Biomedical Engineering Reference
In-Depth Information
because of the advanced understanding of how the human immune system reacts to foreign materials
and the increasing use of synthetic biomaterials for human body repair.
Lee and Chu examined the reactivity of the superoxide ion toward biodegradable biomaterials having
an aliphatic polyester structure at different reaction conditions such as temperature, time, and superox-
ide ion concentration (Lee et al., 1999; Lee and Chu, 2000). Due to the extreme reactivity of the superox-
ide ion, it has been observed that the effect of superoxide ion-induced hydrolytic degradation of PDLLA
and PLLA was significant in terms of changes in molecular weights and thermal properties (Lee et al.,
1999). The superoxide ion-induced fragmentation of PDLLA would result in a mixture of various species
with different chain lengths. A combined GPC method with a chemical tagging method revealed that
the structure of oligomer species formed during the superoxide-induced degradation of PDLLA and
PLLA was linear. The significant reduction in molecular weight of PDLLA by superoxide ion was also
evident in the change of thermal properties like
T
g
. The linear low-molecular species (oligomer, trimers,
and dimers) in the reaction mixture could act as an internal plasticizer to provide the synergetic effects
of lowering
T
g
by increasing free volume. The effect of the superoxide ion-induced hydrolytic degrada-
tion on molecular weight of PLLA was similar to PDLLA but with a much smaller magnitude. The
mechanism of simple hydrolysis of ester by superoxide ion proposed by Forrester et al. was subsequently
modified to interpret the data obtained from the synthetic biodegradable polymers.
In addition to PDLLA and PLLA, superoxide ions also have a significant adverse effect on the hydro-
lytic degradation of synthetic absorbable sutures (Lee and Chu, 2000). A significant reduction in molec-
ular weight has been found along with mechanical and thermal properties of these sutures over a wide
range of superoxide ion concentrations, particularly during the first few hours of contact with superox-
ide ions. For example, the PGA suture lost almost all of its mass at the end of 24 h contact with super-
oxide ions at 25°C, while the same suture would take at least 50 days in an
in vitro
buffer for a complete
mass loss. The surface morphology of these sutures was also altered drastically. The exact mechanism,
however, is not fully known yet; Lee et al. suggested the possibility of simultaneous occurrence of several
main-chain scissions by three different nucleophilic species.
Lee and Chu also reported that the addition of Fenton agent or hydrogen peroxide to the degradation
medium would retard the well-known adverse effect of the conventional γ-irradiation sterilization of
synthetic absorbable sutures (Lee and Chu, 1996). They found that these γ-irradiated sutures retained
better tensile-breaking strength in the Fenton medium than in the regular buffer media. Chu et al.
postulated that the γ-irradiation-induced α-carbon radicals in these sutures react with the hydroxyl
radicals from the Fenton agent medium and hence neutralize the adverse effect of α-carbon radicals on
the backbone chain scission. This mechanism is supported by the observed gradual loss of ESR signal of
the sutures in the presence of the Fenton agent in the medium.
Instead of the adverse effect of free radicals on the degradation properties of synthetic biodegradable
polyesters, Lee and Chu described an innovative approach of covalent bonding nitroxyl radicals onto
these biodegradable polymers so that the nitroxyl radical attached polymers would have biological func-
tions similar to nitric oxide (Lee and Chu, 1996, 1998). The same approach was also used to chemically
attach nitroxyl radicals onto the amino acid-based biodegradable poly(ester amide) copolymers (Chu and
Katsarava, 2003). A preliminary
in vitro
cell culture study of these new biologically active absorbable ali-
phatic polyesters indicated that they could retard the proliferation of human smooth muscle cells as native
nitric oxides do. The full potential of this new biologically active biodegradable polymers is currently under
investigation by Chu for a variety of therapeutic applications like drug-eluting stents and vascular grafts.
5.6 Role of Linear Aliphatic Biodegradable Polyesters in Tissue
Engineering and Regeneration
The use of biodegradable polymers as the temporary scaffolds either to grow cells/tissues
in vitro
for tis-
sue engineering applications or to regenerate tissues
in vivo
has very recently become a highly important
