Constitutive Effects of Hydrolytic Degradation in Electro-Spun Polyester-Urethane Scaffolds for Soft Tissue Regeneration - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 63

4 Discussion

The mechanical properties of the electro-spun DegraPol ® scaffold deteriorated dra-
matically during the degradation period of 34 days. The maximum stress, σ max ,
did not change significantly during the first 14 days of degradation but decreased
steadily thereafter, dropping to 6.4 % of the initial value after 34 days. The strain,
ε max , associated with the maximum stress displayed a different change: from the
start of the degradation it decreased rapidly to 26.4 % during the first 14 days and
decreased markedly slower thereafter to 13.9 % after 34 days. The change in the
strain ε max coincided with the specification of the manufacturer that indicates com-
plete degradation of DegraPol ® DP30 with respect to the mechanical properties
within two weeks. With increasing degradation time, the electro-spun scaffold be-
came more susceptible to load cycling. A reduction of the stress at a strain of 20 %
with repeated loading was observed in all samples. For degradation times of up to
18 days, the stress reduction was non-significant and resembled a mechanical condi-
tioning which is also observed in other materials such as Nitinol [ 32 ] and biological
soft tissues [ 33 ]. For a degradation time of 22 days and longer, the stress reduction
became statistically significant, now governed by the reduced strength of the ma-
terial. The elastic modulus also decreased with degradation time, the change was
however less pronounced as that of the maximum stress and associated strain. The
increase of the elastic modulus with increasing strain observed during the sixth load-
ing cycle is typical feature of non-linear stiffening. Such a mechanical behaviour is
observed in biological soft tissues such as arteries [ 34 ] and may be desirable for im-
plant materials, e.g. biomedical coarse knit fabrics [ 16 , 35 ], that aim at simulating
physiological mechanics. The deterioration of physical properties due to degrada-
tion was not reflected in scaffold mass. The mass of the samples remained nearly
constant over the entire degradation period.
In order to assess the data of mechanical characteristics of the current study
against those reported in literature, scaffold structure and materials used need to be
compared. The fibre thickness of the scaffold was found to be similar to electro-spun
DegraPol ® scaffolds used in previous studies [ 12 , 20 ]. While the amount of fibre
alignment appeared to be similar to that of the scaffold by Riboldi et al. [ 12 ] from
visual inspection of SEM images, the 2D FFT analysis indicated a lower amount of
alignment in the direction of rotation in the scaffold used in the current study. It was
also found that the fibre alignment was more pronounced on the outer surface than
on the inner surface. This can be explained with an increase in tangential velocity
on the target surface during the spinning process that is associated with the increase
in target diameter due to scaffold build-up, and which has been reported to affect
fibre alignment [ 31 ].
The mechanical properties obtained for the non-degraded scaffold were lower
compared to electro-spun DegraPol ® DP15 [ 12 ]. The maximum stress, associ-
ated strain and elastic modulus reported were 4.8-, 1.25- and 6.26-fold ( σ max =
2 . 52

0 . 69 MPa) of the val-
ues obtained in the current study. Since the ratio of hard- and soft segment of the
two DegraPol ® variants was identical (personal communications: S. Mantero, Po-
litecnico di Milano, 23/06/2010; E. Bonavoglia, ab medica S.p.A, 28/06/2010), the

± 0 . 17 MPa, ε max = 220 . 40 ± 57 . 09 %, E = 10 . 15 ±