Biomedical Engineering Reference
In-Depth Information
Williams et al. suggested that these reactive oxygen species may be harmful to polymeric implant
surfaces through their production of highly reactive, potent, and harmful hydroxyl radicals
•
OH in the
presence of metals like iron as shown in the following series of redox reactions (Williams and Zhong,
1991; Ali et al., 1993; Zhong et al., 1994).
i
OM
+ →+
n
+
OM
(
n
− +
1
)
(5.3)
2
2
HO
+ →+
MOHHOM
(
n
−+
)
i
−
+
n
+
(5.4)
22
The net reaction will be
i
i
OHO
−
−
2
+ →+
HHOO
+
(5.5)
22
2
and is often referred to as the metal-catalyzed Haber-Weiss reaction (Haber and Weiss, 1934).
Although the role of free radicals in the hydrolytic degradation of synthetic biodegradable poly-
mers is largely unknown, a very recent study using absorbable sutures like Vicryl in the presence of an
aqueous free radical solution prepared from H
2
O
2
and ferrous sulfate, FeSO
4
, raised the possibility of
the role of free radicals in the biodegradation of synthetic absorbable sutures (Williams and Zhong,
1991; Zhong et al., 1994). As shown below, both
•
OH radicals and OH
−
are formed in the process of
oxidation of Fe
2+
by H
2
O
2
and could exert some influence on the subsequent hydrolytic degradation
of Vicryl sutures.
Fe
2
+
+ →+
HO
Fe
3
+
i
OH
+
OH
−
22
SEM results indicated that Vicryl sutures in the presence of free radical solutions exhibited many
irregular surface cracks at both 7 and 14 days
in vitro
, while the same sutures in the two controls (H
2
O
2
or FeSO
4
solutions) did not have these surface cracks. Surprisingly, the presence of surface cracks of
Vicryl sutures treated in the free radical solutions did not accelerate the tensile-breaking strength-
loss as would be expected. Thermal properties of Vicryl sutures under the free radical and 3% H
2
O
2
media showed the classical well-known maximum pattern of the change of the level of crystallinity
with hydrolysis time. The level of crystallinity of Vicryl sutures peaked at 7 days in both media (free
radical and 3% H
2
O
2
). The time for peak appearance in these two media was considerably earlier than
Vicryl sutures in conventional physiological buffer media. Based on Chu's suggestion of using the time
of the appearance of the crystallinity peak as an indicator of degradation rate, it appears that these two
media accelerated the degradation of Vicryl sutures when compared with regular physiological buffer
solution. Based on their findings, Williams et al. proposed the possible routes of the role of
•
OH radicals
in the hydrolytic degradation of Vicryl sutures (Zhong et al., 1994). Unfortunately, the possible role of
OH
−
, one of the byproducts of Fenton reagents (H
2
O
2
/FeSO
4
), was not considered in the interpretation
of their findings. OH
−
species could be more potent than OH toward hydrolytic degradation of synthetic
absorbable sutures. This is because hydroxyl anions are the sole species which attack carbonyl carbon of
the ester linkages during alkaline hydrolysis. Since an equal amount of
•
OH and OH
−
are generated in
Fenton reagents, the observed changes in morphological, mechanical, and thermal properties could be
partially attributed to OH
−
ions as well as
•
OH radicals.
Besides hydroxyl radicals, the production of superoxide ions and singlet oxygen during phagocytosis
has been well documented (Babior et al., 1973). Although the role of superoxide in simple organic ester
hydrolysis has been known since the 1970s (Johnson, 1976; Mango and Bontempeli, 1976; San Fillipo
et al., 1976; Forrester and Purushotham, 1984, 1987), its role in the hydrolytic degradation of synthetic
biodegradable polyester-based biomaterials has remained largely unknown. Such an understanding of
the superoxide ion role during the biodegradation of foreign materials has become increasingly desirable
