Understanding Composition–Structure–Bioactivity Correlations in Bioactive Glasses

Sammanfattning: Bioactive glasses integrate with bone/tooth tissues by forming a layer of hydroxy-carbonate apatite (HCA), which mimics the composition of bone mineral. In the current thesis, we investigated composition–structure–bioactivity correlations of phosphosilicate and borophosphosilicate (BPS) glasses. Bioactive phosphosilicate glasses extend the compositional space of the ”45S5 Bioglass®”, which has been in clinical use for decades. Recently developed bioactive BPS glasses with SiO2→B2O3 substitutions transform more completely into HCA and their glass dissolution behaviors can be tuned by varying the relative contents of B and Si. It is known that the average silicate network connectivity NSi and the phosphate content (x(P2O5)) affect the apatite formation (in vitro bioactivity) of phosphosilicate glasses, but the details remain poorly explored. Three series of phosphosilicate glasses were designed by independently varying NSi and x(P2O5). After immersion of the glasses in a simulated body fluid (SBF) for 24 hours, different degrees of their apatite formation were quantified by Fourier-transform infrared (FTIR) spectroscopy. The results revealed that a high P content widened the NSi range that generated optimum amounts of apatite and also mitigated the detrimental effects associated with using glass particles with < 50 μm. The amounts of apatite derived from FTIR agreed with those from 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The growth of apatite at bioactive glass surfaces was found to follow a sigmoidal growth model, in which the precursor phase, amorphous calcium phosphate (ACP), formed in the induction period and then crystallized into HCA in the following proliferation period, with an improvement in the structural ordering of HCA in the maturation period. This formation process closely resembles the apatite precipitated spontaneously from supersaturated Ca/P-containing solutions. The simultaneous growth of ACP and HCA is discussed in conjunction with a previously proposed mechanism for explaining in vitro bioactivity and apatite growth from bioactive glasses. The short- and medium- range structures of bioactive borophosphosilicate (BPS) glasses were investigated by solid-state MAS NMR. Two series of BPS glasses were designed by gradually replacing SiO2 with B2O3 in the 45S5 glass, as well as another base glass featuring a more condensed glass network. As the B2O3 content is increased, the glass networks become more polymerized, together with decreased fractions of the dominating BO3 and orthophosphate units. Borate groups are homogeneously mixed with the isolated orthophosphate groups, while the remaining phosphate groups exhibit a slight preference for bonding to BO4 over SiO4 units. Linkages among borate groups are dominated by B[3]–O–B[4] linkages at the expenses of B[3]–O–B[3] and B[4]–O–B[4] linkages, with the latter B[4]–O–B[4] motifs disfavored yet abundant. A similar fashion of borate mixing was observed in P-free Na/Ca-based borosilicate glasses that span a large compositional space. The content of B[4]–O–B[4] linkages was found to be controlled by the relative fractions of BO4 groups and non-bridging oxygen ions.