Biomedical Engineering Reference
In-Depth Information
400
◦
C is reached, when it diffuses into the silica and disrupts the
network. At lower temperatures, the calcium is likely to be precipitated
on the surface of the silica or chelated on carboxylic acid groups of the
polymers (if present).
10.7.2 Calcium-Containing Ormosils
Calcium-containing Ormosils are the first polymers or hybrids for which
apatite formation has been confirmed since the invention of the bioactive
silicate glasses. They release calcium ions into the surrounding body fluid
and favor apatite precipitation. When an Ormosil-type hybrid deposits
apatite in SBF, they deposit apatite within only one day. In contrast, the
Ca-free Ormosils with similar compositions do not deposit an apatite
layer after being soaked in SBF for 30 days, even though Fourier
transform infrared (FTIR) spectrophotometry indicates that phosphate
ions are attached on the Ormosil sample surface. It is perplexing that the
attached phosphate should attract the calcium ions in the surrounding
solution, but still those two kinds of ions would not form HCA. The
HCA formation mechanism on Ca-containing Ormosils is similar to
that on glasses: Si-O-Ca
2
+
bonds are hydrolyzed in SBF, yielding
Si-OH groups and releasing the calcium ions, while hydroxide ions that
increase pH are in the vicinity of the hybrid surface. Some of those
-SiOH groups are likely to give -Si-O
−
due to the hydroxyl groups,
and they form the hydrated silica layer to provide the heterogeneous
nucleation sites for apatite. It is strange, however, that hydrated silica
does not always induce apatite nucleation, and that the active ones are
either on the material surface due to the hydrolysis reaction with the
body fluid or highly porous pure silica gel. Ca-free silica gels derived
through a conventional sol-gel route cannot deposit HCA. Therefore, the
formation of the -Si-OH groups is not the only factor for controlling the
apatite deposition; the way in which those silanol groups are arranged
is important, that is, a specific structure is essential.
10.7.3 Ormotites
Ti-O can substitute Si-O in Ormosils, and the resultant material
might be called Ormotite (organically modified titanate) in analogy
to Ormosils. Some hybrids in the system CaO-SiO
2
-TiO
2
-PDMS are
similar in mechanical characteristics such as stress-strain behavior and
yield strength to those of human spongy bone (cancellous bone) when
