Biomedical Engineering Reference
In-Depth Information
Hybrids cannot be heated to high temperatures, as the polymer will be
damaged. Therefore, another calcium precursor is needed.
Many bioresorbable polymers, for example, poly(lactides), cannot be
simply introduced into the sol, as they are not soluble in the sol. However,
they can be functionalised so that not only are they incorporated in the
sol-gel process, but also they can form covalent bonds with the silica
network, creating a class II hybrid material. The functionalisation of the
polymer involves the introduction of coupling agents.
One example is silica-poly(
-caprolactone) (PCL) hybrids. Hydroxyl
groups at either end of poly(
ε
-caprolactone diol) polymer chains
can be reacted with 3-isocyanatopropyl triethoxysilane (IPTS). This
process results in a polymer end-capped with a triethoxysilyl group.
When the end-capped PCL is introduced into a sol, the siloxane
groups hydrolyse and then Si-OH groups from the end-capped polymer
condense with the Si-OH groups from the hydrolysed TEOS in the sol
to yield an interconnected PCL-silica network.
Alternatives to conventional polyesters are natural polymers, which
can be a closer mimic of bone's natural structure. Bone contains collagen,
which is a structural protein with a triple helix of polypeptides (amino
acid chains), giving it excellent mechanical strength (the structure is
analogous to that of rope).
Collagen would therefore be an ideal choice to use as a natural
polymer in hybrid synthesis. Unfortunately, the triple helix structure
makes it very insoluble. It will dissolve in acetic acid, but only in very
low concentrations. Therefore, it is not possible to produce a hybrid
with significant amounts of polymer using collagen. Where to source
the collagen is also an issue. Currently, it cannot be synthesised in
any significant quantity, so it must be sourced from animals. Although
collagen is unlikely to be rejected by a patient's body, patients may refuse
an implant on religious or moral grounds owing to the animal species
from which it originates, for example, bovine (cow) or porcine (pig).
One of the great benefits of natural polypeptides is that they can be
functionalised. Gelatin has great potential as it is hydrolysed collagen
and it is water-soluble. It also contains -COOH groups (carboxylic acid),
-NH and -NH
2
groups along its backbone that are available for func-
tionalisation. This time, glycidoxypropyl trimethoxysilane (GPTMS) is
used as the coupling agent. The glycidol group (expoxy ring) can open
and react with the functional groups on the polymer chain. The func-
tionalised polymer has short molecules bonded to it with Si-OH groups
on the end of them, ready to undergo condensation with other Si-OH
groups from the silica in the sol, to form Si-O-Si bonds. Any polypeptide
ε
