Biomedical Engineering Reference
In-Depth Information
Aerobic biodegradation:
C
+→
O
CO
+
O
+
C
+
C
(11.1)
polymer
2
2
2
residue
biomass
Anaerobic biodegradation:
C
→+ ++
CO
CH
H O
C
+
C
(11.2)
polymer
2
4
2
residue
biomass
Complete biodegradation occurs when no residue remains, and complete miner-
alization is established when the original substrate, C
polymer
in this example, is
completely converted into gaseous products and salts. However, mineralization is
a very slow process under natural conditions because some of the polymer under-
going biodegradation will initially be turned into biomass [13, 14]. Therefore,
complete biodegradation, and not mineralization, is the measurable goal when
assessing removal from the environment.
11.4
Mechanisms of Polymer Degradation
When working with biodegradable materials, the obvious question is why some
polymers biodegrade and others do not. To understand this, one needs to know
about the mechanisms through which polymeric materials are biodegraded.
Although biodegradation is usually defi ned as degradation caused by biological
activity (especially enzymatic action), it will usually occur simultaneously with - and
is sometimes even initiated by - abiotic degradation such as photodegradation and
simple hydrolysis. The following paragraphs give a brief introduction about the
most important mechanisms of polymer degradation.
11.4.1
Nonbiological Degradation of Polymers
A great number of polymers is subject to hydrolysis, such as polyesters, polyan-
hydrides, polyamides, polycarbonates, polyurethanes, polyureas, polyacetals, and
polyorthoesters. Different mechanisms of hydrolysis have been extensively
reviewed not only for backbone hydrolysis but also for the hydrolysis of pendant
groups [15-17]. The necessary elements for a wide range of catalysis, such as acids
and bases, cations, nucleophiles and micellar, and phase transfer agents are
usually present in most environments. In contrast to enzymatic degradation,
where a material is degraded gradually from the surface inward (primarily because
macromolecular enzymes cannot diffuse into the interior of the material), chemi-
cal hydrolysis of a solid material can take place throughout its cross section except
for few hydrophobic polymers.
Important features affecting chemical polymer degradation and erosion include
(i) the type of chemical bond, (ii) the pH, (iii) the temperature, (iv) the copolymer
