Biomedical Engineering Reference
In-Depth Information
their compositions and preparation conditions are optimized. Those
hybrids show rather weak bioactivity, as it took three days in SBF
before depositing apatite, while the bioactive Ca-Ormosils needed only
a day or so. Other hybrids in the system CaO-SiO
2
-TiO
2
have also
been prepared where Ti-O-Si substitutes a part of their siloxane skele-
tons. They are bioactive enough to deposit apatite within one day in
SBF. Cytocompatibility and blood compatibility of those Ormosils and
Ormotites have been examined. A few papers have been listed in the
Further Reading at the end of this chapter.
10.7.4 Hybrids From Vinylsilanes or Other Bifunctional
Silanes
The silanes listed in Table 10.2 or any other silanes with a functional
group at the end opposite to the -Si(OR)
3
end also give inorganic-
organic hybrids. When vinylsilanes are polymerized,
they yield
polyethylene-type C-C skeletons:
CH(R)-CH
2
-CH
2
-CH(R
)- (10.4)
CH-R
→
R-CH
=
CH
2
+
CH
2
=
The Si-OH groups from the hydrolysis of the alkoxysilane groups appear
randomly on the C-C skeletons. Those Si-OH groups are polymerized
to yield a silica-type -Si-O-Si- skeleton. Therefore, such two active
sites for polymerization can thus provide C-C, C-N (cyanosilanes), or
Si-O-Si skeletons. They offer additional freedom to design a wider range
of hybrids and controlling the resultant hybrids.
The vinylsilane-based hybrids deposit apatite under body condi-
tions when they involve calcium ions. For example, a series of bulk
hybrids derived from the precursor system VTMS-Ca(CH
3
COO)
2
-
C
2
H
5
OH-H
2
O (VTMS
vinyl trimethoxysilane) deposit apatite in SBF
within one day. However, even if they involve calcium ions, those gel
films have an inferior ability to deposit apatite in SBF, that is, they
have longer induction times. This happens partly because the gel films
involved a smaller number of
=
Si-OH groups on their surface that
would serve as nucleation sites for apatite by attracting the relevant
ions, and partly because fewer calcium ions are present in the top sur-
face. The calcium deficiency would be interpreted as a result of the
following: the drying that occurs during the sol film coating process
rapidly shrinks the top layer, drives down the calcium ions, and closes
the paths through which they diffuse up to the top when the gel is soaked
in SBF. This interpretation is supported by the shorter induction time
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