Functional fabrication of recombinant human collagen-phosphorylcholine hydrogels for regenerative medicine applications

In situ UNIversal Orthogonal Network (UNION) bioink deposition for direct delivery of corneal stromal stem cells to corneal wounds

The scarcity of human donor corneal graft tissue worldwide available for corneal transplantation necessitates the development of alternative therapeutic strategies for treating patients with corneal blindness. Corneal stromal stem cells (CSSCs) have the potential to address this global shortage by allowing a single donor cornea to treat multiple patients. To directly deliver CSSCs to corneal defects within an engineered biomatrix, we developed a UNIversal Orthogonal Network (UNION) collagen bioink that crosslinks in situ with a bioorthogonal, covalent chemistry. This cell-gel therapy is optically transparent, stable against contraction forces exerted by CSSCs, and permissive to the efficient growth of corneal epithelial cells. Furthermore, CSSCs remain viable within the UNION collagen gel precursor solution under standard storage and transportation conditions. This approach promoted corneal transparency and re-epithelialization in a rabbit anterior lamellar keratoplasty model, indicating that the UNION collagen bioink serves effectively as an in situ-forming, suture-free therapy for delivering CSSCs to corneal wounds.

Development of silk fibroin/collagen film containing GI-20 peptide-loaded PLGA nanoparticles against corneal herpes simplex virus-1

Herpes simplex virus-1 (HSV-1) is the primary cause of infectious blindness. Despite impressive therapeutic outcomes of conventional treatments, HSV-1 drug resistance can be easily developed. Thus, more constructive strategies should be implemented. Led by this inspiration, this work describes the potential utility of a biodegradable silk fibroin/collagen (SF/Col) film combined with GI-20-loaded poly lactic-co-glycolic acid (PLGA) nanoparticle to provide efficient and sustained delivery platform for synthetic GI-20 peptide against HSV-1. A non-irritant film containing 90 % SF and 10 % Col incorporated with mentioned nanodrug showed some optimum physicochemical properties including loading efficiency (74.15 % ± 1.12), tensile strength (3.16 ± 0.67 MPa), water uptake ability (∼73 %), cytocompatibility (viable up to 35 µg/mL of GI-20), and sustained release paradigm (∼90 % within 14 days). Also, GI-20 peptide at concentration of 35 µg/mL could prophylactically attenuate viral titration by 5 log10 units. In addition, the corneal uptake was improved without vascular irritation. In accordance with in vitro results, no hallmarks of keratitis and significant neovascularization along with ignorable inflammatory responses were obtained. Taken together, these results could guarantee the potential of mentioned multifunctional biomaterial in the healing of infected corneal tissue.

An in vitro 3-dimensional Collagen-based Corneal Construct with Innervation Using Human Corneal Cell Lines

Purpose

To develop a 3-dimensional corneal construct suitable for in vitro studies of disease conditions and therapies.

Design

In vitro human corneal constructs were created using chemically crosslinked collagen and chondroitin sulfate extracellular matrix and seeded with 3 human corneal cell types (epithelial, stromal, and endothelial) together with neural cells. The neural cells were derived from hybrid neuroblastoma cells and the other cells used from immortalized human corneal cell lines. To check the feasibility and characterize the constructs, cytotoxicity, cell proliferation, histology, and protein expression studies were performed.

Results

Optimized culture condition permitted synchronized viability across the cell types within the construct. The construct showed a typical appearance for different cellular layers, including healthy appearing, phenotypically differentiated neurons. The expected protein expression profiles for specific cell types within the construct were confirmed with western blotting.

Conclusions

An in vitro corneal construct was successfully developed with maintenance of individual cell phenotypes with anatomically correct cellular loci. The construct may be useful in evaluation of specific corneal disorders and in developing different corneal disease models. Additionally, the construct can be used in evaluating drug targeting and/or penetration to individual corneal layers, testing novel therapeutics for corneal diseases, and potentially reducing the necessity for animals in corneal research at the early stages of investigation.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Silk fibroin/vitreous humor hydrogel scaffold modified by a carbodiimide crosslinker for wound healing

Natural-derived biomaterials can be used as substrates for the growth, proliferation, and differentiation of cells. In this study, bovine vitreous humor as a biological material was cross-linked to silk fibroin with different concentration ratios to design a suitable substrate for corneal tissue regeneration. The cross-linked samples were evaluated with different analyses such as structural, physical (optical, swelling, and degradation), mechanical, and biological (viability, cell adhesion) assays. The results showed that all samples had excellent transparency, especially those with higher silk fibroin content. Increasing the ratio of vitreous humor to silk fibroin decreased mechanical strength and increased swelling and degradation, respectively. There was no significant difference in the toxicity of the samples, and with the increase in vitreous humor ratio, adhesion and cell proliferation increased. Generally, silk fibroin with vitreous humor can provide desirable characteristics as a transparent film for corneal wound healing.

In Situ UNIversal Orthogonal Network (UNION) Bioink Deposition for Direct Delivery of Corneal Stromal Stem Cells to Corneal Wounds

The scarcity of human donor corneal graft tissue worldwide available for corneal transplantation necessitates the development of alternative therapeutic strategies for treating patients with corneal blindness. Corneal stromal stem cells (CSSCs) have the potential to address this global shortage by allowing a single donor cornea to treat multiple patients. To directly deliver CSSCs to corneal defects within an engineered biomatrix, we developed a UNIversal Orthogonal Network (UNION) collagen bioink that crosslinks in situ with a bioorthogonal, covalent chemistry. This cell-gel therapy is optically transparent, stable against contraction forces exerted by CSSCs, and permissive to the efficient growth of corneal epithelial cells. Furthermore, CSSCs remain viable within the UNION collagen gel precursor solution under standard storage and transportation conditions. This approach promoted corneal transparency and re-epithelialization in a rabbit anterior lamellar keratoplasty model, indicating that the UNION collagen bioink serves effectively as an in situ -forming, suture-free therapy for delivering CSSCs to corneal wounds. TEASER. Corneal stem cells are delivered within chemically crosslinked collagen as a transparent, regenerative biomaterial therapy.

Use of discarded corneo-scleral rims to create cornea-like tissue

BACKGROUND

Corneal disease is a major cause of blindness. Transplantation of cadaver-derived corneas (keratoplasty) is still the current therapy of choice; however, the global shortage of donor corneas continues to drive a search for alternatives. To this end, biosynthetic corneal substitutes have recently begun to gain importance. Here, we present a novel method for the generation of a cornea-like tissue (CLT), using corneo-scleral rims discarded after keratoplasty.

METHODS AND RESULTS

Type I collagen was polymerized within the corneo-scleral rim, which functioned as a 'host' mould, directing the 'guest' collagen to polymerize into disc-shaped cornea-like material (CLM), displaying the shape, curvature, thickness, and transparency of normal cornea. This polymerization of collagen appears to derive from some morphogenetic influence exerted by the corneo-scleral rim. Once the CLM had formed naturally, we used collagen crosslinking to fortify it, and then introduced cells to generate a stratified epithelial layer to create cornea-like tissue (CLT) displaying characteristics of native cornea. Through the excision and reuse of rims, each rim turned out to be useful for the generation of multiple cornea-shaped CLTs.

CONCLUSIONS

The approach effectively helps to shorten the gap between demand and supply of CLMs/CLTs for transplantation. We are exploring the surgical transplantation of this CLT into animal eyes, as keratoprostheses, as a precursor to future applications involving human eyes. It is possible to use either the CLM or CLT, for patients with varying corneal blinding diseases.

Fabrication and performance evaluation of PLCL-hCOLIII small-diameter vascular grafts crosslinked with procyanidins

Cardiovascular disease has become one of the main causes of death. It is the common goal of researchers worldwide to develop small-diameter vascular grafts to meet clinical needs. Collagen is a valuable biomaterial that has been used in the preparation of vascular grafts and has shown good results. Recombinant humanized collagen (RHC) has the advantages of clear chemical structure, batch stability, no virus hazard and low immunogenicity compared with animal-derived collagen, which can be developed as vascular materials. In this study, Poly (l-lactide- ε-caprolactone) with l-lactide/ε-caprolactone (PLCL) and type III recombinant humanized collagen (hCOLIII) were selected as raw materials to prepare vascular grafts, which were prepared by the same-nozzle electrospinning apparatus. Meanwhile, procyanidin (PC), a plant polyphenol, was used to cross-link the vascular grafts. The physicochemical properties and biocompatibility of the fabricated vascular grafts were investigated by comparing with glutaraldehyde (GA) crosslinked vascular grafts and pure PLCL grafts. Finally, the performance of PC cross-linked PLCL-hCOLIII vascular grafts were evaluated by rabbit carotid artery transplantation model. The results indicate that the artificial vascular grafts have good cell compatibility, blood compatibility, and anti-calcification performance, and can remain unobstructed after 30 days carotid artery transplantation in rabbits. The grafts also showed inhibitory effects on the proliferation of SMCs and intimal hyperplasia, demonstrating its excellent performance as small diameter vascular grafts.

Exploring the recent developments of alginate silk fibroin material for hydrogel wound dressing: A review

Hydrogels, a type of polymeric material capable of retaining water within a three-dimensional network, have demonstrated their potential in wound healing, surpassing traditional wound dressings. These hydrogels possess remarkable mechanical, chemical, and biological properties, making them suitable scaffolds for tissue regeneration. This article aims to emphasize the advantages of alginate, silk fibroin, and hydrogel-based wound dressings, specifically highlighting their crucial functions that accelerate the healing process of skin wounds. Noteworthy functions include self-healing ability, water solubility, anti-inflammatory properties, adhesion, antimicrobial properties, drug delivery, conductivity, and responsiveness to stimuli. Moreover, recent advancements in hydrogel technology have resulted in the development of wound dressings with enhanced features for monitoring wound progression, further augmenting their effectiveness. This review emphasizes the utilization of hydrogel membranes for treating excisional and incisional wounds, while exploring recent breakthroughs in hydrogel wound dressings, including nanoparticle composite hydrogels, stem cell hydrogel composites, and curcumin-hydrogel composites. Additionally, the review focuses on diverse synthesis procedures, designs, and potential applications of hydrogels in wound healing dressings.

Protein-Based Hydrogels and Their Biomedical Applications

Hydrogels made from proteins are attractive materials for diverse medical applications, as they are biocompatible, biodegradable, and amenable to chemical and biological modifications. Recent advances in protein engineering, synthetic biology, and material science have enabled the fine-tuning of protein sequences, hydrogel structures, and hydrogel mechanical properties, allowing for a broad range of biomedical applications using protein hydrogels. This article reviews recent progresses on protein hydrogels with special focus on those made of microbially produced proteins. We discuss different hydrogel formation strategies and their associated hydrogel properties. We also review various biomedical applications, categorized by the origin of protein sequences. Lastly, current challenges and future opportunities in engineering protein-based hydrogels are discussed. We hope this review will inspire new ideas in material innovation, leading to advanced protein hydrogels with desirable properties for a wide range of biomedical applications.

Phosphorylcholine and KR12-Containing Corneal Implants in HSV-1-Infected Rabbit Corneas

Severe HSV-1 infection can cause blindness due to tissue damage from severe inflammation. Due to the high risk of graft failure in HSV-1-infected individuals, cornea transplantation to restore vision is often contraindicated. We tested the capacity for cell-free biosynthetic implants made from recombinant human collagen type III and 2-methacryloyloxyethyl phosphorylcholine (RHCIII-MPC) to suppress inflammation and promote tissue regeneration in the damaged corneas. To block viral reactivation, we incorporated silica dioxide nanoparticles releasing KR12, the small bioactive core fragment of LL37, an innate cationic host defense peptide produced by corneal cells. KR12 is more reactive and smaller than LL37, so more KR12 molecules can be incorporated into nanoparticles for delivery. Unlike LL37, which was cytotoxic, KR12 was cell-friendly and showed little cytotoxicity at doses that blocked HSV-1 activity in vitro, instead enabling rapid wound closure in cultures of human epithelial cells. Composite implants released KR12 for up to 3 weeks in vitro. The implant was also tested in vivo on HSV-1-infected rabbit corneas where it was grafted by anterior lamellar keratoplasty. Adding KR12 to RHCIII-MPC did not reduce HSV-1 viral loads or the inflammation resulting in neovascularization. Nevertheless, the composite implants reduced viral spread sufficiently to allow stable corneal epithelium, stroma, and nerve regeneration over a 6-month observation period.

Comparative In Vitro Activity of New Lipoglycopeptides and Vancomycin Against Ocular Staphylococci and Their Toxicity on the Human Corneal Epithelium

PURPOSE

The purpose of this study was to assess the potential of new lipoglycopeptides as novel topical therapies for improved treatment of recalcitrant ocular infections. We evaluated the in vitro antimicrobial activity of oritavancin, dalbavancin, and telavancin compared with vancomycin (VAN) against a large collection of ocular staphylococcal isolates and their cytotoxicity on human corneal epithelial cells (HCECs).

METHODS

Antimicrobial susceptibility testing was performed by broth microdilution against 223 Staphylococcus spp. clinical isolates. Time-kill kinetics were determined for methicillin-resistant strains of Staphylococcus aureus (MRSA) (n = 2) and Staphylococcus epidermidis (MRSE) (n = 1). In vitro cytotoxicity assays were performed with AlamarBlue and live/dead staining on HCECs.

RESULTS

All new lipoglycopeptides showed strong in vitro potency against ocular staphylococci, including multidrug-resistant MRSA strains, with dalbavancin showing a slightly higher potency overall [minimum inhibitory concentration (MIC) 90 0.06 μg/mL] compared with telavancin and oritavancin (MIC 90 0.12 μg/mL), whereas VAN had the lowest potency (MIC 90 2 μg/mL). Oritavancin exerted rapid bactericidal activity within 1 h for MRSA and 2 h for MRSE. All other drugs were bactericidal within 24 h. At a concentration commonly used for topical preparations (25 mg/mL), cytotoxicity was observed for VAN after 5 min of incubation, whereas reduction in HCEC viability was not seen for telavancin and was less affected by oritavancin and dalbavancin. Cytotoxicity at 25 mg/mL was seen for all drugs at 30 and 60 min but was significantly reduced or undetected for lower concentrations.

CONCLUSIONS

Our study demonstrates that new lipoglycopeptides have substantially better in vitro antimicrobial activity against ocular staphylococcal isolates compared with VAN, with a similar or improved toxicity profile on HCECs.

Collagen as a Biomaterial for Skin and Corneal Wound Healing

The cornea and the skin are two organs that form the outer barrier of the human body. When either is injured (e.g., from surgery, physical trauma, or chemical burns), wound healing is initiated to restore integrity. Many cells are activated during wound healing. In particular, fibroblasts that are stimulated often transition into repair fibroblasts or myofibroblasts that synthesize extracellular matrix (ECM) components into the wound area. Control of wound ECM deposition is critical, as a disorganized ECM can block restoration of function. One of the most abundant structural proteins in the mammalian ECM is collagen. Collagen type I is the main component in connective tissues. It can be readily obtained and purified, and short analogs have also been developed for tissue engineering applications, including modulating the wound healing response. This review discusses the effect of several current collagen implants on the stimulation of corneal and skin wound healing. These range from collagen sponges and hydrogels to films and membranes.

Collagen fabricated delivery systems for wound healing: A new roadmap

Collagen is a biopolymer found in the animal body. It is one of the most abundant proteins in the extracellular matrix that provides strength to the skin, joints, and bones in the human body. It is an important source of elasticity and strength in the extracellular matrix and contributes to the structural and physiological integrity of tissues. Collagen plays an important role in regulating the wound healing process. It helps in wound healing by attracting fibroblasts and encouraging new collagen formation in the wound bed. Therefore, it can be used as a supplementary aid for wound treatment to accelerate the healing process. A prominent benefit of incorporating collagen in wound dressings is its ability to enhance the healing process for critical wounds. Not only collagen but various collagen-containing systems are being prepared to boost its efficacy in wound healing. Different strategies like nanoscale reductions, biopolymers, and incorporating anti-inflammatory and antimicrobial drugs with collagen have been reported. This review article emphasizes the use of collagen for wound healing and various collagen fabricated delivery systems such as nanofibres, nanoparticles, hydrogels, films, and sponges that aid in the healing of wounds.

Recombinant and genetic code expanded collagen-like protein as a tailorable biomaterial

Collagen occurs in nature with a dedicated triple helix structure and is the most preferred biomaterial in commercialized medical products. However, concerns on purity, disease transmission, and the reproducibility of animal derived collagen restrict its applications and warrants alternate recombinant sources. The expression of recombinant collagen in different prokaryotic and eukaryotic hosts has been reported with varying degrees of success, however, it is vital to elucidate the structural and biological characteristics of natural collagen. The recombinant production of biologically functional collagen is restricted by its high molecular weight and post-translational modification (PTM), especially the hydroxylation of proline to hydroxyproline. Hydroxyproline plays a key role in the structural stability and higher order self-assembly to form fibrillar matrices. Advancements in synthetic biology and recombinant technology are being explored for improving the yield and biomimicry of recombinant collagen. It emerges as reliable, sustainable source of collagen, promises tailorable properties and thereby custom-made protein biomaterials. Remarkably, the evolutionary existence of collagen-like proteins (CLPs) has been identified in single-cell organisms. Interestingly, CLPs exhibit remarkable ability to form stable triple helical structures similar to animal collagen and have gained increasing attention. Strategies to expand the genetic code of CLPs through the incorporation of unnatural amino acids promise the synthesis of highly tunable next-generation triple helical proteins required for the fabrication of smart biomaterials. The review outlines the importance of collagen, sources and diversification, and animal and recombinant collagen-based biomaterials and highlights the limitations of the existing collagen sources. The emphasis on genetic code expanded tailorable CLPs as the most sought alternate for the production of functional collagen and its advantages as translatable biomaterials has been highlighted.

Improving the properties of fish skin collagen/silk fibroin dressing by chemical treatment for corneal wound healing

Natural biomaterials are crucial in ocular tissue engineering because they allow cells to proliferate, differentiate, and stratify while maintaining the typical epithelial phenotype. In this study, membranes as dressings were formed from silk fibroin and collagen (Co) extracted from fish skin and then modified with carbodiimide chemical cross linker in different concentrations. The samples were evaluated by different analyses such as structural, physical (optical, swelling, denaturation temperature, degradation), mechanical, and biological (viability, cell adhesion, immunocytochemistry) assays. The results showed that all membranes have excellent transparency, especially with higher silk fibroin content. Increasing the cross linker concentration and the ratio of silk fibroin to Co increased the denaturation temperature and mechanical strength and, conversely, reduced the degradation rate and cell adhesion. The samples did not show a significant difference in toxicity with increasing cross linker and silk fibroin ratio. In general, samples with a low silk fibroin ratio combined with cross linker can provide desirable properties as a membrane for corneal wound healing.

Electron-Beam Irradiated Recombinant Human Collagen-Phosphorylcholine Corneal Implants Retain Pro-Regeneration Capacity

Sterilization of biodegradable, collagen-based implants is challenging as irradiation sterilization methods can alter their mechanical properties. Electron beam (EB) irradiation is a terminal sterilization method that has been used for biologically-derived implants. Here, recombinant human collagen type III-phosphorylcholine (RHCIII-MPC) hydrogels were irradiated with EB doses of 17, 19, or 21 kGy and their subsequent biocompatibility and ability to promote regeneration in rabbit corneas was evaluated. Unirradiated hydrogels stored in 1% chloroform in phosphate-buffered saline (C-PBS) were the controls. There were no significant differences between irradiated and non-irradiated samples in optical or physical properties (tensile strength, modulus, elasticity), or the ability to support cell growth. However, irradiated implants were more sensitive to high levels of collagenase than unirradiated controls and the C-PBS implants had increased cell growth compared to EB and controls at 72 h. Corneal implants e-beamed at 17 kGy or e-beamed and subsequently frozen (EB-F) to increase shelf-life showed no adverse biological effects of the irradiation. EB, EB-F, and C-PBS implanted corneas all rapidly re-epithelialized but showed mild neovascularization that resolved over 6 months. The regenerated neo-corneas were transparent at 6 months post-operation. In vivo confocal microscopy confirmed normal morphology for the epithelium, stroma, sub-basal nerves and unoperated endothelium. Histology showed that all the regenerated corneas were morphologically similar to the normal. Immunohistochemistry indicated the presence of a differentiated corneal epithelium and functional tear film. In conclusion, the e-beamed corneal implants performed as well as non-irradiated control implants, resulting in fully regenerated neo-corneas with new nerves and without blood vessels or inflammation that may impede vision or corneal function. Therefore, a complete validation study to establish EB irradiation as an effective means for corneal implant sterilization prior to clinical application is necessary as a next step.

Crosslinker-free collagen gelation for corneal regeneration

Development of an artificial cornea can potentially fulfil the demand of donor corneas for transplantation as the number of donors is far less than needed to treat corneal blindness. Collagen-based artificial corneas stand out as a regenerative option, having promising clinical outcomes. Collagen crosslinked with chemical crosslinkers which modify the parent functional groups of collagen. However, crosslinkers are usually cytotoxic, so crosslinkers need to be removed from implants completely before application in humans. In addition, crosslinked products are mechanically weak and susceptible to enzymatic degradation. We developed a crosslinker free supramolecular gelation strategy using pyrene conjugated dipeptide amphiphile (PyKC) consisting of lysine and cysteine; in which collagen molecules are intertwined inside the PyKC network without any functional group modification of the collagen. The newly developed collagen implants (Coll-PyKC) are optically transparent and can effectively block UV light, are mechanically and enzymatically stable, and can be sutured. The Coll-PyKC implants support the growth and function of all corneal cells, trigger anti-inflammatory differentiation while suppressing the pro-inflammatory differentiation of human monocytes. Coll-PyKC implants can restrict human adenovirus propagation. Therefore, this crosslinker-free strategy can be used for the repair, healing, and regeneration of the cornea, and potentially other damaged organs of the body.

Comparison of Microglial Morphology and Function in Primary Cerebellar Cell Cultures on Collagen and Collagen-Mimetic Hydrogels

Neuronal-glial cell cultures are usually grown attached to or encapsulated in an adhesive environment as evenly distributed networks lacking tissue-like cell density, organization and morphology. In such cultures, microglia have activated amoeboid morphology and do not display extended and intensively branched processes characteristic of the ramified tissue microglia. We have recently described self-assembling functional cerebellar organoids promoted by hydrogels containing collagen-like peptides (CLPs) conjugated to a polyethylene glycol (PEG) core. Spontaneous neuronal activity was accompanied by changes in the microglial morphology and behavior, suggesting the cells might play an essential role in forming the functional neuronal networks in response to the peptide signalling. The present study examines microglial cell morphology and function in cerebellar cell organoid cultures on CLP-PEG hydrogels and compares them to the cultures on crosslinked collagen hydrogels of similar elastomechanical properties. Material characterization suggested more expressed fibril orientation and denser packaging in crosslinked collagen than CLP-PEG. However, CLP-PEG promoted a significantly higher microglial motility (determined by time-lapse imaging) accompanied by highly diverse morphology including the ramified (brightfield and confocal microscopy), more active Ca2+ signalling (intracellular Ca2+ fluorescence recordings), and moderate inflammatory cytokine level (ELISA). On the contrary, on the collagen hydrogels, microglial cells were significantly less active and mostly round-shaped. In addition, the latter hydrogels did not support the neuron synaptic activity. Our findings indicate that the synthetic CLP-PEG hydrogels ensure more tissue-like microglial morphology, motility, and function than the crosslinked collagen substrates.

Recent Progress in Biopolymer-Based Hydrogel Materials for Biomedical Applications

Hydrogels from biopolymers are readily synthesized, can possess various characteristics for different applications, and have been widely used in biomedicine to help with patient treatments and outcomes. Polysaccharides, polypeptides, and nucleic acids can be produced into hydrogels, each for unique purposes depending on their qualities. Examples of polypeptide hydrogels include collagen, gelatin, and elastin, and polysaccharide hydrogels include alginate, cellulose, and glycosaminoglycan. Many different theories have been formulated to research hydrogels, which include Flory-Rehner theory, Rubber Elasticity Theory, and the calculation of porosity and pore size. All these theories take into consideration enthalpy, entropy, and other thermodynamic variables so that the structure and pore sizes of hydrogels can be formulated. Hydrogels can be fabricated in a straightforward process using a homogeneous mixture of different chemicals, depending on the intended purpose of the gel. Different types of hydrogels exist which include pH-sensitive gels, thermogels, electro-sensitive gels, and light-sensitive gels and each has its unique biomedical applications including structural capabilities, regenerative repair, or drug delivery. Major biopolymer-based hydrogels used for cell delivery include encapsulated skeletal muscle cells, osteochondral muscle cells, and stem cells being delivered to desired locations for tissue regeneration. Some examples of hydrogels used for drug and biomolecule delivery include insulin encapsulated hydrogels and hydrogels that encompass cancer drugs for desired controlled release. This review summarizes these newly developed biopolymer-based hydrogel materials that have been mainly made since 2015 and have shown to work and present more avenues for advanced medical applications.

Recent progress and perspectives in applications of 2-methacryloyloxyethyl phosphorylcholine polymers in biodevices at small scales

Bioinspired materials have attracted attention in a wide range of fields. Among these materials, a polymer family containing 2-methacryloyloxyethyl phosphorylcholine (MPC), which has a zwitterionic phosphorylcholine headgroup inspired by the...

Cardiomyogenic Differentiation Potential of Human Dilated Myocardium-Derived Mesenchymal Stem/Stromal Cells: The Impact of HDAC Inhibitor SAHA and Biomimetic Matrices

Dilated cardiomyopathy (DCM) is the most common type of nonischemic cardiomyopathy characterized by left ventricular or biventricular dilation and impaired contraction leading to heart failure and even patients’ death. Therefore, it is important to search for new cardiac tissue regenerating tools. Human mesenchymal stem/stromal cells (hmMSCs) were isolated from post-surgery healthy and DCM myocardial biopsies and their differentiation to the cardiomyogenic direction has been investigated in vitro. Dilated hmMSCs were slightly bigger in size, grew slower, but had almost the same levels of MSC-typical surface markers as healthy hmMSCs. Histone deacetylase (HDAC) activity in dilated hmMSCs was 1.5-fold higher than in healthy ones, which was suppressed by class I and II HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) showing activation of cardiomyogenic differentiation-related genes alpha-cardiac actin (ACTC1) and cardiac troponin T (TNNT2). Both types of hmMSCs cultivated on collagen I hydrogels with hyaluronic acid (HA) or 2-methacryloyloxyethyl phosphorylcholine (MPC) and exposed to SAHA significantly downregulated focal adhesion kinase (PTK2) and activated ACTC1 and TNNT2. Longitudinal cultivation of dilated hmMSC also upregulated alpha-cardiac actin. Thus, HDAC inhibitor SAHA, in combination with collagen I-based hydrogels, can tilt the dilated myocardium hmMSC toward cardiomyogenic direction in vitro with further possible therapeutic application in vivo.

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.

One-step harvest and delivery of micropatterned cell sheets mimicking the multi-cellular microenvironment of vascularized tissue

Techniques for harvest and delivery of cell sheets have been improving for decades. However, cell sheets with complicated patterns closely related to natural tissue architecture were hardly achieved. Here, we developed an efficient method to culture and harvest cell sheets with complex shape (noted as microtissues) using temperature-responsive hydrogel consisting of expandable polyethylene oxide polymer at low temperature. Firstly, a temperature-responsive hydrogel surface with honeycomb patterns (50 and 100 µm in width) were developed through microcontact printing of polydopamine (PD). The human dermal fibroblasts (HDFBs) and human umbilical vein endothelial cells (HUVECs) spontaneously formed honeycomb-shaped microtissues on the patterned hydrogel surface. The microtissues on the hydrogel were able to be harvested and directly delivered to the desired target through thermal expansion of the hydrogel at 4 °C with an efficiency close to 80% within 10 min which is faster than conventional method based on poly(N-isopropylacrylamide). The microtissues maintained their original honeycomb network and intact structures. Honeycomb-patterned cell sheets also were fabricated through serial seeding of various cell lines, including HDFBs, HUVECs, and human adipose-derived stem cells, in which cells were attached along the honeycomb pattern. The underlying honeycomb patterns in the cell sheets were successfully maintained for 3 days, even after delivery. In addition, patterned cell sheets were successfully delivered in vivo while maintaining an intact structure for 7 days. Together, our findings demonstrate that micropatterned temperature-responsive hydrogel is an efficient method of one-step culturing and delivery of complex microtissues and should prove useful in various tissue engineering applications. STATEMENT OF SIGNIFICANCE: Scaffold-free cell delivery techniques, including cell sheet engineering, have been developed for decades. However, there is limited research regarding culture and delivery of microtissues with complex architecture mimicking natural tissue. Herein, we developed a micro-patterned hydrogel platform for the culture and delivery of honeycomb-shaped microtissues. Honeycomb patterns were chemically engineered on the temperature-responsive hydrogel through microcontact printing of polydopamine to selectively allow for human dermal fibroblast or human umbilical vein endothelial cell adhesion. They spontaneously formed honeycomb-shaped microtissues within 24 hr upon cell seeding and directly delivered to various target area including in vivo via thermal expansion of the hydrogel at 4 °C, suggesting that the micro-patterned hydrogel can be an efficient tool for culture and delivery of complex microtissue.

Plant Recombinant Human Collagen Type I Hydrogels for Corneal Regeneration

Purpose

To determine feasibility of plant-derived recombinant human collagen type I (RHCI) for use in corneal regenerative implants

Methods

RHCI was crosslinked with 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) to form hydrogels. Application of shear force to liquid crystalline RHCI aligned the collagen fibrils. Both aligned and random hydrogels were evaluated for mechanical and optical properties, as well as in vitro biocompatibility. Further evaluation was performed in vivo by subcutaneous implantation in rats and corneal implantation in Göttingen minipigs.

Results

Spontaneous crosslinking of randomly aligned RHCI (rRHCI) formed robust, transparent hydrogels that were sufficient for implantation. Aligning the RHCI (aRHCI) resulted in thicker collagen fibrils forming an opaque hydrogel with insufficient transverse mechanical strength for surgical manipulation. rRHCI showed minimal inflammation when implanted subcutaneously in rats. The corneal implants in minipigs showed that rRHCI hydrogels promoted regeneration of corneal epithelium, stroma, and nerves; some myofibroblasts were seen in the regenerated neo-corneas.

Conclusion

Plant-derived RHCI was used to fabricate a hydrogel that is transparent, mechanically stable, and biocompatible when grafted as corneal implants in minipigs. Plant-derived collagen is determined to be a safe alternative to allografts, animal collagens, or yeast-derived recombinant human collagen for tissue engineering applications. The main advantage is that unlike donor corneas or yeast-produced collagen, the RHCI supply is potentially unlimited due to the high yields of this production method.

Lay Summary

A severe shortage of human-donor corneas for transplantation has led scientists to develop synthetic alternatives. Here, recombinant human collagen type I made of tobacco plants through genetic engineering was tested for use in making corneal implants. We made strong, transparent hydrogels that were tested by implanting subcutaneously in rats and in the corneas of minipigs. We showed that the plant collagen was biocompatible and was able to stably regenerate the corneas of minipigs comparable to yeast-produced recombinant collagen that we previously tested in clinical trials. The advantage of the plant collagen is that the supply is potentially limitless.

Hydrogels as Emerging Materials for Cornea Wound Healing

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

Optimization of Collagen Chemical Crosslinking to Restore Biocompatibility of Tissue-Engineered Scaffolds

Collagen scaffolds, one of the most used biomaterials in corneal tissue engineering, are frequently crosslinked to improve mechanical properties, enzyme tolerance, and thermal stability. Crosslinkers such as 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) are compatible with tissues but provide low crosslinking density and reduced mechanical properties. Conversely, crosslinkers such as glutaraldehyde (GTA) can generate mechanically more robust scaffolds; however, they can also induce greater toxicity. Herein, we evaluated the effectivity of double-crosslinking with both EDC and GTA together with the capability of sodium metabisulfite (SM) and sodium borohydride (SB) to neutralize the toxicity and restore biocompatibility after crosslinking. The EDC-crosslinked collagen scaffolds were treated with different concentrations of GTA. To neutralize the free unreacted aldehyde groups, scaffolds were treated with SM or SB. The chemistry involved in these reactions together with the mechanical and functional properties of the collagen scaffolds was evaluated. The viability of the cells grown on the scaffolds was studied using different corneal cell types. The effect of each type of scaffold treatment on human monocyte differentiation was evaluated. One-way ANOVA was used for statistical analysis. The addition of GTA as a double-crosslinking agent significantly improved the mechanical properties and enzymatic stability of the EDC crosslinked collagen scaffold. GTA decreased cell biocompatibility but this effect was reversed by treatment with SB or SM. These agents did not affect the mechanical properties, enzymatic stability, or transparency of the double-crosslinked scaffold. Contact of monocytes with the different scaffolds did not trigger their differentiation into activated macrophages. Our results demonstrate that GTA improves the mechanical properties of EDC crosslinked scaffolds in a dose-dependent manner, and that subsequent treatment with SB or SM partially restores biocompatibility. This novel manufacturing approach would facilitate the translation of collagen-based artificial corneas to the clinical setting.

Collagen analogs with phosphorylcholine are inflammation-suppressing scaffolds for corneal regeneration from alkali burns in mini-pigs

The long-term survival of biomaterial implants is often hampered by surgery-induced inflammation that can lead to graft failure. Considering that most corneas receiving grafts are either pathological or inflamed before implantation, the risk of rejection is heightened. Here, we show that bioengineered, fully synthetic, and robust corneal implants can be manufactured from a collagen analog (collagen-like peptide-polyethylene glycol hybrid, CLP-PEG) and inflammation-suppressing polymeric 2-methacryloyloxyethyl phosphorylcholine (MPC) when stabilized with the triazine-based crosslinker 4-(4,6-Dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride. The resulting CLP-PEG-MPC implants led to reduced corneal swelling, haze, and neovascularization in comparison to CLP-PEG only implants when grafted into a mini-pig cornea alkali burn model of inflammation over 12 months. Implants incorporating MPC allowed for faster nerve regeneration and recovery of corneal sensation. CLP-PEG-MPC implants appear to be at a more advanced stage of regeneration than the CLP-PEG only implants, as evidenced by the presence of higher amounts of cornea-specific type V collagen, and a corresponding decrease in the presence of extracellular vesicles and exosomes in the corneal stroma, in keeping with the amounts present in healthy, unoperated corneas.

Exploring the potential of the recombinant human collagens for biomedical and clinical applications: a short review

Natural animal collagen and its recombinant collagen are favourable replacements in human tissue engineering due to their remarkable biomedical property. However, this exploitation is largely restricted due to the potential of immunogenicity and virus contamination. Exploring new ways to produce human collagen is fundamental to its biomedical and clinical application. All human fibrillar collagen molecules have three polypeptide chains constructed from a repeating Gly-Xaa-Yaa triplet, where Xaa and Yaa represent one random amino acid. Using cDNA techniques to modify several repeat sequences of the cDNA fragment, a novel human collagen, named recombinant human-like collagen (rHLC), with low immunogenicity and little risk from hidden virus can be engineered and notably tailored to specific applications. Human-like collagen (HLC) was initially used as a coating to modify the tissue engineering scaffold, and then used as the scaffold after cross-link agents were added to increase its mechanical strength. Due to its good biocompatibility, low immunogenicity, stabilised property, and the ability of mass production, HLC has been widely used in skin injury treatments, vascular scaffolds engineering, cartilage, bone defect repair, skincare, haemostatic sponge, and drug delivery, including coating with medical nanoparticles. In this review, we symmetrically reviewed the development, recent advances in design and application of HLC, and other recombinant human collagen-based biomedicine potentials. At the end, future improvements are also discussed.

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.

Modeling the cornea in 3-dimensions: Current and future perspectives

The cornea is an avascular, transparent ocular tissue that serves as a refractive and protective structure for the eye. Over 90% of the cornea is composed of a collagenous-rich extracellular matrix within the stroma with the other 10% composed by the corneal epithelium and endothelium layers and their corresponding supporting collagen layers (e.g., Bowman's and Descemet's membranes) at the anterior and posterior cornea, respectively. Due to its prominent role in corneal structure, tissue engineering approaches to model the human cornea in vitro have focused heavily on the cellular and functional properties of the corneal stroma. In this review, we discuss model development in the context of culture dimensionality (e.g., 2-dimensional versus 3-dimensional) and expand on the optical, biomechanical, and cellular functions promoted by the culture microenvironment. We describe current methods to model the human cornea with focus on organotypic approaches, compressed collagen, bioprinting, and self-assembled stromal models. We also expand on co-culture applications with the inclusion of relevant corneal cell types, such as epithelial, stromal keratocyte or fibroblast, endothelial, and neuronal cells. Further advancements in corneal tissue model development will markedly improve our current understanding of corneal wound healing and regeneration.

A reliable animal model of corneal stromal opacity: Development and validation using in vivo imaging

PURPOSE

To validate an animal model of corneal stromal opacity by using objective vision-independent in vivo imaging metrics.

METHODS

This was a prospective study, with two arms: (i) observational human arm which included 14 patients with healed unilateral ulcerative keratitis; and (ii) experimental rabbit arm, which included 6 New Zealand white rabbits. A 3-mm central wound was created in the left eye of the rabbits by manually removing 200-250 μm of the superficial stroma, followed by rotating-burr application. Both groups underwent photography, high-resolution anterior segment optical coherence tomography, and Scheimpflug imaging using similar diagnostic platforms and standardized image capturing protocols. Parameters studied were relative change in (i) corneal thickness; (ii) corneal epithelial: stromal (E:S) reflectivity ratio; (iii) corneal stromal light scattering using densitometry; and (iv) central corneal keratometry.

RESULTS

In the experimental arm, there was a significant decrease in corneal thickness (273 ± 51.3 vs. 407.3 ± 10.3 μm, p = 0.0038), E:S reflectivity ratio (0.71 ± 0.09 vs. 0.99 ± 0.06, p = 0.0018), and keratometry (40.4 ± 2.3 vs. 45.8 ± 0.9D, p = 0.0033) and increase in densitometry (54.2 ± 11.65 vs.18.7 ± 3.8 GSU, p = 0.0001) from baseline, which stabilized at 4 to 8-weeks post-wounding (p > 0.3632). At 8-weeks, the relative change from baseline in corneal thickness (28.4 ± 13.5% vs.22.4 ± 13%, p = 0.368), E:S reflectivity ratio (28.1 ± 11.5% vs. 30.6 ± 8.9%, p = 0.603), corneal densitometry (204.17 ± 97.3% vs. 304.9 ± 113.6%, p = 0.1113), and central corneal keratometry (13.6 ± 6.9% vs. 18.9 ± 7.4%, p = 0.1738) in rabbits was similar to human corneal scars.

CONCLUSION

The animal model of corneal opacification was objectively comparable to human post-keratitis scars and can be valuable for in vivo evaluation of emerging therapies for corneal opacities.

Photografting and Patterning of Poly(ethylene glycol) Methacrylate Hydrogel on Glass for Biochip Applications

An efficient procedure for chemical initiator-free, in situ synthesis of a functional polyethylene glycol methacrylate (PEG MA) hydrogel on regular glass substrates is reported. It is demonstrated that self-initiated photografting and photopolymerization driven by UV irradiation can yield tens of nanometer-thick coatings of carboxy-functionalized PEG MA on the aldehyde-terminated borosilicate glass surface. The most efficient formulation for hydrogel synthesis contained methyl methacrylic acid (MAA), 2-hydroxyethyl methacrylate (HEMA), and PEG methacrylate (PEG₁₀MA) monomers (1:1:1). The resulting HEMA/PEG₁₀MA/MAA (HPMAA) coatings had a defined thickness in the range from 11 to 50 nm. The physicochemical properties of the synthesized HPMAA coatings were analyzed by combining water contact angle measurements, stylus profilometry, imaging null ellipsometry, and atomic force microscopy (AFM). The latter technique was employed in the quantitative imaging mode not only for direct probing of the surface topography but also for swelling behavior characterization in the pH range from 4.5 to 8.0. The estimated high swelling ratios of the HPMAA hydrogel (up to 3.2) together with its good stability and resistance to nonspecific protein binding were advantageous in extracellular matrix mimetics via patterning of fibronectin (FN) at a resolution close to 200 nm. It was shown that the fabricated FN micropatterns on HPMAA were equally suitable for single-cell arraying, as well as controlled cell culture lasting at least for 96 h.

Recent developments in regenerative ophthalmology

Regenerative medicine (RM) is one of the most promising disciplines for advancements in modern medicine, and regenerative ophthalmology (RO) is one of the most active fields of regenerative medicine. This review aims to provide an overview of regenerative ophthalmology, including the range of tools and materials being used, and to describe its application in ophthalmologic subspecialties, with the exception of surgical implantation of artificial tissues or organs (e.g., contact lens, artificial cornea, intraocular lens, artificial retina, and bionic eyes) due to space limitations. In addition, current challenges and limitations of regenerative ophthalmology are discussed and future directions are highlighted.

Specialty Tough Hydrogels and Their Biomedical Applications

Hydrogels have long been explored as attractive materials for biomedical applications given their outstanding biocompatibility, high water content, and versatile fabrication platforms into materials with different physiochemical properties and geometries. Nonetheless, conventional hydrogels suffer from weak mechanical properties, restricting their use in persistent load-bearing applications often required of materials used in medical settings. Thus, the fabrication of mechanically robust hydrogels that can prolong the lifetime of clinically suitable materials under uncompromising in vivo conditions is of great interest. This review focuses on design considerations and strategies to construct such tough hydrogels. Several promising advances in the proposed use of specialty tough hydrogels for soft actuators, drug delivery vehicles, adhesives, coatings, and in tissue engineering settings are highlighted. While challenges remain before these specialty tough hydrogels will be deemed translationally acceptable for clinical applications, promising preliminary results undoubtedly spur great hope in the potential impact this embryonic research field can have on the biomedical community.

Cell Loaded GelMA:HEMA IPN hydrogels for corneal stroma engineering

Stroma is the main refractive element of the cornea and damage to it is one of the main causes of blindness. In this study, cell loaded hydrogels of methacrylated gelatin (GelMA) and poly(2-hydroxyethyl methacrylate) (pHEMA) (8:2) interpenetrating network (IPN) hydrogels were prepared as the corneal stroma substitute and tested in situ and in vitro. Compressive modulus of the GelMA hydrogels was significantly enhanced with the addition of pHEMA in the structure (6.53 vs 155.49 kPa, respectively). More than 90% of the stromal keratocytes were viable in the GelMA and GelMA-HEMA hydrogels as calculated by Live-Dead Assay and NIH Image-J program. Cells synthesized representative collagens and proteoglycans in the hydrogels indicating that they preserved their keratocyte functions. Transparency of the cell loaded GelMA-HEMA hydrogels was increased significantly up to 90% at 700 nm during three weeks of incubation and was comparable with the transparency of native cornea. Cell loaded GelMA-HEMA corneal stroma model is novel and reported for the first time in the literature in terms of introduction of cells during the preparation phase of the hydrogels. The appropriate mechanical strength and high transparency of the cell loaded constructs indicates a viable alternative to the current devices used in the treatment of corneal blindness.

Engineering topography: Effects on corneal cell behavior and integration into corneal tissue engineering

Cell-material interactions are important to tissue engineering. Inspired by the natural topographic structures on the extracellular matrix, a growing number of studies have integrated engineering topography into investigations of cell behavior on biomaterials. Engineering topography has a significant influence on cell behaviors. These cell-topography interactions play an important role in regenerative medicine and tissue engineering. Similarly, cell-topography interactions are important to corneal reconstruction and regeneration. In this review, we primarily summarized the effects of topographic cues on the behaviors of corneal cells, including cell morphology, adhesion, migration, and proliferation. Furthermore, the integration of engineering surface topography into corneal tissue engineering was also discussed.

SPARC promotes self-renewal of limbal epithelial stem cells and ocular surface restoration through JNK and p38-MAPK signaling pathways

The purpose of this study was to investigate the effects of secreted protein acidic and rich in cysteine (SPARC) on the maintenance of limbal epithelial stem cell (LESC) stemness and restoration of ocular surface. To determine the suitable concentration of SPARC for LESC culture, the marker expression, mitogenic effect, and holoclone-forming capacity of LESCs treated with different concentrations of SPARC were analyzed. To investigate the mechanism of SPARC's action on the preservation of LESCs stemness, the phosphorylation of related signaling pathways was evaluated by Western blotting. A corneal wound model was established to verify the function of SPARC in ocular surface repair. Consecutive subculturing, colony-forming efficiency, immunofluorescence, and 5-ethynyl-2-deoxyuridine incorporation assays indicated that 1 μg/mL SPARC was a suitable concentration to stimulate LESC proliferation and preserve their proliferative potential. Compared with a control group, 1 μg/mL SPARC effectively increased the expression of ABCG-2, Bmi-1, and Ki67, while decreasing that of CK3/12. The mitogenic effect of SPARC on LESCs was found to be mediated by the phosphorylation of c-Jun N-terminal kinase (JNK) and p38-MAPK signaling pathways, whereas the inhibitors of JNK and p38 MAPK reduced the marker expression and mitogenic capacity of LESCs. In a corneal injury model, SPARC facilitated corneal epithelial wound healing and promoted the proliferation of p63α-positive cells both in the limbus and in the epithelial healing front. SPARC promotes proliferation while suppressing spontaneous differentiation of LESCs through JNK and p38-MAPK signaling pathways, suggesting that SPARC is a promising factor for the improvement of LESCs culture in vitro and in vivo.

Effects of gamma radiation sterilization on the structural and biological properties of decellularized corneal xenografts

To address the shortcomings associated with corneal transplants, substantial efforts have been focused on developing new modalities such as xenotransplantion. Xenogeneic corneas are anatomically and biomechanically similar to the human cornea, yet their applications require prior decellularization to remove the antigenic components to avoid rejection. In the context of bringing decellularized corneas into clinical use, sterilization is a crucial step that determines the success of the transplantation. Well-standardized sterilization methods, such as gamma irradiation (GI), have been applied to decellularized porcine corneas (DPC) to avoid graft-associated infections in human recipients. However, little is known about the effect of GI on decellularized corneal xenografts. Here, we evaluated the radiation effect on the ultrastructure, optical, mechanical and biological properties of DPC. Transmission electron microscopy revealed that gamma irradiated decellularized porcine cornea (G-DPC) preserved its structural integrity. Moreover, the radiation did not reduce the optical properties of the tissue. Neither DPC nor G-DPC led to further activation of complement system compared to native porcine cornea when exposed to plasma. Although, DPC were mechanically comparable to the native tissue, GI increased the mechanical strength, tissue hydrophobicity and resistance to enzymatic degradation. Despite these changes, human corneal epithelial, stromal, endothelial and hybrid neuroblastoma cells grew and differentiated on DPC and G-DPC. Thus, GI may achieve effective tissue sterilization without affecting critical properties that are essential for corneal transplant survival.

Outlooks on Three-Dimensional Printing for Ocular Biomaterials Research

Given its potential for high-resolution, customizable, and waste-free fabrication of medical devices and in vitro biological models, 3-dimensional (3D) bioprinting has broad utility within the biomaterials field. Indeed, 3D bioprinting has to date been successfully used for the development of drug delivery systems, the recapitulation of hard biological tissues, and the fabrication of cellularized organ and tissue-mimics, among other applications. In this study, we highlight convergent efforts within engineering, cell biology, soft matter, and chemistry in an overview of the 3D bioprinting field, and we then conclude our work with outlooks toward the application of 3D bioprinting for ocular research in vitro and in vivo.

In Vitro Cultivation of Limbal Epithelial Stem Cells on Surface-Modified Crosslinked Collagen Scaffolds

PURPOSE

To investigate the efficacy of recombinant human collagen type I (RHC I) and collagen-like peptide (CLP) hydrogels as alternative carrier substrates for the cultivation of limbal epithelial stem cells (LESC) under xeno-free culture conditions.

METHODS

Human LESC were cultivated on seven different collagen-derived hydrogels: (1) unmodified RHC I, (2) fibronectin-patterned RHC I, (3) carbodiimide-crosslinked CLP (CLP-12 EDC), (4) DMTMM- (4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methyl-morpholinium-) crosslinked CLP (CLP-12), (5) fibronectin-patterned CLP-12, (6) "3D limbal niche-mimicking" CLP-12, and (7) DMTMM-crosslinked CLP made from higher CLP concentration solution. Cell proliferation, cell morphology, and expression of LESC markers were analyzed. All data were compared to cultures on human amniotic membrane (HAM).

RESULTS

Human LESC were successfully cultivated on six out of seven hydrogel formulations, with primary cell cultures on CLP-12 EDC being deemed unsuccessful since the area of outgrowth did not meet quality standards (i.e., inconsistence in outgrowth and confluence) after 14 days of culture. Upon confluence, primary LESC showed high expression of the stem cell marker ΔNp63, proliferation marker cytokeratin (KRT) 14, adhesion markers integrin-β4 and E-cadherin, and LESC-specific extracellular matrix proteins laminin-α1, and collagen type IV. Cells showed low expression of differentiation markers KRT3 and desmoglein 3 (DSG3). Significantly higher gene expression of KRT3 was observed for cells cultured on CLP hydrogels compared to RHC I and HAM. Surface patterning of hydrogels influenced the pattern of proliferation but had no significant effect on the phenotype or genotype of cultures. Overall, the performance of RHC I and DMTMM-crosslinked CLP hydrogels was equivalent to that of HAM.

CONCLUSION

RHC I and DMTMM-crosslinked CLP hydrogels, irrespective of surface modification, support successful cultivation of primary human LESC using a xeno-free cultivation protocol. The regenerated epithelium maintained similar characteristics to HAM-based cultures.

Combined experimental and computational characterization of crosslinked collagen-based hydrogels

Collagen hydrogels are widely used for in-vitro experiments and tissue engineering applications. Their use has been extended due to their biocompatibility with cells and their capacity to mimic biological tissues; nevertheless their mechanical properties are not always optimal for these purposes. Hydrogels are formed by a network of polymer filaments embedded on an aqueous substrate and their mechanical properties are mainly defined by the filament network architecture and the individual filament properties. To increase properties of native collagen, such as stiffness or strain-stiffening, these networks can be modified by adding crosslinking agents that alter the network architecture, increasing the unions between filaments. In this work, we have investigated the effect of one crosslinking agent, transglutaminase, in collagen hydrogels with varying collagen concentration. We have observed a linear dependency of the gel rigidity on the collagen concentration. Moreover, the addition of transglutaminase has induced an earlier strain-stiffening of the collagen gels. In addition, to better understand the mechanical implications of collagen concentration and crosslinkers inclusion, we have adapted an existing computational model, based on the worm-like chain model (WLC), to reproduce the mechanical behavior of the collagen gels. With this model we can estimate the parameters of the biopolymer networks without more sophisticated techniques, such as image processing or network reconstruction, or, inversely, predict the mechanical properties of a defined collagen network.

Improving the practicality and safety of artificial corneas: Pre-assembly and gamma-rays sterilization of the Boston Keratoprosthesis

PURPOSE

To make the Boston keratoprosthesis (B-KPro), together with its carrier corneal graft, more easily procured, transported and stored, as well as less expensive, easier for the surgeon to implant and safer for the patient, it is proposed that the B-KPro-graft combination be pre-assembled by an expert technician, followed by sterilization with gamma ray irradiation (GI) allowing long-term storage at room temperature. For this to be possible, it must be shown that the B-KPro itself (not only the graft) remains unharmed by the irradiation.

METHODS

Polymethyl methacrylate (PMMA) discs and B-KPros were submitted to either ethylene oxide sterilization or different doses of GI. Cell biocompatibility, mechanical strength and optical quality were evaluated. The feasibility of assembling the B-KPro to a corneal graft, and gamma-radiate afterwards, was also assessed.

RESULTS

There were no differences in cell biocompatibility between the samples. The optical evaluation showed high levels of transparency for all the groups. The absorbance of ultraviolet was higher for the groups treated with GI. The mechanical evaluation by nanoindentation showed no alterations of the PMMA discs after GI. The flexure test revealed a similar mechanical behavior. Technically, pre-assembly and GI of the B-KPro revealed no problems.

CONCLUSIONS

Sterilization of B-KPro using GI has no detrimental influence on the device. The pre-assembly of B-KPro to a donor cornea, followed by gamma sterilization, emerges as an efficient and safe procedure.

3D Microfabricated Scaffolds and Microfluidic Devices for Ocular Surface Replacement: a Review

In recent years, there has been increased research interest in generating corneal substitutes, either for use in the clinic or as in vitro corneal models. The advancement of 3D microfabrication technologies has allowed the reconstruction of the native microarchitecture that controls epithelial cell adhesion, migration and differentiation. In addition, such technology has allowed the inclusion of a dynamic fluid flow that better mimics the physiology of the native cornea. We review the latest innovative products in development in this field, from 3D microfabricated hydrogels to microfluidic devices.

Short peptide analogs as alternatives to collagen in pro-regenerative corneal implants

Short collagen-like peptides (CLPs) are being proposed as alternatives to full-length collagen for use in tissue engineering, on their own as soft hydrogels, or conjugated to synthetic polymer for mechanical strength. However, despite intended clinical use, little is known about their safety and efficacy, mechanism of action or degree of similarity to the full-length counterparts they mimic. Here, we show the functional equivalence of a CLP conjugated to polyethylene glycol (CLP-PEG) to full-length recombinant human collagen in vitro and in promoting stable regeneration of corneal tissue and nerves in a pre-clinical mini-pig model. We also show that these peptide analogs exerted their pro-regeneration effects through stimulating extracellular vesicle production by host cells. Our results support future use of CLP-PEG implants for corneal regeneration, suggesting the feasibility of these or similar peptide analogs in clinical application in the eye and other tissues.

Statement of significance

Although biomaterials comprising full-length recombinant human collagen and extracted animal collagen have been evaluated and used clinically, these macromolecules provide only a limited number of functional groups amenable to chemical modification or crosslinking and are demanding to process. Synthetic, customizable analogs that are functionally equivalent, and can be readily scaled-up are therefore very desirable for pre-clinical to clinical translation. Here, we demonstrate, using cornea regeneration as our test bed, that collagen-like-peptides conjugated to multifunctional polyethylene glycol (CLP-PEG) when grafted into mini-pigs as corneal implants were functionally equivalent to recombinant human collagen-based implants that were successfully tested in patients. We also show for the first time that these materials affected regeneration through stimulation of extracellular vesicle production by endogenous host cells that have migrated into the CLP-PEG scaffolds.

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.