3 Study on Improving the Performance of Nitinol Cardiovascular Stent …
37
3.3.3 Dross
As shown in Fig. 3.7a, a large amount of dross appeared on the inner surface of
the stent after laser cutting, which was caused by the melting of materials in the
laser processing area and the accumulation of molten materials on the inner surface
under the action of gravity and argon pressure. The dross seriously affected the
performance of the stent. However, there was no dross on the inner surface of the
stent after electropolishing as shown in Fig. 3.7b, and the quality of the inner surface
was greatly improved.
3.3.4 Surface Roughness
Surface roughness plays an important role in the fatigue life and biocompatibility of
cardiovascular stents, and it is also an important index to measure the surface quality.
After laser cutting, the mean value of the surface roughness measured by the white
light interferometer was about Ra 5.2543 µm. After laser cutting, the surface of the
stent was relatively rough, which could also be seen from Figs. 3.5a, 3.6a and 3.7a.
The surface roughness of the stent had been greatly improved with Ra 53.4 nm, as
shown in in Fig. 3.8. This also showed that the electropolishing could improve the
surface roughness of the stent very well.
3.3.5 Surface Chemical Composition
The chemical composition and content of the stents and nitinol tube are shown in
Table 3.3. It could be seen from the table that the surface chemical composition of
the stent after laser cutting had basically not changed, while the surface chemical
composition of the stent after electropolishing had changed a lot, and the titanium
dioxide (TiO 2 ) was generated on the surface of the stent, as shown in Fig. 3.9. In
the process of electropolishing, part of titanium ions generated by anodic dissolution
were combined with chloride ions in the electrolyte to form titanium tetrachloride,
which reacted with water to form more stable titanium dioxide in turn, as shown in
Eqs. (3.1) and (3.2).
Ti
4+
+ 4Cl
− TiCl 4 (l)
(3.1)
TiCl 4 (l) + 2H 2 OTiO 2 (s) + 4H
+
+ 4Cl
−
(3.2)
In addition, there was a large number of glycol in the electrolyte. Titanium tetrachloride reacted with glycol to form titanium compounds in the process of electropolishing. With the progress of electropolishing, titanium compounds interacted with
37
3.3.3 Dross
As shown in Fig. 3.7a, a large amount of dross appeared on the inner surface of
the stent after laser cutting, which was caused by the melting of materials in the
laser processing area and the accumulation of molten materials on the inner surface
under the action of gravity and argon pressure. The dross seriously affected the
performance of the stent. However, there was no dross on the inner surface of the
stent after electropolishing as shown in Fig. 3.7b, and the quality of the inner surface
was greatly improved.
3.3.4 Surface Roughness
Surface roughness plays an important role in the fatigue life and biocompatibility of
cardiovascular stents, and it is also an important index to measure the surface quality.
After laser cutting, the mean value of the surface roughness measured by the white
light interferometer was about Ra 5.2543 µm. After laser cutting, the surface of the
stent was relatively rough, which could also be seen from Figs. 3.5a, 3.6a and 3.7a.
The surface roughness of the stent had been greatly improved with Ra 53.4 nm, as
shown in in Fig. 3.8. This also showed that the electropolishing could improve the
surface roughness of the stent very well.
3.3.5 Surface Chemical Composition
The chemical composition and content of the stents and nitinol tube are shown in
Table 3.3. It could be seen from the table that the surface chemical composition of
the stent after laser cutting had basically not changed, while the surface chemical
composition of the stent after electropolishing had changed a lot, and the titanium
dioxide (TiO 2 ) was generated on the surface of the stent, as shown in Fig. 3.9. In
the process of electropolishing, part of titanium ions generated by anodic dissolution
were combined with chloride ions in the electrolyte to form titanium tetrachloride,
which reacted with water to form more stable titanium dioxide in turn, as shown in
Eqs. (3.1) and (3.2).
Ti
4+
+ 4Cl
− TiCl 4 (l)
(3.1)
TiCl 4 (l) + 2H 2 OTiO 2 (s) + 4H
+
+ 4Cl
−
(3.2)
In addition, there was a large number of glycol in the electrolyte. Titanium tetrachloride reacted with glycol to form titanium compounds in the process of electropolishing. With the progress of electropolishing, titanium compounds interacted with