ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

ecoiled. Finally, both expanded samples were immersed in the D-Hank’s solution
(Table 1) with 20ml per stent for a corrosion test lasting 7 days. The samples were
observed by the unaided eyes for the check of the structural integrity during corrosion.
After 7 days the samples were taken out for the observation of scanning electronic
microscope (SEM) to review the morphological changes and broken points due to
corrosion.
Table 1: Composition of D-Hank’s solution (without Mg, Ca ions) (g/L)
KCl KH2PO4 NaCl NaHCO3 Na2HPO4•7H2O Phenol red
0.4 0.06 8.0 0.35 0.06 0.02
4. RESULTS
4.1 Simulation of corrosion for both models
After the expansion step, the Model A had the lower values in the maximum principal
strain and stress than the Model B. The distribution of maximum principal stress for
both designs at beginning of degradation were concentrated mainly at the more
deformed locations during stent expansion (Fig. 4). The Model A had more uniform
stress distribution than Model B. The highest maximum principal stress of the Model B
was concentrated at the inner surface near the bow.
Fig. 4. The maximum principal stress distribution for (a) Model A and (b) Model B after
stent implanting process. The areas of high stress concentration are indicated by circles.
During simulation of degradation, Model A and Model B lost their scaffolding ability at
normalized time unit t* of 1 and 0.7, respectively, which means that the Model A had a
longer time prior to loss of scaffolding ability. The detailed corrosion evolution of the
two designs showed that the degradation was due to combined effects of uniform and
stress corrosion. As depicted in Fig. 5, the element deletion occurred first at simulated
locations of concentrated high stresses (indicated in Fig. 4) for both models (t* equals
0.19), and Model B had faster and concentrated stress corrosion near the bows. The
uniform corrosion evolved at the outer surfaces throughout the whole degradation
process. When t* equals 0.57, the structure of the Model B was severely damaged by
stress corrosion near the bows. The damages near the Model B bows led to a lack of
structural integrity at t* of 0.7, the same time as its loss of scaffolding ability. The
Model A kept more uniform stress corrosion than the Model B. When t* equals 0.95, a
layer of elements at the original outer surface of the Model A was fully deleted and
some locations were severely damaged; however, the Model A still conserved its
structural integrity.