Biomedical Engineering Reference
In-Depth Information
9.2 BIODEGRADABLE POLYMERS
Bone scaffolds provide a suitable environment for new bone formation
and they act as a temporary substrate to support cell attachment and
subsequent bone matrix production. In order to prevent any issues
associated with the long-term persistence of foreign bodies, scaffolds
must be made from biodegradable materials; for example, biodegradable
polymers are key components in scaffold development [4]. While the
scaffold is degrading, the physical support provided by the 3D scaffold
must be maintained until the new tissue formed has sufficient mechanical
integrity to support itself. As the scaffold degrades, the new bone should
remodel into mature bone. The degradation time profile of the scaffold
must therefore be accurately controlled. Several biodegradable polymers
(natural and synthetic) are being considered for bone tissue scaffolds, and
detailed descriptions of the synthesis and properties of biodegradable
polymers are available in the specialised literature. A brief overview
of relevant polymers used in combination with bioactive glasses is
presented next.
9.2.1 Natural Polymers
Natural polymers can be classified as proteins (e.g. silk, collagen, gelatin,
fibrinogen, elastin, keratin, actin and myosin), polysaccharides (i.e.
carbohydrates, e.g. cellulose, amylose, dextran, chitin, chitosan and gly-
cosaminoglycans) and polynucleotides (DNA, RNA). Natural polymers
exhibit similar molecular structure to the components of tissues, thus
enabling easy recognition by the biological system. The obvious choice
for mimicking of bone would be to use collagen. Issues related to toxicity
and stimulation of inflammatory reactions, as well as lack of recognition
by cells, which may be provoked by synthetic polymers, can be avoided
using natural polymers. A benefit of natural polymers is that they can
degrade by natural mechanisms in the body, for example, enzyme degra-
dation, which can yield natural degradation and remodelling rates. A
drawback of natural polymers is that they are difficult to produce by
Nature and are difficult to produce by synthetic chemistry.
This means that natural polymers often have to be harvested from
natural tissue; for example, collagen and gelatin are usually harvested
from pigs. When it comes to producing medical devices and prod-
ucts, it can be difficult to obtain exactly the same polymer each time
(e.g. inherent properties such as molecular weight), which translates
into more complicated routes to regulatory approval. Patients may also
