4D Numerical Analysis of Scaffolds: A New Approach - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 82

Fig. 6 Tensile test results for
different degradation time of
PLA-PCL fibers (400 µm)
under PBS

in tubes and submitted to different degradation stages, under PBS (Phosphate Buffer
Solution) at 37 °C. The duration of stages was previously determined, according to
the supplier durability claims, until a maximum of 7 months. At the end of each
degradation stage, pH of the media was measured, then test pieces were weighted
after and before drying, further submitted to tensile tests and finally to GPC (Gel
Permeation Chromatography) to measure molecular weight. The initial pH of the
PBS solution was 8 (eight) and did not change significantly during degradation. As
can be seen in Fig. 6 , PLA-PCL has become brittle only after 16 weeks, lost its plas-
ticity region, and strength has progressively decreased. The almost constant slope
of the linear elastic stage indicates that no significant variation in Young modulus
occurred during degradation.
For these PLA-PCL fibers, no significant differences were observed among the
different dimensions tested, either in terms of strength and molecular weight evo-
lutions during degradation (see Fig. 7 ). One can conclude that, in the present case,
water diffusion can be assumed instantaneous and that hydrolysis takes place ho-
mogeneously throughout the samples (bulk degradation without autocatalysis) [ 2 ].
For highly heterogeneous degradation, the rate will not be globally similar, inde-
pendently of dimensions, and the water concentration will locally depend on the
position and time. As can be seen in Fig. 8 , while in the first 16 weeks the fiber only
looses 10 % of mass, it presents 80 % of strength loss.
From Fig. 9 , one can see that the measured strength follows the same trend as
the molecular weight, in a semi-logarithmic representation. The slope of this linear
fitting, that includes all experimental results normalized to the initial value and in
semi-logarithmic scale, represents the degradation rate. Instead of Eq. ( 8 ), a rela-
tionship similar to the one obtained for the molecular weight, Eq. ( 7 ), can be used,

σ 0 e − u s t σ = (18)

where u s is the strength decrease rate of the material. This parameter, u s , seems to be
directly related to the molecular weight decrease rate of the material, u m , as can be