Tissue engineering: current and future approaches to ocular surface reconstruction

Next-generation nanomaterials: advancing ocular anti-inflammatory drug therapy

Ophthalmic inflammatory diseases, including conjunctivitis, keratitis, uveitis, scleritis, and related conditions, pose considerable challenges to effective management and treatment. This review article investigates the potential of advanced nanomaterials in revolutionizing ocular anti-inflammatory drug interventions. By conducting an exhaustive analysis of recent advancements and assessing the potential benefits and limitations, this review aims to identify promising avenues for future research and clinical applications. The review commences with a detailed exploration of various nanomaterial categories, such as liposomes, dendrimers, nanoparticles (NPs), and hydrogels, emphasizing their unique properties and capabilities for accurate drug delivery. Subsequently, we explore the etiology and pathophysiology of ophthalmic inflammatory disorders, highlighting the urgent necessity for innovative therapeutic strategies and examining recent preclinical and clinical investigations employing nanomaterial-based drug delivery systems. We discuss the advantages of these cutting-edge systems, such as biocompatibility, bioavailability, controlled release, and targeted delivery, alongside potential challenges, which encompass immunogenicity, toxicity, and regulatory hurdles. Furthermore, we emphasize the significance of interdisciplinary collaborations among material scientists, pharmacologists, and clinicians in expediting the translation of these breakthroughs from laboratory environments to clinical practice. In summary, this review accentuates the remarkable potential of advanced nanomaterials in redefining ocular anti-inflammatory drug therapy. We fervently support continued research and development in this rapidly evolving field to overcome existing barriers and improve patient outcomes for ophthalmic inflammatory disorders.

Aspartic acid and epidermal growth factor modified decellularized rabbit conjunctiva for conjunctival reconstruction

Conjunctival reconstruction is an indispensable part of ocular surface regeneration. Decellularized matrix has been considered as an ideal conjunctival substitution for conjunctival reconstruction. In the present study, we report the use of a decellularized rabbit conjunctiva (DRC) for conjunctival reconstruction in the rabbit surgical trauma model. Prepared by the phospholipase A₂ decellularized method, the DRC was nearly DNA free while the collagen structure and natural extracellular matrix (ECM) were well preserved. In order to improve the performance of DRC, aspartic acid (Asp) was used as a spacer arm to crosslink epidermal growth factor (EGF) on the DRC to obtain DRC-Asp-EGF. The conjunctival epithelial cells cultured on the DRC-Asp-EGF showed a higher survival rates and a greater potential to differentiate into conjunctival goblet cells (CGCs) than those on the DRC. Finally, three groups were set to evaluate the transplantation effects in the rabbit surgical trauma model for 28 days: DRC-Asp-EGF group, amniotic membrane (AM) group, and ungrafted group. The DRC-Asp-EGF group was completely re-epithelized, and more CGCs were regenerated than the AM group, while no significant improvements were observed in the ungrafted group. Intact collagen structure, angiogenesis, and no scar formation were also observed in the DRC-Asp-EGF group. These results suggest that DRC-Asp-EGF is a feasible and effective transplant for conjunctival reconstruction and ocular surface regeneration.

[Modern prerequisites for creating a collagen-based artificial analogue of the corneal stroma]

Despite the fact that various collagen biomaterials have been actively used in ophthalmology for more than 30 years, the problem of creating a material that could replace the donor cornea have not been solved. Recent advances in the field of tissue engineering and regenerative medicine have shifted the focus of approaches to solving the problem of creating an artificial cornea towards laying conditions for the restoration of its specific layers through mechanisms of its own cellular regeneration. In this regard, extracellular matrices based on collagen are gaining popularity. This review discusses general limitations and advantages of collagen for creating an artificial cornea.

Surface modified electrospun poly(lactic acid) fibrous scaffold with cellulose nanofibrils and Ag nanoparticles for ocular cell proliferation and antimicrobial application

Corneal and conjunctival infections are common ocular diseases, sometimes, causing severe and refractory drug-resistant bacteria infections. Fungal keratitis is a leading cause for blindness and traditional medical treatment is unsatisfactory. Thus, there is an urge to develop a new therapy to deal with these cases. In this study, we developed surface modified poly(lactic acid) (PLA) electrospun nanofibrous membranes (EFMs) with silver nanoparticles (AgNPs) and cellulose nanofibrils (CNF) as scaffolds for cell proliferation and antimicrobial application. The AgNPs with a very low content (below 0.1%) were easily anchored on the surface of PLA EFMs by CNF, which endowed the scaffold with hydrophilicity and antibacterial ability. The in-vitro cell co-culture experiments showed that the scaffold had great biocompatibility to ocular epithelial cells, especially the scaffolds coated by CNF, which significantly proliferated cells. Furthermore, the antibacterial activity could reach >95% inhibiting Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) due to the implantation of AgNPs, and the antifungal activity was also outstanding with most of the Fusarium spp. inhibited. Hence, the developed PLA EFMs with CNF and AgNPs are promising ocular bandages to promote cell proliferation and kill infectious pathogens, exhibiting potential applications in ocular wound healing in the future.

Exploitation of human mesenchymal stromal cell derived matrix towards the structural and functional restoration of the ocular surface

Decellularized matrix of ADMSCs is a promising conjunctival substitute with superb wound repairing property by promoting proliferation of conjunctival epithelial cells and restoring goblet cells without causing cosmetic differences.

A novel application of electrospun silk fibroin/poly(l-lactic acid-co-ε-caprolactone) scaffolds for conjunctiva reconstruction

Electrospun hybrid nanofibers prepared using combinations of natural and synthetic polymers have been widely investigated in tissue engineering. In this study, silk fibroin (SF) and poly(l-lactic acid-co-ε-caprolactone) (PLCL) hybrid scaffolds were successfully prepared by electrospinning. Scanning electron micrographs (SEM) showed that SF/PLCL scaffolds were composed of defect-free nanofibers with a smooth and homogeneous fiber morphology. Water contact angle measurements demonstrated that the scaffolds were hydrophilic. To assess the cell affinity of SF/PLCL scaffolds, rabbit conjunctival epithelial cells (rCjECs) were cultured on the electrospun scaffolds. Scanning electron micrographs and in vitro proliferation assays showed that the cells adhered and proliferated well on the scaffolds. The quantitative polymerase chain reaction (qPCR) results showed excellent expression of CjEC genes, with reduced expression of inflammatory mediators. Hematoxylin and eosin (H&E) staining showed that the engineered conjunctiva constructed with SF/PLCL scaffolds consisted of 2–4 layers of epithelium. Furthermore, SF/PLCL scaffolds transplanted subcutaneously exhibited excellent biocompatibility. Therefore, SF/PLCL scaffolds may find biomedical applications in conjunctival reconstruction in the near future.

We present a promising scaffold with favorable mechanical and biological properties for conjunctival regeneration.

Long-term Outcomes of Ocular Surface Stem Cell Allograft Transplantation

PURPOSE

To investigate the long-term outcomes of ocular surface stem cell allograft transplantation (OSST) in patients with total limbal stem cell deficiency (LSCD) owing to various etiologies with a follow-up ≥ 5 years.

DESIGN

Retrospective interventional cohort.

METHODS

Setting: Single tertiary referral hospital.

STUDY POPULATION

Patients who had (1) presence of total LSCD, (2) surgical treatment with at least 1 allograft OSST procedure, and (3) minimum follow-up ≥ 5 years after OSST.

INTERVENTION

All patients underwent allograft OSST from March 1998 to June 2009. All patients received systemic immunosuppression.

MAIN OUTCOME MEASURES

Ocular surface stability, best-corrected visual acuity (BCVA).

RESULTS

A total of 165 eyes of 110 patients fulfilled the inclusion criteria with a mean follow-up period of 109.22 ± 35.7 months or approximately 9.1 years (range 5.2-17.7 years). Ocular surface stability was achieved in 72.7% (120/165) of eyes at last follow-up, while 15.2% (25/165) maintained an improved ocular surface and 12.1% (20/165) developed total surface failure. Additional OSST surgery was necessary in 30.9% (51/165 eyes) to maintain a stable ocular surface. There was ≥ 2 lines BCVA improvement in 62.1%, no change in 7.7%, and a worsened BCVA in 18.6% at last follow-up.

CONCLUSIONS

With proper immunosuppression and repeat procedure in case of failure, allograft OSST can provide true long-term ocular surface stability and successful visual outcomes.

Electrospun collagen/poly(L-lactic acid-co-ε-caprolactone) scaffolds for conjunctival tissue engineering

Conjunctival injuries are general but intractable ocular surface diseases, the sequelae of which are particularly challenging to treat. A promising therapy for conjunctival injuries is to employ biodegradable scaffolds to deliver conjunctival epithelial cells for repairing damaged or diseased conjunctiva. In the present study, an ultrathin porous nanofibrous scaffold was fabricated by using collagen and poly(L-lactic acid-co-ε-caprolactone) (PLCL) and displayed a thickness of 20 µm, with a high porosity and an average fiber diameter of 248.83±26.44 nm. Conjunctival epithelial cells seeded on the scaffolds proliferated well and had a high cell viability. Reverse-transcription quantitative PCR showed the expression of conjunctival epithelial cell-specific genes; in addition, there was no significant difference in the inflammatory gene expression between cells grown on collagen/PLCL scaffolds and tricalcium phosphate scaffolds. After co-culture for 2 weeks in vitro, epithelial cell stratification was observed using hematoxylin and eosin staining, exhibiting three to four epithelial-cell layers. In conclusion, these results suggested that collagen/PLCL scaffolds have potential application for repairing conjunctival epithelial coloboma.

A hyaluronan hydrogel scaffold-based xeno-free culture system for ex vivo expansion of human corneal epithelial stem cells

PurposeTo develop a hyaluronan hydrogel scaffold-based xeno-free culture system for ex vivo cultivation of human corneal epithelial stem cells (CESCs).Patients and MethodsCESCs were cultivated from donor limbal explants on the HyStem-C Hydrogel bio-scaffold in 12-well plates for 3 weeks. Group A used the traditional supplemented hormonal epidermal medium (SHEM) and group B used the defined SHEM (without fetal bovine serum and toxin A, adding 20% serum replacement). The growth and morphology of the cultured cells were assessed by phase contrast microscope. The expressions of specific cell markers were assessed by immunofluorescence staining and quantitative real-time PCR (qRT-PCR).ResultsSuccessful cultures of CESCs were obtained in both groups, resulting in multilayered stratified epithelia. Comparing to group A, the cells in group B was grown slightly slower and formed less cellular layers at the end of culture. The corneal specific cytokeratin (K) 12 and differentiation markers, involucrin, and connexin 43, were mainly expressed in the superficial cellular layers in both groups. Interestingly, certain basal cells were immune-positive to proposed stem cell markers such as K19, ABCG2, and integrin β1 in both groups. There was no significant difference between the two groups with regard to the gene expression levels of all these selected corneal markers (all P>0.05).ConclusionsThe hyaluronan hydrogel scaffold-based xeno-free culture system may support the expansion of regenerative CESCs without the risk of xeno component contamination. The regenerated epithelium maintains similar characteristics of native corneal epithelium.

Biomaterials and Tissue Engineering Strategies for Conjunctival Reconstruction and Dry Eye Treatment

The ocular surface is a component of the anterior segment of the eye and is covered by the tear film. Together, they protect the vital external components of the eye from the environment. Injuries, surgical trauma, and autoimmune diseases can damage this system, and in severe cases, tissue engineering strategies are necessary to ensure proper wound healing and recovery. Dry eye is another major concern and a complicated disease affecting the ocular surface. More effective and innovative therapies are required for better outcomes in treating dry eye. This review focuses on the regenerative medicine of the conjunctiva, which is an essential part of the ocular surface system. Features and advances of different types of biomolecular materials, and autologous and allogeneic tissue grafts are summarized and compared. Specifically, vitrigel, a collagen membrane and novel material for use on the ocular surface, offers significant advantages over other biomaterials. This review also discusses a breakthrough microfluidic technology, “organ-on-a-chip” and its potential application in investigating new therapies for dry eye.

Electrospun mat with eyelid fat-derived stem cells as a scaffold for ocular epithelial regeneration

The aim of this study was to develop nanofibrous gelatin substrates for eyelid fat stem cell (EFSC) expansion that can serve as a potential alternative substrate to replace human amniotic membrane. Biocompatibility results indicated that all substrates were highly biocompatible, as EFSCs could favorably attach and proliferate on the nanofibrous surfaces. Microscopic figures showed that the EFSC were firmly anchored to the substrates and were able to retain a normal stem cell phenotype. Immunocytochemistry (ICC) and real time-PCR results revealed change in the expression profile of EFSCs grown on nanofibrous substrates when compared to those grown on control in epithelial induction condition. In addition, electrospun gelatin mats especially oriented scaffold provides not only a milieu supporting EFSCs expansion, but also serves as a useful alternative carrier for ocular surface tissue engineering and could be used as an alternative substrate to amniotic membrane.

Vitrified collagen-based conjunctival equivalent for ocular surface reconstruction

The main functions of the conjunctiva, an essential part of the ocular surface, are to maintain the equilibrium of the tear film and to protect the eye. Upon injuries, the prerequisite to successful ocular surface repair is conjunctival reconstruction. Tissue engineering techniques, including transplantation of autografts, amniotic membranes and numerous synthetic/natural materials, have been developed. However, none of these strategies is completely satisfactory due to lack of goblet cell repopulation, poor mechanical properties or non-standardized preparation procedure. Here, we cultured conjunctival epithelial cells on vitrified collagen membranes and developed a tissue equivalent for repairing damaged conjunctiva. Optimized vitrified collagen has superior mechanical and optical properties to previous biomaterials for ocular surface application, and its unique fibrillar structure significantly benefited conjunctival epithelial cell growth and the phenotypic development in vitro. In a rabbit model, vitrified collagen greatly promoted conjunctival regeneration with rapid re-epithelization, sufficient repopulation of goblet cells and minimized fibrosis and wound contracture, proved by gene expression analyses and histological staining. In conclusion, we have demonstrated the potential suitability of utilizing vitrified collagen-based tissue equivalent in ocular surface reconstruction.

Fabricated micro-nano devices for in vivo and in vitro biomedical applications

In recent years, the innovative use of microelectromechanical systems (MEMSs) and nanoelectromechanical systems (NEMSs) in biomedical applications has opened wide opportunities for precise and accurate human diagnostics and therapeutics. The introduction of nanotechnology in biomedical applications has facilitated the exact control and regulation of biological environments. This ability is derived from the small size of the devices and their multifunctional capabilities to operate at specific sites for selected durations of time. Researchers have developed wide varieties of unique and multifunctional MEMS/NEMS devices with micro and nano features for biomedical applications (BioMEMS/NEMS) using the state of the art microfabrication techniques and biocompatible materials. However, the integration of devices with the biological milieu is still a fundamental issue to be addressed. Devices often fail to operate due to loss of functionality, or generate adverse toxic effects inside the body. The in vitro and in vivo performance of implantable BioMEMS such as biosensors, smart stents, drug delivery systems, and actuation systems are researched extensively to understand the interaction of the BioMEMS devices with physiological environments. BioMEMS developed for drug delivery applications include microneedles, microreservoirs, and micropumps to achieve targeted drug delivery. The biocompatibility of BioMEMS is further enhanced through the application of tissue and smart surface engineering. This involves the application of nanotechnology, which includes the modification of surfaces with polymers or the self-assembly of monolayers of molecules. Thereby, the adverse effects of biofouling can be reduced and the performance of devices can be improved in in vivo and in vitro conditions.

A native-like corneal construct using donor corneal stroma for tissue engineering

Tissue engineering holds great promise for corneal transplantation to treat blinding diseases. This study was to explore the use of natural corneal stroma as an optimal substrate to construct a native like corneal equivalent. Human corneal epithelium was cultivated from donor limbal explants on corneal stromal discs prepared by FDA approved Horizon Epikeratome system. The morphology, phenotype, regenerative capacity and transplantation potential were evaluated by hematoxylin eosin and immunofluorescent staining, a wound healing model, and the xeno-transplantation of the corneal constructs to nude mice. An optically transparent and stratified epithelium was rapidly generated on donor corneal stromal substrate and displayed native-like morphology and structure. The cells were polygonal in the basal layer and became flattened in superficial layers. The epithelium displayed a phenotype similar to human corneal epithelium in vivo. The differentiation markers, keratin 3, involucrin and connexin 43, were expressed in full or superficial layers. Interestingly, certain basal cells were immunopositive to antibodies against limbal stem/progenitor cell markers ABCG2 and p63, which are usually negative in corneal epithelium in vivo. It suggests that this bioengineered corneal epithelium shared some characteristics of human limbal epithelium in vivo. This engineered epithelium was able to regenerate in 4 days following from a 4mm-diameter wound created by a filter paper soaked with 1 N NaOH. This corneal construct survived well after xeno-transplantation to the back of a nude mouse. The transplanted epithelium remained multilayer and became thicker with a phenotype similar to human corneal epithelium. Our findings demonstrate that natural corneal stroma is an optimal substrate for tissue bioengineering, and a native-like corneal construct has been created with epithelium containing limbal stem cells. This construct may have great potential for clinical use in corneal reconstruction.

Ocular surface reconstruction: recent advances and future outlook

PURPOSE OF REVIEW

Ocular surface disorder underlies a diverse group of prevalent diseases in the United States, caused by biological aging, autoimmune conditions, trauma, or iatrogenic factors. Left untreated, these conditions can progress to vision loss or destruction of the globe itself. This review discusses the most recent and relevant clinical and experimental advances in the treatment options for ocular surface disorders.

RECENT FINDINGS

Current literature suggests that recent progress in tissue bioengineering, and molecular and cellular biology research presents many potential interventional therapies for ocular surface diseases. Depending on the pathogenesis of each condition, treatment options include bioengineered amniotic membrane graft, limbal stem cell transplantation, conjunctival and extraocular tissue transplantation, multiagent immunosuppressant therapy, and bioartificial devices such as lacrimal gland microdevices and keratoprostheses, or tissue adhesives.

SUMMARY

Much progress has been made in the fields of microbiology, stem-cell research, tissue engineering, and bioartificial devices for the treatment of the heterogeneous group of ocular surface disorders. Intensive efforts are underway to ensure the adaptation and accessibility of these therapeutic options to the general population.

Silk as Substratum for Cell Attachment and Proliferation

Silk fibroin (SF), isolated from silkworm (Bombyx mori) cocoons, is a natural biodegradable polymer. Over the past decade, there was some interest in using SF as a biomedical material. As part of a project to develop tissue-engineered constructs for the surgical restoration of the ocular surface (cornea, conjunctiva), we have investigated the capacity of SF to function as a substratum for the attachment and growth of corneal stem/progenitor cells harvested from the corneoscleral limbus of donor human corneal tissue. SF membranes were produced from cocoons following a protocol involving successive dissolution steps, filtration, dialysis, evaporation, and methanol treatment. Human limbal epithelial cells were harvested from donor tissue and seeded onto SF membranes. After 5 days, the culture was fixed and stained with specific agents to visualize the cells. The study indicated profuse cellular attachment and growth. SF membranes appear to be suitable as a substratum for the repair of damaged ocular surface.

Transepithelial bioelectrical properties of rabbit acinar cell monolayers on polyester membrane scaffolds

In our quest to develop a tissue-engineered tear secretory system, we have tried to demonstrate active transepithelial ion fluxes across rabbit lacrimal acinar cell monolayers on polyester membrane scaffolds to evaluate the bioelectrical properties of the cultured cells. Purified lacrimal gland acinar cells were seeded onto polyester membrane inserts and cultured to confluency. Morphological properties of the cell monolayers were evaluated by transmission electron microscopy and immunofluorescence staining for Na(+),K(+)-ATPase and the tight junction-associated protein occludin. Sections revealed cell monolayers with well-maintained epithelial cell polarity, i.e., presence of apical (AP) secretory granules, microvilli, and junctional complexes. Na(+),K(+)-ATPase was localized on both the basal-lateral and apical plasma membranes. The presence of tight cell junctions was demonstrated by a positive circumferential stain for occludin. Bioelectrical properties of the cell monolayers were studied in Ussing chambers under short-circuit conditions. Active ion fluxes were evaluated by inhibiting the short-circuit current (I(sc)) with a Na(+),K(+)-ATPase inhibitor, ouabain (100 microM; basal-lateral, BL), and under Cl(-)-free buffer conditions after carbachol stimulation (CCh; 100 microM). The directional apical secretion of Cl(-) was demonstrated through pharmacological analysis, using amiloride (1 mM; BL) and bumetanide (0.1 mM; BL), respectively. Regulated protein secretion was evaluated by measuring the beta-hexosaminidase catalytic activity in the AP culture medium in response to 100 microM basal CCh. In summary, rabbit lacrimal acinar cell monolayers generate a Cl(-)-dependent, ouabain-sensitive AP --> BL I(sc) in response to CCh, consistent with current models for Na(+)-dependent Cl(-) secretion.

Corneal gene therapy

Gene therapy to the cornea can potentially correct inherited and acquired diseases of the cornea. Factors that facilitate corneal gene delivery are the accessibility and transparency of the cornea, its stability ex vivo and the immune privilege of the eye. Initial corneal gene delivery studies characterized the relationship between intraocular modes of administration and location of reporter gene expression. The challenge of achieving effective topical gene transfer, presumably due to tear flow, blinking and low penetration of the vector through epithlelial tight junctions left no alternative but invasive administration to the anterior chamber and corneal stroma. DNA vaccination, RNA interference and gene transfer of cytokines, growth factors and enzymes modulated the corneal microenvironment. Positive results were obtained in preclinical studies for prevention and treatment of corneal graft rejection, neovascularization, haze and herpetic stromal keratitis. These studies, corneal gene delivery systems and modes of administration, and considerations regarding the choice of animal species used are the focus of this review. Opportunities in the field of corneal gene therapy lie in expanding the array of corneal diseases investigated and in the implementation of recent designs of safer vectors with reduced immunogenicity and longer duration of gene expression.