Mineralized nanofibril arrays as building units for tissue engineering of long bones

Bone tissue engineering has emerged as a future alternative to bone grafting for the treatment of long bone defects. Numerous strategies involving biomolecules, stem cells, and engineered scaffolds have shown promise in inducing sufficient bone regeneration. However, a common limitation of these strategies lies in their inability to generate bone tissue that mimics the organized microstructure of cortical bone. Thus, the objective of this study is to develop highly-aligned nanofibril scaffolds, which closely mimic the microstructure of the cortical bone. The idea behind this goal is that if a more organized bone structure is formed during the healing process, the time for the remodeling process to occur would be reduced. Electrospun polymer nanofibril arrays are often used in tissue engineering applications due to their ability to support cell attachment, growth, and tissue formation. Using an electrospinning apparatus paired with a customized motorized collecting device developed in our lab, we are able to spin small diameter, highly-aligned, loose nanofibril arrays that closely mimic the structure and arrangement of collagen fibrils in bones' ECM. This is advantageous as it will allow cells to penetrate and migrate within the scaffold in order to regenerate the tissue. In this study, polycaprolactone (PCL) was used due to its extended biodegradability as well as excellent biocompatibility, though other polymers can be used with this device. To further mimic the structure, composition, and mechanical properties of the lowest hierarchical level of natural cortical bone, we have biomineralized the surface of the nanofibril arrays in order to direct osteoblast behavior and facilitate bone tissue formation, organization, and regeneration. Our present study determines the proper treatment methods and parameters for optimal mineralization of the nanofibril scaffolds, in order to achieve a composition and microstructure that mimics that of natural bone. Results indicate that a hydrolysis treatment for 30 minutes prior to incubating with 5X simulated body fluid (SBF) for 24 hours leads to a coating with ideal compositional and structural characteristics. This structure resulted in enhanced mechanical properties, as well as improved osteoconductivity and bioactivity, which allowed bone progenitor cells to attach, align, and proliferate on. Future preclinical studies will include the incorporation of angiogenic and osteogenic factors in order to further promote vascularization and enhance bone regeneration. As current strategies have yet to produce a suitable candidate for a clinical cortical bone substitute, this strategy has a great deal of clinical relevance, as it can potentially serve as an alternative to autologous bone grafts for human long bone regeneration.