Page 29 - Biodegradable Polyesters

1.3 Biodegradable Polyesters 7

homopolymer of glycolic acid which is suitable for use as a binder in explosives.
In 1962, Bowman showed that this polymer could be used as a binder for solid
propellants [23]. Poly(glycolic acid) (PGA)was found easily thermally degraded
[24] and its poor thermal and hydrolytic stabilities were problematic for any
permanent application. Choju et al. [25] mentioned that this PGA homopolymer
∘
is an unstabilized polymer and weight loss under heating begins at 240 C [25].
It was later realized that one could take advantage of the hydrolytic sensitivity
of PGA to make polymeric devices which can degrade in the human body
[23, 26]. Schmitt and Polistina [27] made use of the hydrolytic degradation
of PGA to make absorbable surgical dressings. This degradability also made
PGA the first bioresorbable suture material [28, 29]. In Li’s review [30], it was
summarized that people tend to use the word “degradable” as a general term
and reserve “biodegradable” for polymers which are biologically degraded by
enzymes introduced in vitro or generated by surrounding living cells. A polymer
able to degrade, and to have its degradation by-products assimilated or excreted
by a living system, is then designated as “bioresorbable.” Most degradable and
biodegradable polymers contain hydrolyzable linkages, namely, ester, orthoester,
anhydride, carbonate, amide, urea, and urethane, in their backbones. The ester
bond-containing aliphatic polyesters are particularly interesting because of their
outstanding biocompatibility and variable physical, chemical, and biological
properties. The main members of aliphatic polyesters, their acronyms, and
chemical structures are listed in Table 1.2. Among the aliphatic polyesters, PLA,
PGA , and PCL are the most investigated [30, 31].

1.3.1
Biodegradable Aliphatic Polyesters and Their Copolymers

Biodegradable polyesters can degrade in the environment because of the charac-
teristics of their main-chain structure and a certain extent of hydrophilicity and
crystallinity. Latest investigations have shown that the hydrophilic/hydrophobic
balance of polyester molecules seems to be crucial for the enzyme to bind to
the substrate and the subsequent hydrolytic action of the enzyme. Interestingly,
lipases are not able to hydrolyze polyesters having an optically active carbon
such as PHB and PLLA (poly-L-lactide) [32]. Lipases are an important group of
esterases for biodegradation of aliphatic polyesters. These enzymes are known
to hydrolyze triacylglycerols (fat) to fatty acid and glycerol. It seems probable
that lipase can hydrolyze aliphatic polyestersin contrast to aromatic polyesters
because the flexibility of the main chain and the hydrophilicity of aliphatic
polyesters is so high that it allows intimate contact between the polyester chain
and the active site of lipases. This is in marked contrast to the rigid main chain
and hydrophobicity of aromatic polyesters [32].

1.3.1.1 Poly(lactic acid)
Lactic acid is the smallest optically active organic molecule of natural origin
with either L(+)or D(−) stereoisomers; it is produced by animals, in plants, and