hPSC-derived LESC-like cells on bioengineered collagen matrices in serum-free conditions, aiming for clinical applications to reconstruct the corneal epithelium and partially replace the damaged stroma. Differentiation of hPSCs towards LESC-like cells was directed using small-molecule induction followed by maturation in corneal epithelium culture medium. After four to five weeks of culture, differentiated cells were seeded onto bioengineered matrices fabricated as transparent membranes of uniform thickness, using medical-grade porcine collagen type I and a hybrid cross-linking technology. The bioengineered matrices were fully transparent, with high water content and swelling capacity, and parallel lamellar microstructure. Cell proliferation of hPSC-LESCs was significantly higher on bioengineered matrices than on collagen-coated control wells after two weeks of culture, and LESC markers p63 and cytokeratin 15, along with proliferation marker Ki67 were expressed even after 30 days in culture. Overall, hPSC-LESCs retained their capacity to self-renew and proliferate, but were also able to terminally differentiate upon stimulation, as suggested by protein expression of cytokeratins 3 and 12. We propose the use of bioengineered collagen matrices as carriers for the clinically-relevant hPSC-derived LESC-like cells, as a novel tissue engineering approach for corneal reconstruction.

Human corneal stromal stem cells support limbal epithelial cells cultured on RAFT tissue equivalents

Human limbal epithelial cells (HLE) and corneal stromal stem cells (CSSC) reside in close proximity in vivo in the corneal limbal stem cell niche. However, HLE are typically cultured in vitro without supporting niche cells. Here, we re-create the cell-cell juxtaposition of the native environment in vitro, to provide a tool for investigation of epithelial-stromal cell interactions and to optimize HLE culture conditions for potential therapeutic application. RAFT (Real Architecture For 3D Tissue) tissue equivalents (TEs) were used as a 3-dimensional substrate for co-culturing HLE and CSSC. Our results demonstrate that a monolayer of HLE that maintained expression of p63α, ABCB5, CK8 and CK15 (HLE markers), formed on the surface of RAFT TEs within 13 days of culture. CSSC remained in close proximity to HLE and maintained expression of mesenchymal stem cell markers. This simple technique has a short preparation time of only 15 days with the onset of HLE layering and differentiation observed. Furthermore, co-cultivation of HLE with another niche cell type (CSSC) directly on RAFT TEs, eliminates the requirement for animal-derived feeder cells. RAFT TEs may be useful for future therapeutic delivery of multiple cell types to restore the limbal niche following ocular surface injury or disease.

Optimization of optical and mechanical properties of real architecture for 3-dimensional tissue equivalents: Towards treatment of limbal epithelial stem cell deficiency

Limbal epithelial stem cell (LESC) deficiency can cause blindness. Transplantation of cultured human limbal epithelial cells (hLE) on human amniotic membrane (HAM) can restore vision but clinical graft manufacture can be unreliable. We have developed a reliable and robust tissue equivalent (TE) alternative to HAM, Real Architecture for 3D Tissue (RAFT). Here, we aimed to optimize the optical and mechanical properties of RAFT TE for treatment of LESC deficiency in clinical application. The RAFT TE protocol is tunable; varying collagen concentration and volume produces differing RAFT TEs. These were compared with HAM samples taken from locations proximal and distal to the placental disc. Outcomes assessed were transparency, thickness, light transmission, tensile strength, ease of handling, degradation rates and suitability as substrate for hLE culture. Proximal HAM samples were thicker and stronger with poorer optical properties than distal HAM samples. RAFT TEs produced using higher amounts of collagen were thicker and stronger with poorer optical properties than those produced using lower amounts of collagen. The ‘optimal’ RAFT TE was thin, transparent but still handleable and was produced using 0.6 ml of 3 mg/ml collagen. Degradation rates of the ‘optimal’ RAFT TE and HAM were similar. hLE achieved confluency on ‘optimal’ RAFT TEs at comparable rates to HAM and cells expressed high levels of putative stem cell marker p63α. These findings support the use of RAFT TE for hLE transplantation towards treatment of LESC deficiency.

Early Observation of Extracellular Matrix-Derived Hydrogels for Corneal Stroma Regeneration

A lack of healthy transplantable tissue to treat corneal blindness has led researchers to investigate the development and application of different scaffold materials for corneal tissue engineering and regeneration. In this study, hydrogels fabricated from decellularized corneal extracellular matrix were developed as a new approach to corneal stromal tissue regeneration. Porcine corneas were decellularized using a technique that combined freeze-thaw cycles with a nuclease treatment. The corneas were then freeze-dried, milled, and digested in an acidic pepsin solution that was used to form a hydrogel after adjusting the pH and gelation temperature. The resultant corneal matrix hydrogels (CMHs) were seeded with human corneal stromal cells and cultured for several days. When compared to collagens hydrogels, CMHs had superior optical transparency, similar mechanical properties, and were better able to retain the stromal cells native keratocyte phenotype. The CMHs also supported cell viability and proliferation and contained sulfated glycosaminoglycan, a vital constituent, for maintaining corneal transparency. These results suggest that the CMHs could provide an exceptional biomaterial for corneal stroma regeneration.

Electrospun nanofibrous SF/P(LLA-CL) membrane: a potential substratum for endothelial keratoplasty

Background

Cornea transplant technology has progressed markedly in recent decades, allowing surgeons to replace diseased corneal endothelium by a thin lamellar structure. A thin, transparent, biocompatible, tissue-engineered substratum with corneal endothelial cells for endothelial keratoplasty is currently of interest. Electrospinning a nanofibrous structure can simulate the extracellular matrix and have beneficial effects for cell culture. Silk fibroin (SF) has good biocompatibility but poor mechanical properties, while poly(l-lactic acid-co-ε-caprolactone) (P(LLA-CL)) has good mechanical properties but poor biocompatibility. Blending SF with P(LLA-CL) can maintain the advantages of both these materials and overcome their disadvantages. Blended electrospun nanofibrous membranes may be suitable for regeneration of the corneal endothelium. The aim of this study was to produce a tissue-engineered construct suitable for endothelial keratoplasty.

Methods

Five scaffolds containing different SF:P(LLA-CL) blended ratios (100:0, 75:25, 50:50, 25:75, 0:100) were manufactured. A human corneal endothelial (B4G12) cell line was cultured on the membranes. Light transmission, speed of cell adherence, cell viability (live-dead test), cell proliferation (Ki-67, BrdU staining), and cell monolayer formation were detected on membranes with the different blended ratios, and expression of some functional genes was also detected by real-time polymerase chain reaction.

Results

Different blended ratios of scaffolds had different light transmittance properties. The 25:75 blended ratio membrane had the best transmittance among these scaffolds. All electrospun nanofibrous membranes showed improved speed of cell adherence when compared with the control group, especially when the P(LLA-CL) ratio increased. The 25:75 blended ratio membranes also had the highest cell proliferation. B4G12 cells could form a monolayer on all scaffolds, and most functional genes were also stably expressed on all scaffolds. Only two genes showed changes in expression.

Conclusion

All blended ratios of SF:P(LLA-CL) scaffolds were evaluated and showed good biocompatibility for cell adherence and monolayer formation. Among them, the 25:75 blended ratio SF:P(LLA-CL) scaffold had the best transmittance and the highest cell proliferation. These attributes further the potential application of the SF:P(LLA-CL) scaffold for corneal endothelial transplantation.

Optimization of silk films as substrate for functional corneal epithelium growth

The corneal epithelium is the first cellular barrier to protect the cornea. Thus, functional tissue engineering of the corneal epithelium is a strategy for clinical transplantation. In this study, the optimization of silk films (SFs) as substrates for functional human corneal epithelium growth was investigated with primary human corneal epithelial cells on SFs, poly-D-lysine (PDL) coated SFs, arginine-glycine-aspartic acid (RGD) modified SFs and PDL blended SFs. PDL coated SFs significantly promoted cell adhesion at early phases in comparison to the other study groups, while PDL blended SF significantly promoted cell migration in a "wound healing" model. All film modifications promoted cell proliferation and viability, and a multi-layered epithelium was achieved in 4 weeks of culture. The epithelia formed were tightly apposed and maintained an intact barrier function against rose bengal dye penetration. The results suggested that a differentiated human corneal epithelium can be established with primary corneal epithelial cells on SFs in vitro, by optimizing SF composition with PDL.

Clinical outcomes of xeno-free expansion and transplantation of autologous ocular surface epithelial stem cells via contact lens delivery: a prospective case series

Introduction

Depletion of limbal stem cells leads to a debilitating condition known as limbal stem cell deficiency, characterised by impaired corneal wound healing and poor vision. The aim of this study was to determine whether delivering progenitor cells on a contact lens is a viable and effective alternative to current transplantation techniques, which are complicated by biological and xenogeneic materials.

Methods

Sixteen eyes of 16 patients who had total (n = 14) and partial (n = 2) limbal stem cell deficiency (chemical burns, five eyes; iatrogenic causes, four eyes; aniridia, three eyes; trachoma-induced, two eyes; contact lens over-wear, one eye; and cicatrising conjunctivitis, one eye) and who had failed prior therapy were recruited prospectively into the study. Autologous limbal (n = 7) or conjunctival epithelial (n = 9) biopsies were harvested from patients and placed on the concave surface of silicone hydrogel contact lenses. Cells were expanded in culture with autologous serum and transplanted onto the ocular surface.

Results

Restoration of a transparent avascular and clinically stable corneal epithelium was attained in 10 of 16 eyes (63%) at a median follow-up time of 2.5 years (range of 0.8 to 5.8 years). Although minor complications occurred in two eyes of two patients because of contact lens insertion or removal, these were not associated with long-term sequelae.

Conclusions

This is the first and largest study to evaluate the mid-term outcomes of autologous limbal/conjunctival stem cell transplantation via a US Food and Drug Administration-approved contact lens, demonstrating that delivery of ocular progenitor cells via this procedure offers a viable, effective, and xeno-free alternative to current transplantation methodologies.

Trial registration

Australian New Zealand Clinical Trials Registry ACTRN012607000211460. Registered 17 April 2007.

Clinical applications of naturally derived biopolymer-based scaffolds for regenerative medicine

Naturally derived polymeric biomaterials, such as collagens, silks, elastins, alginates, and fibrins are utilized in tissue engineering due to their biocompatibility, bioactivity, and tunable mechanical and degradation kinetics. The use of these natural biopolymers in biomedical applications is advantageous because they do not release cytotoxic degradation products, are often processed using environmentally-friendly aqueous-based methods, and their degradation rates within biological systems can be manipulated by modifying the starting formulation or processing conditions. For these reasons, many recent in vivo investigations and FDA-approval of new biomaterials for clinical use have utilized natural biopolymers as matrices for cell delivery and as scaffolds for cell-free support of native tissues. This review highlights biopolymer-based scaffolds used in clinical applications for the regeneration and repair of native tissues, with a focus on bone, skeletal muscle, peripheral nerve, cardiac muscle, and cornea substitutes.

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.