Biomedical Engineering Reference
In-Depth Information
arrangement, are effective in inducing apatite nucleation. The crucial part
is how well the interface between the organized hydroxylated surface and
HAp nuclei structurally match, and also the surface charge (Uchida et al.
2001). Hydroxylated 110 (anatase) and 0001 (HAp) match their interface via
three parts: (1) hydrogen bond interaction, (2) crystal lattice matching, and
(3) stereochemical matching. Lindberg et al. (2008) reported experimental
observations of early growth and growth of apatite on single-crystal rutile
substrates (100), (001), and (110). The adsorption of calcium and phosphate
ions was faster on the (001) and (100) surfaces than on the (110) surface in
the early stage (Lindahl et al. 2010). After a long time soaking, the hydroxy-
apatite precipitate nucleus on the (001) surface led to faster coverage of this
surface compared to the (110) and (100) rutile surfaces. The ion-substituted
apatite can form on pretreated rutile surfaces. Xia, Lindahl, Persson, et al.
(2010) reported that the ion doping not only changed the composition but
also influenced the morphology. Strontium-, silicon-, and fluoride-substi-
tuted apatite appeared spherical, nano-flake, and needle-like, respectively.
5.3.2 Biomineralization of Silicate- and Aluminate-Based Bioceramics
It is known that simulated body fluid contains calcium and phosphate ions that
are supersaturated with respect to apatite. However, apatite could not sponta-
neously precipitate under a normal condition. As described in FigureĀ 5.1, the
energy barrier for the apatite nucleation inhibits the initial formation. Hench
(1991) have reported that the formation of Si-OH group on bioactive glass sur-
faces plays the key role in the nucleation of apatite. The formation of Si-OH
group is triggered by the release of calcium and sodium ions from bioactive
glasses. The release of cations also increases the degree of supersaturation of
SBF. Both the formation of Si-OH and the release of cations help the nucle-
ation and growth of apatite. FigureĀ 5.3 illustrates the nucleation and growth
of bonelike apatite (Lee et al. 2006). Normally other ions, such as carbonate
and Mg, will spontaneously precipitate with calcium and phosphate.
Except for bioactive glasses, in terms of silicate-based bioceramics, mesopo-
rous bioactive glass (MBG) is another interesting material. MBGs constitute a
new family of bioceramics with the fastest in vitro bioactivity studied so far. The
interest of MBG is due to their specific structure with high surface area and
suitable pore size and pore volume. The extreme high surface area can offer
more places for the formation of Si-OH group that help the nucleation of apa-
tite. Those result in a fast precipitation of apatite on MBG surface. A sequential
transition from amorphous calcium phosphate (ACP) to octacalcium phosphate
(OCP) and to calcium deficient carbonate hydroxyapatite (CDHA) maturation,
similar to the in vivo bone biomineralization, has been observed in a MBG sys-
tem with 2D hexagonal structure and high Ca content (37 mol%) (Izquierdo-
Barba, Arcos, et al. 2008). It was the first time to find the ACP-OCP-CDHA
maturation sequence in a bioactive ceramic system. The intense exchange of
Ca 2+ and H 3 O + decreases the local pH value that favors the formation of OCP.
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