Biomedical Engineering Reference
In-Depth Information
composition is totally amorphous. IR bands associated with Vicryl molecules in the amorphous domains
are 560, 710, 850, and 888 cm
−1
, whereas 590, 626, 808, 900, and 972 cm
−1
are associated with the crys-
talline domains (Fredericks et al., 1984). Like PGA, these IR bands could be used to assess the extent of
hydrolysis.
A relatively new block copolymer of glycolide and carbonates, such as TMC, has been commercial-
ized. Maxon is made from a block copolymer of glycolide and 1,3-dioxan-2-one (trimethylene carbon-
ate or GTMC) and consists of 32.5 wt% by weight (or 36 mol%) of TMC (Casey and Roby, 1984; Katz
et al., 1985). Maxon is a poly(ester-carbonate). The polymerization process of Maxon is divided into
two stages. The first stage is the formation of a middle block which is a random copolymer of gly-
colide and 1,3-dioxan-2-one. Diethylene glycol is used as an initiator and stannous chloride dihydrate
(SnCl
2
⋅ 2H
2
O) serves as the catalyst. The polymerization is conducted at about 180°C. The weight ratio
of glycolide to TMC in the middle block is 15:85. After the synthesis of the middle block, the tempera-
ture of the reactive bath is raised to about 220°C to prevent the crystallization of the copolymer, and
additional glycolide monomers as the end blocks are added into the reaction bath to form the final
triblock copolymer.
The latest glycolide-based copolymer that has become commercially successful is Monocryl
®
suture.
It is a segmented block copolymer consisting of both soft and hard segments. The purpose of having soft
segments in the copolymer is to provide good handling properties such as pliability, whereas the hard
segments are used to provide adequate strength. The generic copolymerization process between glycolic
acid and ε-caprolactone was recently reported by Fukuzaki et al. (1989, 1991) in Japan. The resulting
copolymers were low-molecular-weight biodegradable copolymers of glycolic acid and various lactones
for potential drug delivery purposes. The composition of lactone ranged from as low as 15 to as high as
50 mol% and the weight average for molecular weight ranged from 4510 to 16,500. The glass transition
temperature ranged from 18 to −43°C, depending on the copolymer composition and molecular weight.
Monocryl is made from two stages of the polymerization process (Bezwada et al., 1995). In the first
stage, soft segments of prepolymer of glycolide and ε-caprolactone are made. This soft segmented pre-
polymer is further polymerized with glycolides to provide hard segments of polyglycolide. Monocryl
has a composition of 75% glycolide and 25% ε-caprolactone and should have a higher molecular weight
than those glycolide/ε-caprolactone copolymers reported by Fukuzaki et al., for adequate mechanical
properties required by sutures. The most unique aspect of Monocryl monofilament suture is its pliabil-
ity as claimed by Ethicon (Bezwada et al., 1995). The force required to bend a 2/0 suture is only about
2.8 × 104 lb-in.
2
for Monocryl, while the same sized PDSII
®
and Maxon
®
monofilament sutures require
about 3.9 and 11.6 × 104 lb-in.
2
force, respectively. This inherent pliability of Monocryl is due to the
presence of soft segments and
T
g
resulting from the ε-caprolactone co-monomer unit. Its
T
g
is expected
to be between 15°C and −36°C.
5.2.3 Glycolide-Based Biodegradable Copolyesters with Nonaliphatic
Polyester-Based Co-Monomers
In this category, the most important one is the glycolide copolymer consisting of poly(ethylene
1,4-phenylene-bis-oxyacetate) (PEPBO) (Jamiokowski and Shalaby, 1991). The development of this
type of glycolide-based copolymer was initiated because of the adverse effect of γ-irradiation on the
mechanical properties of glycolide-based synthetic absorbable sutures. There is a great desire to develop
γ-irradiation sterilizable, synthetic, absorbable polymers to take advantage of the highly convenient and
reliable method of sterilization.
Shalaby et al. recently reported that incorporation of about 10 mol% of a polymeric radiostabilizer
like PEPBO into PGA backbone chains would make the copolymer sterilizable by γ-irradiation with-
out a significant accelerated loss of mechanical properties upon hydrolysis when compared with the
unirradiated copolymer control (MPG) (Jamiokowski et al., 1991). The changes in tensile-breaking
force of both MPG and PGA sutures implanted intramuscularly and subcutaneously in rats for various
