4D Numerical Analysis of Scaffolds: A New Approach - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 71

Fig. 1 Scheme of erosion

process

the degradation. Usually, the most important degradation mechanism of biodegrad-
able polymers is chemical degradation via hydrolysis or enzyme-catalyzed hydrol-
ysis [ 14 ]. The most important factor is its chemical structure and the occurrence of
specific bonds along its chains. Like those in groups of esters, amides, etc., which
might be susceptible to hydrolysis when exposed to water [ 18 , 33 ].
Another important distinction must be made between erosion and degradation.
Both are irreversible processes. However, while the degree of erosion is estimated
from the mass loss, the degree of degradation can be estimated by measuring the
evolution of molecular weight, by SEC (Size Exclusion Chromatography) or GPC
(Gel Permeation Chromatography), or the tensile strength evolution (by universal
tensile testing). Hence, the hydrolytic degradation process is included on the erosion
process.
The erosion process can be described by phenomenological diffusion-reaction
mechanisms presented in Fig. 1 . An aqueous media diffuses into the polymeric ma-
terial while oligomeric products diffuse outwards to be then bio-assimilated by the
host environment. Then, there is material erosion with correspondent mass loss. On
the other hand, degradation refers to mechanical damage and depends on hydroly-
sis. Within the polymeric matrix, hydrolytic reactions take place, mediated by water
and/or enzymes. While water diffuses rapidly well inside the material, enzymes
are large molecules unable to do it, and so they degrade at surface. The degrada-
tion of polyesters by micro-organisms is initiated by extracellular hydrolases, which
are secreted by the organisms to reduce the molar mass of the polymeric substrate
and to make it bio-available and bio-assimilated. It was demonstrated by Tokiwa
and Suzuki [ 44 ] that synthetic polyesters can be attacked by hydrolases (lipases).
However, for most biodegradable materials, especially synthetic polymers, passive
hydrolysis due to the presence of water molecules is the most important mode of
degradation.