Biomedical Engineering Reference
In-Depth Information
that the mechanism involves ion exchange (e.g. Ca
2
+
ions leave the
glass, leaving Si-OH). The negative charge on the surface attracts Ca
2
+
and PO
4
3
−
Si-O
−
ions. The
>
Si-OH and
>
form a hydrated silica
Si-O
−
gel layer, in which the
>
content depends on the pH of the
surrounding plasma. (The symbol
stands for additional Si-O bonds to
accommodate the quadruple valence state of Si.) Since the point of zero
charge (pzc) of soluble silica (SiO
2
)is
>
∼
pH 2, most of the silanol groups
Si-O
−
groups under the body's environment at
pH 7.4. Bioactive glasses can be made by the melt or sol-gel routes, and
sol-gel glasses form HCA layers more rapidly than melt-derived glasses
of similar nominal compositions, as they have higher surface area and
lower network connectivity.
Although original bioactive glasses contained phosphate, it is not
necessary for a glass to contain phosphate to be bioactive, as the blood
plasma can supply phosphate (and additional calcium) ions since it is
supersaturated with Ca
2
+
and PO
4
3-
ions. An increase in the calcium
ions and hydroxyl groups at the glass-blood interface triggers HCA
nucleation and growth. Therefore, hybrids or composites involving
>
are actually present as
>
Si-OH and Ca
2
+
in their matrix are very much compatible with bone
cells and surely deposit apatite when embedded in the body.
Cell-material interactions, for example, attachment, proliferation,
and differentiation, result from a series of complicated surface phe-
nomena, and depend on the material and its structure (the components
from which the matrix is constructed and how they are integrated) and
microstructure, such as porosity, surface charge, and surface roughness
(topography). Its chemical properties also contribute to the interaction.
Those factors control blood protein adsorption, that is, protein affinity,
or dissolution of the components, which might affect gene expression.
Adjusting the content of the silanol groups as well as the microstruc-
ture of the matrix will modify the compatibility of the hybrids with
fibroblasts or nerve cells as well as osteoblasts.
10.7.1 Calcium Incorporation in Sol-Gel Hybrids
So far the hybrids discussed have largely been silica-polymer systems.
However, they are not necessarily bioactive. Calcium nitrate and calcium
chloride have been used in some hybrids, but the problem is that the
calcium is not incorporated into the silicate network during the low-
temperature processes used. Hybrids are not usually heated much above
100
◦
C, as higher temperatures would damage the polymer, but calcium
is not incorporated into the silicate network until a temperature of
