Abstract
Body-fluid-exposed bioactive glasses (BGs) integrate with living tissues due to the formation of a biomimetic surface layer of calcium hydroxy-carbonate apatite (HCA) with a close composition to bone mineral. Vast efforts have been spent to understand the mechanisms underlying in vitro apatite mineralization, as either formed by direct precipitation from supersaturated solutions, or from BG substrates in a simulated body fluid (SBF). Formally, these two scenarios are distinct and have hitherto been discussed as such. Herein, we contrast them and identify several shared features. We monitored the formation of amorphous calcium phosphate (ACP) and its crystallization into HCA from a NaO–CaO–SiO–PO glass exposed to SBF for variable periods out to 28 days. The HCA growth was assessed semi-quantitatively by Fourier transform infrared spectroscopy and powder X-ray diffraction, with the evolution of the relative apatite content for increasing SBF-exposure periods evaluated against trends in Ca and P concentrations in the accompanying solutions. This revealed a sigmoidal apatite growth behavior, well-known to apply to spontaneously precipitated apatite. The results are discussed in relation to the prevailing mechanism proposed for in vitro HCA formation from silicate-based BGs, where we highlight largely simultaneous growth processes of ACP and HCA.
Highlights
Calcium hydroxyapatite (“apatite”; Ca5 (PO4 )3 OH) is a naturally occurring mineral, whose carbonatesubstituted form constitutes the inorganic part of mammalian bone and tooth [1]
Notwithstanding these formal differences, a key finding is that calcium phosphate (CaP) formed by heterogenous nucleation at simulated body fluid (SBF)-exposed bioactive glasses (BGs) surfaces exhibits identical sigmoidal growth characteristics as those well-established for spontaneous H(C)A precipitation [4,5,6,7], which comprise three stages of
The five-step reaction mechanism accounting for the main events leading to hydroxy-carbonate apatite (HCA) formation when a Na–Ca–Si–O–(P) glass is exposed to aqueous solutions was proposed by Hench [2] mainly on the basis of results from in vitro testing in water [27,28,67]: while formally only glasses that incorporate both Ca and phosphate species may produce HCA, the Hench mechanism” (HM) is valid for in vitro testing in SBF media [56,57] as well as for real in vivo applications, for which the fluid surrounding the glass will provide the main Ca/P reservoirs of the biomimetic CaP layer
Summary
Calcium hydroxyapatite (“apatite”; Ca5 (PO4 ) OH) is a naturally occurring mineral, whose carbonatesubstituted form constitutes the inorganic part of mammalian bone and tooth [1]. Such a biomimetic hydroxy-carbonate apatite (HCA) phase forms at the surface of a silicate-based bioactive glass (BG) on its exposure to body fluids [2,3]. By “ACP”, we mean an amorphous CaP phase with a stoichiometry close to Ca3 (PO4 ) and comprising minor (and variable) HPO4− contents and water molecules [7,20,21,22]
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