Mesoporous Bioactive Glasses for Drug Delivery and Bone Tissue Regeneration - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 2

at high temperature (1300°C-1500°C). The 45S5 bioglass has been regarded
as bioactive bone regeneration materials that are able to bond closely with
the host bone tissue (Hench 1991). The mechanism behind new bone forma-
tion on bioactive glasses is closely associated with the release of Na + and Ca 2+
ions and the deposition of a carbonated hydroxyapatite (CHAp) layer. The
apatite layer forms a strong chemical bond between 45S5 bioglass and the
host bone (Hench 1991). Further studies have also shown that the Ca and
Si released from the 45S5 contribute to its bioactivity, as both Ca and Si are
found to stimulate osteoblast proliferation and differentiation (Xynos et al.
2000; Gough, Jones, et al. 2004; Gough, Notingher, et al. 2004; Jones et al. 2006,
2007; Hoppe et al. 2011). Xynos et al. (2000) further found that 45S5 bioglass is
able to enhance the expression of a potent osteoblast mitogenic growth fac-
tor, insulin-like growth factor II (IGF-II) (Xynos et al. 2000; Wu, Chang, et al.
2011). The 45S5 bioglass is still considered to be the gold standard for bioactive
glasses, although melt-derived bioactive glasses have a number of limitations
(Wu, Chang, et al. 2011). One of these limitations is the fact that it needs to be
melted at a very high temperature (>1300°C) and another is its lack of micro-
porous structure inside the materials with low specific surface area; therefore,
the bioactivity of melt-derived bioactive glasses will mainly depend on the
contents of SiO 2 (Wu, Chang, et al. 2011). Generally, the bioactivity of melt-
derived bioactive glasses will decrease with the increase of SiO 2 contents
(Hench 1998; Hench and Polak 2002; Hench and Thompson 2010). When the
SiO 2 content exceeds 60%, the bioactive glass is not able to induce the forma-
tion of CHAp layers even after several weeks in simulated body fluid (SBF)
and it fails to bond to either bone or soft tissues (Arcos and Vallet-Regi 2010).
The main reason is that high SiO 2 -containing glasses prepared by the melt-
derived method have a stable net structure and do not easily release Na + and
Ca 2+ , leading to insufficient OH - groups on the surface of glasses to induce
apatite formation. In the early 1990s, in an effort to overcome the limitation of
melt-derived bioactive glasses, Li et al. (1991) prepared sol-gel-derived bioac-
tive glasses. They demonstrated that this class of materials was bioactive in
a wider compositional range when compared with traditional melt-derived
bioactive glasses. The glasses in the SiO 2 -CaO-P 2 O 5 system had a silica content
of up to 90% and were still capable of inducing the formation of apatite lay-
ers, compared to the 60% SiO 2 boundary of the melt-derived bioactive glasses
(Cerruti and Sahai 2006). Due to its larger surface area and porosity proper-
ties derived from the sol-gel process, the range of bioactive compositions for
sol-gel-derived bioglasses is wider, and as compared to melt-derived bioac-
tive glasses, these bioactive glasses exhibit higher bone bonding rates coupled
with excellent degradation and resorption properties (Zhong and Greenspan
2000; Hamadouche et al. 2001; Arcos and Vallet-Regi 2010). Although sol-gel-
derived bioactive glasses have better composition range and bioactivity than
melt-derived bioactive glasses, the micropore distribution is not uniform and
is inadequate for efficient drug loading and release (Arcos et al. 2009; Wu,
Chang, et al. 2011; Wu and Chang 2012).