Biomedical Engineering Reference
In-Depth Information
improving toughness and flexibility. Organic-inorganic hybrids involving
inorganic skeletons, Si-O and Ti-O, together with organic polymer
skeletons are sometimes called ''Ceramer'' because they are ceramic-
modified polymers. A typical silicate sol is made from the hydrolysis of
TEOS. As condensation begins, nanoparticles form (Chapter 3). Organic
polymers can then be added to the sol before the nanoparticles coalesce
and gel together. This creates nanoscale interactions between the organic
chains and the forming silica network. Interactions between the silica
and the polymer chains can either be physical interactions such as Van
der Waals forces and hydrogen bonding, or chemical covalent bonds.
The latter are important if the hybrid is to have controlled degradation
rates and tailored mechanical properties. Hybrids with only physical
interactions are likely to dissociate rapidly in aqueous solutions, rather
like sugar lumps. Covalent bonds are therefore necessary between the
inorganic and organic components.
10.5 ORMOSILS
One method for obtaining covalent bonds between organic and
inorganic components in a hybrid is to use an organosilane (a
molecule that contains organic and silane components), for example,
poly(dimethylsiloxane) (PDMS). PDMS oligomers are subject to
hetero-condensation with the Si-OH groups derived from TEOS, and
then the product is a typical example of an organically modified silicate,
or Ormosil. Figure 10.4 represents the structure concept of Ormosils.
The PDMS chains of various lengths bind the silica microblocks, derived
from TEOS.
The silica blocks consist of a few silicate units, which are labeled as
Q, as in glasses (Chapters 2 and 3). Each Si atom in the Q units bonds
to four oxygen atoms. When an oxygen atom in a unit is bonded to a
Si atom (Si-O-Si) belonging to another Q unit, it is a bridging oxygen
atom. A non-bridging oxygen atom satisfies its valence by bonding to an
atom other than Si, like H in Si-OH. The non-bridging oxygen can be
ionically or electrostatically interacted with mono- or divalent cations:
Si-O
−
•
Ca
2
+
. The Q units can hold different numbers of bridging
and non-bridging oxygen atoms. In contrast, the PDMS chains are
composed of D units: -Si(CH
3
)
2
-O- (D units are bifunctional) where
-O- represents a bridging oxygen. Note that some of the
<
bridging bonds in the PDMS oligomer chains are decomposed by the
hydrolyzing activity of HCl, added as the catalyst, during the sol-gel
procedure to yield extra Si-OH groups. They are active and condensed
>
Si-O-Si
