Biomimetic scaffold-mediated in situ bone remodeling and rapid bone regeneration: Whitlockite nanoparticles and biopolymers

Abstract

Various strategies have been explored to overcome critically sized bone defects via bone tissue engineering approaches that incorporate biomimetic scaffolds. Biomimetic scaffolds may provide a novel platform for phenotypically stable tissue formation and stem cell differentiation. In recent years, osteoinductive and inorganic biomimetic scaffold materials have been optimized to offer an osteo-friendly microenvironment for the osteogenic commitment of stem cells. Furthermore, scaffold structures with a microarchitecture design similar to native bone tissue are necessary for successful bone tissue regeneration. For this reason, various methods for fabricating threedimensional porous structures have been developed. Innovative techniques, such as 3D printing methods, are currently being utilized for optimal host stem cell infiltration, vascularization, nutrient transfer, and stem cell differentiation. In the first part of this dissertation, we will review biomimetic materials and fabrication approaches that are currently being utilized for biomimetic scaffold design. For successful bone tissue regeneration, a variety of materials has been investigated to fabricate the optimal scaffolds. A lot of biocompatible and biodegradable polymers have been explored to make bone scaffold frame, and osteoinductive materials such as metal ions, growth factors have been incorporated to bone scaffolds. Furthermore, various techniques have been developed to construct the three dimensional porous bone scaffolds like natural bone tissue. Those traditional strategies have been innovated for years, and various cell sources optimized for osteogenesis have been applied to bone tissue engineering. In this way, we expected to overcome current challenges and critical-sized bone defects from accidents and diseases.

In the second part of the thesis, we demonstrate that synthetic whitlockite (WH: Ca18Mg2(HPO4)2(PO4)12) nanoparticles can recapitulate early-stage of bone regeneration through stimulating osteogenic differentiation, prohibiting osteoclastic activity, and transforming into mechanically enhanced hydroxyapatite (HAP)-neo bone tissues by continuous supply of PO4 3- and Mg2+ under physiological conditions. In addition, based on their structural analysis, the dynamic phase transformation from WH into HAP contributed as a key factor for rapid bone regeneration with denser hierarchical neo-bone structure. Our findings suggest a groundbreaking concept of 'living bone minerals' that actively communicate with the surrounding system to induce self-healing, while previous notions about bone minerals have been limited to passive products of cellular mineralization.

In the third part of the thesis, we fabricated methacrylated PEGDA/CS-based hydrogels with varying CS concentration and investigated them as biomineralizing three-dimensional scaffolds for charged ion binding and depositions. Due to its negative charge from the sulfate group, CS exhibited an osteogenically favorable microenvironment by binding charged ions such as calcium and phosphate. Particularly, ion binding and distribution within negatively charged hydrogel was dependent on CS concentration. Furthermore, CS dependent biomineralizing microenvironment induced osteogenic differentiation of human tonsil-derived mesenchymal stem cells in vitro. This PEGDA/CS-based biomineralizing hydrogel platform can be utilized for in situ bone formation in addition to being an investigational tool for in vivo bone mineralization and resorption mechanisms.

In the last part of the thesis, our aim is to explore the potential use of human vascular endothelial growth factors (hVEGF) and Whitlockite (WH) for bone tissue engineering. Various strategies have been explored to stimulate a new bone formation. Among these strategies, includes using angiogenic stimulants in combination with inorganic biomaterials. Neovascularization during the neo-bone formation provides nutrients along with bone forming minerals. Therefore, it is crucial to design a bone stimulating microenvironment composed of both pro-angiogenic and osteogenic factors. In this respect, hVEGF have been shown to promote blood vessel formation and bone formation. In this study, we demonstrated that hVEGF and WH synergistically stimulated osteogenic commitment of mesenchymal stem cells both in vitro and in vivo.

Biomimetic scaffold-mediated in situ bone remodeling and rapid bone regeneration in this thesis will be useful to understand the mechanism and the role of biomineral and biopolymer in the bone regeneration system. We believe that this study will directly contribute to make more bone-like scaffold or implants and provide inspiration and foundation knowledge to the other various research fields.