Design and Development of Biomimetic Hydrogel Scaffold for Bone Tissue Engineering

Abstract

The use of injectable hydrogels is currently restricted by the challenge of achieving fast gelation, good mechanical strength, and cytocompatibility. Perhaps the greatest challenge is the need for biomaterials to satisfy the bone's high compressive properties, a prerequisite for in vivo functioning. One of the drawbacks of biopolymer-based scaffolds is that under aqueous conditions, their mechanical characteristics reduce significantly. We developed an injectable, thermosensitive polyelectrolyte complex-based hydrogel to mimic the native extracellular matrix using a biomimetic approach for the repair and regeneration of defective tissue. We synthesized self-assembled structures of polyelectrolyte complexes (PECs) of polyanionic sodium alginate with the polycationic chitosan at room temperature. The PECs prepared at different pH values exhibited two distinct morphologies. The chitosan-alginate PECs self-assembled into the fibrous structure in a low pH range of pH 3 to 7. The PECs obtained at high pH series around pH 8 and above resulted in the formation of colloidal nanoparticles in the range of 120±9 nm to 46±17 nm. Furthermore, the practicability of developing a chitosan-based thermogelling solution using hydroxyapatite and polyelectrolyte complex (PEC) self-assembled fibers was evaluated. The effect of βGP concentration on gelation time was studied by varying the concentration of βGP added to the chitosan solution. Various combinations were tested to create a suitable hydrogel environment for cell encapsulation, growth, and proliferation at physiological pH and temperature. We investigated interfacial bonding between PEC fibers with βGP, NaHCO3, and HAp. The combination of hydroxyapatite and polymer self-assembly techniques improved the efficiency of injectable hydrogels which are helpful in minimally invasive applications. Later we fabricated nanocomposite scaffolds using hydrothermal treatment of polyelectrolyte complex (PEC) of chitosan and polygalacturonic acid. PEC fibers with Hap 5% by weight are obtained as 3D porous scaffolds by freeze-drying. FTIR analysis revealed the ability of the thermal treatment to set the interaction of HAp with polymeric PEC fibers. FESEM analysis suggests the influence of hydrothermal gelation on pore arrangement and unique molecular organization. Mechanical tests revealed that thermal heating exhibits a beneficial effect on PEC fibers and the molecular structuration of PEC-HAp, improving their stiffness and compressive strength. Hence, the hydrothermal treatment proved an effective crosslinker-free gelation with improved mechanical strength and nanofibrous structure. Later, we developed a novel process to fabricate thermosensitive hydrogels mimicking the ECM of the native tissue with a homogeneous, interconnected porous fibrous structure for tissue engineering applications without using organic solvents. Moreover, the hydrogel formed at physiological temperature and pH exhibits potential for cell encapsulation and injectability for tissue engineering applications. It contained self-assembled fibrous PEC with HAp-gelatin interspersed with chitosan matrix. The introduction of hydrothermal treatment resulted in the enhanced overall stiffness of the hydrogels. The hydrogels' fibrous and interconnected porous structure offered ample space for the embedded cells, which enhanced the direct interaction with the hydrogel matrix. Thus, the increase in ALP activity and collagen production confirmed the differentiation of preosteoblasts cultured in the hydrogels. This indicates that these hydrogels are promising biomaterial for the repair and regeneration of target bone tissue (any shape) and clinical application. These hydrogels hold great potential to accommodate MSCs for bone repair and regeneration purposes, which warrants future in- vivo studies.