Biomedical Engineering Reference
In-Depth Information
Copolymer fibers made from PDS and monomers other than linear aliphatic polyester such as
morpholine2,5-dione (MD) exhibit rather interesting biodegradation properties. This copolymer fiber
was absorbed 10−25% earlier than PDS. The copolymer, however, retained a tensile-breaking strength
profile similar to PDS with a slightly faster strength loss during the earlier stage, that is, the first 14 days
(Shalaby, 1994). This ability to break the inherent fiber structure-property relationship through copoly-
merization is a major improvement in biodegradation properties of absorbable sutures. It is interesting
to recognize that a small percentage (3%) of MD in the copolymer suture is sufficient to result in a faster
mass-loss profile without the expense of its tensile strength-loss profile. The ability to achieve this ideal
biodegradation property might be attributed to both an increasing hydrophilicity of the copolymer
and the disruption of crystalline domains due to MD moiety. As described later, the loss of suture mass
is mainly due to the destruction of crystalline domains, whereas the loss of tensile-breaking strength
is chiefly due to the scission of tie-chain segments located in the amorphous domains. The question is
why MD-PDS copolymeric suture retains its strength-loss similar to PDS. The possible explanation is
that the amide functional groups in MD could form stronger intermolecular hydrogen bonds than ester
functional groups. This stronger hydrogen bond contributes to the strength retention of the copolymer
of PDS and MD during
in vivo
biodegradation. The incorporation of MD moiety into PDS also lowers
the unknot and knot strength of unhydrolyzed specimens, but increases elongation at break. This sug-
gests that the copolymer of PDS and MD should have a lower level of crystallinity than PDS which is
consistent with its observed faster mass loss
in vivo
.
To improve γ-irradiation stability of PDS, radiostabilizers such as PEPBO have been copolymer-
ized with PDS to form segmented copolymers the same way as PEPBO with glycolide described above
(Koelmel et al., 1991; Shalaby, 1994). The incorporation of 5−10% of such stabilizer in PDS has been
shown not only to improve γ-irradiation resistance considerably, but to also increase the compliance
of the material. For example, PEPBO-PDS copolymer retained 79, 72, and 57% of its original tensile-
breaking strength at 2, 3, and 4 weeks in
in vivo
implantation, whereas PDS homopolymer retained only
43, 30, and 25% at the corresponding periods, respectively. It appears that an increasing (CH
2
) group
between the two ester functional groups of the radiation stabilizers improves the copolymer resistance
toward γ-irradiation.
5.2.5 Lactide Biodegradable Homopolymers and Copolymers
Polylactides, particularly PLLA, and copolymers having >50% l- or d,l-lactide have been explored for
medical use without much success mainly due to their much slower absorption and difficulty in melt
processing. PLLAs are prepared in solid state through ring-opening polymerization due to their ther-
mal instability and should be melt-processed at the lowest possible temperature (Shalaby, 1994). Other
methods such as solution spinning, particularly for high molecular weight, and suspension polymeriza-
tion have been reported as better alternatives. PLLA is a semi-crystalline polymer with
T
m
= 170°C and
T
g
= 56°C. This high
T
g
is mainly responsible for the extremely slow biodegradation rate reported in the
literature. The molecular weight of lactide-based biodegradable polymers suitable for medical use ranges
from 1.5 to 5.0 dL/g inherent viscosity in chloroform. Ultra-high-molecular weight of polylactides have
been reported (Tunc, 1983; Leenslag and Pennings, 1984). For example, an intrinsic viscosity as high
as 13 dL/g was reported by Leenslag et al. High-strength PLLA fibers from this ultra-high-molecular-
weight polylactide was made by hot-drawing fibers from solutions of good solvents. The resulting fibers
had tensile-breaking strength close to 1.2 GPa (Gogolewski and Pennings, 1983). Due to a dissymmetric
nature of lactic acid, the polymer made from the optically inactive racemic mixture of d and l enantio-
mers, poly-dl-lactide, however, is an amorphous polymer.
Lactide-based copolymers having a high percentage of lactide have recently been reported, particu-
larly those copolymerized with aliphatic polycarbonates such as TMC or 3,3-dimethyltrimethylene car-
bonate (DMTMC) (Shieh et al., 1990). The major advantage of incorporating TMC or DMTMC units
into lactide is that the degradation products from TMC or DMTMC are largely neutral pH and hence are
