Biomedical Engineering Reference
In-Depth Information
of hybrids over composites is that the mechanical properties should
decrease in line with the degradation rate.
So, the first criterion is that the polymer should be biodegradable.
The first place to look for candidate polymers would be commercially
available biomedical polymers, but there are few synthetic biodegradable
polymers that have been approved for clinical use. They are mainly based
on polyesters such as the lactides and glycolides and their copolymers.
The problem with these polymers is that, once they start to degrade, the
degradation rate is rapid and mechanical properties are lost rapidly. The
reason for this is the mechanism of degradation, which is chain scission
by hydrolysis (the ester linkage is broken by reaction with water).
The problem is that the chain scission is catalyzed by the degradation
products (carboxylic acid groups).
The next criteria are these: Can covalent bonds be formed between
the polymer and the silica? For sol-gel hybrids, can the polymer be
incorporated into the sol, that is, is it soluble?
For tissue repair, a common strategy is to look at the natural tissue
for inspiration. Most connective tissues contain collagen, and therefore
collagen would be an obvious candidate, but collagen has very low
solubility and is difficult to process. However, gelatin is hydrolyzed
collagen, and this and other natural polymers can be used, for example,
polysaccharides such as chitosan.
There are essentially two methods for obtaining covalent bonds
between silica and natural polymers: using a siloxane that can introduce
silica to a polymer, or using a siloxane coupling agent to bridge between
a silicate network and the polymer. An example of such as siloxane is
3-glycidoxypropyl trimethoxysilane (GPTMS). A GPTMS molecule con-
tains a glycidoxy group (epoxy ring) at one end and three methoxysilane
bonds at the other (Figure 10.5). The epoxy ring can be opened by
Organic
chain
H
C
O
+
Si
O
Si
O
Si
Si
O
O
O
O
O
Si
(a)
(b)
Figure 10.5
(a) The 3-glycidoxypropyl trimethoxysilane (GPTMS) molecule and
(b) the T
3
unit.
