Page 358 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOPOLYMERS 335

Polycaprolactone (PCL) is a biodegradable, semicrystalline polyester that is synthesized from capro-
lactone using stannous octoate in a similar manner to polylactide or polyglycolide. PCL has a very
low modulus of around 50 KSI since it has a low T g of –60°C. PCL degrades very slowly, and, there-
fore, it is usually not used as a homopolymer. Caprolactone, however, is copolymerized with glycolide
to make a flexible suture material (trade name MONOCRYL). 33

Polyhydroxyalkanoates. Degradation: bioerosion.

O R
C CH 2 CH 2 O
x
n
Polyhydroxyalkanoates (PHAs) are another class of thermoplastic degradable polyesters like poly-
lactide, polyglycolide, and polycaprolactone. Materials with properties ranging from elastomers to
strong, rigid polymers can be produced. PHAs are widely studied in the scientific literature, but are
just beginning to be commercialized for use in medical devices. The figure above shows the struc-
ture of PHAs, which can vary widely. The alkyl group within the polymer chain [CH ] typically
2 x
contains 1 to 4 CH groups. R is most commonly a short-chain alkyl group such as methyl (CH ) or
2 3
ethyl (CH CH ). A variety of different mechanical properties and degradation rates can be achieved
2 3
by varying x and R. The ranges of degradation rates, mechanical properties, and applications for
PHAs are similar to polylactide, polyglycolide, and their copolymers. 34
PHAs are produced in nature by many different microorganisms, which use them for energy storage.
Tepha (Cambridge, MA) has isolated the DNA sequences responsible for production of PHAs
in bacteria and has developed methods to produce PHAs with tailored chemistry and properties by
fermentation using transgenic bacteria. Tepha received FDA clearance for a suture made from poly-
4-hydroxybutyrate [poly(4HB)] in February 2007. Figure 13.15 shows some PHAs that have been

Poly-3-hydroxybutyrate O CH 3
C CH 2 CH 2 O
(Poly(3HB))
n

Poly-4-hydroxybutyrate O
C CH 2 CH 2 CH 2 O
(Poly(4HB))
n

Poly-3-hydroxybutyrate-
O CH 3 O
co-4-hydroxybutyrate
C CH 2 CH 2 O C CH 2 CH 2 CH 2 O
Poly(3HB-co-4HB)
n m

Poly-3-hydroxybutyrate-
co-3-hydroxyvalerate O CH 3 O CH 2 CH 3
C CH 2 CH 2 O C CH 2 CH 2 O
(Poly(3HB-co-3HV))
n m
FIGURE 13.15 Chemical structures of different PHAs.