Hydrogel membranes based on genipin-cross-linked chitosan blends for corneal epithelium tissue engineering

Advances in Cell Transplantation Therapy for Limbal Stem Cell Deficiency

BACKGROUND

Limbal stem cells (LSCs) are essential for maintaining corneal transparency and ocular surface integrity. Many external factors or genetic diseases can lead to corneal limbal stem cell deficiency (LSCD), resulting in the loss of barrier and corneal epithelial cell renewal functions. Stem cell transplantation is one of the primary treatments for LSCD, including limbal transplantation and cultivated limbal epithelial transplantation. In addition, a variety of non-limbal stem cell lines have been experimented with for LSCD treatment. Biological scaffolds are also used to support in vitro stem cell culture and transplantation. Here, we review the mechanisms of corneal maintenance by LSCs, the clinical stage and surgical treatment of LSCD, the source of stem cells, and the biological scaffolds required for in vitro culture.

METHODS

This study is a narrative retrospective study aimed at collecting available information on various aspects of surgical treatments for LSCD. Relevant literature was searched in a range of online databases, including Web of Science, Scopus, and PubMed from 2005 to March, 2023.

RESULTS

A total of 397 relevant articles were found, and 49 articles with strong relevance to the studies in this paper were obtained and analyzed. Moreover, 11 of these articles were on the concept of LSCD and the mechanism of LESCs maintaining the corneal epithelium, 3 articles on the staging and grading of LSCD, 17 articles on cell transplantation methods and donor cell sources, and 18 articles on scaffolds for delivering stem cells. We also summarized the advantages and disadvantages of different cell transplantation methods and the benefits and limitations of scaffolds based on the above literature.

CONCLUSION

The treatment of LSCD is determined by the clinical stage and whether it involves monocular or binocular eyes. Appropriate surgical techniques should be taken for LSCD patients in order to reconstruct the ocular surface, relieve symptoms, and restore visual function. Meanwhile, biological scaffolds assist in the ex vivo culture and implantation of stem cells.

Dendrimers in Corneal Drug Delivery: Recent Developments and Translational Opportunities

Dendrimers are biocompatible organic nanomaterials with unique physicochemical properties, making them the focus of recent research in drug delivery. The cornea of the human eye presents a challenge for drug transit due to its inherently impenetrable nature, requiring nanocarrier-mediated targeted drug delivery. This review intends to examine recent advancements in the use of dendrimers for corneal drug delivery, including their properties and their potential for treating various ocular diseases. The review will also highlight the benefit of the novel technologies that have been developed and applied in the field, such as corneal targeting, drug release kinetics, treatments for dry eye disease, antibacterial drug delivery, corneal inflammation, and corneal tissue engineering. The review seeks to provide a comprehensive overview of the current state of research in this field, along with the translational developments in the field of dendrimer-based therapeutics and imaging agents and inspire the potential for future developments and translational opportunities in dendrimers based corneal drug delivery.

The progress in techniques for culturing human limbal epithelial stem cells

In vitro culture of human limbal epithelial stem cells (hLESCs) is crucial to cell therapy in the treatment of limbal stem cell deficiency, a potentially vision-threatening disease that is characterized by persistent corneal epithelial defects and corneal epithelium conjunctivalization. Traditionally, hLESCs are cultivated based on either limbal tissue explants or single-cell suspensions in culture media containing xenogenous components, such as fetal bovine serum and murine 3T3 feeder cells. Plastic culture dishes and human amniotic membranes are classical growth substrates used in conventional hLESC culture systems. The past few decades have witnessed considerable progress and innovations in hLESC culture techniques to ensure a higher level of biosafety and lower immunogenicity for further cell treatment, including complete removal of xenogenous components from culture media, the application of human-derived feeder cells, and the development of novel scaffolds. Three-dimensional artificial niches and three-dimensional culture techniques have also been established to simulate the real microenvironment of limbal crypts for better cell outgrowth and proliferation. All these progresses ensure that in vitro cultured hLESCs are more adaptable to translational stem cell therapy for limbal stem cell deficiency.

[Regenerative medicine for the corneal epithelium : Cell therapy from bench to bedside]

In the case of thermal or caustic burns of the ocular surface, loss of limbal epithelial stem cells leads to compromised self-renewal of the corneal epithelium. This results in permanent loss of vision. In these situations, transplantation of cultured limbal epithelial cells on an amniotic membrane or fibrin gel as substrate (Holoclar®) can help to regenerate the corneal surface. The required cells are obtained from the healthy partner eye, if available. Adult stem cells from other parts of the body potentially serve as alternative cell sources: hair follicles, oral mucosa, mesenchymal stromal cells, or induced pluripotent stem cells (originally, e.g., skin fibroblasts). The reprogramming of such cells can be achieved with the help of transcription factors. In addition, work is being done on biosynthetic or synthetic matrices, which not only serve as substrate material for the transplantation but also support the functional properties of these cells (self-renewal, corneal epithelial-typical phenotype).

Membranes for the life sciences and their future roles in medicine

Since the global outbreak of COVID-19, membrane technology for clinical treatments, including extracorporeal membrane oxygenation (ECMO) and protective masks and clothing, has attracted intense research attention for its irreplaceable abilities. Membrane research and applications are now playing an increasingly important role in various fields of life science. In addition to intrinsic properties such as size sieving, dissolution and diffusion, membranes are often endowed with additional functions as cell scaffolds, catalysts or sensors to satisfy the specific requirements of different clinical applications. In this review, we will introduce and discuss state-of-the-art membranes and their respective functions in four typical areas of life science: artificial organs, tissue engineering, in vitro blood diagnosis and medical support. Emphasis will be given to the description of certain specific functions required of membranes in each field to provide guidance for the selection and fabrication of the membrane material. The advantages and disadvantages of these membranes have been compared to indicate further development directions for different clinical applications. Finally, we propose challenges and outlooks for future development.

Polysaccharides, as biological macromolecule-based scaffolding biomaterials in cornea tissue engineering: A review

Corneal-related diseases and injuries are the leading causes of vision loss, estimated to affect over 10 million people worldwide. Currently, cadaveric corneal grafts are considered the gold standard of treatment to restore cornea-related vision. However, this treatment modality faces different challenges such as donor shortage and graft failure. Therefore, the need for alternative solutions continues to grow. Tissue engineering has dramatically progressed to produce artificial cornea implants in order to repair, regenerate, or replace the damaged cornea. In this regard, a variety of polysaccharides such as cellulose, chitosan, alginate, agarose, and hyaluronic acid have been widely explored as scaffolding biomaterials for the production of tissue-engineered cornea. These polymers are known for their excellent biocompatibility, versatile properties, and processability. Recent progress and future perspectives of polysaccharide-based biomaterials in cornea tissue engineering is reviewed here.

Goals and Challenges of Stem Cell-Based Therapy for Corneal Blindness Due to Limbal Deficiency

Corneal failure is a highly prevalent cause of blindness. One special cause of corneal failure occurs due to malfunction or destruction of the limbal stem cell niche, upon which the superficial cornea depends for homeostatic maintenance and wound healing. Failure of the limbal niche is referred to as limbal stem cell deficiency. As the corneal epithelial stem cell niche is easily accessible, limbal stem cell-based therapy and regenerative medicine applied to the ocular surface are among the most highly advanced forms of this novel approach to disease therapy. However, the challenges are still great, including the development of cell-based products and understanding how they work in the patient's eye. Advances are being made at the molecular, cellular, and tissue levels to alter disease processes and to reduce or eliminate blindness. Efforts must be coordinated from the most basic research to the most clinically oriented projects so that cell-based therapies can become an integrated part of the therapeutic armamentarium to fight corneal blindness. We undoubtedly are progressing along the right path because cell-based therapy for eye diseases is one of the most successful examples of global regenerative medicine.

Hydrogels as Emerging Materials for Cornea Wound Healing

Hydrogel biomaterials have many favorable characteristics including tuneable mechanical behavior, cytocompatibility, optical properties suitable for regeneration and restoration of the damaged cornea tissue. The cornea is a tissue susceptible to various injuries and traumas with a complicated healing cascade, in which conserving its transparency and integrity is critical. Accordingly, the hydrogels' known properties along with the stimulation of nerve and cell regeneration make them ideal scaffold for corneal tissue engineering. Hydrogels have been used extensively in clinical applications for the repair and replacement of diseased organs. The development and optimizing of novel hydrogels to repair/replace corneal injuries have been the main focus of researches within the last decade. This research aims to critically review in vitro, preclinical, as well as clinical trial studies related to corneal wound healing using hydrogels in the past 10 years, as this is considered as an emerging technology for corneal treatment. Several unique modifications of hydrogels with smart behaviors have undergone early phase clinical trials and showed promising outcomes. Financially, this considers a multibillion dollars industry and with huge interest from medical devices as well as pharmaceutical industries with several products may emerge within the next five years.

Long-Term Study of Corneal Stroma and Endothelium on Structure and Cells After Genipin Treatment of Rabbit Corneas

Purpose

To study the long-term safety of genipin treatment using a vacuum device with or without epithelial cells at different crosslinking times.

Methods

Twenty-five healthy New Zealand white rabbits were separated into five treatment groups: 0.25% genipin with epithelial cells for 5 minutes (G1), 0.25% genipin without epithelial cells for 5 minutes (G2), 0.25% genipin without epithelial cells for 10 minutes (G3), ultraviolet A–riboflavin collagen crosslinking (UVA), and controls (C). Before and 2, 4, 6, and 8 weeks after crosslinking treatment, anterior segment optical coherence tomography (ASOCT), in vivo confocal microscopy (IVCM), and the Pentacam system were used to evaluate the right eyes.

Results

A demarcation line (DL) was observed in the corneal stroma in the G2, G3, and UVA groups. The DL depths in the G2 and G3 groups were stable but decreased in the UVA group over time. The density of keratocytes in these groups increased. Endothelial cell density was decreased in the UVA group. There were no differences in the endothelium before and after treatment in the G1, G2, G3, and C groups. The densitometry, as determined using the Pentacam system, significantly increased in the G2, G3, and UVA groups and was positively correlated with keratocyte densities.

Conclusions

A vacuum ring assisting local genipin immersion crosslinking without corneal epithelium can activate the keratocytes in the corneal stroma and was safe enough for the thin cornea.

Translational Relevance

Genipin can not only crosslink the collagen fibers but also activate the keratocytes and even may promote collagen fiber secretion.

Significance of Crosslinking Approaches in the Development of Next Generation Hydrogels for Corneal Tissue Engineering

Medical conditions such as trachoma, keratoconus and Fuchs endothelial dystrophy can damage the cornea, leading to visual deterioration and blindness and necessitating a cornea transplant. Due to the shortage of donor corneas, hydrogels have been investigated as potential corneal replacements. A key factor that influences the physical and biochemical properties of these hydrogels is how they are crosslinked. In this paper, an overview is provided of different crosslinking techniques and crosslinking chemical additives that have been applied to hydrogels for the purposes of corneal tissue engineering, drug delivery or corneal repair. Factors that influence the success of a crosslinker are considered that include material composition, dosage, fabrication method, immunogenicity and toxicity. Different crosslinking techniques that have been used to develop injectable hydrogels for corneal regeneration are summarized. The limitations and future prospects of crosslinking strategies for use in corneal tissue engineering are discussed. It is demonstrated that the choice of crosslinking technique has a significant influence on the biocompatibility, mechanical properties and chemical structure of hydrogels that may be suitable for corneal tissue engineering and regenerative applications.

Cytotoxicity and UV Light Absorption in Biopolymeric Membranes from Native Vegetation of Mexico

Sustainable processing from native vegetation such as agave, nopal and aloe, is mainly centered on the components of lignin, cellulose, and hemicellulose, defined generally as lignocellulosic. Cytotoxicity of lignocellulosic biopolymer membranes (LBMs) of mucilages of Opuntia ficus-indica, Aloe barbadensis Miller, fructans, and residual Agave tequilana Weber fibers at different concentrations (30, 50 and 70%), was evaluated in contact with human fibroblast cells (ATCC® PCS-201-010), cultured in vitro. Long and short LMBs were formed from the fibrous material. The interaction showed mild cytotoxicity according to ISO 10993-5: 2009. Cytotoxic activity demonstrated in the presence of fibroblasts implied a maximum cytotoxicity of close to 46% and a minimum of 7% in LBMs. UV light absorption results of the large fiber LMBs showed the highest prevention of passage of light, having a protective effect mechanism of delaying the response of hypersensitivity as a result of exposure to UV radiation. The morphology and structure were characterized using SEM, FT-IR, and image texture analysis. According to the LBMs reactivity measured, the feasible uses of these membranes can be suggested as materials for biomedical applications over a short period of time, with feasibility as a support to provide some beneficial solutions.

Gradient Chitosan Hydrogels Modified with Graphene Derivatives and Hydroxyapatite: Physiochemical Properties and Initial Cytocompatibility Evaluation

In this study, we investigated preparation of gradient chitosan-matrix hydrogels through a novel freezing-gelling-thawing method. The influence of three types of graphene family materials (GFM), i.e., graphene oxide (GO), reduced graphene oxide (rGO), and poly(ethylene glycol) grafted graphene oxide (GO-PEG), as well as hydroxyapatite (HAp) on the physicochemical and biological properties of the composite hydrogels was examined in view of their potential applicability as tissue engineering scaffolds. The substrates and the hydrogel samples were thoroughly characterized by X-ray photoelectron spectroscopy, X-ray diffractometry, infrared spectroscopy, digital and scanning electron microscopy, rheological and mechanical analysis, in vitro chemical stability and bioactivity assays, as well as initial cytocompatibility evaluation with human umbilical cord Wharton's jelly mesenchymal stem cells (hUC-MSCs). We followed the green-chemistry approach and avoided toxic cross-linking agents, using instead specific interactions of our polymer matrix with tannic acid, non-toxic physical cross-linker, and graphene derivatives. It was shown that the most promising are the gradient hydrogels modified with GO-PEG and HAp.

Dual Crosslinked Collagen/Chitosan Film for Potential Biomedical Applications

The application of polymeric biomaterial scaffolds utilizing crosslinking strategy has become an effective approach in these days. In the present study, the development and characterization of collagen-chitosan hydrogel film has been reported on using dual crosslinking agent's, i.e., tannic acid and genipin simultaneously. Incorporation of genipin imparts a greenish-blue color to the polymeric film. The effect of dual crosslinking and their successful interaction within the matrix was evaluated by infrared analysis spectroscopy. The porosity of the film was examined using scanning electron microscopy (SEM). Results of TGA determine the intermediate thermal degradation. Further, the crosslinking phenomenon has found primary impact on the strength of the films. Enzymatic degradation for the films was performed with lysozyme and lipase. The cell adhesion and proliferation was also accomplished using mouse embryonic cell lines wherein the cells cultured on the dual crosslinked film. The thriving utilization of such dual crosslinked polymeric film finds their applications in ophthalmology especially as an implant for temporary injured cornea and skin tissue regeneration.

Decellularized Annulus Fibrosus Matrix/Chitosan Hybrid Hydrogels with Basic Fibroblast Growth Factor for Annulus Fibrosus Tissue Engineering

Low back pain caused by degenerative disc disease affects many people worldwide and brings huge economical burden. Thus, attentions have focused on annulus fibrosus (AF) tissue engineering for treatment of intervertebral disc degeneration. To engineer a functional replacement for the AF, it is important to fabricate scaffolds that mimic the structural and mechanical properties of native tissue. AF-derived stem cells are promising seed cells for AF tissue engineering due to their tissue specificity. In the present study, decellularized AF matrix (DAFM)/chitosan hybrid hydrogels were fabricated using genipin as a crosslinker. AF stem cells were cultured on hydrogel scaffolds with or without basic fibroblast growth factor (bFGF), and cell proliferation, morphology, gene expression, and AF tissue synthesis were examined. Overall, more collagen-I, collagen-II, and aggrecan were secreted by AF stem cells grown on hydrogels with bFGF compared to those without. These results support the application of DAFM/chitosan hybrid hydrogels as an appropriate candidate for AF tissue engineering. Furthermore, incorporation of bFGF into hydrogels promoted AF-related tissue synthesis.

A study of corneal structure and biomechanical properties after collagen crosslinking with genipin in rabbit corneas

Purpose

Aim to assess the short-term effect of genipin collagen crosslinking (G-CXL) on corneal structure and biomechanical properties compared with ultraviolet A/riboflavin collagen crosslinking (UVA-CXL) in rabbit corneas.

Methods

Right eyes of 40 healthy rabbits were divided into the 0.20% G-CXL group, 0.25% G-CXL group, UVA-CXL group, and control group. Anterior segment optical coherence tomography (ASOCT) and in vivo confocal microscopy (IVCM) were performed before, 7 days after, and 14 days after the CXL treatment. Corneal strips were harvested for tensile strain measurements 7 and 14 days after the CXL treatment.

Results

ASOCT showed the demarcation line (DL) in the UVA-CXL group was deeper than in the 0.20% G-CXL group and the 0.25% G-CXL group on day 7 (p=0.014) and day 14 (p=0.012). Nerve and keratocyte density in all CXL groups decreased, but was more obvious in the UVA-CXL group (p<0.001). Endothelial cell loss in the 0.20% G-CXL group, 0.25% G-CXL group, UVA-CXL group, and control group was 11.7%, 6.8%, 32.8%, and 2.0% 14 days after CXL, respectively. Young's modulus and stress in the 0.25% G-CXL group and the UVA-CXL group were statistically significantly higher than in the control group (p<0.05) 7 and 14 days after CXL. No statistically significant differences were observed between the 0.25% G-CXL group and the UVA-CXL group (p>0.05). The DL depth was positively correlated with Young's modulus (r=0.426, p=0.042) and stress (r=0.469, p=0.024).

Conclusions

The administration of 0.25% genipin enhances corneal biomechanical properties as long as 14 days after the CXL treatment with low toxicity. The DL exists in CXL-treated corneas, and the depth is related to the biomechanical properties.

The Short-Term Safety Evaluation of Corneal Crosslinking Agent-Genipin

BACKGROUND

Genipin (GP) is a safe method for corneal crosslinking, even for very thin corneas. However, there have been no reports on the optimal GP concentration range to use in vivo for corneal crosslinking.

OBJECTIVES

To investigate the safety of corneal crosslinking after a 24-h incubation with different concentrations of GP.

METHODS

Twenty New Zealand white rabbits were divided into a phosphate-buffered saline (PBS) group, 0.2% GP crosslinking (GP-CXL) group, 0.25% GP-CXL group, and 0.3% GP-CXL group. Before and after surgery, the operated eyes of each group were characterized by confocal microscopy, and corneal buttons were excised for endothelium staining and electron microscopy.

RESULTS

The keratocyte structures in each GP group appeared to be similar to those in the PBS group. Through the confocal microscopy, the changes in corneal endothelial cell density also did not significantly differ among groups. There was a significant difference in apoptosis between the 0.3% GP-CXL and PBS groups (p < 0.05) and between the 0.3% GP-CXL and 0.25% GP-CXL groups (p < 0.05), but there were no significant differences between the 0.2 and 0.25% GP-CXL groups compared to the PBS group. Transmission electron microscopy showed endothelial cell damage in the 0.3% GP-CXL group, with minimal endothelial cell damage in the other groups.

CONCLUSIONS

Treatment of rabbit corneas with ≤0.25% GP resulted in minimal toxicity to keratocytes and endothelial cells, suggesting that it is a safe crosslinking agent at those concentrations.

Decellularized corneal lenticule embedded compressed collagen: toward a suturable collagenous construct for limbal reconstruction

Recently, compressed collagen has attracted much attention as a potential alternative for a limbal epithelial stem cell (LESC) carrier to treat limbal stem cell deficiency (LSCD), in that it can provide mechanically improved collagen fibrillar structures compared to conventional collagen hydrogel. However, its clinical efficacy as an LESC carrier has not yet been studied through in vivo transplantation due to limited mechanical strength that cannot withstand a force induced by surgical suturing and low resistance to enzymatic degradation. This study firstly presents a suturable LESC carrier based on compressed collagen in the form of a biocomposite. The biocomposite was achieved by integrating a decellularized corneal lenticule, which is a decellularized stromal tissue obtained from corneal refractive surgery, inside a compressed collagen to form a sandwich structure. A suture retention test verified that the biocomposite has a much higher suture retention strength (0.56 ± 0.12 N) compared to the compressed collagen (0.02 ± 0.01 N). The biocomposite also exhibited more than 3 times higher resistance to enzymatic degradation, indicating long-term stability after transplantation. In vitro cell culture results revealed that the biocomposite effectively supported the expansion and stratification of the LESCs with expressions of putative stem cell and differentiated corneal epithelial cell markers. Finally, the biocomposite verified its clinical efficacy by stably delivering the LESCs onto an eye of a rabbit model of LSCD and effectively reconstructing the ocular surface.

Ophthalmic gels: Past, present and future

Aqueous gels formulated using hydrophilic polymers (hydrogels) along with those based on stimuli responsive polymers (in situ gelling or gel forming systems) continue to attract increasing interest for various eye health-related applications. They allow the incorporation of a variety of ophthalmic pharmaceuticals to achieve therapeutic levels of drugs and bioactives at target ocular sites. The integration of sophisticated drug delivery technologies such as nanotechnology-based ones with intelligent and environment responsive systems can extend current treatment duration to provide more clinically relevant time courses (weeks and months instead of hours and days) which will inevitably reduce dose frequency, increase patient compliance and improve clinical outcomes. Novel applications and design of contact lenses and intracanalicular delivery devices along with the move towards integrating gels into various drug delivery devices like intraocular pumps, injections and implants has the potential to reduce comorbidities caused by glaucoma, corneal keratopathy, cataract, diabetic retinopathies and age-related macular degeneration. This review describes ophthalmic gelling systems with emphasis on mechanism of gel formation and application in ophthalmology. It provides a critical appraisal of the techniques and methods used in the characterization of ophthalmic preformed gels and in situ gelling systems along with a thorough insight into the safety and biocompatibility of these systems. Newly developed ophthalmic gels, hydrogels, preformed gels and in situ gelling systems including the latest in the area of stimuli responsive gels, molecularly imprinted gels, nanogels, 3D printed hydrogels; 3D printed devices comprising ophthalmic gels are covered. Finally, new applications of gels in the production of artificial corneas, corneal wound healing and hydrogel contact lenses are described.

Native and synthetic scaffolds for limbal epithelial stem cell transplantation

Limbal stem cell deficiency (LSCD) is a complex blinding disease of the cornea, which cannot be treated with conventional corneal transplants. Instead, a stem cell (SC) graft is required to replenish the limbal epithelial stem cell (LESC) reservoir, which is ultimately responsible for regenerating the corneal epithelium. Current therapies utilize limbal tissue biopsies that harbor LESCs as well as tissue culture expanded cells. Typically, this tissue is placed on a scaffold that supports the formation of corneal epithelial cell sheets, which are then transferred to diseased eyes. A wide range of biological and synthetic materials have been identified as carrier substrates for LESC, some of which have been used in the clinic, including amniotic membrane, fibrin, and silicon hydrogel contact lenses, each with their own advantages and limitations. This review will provide a brief background of LSCD, focusing on bio-scaffolds that have been utilized in limbal stem cell transplantation (LSCT) and materials that are being developed as potentially novel therapeutics for patients with this disease.

STATEMENT OF SIGNIFICANCE

The outcome of patients with corneal blindness that receive stem cell grafts to restore eye health and correct vision varies considerably and may be due to the different biological and synthetic scaffolds used to deliver these cells to the ocular surface. This review will highlight the positive attributes and limitations of the myriad of carriers developed for clinical use as well as those that are being trialled in pre-clinical models. The overall focus is on developing a standardized therapy for patients, however due to the multiple causes of corneal blindness, a personal regenerative medicine approach may be the best option.

Chitosan coating as an antibacterial surface for biomedical applications

Background and objectives

A current public health issue is preventing post-surgical complications by designing antibacterial implants. To achieve this goal, in this study we evaluated the antibacterial activity of an animal-free chitosan grafted onto a titanium alloy.

Methods

Animal-free chitosan binding on the substrate was performed by covalent link via a two-step process using TriEthoxySilylPropyl Succinic Anhydride (TESPSA) as the coupling agent. All grafting steps were studied and validated by means of X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) analyses and Dynamic-mode Secondary Ion Mass Spectrometry (DSIMS). The antibacterial activity against Escherichia coli and Staphylococcus aureus strains of the developed coating was assessed using the number of colony forming units (CFU).

Results

XPS showed a significant increase in the C and N atomic percentages assigned to the presence of chitosan. A thick layer of polymer deposit was detected by ToF-SIMS and the results obtained by DSIMS measurements are in agreement with ToF-SIMS and XPS analyses and confirms that the coating synthesis was a success. The developed coating was active against both gram negative and gram positive tested bacteria.

Conclusion

The success of the chitosan immobilization was proven using the surface characterization techniques applied in this study. The coating was found to be effective against Escherichia coli and Staphylococcus aureus strains.

The comparative safety of genipin versus UVA-riboflavin crosslinking of rabbit corneas

PURPOSE

To investigate, after 24 h, the safety of genipin or ultraviolet A (UVA)-riboflavin crosslinking of keratocytes and endothelial cells.

METHODS

Fifteen New Zealand white rabbits were selected and divided into a PBS group (five rabbits), a 0.2% genipin crosslinking (GP-CXL) group (five rabbits), and a UVA-riboflavin crosslinking (UVA-CXL) group (five rabbits). In the GP-CXL and PBS groups, 0.2% genipin or PBS was applied to the corneal surface of the right eyes. In the UVA-CXL group, a clinical crosslinking procedure was used. Before and after surgery, the operated eyes of each group were characterized with confocal microscopy, and the corneal buttons were excised for endothelium staining and electron microscopy.

RESULTS

The corneal endothelial cell density of the GP-CXL, UVA-CLX, and PBS groups changed. There was a statistically significant difference in thickness and changes in corneal endothelial cell density between the UVA-CXL group and the PBS group (p<0.05), and between the UVA-CXL group and the GP-CXL group (p<0.05), but no statistically significant difference between the GP-CXL group and the PBS group. Confocal microscopy, transmission electron microscopy, and hematoxylin and eosin staining showed that there was keratocyte apoptosis in the anterior and middle stroma and endothelial cell damage in the UVA-CXL group. In the GP-CXL group, only active keratocytes were found and minimal endothelial cell damage.

CONCLUSIONS

Treatment of rabbit corneas with 0.2% genipin showed minimal toxicity toward keratocytes and endothelial cells. Genipin is safer than UVA-CXL for crosslinking of thin corneas.

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.

Hydrogels for ocular drug delivery and tissue engineering

Hydrogels, as crosslinked polymeric three dimensional networks, possess unique structure and behavior in response to the internal and/or external stimuli. As a result, they offer great prospective applications in drug delivery, cell therapy and human tissue engineering. Here, we highlight the potential of hydrogels in prolonged intraocular drug delivery and ocular surface therapy using stem cells incorporated hydrogels.

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.

Use of Mesothelial Cells and Biological Matrices for Tissue Engineering of Simple Epithelium Surrogates

Tissue-engineering technologies have progressed rapidly through last decades resulting in the manufacture of quite complex bioartificial tissues with potential use for human organ and tissue regeneration. The manufacture of avascular monolayered tissues such as simple squamous epithelia was initiated a few decades ago and is attracting increasing interest. Their relative morphostructural simplicity makes of their biomimetization a goal, which is currently accessible. The mesothelium is a simple squamous epithelium in nature and is the monolayered tissue lining the walls of large celomic cavities (peritoneal, pericardial, and pleural) and internal organs housed inside. Interestingly, mesothelial cells can be harvested in clinically relevant numbers from several anatomical sources and not less important, they also display high transdifferentiation capacities and are low immunogenic characteristics, which endow these cells with therapeutic interest. Their combination with a suitable scaffold (biocompatible, degradable, and non-immunogenic) may allow the manufacture of tailored serosal membranes biomimetics with potential spanning a wide range of therapeutic applications, principally for the regeneration of simple squamous-like epithelia such as the visceral and parietal mesothelium vascular endothelium and corneal endothelium among others. Herein, we review recent research progresses in mesothelial cells biology and their clinical sources. We make a particular emphasis on reviewing the different types of biological scaffolds suitable for the manufacture of serosal mesothelial membranes biomimetics. Finally, we also review progresses made in mesothelial cells-based therapeutic applications and propose some possible future directions.

Ocular surface reconstruction using stem cell and tissue engineering

Most human sensory information is gained through eyesight, and integrity of the ocular surface, including cornea and conjunctiva, is known to be indispensable for good vision. It is believed that severe damage to corneal epithelial stem cells results in devastating ocular surface disease, and many researchers and scientists have tried to reconstruct the ocular surface using medical and surgical approaches. Ocular surface reconstruction via regenerative therapy is a newly developed medical field that promises to be the next generation of therapeutic modalities, based on the use of tissue-specific stem cells to generate biological substitutes and improve tissue functions. The accomplishment of these objectives depends on three key factors: stem cells, which have highly proliferative capacities and longevities; the substrates determining the environmental niche; and growth factors that support them appropriately. This manuscript describes the diligent development of ocular surface reconstruction using tissue engineering techniques, both past and present, and discusses and validates their future use for regenerative therapy in this field.

Tissue Engineering the Cornea: The Evolution of RAFT

Corneal blindness affects over 10 million people worldwide and current treatment strategies often involve replacement of the defective layer with healthy tissue. Due to a worldwide donor cornea shortage and the absence of suitable biological scaffolds, recent research has focused on the development of tissue engineering techniques to create alternative therapies. This review will detail how we have refined the simple engineering technique of plastic compression of collagen to a process we now call Real Architecture for 3D Tissues (RAFT). The RAFT production process has been standardised, and steps have been taken to consider Good Manufacturing Practice compliance. The evolution of this process has allowed us to create biomimetic epithelial and endothelial tissue equivalents suitable for transplantation and ideal for studying cell-cell interactions in vitro.

Characterization of the modified chitosan membrane cross-linked with genipin for the cultured corneal epithelial cells

OBJECTIVES

To modify a chitosan membrane (CM) by cross-linking the chitosan with genipin, a naturally occurring cross-linker extracted from Gardenia jasminoides fructus, with the aim of developing a new cell culture support and to observe the phenotypes of cultured human corneal epithelial cells (HCECs) on genipin-cross-linked chitosan membrane (GCM).

METHODS

We tested the cross-linking characteristics and mechanical strength of the GCM. CMs modified by cross-linking with different concentrations of genipin were prepared to investigate the rate of membrane degradation. The biocompatibility of the GCMs was investigated by determining the viability of HCECs cultured on them in vitro. The morphology of the HCECs cultured on CM or GCM was analyzed by confocal microscopy and scanning electron microscopy (SEM). Immunocytochemical staining was conducted to determine the phenotypes of the cultured cells.

RESULTS

The fixation index of the GCM was 31 ± 3% after treatment of CM with 0.5mM genipin. A stress-strain test showed that the GCM could tolerate three times the mechanical force of noncross-linked CM. The biodegradation rate of GCM was much slower than for CM. A 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay showed that cell viability was not affected by cross-linking with 5.0mM genipin. SEM showed that the cultured HCECs adhered to and grew well on the surface of the GCM. Immunocytochemical staining showed keratin 3 (K3) and connexin 43 (Cx-43) immunoreactive HCECs on the GCM and their proliferative ability was not significantly affected by strong immunoreactivity of Ki-67 and p63 markers.

CONCLUSIONS

GCM has potential as a scaffold for corneal epithelium in ocular surface surgery and greater mechanical strength and slower degradation than unmodified CM.

Response of human limbal epithelial cells to wounding on 3D RAFT tissue equivalents: effect of airlifting and human limbal fibroblasts

Limbal epithelial stem cell deficiency can cause blindness but may be treated by human limbal epithelial cell (hLE) transplantation, normally on human amniotic membrane. Clinical outcomes using amnion can be unreliable and so we have developed an alternative tissue equivalent (TE), RAFT (Real Architecture for 3D Tissue), which supports hLE expansion, and stratification when airlifted. Human limbal fibroblasts (hLF) may be incorporated into RAFT TEs, where they support overlying hLE and improve phenotype. However, the impact of neither airlifting nor hLF on hLE function has been investigated. hLE on RAFT TEs (±hLF and airlifting) were wounded using heptanol and re-epithelialisation (fluorescein diacetate staining), and percentage putative stem cell marker p63α and proliferative marker Ki67 expression (wholemount immunohistochemistry), measured. Airlifted, hLF- RAFT TEs were unable to close the wound and p63α expression was 7 ± 0.2% after wounding. Conversely, non-airlifted, hLF- RAFT TEs closed the wound within 9 days and p63α expression was higher at 22 ± 5% (p < 0.01). hLE on both hLF- and hLF+ RAFT TEs (non-airlifted) closed the wound and p63α expression was 26 ± 8% and 36 ± 3% respectively (ns). Ki67 expression by hLE increased from 1.3 ± 0.5% before wounding to 7.89 ± 2.53% post-wounding for hLF- RAFT TEs (p < 0.01), and 0.8 ± 0.08% to 17.68 ± 10.88% for hLF+ RAFT TEs (p < 0.05), suggesting that re-epithelialisation was a result of proliferation. These data suggest that neither airlifting nor hLF are necessarily required to maintain a functional epithelium on RAFT TEs, thus simplifying and shortening the production process. This is important when working towards clinical application of regenerative medicine products.

Stem cell-based therapy for treating limbal stem cells deficiency: A review of different strategies

The self renewal capability of limbal epithelial stem (LEST) cells is fundamental to the maintenance and healing of corneal epithelium. Limbal stem cell deficiency (LSCD), due to dysfunction or loss of LEST cells, therefore presents as persistent epithelial defects, corneal vascularization, conjunctivalization etc. Stem cell-based therapy, in its simplest form - limbal autograft, has been used successfully for more than a decade. For bilateral LSCD, similar approaches with limbal allografts have been unsuccessful largely due to strong immune rejection. Therefore, as an alternate strategy for treating bilateral LSCD, ex vivo expansion of the remaining LEST cells or autologous stem cells sourced from other potential sites is being explored. Different culture systems (with and without xenobiotic supplements) using substrates like amniotic membrane or fibrin gels have been used successfully for ex vivo LEST cell maintenance and reproduction by imitating the stem cell niche. This paper is organized into sections reviewing the LEST cells, LSCD and various stem cell-based approaches for treating LSCD and discussing future direction and challenges.

Thermally triggered release of a pro-osteogenic peptide from a functionalized collagen-based scaffold using thermosensitive liposomes

Collagen is one of the most attractive materials for the development of matrices for tissue engineering, due to its excellent biocompatibility and non-toxic bioresorption. The present work describes a collagen-based externally controlled drug-eluting scaffold which consists of drug encapsulated thermoresponsive liposomes covalently attached to the surface of a functionalized collagen-based scaffold. The model drug used in this work was PTHrP 107-111, a pentapeptide with pro-osteogenic and antiosteoclastic activity. An osteoconductive collagen-hydroxyapatite scaffold, designed specifically for bone repair, was used as a model scaffold. The results demonstrate that it is possible to modify the kinetics of release of the drug from the scaffold with the application of an external thermal stimulus (42°C, 20min). In vitro studies carried out with pre-osteoblastic MC3T3-E1 cells demonstrated that neither the attachment of liposomes to the surface of the scaffolds nor the hyperthermic pulse negatively affected the ability of cells to attach and proliferate on the scaffolds. Importantly, the on-demand release of PTHrP 107-111 had a pro-osteogenic effect, as shown by the enhancement of alkaline phosphatase activity, an early osteogenic marker, which correlated with increased expression of the osteogenic genes osteopontin and osteocalcin. In conclusion, the scaffold-based release system developed in this study has immense potential for tuning the delivery of a diverse range of drugs which can be applied for the regeneration of a variety of tissue types.

Gelatin-Based Materials in Ocular Tissue Engineering

Gelatin has been used for many years in pharmaceutical formulation, cell culture and tissue engineering on account of its excellent biocompatibility, ease of processing and availability at low cost. Over the last decade gelatin has been extensively evaluated for numerous ocular applications serving as cell-sheet carriers, bio-adhesives and bio-artificial grafts. These different applications naturally have diverse physical, chemical and biological requirements and this has prompted research into the modification of gelatin and its derivatives. The crosslinking of gelatin alone or in combination with natural or synthetic biopolymers has produced a variety of scaffolds that could be suitable for ocular applications. This review focuses on methods to crosslink gelatin-based materials and how the resulting materials have been applied in ocular tissue engineering. Critical discussion of recent innovations in tissue engineering and regenerative medicine will highlight future opportunities for gelatin-based materials in ophthalmology.

Stem cell-based therapy for corneal epithelial reconstruction: present and future

Limbal stem cell deficiency is a painful and potentially blinding disease. Cultured limbal epithelial transplantation (CLET) is frequently performed for corneal surface reconstruction with variable clinical success. This work summarizes recent developments and trends that have the potential to increase safety and efficacy of CLET in the future. Apart from gradual transition to xenobiotic-free culture systems, novel biofunctional scaffolds presenting components of stem cell microenvironments aim at promoting long-term maintenance of stem cells in vitro and after transplantation. Hair follicles and other tissues may serve as autologous sources of adult stem cells in bilateral ocular surface disease. However, despite all progress made in the fields of tissue engineering and cell therapy, it is unlikely that CLET will yield fully satisfactory clinical results until the factors that govern limbal stem cell maintenance and differentiation are identified.

Biocompatibility of genipin and glutaraldehyde cross-linked chitosan materials in the anterior chamber of the eye

Chitosan is a naturally occurring cationic polysaccharide and has attracted much attention in the past decade as an important ophthalmic biomaterial. We recently demonstrated that the genipin (GP) cross-linked chitosan is compatible with human retinal pigment epithelial cells. The present work aims to further investigate the in vivo biocompatibility of GP-treated chitosan (GP-chi group) by adopting the anterior chamber of a rabbit eye model. The glutaraldehyde (GTA) cross-linked samples (GTA-chi group) were used for comparison. The 7-mm-diameter membrane implants made from either non-cross-linked chitosan or chemically modified materials with a cross-linking degree of around 80% were inserted in the ocular anterior chamber for 24 weeks and characterized by slit-lamp and specular microscopic examinations, intraocular pressure measurements, and corneal thickness measurements. The interleukin-6 expressions at mRNA level were also detected by quantitative real-time reverse transcription polymerase chain reaction. Results of clinical observations showed that the overall ocular scores in the GTA-chi groups were relatively high. In contrast, the rabbits bearing GP-chi implants in the anterior chamber of the eye exhibited no signs of ocular inflammation. As compared to the non-cross-linked counterparts, the GP-chi samples improved the preservation of corneal endothelial cell density and possessed better anti-inflammatory activities, indicating the benefit action of the GP cross-linker. In summary, the intracameral tissue response to the chemically modified chitosan materials strongly depends on the selection of cross-linking agents.