Biomedical Engineering Reference
In-Depth Information
measurements are possible: those pertaining to the substrates, to the microorgan-
isms, or to the reaction products. Four common approaches available for studying
biodegradation processes have been reviewed in detail by Andrady [13, 14]:
1)
monitoring accumulation of biomass
2)
monitoring the depletion of substrates
3)
monitoring reaction products
4)
monitoring changes in substrate properties.
In the following sections, different test methods for the assessment of polymer
biodegradability are presented. Measurements are usually based on one of the four
approaches given above, but combinations also occur. Before choosing an assay
to simulate environmental effects in an accelerated manner, it is critical to con-
sider the closeness of fi t that the assay will provide between substrate, microorgan-
isms, or enzymes, and the application or environment in which biodegradation
should take place [23].
11.5.1
Enzyme Assays
11.5.1.1
Principle
In enzyme assays, the polymer substrate is added to a buffered or pH-controlled
system, containing one or several types of purifi ed enzymes. These assays are very
useful in examining the kinetics of depolymerization, or oligomer or monomer
release from a polymer chain under different assay conditions. The method is very
rapid (minutes to hours) and can give quantitative information. However, miner-
alization rates cannot be determined with enzyme assays.
11.5.1.2
Applications
The type of enzyme to be used, and quantifi cation of degradation, will depend on
the polymer being screened. For example, Mochizuki
et al.
[24] studied the effects
of draw ratio of polycaprolactone fi bers on enzymatic hydrolysis by lipase. Degrad-
ability of PCL fi bers was monitored by dissolved organic carbon (DOC) formation
and weight loss. Similar systems with lipases have been used for studying the
hydrolysis of broad ranges of aliphatic polyesters [25-30], copolyesters with aro-
matic segments [26, 31 - 33] , and copolyesteramides [34, 35] . Other enzymes such
as
-trypsin have also been applied for these polymers [36,
37]. Biodegradability of poly(vinyl alcohol) segments with respect to block length
and stereochemical confi guration has been studied using isolated poly(vinyl
alcohol)-dehydrogenase [38]. Cellulolytic enzymes have been used to study the
biodegradability of cellulose ester derivatives as a function of degree of substitution
and the substituent size [39]. Similar work has been performed with starch esters
using amylolytic enzymes such as
α
- chymotrypsin and
α
- amylases, glucoamylases, and
amyloglucosidases [40]. Enzymatic methods have also been used to study the
biodegradability of starch plastics or packaging materials containing cellulose
[41 - 46] .
α
- amylases,
β
