Biomedical Engineering Reference
In-Depth Information
nucleophilic attack and condensation can then occur with the -NH
2
unit of an amino acid residue on a polypeptide chain, that is, the
GPTMS molecule is grafted to a gelatin molecule. The trimethoxysilane
group at the other end yields -Si-O-Si bridges or the siloxane units
due to hydrolysis and condensation, similar to the reactions in the
TEOS-based sol-gel process. Silica-organic hybrids with covalent bonds
between the components consists of T-structure units (R-Si(-O-)
3
),
where T stands for trifunctional, and -O- stands for a bridging oxygen
atom that is shared by two Si atoms forming a bridging bond -Si-O-Si-.
Figure 10.5(b) shows a T
3
unit with an Si-C bond and three -Si-O-Si-
bridging oxygen bonds.
10.6.1 Silica/Gelatin
A series of gelatin-siloxane hybrids have been prepared from gelatin
and GPTMS, where the epoxy ring on the GPTMS molecules open and
the GPTMS is grafted onto the gelatin. Meanwhile, the trimethoxysilane
group at the other end hydrolyzes, and condensation of the resulting
Si-OH bonds occurs to yield -Si-O-Si bridges to other GPTMS bonds.
In consequence, cross-links of the gelatin chains are formed. Mechanical
and viscoelastic properties naturally depend on the cross-link density.
For example, consider the glass transition temperature (
T
g
) at which the
structure of a substance changes from more rigid to more flexible and
fluid. Hybrids are hard and stiff below
T
g
, while they are soft and flexible
above
T
g
.The
T
g
for the gelatin-GPTMS hybrids increases with increase
in the GPTMS content, as expected from the cross-linking effects of
GPTMS. When calcium is introduced by adding calcium nitrate, the
presence of the calcium decreases
T
g
and hence gives flexibility. The
effect of the calcium ions is interpreted in terms of the interaction among
the gelatin-composing chains. Gelatin consists of triple helical strands,
and these are uncoiled into random coils by the Ca
2
+
ions, and such a
change weakened the intermolecular or intramolecular bonding among
the gelatin chains with which the helix structure is stabilized.
Gelatin functionalized (grafted) with GPTMS can also be incorpo-
rated into the TEOS-based sol-gel process. The GPTMS is used to
form covalent links between interpenetrating networks of gelatin and
silica. The gelatin is first reacted with some GPTMS and then this
hybrid is added into the early stage of the sol-gel process so that the
trimethoxysilane groups on the GPTMS hydrolyze and condensation
occurs between the hydrolyzed GPTMS and the nanoparticles in the
colloidal sol (Figure 10.6 and see Figure 12 in colour section).
