Page 356 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOPOLYMERS 333

Perfluorinated Ethylene-Propylene Polymer. Degradation: bioinert.
CF 3
CF CF CF
CF 2 2 2
n m

Perfluorinated ethylene-propylene polymer (FEP) is a copolymer of tetrafluoroethylene (TFE) and
hexafluoropropylene (HFP). FEP has similar properties to PTFE, but its lower melting temperature
of 275°C allows it to be melt processed. 31

Polylactide, Polyglycolide, and Copolymers. (Also known as polylactic acid and polyglycolic acid.)
Degradation: bioerosion.

O R
O C CH
n

Polylactide: R = CH
3
Polyglycolide: R = H
Polylactide and polyglycolide are the most widely used synthetic degradable biopolymers. They are
popular since they have good mechanical properties and degrade to nontoxic metabolites: glycolic
or lactic acid. Polylactide, polyglycolide, and copolymers of the two find clinical use in degradable
sutures and orthopedic pins and screws. Recent research has focused on their use as a drug delivery
matrix since sustained release of drugs can be achieved as the materials degrade. Drug delivery
matrices include monoliths and microspheres. Microspheres are routinely prepared by dissolution of
polymer and drug in chloroform (or dichloromethane), suspension in aqueous polyvinyl alcohol
(to form an oil in water emulsion), and evaporation to form drug-entrapped microspheres or nanos-
pheres. Stirring speed and polymer/drug concentration in the oil phase (chloroform or dichloromethane
solution) are the primary controls of sphere size.
Polylactide differs from polyglycolide in that R is a methyl group (CH ) for polylactide and a
3
hydrogen for polyglycolide. Polylactide and polyglycolide are usually synthesized from lactide and
glycolide cyclic monomers using initiators such as stannous 2-ethyl hexanoate (stannous octoate).
Like polypropylene, the stereochemistry of the repeat unit has a large effect on the structure and
properties of polylactide. Poly(DL-lactide) is atactic, meaning it has no regular stereostructure and as a
result is purely amorphous. Poly(D-lactide) and poly(L-lactide) are isotactic and consequently are
approximately 35 percent decrystalline. Poly(D-lactide) is seldom used commercially since D-lactic
acid (degradation product of D-lactide) does not occur naturally in the human body, while L-lactic acid
is a common metabolite. Poly(L-lactide) has a higher modulus and tensile strength than the amorphous
poly(DL-lactide). Similarly, the crystalline poly(L-lactide) degrades completely in vivo in 20 months to
5 years, while poly(DL-lactide) degrades much faster, in 6 to 17 weeks. 32
Copolymers of glycolide and lactide [poly (lactide-co-glycolide)] are amorphous and have similar
mechanical properties and degradation rates as poly(DL-lactide). Pure polyglycolide is very strong
and stiff, yet has similar degradation as the poly(DL-lactides) and the lactide/glycolide copolymers.
32
Polyglycolide is highly crystalline, with crystallinities between 35 and 70 percent. Figures 13.13 and
13.14 show degradation rates for polyglycolide and poly(L-lactide).
Polylactide, polyglycolide, and poly(lactide-co-glycolide) are often called polylactic acid, pol-
glycolic acid, and poly(lactic-co-glycolic acid) since their structures can be deduced by the direct
condensation of lactic and glycolic acid. Though it is rare, synthesis of polylactic and glycolic acids
can be achieved by direct condensation, but this results in low-molecular-weight polymer (on the
order of 2000 g/mol), with poor mechanical properties but increased degradation rates.