Understanding crystallization in alginate-calcium phosphate composites

Summary of thesis: Tissue engineering seeks to mimic the properties and functionalities of biological tissue. In the context of bone repair, tissue engineering is becoming an increasingly important science in the active search for synthetic alternatives to autografts. The purpose of this work has been to develop and characterize an organic-inorganic composite material with potential applications within the field of bone tissue engineering. A simple, fast and cell-friendly fabrication of an alginate and calcium phosphate based composite was comprehensively explored and emphasis was put on the development of a thorough characterization protocol. Alginate, a biopolymer well-known for its ionotropic gelation and cell-encapsulation abilities, has been combined with calcium phosphate minerals to explore its potential as a scaffold material for hard tissue applications and as a model system to study mineral formation in the presence of an organic matrix. A one step counter-diffusion method was applied to simultaneously gel the alginate and precipitate the mineral. Characterization of this composite was performed using bulk, end-point techniques such as X-ray diffraction, and thermal gravimetric analysis to investigate mineral phase and content, respectively, as well as in situ optical characterization of gelling kinetics and mineralization. The in situ studies were performed in a model system to map the spatial distribution of mineral within the composite and the formation and maturation of the mineral crystals primarily using confocal Raman microspectroscopy and confocal laser scanning microscopy. A robust, cell-friendly fabrication protocol for a phase-pure brushite-alginate composite has been developed using brushite seed crystals. Using a correlative characterization scheme with spatiotemporal resolution developed in this work, the mineralization process during the formation of this composite was comprehensively described and shown to include the presence of amorphous calcium phosphate, which quickly dissolved and led to brushite growth in the presence of seeds and conversion to octacalcium phosphate in the absence of brushite seeds. Further maturation led to the formation of thermodynamically stable hydroxyapatite in both cases. The findings in this study may improve the design of scaffold materials based on alginate-calcium phosphate composites and contributes to the understanding of crystallization within hydrogel matrices