Alginate Hydrogel Integrated with a Human Fibroblast-Derived Extracellular Matrix Supports Corneal Endothelial Cell Functionality and Suppresses Endothelial-Mesenchymal Transition

Human corneal transplantation is still the only option to restore the function of corneal endothelial cells (CECs). Therefore, there is an urgent need for hCEC delivery systems to replace the human donor cornea. Here, we propose an alginate hydrogel (AH)-based delivery system, where a human fibroblast-derived, decellularized extracellular matrix (ECM) was physically integrated with AH. This AH securely combined with the ECM (ECM-AH) was approximately 50 μm thick, transparent, and permeable. The surface roughness and surface potential provided ECM-AH with a favorable microenvironment for CEC adhesion and growth in vitro. More importantly, ECM-AH could support the structural (ZO-1) and functional (Na+/K+-ATPase) markers of hCECs, as assessed via western blotting and quantitative polymerase chain reaction, which were comparable with those of a ferritic nitrocarburizing (FNC)-coated substrate (a positive control). The cell density per unit area was also significantly better with ECM-AH than the FNC substrate at day 7. A simulation test of cell engraftment in vitro showed that hCECs were successfully transferred into the decellularized porcine corneal tissue, where they were mostly alive. Furthermore, we found out that the endothelial-mesenchymal transition (EnMT)-inductive factors (Smad2 and vimentin) were largely declined with the hCECs grown on ECM-AH, whereas the EnMT inhibitory factor (Smad7) was significantly elevated. The difference was statistically significant compared to that of the FNC substrate. Moreover, we also observed that TGF-β1-treated hCECs showed faster recovery of cell phenotype on the ECM. Taken together, our study demonstrates that ECM-AH is a very promising material for hCEC culture and delivery, which endows an excellent microenvironment for cell function and phenotype maintenance.

Sacrificial biomaterials in 3D fabrication of scaffolds for tissue engineering applications

Three-dimensional (3D) printing technology has progressed exceedingly in the area of tissue engineering. Despite the tremendous potential of 3D printing, building scaffolds with complex 3D structure, especially with soft materials, still exist as a challenge due to the low mechanical strength of the materials. Recently, sacrificial materials have emerged as a possible solution to address this issue, as they could serve as temporary support or templates to fabricate scaffolds with intricate geometries, porous structures, and interconnected channels without deformation or collapse. Here, we outline the various types of scaffold biomaterials with sacrificial materials, their pros and cons, and mechanisms behind the sacrificial material removal, compare the manufacturing methods such as salt leaching, electrospinning, injection-molding, bioprinting with advantages and disadvantages, and discuss how sacrificial materials could be applied in tissue-specific applications to achieve desired structures. We finally conclude with future challenges and potential research directions.

Development of chondrocyte-laden alginate hydrogels with modulated microstructure and properties for cartilage regeneration

Alginate hydrogel is an attractive biomaterial for cell microencapsulation. The microarchitecture of hydrogels can regulate cellular functions. This study aims to investigate the applicability of sodium citrate buffer (SCB) as a culture medium supplement for modulating the microstructure of alginate microbeads to provide a favorable microenvironment for chondrogenic induction. The chondrocyte-laden microbeads, with and without TGF-β3 incorporation, were produced through an encapsulator. The obtained small-sized microbeads (~300 μm) were exposed to a treatment medium containing SCB, composed of varied concentrations of sodium citrate (1.10-1.57 mM), sodium chloride (3.00-4.29 mM), and ethylenediaminetetraacetic acid (0.60-0.86 mM) to partially degrade their crosslinked structure for 3 days, followed by culture in a normal medium until day 21. Scanning electron microscope micrographs demonstrated a loose hydrogel network with an enhanced pore size in the SCB-treated microbeads. Increasing the concentration of SCB in the treatment medium reduced the calcium content of the microbeads via a Na⁺ /Ca²⁺ exchange process and improved the water absorption of the microbeads, resulting in a higher swelling ratio. All the tested SCB concentrations were non-cytotoxic. Increases in aggrecan and type II collagen gene expression and their corresponding extracellular matrix accumulation, glycosaminoglycans, and type II collagen were vividly detected in the TGF-β3-containing microbeads with increasing SCB concentrations in the treatment medium. Our findings highlighted that the combination of SCB treatment and TGF-β3 incorporation in the chondrocyte-laden microbeads is a promising strategy for enhancing cartilage regeneration, which may contribute to a versatile application in cell delivery and tissue engineering.

Genipin-crosslinked fibrin seeded with oxidized alginate microbeads as a novel composite biomaterial strategy for intervertebral disc cell therapy

Discectomy procedures alleviate disability caused by intervertebral disc (IVD) herniation, but do not repair herniation-induced annulus fibrosus (AF) defects. Cell therapy shows promise for IVD repair, yet cell delivery biomaterials capable of sealing AF defects and restoring biomechanical function have poor biological performance. To balance the biomechanical and biological demands of IVD cell delivery biomaterials, we engineered an injectable composite biomaterial using cell-laden, degradable oxidized alginate (OxAlg) microbeads (MBs) to deliver AF cells within high-modulus genipin-crosslinked fibrin (FibGen) hydrogels (FibGen + MB composites). Conceptually, the high-modulus FibGen would immediately stabilize injured IVDs, while OxAlg MBs would protect and release cells required for long-term healing. We first showed that AF cells microencapsulated in OxAlg MBs maintained high viability and, upon release, displayed phenotypic AF cell morphology and gene expression. Next, we created cell-laden FibGen + MB composites and demonstrated that OxAlg MBs functionalized with RGD peptides (MB-RGD) minimized AF cell apoptosis and retained phenotypic gene expression. Further, we showed that cell-laden FibGen + MB composites are biomechanically stable and promote extracellular matrix (ECM) synthesis in long-term in vitro culture. Lastly, we evaluated cell-laden FibGen + MB-RGD composites in a long-term bovine caudal IVD organ culture bioreactor and found that composites had low herniation risk, provided superior biomechanical and biological repair to discectomy controls, and retained anabolic cells within the IVD injury space. This novel injectable composite hydrogel strategy shows promise as an IVD cell delivery sealant with potentially broad applications for its capacity to balance biomechanical and biological performance.

Three-Dimensional-Printed Flexible Scaffolds Have Tunable Biomimetic Mechanical Properties for Intervertebral Disc Tissue Engineering

The intervertebral disc (IVD) exhibits complex structure and biomechanical function, which supports the weight of the body and permits motion. Surgical treatments for IVD degeneration (e.g., lumbar fusion, disc replacement) often disrupt the mechanical environment of the spine which lead to adjacent segment disease. Alternatively, disc tissue engineering strategies, where cell-seeded hydrogels or fibrous biomaterials are cultured in vitro to promote matrix deposition, do not recapitulate the complex IVD mechanical properties. In this study, we use 3D printing of flexible polylactic acid (FPLA) to fabricate a viscoelastic scaffold with tunable biomimetic mechanics for whole spine motion segment applications. We optimized the mechanical properties of the scaffolds for equilibrium and dynamic moduli in compression and tension by varying fiber spacing or porosity, generating scaffolds with de novo mechanical properties within the physiological range of spine motion segments. The biodegradation analysis of the 3D printed scaffolds showed that FPLA exhibits lower degradation rate and thus has longer mechanical stability than standard PLA. FPLA scaffolds were biocompatible, supporting viability of nucleus pulposus (NP) cells in 2D and in FPLA+hydrogel composites. Composite scaffolds cultured with NP cells maintained baseline physiological mechanical properties and promoted matrix deposition up to 8 weeks in culture. Mesenchymal stromal cells (MSCs) cultured on FPLA adhered to the scaffold and exhibited fibrocartilaginous differentiation. These results demonstrate for the first time that 3D printed FPLA scaffolds have de novo viscoelastic mechanical properties that match the native IVD motion segment in both tension and compression and have the potential to be used as a mechanically stable and biocompatible biomaterial for engineered disc replacement.

Macroporous Hydrogel Scaffolds for Three-Dimensional Cell Culture and Tissue Engineering

Hydrogels have been promising candidate scaffolds for cell delivery and tissue engineering due to their tissue-like physical properties and capability for homogeneous cell loading. However, the encapsulated cells are generally entrapped and constrained in the submicron- or nanosized gel networks, seriously limiting cell growth and tissue formation. Meanwhile, the spatially confined settlement inhibits attachment and spreading of anchorage-dependent cells, leading to their apoptosis. In recent years, macroporous hydrogels have attracted increasing attention in use as cell delivery vehicles and tissue engineering scaffolds. The introduction of macropores within gel scaffolds not only improves their permeability for better nutrient transport but also creates space/interface for cell adhesion, proliferation, and extracellular matrix deposition. Herein, we will first review the development of macroporous gel scaffolds and outline the impact of macropores on cell behaviors. In the first part, the advantages and challenges of hydrogels as three-dimensional (3D) cell culture scaffolds will be described. In the second part, the fabrication of various macroporous hydrogels will be presented. Third, the enhancement of cell activities within macroporous gel scaffolds will be discussed. Finally, several crucial factors that are envisaged to propel the improvement of macroporous gel scaffolds are proposed for 3D cell culture and tissue engineering.

Synthesis, characterization and chondrocyte culture of polyhedral oligomeric silsesquioxane (POSS)-containing hybrid hydrogels

Polyhedral oligomeric silsesquioxanes (POSS)-based hybrid hydrogels were successfully prepared via a fast azide-alkyne click reaction between octa-azido-functionalized POSS (OAPOSS) and alkyne-functionalized poly(ethylene glycol) (PEG).