Sequentially-crosslinked bioactive hydrogels as nano-patterned substrates with customizable stiffness and degradation for corneal tissue engineering applications

Bioprinting of anisotropic functional corneal stroma using mechanically robust multi-material bioink based on decellularized cornea matrix

Corneal scarring is a common cause of blindness, affecting millions globally each year. A huge gap between the demand and supply of donor tissue currently limits corneal transplantation, the only definitive therapy for patients with corneal scarring. To overcome this challenge, researchers have harnessed the efficacy of 3D bioprinting to fabricate artificial corneal stromal constructs. With all the different bioinks available, the decellularized corneal matrix-based bioprinted construct can fulfill the required biological functionality but is limited by the lack of mechanical stiffness. Additionally, from a biophysical standpoint, it is necessary for an ideal corneal substitute to mimic the anisotropy of the cornea from the central optic zone to the surrounding periphery. In this study, we enhanced the mechanical robustness of decellularized cornea matrix (DCM) hydrogel by blending it with another natural polymer, sonicated silk fibroin solution in a defined ratio. Although hybrid hydrogel has an increased complex modulus than DCM hydrogel, it has a lower in vitro degradation rate and increased opaqueness due to the presence of crystalline beta-sheet conformation within the hydrogel. Therefore, we used this multi-material bioink-based approach to fabricate a corneal stromal equivalent where the outer peripheral corneal rim was printed with a mechanically robust polymeric blend of DCM and sonicated silk fibroin and the central optic zone was printed with only DCM. The bioprinted corneal stroma thus maintained its structural integrity and did not break when lifted with forceps. The two different bioinks were encapsulated with human limbus-derived mesenchymal stem cells (hLMSC) individually and 3D bioprinted in different patterns (concentric and parallel) to attain a native-like structure in terms of architecture and transparency. Thus, the bilayer cornea constructs maintained high cell viability and expressed keratocyte core proteins indicating optimal functionality. This approach helped to gain insight into bioprinting corneas with heterogeneous mechanical property without disturbing the structural clarity of the central optic zone.

Advancements in Polymer Biomaterials as Scaffolds for Corneal Endothelium Tissue Engineering

Corneal endothelial dysfunction is a leading cause of vision loss globally, frequently requiring corneal transplantation. However, the limited availability of donor tissues, particularly in developing countries, has spurred on the exploration of tissue engineering strategies, with a focus on polymer biomaterials as scaffolds for corneal endotlhelium regeneration. This review provides a comprehensive overview of the advancements in polymer biomaterials, focusing on their role in supporting the growth, differentiation, and functional maintenance of human corneal endothelial cells (CECs). Key properties of scaffold materials, including optical clarity, biocompatibility, biodegradability, mechanical stability, permeability, and surface wettability, are discussed in detail. The review also explores the latest innovations in micro- and nano-topological morphologies, fabrication techniques such as electrospinning and 3D/4D bioprinting, and the integration of drug delivery systems into scaffolds. Despite significant progress, challenges remain in translating these technologies to clinical applications. Future directions for research are highlighted, including the need for improved biomaterial combinations, a deeper understanding of CEC biology, and the development of scalable manufacturing processes. This review aims to serve as a resource for researchers and clinician-scientists seeking to advance the field of corneal endothelium tissue engineering.

Bioprinting of Cells, Organoids and Organs-on-a-Chip Together with Hydrogels Improves Structural and Mechanical Cues

The 3D bioprinting technique has made enormous progress in tissue engineering, regenerative medicine and research into diseases such as cancer. Apart from individual cells, a collection of cells, such as organoids, can be printed in combination with various hydrogels. It can be hypothesized that 3D bioprinting will even become a promising tool for mechanobiological analyses of cells, organoids and their matrix environments in highly defined and precisely structured 3D environments, in which the mechanical properties of the cell environment can be individually adjusted. Mechanical obstacles or bead markers can be integrated into bioprinted samples to analyze mechanical deformations and forces within these bioprinted constructs, such as 3D organoids, and to perform biophysical analysis in complex 3D systems, which are still not standard techniques. The review highlights the advances of 3D and 4D printing technologies in integrating mechanobiological cues so that the next step will be a detailed analysis of key future biophysical research directions in organoid generation for the development of disease model systems, tissue regeneration and drug testing from a biophysical perspective. Finally, the review highlights the combination of bioprinted hydrogels, such as pure natural or synthetic hydrogels and mixtures, with organoids, organoid-cell co-cultures, organ-on-a-chip systems and organoid-organ-on-a chip combinations and introduces the use of assembloids to determine the mutual interactions of different cell types and cell-matrix interferences in specific biological and mechanical environments.

Exploring single-cell RNA sequencing as a decision-making tool in the clinical management of Fuchs' endothelial corneal dystrophy

Single-cell RNA sequencing (scRNA-seq) has enabled the identification of novel gene signatures and cell heterogeneity in numerous tissues and diseases. Here we review the use of this technology for Fuchs' Endothelial Corneal Dystrophy (FECD). FECD is the most common indication for corneal endothelial transplantation worldwide. FECD is challenging to manage because it is genetically heterogenous, can be autosomal dominant or sporadic, and progress at different rates. Single-cell RNA sequencing has enabled the discovery of several FECD subtypes, each with associated gene signatures, and cell heterogeneity. Current FECD treatments are mainly surgical, with various Rho kinase (ROCK) inhibitors used to promote endothelial cell metabolism and proliferation following surgery. A range of emerging therapies for FECD including cell therapies, gene therapies, tissue engineered scaffolds, and pharmaceuticals are in preclinical and clinical trials. Unlike conventional disease management methods based on clinical presentations and family history, targeting FECD using scRNA-seq based precision-medicine has the potential to pinpoint the disease subtypes, mechanisms, stages, severities, and help clinicians in making the best decision for surgeries and the applications of therapeutics. In this review, we first discuss the feasibility and potential of using scRNA-seq in clinical diagnostics for FECD, highlight advances from the latest clinical treatments and emerging therapies for FECD, integrate scRNA-seq results and clinical notes from our FECD patients and discuss the potential of applying alternative therapies to manage these cases clinically.

Photocrosslinked carboxymethylcellulose-based hydrogels: Synthesis, characterization for curcumin delivery and wound healing

Skin could protect our body and regenerate itself to against dysfunctional and disfiguring scars when faced with external injury. As wound dressings, hydrogels are biocompatible, hydrophilic and have a 3D structure similar to the extracellular matrix (ECM). In particular, hydrogels with drug-releasing capabilities are in acute wound healing. In this paper, photocrosslinked hydrogels served as wound dressing based on sodium carboxymethylcellulose (CMC) were prepared to promote wound healing. Photocrosslinked hydrogels were prepared by grafting lysine and allyl glycidyl ether (AGE) onto CMC and encapsulating curcumin (Cur). The synthesized hydrogels had the unique 3D porous structure with a swelling ratio up to 1300 % in aqueous solution. The drug release ratios of the hydrogels were 20.8 % in acid environment, and 14.4 % in alkaline environment. Notably, the hydrogels showed good biocompatibility and antibacterial properties and also exhibited the ability to accelerate the process of skin wound healing while prevent inflammation and scar formation when applied to a mouse skin wound model. As a result, the prepared hydrogels Gel-CLA@Cur showed great potential in wound healing.

Potential of an Amphiphilic Artificial Corneal Endothelial Layer as a Replacement Option for Damaged Corneal Endothelium

Bullous keratopathy, a condition severely impacting vision and potentially leading to corneal blindness, necessitates corneal transplantation. However, the shortage of donor corneas and complex surgical procedures drive the exploration of tissue-engineered corneal endothelial layers. This study develops a transparent, amphiphilic, and cell-free membrane for corneal endothelial replacement. The membrane, securely attached to the posterior surface of the cornea, is created by mixing hydroxyethyl methacrylate (HEMA) and ethylene glycol dimethylacrylate (EGDMA) in a 10:1 ratio. A 50 µL volume is used to obtain a 60 µm hydrophobic membrane on both sides, with one side treated with a polyvinylpyrrolidone (PVP) solution. The resulting membrane is transparent, foldable, biocompatible, amphiphilic, and easily handled. When exposed to 20% sulfur hexafluoride (SF6), the hydrophilic side of the membrane adheres tightly to the corneal Descemet's membrane, preventing water absorption into the corneal stroma, and thus treating bullous keratopathy. Histological test confirms its effectiveness, showing normal corneal structure and low inflammation when implanted in rabbits for up to 100 d. This study showcases the potential of this membrane as a viable option for corneal endothelial replacement, offering a novel approach to address donor tissue scarcity in corneal transplantation.

Drug-eluting contact lenses: Progress, challenges, and prospects

Topical ophthalmic solutions (eye drops) are becoming increasingly popular in treating and preventing ocular diseases for their safety, noninvasiveness, and ease of handling. However, the static and dynamic barriers of eyes cause the extremely low bioavailability (<5%) of eye drops, making ocular therapy challenging. Thus, drug-eluting corneal contact lenses (DECLs) have been intensively investigated as a drug delivery device for their attractive properties, such as sustained drug release and improved bioavailability. In order to promote the clinical application of DECLs, multiple aspects, i.e., drug release and penetration, safety, and biocompatibility, of these drug delivery systems were thoroughly examined. In this review, we systematically discussed advances in DECLs, including types of preparation materials, drug-loading strategies, drug release mechanisms, strategies for penetrating ocular barriers, in vitro and in vivo drug delivery and penetration detection, safety, and biocompatibility validation methods, as well as challenges and future perspectives.

Injectable Hydrogels in Cardiovascular Tissue Engineering

Heart problems are quite prevalent worldwide. Cardiomyocytes and stem cells are two examples of the cells and supporting matrix that are used in the integrated process of cardiac tissue regeneration. The objective is to create innovative materials that can effectively replace or repair damaged cardiac muscle. One of the most effective and appealing 3D/4D scaffolds for creating an appropriate milieu for damaged tissue growth and healing is hydrogel. In order to successfully regenerate heart tissue, bioactive and biocompatible hydrogels are required to preserve cells in the infarcted region and to bid support for the restoration of myocardial wall stress, cell survival and function. Heart tissue engineering uses a variety of hydrogels, such as natural or synthetic polymeric hydrogels. This article provides a quick overview of the various hydrogel types employed in cardiac tissue engineering. Their benefits and drawbacks are discussed. Hydrogel-based techniques for heart regeneration are also addressed, along with their clinical application and future in cardiac tissue engineering.

Unveiling the versatility of gelatin methacryloyl hydrogels: a comprehensive journey into biomedical applications

Gelatin methacryloyl (GelMA) hydrogels have gained significant recognition as versatile biomaterials in the biomedical domain. GelMA hydrogels emulate vital characteristics of the innate extracellular matrix by integrating cell-adhering and matrix metalloproteinase-responsive peptide motifs. These features enable cellular proliferation and spreading within GelMA-based hydrogel scaffolds. Moreover, GelMA displays flexibility in processing, as it experiences crosslinking when exposed to light irradiation, supporting the development of hydrogels with adjustable mechanical characteristics. The drug delivery landscape has been reshaped by GelMA hydrogels, offering a favorable platform for the controlled and sustained release of therapeutic actives. The tunable physicochemical characteristics of GelMA enable precise modulation of the kinetics of drug release, ensuring optimal therapeutic effectiveness. In tissue engineering, GelMA hydrogels perform an essential role in the design of the scaffold, providing a biomimetic environment conducive to cell adhesion, proliferation, and differentiation. Incorporating GelMA in three-dimensional printing further improves its applicability in drug delivery and developing complicated tissue constructs with spatial precision. Wound healing applications showcase GelMA hydrogels as bioactive dressings, fostering a conducive microenvironment for tissue regeneration. The inherent biocompatibility and tunable mechanical characteristics of GelMA provide its efficiency in the closure of wounds and tissue repair. GelMA hydrogels stand at the forefront of biomedical innovation, offering a versatile platform for addressing diverse challenges in drug delivery, tissue engineering, and wound healing. This review provides a comprehensive overview, fostering an in-depth understanding of GelMA hydrogel's potential impact on progressing biomedical sciences.

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.

Gelatin Methacryloyl (GelMA)-Based Biomaterial Inks: Process Science for 3D/4D Printing and Current Status

Tissue engineering for injured tissue replacement and regeneration has been a subject of investigation over the last 30 years, and there has been considerable interest in using additive manufacturing to achieve these goals. Despite such efforts, many key questions remain unanswered, particularly in the area of biomaterial selection for these applications as well as quantitative understanding of the process science. The strategic utilization of biological macromolecules provides a versatile approach to meet diverse requirements in 3D printing, such as printability, buildability, and biocompatibility. These molecules play a pivotal role in both physical and chemical cross-linking processes throughout the biofabrication, contributing significantly to the overall success of the 3D printing process. Among the several bioprintable materials, gelatin methacryloyl (GelMA) has been widely utilized for diverse tissue engineering applications, with some degree of success. In this context, this review will discuss the key bioengineering approaches to identify the gelation and cross-linking strategies that are appropriate to control the rheology, printability, and buildability of biomaterial inks. This review will focus on the GelMA as the structural (scaffold) biomaterial for different tissues and as a potential carrier vehicle for the transport of living cells as well as their maintenance and viability in the physiological system. Recognizing the importance of printability toward shape fidelity and biophysical properties, a major focus in this review has been to discuss the qualitative and quantitative impact of the key factors, including microrheological, viscoelastic, gelation, shear thinning properties of biomaterial inks, and printing parameters, in particular, reference to 3D extrusion printing of GelMA-based biomaterial inks. Specifically, we emphasize the different possibilities to regulate mechanical, swelling, biodegradation, and cellular functionalities of GelMA-based bio(material) inks, by hybridization techniques, including different synthetic and natural biopolymers, inorganic nanofillers, and microcarriers. At the close, the potential possibility of the integration of experimental data sets and artificial intelligence/machine learning approaches is emphasized to predict the printability, shape fidelity, or biophysical properties of GelMA bio(material) inks for clinically relevant tissues.

Leveraging the Recent Advancements in GelMA Scaffolds for Bone Tissue Engineering: An Assessment of Challenges and Opportunities

The field of bone tissue engineering has seen significant advancements in recent years. Each year, over two million bone transplants are performed globally, and conventional treatments, such as bone grafts and metallic implants, have their limitations. Tissue engineering offers a new level of treatment, allowing for the creation of living tissue within a biomaterial framework. Recent advances in biomaterials have provided innovative approaches to rebuilding bone tissue function after damage. Among them, gelatin methacryloyl (GelMA) hydrogel is emerging as a promising biomaterial for supporting cell proliferation and tissue regeneration, and GelMA has exhibited exceptional physicochemical and biological properties, making it a viable option for clinical translation. Various methods and classes of additives have been used in the application of GelMA for bone regeneration, with the incorporation of nanofillers or other polymers enhancing its resilience and functional performance. Despite promising results, the fabrication of complex structures that mimic the bone architecture and the provision of balanced physical properties for both cell and vasculature growth and proper stiffness for load bearing remain as challenges. In terms of utilizing osteogenic additives, the priority should be on versatile components that promote angiogenesis and osteogenesis while reinforcing the structure for bone tissue engineering applications. This review focuses on recent efforts and advantages of GelMA-based composite biomaterials for bone tissue engineering, covering the literature from the last five years.

Biofabrication paradigms in corneal regeneration: bridging bioprinting techniques, natural bioinks, and stem cell therapeutics

Corneal diseases are a major cause of vision loss worldwide. Traditional methods like corneal transplants from donors are effective but face challenges like limited donor availability and the risk of graft rejection. Therefore, new treatment methods are essential. This review examines the growing field of bioprinting and biofabrication in corneal tissue engineering. We begin by discussing various bioprinting methods such as stereolithography, inkjet, and extrusion printing, highlighting their strengths and weaknesses for eye-related uses. We also explore how biological tissues are made suitable for bioprinting through a process called decellularization, which can be achieved using chemical, physical, or biological methods. The review then looks at natural materials, known as bioinks, used in bioprinting. We focus on materials like gelatin, collagen, fibrin, chitin, chitosan, silk fibroin, and alginate, examining their mechanical and biological properties. The importance of hydrogel scaffolds, particularly those based on collagen and other materials, is also discussed in the context of repairing corneal tissue. Another key area we cover is the use of stem cells in corneal regeneration. We pay special attention to limbal epithelial stem cells and mesenchymal stromal cells, highlighting their roles in this process. The review concludes with an overview of the latest advancements in corneal tissue bioprinting, from early techniques to advanced methods of delivering stem cells using bioengineered materials. In summary, this review presents the current state and future potential of bioprinting and biofabrication in creating functional corneal tissues, highlighting new developments and ongoing challenges with a view towards restoring vision.

Ultra-low content physio-chemically crosslinked gelatin hydrogel improves encapsulated 3D cell culture

Gelatin-based hydrogels are extensively used for 3D cell culture, bioprinting, and tissue engineering due to their cell-adhesive nature and tunable physio-chemical properties. Gelatin hydrogels for 3D cell culture are often developed using high-gelatin content (frequently 10-15 % w/v) to ensure fast gelation and improved stability. While highly stable, such matrices restrict the growth of encapsulated cells due to creating a dense, restrictive environment around the encapsulated cells. Hydrogels with lower polymer content are known to improve 3D cell growth, yet fabrication of ultra-low concentration gelatin hydrogels is challenging while ensuring fast gelation and stability. Here, we demonstrate that physical gelation and photo-crosslinking in gelatin results in a fast-gelling hydrogel at a remarkably low gelatin concentration of 1 % w/v (GelPhy/Photo). The GelPhy/Photo hydrogel was highly stable, allowed uniform 3D distribution of cells, and significantly improved the spreading of encapsulated 3T3 fibroblast cells. Moreover, human cholangiocarcinoma (HuCCT-1) cells encapsulated in 1 % GelPhy/Photo matrix grew and self-assembled into epithelial cysts with lumen, which could not be achieved in a traditional high-concentration gelatin hydrogel. These findings pave the way to significantly improve existing gelatin hydrogels for 3D cell culture applications.

Recent Progress and Challenges of Implantable Biodegradable Biosensors

Implantable biosensors have evolved to the cutting-edge technology of personalized health care and provide promise for future directions in precision medicine. This is the reason why these devices stand to revolutionize our approach to health and disease management and offer insights into our bodily functions in ways that have never been possible before. This review article tries to delve into the important developments, new materials, and multifarious applications of these biosensors, along with a frank discussion on the challenges that the devices will face in their clinical deployment. In addition, techniques that have been employed for the improvement of the sensitivity and specificity of the biosensors alike are focused on in this article, like new biomarkers and advanced computational and data communicational models. A significant challenge of miniaturized in situ implants is that they need to be removed after serving their purpose. Surgical expulsion provokes discomfort to patients, potentially leading to post-operative complications. Therefore, the biodegradability of implants is an alternative method for removal through natural biological processes. This includes biocompatible materials to develop sensors that remain in the body over longer periods with a much-reduced immune response and better device longevity. However, the biodegradability of implantable sensors is still in its infancy compared to conventional non-biodegradable ones. Sensor design, morphology, fabrication, power, electronics, and data transmission all play a pivotal role in developing medically approved implantable biodegradable biosensors. Advanced material science and nanotechnology extended the capacity of different research groups to implement novel courses of action to design implantable and biodegradable sensor components. But the actualization of such potential for the transformative nature of the health sector, in the first place, will have to surmount the challenges related to biofouling, managing power, guaranteeing data security, and meeting today's rules and regulations. Solving these problems will, therefore, not only enhance the performance and reliability of implantable biodegradable biosensors but also facilitate the translation of laboratory development into clinics, serving patients worldwide in their better disease management and personalized therapeutic interventions.

Biopolymeric corneal lenticules by digital light processing based bioprinting: a dynamic substitute for corneal transplant

Digital light processing (DLP) technology has gained significant attention for its ability to construct intricate structures for various applications in tissue modeling and regeneration. In this study, we aimed to design corneal lenticules using DLP bioprinting technology, utilizing dual network bioinks to mimic the characteristics of the human cornea. The bioink was prepared using methacrylated hyaluronic acid and methacrylated gelatin, where ruthenium salt and sodium persulfate were included for mediating photo-crosslinking while tartrazine was used as a photoabsorber. The bioprinted lenticules were optically transparent (85.45% ± 0.14%), exhibited adhesive strength (58.67 ± 17.5 kPa), and compressive modulus (535.42 ± 29.05 kPa) sufficient for supporting corneal tissue integration and regeneration. Puncture resistance tests and drag force analysis further confirmed the excellent mechanical performance of the lenticules enabling their application as potential corneal implants. Additionally, the lenticules demonstrated outstanding support for re-epithelialization and stromal regeneration when assessed with human corneal stromal cells. We generated implant ready corneal lenticules while optimizing bioink and bioprinting parameters, providing valuable solution for individuals suffering from various corneal defects and waiting for corneal transplants.

Advancing Peripheral Nerve Regeneration: 3D Bioprinting of GelMA-Based Cell-Laden Electroactive Bioinks for Nerve Conduits

Peripheral nerve injuries often result in substantial impairment of the neurostimulatory organs. While the autograft is still largely used as the "gold standard" clinical treatment option, nerve guidance conduits (NGCs) are currently considered a promising approach for promoting peripheral nerve regeneration. While several attempts have been made to construct NGCs using various biomaterial combinations, a comprehensive exploration of the process science associated with three-dimensional (3D) extrusion printing of NGCs with clinically relevant sizes (length: 20 mm; diameter: 2-8 mm), while focusing on tunable buildability using electroactive biomaterial inks, remains unexplored. In addressing this gap, we present here the results of the viscoelastic properties of a range of a multifunctional gelatin methacrylate (GelMA)/poly(ethylene glycol) diacrylate (PEGDA)/carbon nanofiber (CNF)/gellan gum (GG) hydrogel bioink formulations and printability assessment using experiments and quantitative models. Our results clearly established the positive impact of the gellan gum on the enhancement of the rheological properties. Interestingly, the strategic incorporation of PEGDA as a secondary cross-linker led to a remarkable enhancement in the strength and modulus by 3 and 8-fold, respectively. Moreover, conductive CNF addition resulted in a 4-fold improvement in measured electrical conductivity. The use of four-component electroactive biomaterial ink allowed us to obtain high neural cell viability in 3D bioprinted constructs. While the conventionally cast scaffolds can support the differentiation of neuro-2a cells, the most important result has been the excellent cell viability of neural cells in 3D encapsulated structures. Taken together, our findings demonstrate the potential of 3D bioprinting and multimodal biophysical cues in developing functional yet critical-sized nerve conduits for peripheral nerve tissue regeneration.

Emerging granular hydrogel bioinks to improve biological function in bioprinted constructs

Advancements in 3D bioprinting have been hindered by the trade-off between printability and biological functionality. Existing bioinks struggle to meet both requirements simultaneously. However, new types of bioinks composed of densely packed microgels promise to address this challenge. These bioinks possess intrinsic porosity, allowing for cell growth, oxygen and nutrient transport, and better immunomodulatory properties, leading to superior biological functions. In this review, we highlight key trends in the development of these granular bioinks. Using examples, we demonstrate how granular bioinks overcome the trade-off between printability and cell function. Granular bioinks show promise in 3D bioprinting, yet understanding their unique structure-property-function relationships is crucial to fully leverage the transformative capabilities of these new types of bioinks in bioprinting.

Fabrication and Characterization of an Enzyme-Triggered, Therapeutic-Releasing Hydrogel Bandage Contact Lens Material

PURPOSE

The purpose of this study was to develop an enzyme-triggered, therapeutic-releasing bandage contact lens material using a unique gelatin methacrylate formulation (GelMA+).

METHODS

Two GelMA+ formulations, 20% w/v, and 30% w/v concentrations, were prepared through UV polymerization. The physical properties of the material, including porosity, tensile strain, and swelling ratio, were characterized. The enzymatic degradation of the material was assessed in the presence of matrix metalloproteinase-9 (MMP-9) at concentrations ranging from 0 to 300 µg/mL. Cell viability, cell growth, and cytotoxicity on the GelMA+ gels were evaluated using the AlamarBlueTM assay and the LIVE/DEADTM Viability/Cytotoxicity kit staining with immortalized human corneal epithelial cells over 5 days. For drug release analysis, the 30% w/v gels were loaded with 3 µg of bovine lactoferrin (BLF) as a model drug, and its release was examined over 5 days under various MMP-9 concentrations.

RESULTS

The 30% w/v GelMA+ demonstrated higher crosslinking density, increased tensile strength, smaller pore size, and lower swelling ratio (p < 0.05). In contrast, the 20% w/v GelMA+ degraded at a significantly faster rate (p < 0.001), reaching almost complete degradation within 48 h in the presence of 300 µg/mL of MMP-9. No signs of cytotoxic effects were observed in the live/dead staining assay for either concentration after 5 days. However, the 30% w/v GelMA+ exhibited significantly higher cell viability (p < 0.05). The 30% w/v GelMA+ demonstrated sustained release of the BLF over 5 days. The release rate of BLF increased significantly with higher concentrations of MMP-9 (p < 0.001), corresponding to the degradation rate of the gels.

DISCUSSION

The release of BLF from GelMA+ gels was driven by a combination of diffusion and degradation of the material by MMP-9 enzymes. This work demonstrated that a GelMA+-based material that releases a therapeutic agent can be triggered by enzymes found in the tear fluid.

A Rapid Screening Platform for Simultaneous Evaluation of Biodegradation and Therapeutic Release of an Ocular Hydrogel

This study attempts to address the challenge of accurately measuring the degradation of biodegradable hydrogels, which are frequently employed in drug delivery for controlled and sustained release. The traditional method utilizes a mass-loss approach, which is cumbersome and time consuming. The aim of this study was to develop an innovative screening platform using a millifluidic device coupled with automated image analysis to measure the degradation of Gelatin methacrylate (GelMA) and the subsequent release of an entrapped wetting agent, polyvinyl alcohol (PVA). Gel samples were placed within circular wells on a custom millifluidic chip and stained with a red dye for enhanced visualization. A camera module captured time-lapse images of the gels throughout their degradation. An image-analysis algorithm was used to translate the image data into degradation rates. Simultaneously, the eluate from the chip was collected to quantify the amount of GelMA degraded and PVA released at various time points. The visual method was validated by comparing it with the mass-loss approach (R = 0.91), as well as the amount of GelMA eluted (R = 0.97). The degradation of the GelMA gels was also facilitated with matrix metalloproteinases 9. Notably, as the gels degraded, there was an increase in the amount of PVA released. Overall, these results support the use of the screening platform to assess hydrogel degradation and the subsequent release of entrapped therapeutic compounds.

The Innovative Biomaterials and Technologies for Developing Corneal Endothelium Tissue Engineering Scaffolds: A Review and Prospect

Corneal transplantation is the only treatment for corneal endothelial blindness. However, there is an urgent need to find substitutes for corneal endothelium grafts due to the global shortage of donor corneas. An emerging research field focuses on the construction of scaffold-based corneal endothelium tissue engineering (CETE). Long-term success in CETE transplantation may be achieved by selecting the appropriate biomaterials as scaffolds of corneal endothelial cells and adding bioactive materials to promote cell activity. This article reviews the research progress of CETE biomaterials in the past 20 years, describes the key characteristics required for corneal endothelial scaffolds, and summarizes the types of materials that have been reported. Based on these, we list feasible improvement strategies for biomaterials innovation. In addition, we describe the improved techniques for the scaffolds' surface topography and drug delivery system. Some promising technologies for constructing CETE are proposed. However, some questions have not been answered yet, and clinical trials and industrialization should be carried out with caution.

Establishing the Callus-Based Isolation of Extracellular Vesicles from Cissus quadrangularis and Elucidating Their Role in Osteogenic Differentiation

Extracellular vesicles (EVs) are nano-sized vehicles secreted by all live cells to establish communication with adjacent cells. In recent years, mammalian EVs (MEVs) have been widely investigated for their therapeutic implications in human disease conditions. As the understanding of MEV composition and nature is advancing, scientists are constantly exploring alternatives for EV production with similar therapeutic potential. Plant-derived exosome-like nanovesicles (PDEVs) may be a better substitute for MEVs because of their widespread sources, cost-effectiveness, and ease of access. Cissus quadrangularis (CQ), known as "bone setter or Hadjod", is a perennial plant utilized for its osteogenic potential. Its crude powder extract formulations are widely used as tablets and syrups. The present work elucidates the isolation of exosome-like nanovesicles (henceforth exosomes) from the culture supernatants of an in vitro cultured callus tissue derived from CQ. The physical and biological properties of the exosomes were successfully investigated using different characterization techniques. The therapeutic potential of the CQ exosomes was found to ameliorate the wound scratch injury and oxidative stress conditions in human-derived mesenchymal stem cells (hMSCs) and the pre-osteoblast (MC3T3) cell line. These exosomes also induced the proliferation and differentiation of hMSCs, as observed by alkaline phosphatase activity. These findings may serve as a proof of concept for further investigating the CQ exosomes as a nanocarrier for drug molecules in various therapeutic bone applications.

A suture-free, shape self-adaptive and bioactive PEG-Lysozyme implant for Corneal stroma defect repair and rapid vision restoration

Corneal transplantation is a prevailing treatment to repair injured cornea and restore vision but faces the limitation of donor tissue shortage clinically. In addition, suturing-needed transplantation potentially causes postoperative complications. Herein, we design a PEG-Lysozyme injective hydrogel as a suture-free, shape self-adaptive, bioactive implant for corneal stroma defect repair. This implant experiences a sol-gel phase transition via an in situ amidation reaction between 4-arm-PEG-NHS and lysozyme. The physicochemical properties of PEG-Lysozyme can be tuned by the components ratio, which confers the implant mimetic corneal modulus and provides tissue adhesion to endure increased intraocular pressure. In vitro tests prove that the implant is beneficial to Human corneal epithelial cells growth and migration due to the bioactivity of lysozyme. Rabbit lamellar keratoplasty experiment demonstrates that the hydrogel can be filled into defect to form a shape-adaptive implant adhered to native stroma. The implant promotes epithelialization and stroma integrity, recovering the topology of injured cornea to normal. A newly established animal forging behavior test prove a rapid visual restoration of rabbits when use implant in a suture free manner. In general, this work provides a promising preclinical practice by applicating a self-curing, shape self-adaptive and bioactive PEG-Lysozyme implant for suture-free stroma repair.

Biomaterials used for tissue engineering of barrier-forming cell monolayers in the eye

Cell monolayers that form a barrier between two structures play an important role for the maintenance of tissue functionality. In the anterior portion of the eye, the corneal endothelium forms a barrier that controls fluid exchange between the aqueous humor of the anterior chamber and the corneal stroma. This monolayer is central in the pathogenesis of Fuchs endothelial corneal dystrophy (FECD). FECD is a common corneal disease, in which corneal endothelial cells deposit extracellular matrix that increases the thickness of its basal membrane (Descemet's membrane), and forms excrescences (guttae). With time, there is a decrease in endothelial cell density that generates vision loss. Transplantation of a monolayer of healthy corneal endothelial cells on a Descemet membrane substitute could become an interesting alternative for the treatment of this pathology. In the back of the eye, the retinal pigment epithelium (RPE) forms the blood-retinal barrier, controlling fluid exchange between the choriocapillaris and the photoreceptors of the outer retina. In the retinal disease dry age-related macular degeneration (dry AMD), deposits (drusen) form between the RPE and its basal membrane (Bruch's membrane). These deposits hinder fluid exchange, resulting in progressive RPE cell death, which in turn generates photoreceptor cell death, and vision loss. Transplantation of a RPE monolayer on a Bruch's membrane/choroidal stromal substitute to replace the RPE before photoreceptor cell death could become a treatment alternative for this eye disease. This review will present the different biomaterials that are proposed for the engineering of a monolayer of corneal endothelium for the treatment of FECD, and a RPE monolayer for the treatment of dry AMD.

Application of biomaterials and nanotechnology in corneal tissue engineering

Corneal diseases are among the most common causes of blindness worldwide. Regardless of the etiology, corneal opacity- or globe integrity-threatening conditions may necessitate corneal replacement procedures. Several procedure types are currently available to address these issues, based on the complexity and extent of injury. Corneal allograft or keratoplasty is considered to be first-line treatment in many cases. However, a significant proportion of the world's population are reported to have no access to this option due to limitations in donor preparation. Thus, providing an appropriate, safe, and efficient synthetic implant (e.g., artificial cornea) may revolutionize this field. Nanotechnology, with its potential applications, has garnered a lot of recent attention in this area, however, there is seemingly a long way to go. This narrative review provides a brief overview of the therapeutic interventions for corneal pathologies, followed by a summary of current biomaterials used in corneal regeneration and a discussion of the nanotechnologies that can aid in the production of superior implants.

Dynamic Stimulations with Bioengineered Extracellular Matrix-Mimicking Hydrogels for Mechano Cell Reprogramming and Therapy

Cells interact with their surrounding environment through a combination of static and dynamic mechanical signals that vary over stimulus types, intensity, space, and time. Compared to static mechanical signals such as stiffness, porosity, and topography, the current understanding on the effects of dynamic mechanical stimulations on cells remains limited, attributing to a lack of access to devices, the complexity of experimental set-up, and data interpretation. Yet, in the pursuit of emerging translational applications (e.g., cell manufacturing for clinical treatment), it is crucial to understand how cells respond to a variety of dynamic forces that are omnipresent in vivo so that they can be exploited to enhance manufacturing and therapeutic outcomes. With a rising appreciation of the extracellular matrix (ECM) as a key regulator of biofunctions, researchers have bioengineered a suite of ECM-mimicking hydrogels, which can be fine-tuned with spatiotemporal mechanical cues to model complex static and dynamic mechanical profiles. This review first discusses how mechanical stimuli may impact different cellular components and the various mechanobiology pathways involved. Then, how hydrogels can be designed to incorporate static and dynamic mechanical parameters to influence cell behaviors are described. The Scopus database is also used to analyze the relative strength in evidence, ranging from strong to weak, based on number of published literatures, associated citations, and treatment significance. Additionally, the impacts of static and dynamic mechanical stimulations on clinically relevant cell types including mesenchymal stem cells, fibroblasts, and immune cells, are evaluated. The aim is to draw attention to the paucity of studies on the effects of dynamic mechanical stimuli on cells, as well as to highlight the potential of using a cocktail of various types and intensities of mechanical stimulations to influence cell fates (similar to the concept of biochemical cocktail to direct cell fate). It is envisioned that this progress report will inspire more exciting translational development of mechanoresponsive hydrogels for biomedical applications.

Advances in Gelatin Bioinks to Optimize Bioprinted Cell Functions

Gelatin is a widely utilized bioprinting biomaterial due to its cell-adhesive and enzymatically cleavable properties, which improve cell adhesion and growth. Gelatin is often covalently cross-linked to stabilize bioprinted structures, yet the covalently cross-linked matrix is unable to recapitulate the dynamic microenvironment of the natural extracellular matrix (ECM), thereby limiting the functions of bioprinted cells. To some extent, a double network bioink can provide a more ECM-mimetic, bioprinted niche for cell growth. More recently, gelatin matrices are being designed using reversible cross-linking methods that can emulate the dynamic mechanical properties of the ECM. This review analyzes the progress in developing gelatin bioink formulations for 3D cell culture, and critically analyzes the bioprinting and cross-linking techniques, with a focus on strategies to optimize the functions of bioprinted cells. This review discusses new cross-linking chemistries that recapitulate the viscoelastic, stress-relaxing microenvironment of the ECM, and enable advanced cell functions, yet are less explored in engineering the gelatin bioink. Finally, this work presents the perspective on the areas of future research and argues that the next generation of gelatin bioinks should be designed by considering cell-matrix interactions, and bioprinted constructs should be validated against currently established 3D cell culture standards to achieve improved therapeutic outcomes.

Research Progress of Polymer Biomaterials as Scaffolds for Corneal Endothelium Tissue Engineering

Nowadays, treating corneal diseases arising from injury to the corneal endothelium necessitates donor tissue, but these corneas are extremely scarce. As a result, researchers are dedicating significant efforts to exploring alternative approaches that do not rely on donor tissues. Among these, creating a tissue-engineered scaffold on which corneal endothelial cells can be transplanted holds particular fascination. Numerous functional materials, encompassing natural, semi-synthetic, and synthetic polymers, have already been studied in this regard. In this review, we present a comprehensive overview of recent advancements in using polymer biomaterials as scaffolds for corneal endothelium tissue engineering. Initially, we analyze and present the key properties necessary for an effective corneal endothelial implant utilizing polymer biomaterials. Subsequently, we focus on various emerging biomaterials as scaffolds for corneal endothelium tissue engineering. We discuss their modifications (including natural and synthetic composites) and analyze the effect of micro- and nano-topological morphology on corneal endothelial scaffolds. Lastly, we highlight the challenges and prospects of these materials in corneal endothelium tissue engineering.

Fabrication Strategies for Engineered Thin Membranous Tissues

Thin membranous tissues (TMTs) are anatomical structures consisting of multiple stratified cell layers, each less than 100 μm in thickness. While these tissues are small in scale, they play critical roles in normal tissue function and healing. Examples of TMTs include the tympanic membrane, cornea, periosteum, and epidermis. Damage to these structures can be caused by trauma or congenital disabilities, resulting in hearing loss, blindness, dysfunctional bone development, and impaired wound repair, respectively. While autologous and allogeneic tissue sources for these membranes exist, they are significantly limited by availability and patient complications. Tissue engineering has therefore become a popular strategy for TMT replacement. However, due to their complex microscale architecture, TMTs are often difficult to replicate in a biomimetic manner. The critical challenge in TMT fabrication is balancing fine resolution with the ability to mimic complex target tissue anatomy. This Review reports existing TMT fabrication strategies, their resolution and material capabilities, cell and tissue response, and the advantages and disadvantages of each technique.

Influence of Gelatin Source and Bloom Number on Gelatin Methacryloyl Hydrogels Mechanical and Biological Properties for Muscle Regeneration

Approximately half of an adult human's body weight is made up of muscles. Thus, restoring the functionality and aesthetics of lost muscle tissue is critical. The body is usually able to repair minor muscle injuries. However, when volumetric muscle loss occurs due to tumour extraction, for instance, the body will form fibrous tissue instead. Gelatin methacryloyl (GelMA) hydrogels have been applied for drug delivery, tissue adhesive, and various tissue engineering applications due to their tuneable mechanical properties. Here, we have synthesised GelMA from different gelatin sources (i.e., porcine, bovine, and fish) with varying bloom numbers, which refers to the gel strength, and investigated for the influence of the source of gelatin and the bloom number on biological activities and mechanical properties. The results indicated that the source of the gelatin and variable bloom numbers have an impact on GelMA hydrogel properties. Furthermore, our findings established that the bovine-derived gelatin methacryloyl (B-GelMA) has better mechanical properties than the other varieties composed of porcine and fish with 60 kPa, 40 kPa, and 10 kPa in bovine, porcine, and fish, respectively. Additionally, it showed a noticeably greater swelling ratio (SR) ~1100% and a reduced rate of degradation, improving the stability of hydrogels and giving cells adequate time to divide and proliferate to compensate for muscle loss. Furthermore, the bloom number of gelatin was also proven to influence the mechanical properties of GelMA. Interestingly, although GelMA made of fish had the lowest mechanical strength and gel stability, it demonstrated excellent biological properties. Overall, the results emphasise the importance of gelatin source and bloom number, allowing GelMA hydrogels to have a wide range of mechanical and excellent biological properties and making them suitable for various muscle tissue regeneration applications.

Understanding the Molecular Conformation and Viscoelasticity of Low Sol-Gel Transition Temperature Gelatin Methacryloyl Suspensions

For biomedical applications, gelatin is usually modified with methacryloyl groups to obtain gelatin methacryloyl (GelMA), which can be crosslinked by a radical reaction induced by low wavelength light to form mechanically stable hydrogels. The potential of GelMA hydrogels for tissue engineering has been well established, however, one of the main disadvantages of mammalian-origin gelatins is that their sol-gel transitions are close to room temperature, resulting in significant variations in viscosity that can be a problem for biofabrication applications. For these applications, cold-water fish-derived gelatins, such as salmon gelatin, are a good alternative due to their lower viscosity, viscoelastic and mechanical properties, as well as lower sol-gel transition temperatures, when compared with mammalian gelatins. However, information regarding GelMA (with special focus on salmon GelMA as a model for cold-water species) molecular conformation and the effect of pH prior to crosslinking, which is key for fabrication purposes since it will determine final hydrogel's structure, remains scarce. The aim of this work is to characterize salmon gelatin (SGel) and salmon methacryloyl gelatin (SGelMA) molecular configuration at two different acidic pHs (3.6 and 4.8) and to compare them to commercial porcine gelatin (PGel) and methacryloyl porcine gelatin (PGelMA), usually used for biomedical applications. Specifically, we evaluated gelatin and GelMA samples' molecular weight, isoelectric point (IEP), their molecular configuration by circular dichroism (CD), and determined their rheological and thermophysical properties. Results showed that functionalization affected gelatin molecular weight and IEP. Additionally, functionalization and pH affected gelatin molecular structure and rheological and thermal properties. Interestingly, the SGel and SGelMA molecular structure was more sensitive to pH changes, showing differences in gelation temperatures and triple helix formation than PGelMA. This work suggests that SGelMA presents high tunability as a biomaterial for biofabrication, highlighting the importance of a proper GelMA molecular configuration characterization prior to hydrogel fabrication.

Arrested Phase Separation Enables High-Performance Keratoprostheses

Corneal transplantation is impeded by donor shortages, immune rejection, and ethical reservations. Pre-made cornea prostheses (keratoprostheses) offer a proven option to alleviate these issues. Ideal keratoprostheses must possess optical clarity and mechanical robustness, but also high permeability, processability, and recyclability. Here, it is shown that rationally controlling the extent of arrested phase separation can lead to optimized multiscale structure that reconciles permeability and transparency, a previously conflicting goal by common pore-forming strategies. The process is simply accomplished by hydrothermally treating a dense and transparent hydrophobic association hydrogel. The examination of multiscale structure evolution during hydrothermal treatment reveals that the phase separation with upper miscibility gap evolves to confer time-dependent pore growth due to slow dynamics of polymer-rich phase which is close to vitrification. Such a process can render a combination of multiple desired properties that equal or surpass those of the state-of-the-art keratoprostheses. In vivo tests confirm that the keratoprosthesis can effectively repair corneal perforation and restore a transparent cornea with treatment outcomes akin to that of allo-keratoplasty. The keratoprosthesis is easy to access and convenient to carry, and thus would be an effective temporary substitute for a corneal allograft in emergency conditions.

Electrically responsive release of proteins from conducting polymer hydrogels

Electrically modulated delivery of proteins provides an avenue to target local tissues specifically and tune the dose to the application. This approach prolongs and enhances activity at the target site whilst reducing off-target effects associated with systemic drug delivery. The work presented here explores an electrically active composite material comprising of a biocompatible hydrogel, gelatin methacryloyl (GelMA) and a conducting polymer, poly(3,4-ethylenedioxythiophene), generating a conducting polymer hydrogel. In this paper, the key characteristics of electroactivity, mechanical properties, and morphology are characterized using electrochemistry techniques, atomic force, and scanning electron microscopy. Cytocompatibility is established through exposure of human cells to the materials. By applying different electrical-stimuli, the short-term release profiles of a model protein can be controlled over 4 h, demonstrating tunable delivery patterns. This is followed by extended-release studies over 21 days which reveal a bimodal delivery mechanism influenced by both GelMA degradation and electrical stimulation events. This data demonstrates an electroactive and cytocompatible material suitable for the delivery of protein payloads over 3 weeks. This material is well suited for use as a treatment delivery platform in tissue engineering applications where targeted and spatio-temporal controlled delivery of therapeutic proteins is required. STATEMENT OF SIGNIFICANCE: Growth factor use in tissue engineering typically requires sustained and tunable delivery to generate optimal outcomes. While conducting polymer hydrogels (CPH) have been explored for the electrically responsive release of small bioactives, we report on a CPH capable of releasing a protein payload in response to electrical stimulus. The composite material combines the benefits of soft hydrogels acting as a drug reservoir and redox-active properties from the conducting polymer enabling electrical responsiveness. The CPH is able to sustain protein delivery over 3 weeks, with electrical stimulus used to modulate release. The described material is well suited as a treatment delivery platform to deliver large quantities of proteins in applications where spatio-temporal delivery patterns are paramount.

Photocrosslinked Fish Collagen Peptide/Chitin Nanofiber Composite Hydrogels from Marine Resources: Preparation, Mechanical Properties, and an In Vitro Study

Fish collagen peptide (FCP) is a water-soluble polymer with easy accessibility, bioactivity, and reactivity due to its solubility. The gelation of FCP can be carried out by chemical crosslinking, but the mechanical strength of FCP hydrogel is very low because of its intrinsically low molecular weight. Therefore, the mechanical properties of FCP gel should be improved for its wider application as a biomaterial. In this study, we investigated the mechanical properties of M-FCP gel in the context of understanding the influence of chitin nanofibers (CHNFs) on FCP hydrogels. FCP with a number average molecular weight (M_n) of ca. 5000 was reacted with glycidyl methacrylate (GMA) and used for the preparation of photocrosslinked hydrogels. Subsequently, composite hydrogels of methacrylate-modified FCP (M-FCP) and CHNF were prepared by the photoirradiation of a solution of M-FCP containing dispersed CHNF at an intensity of ~60 mW/cm² for 450 s in the presence of 2-hydroxy-1-[4-(hydroxyethoxy)phenyl]-2-methyl-1-propanone (Irgacure 2959) as a photoinitiator. Compression and tensile tests of the FCP hydrogels were carried out using a universal tester. The compression and tensile strength of the hydrogel increased 10-fold and 4-fold, respectively, by the addition of 0.6% CHNF (20% M-FCP), and Young's modulus increased 2.5-fold (20% M-FCP). The highest compression strength of the M-FCP/CHNF hydrogel was ~300 kPa. Cell proliferation tests using fibroblast cells revealed that the hydrogel with CHNF showed good cell compatibility. The cells showed good adhesion on the M-FCP gel with CHNF, and the growth of fibroblast cells after 7 days was higher on the M-FCP/CHNF gel than on the M-FCP gel without CHNF. In conclusion, we found that CHNF improved the mechanical properties as well as the fibroblast cell compatibility, indicating that M-FCP hydrogels reinforced with CHNF are useful as scaffolds and wound-dressing materials.

Unraveling the mechanobiology of cornea: From bench side to the clinic

The cornea is a transparent, dome-shaped structure on the front part of the eye that serves as a major optic element and a protector from the external environment. Recent evidence shows aberrant alterations of the corneal mechano-environment in development and progression of various corneal diseases. It is, thus, critical to understand how corneal cells sense and respond to mechanical signals in physiological and pathological conditions. In this review, we summarize the corneal mechano-environment and discuss the impact of these mechanical cues on cellular functions from the bench side (in a laboratory research setting). From a clinical perspective, we comprehensively review the mechanical changes of corneal tissue in several cornea-related diseases, including keratoconus, myopia, and keratectasia, following refractive surgery. The findings from the bench side and clinic underscore the involvement of mechanical cues in corneal disorders, which may open a new avenue for development of novel therapeutic strategies by targeting corneal mechanics.

Design of functional biomaterials as substrates for corneal endothelium tissue engineering

Corneal endothelium defects are one of the leading causes of blindness worldwide. The actual treatment is transplantation, which requires the use of human cadaveric donors, but it faces several problems, such as global shortage of donors. Therefore, new alternatives are being developed and, among them, cell therapy has gained interest in the last years due to its promising results in tissue regeneration. Nevertheless, the direct administration of cells may sometimes have limited success due to the immune response, hence requiring the combination with extracellular mimicking materials. In this review, we present different methods to obtain corneal endothelial cells from diverse cell sources such as pluripotent or multipotent stem cells. Moreover, we discuss different substrates in order to allow a correct implantation as a cell sheet and to promote an enhanced cell behavior. For this reason, natural or synthetic matrixes that mimic the native environment have been developed. These matrixes have been optimized in terms of their physicochemical properties, such as stiffness, topography, composition and transparency. To further enhance the matrixes properties, these can be tuned by incorporating certain molecules that can be delivered in a sustained manner in order to enhance biological behavior. Finally, we elucidate future directions for corneal endothelial regeneration, such as 3D printing, in order to obtain patient-specific substrates.

Visible-Light Stiffness Patterning of GelMA Hydrogels Towards In Vitro Scar Tissue Models

Variations in mechanical properties of the extracellular matrix occurs in various processes, such as tissue fibrosis. The impact of changes in tissue stiffness on cell behaviour are studied in vitro using various types of biomaterials and methods. Stiffness patterning of hydrogel scaffolds, through the use of stiffness gradients for instance, allows the modelling and studying of cellular responses to fibrotic mechanisms. Gelatine methacryloyl (GelMA) has been used extensively in tissue engineering for its inherent biocompatibility and the ability to precisely tune its mechanical properties. Visible light is now increasingly employed for crosslinking GelMA hydrogels as it enables improved cell survival when performing cell encapsulation. We report here, the photopatterning of mechanical properties of GelMA hydrogels with visible light and eosin Y as the photoinitiator using physical photomasks and projection with a digital micromirror device. Using both methods, binary hydrogels with areas of different stiffnesses and hydrogels with stiffness gradients were fabricated. Their mechanical properties were characterised using force indentation with atomic force microscopy, which showed the efficiency of both methods to spatially pattern the elastic modulus of GelMA according to the photomask or the projected pattern. Crosslinking through projection was also used to build constructs with complex shapes. Overall, this work shows the feasibility of patterning the stiffness of GelMA scaffolds, in the range from healthy to pathological stiffness, with visible light. Consequently, this method could be used to build in vitro models of healthy and fibrotic tissue and study the cellular behaviours involved at the interface between the two.

3D Printed Hydrogels for Ocular Wound Healing

Corneal disease is one of the most significant causes of blindness around the world. Presently, corneal transplantation is the only way to treat cornea blindness. It should be noted that the amount of cornea that people donate is so much less than that required (1:70). Therefore, scientists have tried to resolve this problem with tissue engineering and regenerative medicine. Fabricating cornea with traditional methods is difficult due to their unique properties, such as transparency and geometry. Bioprinting is a technology based on additive manufacturing that can use different biomaterials as bioink for tissue engineering, and the emergence of 3D bioprinting presents a clear possibility to overcome this problem. This new technology requires special materials for printing scaffolds with acceptable biocompatibility. Hydrogels have received significant attention in the past 50 years, and they have been distinguished from other materials because of their unique and outstanding properties. Therefore, hydrogels could be a good bioink for the bioprinting of different scaffolds for corneal tissue engineering. In this review, we discuss the use of different types of hydrogel for bioink for corneal tissue engineering and various methods that have been used for bioprinting. Furthermore, the properties of hydrogels and different types of hydrogels are described.

Ocular Nanomedicine

Intrinsic shortcomings associated with conventional therapeutic strategies often compromise treatment efficacy in clinical ophthalmology, prompting the rapid development of versatile alternatives for satisfactory diagnostics and therapeutics. Given advances in material science, nanochemistry, and nanobiotechnology, a broad spectrum of functional nanosystems has been explored to satisfy the extensive requirements of ophthalmologic applications. In the present review, the recent progress in nanosystems, both conventional and emerging nanomaterials in ophthalmology from state-of-the-art studies, are comprehensively examined and the role of their fundamental physicochemical properties in bioavailability, tissue penetration, biodistribution, and elimination after interacting with the ophthalmologic microenvironment emphasized. Furthermore, along with the development of surface engineering of nanomaterials, emerging theranostic methodologies are promoted as potential alternatives for multipurpose ocular applications, such as emerging biomimetic ophthalmology (e.g., smart electrochemical eye), thus provoking a holistic review of "ocular nanomedicine." By affording insight into challenges encountered by ocular nanomedicine and further highlighting the direction of future studies, this review provides an incentive for enriching ocular nanomedicine-based fundamental research and future clinical translation.

Multiscale Anisotropic Tissue Biofabrication via Bulk Acoustic Patterning of Cells and Functional Additives in Hybrid Bioinks

Recapitulation of the microstructural organization of cellular and extracellular components found in natural tissues is an important but challenging feat for tissue engineering, which demands innovation across both process and material fronts. In this work, a highly versatile ultrasound-assisted biofabrication (UAB) approach is demonstrated that utilizes radiation forces generated by superimposing ultrasonic bulk acoustic waves to rapidly organize arrays of cells and other biomaterial additives within single and multilayered hydrogel constructs. UAB is used in conjunction with a novel hybrid bioink system, comprising of cartilage-forming cells (human adipose-derived stem cells or chondrocytes) and additives to promote cell adhesion (collagen microaggregates or polycaprolactone microfibers) encapsulated within gelatin methacryloyl (GelMA) hydrogels, to fabricate cartilaginous tissue constructs featuring bulk anisotropy. The hybrid matrices fabricated under the appropriate synergistic thermo-reversible and photocrosslinking conditions demonstrate enhanced mechanical stiffness, stretchability, strength, construct shape fidelity and aligned encapsulated cell morphology and collagen II secretion in long-term culture. Hybridization of UAB is also shown with extrusion and stereolithography printing to fabricate constructs featuring 3D perfusable channels for vasculature combined with a crisscross or circumferential organization of cells and adhesive bioadditives, which is relevant for further translation of UAB toward complex physiological-scale biomimetic tissue fabrication.

Tissue engineered scaffolds for corneal endothelial regeneration: a material's perspective

Currently, the treatment of corneal diseases caused by damage to the corneal endothelium requires a donor cornea. Because of their limited availability (1 donor cornea for 70 patients in need), researchers are investigating alternative approaches that are independent of donor tissue. One of them includes the development of a tissue engineered scaffold onto which corneal endothelial cells are seeded. In order to function as a suitable substrate, some of its essential properties including thickness, permeability, transparency and mechanical strength should meet certain demands. Additionally, the membrane should be biocompatible and allow the formation of a functional endothelium on the surface. Many materials have already been investigated in this regard including natural, semi-synthetic and synthetic polymers. In the current review, we present an overview of their characteristics and provide a critical view on the methods exploited for material characterization. Next, also the suitability of scaffolds to serve their purpose is discussed along with an overview of natural tissues (e.g. amniotic membrane and lens capsule) previously investigated for this application. Eventually, we propose a consistent approach to be exploited ideally for membrane characterization in future research. This will allow a scientifically sound comparison of materials and membranes investigated by different research groups, hence benefitting research towards the creation of a suitable/optimal tissue engineered endothelial graft.

Novel Corneal Endothelial Cell Carrier Couples a Biodegradable Polymer and a Mesenchymal Stem Cell-Derived Extracellular Matrix

Here, we report a transparent, biodegradable, and cell-adhesive carrier that is securely coupled with the extracellular matrix (ECM) for corneal endothelial cell (CEC) transplantation. To fabricate a CEC carrier, poly(lactide-co-caprolactone) (PLCL) solution was poured onto the decellularized ECM (UMDM) derived from in vitro cultured umbilical cord blood-MSCs. Once completely dried, ECM-PLCL was then peeled off from the substrate. It was 20 μm thick, transparent, rich in fibronectin and collagen type IV, and easy to handle. Surface characterizations exhibited that ECM-PLCL was very rough (54.0 ± 4.50 nm) and uniformly covered in high density by ECM and retained a positive surface charge (65.2 ± 57.8 mV), as assessed via atomic force microscopy. Human CECs (B4G12) on the ECM-PLCL showed good cell attachment, with a cell density similar to the normal cornea. They could also maintain a cell phenotype, with nicely formed cell-cell junctions as assessed via ZO-1 and N-cadherin at 14 days. This was in sharp contrast to the CEC behaviors on the FNC-coated PLCL (positive control). A function-related marker, Na⁺/K⁺-ATPase, was also identified via western blot and immunofluorescence. In addition, primary rabbit CECs showed a normal shape and they could express structural and functional proteins on the ECM-PLCL. A simulation test confirmed that CECs loaded on the ECM-PLCL were successfully engrafted into the decellularized porcine corneal tissue, with a high engraftment level and cell viability. Moreover, ECM-PLCL transplantation into the anterior chamber of the rabbit eye for 8 weeks proved the maintenance of normal cornea properties. Taken together, this study demonstrates that our ECM-PLCL can be a promising cornea endothelium graft with an excellent ECM microenvironment for CECs.

Tissue-Engineered Corneal Endothelial Sheets Using Ultrathin Acellular Porcine Corneal Stroma Substrates for Endothelial Keratoplasty

Tissue-engineered cornea endothelial sheets (TECES), created using a biocompatible thin and transparent carrier with corneal endothelial cells, could alleviate the shortage of donor corneas and provide abundant functional endothelial cells. In our previous clinical trials, the effectiveness and safety of the acellular porcine corneal stroma (APCS) applied in lamellar keratoplasty have been confirmed. In this study, we optimized the method to cut APCS into multiple 20 μm ultrathin lamellae by a cryostat microtome and investigated the feasibility of TECES by seeding rabbit corneal endothelial cells (RCECs) on ultrathin APCS. Cell adhesion, proliferation, and functional gene expression of RCECs on tissue-culture plastic and APCS of different thicknesses were compared. The results indicated that ultrathin lamellae were superior in increasing cell viability and maintaining cell functions. Analyzing with histology, electron microscopy, and immunofluorescence, we found that RCECs cultured on 20 μm ultrathin APCS for 5 days grew into a confluent monolayer with a density of 3726 ± 223 cells/mm² and expressed functional biomarkers Na⁺/K⁺-ATPase and zonula occludens. After 14 days, RCECs formed an early stage of Descemet's membrane-like structure by synthesizing collagen IV and laminin. Human corneal endothelial cells were also used to further validate the supportive effect of ultrathin APCS on cells. The resulting constructs were flexible and tough enough to implant into rabbits' anterior chambers through small incisions. TECES adhered to the posterior corneal stroma, and the thickness of cornea gradually reduced to normal after grafting. These results indicate that the ultrathin APCS can serve as a tissue engineering carrier and might be a suitable alternative for endothelial cells expansion in endothelial keratoplasty.

Photo-crosslinked hydrogels for tissue engineering of corneal epithelium

The vast majority of patients with corneal blindness cannot recover their vision due to the serious shortage of donor cornea. However, the technology to construct a feasible corneal substitute is a promising treatment method for corneal blindness. In this paper, methacrylated gelatin (GelMA)-methacrylated hyaluronic acid (HAMA) double network (GHDN) hydrogels were prepared by modifying gelatin and hyaluronic acid with methacrylate anhydride (MA). GHDN hydrogel was compared with GelMA single network and HAMA single network hydrogels through characterization experiments of mechanical properties, optical properties, hydrophilicity and in-situ degradation in vitro. At the same time, the biocompatibility of hydrogel was tested by inoculating rabbit corneal epithelial cells (CEpCs) epidermal cells on hydrogels using CCK-8 test, live/dead staining, immunofluorescence staining and qRT-PCR. It was found that the GHDN hydrogel has optical transparency in the visible region, and its mechanical properties are better than those of GelMA and HAMA hydrogels, and its hydrophilicity is similar to that of normal human corneas. The results of in vitro hydrogel culture of CEpCs showed that the proliferation of CEpCs on GHDN hydrogel was two times higher than that of HAMA hydrogel, and the expression of specific marker Cytokeratin 3 (CK3) and Cytokeratin 12 (CK12) could be better maintained on GHDN hydrogel. All the experimental results proved that GHDN hydrogel has good physical properties and biocompatibility and is a potential candidate for corneal tissue engineering scaffolds.

Cells on Hydrogels with Micron-Scaled Stiffness Patterns Demonstrate Local Stiffness Sensing

Cell rigidity sensing-a basic cellular process allowing cells to adapt to mechanical cues-involves cell capabilities exerting force on the extracellular environment. In vivo, cells are exposed to multi-scaled heterogeneities in the mechanical properties of the surroundings. Here, we investigate whether cells are able to sense micron-scaled stiffness textures by measuring the forces they transmit to the extracellular matrix. To this end, we propose an efficient photochemistry of polyacrylamide hydrogels to design micron-scale stiffness patterns with kPa/µm gradients. Additionally, we propose an original protocol for the surface coating of adhesion proteins, which allows tuning the surface density from fully coupled to fully independent of the stiffness pattern. This evidences that cells pull on their surroundings by adjusting the level of stress to the micron-scaled stiffness. This conclusion was achieved through improvements in the traction force microscopy technique, e.g., adapting to substrates with a non-uniform stiffness and achieving a submicron resolution thanks to the implementation of a pyramidal optical flow algorithm. These developments provide tools for enhancing the current understanding of the contribution of stiffness alterations in many pathologies, including cancer.

Use of biomaterials in corneal endothelial repair

Human corneal endothelium (HCE) is a single layer of hexagonal cells that lines the posterior surface of the cornea. It forms the barrier that separates the aqueous humor from the rest of the corneal layers (stroma and epithelium layer). This layer plays a fundamental role in maintaining the hydration and transparency of the cornea, which in turn ensures a clear vision. In vivo, human corneal endothelial cells (HCECs) are generally believed to be nonproliferating. In many cases, due to their nonproliferative nature, any damage to these cells can lead to further issues with Descemet’s membrane (DM), stroma and epithelium which may ultimately lead to hazy vision and blindness. Endothelial keratoplasties such as Descemet’s stripping automated endothelial keratoplasty (DSAEK) and Descemet’s membrane endothelial keratoplasty (DEK) are the standard surgeries routinely used to restore vision following endothelial failure. Basically, these two similar surgical techniques involve the replacement of the diseased endothelial layer in the center of the cornea by a healthy layer taken from a donor cornea. Globally, eye banks are facing an increased demand to provide corneas that have suitable features for transplantation. Consequently, it can be stated that there is a significant shortage of corneal grafting tissue; for every 70 corneas required, only 1 is available. Nowadays, eye banks face long waiting lists due to shortage of donors, seriously aggravated when compared with previous years, due to the global COVID-19 pandemic. Thus, there is an urgent need to find alternative and more sustainable sources for treating endothelial diseases, such as utilizing bioengineering to use of biomaterials as a remedy. The current review focuses on the use of biomaterials to repair the corneal endothelium. A range of biomaterials have been considered based on their promising results and outstanding features, including previous studies and their key findings in the context of each biomaterial.

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

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

Characterizing the Effects of Synergistic Thermal and Photo-Cross-Linking during Biofabrication on the Structural and Functional Properties of Gelatin Methacryloyl (GelMA) Hydrogels

Gelatin methacryloyl (GelMA) hydrogels have emerged as promising and versatile biomaterial matrices with applications spanning drug delivery, disease modeling, and tissue engineering and regenerative medicine. GelMA exhibits reversible thermal cross-linking at temperatures below 37 °C due to the entanglement of constitutive polymeric chains, and subsequent ultraviolet (UV) photo-cross-linking can covalently bind neighboring chains to create irreversibly cross-linked hydrogels. However, how these cross-linking modalities interact and can be modulated during biofabrication to control the structural and functional characteristics of this versatile biomaterial is not well explored yet. Accordingly, this work characterizes the effects of synergistic thermal and photo-cross-linking as a function of GelMA solution temperature and UV photo-cross-linking duration during biofabrication on the hydrogels' stiffness, microstructure, proteolytic degradation, and responses of NIH 3T3 and human adipose-derived stem cells (hASC). Smaller pore size, lower degradation rate, and increased stiffness are reported in hydrogels processed at lower temperature or prolonged UV exposure. In hydrogels with low stiffness, the cells were found to shear the matrix and cluster into microspheroids, while poor cell attachment was noted in high stiffness hydrogels. In hydrogels with moderate stiffness, ones processed at lower temperature demonstrated better shape fidelity and cell proliferation over time. Analysis of gene expression of hASC encapsulated within the hydrogels showed that, while the GelMA matrix assisted in maintenance of stem cell phenotype (CD44), a higher matrix stiffness resulted in higher pro-inflammatory marker (ICAM1) and markers for cell-matrix interaction (ITGA1 and ITGA10). Analysis of constructs with ultrasonically patterned hASC showed that hydrogels processed at higher temperature possessed lower structural fidelity but resulted in more cell elongation and greater anisotropy over time. These findings demonstrate the significant impact of GelMA material formulation and processing conditions on the structural and functional properties of the hydrogels. The understanding of these material-process-structure-function interactions is critical toward optimizing the functional properties of GelMA hydrogels for different targeted applications.