Nanoscale modification of porous gelatin scaffolds with chondroitin sulfate for corneal stromal tissue engineering

Sulfated polysaccharide as biomimetic biopolymers for tissue engineering scaffolds fabrication: Challenges and opportunities

Sulfated polysaccharides play important roles in tissue engineering applications because of their high growth factor preservation ability and their native-like biological features. There are different sulfated polysaccharides based on different repeating units in the carbohydrate backbone, the position of the sulfate group, and the sulfation degree of the polysaccharide. These led to various sulfated polymers with different negative charge densities and resultant structure-property relationships. Since numerous reports are presented related to sulfated polysaccharide applications in tissue engineering, it is crucial to review the role of effective physicochemical and biological parameters in their usage; as well as their structure-property relationships. Within this review, we focused on the effect of naturally occurring and synthetic sulfated polysaccharides in tissue engineering applications reported in the last years, highlighting the challenges of the scaffold fabrication process, the position, and the degree of sulfate on biomedical activity. Additionally, we discussed their use in numerous in vitro and in vivo model systems.

Development of 3D Printable Gelatin Methacryloyl/Chondroitin Sulfate/Hyaluronic Acid Hydrogels as Implantable Scaffolds

The development of biomaterials tailored for various tissue engineering applications has been increasingly researched in recent years; however, stimulating cells to synthesise the extracellular matrix (ECM) is still a significant challenge. In this study, we investigate the use of ECM-like hydrogel materials composed of Gelatin methacryloyl (GelMA) and glycosaminoglycans (GAG), such as hyaluronic acid (HA) and chondroitin sulphate (CS), to provide a biomimetic environment for tissue repair. These hydrogels are fully characterised in terms of physico-chemical properties, including compression, swelling behaviour, rheological behaviour and via 3D printing trials. Furthermore, porous scaffolds were developed through freeze drying, producing a scaffold morphology that better promotes cell proliferation, as shown by in vitro analysis with fibroblast cells. We show that after cell seeding, freeze-dried hydrogels resulted in significantly greater amounts of DNA by day 7 compared to the GelMA hydrogel. Furthermore, freeze-dried constructs containing HA or HA/CS were found to have a significantly higher metabolic activity than GelMA alone.

Impact of Composition and Autoclave Sterilization on the Mechanical and Biological Properties of ECM-Mimicking Cryogels

Cryogels represent a valid strategy as scaffolds for tissue engineering. In order to adequately support adhesion and proliferation of anchorage-dependent cells, different polymers need to be combined within the same scaffold trying to mimic the complex features of a natural extracellular matrix (ECM). For this reason, in this work, gelatin (Gel) and chondroitin sulfate (CS), both functionalized with methacrylic groups to produce CSMA and GelMA derivatives, were selected to prepare cryogel networks. Both homopolymer and heteropolymer cryogels were produced, via radical crosslinking reactions carried out at -12 °C for 2 h. All the scaffolds were characterized for their mechanical, swelling and morphological properties, before and after autoclave sterilization. Moreover, they were evaluated for their biocompatibility and ability to support the adhesion of human gingival fibroblasts and tenocytes. GelMA-based homopolymer networks better withstood the autoclave sterilization process, compared to CSMA cryogels. Indeed, GelMA cryogels showed a decrease in stiffness of approximately 30%, whereas CSMA cryogels of approximately 80%. When GelMA and CSMA were blended in the same network, an intermediate outcome was observed. However, the hybrid scaffolds showed a general worsening of the biological performance. Indeed, despite their ability to withstand autoclave sterilization with limited modification of the mechanical and morphological properties, the hybrid cryogels exhibited poor cell adhesion and high LDH leakage. Therefore, not only do network components need to be properly selected, but also their combination and ability to withstand effective sterilization process should be carefully evaluated for the development of efficient scaffolds designed for tissue engineering purposes.

Kuragel: A biomimetic hydrogel scaffold designed to promote corneal regeneration

Cornea-related injuries are the most common cause of blindness worldwide. Transplantation remains the primary approach for addressing corneal blindness, though the demand for donor corneas outmatches the supply by millions. Tissue adhesives employed to seal corneal wounds have shown inefficient healing and incomplete vision restoration. We have developed a biodegradable hydrogel - Kuragel, with the ability to promote corneal regeneration. Functionalized gelatin and hyaluronic acid form photo-crosslinkable hydrogel with transparency and compressive modulus similar to healthy human cornea. Kuragel composition was tuned to achieve sufficient adhesive strength for sutureless integration to host tissue, with minimal swelling post-administration. Studies in the New Zealand rabbit mechanical injury model affecting corneal epithelium and stroma demonstrate that Kuragel efficiently promotes re-epithelialization within 1 month of administration, while stroma and sub-basal nerve plexus regenerate within 3 months. We propose Kuragel as a regenerative treatment for patients suffering from corneal defects including thinning, by restoration of transparency and thickness.

Introducing Dynamicity: Engineering Stress Relaxation Into Hydrogels Via Thiol-Ene Modified Alginate for Mechanobiological in vitro Modeling of the Cornea

Developing biomaterials for corneal repair and regeneration is crucial for maintaining clear vision. The cornea, a specialized tissue, relies on corneal keratocytes, that respond to their mechanical environment. Altering stiffness affects keratocyte behavior, but static stiffness alone cannot capture the dynamic properties of in vivo tissue. This study proposes that the cornea exhibits time-dependent mechanical properties, similar to other tissues, and aims to replicate these properties in potential therapeutic matrices. First, the cornea's stress relaxation properties are investigated using nanoindentation, revealing 15% relaxation within 10 seconds. Hydrogel dynamicity is then modulated using a specially formulated alginate-PEG and alginate-norbornene mixture. The tuning of the hydrogel's dynamicity is achieved through a photoinitiated norbornene-norbornene dimerization reaction, resulting in relaxation times ranging from 30 seconds to 10 minutes. Human primary corneal keratocytes are cultured on these hydrogels, demonstrating reduced αSMA (alpha smooth muscle actin) expression and increased filopodia formation on slower relaxing hydrogels, resembling their native phenotype. This in vitro model can enable the optimization of stress relaxation for various cell types, including corneal keratocytes, to control tissue formation. Combining stress relaxation optimization with stiffness assessment provides a more accurate tool for studying cell behavior and reduces mechanical mismatch with native tissues in implanted constructs.

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.

A Review of Chondroitin Sulfate's Preparation, Properties, Functions, and Applications

Chondroitin sulfate (CS) is a natural macromolecule polysaccharide that is extensively distributed in a wide variety of organisms. CS is of great interest to researchers due to its many in vitro and in vivo functions. CS production derives from a diverse number of sources, including but not limited to extraction from various animals or fish, bio-synthesis, and fermentation, and its purity and homogeneity can vary greatly. The structural diversity of CS with respect to sulfation and saccharide content endows this molecule with distinct complexity, allowing for functional modification. These multiple functions contribute to the application of CS in medicines, biomaterials, and functional foods. In this article, we discuss the preparation of CS from different sources, the structure of various forms of CS, and its binding to other relevant molecules. Moreover, for the creation of this article, the functions and applications of CS were reviewed, with an emphasis on drug discovery, hydrogel formation, delivery systems, and food supplements. We conclude that analyzing some perspectives on structural modifications and preparation methods could potentially influence future applications of CS in medical and biomaterial research.

Preparation and Characterization of a Photo-Crosslinked Methacryloyl-Collagen Composite Film to Promote Corneal Nerve Regeneration via Surface Grafting of Taurine Molecules

Blindness is frequently caused by corneal abnormalities, and corneal transplantation is the most effective treatment method. It is extremely important to develop high-quality artificial corneas because there are not enough donor corneas accessible for cornea transplantation. One of the most-often utilized materials is collagen, which is the primary component of natural cornea. Collagen-based corneal repair materials have good physicochemical properties and excellent biocompatibility, but how to promote the regeneration of the corneal nerve after keratoplasty is still a big challenge. In this research, in order to promote the growth of nerve cells on a collagen (Col) substrate, a novel collagen-based material was synthesized starting from the functionalization of collagen with unsaturated methacryloyl groups that three-dimensionally photopolymerize to a 3D network of chemically crosslinked collagen (ColMA), onto which taurine molecules were eventually grafted (ColMA-Tr). The physicochemical properties and biocompatibility of the Col, ColMA and ColMA-Tr films were evaluated. By analyzing the results, we found that all the three samples had good moisture retention and aq high covalent attachment of methacryloyl groups followed by their photopolymerization improved the mechanical properties of the ColMA and ColMA-Tr. Most importantly, compared with ColMA, the taurine-modified collagen-MA film significantly promoted the growth of nerve cells and corneal epithelial cells on its surface. Our preliminary results suggest that this novel ColMA-Tr film may have potential use in cornea tissue engineering in the future.

Preparation and Physicochemical Characterization of Gelatin-Aldehyde Derivatives

The present study aimed at preparing novel free-radical scavenging and water-soluble compounds derived from gelatin. Specifically, gelatin−syringaldehyde, gelatin−anisaldehyde, and gelatin−vanillin were synthesized and thoroughly studied for their physicochemical properties. In particular, the compounds were characterized by UV-Vis spectroscopy, Fourier-transform infrared spectroscopy, and scanning electron microscopy. Notably, as demonstrated by thermogravimetry and differential scanning calorimetry, all three derivatives exhibited higher thermal stability than gelatin itself. Free-radical scavenging activities of the examined compounds were explored by (i) a standard spectrophotometric ABTS assay and (ii) an assay of oxidative degradation of hyaluronic acid monitored by rotational viscometry. We found that gelatin and gelatin−syringaldehyde demonstrated the highest efficacy in scavenging •OH radicals, whereas gelatin−anisaldehyde was the least effective. The efficacy of scavenging alkyloxy- and alkylperoxy-type free radicals via hydrogen-atom-transferring property was in the following order: gelatin > gelatin−vanillin > gelatin−syringaldehyde > gelatin−anisaldehyde. Electron-donor properties determined using the ABTS assay revealed the following order in one-electron reduction of ABTS•+: gelatin > gelatin−anisaldehyde > gelatin−vanillin > gelatin−syringaldehyde.

3D printed gelatin/decellularized bone composite scaffolds for bone tissue engineering: Fabrication, characterization and cytocompatibility study

Three-dimensional (3D) printing technology enables the design of personalized scaffolds with tunable pore size and composition. Combining decellularization and 3D printing techniques provides the opportunity to fabricate scaffolds with high potential to mimic native tissue. The aim of this study is to produce novel decellularized bone extracellular matrix (dbECM)-reinforced composite-scaffold that can be used as a biomaterial for bone tissue engineering. Decellularized bone particles (dbPTs, ∼100 ​μm diameter) were obtained from rabbit femur and used as a reinforcement agent by mixing with gelatin (GEL) in different concentrations. 3D scaffolds were fabricated by using an extrusion-based bioprinter and crosslinking with microbial transglutaminase (mTG) enzyme, followed by freeze-drying to obtain porous structures. Fabricated 3D scaffolds were characterized morphologically, mechanically, and chemically. Furthermore, MC3T3-E1 mouse pre-osteoblast cells were seeded on the dbPTs reinforced GEL scaffolds (GEL/dbPTs) and cultured for 21 days to assess cytocompatibility and cell attachment. We demonstrate the 3D-printability of dbPTs-reinforced GEL hydrogels and the achievement of homogenous distribution of the dbPTs in the whole scaffold structure, as well as bioactivity and cytocompatibility of GEL/dbPTs scaffolds. It was shown that Young's modulus and degradation rate of scaffolds were enhanced with increasing dbPTs content. Multiphoton microscopy imaging displayed the interaction of cells with dbPTs, indicating attachment and proliferation of cells around the particles as well as into the GEL-particle hydrogels. Our results demonstrate that GEL/dbPTs hydrogel formulations have potential for bone tissue engineering.

Fabrication and In Vitro Evaluation of A Chondroitin Sulphate-Polycaprolactone Composite Nanofibrous Scaffold for Potential Use in Dermal Tissue Engineering

OBJECTIVE

Poly(ε-caprolactone) (PCL) has considerable mechanical and biological properties that make it a good candidate for tissue engineering applications. PCL alongside proteins and polysaccharides, like gelatin (GEL) and chondroitin sulphate (CS), can be used to fabricate composite scaffolds that provide mechanical and biological requirements for skin tissue engineering scaffolds. The aim of this study was fabricating novel composite nanofibrous scaffold containing various ratios of GEL/CS and PCL using co-electrospinning process.

MATERIALS AND METHODS

In this experimental study, PCL mixed with a GEL/CS blend has limitations in miscibility and the lack of a common solvent. Here, we electrospun PCL and GEL/CS coincide separately on the same drum by using different nozzles to create composite nanofibrous scaffolds with different ratios (2:1, 1:1 and 1:2) of GEL to CSPCL, and we mixed them at the micro/nanoscale. Morphology, porosity, phosphate-buffered saline (PBS) absorption, chemical structure, mechanical property and in vitro bioactivity of the prepared composite scaffolds were analysed.

RESULTS

Scanning electron microscopy (SEM) images showed beadless nanofibres at all ratios of GEL to CS-PCL. The composite scaffolds (2:1, 1:1 and 1:2) had increased porosity compared to the PCL nanofibrous scaffolds, in addition to a significant increase in PBS absorption. The mechanical properties of the composite scaffolds were investigated under different conditions. The results demonstrated that all of the composite specimens had better strength when compared with the GEL/CS nanofibres. The increase in PCL ratio led to an increase in tensile strength of the nanofibres. Human dermal fibroblasts (HDF) were cultured on the fabricated composite scaffolds and evaluated by 3-(4,5-dimethylthiazol- 2-yl)-5-(3 carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) analysis and SEM. The results showed the bioactivity of these nanofibres and the potential for these scaffolds to be used for skin tissue engineering applications.

CONCLUSION

The fabricated co-electrospun composite scaffolds had higher porosity and PBS absorption in comparison with the PCL nanofibrous scaffolds, in addition to significant improvements in mechanical properties under wet and dry conditions compared to the GEL/CS scaffold.

Photo-cross-linked Gelatin Glycidyl Methacrylate/N-Vinylpyrrolidone Copolymeric Hydrogel with Tunable Mechanical Properties for Ocular Tissue Engineering Applications

Corneal transplantation is currently the primary treatment for corneal blindness. However, severe global scarcity of donor corneas is driving the scientific community to find novel solutions. One potential solution is to replace the damaged tissue with a biocompatible artificial cornea. Here, gelatin glycidyl methacrylate (GM) and N-vinylpyrrolidone (VP) were cocrosslinked to afford a hybrid bicomponent copolymeric hydrogel with excellent mechanical, structural, and biological properties. Our studies showed that the GM/VP ratio can be adjusted to generate a construct with high tensile modulus and strength of 1.6 and 1.0 MPa, respectively, compared to 14 and 7.5 MPa for human cornea. The construct can tolerate up to 22.4 kPa pressure before retention sutures can tear through it. Due to the presence of a synthetic component, it has a significantly higher stability against collagenase induced degradation, yet it is biocompatible and promotes cellular adhesion, proliferation, and migration under in vitro settings.

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.

A chondroitin sulfate based injectable hydrogel for delivery of stem cells in cartilage regeneration

Chondroitin sulfate (CS), as a popular material for cartilage tissue engineering scaffolds, has been extensively studied and reported for its safety and excellent biocompatibility. However, the rapid degradation of pure CS scaffolds has brought a challenge to regenerate neo-tissue similar to natural articular cartilage effectively. Meanwhile, the poly(ethene glycol) (PEG) -based biopolymer is frequently applied as a structural constituent material because of its remarkable mechanical properties, long-lasting in vivo stability, and hypo-immunity. Here, we report that the combination of CS and hyperbranched multifunctional PEG copolymer (HB-PEG) could synergistically promote cartilage repair. The thiol functionalised CS (CS-SH)/HB-PEG hydrogel scaffolds were fabricated via thiol-ene reaction, which exhibits rapid gelation, excellent mechanical properties and prolonged degradation properties. We found that rat adipose-derived mesenchymal stem cells presented great cell viability and improved chondrogenesis in CS-SH/HB-PEG hydrogels. Moreover, the injectable hydrogel scaffolds reduced stem cell inflammatory response, consistent with the well-documented anti-inflammatory activities of CS.

Tuning gelatin-based hydrogel towards bioadhesive ocular tissue engineering applications

Gelatin based adhesives have been used in the last decades in different biomedical applications due to the excellent biocompatibility, easy processability, transparency, non-toxicity, and reasonable mechanical properties to mimic the extracellular matrix (ECM). Gelatin adhesives can be easily tuned to gain different viscoelastic and mechanical properties that facilitate its ocular application. We herein grafted glycidyl methacrylate on the gelatin backbone with a simple chemical modification of the precursor, utilizing epoxide ring-opening reactions and visible light-crosslinking. This chemical modification allows the obtaining of an elastic protein-based hydrogel (GELGYM) with excellent biomimetic properties, approaching those of the native tissue. GELGYM can be modulated to be stretched up to 4 times its initial length and withstand high tensile stresses up to 1.95 MPa with compressive strains as high as 80% compared to Gelatin-methacryloyl (GeIMA), the most studied derivative of gelatin used as a bioadhesive. GELGYM is also highly biocompatible and supports cellular adhesion, proliferation, and migration in both 2 and 3-dimensional cell-cultures. These characteristics along with its super adhesion to biological tissues such as cornea, aorta, heart, muscle, kidney, liver, and spleen suggest widespread applications of this hydrogel in many biomedical areas such as transplantation, tissue adhesive, wound dressing, bioprinting, and drug and cell delivery.

Chondroitin Sulfate/Polycaprolactone/Gelatin Electrospun Nanofibers with Antithrombogenicity and Enhanced Endothelial Cell Affinity as a Potential Scaffold for Blood Vessel Tissue Engineering

Electrospun polymer nanofibers have gained much attention in blood vessel tissue engineering. However, conventional nanofiber materials with the deficiencies of slow endothelialization and thrombosis are not effective in promoting blood vessel tissue repair and regeneration. Herein, biomimetic gelatin (Gt)/polycaprolactone (PCL) composite nanofibers incorporating a different amount of chondroitin sulfate (CS) were developed via electrospinning technology to investigate their effects on antithrombogenicity and endothelial cell affinity. Varying CS concentrations in PG nanofibers affects fiber morphology and diameter. The CS/Gt/PCL nanofibers have suitable porosity (~ 80%) and PBS solution absorption (up to 650%). The introduction of CS in Gt/PCL nanofibers greatly enhances their anticoagulant properties, prolongs their coagulation time, and facilitates cell responses. Particularly, 10%CS/Gt/PCL nanofibers display favorable cell attachment, elongation, and proliferation. Thus, the Gt/PCL nanofibers containing a certain amount of CS could be excellent candidates as a promising tissue-engineering scaffold in blood vessel repair and regeneration.

Micro-mechanical properties of corneal scaffolds from two different bio-models obtained by an efficient chemical decellularization

The present study elucidates the impact of detergent-based chemical decellularization on the micro-mechanical properties of porcine and rabbit corneas for the purpose of extracellular matrix (ECM) derived scaffolds. Aiming to optimize the decellularization process, different concentrations of Sodium Dodecyl Sulfate (SDS), Triton X-100 and CHAPS detergents were assessed on their ability to decellularize corneas from both bio-models at incubation periods of 12 and 24h. We evaluated the effect of decellularization on corneal ECM Young's Modulus and various area's roughness parameters (topography features) at a microscale by using Atomic Force Microscopy (AFM). Only SDS presented adequate decellularization properties at the selected concentrations (0.2, 0.5 and 1%) and incubation periods. All topography features displayed by native corneas were preserved after SDS treatments, while no statistically significant differences were identified for the average value of Young's Modulus between the control samples and those treated with 0.2% SDS (rabbit corneas) and 0.5% SDS (porcine corneas) after 12h. In this sense, cornea decellularization procedures can be improved by simultaneously reducing SDS concentration and incubation period. AFM is a useful tool to perform biomechanical analysis of the effect of decellularization on scaffold micro-mechanics. Evaluation of the scaffold mechanical behavior at a microscale could help in understanding cell-scaffold interactions in terms of mechanotransduction, complementing macroscale techniques (e.g. tensile tests) relevant for tissue engineering quality control and decision-making.

Cell sheet technology: Influence of culture conditions on in vitro-cultivated corneal stromal tissue for regenerative therapies of the ocular surface

The in vitro reconstruction of stromal tissue by long-term cultivation of corneal fibroblasts is a smart approach for regenerative therapies of ocular surface diseases. However, systematic investigations evaluating optimized cultivation protocols for the realization of a biomaterial are lacking. This study investigated the influence of supplements to the culture media of human corneal fibroblasts on the formation of a cell sheet consisting of cells and extracellular matrix. Among the supplements studied are vitamin C, fetal bovine serum, L-glutamine, components of collagen such as L-proline, L-4-hydroxyproline and glycine, and TGF-β1, bFGF, IGF-2, PDGF-BB and insulin. After long-term cultivation, the proliferation, collagen and glycosaminoglycan content and light transmission of the cell sheets were examined. Biomechanical properties were investigated by tensile tests and the ultrastructure was characterized by electron microscopy, small-angle X-ray scattering, antibody staining and ELISA. The synthesis of extracellular matrix was significantly increased by cultivation with insulin or TGF-β1, each with vitamin C. The sheets exhibited a high transparency and suitable material properties. The production of a transparent, scaffold-free, potentially autologous, in vitro-generated construct by culturing fibroblasts with extracellular matrix synthesis-stimulating supplements represents a promising approach for a biomaterial that can be used for ocular surface reconstruction in slowly progressing diseases.

The progress in corneal translational medicine

Cornea tissue is in high demand by tissue donation centres globally, and thus tissue engineering cornea, which is the main topic of corneal translational medicine, can serve as a limitless alternative to a donated human cornea tissue. Tissue engineering aims to produce solutions to the challenges associated with conventional cornea tissue, including transplantation and use of human amniotic membrane (HAM), which have issues with storage and immune rejection in patients. Accordingly, by carefully selecting biomaterials and fabrication methods to produce these therapeutic tissues, the demand for cornea tissue can be met, with an improved healing outcome for recipients with less associated harmful risks. In this review paper, we aim to present the recent advancements in the research and clinical applications of cornea tissue, applications including biomaterial selection, fabrication methods, scaffold structure, cellular response to these scaffolds, and future advancements of these techniques.

Advanced Polymer-Based Drug Delivery Strategies for Meniscal Regeneration

The meniscus plays a critical role in maintaining knee joint homeostasis. Injuries to the meniscus, especially considering the limited self-healing capacity of the avascular region, continue to be a challenge and are often treated by (partial) meniscectomy, which has been identified to cause osteoarthritis. Currently, meniscus tissue engineering focuses on providing extracellular matrix (ECM)-mimicking scaffolds to direct the inherent meniscal regeneration process, and it has been found that various stimuli are essential. Numerous bioactive factors present benefits in regulating cell fate, tissue development, and healing, but lack an optimal delivery system. More recently, bioengineers have developed various polymer-based drug delivery systems (PDDSs), which are beneficial in terms of the favorable properties of polymers as well as novel delivery strategies. Engineered PDDSs aim to provide not only an ECM-mimicking microenvironment but also the controlled release of bioactive factors with release profiles tailored according to the biological concerns and properties of the factors. In this review, both different polymers and bioactive factors involved in meniscal regeneration are discussed, as well as potential candidate systems, with examples of recent progress. This article aims to summarize drug delivery strategies in meniscal regeneration, with a focus on novel delivery strategies rather than on specific delivery carriers. The current challenges and future prospects for the structural and functional regeneration of the meniscus are also discussed. Impact statement Meniscal injury remains a clinical Gordian knot owing to the limited healing potential of the region, restricted surgical approaches, and risk of inducing osteoarthritis. Existing tissue engineering scaffolds that provide mechanical support and a favorable microenvironment also lack biological cues. Advanced polymer-based delivery strategies consisting of polymers incorporating bioactive factors have emerged as a promising direction. This article primarily reviews the types and applications of biopolymers and bioactive factors in meniscal regeneration. Importantly, various carrier systems and drug delivery strategies are discussed with the hope of inspiring further advancements in this field.

Corneal stroma regeneration: Preclinical studies

Corneal grafting is one of the most common and successful forms of human tissue transplantation in the world, but the need for corneal grafting is growing and availability of human corneal donor tissue to fulfill this increasing demand is not assured worldwide. The stroma is responsible for many features of the cornea, including its strength, refractive power and transparency, so enormous efforts have been put into replicating the corneal stroma in the laboratory to find an alternative to classical corneal transplantation. Unfortunately this has not been yet accomplished due to the extreme difficulty in mimicking the highly complex ultrastructure of the corneal stroma, and none of the obtained substitutes that have been assayed has been able to replicate this complexity yet. In general, they can neither match the mechanical properties nor recreate the local nanoscale organization and thus the transparency and optical properties of a normal cornea. In this context, there is an increasing interest in cellular therapy of the corneal stroma using Induced Pluripotent Stem Cells (iPSCs) or mesenchymal stem cells (MSCs) from either ocular or extraocular sources, as they have proven to be capable of producing new collagen within the host stroma, modulate preexisting scars and enhance transparency by corneal stroma remodeling. Despite some early clinical data is already available, in the current article we will summary the available preclinical evidence about the topic corneal stroma regeneration. Both, in vitro and in vivo experiments in the animal model will be shown.

Bioengineering Approaches for Corneal Regenerative Medicine

BACKGROUND

Since the cornea is responsible for transmitting and focusing light into the eye, injury or pathology affecting any layer of the cornea can cause a detrimental effect on visual acuity. Aging is also a reason for corneal degeneration. Depending on the level of the injury, conservative therapies and donor tissue transplantation are the most common treatments for corneal diseases. Not only is there a lack of donor tissue and risk of infection/rejection, but the inherent ability of corneal cells and layers to regenerate has led to research in regenerative approaches and treatments.

METHODS

In this review, we first discussed the anatomy of the cornea and the required properties for reconstructing layers of the cornea. Regenerative approaches are divided into two main categories; using direct cell/growth factor delivery or using scaffold-based cell delivery. It is expected delivered cells migrate and integrate into the host tissue and restore its structure and function to restore vision. Growth factor delivery also has shown promising results for corneal surface regeneration. Scaffold-based approaches are categorized based on the type of scaffold, since it has a significant impact on the efficiency of regeneration, into the hydrogel and non-hydrogel based scaffolds. Various types of cells, biomaterials, and techniques are well covered.

RESULTS

The most important characteristics to be considered for biomaterials in corneal regeneration are suitable mechanical properties, biocompatibility, biodegradability, and transparency. Moreover, a curved shape structure and spatial arrangement of the fibrils have been shown to mimic the corneal extracellular matrix for cells and enhance cell differentiation.

CONCLUSION

Tissue engineering and regenerative medicine approaches showed to have promising outcomes for corneal regeneration. However, besides proper mechanical and optical properties, other factors such as appropriate sterilization method, storage, shelf life and etc. should be taken into account in order to develop an engineered cornea for clinical trials.

Gelatin Promotes Cell Retention Within Decellularized Heart Extracellular Matrix Vasculature and Parenchyma

Introduction

Recellularization of organ decellularized extracellular matrix (dECM) offers a potential solution for organ shortage in allograft transplantation. Cell retention rates have ranged from 10 to 54% in varying approaches for reseeding cells in whole organ dECM scaffolds. We aimed to improve recellularization by using soluble gelatin as a cell carrier to deliver endothelial cells to the coronary vasculature and cardiomyocytes to the parenchyma in a whole decellularized rat heart.

Methods

Rat aortic endothelial cells (RAECs) were perfused over decellularized porcine aorta in low (1%) and high (5%) concentrations of gelatin to assess attachment to a vascular dECM model. After establishing cell viability and proliferation in 1% gelatin, we used 1% gelatin as a carrier to deliver RAECs and neonatal rat cardiomyocytes (NRCMs) to decellularized adult rat hearts. Immediate cell retention in the matrix was quantified, and recellularized hearts were evaluated for visible contractions up to 35 days after recellularization.

Results

We demonstrated that gelatin increased RAEC attachment to decellularized porcine aorta; blocking integrin receptors reversed this effect. In the whole rat heart gelatin (1%) increased retention of both RAECs and NRCMs respectively, compared with the control group (no gelatin). Gelatin was associated with visible contractions of NRCMs within hearts (87% with gelatin vs. 13% control).

Conclusions

Gelatin was an effective cell carrier for increasing cell retention and contraction in dECM. The gelatin-cell-ECM interactions likely mediated by integrin.

Prospects and Challenges of Translational Corneal Bioprinting

Corneal transplantation remains the ultimate treatment option for advanced stromal and endothelial disorders. Corneal tissue engineering has gained increasing interest in recent years, as it can bypass many complications of conventional corneal transplantation. The human cornea is an ideal organ for tissue engineering, as it is avascular and immune-privileged. Mimicking the complex mechanical properties, the surface curvature, and stromal cytoarchitecure of the in vivo corneal tissue remains a great challenge for tissue engineering approaches. For this reason, automated biofabrication strategies, such as bioprinting, may offer additional spatial control during the manufacturing process to generate full-thickness cell-laden 3D corneal constructs. In this review, we discuss recent advances in bioprinting and biomaterials used for in vitro and ex vivo corneal tissue engineering, corneal cell-biomaterial interactions after bioprinting, and future directions of corneal bioprinting aiming at engineering a full-thickness human cornea in the lab.

NIR-Emitting Gold Nanoclusters-Modified Gelatin Nanoparticles as a Bioimaging Agent in Tissue

Gold nanocluster (AuNC) synthesis using a well-distinguished polymer for nanoparticle-mediated drug delivery paves the way for developing efficient theranostics based on pharmaceutically accepted materials. Gelatin-stabilized AuNCs are synthesized and modified by glutathione for tuning the emission spectra. Addition of silver ions enhances the fluorescence, reaching also high quantum yield (26.7%). A simplified model can be proposed describing the nanoclusters' properties-structure relationship based on X-ray photoelectron spectroscopy data and synthesis sequence. Furthermore, these modifications improve fluorescence stability toward pH changes and enzymatic degradation, offering different AuNCs for various applications. The impact of nanocluster formation on gelatin structure integrity is investigated by Fourier transform infrared spectrometry and matrix-assisted laser desorption/ionization time of flight mass spectroscopy, being important to further formulate gelatin nanoparticles (GNPs). The 218 nm-sized NPs show no cytotoxicity up to 600 µg mL^-1 and are imaged in skin, as a challenging autofluorescent tissue, by confocal microscopy, when transcutaneously delivered using dissolving microneedles. Linear unmixing allows simultaneous imaging of AuNCs-GNPs and skin with accurate signal separation. This underlines the great potential for bioimaging of this system to better understand nanomaterials' behavior in tissue. Additionally, it is drug delivery system also potentially serving as a theranostic system.

The Effect of Matrix Stiffness of Biomimetic Gelatin Nanofibrous Scaffolds on Human Cardiac Pericyte Behavior

Congenital heart disease (CHD) is the most common and deadly congenital anomaly, accounting for up to 7.5% of all infant deaths. Survival in children born with CHD has improved dramatically over the past several decades (this positive trend being counterbalanced by the fact that more patients develop heart failure). Seminal data indicate an alteration of the extracellular matrix occurs with time in these hearts due to diffuse and abundant interstitial fibrosis. This results in an escalation in the stiffness of the local myocardial microenvironment. However, the influence of matrix stiffness in regulating the function of resident human stromal cells has not been reported. The objective of this study was to determine the impact of scaffold stiffness on the antigenic and functional profile of cardiac pericytes (CPs) isolated from patients with CHD. To this end, we have first manufactured gelatin nanofibrous scaffolds with varying degrees of stiffness using an in situ cross-linking electrospinning technique in a pure water solvent system. We assessed Young's modulus and performed a comprehensive physicochemical characterization of the scaffolds employing scanning electron microscopy and Fourier transform infrared spectroscopy. We next evaluated the changes induced by a different scaffold stiffness on CP morphology, antigenic profile, viability, proliferation, angiocrine activity, and induced differentiation. Results indicate that soft matrixes with a fiber diameter of ∼400 nm increase CP proliferation, secretion of angiopoietin 2, and F-actin stress fiber formation, without affecting the antigenic profile, viability, or differentiation. These data indicate for the first time that human CPs can be functionally influenced by slight changes in matrix stiffness. The study elucidates the importance of mechanical/morphological cues in modulating the behavior of stromal cells isolated from patients with CHD.

Fabrication and Biocompatibility Evaluation of Nanodiamonds-Gelatin Electrospun Materials Designed for Prospective Tissue Regeneration Applications

Due to the reduced ability of most harmed tissues to self-regenerate, new strategies are being developed in order to promote self-repair assisted or not by biomaterials, among these tissue engineering (TE). Human adipose-derived mesenchymal stem cells (hASCs) currently represent a promising tool for tissue reconstruction, due to their low immunogenicity, high differentiation potential to multiple cell types and easy harvesting. Gelatin is a natural biocompatible polymer used for regenerative applications, while nanodiamond particles (NDs) are used as reinforcing nanomaterial that might modulate cell behavior, namely cell adhesion, viability, and proliferation. The development of electrospun microfibers loaded with NDs is expected to allow nanomechanical sensing due to local modifications of both nanostructure and stiffness. Two aqueous suspensions with 0.5 and 1% w/v NDs in gelatin from cold water fish skin (FG) were used to generate electrospun meshes. Advanced morpho- and micro-structural characterization revealed homogeneous microfibers. Nanoindentation tests confirmed the reinforcing effect of NDs. Biocompatibility assays showed an increased viability and proliferation profile of hASCs in contact with FG_NDs, correlated with very low cytotoxic effects of the materials. Moreover, hASCs developed an elongated cytoskeleton, suggesting that NDs addition to FG materials encouraged cell adhesion. This study showed the FG_NDs fibrous scaffolds potential for advanced TE applications.

Chondroitin sulfate immobilized PCL nanofibers enhance chondrogenic differentiation of mesenchymal stem cells

Cold Atmospheric Plasma (CAP) is used as a promising method in surface modification for immobilization of chondroitin sulfate functional biomacromolecules on PCL nanofibrous substrates for cartilage tissue engineering. The GAG-grafted scaffolds are able to successfully support the attachment and proliferation of mesenchymal stem cells (MSCs). The seeded scaffolds show the chondro-differentiation of MSCs during a 21-days cell culture in a non-differential medium. Expression of SOX9, Collagen10 and Collagen2 proved the chondro-inductive effect of GAG-grafted scaffolds. Besides, no external chondro-genic differential agent was used in the differentiation of MSCs to chondrocyte. The cells passed the last phase of chondrogenesis after 14 days of incubation. Thus, the GAG-fabricated fibrous scaffolds using CAP are potential candidates for cartilage tissue engineering.

Extracellular Matrix Protein Coating of Processed Fish Scales Improves Human Corneal Endothelial Cell Adhesion and Proliferation

Purpose

Corneal transplantation can treat corneal endothelial diseases. Implanting cultivated human corneal endothelial cells (HCECs) via a cell carrier has clinical value as an alternative therapeutic strategy. This study was performed to compare the feasibility of fish scales and other biomaterials (gelatin and chitosan) as cell carriers and to investigate the effects of an extracellular matrix (ECM) protein coating to improve the cytocompatibility of fish scales.

Methods

The physical properties of gelatin, chitosan, and fish scales were compared. Immortalized HCECs (B4G12) were cultured on processed fish scales, which were coated with fibronectin, laminin, collagen IV, or FNC Coating Mix. Cell attachment and proliferation were evaluated by immunofluorescence, cell counting, and bromodeoxyuridine (BrdU) labeling assays. Immunoblots were used to examine the expression levels of integrin-linked kinase (ILK), phosphate-ILK, β-catenin, p63, and cell cycle mediators (cyclin D1 and p27^Kip1).

Results

The transparency of processed fish scales was better than that of chitosan, while the strength was higher than that of gelatin. The laminin, collagen IV, and FNC coatings facilitated B4G12 cell adhesion and proliferation, while fibronectin only facilitated cell adhesion. The laminin, collagen IV, and FNC coatings also upregulated phosphate-ILK and p63 expression. In addition, the FNC coating activated cell cycle mediators.

Conclusion

ECM protein-coated processed fish scales can serve as a novel cell carrier to facilitate the development of HCEC transplantation.

Translational Relevance

Improving the physical properties and cytocompatibility of fish scales as a cell carrier will facilitate the transplantation of HCECs into corneas for the purpose of cell therapy.

Enhanced chondrogenic differentiation of bone marrow mesenchymal stem cells on gelatin/glycosaminoglycan electrospun nanofibers with different amount of glycosaminoglycan

Tissue engineering is a new technique to help damaged cartilage treatment using cells and scaffolds. In this study we tried to evaluate electrospun scaffolds composed of gelatin/glycosaminoglycan (G/GAG) blend nanofibers in chondrogenesis of bone marrow-derived mesenchymal stem cells (BMMSCs). Scaffolds were fabricated by electrospinning technique with different concentration of glycosaminoglycan (0%, 5%, 10%, and 15%) in gelatin matrix. BMMSCs were cultured on the scaffolds for chondrogenesis process. MTT assay was done for scaffold's biocompatibility and cells viability evaluation. Alcian blue staining was carried out to determine the release of GAG and reverse transcription polymerase chain reaction (RT-PCR) was done for expression of COL2A1 and also immunocytochemistry assay were used to confirm expression of type II collagen. Scaffold with 15% GAG showed better result for biocompatibility (p =0.02). Scanning electron microscopy (SEM) micrographs showed that MSCs have good attachment to the scaffolds. Alcian blue staining result confirmed that cells produce GAG during differentiation time different from GAG in the scaffolds. Also the results for RT-PCR showed the expression of COL2A1 marker. Immunocytochemistry assay for type II collagen confirm that this protein expressed. Scaffold comprising 15% GAG is better results for chondrogenesis and it can be a good applicant for cartilage tissue engineering. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 38-48, 2019.

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

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

Non-invasive characterization of hybrid gelatin:poly-l-lactide electrospun scaffolds using second harmonic generation and multiphoton imaging

Hybrid scaffolds composed of synthetic polymers and naturally occurring components have become more relevant in the field of tissue engineering and regenerative medicine. Synthetic polymers are responsible for scaffold durability, strength and structural integrity; however, often do not provide biological signals. Introducing a biological component leads to more advanced and biocompatible scaffolds. In order to use these scaffolds as implants, a deeper knowledge of material characteristics and the impact of the biological component on the scaffold mechanical properties are required. Furthermore, it is necessary to implement fast, easy and non-invasive methods to determine material characteristics. In this work, we aimed to generate gelatin-poly-l-lactide (PLA) hybrids via electrospinning with defined, controllable and tunable scaffold characteristics. Using Raman microspectroscopy, we demonstrated the effectiveness of the cross-linking reaction and evaluated the increasing PLA content in the hybrid scaffolds with a non-invasive approach. Using multiphoton microscopy, we showed that gelatin fibers electrospun from a fluorinated solvent exhibit a second harmonic generation (SHG) signal typical for collagen-like structures. Compared to pure gelatin, where the SHG signal vanishes after cross-linking, the signal could be preserved in the hybrid scaffolds even after cross-linking. Furthermore, we non-invasively imaged cellular growth of human dermal fibroblasts on the hybrid electrospun scaffolds and performed fluorescence lifetime imaging microscopy on the cell-seeded hybrids, where we were able to discriminate between cells and scaffolds. Here, we successfully employed non-invasive methods to evaluate scaffold characteristics and investigate cell-material interactions.

Tailoring the gelatin/chitosan electrospun scaffold for application in skin tissue engineering: an in vitro study

The nanofibrous structure containing protein and polysaccharide has good potential in tissue engineering. The present work aims to study the role of chitosan in gelatin/chitosan nanofibrous scaffolds fabricated through electrospinning process under optimized condition. The performance of chitosan in gelatin/chitosan nanofibrous scaffolds was evaluated by mechanical tests, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and in vitro cell culture on scaffolds with different gelatin/chitosan blend ratios. To assay the influence of chitosan ratio on biocompatibility of the electrospun gelatin/chitosan scaffolds for skin tissue engineering, the culturing of the human dermal fibroblast cells (HDF) on nanofibers in terms of attachment, morphology and proliferation was evaluated. Morphological observation showed that HDF cells were attached and spread well on highly porous gelatin/chitosan nanofibrous scaffolds displaying spindle-like shapes and stretching. The fibrous morphologies of electrospun gelatin/chitosan scaffolds in culture medium were maintained during 7 days. Cell proliferation on electrospun gelatin/chitosan scaffolds was quantified by MTS assay, which revealed the positive effect of chitosan content (around 30%) as well as the nanofibrous structure on the biocompatibility (cell proliferation and attachment) of substrates.

3D-Printed membrane as an alternative to amniotic membrane for ocular surface/conjunctival defect reconstruction: An in vitro & in vivo study

BACKGROUND

The aim of this study was to evaluate the surgical handling and clinical applicability of a specific 3D-printed membrane design fabricated using a gelatin, elastin and sodium hyaluronate blend for conjunctival reconstruction and compare it with amniotic membrane (AM), which is normally used in such surgeries.

METHODS

3D printing technique was employed to fabricate the membrane based on gradient design. Prior to printing, rheometry was employed to optimize the ink composition. The printed membranes were then fully characterized in terms of physical and mechanical properties. In vitro viability, proliferation and adhesion of human limbal epithelial cells were assessed using MTT assay and scanning electron microscopy (SEM), respectively. Prior to in vivo experiment, surgical handling of each membrane was evaluated by three surgeons. In vivo evaluation was conducted through implanting the gelatin-based membranes and AM on induced conjunctival defects in rabbits (n = 8). Clinical observations, including epithelialization, inflammation severity, scar tissue formation and presence of granulation tissue, were recorded from day 1 through day 28. Histological examination was performed on all enucleated eyes on day 28. In addition to H&E staining, specific stains including Periodic Acid Schiff staining, Masson's Trichrome staining and immuno-histochemical staining for α-SMA were further used to assess goblet cell proliferation, healed sub-epithelial stroma and scar tissue formation and the presence of myofibroblasts, respectively.

RESULTS

Among all the examined compositions, a blend of 8% w/v gelatin, 2% w/v elastin and 0.5% w/v sodium hyaluronate was found to be appropriate for printing. The printed membranes had favorable optical characteristics (colorless and transparent), and the surgical handling was significantly easier compared to AM. Epithelial cells cultivated on the membranes indicated suitable viability and proliferation, and SEM images presented appropriate cell adhesion on the surface of the membranes. Clinical observations suggested similar epithelialization time (approximately 3 weeks) for both the membrane and AM grafted eyes but significantly lower levels of clinical inflammation in the membrane group from day 1 through day 28 (p = 0.01), which is a key advantage of using the printed membranes over the AM. Histological examination showed similar qualities in the healed epithelium in terms of cell morphology and cell layers. However, twice the density of goblet cells per 100 cells was observed in the gelatin-based membrane grafted group. Remnant of the degraded implant was seen in only 3 of the membranes, but in 7 of the AM grafted eyes. Inflammation and granulomatous reaction was significantly higher in sections containing the AM compared to membrane (p < 0.01 and p = 0.01, respectively). α-SMA staining was more evident, but not significantly different from the gelatin-based membrane, for the AM group (p = 0.25).

CONCLUSION

The designed gelatin-based membrane offers the necessary physical and mechanical characteristics needed for successful ocular surface/conjunctival defect construction and may be considered a promising alternative to AM due to a more predictable degradation pattern, higher goblet cell density on the healed epithelium, less inflammation and reduced scar tissue formation.

Modulation of Macrophage Phenotype, Maturation, and Graft Integration through Chondroitin Sulfate Cross-Linking to Decellularized Cornea

Decellularized corneas obtained from other species have gained intense popularity in the field of tissue engineering due to its role to serve as an alternative to the limited availability of high-quality donor tissues. However, the decellularized cornea is found to evoke an immune response inspite of the removal of the cellular contents and antigens due to the distortion of the collagen fibrils that exposes certain antigenic sites, which often lead to graft rejection. Therefore, in this study we tested the hypothesis that cross-linking the decellularized corneas with chondroitin sulfate may help in restoring the distorted conformationation changes of fibrous matrix and thus help in reducing the occurrence of graft rejection. Cross-linking of the decellularized cornea with oxidized chondroitin sulfate was validated by ATR-FTIR analysis. An in vitro immune response study involving healthy monocytes and differentiated macrophages with their surface marker analysis by pHrodo red, Lysotracker red, ER tracker, and CD63, LAMP-2 antibodies confirmed that the cross-linked decellularized matrices elicited the least immune response compared to the decellularized ones. We implanted three sets of corneal scaffolds obtained from goat, i.e., native, decellularized, and decellularized corneas conjugated with chondroitin sulfate into the rabbit stroma. Histology analysis, three months after implantation into the rabbit corneal stromal region, confirmed the restoration of the collagen fibril conformation and the migration of cells to the implanted constructs, affirming proper graft integration. Hence we conclude that the chondroitin sulfate cross-linked decellularized corneal matrix may serve as an efficient alternative to the allograft and human cadaveric corneas.

Hydrogels from feather keratin show higher viscoelastic properties and cell proliferation than those from hair and wool keratins

Hydrogel prepared from keratin shows potential applications in tissue engineering. However, the importance of the keratin sources has not been considered. The objectives of this study were to characterize and compare the rheological (storage modulus), physical (porosity, pore size, swelling capacity, and water contact angle) and in vitro cell compatibility of hydrogel scaffolds prepared from various keratin sources. Keratins were characterized by means of their molecular weight, amino acid composition, thermal and conformational properties. Hydrogels from chicken feather keratins demonstrated substantially higher storage modulus (G') than hair and wool keratin hydrogels. However, higher swelling capacity (>3000%) was determined in hair and wool over feather keratin (1500%) hydrogels. Our results suggest that small molecular weight and β-sheet conformation of feather keratin (~10 kDa) facilitated the self-assembly of rigid hydrogels through disulfide bond re-oxidation. Whereas, high molecular weight (10-75 kDa) stretchable α-helix conformation in hair and wool keratins resulted in weaker hydrogels. The cell cultures using fibroblasts showed the highest proliferation rate on chicken feather keratin hydrogel scaffolds. After 15 days of culture, partial breakdown of keratin fibers was observed. Results indicate that stiffer avian keratins can be used to fabricate more mechanically robust biomaterials than mammalian keratins.

Interplay between stiffness and degradation of architectured gelatin hydrogels leads to differential modulation of chondrogenesis in vitro and in vivo

The limited capacity of cartilage to heal large lesions through endogenous mechanisms has led to extensive effort to develop materials to facilitate chondrogenesis. Although physical-chemical properties of biomaterials have been shown to impact in vitro chondrogenesis, whether these findings are translatable in vivo is subject of debate. Herein, architectured 3D hydrogel scaffolds (ArcGel) (produced by crosslinking gelatin with ethyl lysine diisocyanate (LDI)) were used as a model system to investigate the interplay between scaffold mechanical properties and degradation on matrix deposition by human articular chondrocytes (HAC) from healthy donors in vitro and in vivo. Using ArcGel scaffolds of different tensile and shear modulus, and degradation behavior; in this study, we compared the fate of ex vivo engineered ArcGels-chondrocytes constructs, i.e. the traditional tissue engineering approach, with thede novoformation of cartilaginous tissue in HAC laden ArcGels in an ectopic nude mouse model. While the softer and fast degrading ArcGel (LNCO3) was more efficient at promoting chondrogenic differentiation in vitro, upon ectopic implantation, the stiffer and slow degrading ArcGel (LNCO8) was superior in maintaining chondrogenic phenotype in HAC and retention of cartilaginous matrix. Furthermore, surprisingly the de novo formation of cartilage tissue was promoted only in LNCO8. Since HAC cultured for only three days in the LNCO8 environment showed upregulation of hypoxia-associated genes, this suggests a potential role for hypoxia in the observed in vivo outcomes. In summary, this study sheds light on how immediate environment (in vivo versus in vitro) can significantly impact the outcomes of cell-laden biomaterials.

STATEMENT OF SIGNIFICANCE

In this study, 3D architectured hydrogels (ArcGels) with different mechanical and biodegradation properties were investigated for their potential to promote formation of cartilaginous matrix by human articular chondrocytes in vitro and in vivo. Two paradigms were explored (i) ex vivo engineering followed by in vivo implantation in ectopic site of nude mice and (ii) short in vitro culture (3 days) followed by implantation to induce de novo cartilage formation. Softer and fast degrading ArcGel were better at promoting chondrogenesis in vitro, while stiffer and slow degrading ArcGel were strikingly superior in both maintaining chondrogenesis in vivo and inducing de novo formation of cartilage. Our findings highlight the importance of the interplay between scaffold mechanics and degradation in chondrogenesis.

Corneal regeneration: A review of stromal replacements

Corneal blindness is traditionally treated by transplantation of a donor cornea, or in severe cases by implantation of an artificial cornea or keratoprosthesis. Due to severe donor shortages and the risks of complications that come with artificial corneas, tissue engineering in ophthalmology has become more focused on regenerative strategies using biocompatible materials either with or without cells. The stroma makes up the bulk of the corneal thickness and mainly consists of a tightly interwoven network of collagen type I, making it notoriously difficult to recreate in a laboratory setting. Despite the challenges that come with corneal stromal tissue engineering, there has recently been enormous progress in this field. A large number of research groups are working towards developing the ideal biomimetic, cytocompatible and transplantable stromal replacement. Here we provide an overview of the approaches directed towards tissue engineering the corneal stroma, from classical collagen gels, films and sponges to less traditional components such as silk, fish scales, gelatin and polymers. The perfect stromal replacement has yet to be identified and future research should be directed at combined approaches, in order to not only host native stromal cells but also restore functionality.

STATEMENT OF SIGNIFICANCE

In the field of tissue engineering and regenerative medicine in ophthalmology the focus has shifted towards a common goal: to restore the corneal stroma and thereby provide a new treatment option for patients who are currently blind due to corneal opacification. Currently the waiting lists for corneal transplantation include more than 10 million patients, due to severe donor shortages. Alternatives to the transplantation of a donor cornea include the use of artificial cornea, but these are by no means biomimetic and therefore do not provide good outcomes. In recent years a lot of work has gone into the development of tissue engineered scaffolds and other biomaterials suitable to replace the native stromal tissue. Looking at all the different approaches separately is a daunting task and up until now there was no review article in which every approach is discussed. This review does include all approaches, from classical tissue engineering with collagen to the use of various alternative biomaterials and even fish scales. Therefore, this review can serve as a reference work for those starting in the field and but also to stimulate collaborative efforts in the future.

Electrospun Gelatin⁻Chondroitin Sulfate Scaffolds Loaded with Platelet Lysate Promote Immature Cardiomyocyte Proliferation

The aim of the present work was the development of heart patches based on gelatin (G) and chondroitin sulfate (CS) to be used as implants to improve heart recovery after corrective surgery for critical congenital heart defects (CHD). Patches were prepared by means of electrospinning to obtain nanofibrous scaffolds and they were loaded with platelet lysate (PL) as a source of growth factors to further enhance the repair process. Scaffolds were characterized for morphology and mechanical properties and for the capability to support in vitro adhesion and proliferation of dermal fibroblasts in order to assess the system’s general biocompatibility. Adhesion and proliferation of endothelial cells and cardiac cells (cardiomyocytes and cardiac fibroblasts from rat fetuses) onto PL-loaded patches was evaluated. Patches presented good elasticity and high stiffness suitable for in vivo adaptation to heart contraction. CS improved adhesion and proliferation of dermal fibroblasts, as proof of their biocompatibility. Moreover, they enhanced the adhesion and proliferation of endothelial cells, a crucial mediator of cardiac repair. Cell adhesion and proliferation could be related to elastic properties, which could favor cell motility. The presence of platelet lysate and CS was crucial for the adhesion and proliferation of cardiac cells and, in particular, of cardiomyocytes: G/CS scaffold embedded with PL appeared to selectively promote proliferation in cardiomyocytes but not cardiac fibroblasts. In conclusion, G/CS scaffold seems to be a promising system to assist myocardial-repair processes in young patient, preserving cardiomyocyte viability and preventing cardiac fibroblast proliferation, likely reducing subsequent uncontrolled collagen deposition by fibroblasts following repair.

Design and evaluation of chitosan/poly(l-lactide)/pectin based composite scaffolds for cartilage tissue regeneration

Poor regenerative potential of cartilage tissue due to the avascular nature and lack of supplementation of reparative cells impose an important challenge in recent medical practice towards development of artificial extracellular matrix with enhanced neo-cartilage tissue regeneration potential. Chitosan (CH), poly (l-lactide) (PLLA), and pectin (PC) compositions were tailored to generate polyelectrolyte complex based porous scaffolds using freeze drying method and crosslinked by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), N-hydroxysuccinimide (NHS) solution containing chondroitin sulfate (CS) to mimic the composition as well as architecture of the cartilage extracellular matrix (ECM). The physical, chemical, thermal, and mechanical behaviors of developed scaffolds were done. The scaffolds were porous with homogeneous pore structure with pore size 49-170μm and porosities in the range of 79 to 84%. Fourier transform infrared study confirmed the presence of polymers (CH, PLLA and PC) within the scaffolds. The crystallinity of the scaffold was examined by the X-ray diffraction studies. Furthermore, scaffold shows suitable swelling property, moderate biodegradation and hemocompatibility in nature and possess suitable mechanical strength for cartilage tissue regeneration. MTT assay, GAG content, and attachment of chondrocyte confirmed the regenerative potential of the cell seeded scaffold. The histopathological analysis defines the suitability of scaffold for cartilage tissue regeneration.

Mimicking corneal stroma using keratocyte-loaded photopolymerizable methacrylated gelatin hydrogels

Cell-laden methacrylated gelatin (GelMA) hydrogels with high (approximately 90%) transparency were prepared to mimic the natural form and function of corneal stroma. They were synthesized from GelMA with a methacrylation degree of 70% as determined by nuclear magnetic resonance. Hydrogels were strong enough to withstand handling. Stability studies showed that 87% of the GelMA hydrogels remained after 21 days in phosphate buffered saline (PBS). Cell viability in the first 2 days was over 90% for the human keratocytes loaded in the gels as determined with the live-dead analysis. Cells in the hydrogel elongated and connected to each other as observed by confocal laser scanning microscopy (CLSM) images and scanning electron microscope analysis after 3 weeks in the culture medium and cells were seen to be distributed throughout the hydrogel bulk. Cells were found to synthesize collagen Types I and V, decorin, and biglycan (representative collagens and proteoglycans of human corneal stroma, respectively) showing that keratocytes maintained their functions and preserved their phenotypes in the hydrogels. Transparency of cell-loaded and cell-free hydrogels after 21 days was found to be over 90% at all time points in the visible light range and was comparable to the transparency of the native cornea. The corneal stroma equivalent produced in this study that has cells entrapped in it leads to a product with homogenous distribution of cells. It was transparent at the very beginning and is expected to allow better vision than nontransparent substrates. It, therefore, has a significant potential to be used as an alternative to the current products used to treat corneal blindness.

Harnessing chondroitin sulphate in composite scaffolds to direct progenitor and stem cell function for tissue repair

This review details the inclusion of chondroitin sulphate in bioscaffolds for superior functional properties in tissue regenerative applications.

Promising Biomolecules

The osteochondral defect (OD) comprises the articular cartilage and its subchondral bone. The treatment of these lesions remains as one of the most problematic clinical issues, since these defects include different tissues, requiring distinct healing approaches. Among the growing applications of regenerative medicine, clinical articular cartilage repair has been used for two decades, and it is an effective example of translational medicine; one of the most used cell-based repair strategies includes implantation of autologous cells in degradable scaffolds such as alginate, agarose, collagen, chitosan, chondroitin sulfate, cellulose, silk fibroin, hyaluronic acid, and gelatin, among others. Concerning the repair of osteochondral defects, tissue engineering and regenerative medicine started to design single- or bi-phased scaffold constructs, often containing hydroxyapatite-collagen composites, usually used as a bone substitute. Biomolecules such as natural and synthetic have been explored to recreate the cartilage-bone interface through multilayered biomimetic scaffolds. In this chapter, a succinct description about the most relevant natural and synthetic biomolecules used on cartilage and bone repair, describing the procedures to obtain these biomolecules, their chemical structure, common modifications to improve its characteristics, and also their application in the biomedical fields, is given.

New Paradigms for the Study of Ocular Alphaherpesvirus Infections: Insights into the Use of Non-Traditional Host Model Systems

Ocular herpesviruses, most notably human alphaherpesvirus 1 (HSV-1), canid alphaherpesvirus 1 (CHV-1) and felid alphaherpesvirus 1 (FHV-1), infect and cause severe disease that may lead to blindness. CHV-1 and FHV-1 have a pathogenesis and induce clinical disease in their hosts that is similar to HSV-1 ocular infections in humans, suggesting that infection of dogs and cats with CHV-1 and FHV-1, respectively, can be used as a comparative natural host model of herpesvirus-induced ocular disease. In this review, we discuss both strengths and limitations of the various available model systems to study ocular herpesvirus infection, with a focus on the use of these non-traditional virus-natural host models. Recent work has demonstrated the robustness and reproducibility of experimental ocular herpesvirus infections in dogs and cats, and, therefore, these non-traditional models can provide additional insights into the pathogenesis of ocular herpesvirus infections.