Potential for combined delivery of riboflavin and all-trans retinoic acid, from silk fibroin for corneal bioengineering

Bioengineered Silk Fibroin Hydrogel Reinforced with Collagen-Like Protein Chimeras for Improved Wound Healing

The study investigates the potentials of the rapid crosslinking hydrogel concoction comprising of natural silk fibroin (SF) and recombinant tailorable collagen-like protein with binding domains for wound repair. The formation of dityrosine crosslinks between the tyrosine moieties augments the formation of stable hydrogels, in the presence of the cytocompatible photo-initiator riboflavin and visible light. This uniquely engineered PASCH (Photo-activated silk fibroin and tailor-made collagen-like protein hydrogel) confers the key advantage of improved biological properties over the control hydrogels comprising only of SF. The physico-chemical characterization of the hydrogels with respect to crosslinking, modulus, and thermal stability delineates the ascendancy of PASCH 7:3 over other combinations. Furthermore, the hybrid protein hydrogel proves to be a favorable cellular matrix as it enhances cell adhesion, elongation, growth, and proliferation in vitro. Time-lapse microscopy studies reveal an enhanced wound closure in human endothelial cell monolayer (EA.hy926), while the gene expression studies portray the dynamic interplay of cytokines and growth factors in the wound milieu facilitating the repair and regeneration of cells, sculpted by the proteins. The results demonstrate the improved physical and biological properties of fabricated PASCH, depicting their synergism, and implying their competency for use in tissue engineering applications.

Use of photosensitive molecules in the crosslinking of biopolymers: applications and considerations in biomaterials development

The development of diverse types of biomaterials has significantly contributed to bringing new biomedical strategies to treat clinical conditions. Applications of these biomaterials can range from mechanical support and protection of injured tissues to joint replacement, tissue implants, and drug delivery systems. Among the strategies commonly used to prepare biomaterials, the use of electromagnetic radiation to initiate crosslinking stands out. The predominance of photo-induced polymerization methods relies on a fast, efficient, and straightforward process that can be easily adjusted to clinical needs. This strategy consists of irradiating the components that form the material with photons in the near ultraviolet-visible wavelength range (i.e., ∼310 to 750 nm) in the presence of a photoactive molecule. Upon photon absorption, photosensitive molecules can generate excited species that initiate photopolymerization through different reaction mechanisms. However, this process could promote undesired side reactions depending on the target zone or treatment type (e.g., oxidative stress and modification of biomolecules such as proteins and lipids). This review explores the basic concepts behind the photopolymerization process of ex situ and in situ biomaterials. Particular emphasis was put on the photosensitization initiated by the most employed photosensitizers and the photoreactions that they mediate in aqueous media. Finally, the undesired oxidation reactions at the bio-interface and potential solutions are presented.

Cross-Linking Methods of the Silk Protein Hydrogel in Oral and Craniomaxillofacial Tissue Regeneration

BACKGROUND

Craniomaxillofacial tissue defects are clinical defects involving craniomaxillofacial and oral soft and hard tissues. They are characterized by defect-shaped irregularities, bacterial and inflammatory environments, and the need for functional recovery. Conventional clinical treatments are currently unable to achieve regeneration of high-quality oral craniomaxillofacial tissue. As a natural biomaterial, silk fibroin (SF) has been widely studied in biomedicine and has broad prospects for use in tissue regeneration. Hydrogels made of SF showed excellent water retention, biocompatibility, safety and the ability to combine with other materials.

METHODS

To gain an in-depth understanding of the current development of SF, this article reviews the structure, preparation and application prospects in oral and craniomaxillofacial tissue regenerative medicine. It first briefly introduces the structure of SF and then summarizes the principles, advantages and disadvantages of the different cross-linking methods (physical cross-linking, chemical cross-linking and double network structure) of SF. Finally, the existing research on the use of SF in tissue engineering and the prospects of using SF with different cross-linking methods in oral and craniomaxillofacial tissue regeneration are also discussed.

CONCLUSIONS

This review is intended to show the advantages of SF hydrogels in tissue engineering and provides theoretical support for establishing novel and viable silk protein hydrogels for regeneration.

Hydrophilic, Porous, Fiber-Reinforced Collagen-Based Membrane for Corneal Repair

Collagen membrane with outstanding biocompatibility exhibits immense potential in the field of corneal repair and reconstruction, but the poor mechanical properties limit its clinical application. Polycaprolactone (PCL) is a biodegradable polymer widely explored for application in corneal reconstruction due to its excellent mechanical properties, biocompatibility, easy processability, and flexibility. In this study, a PCL/collagen composite membrane with reinforced mechanical properties is developed. The membrane has a strong composite structure with collagen by utilizing a porous and hydrophilic PCL scaffold, maintaining its integrity even after immersion. The suture retention and mechanical tests demonstrate that compared with the pure collagen membrane, the prepared membrane has a greater tensile strength and twice the modulus of elasticity. Further, the suture retention strength is improved by almost two times. In addition, the membrane remains fully intact on the implant bed in an in vitro corneal defect model. Moreover, the membrane can be tightly sutured to a rabbit corneal defect, progressively achieve epithelialization, and remain unchanged during observation. Overall, the PCL/collagen composite membrane is a promising candidate as a suturable corneal restoration material in clinical keratoplasty.

Optimization and evaluation of oxygen-plasma-modified, aligned, poly (Є-caprolactone) and silk fibroin nanofibrous scaffold for corneal stromal regeneration

The shortage of human donor corneas for transplantation necessitates the exploration of tissue engineering approaches to develop corneal substitutes. However, these substitutes must possess the necessary strength, transparency, and ability to regulate cell behaviour before they can be used in patients. In this study, we investigated the effectiveness of an oxygen plasma surface-modified poly-ε-caprolactone (PCL) combined with silk fibroin (SF) nanofibrous scaffold for corneal stromal regeneration. To fabricate the electrospun scaffolds, PCL and SF blends were used on a rotating mandrel. The optimization of the blend aimed to replicate the structural and functional properties of the human cornea, focusing on nanofibre alignment, mechanical characteristics, and in vitro cytocompatibility with human corneal stromal keratocytes. Surface modification of the scaffold resulted in improved transparency and enhanced cell interaction. Based on the evaluation, a composite nanofibrous scaffold with a 1:1 blend of PCL and SF was selected for a more comprehensive analysis. The biological response of keratocytes to the scaffold was assessed through cellular adhesion, proliferation, cytoskeletal organization, gene expression, and immunocytochemical staining. The scaffold facilitated the adhesion of corneal stromal cells, supporting cell proliferation, maintaining normal cytoskeletal organization, and promoting increased expression of genes associated with healthy corneal stromal keratocytes. These findings highlight the potential of a surface-modified PCL/SF blend (1:1) as a promising scaffolding material for corneal stromal regeneration. The developed scaffold not only demonstrated favourable biological interactions with corneal stromal cells but also exhibited characteristics aligned with the requirements for successful corneal tissue engineering. Further research and refinement of these constructs could lead to significant advancements in addressing the shortage of corneas for transplantation, ultimately improving the treatment outcomes for patients in need.

Silk fibroin based interpenetrating network hydrogel for corneal stromal regeneration

There is a need to develop tissue engineering based approaches to address the shortage of donor corneas worldwide for transplantation. To do this a novel approach to fabricate three-dimensional hydrogels using free-radical polymerization was investigated to generate constructs for corneal stromal tissue regeneration. Different ratios of silk fibroin (SF) to polyacrylamide (PA) were used to fabricate semi-interpenetrating hydrogels. Scanning electron micrograph displayed the interconnectivity of pores within the fabricated hydrogels. Pore sizes ranged from 25 to 66 μm. Scaffolds with increasing concentration of SF had enhanced β-sheet structure (verified by Fourier transform infrared spectroscopy). The biological response of human corneal stromal cells to these hydrogels was examined using cellular adhesion, proliferation, cytoskeleton organization, gene expression and immunocytochemical analysis. The fabricated hydrogels possess rapid gelation (∼3 min) at 37 °C, 84 % porosity facilitating keratocyte migration during healing, improved cellular adhesion and no cytotoxicity, indicating their efficiency for in-situ corneal tissue regeneration. Presence of SF in semi-interpenetrating network hydrogel enhanced cellular proliferation, elevated GAG deposition, and increased expression of keratocyte genes, normally associated with healthy corneal stromal tissue. This study acts as an initial step towards fabricating SF based semi-interpenetrating network hydrogels for developing clinically applicable ocular implants.

Influence of spiral topographies on human limbal-derived immortalized corneal epithelial cells

Cells migrate from the limbus to the corneal epithelium following a centripetal pathway. Corneal epithelial cells tend to orientate in spiral or vortex patterns. However, when cultured in-vitro, limbal derived corneal epithelia do not tend to align or migrate in a spiral pattern. Here, we used soft lithography to create silk fibroin substrates with spiral topographies that direct the human limbal-derived immortalized corneal epithelial cells (hTCEpi) to form a spiral orientation. The impact of this topography on the cells was then characterized. The spiral patterns affected cytoskeletal organization, cell spreading, and nuclei shapes. Spiral width and numbers had a significant impact on proliferation of cells, their focal adhesion, their chromatin condensation, and number of actin filaments. Immunocytochemical staining showed that the spiral pattern enhanced the expression of markers associated with limbal stem cells. The current work illustrates micro spiral patterns can serve to control the nature of limbal derived epithelial cells by providing relevant biophysical cues.

Photocurable antimicrobial silk-based hydrogels for corneal repair

Corneal transplantation is the current gold standard treatment to restore visual acuity to patients with severe corneal diseases and injuries. Due to severe donor tissue shortage, efforts to develop a corneal equivalent have been made but the challenge remains unmet. Another issue of concern in ocular surgery is the difficult instillation and fast drainage of antibiotic ocular eye drops as bacterial infections can jeopardize implant success by delaying or impairing tissue healing. In this study, we developed antimicrobial silk-based hydrogels that have the potential to be photoactivated in situ, fully adapting to the corneal injury shape. Gentamicin-loaded methacrylated-silk (SilkMA) hydrogels were prepared within minutes using low UV intensity (3 mW/cm2 ). SilkMA gels provided a Young's modulus between 21 and 79 kPa together with a light transmittance spectrum and water content (83%-90%) similar to the human cornea. Polymer concentration (15%-25%) was found to offer a tool for tailoring the physical properties of the hydrogels. We confirmed that the methacrylation did not affect the material's in vitro degradation and biocompatibility by observing fibroblast adhesion and proliferation. Importantly, agar diffusion tests showed that the synthesized hydrogels were able to inhibit Staphylococcus aureus and Pseudomonas aeruginosa growth for 72 h. These characteristics along with their injectability and viscoelasticity demonstrate the potential of SilkMA hydrogels to be applied in several soft tissue engineering fields. As such, for the first time we demonstrate the potential of photocurable antimicrobial SilkMA hydrogels as a novel biomaterial to facilitate corneal regeneration.

Intravitreal Injectable Hydrogels for Sustained Drug Delivery in Glaucoma Treatment and Therapy

Glaucoma is extensively treated with topical eye drops containing drugs. However, the retention time of the loaded drugs and the in vivo bioavailability of the drugs are highly influenced before reaching the targeted area sufficiently, due to physiological and anatomical barriers of the eye, such as rapid nasolacrimal drainage. Poor intraocular penetration and frequent administration may also cause ocular cytotoxicity. A novel approach to overcome these drawbacks is the use of injectable hydrogels administered intravitreously for sustained drug delivery to the target site. These injectable hydrogels are used as nanocarriers to intimately interact with specific diseased ocular tissues to increase the therapeutic efficacy and drug bioavailability of the anti-glaucomic drugs. The human eye is very delicate, and is sensitive to contact with any foreign body material. However, natural biopolymers are non-reactive, biocompatible, biodegradable, and lack immunogenic and inflammatory responses to the host whenever they are incorporated in drug delivery systems. These favorable biomaterial properties have made them widely applicable in biomedical applications, with minimal adversity. This review highlights the importance of using natural biopolymer-based intravitreal hydrogel drug delivery systems for glaucoma treatment over conventional methods.

Photocurable GelMA Adhesives for Corneal Perforations

The current treatments for the management of corneal and scleral perforations include sutures and adhesives. While sutures are invasive, induce astigmatism and carry a risk of infection, cyanoacrylate glues are toxic, proinflammatory and form an opaque and rough surface that precludes vision. Consequently, the clinical need for a fast curing and strong tissue adhesive with minimised cytotoxicity and host inflammation remains unmet. In this paper, we engineer a gelatine methacryloyl (GelMA) adhesive that can be crosslinked in situ within 2 min using UV or visible light and a riboflavin (RF)/sodium persulfate (SPS) system. Optical coherence tomography (OCT) images demonstrated that the flowable GelMA adhesive could completely fill corneal wounds and restore the ocular curvature by forming a smooth contour on the ocular surface. Further, ex vivo studies in porcine eyes showed that GelMA bioadhesives exhibited burst pressures that were comparable to cyanoacrylates (49 ± 9 kPa), with the hydrogels exhibiting a transmittance (90%), water content (85%) and storage modulus (5 kPa) similar to the human cornea. Finally, using human dermal fibroblasts, we showed that our GelMA adhesive was non-toxic and could effectively support cell adhesion and proliferation. Taken together, the adhesive’s performance, injectability and ease of administration, together with gelatin’s availability and cost-effectiveness, make it a potential stromal filler or sealant for corneal and conjunctival applications.

Photocurable silk fibroin-based tissue sealants with enhanced adhesive property for the treatment of corneal perforations

Corneal defects are associated with corneal tissue engineering in terms of vision loss. The treatment of corneal defects is an important clinical challenge due to uniform corneal thickness and the...

Methacrylated Silk Fibroin Hydrogels: pH as a Tool to Control Functionality

The last decade has witnessed significant progress in the development of photosensitive polymers for in situ polymerization and 3D printing applications. Light-mediated sol-gel transitions have immense potential for tissue engineering applications as cell-laden materials can be crosslinked within minutes under mild environmental conditions. Silk fibroin (SF) is extensively explored in regenerative medicine applications due to its ease of modification and exceptional mechanical properties along with cytocompatibility. To efficiently design SF materials, the in vivo assembly of SF proteins must be considered. During SF biosynthesis, changes in pH, water content, and metal ion concentrations throughout the silkworm gland divisions drive the transition from liquid silk to its fiber form. Herein, we study the effect of the glycidyl-methacrylate-modified SF (SilkMA) solution pH on the properties and secondary structure of SilkMA hydrogels by testing formulations prepared at pH 5, 7, and 8. Our results demonstrate an influence of the prepolymer solution pH on the hydrogel rheological properties, compressive modulus, optical transmittance, and network swellability. The hydrogel pH did not affect the in vitro viability and morphology of human dermal fibroblasts. This work demonstrates the utility of the solution pH to tailor the SilkMA conformational structure development toward utility and function and shows the need to strictly control the pH to reduce batch-to-batch variability and ensure reproducibility.

Mechanical Properties of Bioengineered Corneal Stroma

For the majority of patients with severe corneal injury or disease, corneal transplantation is the only suitable treatment option. Unfortunately, the demand for donor corneas greatly exceeds the availability. To overcome shortage issues, a myriad of bioengineered constructs have been developed as mimetics of the corneal stroma over the last few decades. Despite the sheer number of bioengineered stromas developed , these implants fail clinical trials exhibiting poor tissue integration and adverse effects in vivo. Such shortcomings can partially be ascribed to poor biomechanical performance. In this review, existing approaches for bioengineering corneal stromal constructs and their mechanical properties are described. The information collected in this review can be used to critically analyze the biomechanical properties of future stromal constructs, which are often overlooked, but can determine the failure or success of corresponding implants.

Photo Cross-linkable Biopolymers for Cornea Tissue Healing

Light can act as an effective and strong agent for the cross-linking of biomaterials and tissues and is recognized as a safe substitute for chemical cross-linkers to modify mechanical and physical properties and promote biocompatibility. This review focuses on the research about cross-linked biomaterials with different radiation sources such as Laser or Ultraviolet (UV) that can be applied as scaffolds, controlled release systems, and tissue adhesives for cornea healing and tissue regeneration.

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.

Silk fibroin safety in the eye: a review that highlights a concern

The biomedical use of silk as a suture dates back to antiquity. Fibroin is the structural element that determines the strength of silk and here we consider the safety of fibroin in its role in ophthalmology. The high mechanical strength of silk meant sufficiently thin threads could be made for eye microsurgery, but such usage was all but superseded by synthetic polymer sutures, primarily because silk in its entirety was more inflammatory. Significant immunological response can normally be avoided by careful manufacturing to provide high purity fibroin, and it has been utilised in this form for tissue engineering an array of fibre and film substrata deployed in research with cells of the eye. Films of fibroin can also be made transparent, which is a required property in the visual pathway. Transparent layers of corneal epithelial, stromal and endothelial cells have all been demonstrated with maintenance of phenotype, as have constructs supporting retinal cells. Fibroin has a lack of demonstrable infectious agent transfer, an ability to be sterilised and prepared with minimal contamination, long-term predictable degradation and low direct cytotoxicity. However, there remains a known ability to be involved in amyloid formation and potential amyloidosis which, without further examination, is enough to currently question whether fibroin should be employed in the eye given its innervation into the brain.

Construction and Evaluation of Collagen-Based Corneal Grafts Using Polycaprolactone To Improve Tension Stress

The emergence of innovative surgical procedures using partial thickness corneal transplant has created a need for the development of corneal grafts to replace pathologic corneal tissue. Corneal repair materials have been successfully prepared in the past 10 years, but they were difficult to be used in clinics because of the unbearable tension caused by interrupted suture during routine surgery. However, polycaprolactone (PCL), a medical polymer material, can solve this problem. Therefore, a hierarchical collagen (Col)-based corneal graft with curvature, consisting of a transparent core part composed of collagen in the center and a mechanically robust fixed part containing collagen and polycaprolactone in the edge, was used as a potential corneal graft for corneal repair and regeneration in this study. The hierarchical collagen-based corneal grafts [collagen-polycaprolactone (Col-PCL) membranes] that are capable of mimicking the native cornea were developed based on chemical and thermal crosslinking mechanisms. The water adsorption of Col-PCL membranes could reach over 80% similar to that of human cornea, and its swelling could reach over 400%. More importantly, the formed Col-PCL membranes could resist a larger tensile strength (1.1 ± 0.03 MPa) before rupturing in comparison with pure collagen membranes and polycaprolactone membranes. Furthermore, the biodegradable Col-PCL membranes could facilitate cell adhesion and proliferation as well as cell migration (exhibiting epithelial wound coverage in <5 days), which showed promise as corneal grafts for cornea tissue engineering.