Biomedical Engineering Reference
In-Depth Information
object to having animal-derived products implanted on religious or
cultural grounds, which would limit the market for the device. Some
polymers can be grown by bacteria via biotechnology routes, but the
presence of the bacteria is also a concern for regulatory bodies - and,
again, reproducibility of the polymer specification is a concern. Human
recombinant proteins such as collagen are being developed, but at
present the process is expensive and the yields are low. The intrinsically
complex structure of natural polymers, in comparison to synthetic poly-
mers, also complicates the manufacturing techniques for the fabrication
of complex structures (e.g. porous scaffolds). For example, collagen has
excellent mechanical properties as it is a triple helix of three peptide
chains, but its structure makes it difficult to manipulate for process-
ing scaffolds. Collagen is used in some applications as well as gelatin
(hydrolysed collagen). However, biomedical device companies prefer to
make devices from synthetic polymers so that they have better control
over the polymer properties, satisfying the requirements of regulatory
bodies and reducing long-term risk. The problem is that there is not yet
a synthetic polymer that can fully mimic the structure of polypeptides
like gelatin.
9.2.2 Synthetic Polymers
Synthetic polymers represent the largest group of biodegradable poly-
mers, exhibiting predictable and reproducible mechanical and physical
properties such as tensile strength, elastic modulus and degradation rate.
Unfortunately, current biodegradable synthetic polymers may degrade a
bit too suddenly once the degradation process gets going (Figure 9.3).
A large proportion of the currently investigated synthetic degradable
polymers for tissue engineering scaffolds are polyesters, as shown in
Table 9.2. This is primarily because many biomolecules in the body and
foods are esters, for example, fats and oils, which thus meet the most
essential requirement on biomaterials, that is, biocompatibility.
Poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and poly(lactic-
co
-
glycolide) (PLGA) copolymers (Table 9.2) are among the most commonly
used synthetic polymers as biomedical devices. These materials degrade
on contact with water into natural products such as lactic acid and
glycolic acid, and are highly biocompatible, considering that the human
body already contains mechanisms for completely removing lactic and
glycolic acids. PLA, PGA and their copolymers (a copolymer is when the
polymer chains contain mixed species, e.g. glycolic acid and lactic acid
segments) have the approval of the US Food and Drug Administration
