Mesoporous Bioactive Glasses for Drug Delivery and Bone Tissue Regeneration - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 15

1.4 Bone Tissue Engineering of MBG
1.4.1 Excellent Apatite-Mineralization Ability of MBG
One significant advantage of MBG is that it possesses superior apatite-
mineralization ability in biological solution. The characteristics of MBG
indicate that it may be used for bone regeneration applications. In the past
several years, a great number of studies have focused on the in vitro bioac-
tivity of MBG with different forms, including particles and scaffolds (Wu
and Chang 2012). Yan et al . (2004) investigated the apatite mineralization of
MBG particles in simulated body fluids and found that apatite formed only
after soaking for 4 h. The apatite-mineralization ability of MBG was signifi-
cantly higher than that of conventional nonmesoporous bioactive glasses.
It is speculated that the high specific surface area and pore volume of MBG
play an important role in the enhancement of the bioactive behavior of the
MBG materials (Yan et al. 2004). They further optimized the composition
structure-bioactivity correlation of MBG and found that the in vitro bioac-
tivity of MBG is dependent on the Si to Ca ratio in the glass network. MBG
(80Si15Ca5P) with relatively lower calcium content exhibits the best in vitro
bioactivity in contrast to conventional melt-derived NBG where usually
higher calcium percentage BG (e.g., 60Si35Ca) show better bioactivity (Yan et
al. 2006). The mesopore size is also of importance to influence the apatite for-
mation of MBG (Deng et al. 2009; Yu et al. 2010, 2011). Gunawidjaja et al. (2010)
studied the mechanism of apatite mineralization of MBG by using nuclear
magnetic resonance spectroscopy. It was found that the significant differ-
ence of the apatite formation mechanism between MBG and conventional
NBG is that MBG does not require the typical “first 3 stages” (Gunawidjaja
et al. 2010), but conventional NBG does (Hench and Polak 2002). In the first 3
stages, conventional NBG releases M + ions and form Si-OH groups and then
Si-OH groups form networks by repolymerization. However, the surface of
MBG is already inherently “prepared” to accelerate the first 3 stages of the
conventional NBG (Gunawidjaja et al. 2010).
The apatite-mineralization ability of nanosized MBG particles, large-
sized MBG beads, and a series of 3D MBG scaffolds with varied chemi-
cal compositions were investigated. All of them showed excellent apatite
mineralization ability in biological solutions (Figure 1.7). The apatite-min-
eralization ability of MBG was significantly influenced by their chemical
compositions. Typically, MBG with Ca-P-Si compositions showed optimal
apatite mineralization ability. It was found that the incorporation of parts
of Fe, Co, Sr, or Zr ions into MBG scaffolds could decrease their apatite-
mineralization ability (Zhu and Kaskel 2009; Wu, Fan, Zhu, et al. 2011; Wu,
Luo, et al. 2011; Wu, Miron, et al. 2011; Wu and Chang 2012); however, the
incorporation of these ions into MBG scaffolds could enhance their in vitro
cytocompatibility.