Compression Behaviors of Stainless Steel Braided Coronary Stents and
Polyvinyl Alcohol/Stainless Steel Braided Coronary Stents
Jia-Horng Lin1, 2, 3, Shih-Peng Wen1, Ching-Wen Lou4, b and
Kwo-Chang Ueng5, a
1
Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials,
Feng Chia University, Taichung City 40724, Taiwan, R.O.C.
2
School of Chinese Medicine, China Medical University, Taichung 40402, Taiwan, R.O.C.
3
Department of Fashion Design, Asia University, Taichung 41354, Taiwan, R.O.C.
4
Institute of Biomedical Engineering and Materials Science, Central Taiwan University of Science
and Technology, Taichung 40601, Taiwan, R.O.C.
5
Division of Cardiology, Department of Internal Medicine, Chung Shan Medical University Hospital,
School of Medicine, Chung Shan Medical University, Taichung, Taiwan, R.O.C.
a
kcueng@gmail.com, bcwlou@ctust.edu.tw
Keywords: stainless steel, polyvinyl alcohol (PVA), braid, compression, coronary stent
Abstract. With the appearance of reticular tubes, coronary stents can resist the compressive
strength caused by vascular pulsation. This study braids stainless steel fibers with diameters of 0.12
mm and 0.08 mm with a braiding technique, and the resulting braids are then combined with
polyvinyl alcohol (PVA) solution to form three stent types－S12, PVA/S12, and PVA/S8. S12 is
braids that are made of 0.12-mm-diameter stainless steel fibers, PVA/S12 is S12 coated with PVA.
PVA/S8 is braids made with 0.08-mm-diameter stainless steel fibers and then coated with PVA.
Surface, braiding angle, and compression behavior of the coronary stents are observed by a
stereomicroscope, analyzed by Motic Images Plus 2.0 software, and examined by an Instron 5566,
respectively. The experiment results show that compared to S12 and PVA/S8, PVA/S12 has a
smaller braiding angle, indicating its manufacturing process is not stable. Of the three coronary
stents, PVA/S8 possesses the greatest recovery from the compression, and thus this study yields
optimal coronary stents with satisfactory surface, braiding angle, and recovery ability.
Introduction
Yielded by braiding fibers via a braiding technique, coronary stents are in the form of reticular
tubes, and the results of their relevant studies have been applied to self-expandable stents [1] and
bone scaffolds [2]. When used in the treatment for coronary artery stenosis, the stents have to be
able to resist the systolic pressure from the blood vessels. 316L stainless steel is commonly used for
coronary stents due to its good biocompatibility and non-corrosiveness [3]. However, when
stainless steel fibers are braided into reticular and tubular coronary stents, they lack ability to
recover from the compression. In order to provide the coronary stents with good recovery, this study
combines PVA and stainless steel fibers, the former of which possesses good biocompatibility and
film forming ability [4, 5], which fortify the compression resistance and recovery of the resulting
coronary stents.
Experimental
Materials
316L stainless steel fibers (Yuen Neng Co., Ltd, R.O.C.) have a diameter of 0.12 mm and 0.08
mm. Polyvinyl alcohol (PVA) powder (Sigma-Aldrich Co. LLC., USA) has a Mw between
89,000-98,000, and is 99+% hydrolyzed.
Preparation of Braid Coronary Stents
Stainless steel fibers with a diameter of 0.12 mm and 0.08 mm are separately combined on a
doubler (Shang Yang Co., Ltd., R.O.C.) to form 2-ply fibers, and then braided around a stainless
steel bar with a diameter of 3 mm. A batch of the 0.12-mm-diameter braids is removed from the
bars to form the pure stainless steel braids, S12, which serves as the control group. Two remaining
braid types made of 0.08- and 0.12-mm-diameter stainless steel fibers on the bars are then inserted
into a hollow stainless steel tube with an inner diameter of 5 mm, followed by the infusion of 20
wt% PVA solution. After PVA on the braids dries, the stainless steel braids are removed from the
bars to form PVA/S12 and PVA/S8, where the digit refers to the diameter of the stainless steel
fibers.
Tests
Surface Observation and Braiding Angle Measurement
A stereomicroscope (SMZ-10A, Nikon Instruments Inc., Japan) takes pictures of S12, PVA/S12,
and PVA/S8 for a surface observation. Afterwards, Motic Images Plus 2.0 software (Motic Group
Co., Ltd., USA) measures the braiding angles of various coronary stents.
Compression Test
The test conducted by Kim et al., [1] is modified for use in this study. Various coronary stents are
placed on the platform of the tester and tested for compression behaviors three times with an Instron
(Insron 5566, U.S.A.) at a speed of 2 mm/min.
compression
mold
braided coronary stents
Figure 1. Illustration for compression test.
Results and Discussion
Surface Observation and Braiding Angles
The resulting reticular and tubular braided coronary stents have the same look as that in the study
conducted by Kim et al. [1]. Shown in Figure 3, the braiding angles (coronary stents) are 47 degrees
(PVA/S12), 61.4 degrees (S12), and 65.7 degrees (PVA/S8). Such results are due to the fineness of
stainless steel fibers, when it is 0.12 mm, the fibers are thick and cannot be easily inserted into the
hollow stainless steel tube. The braiding angles become small as a result of the fibers’ glide during
the insertion.
a
b
c
Figure 2. Microscopic images (7.5 ×) of braided coronary stents of a) S12, b) PVA/S12, and c)
PVA/S8. The scale bar is 3 mm.
Figure 3. Braiding angles of the coronary stents.
Compression Test
a
c
b
Figure 4. Compression behavior of a) S12, b) PVA/S12, and c) PVA/S8.
Table 1. Summary of the compression behavior yielded from a consecutive 3-time test.
Samples
1st test
2nd test
3rd test
S12
PVA/S12
PVA/S8
13.59 N
19.02 N
2.14 N
11.88 N
16.95 N
2.09 N
11.49 N
15.83 N
2.08 N
Figure 4 shows that PVA/S12 have the greatest compression resistance, followed by S12 and
then PVA/S8. A greater fineness of stainless steel fiber results in a higher compression resistance of
the resulting coronary stents; likewise, coating with PVA also contributes to greater compression
resistance due to PVA’s good film forming ability. In addition, the compression resistance of S12
and PVA/S12 is inversely proportional to the frequency of the test. This significant decrease is
ascribed to the lack of recovery for these two coronary stent types, and therefore, the compression
resistance offered by the braiding structure decreases with the increasing number of the test.
Although PVA/S8 possesses a lower compression resistance, its compression behavior is the closest
to requirement of a coronary stent, and can also be indicated with a relative small curve area, which
is in line with the finding of the study by Kim et al [1].
Conclusion
This study successfully braids stainless steel fibers with a diameter of 0.12 mm and 0.08 mm into
coronary stents with recovery. Made with the same braiding parameters, the braiding angle of S12 is
smaller than that of PVA/S8 by 7 %; however, the braiding angle of PVA/S12 is smaller than that of
PVA/S8 by 39 %. This shows that a smaller fineness (0.08 mm) of stainless steel fibers can lead to a
greater stability of the braiding structure. Finally, the compression resistance of S12, PVA/S12, and
PVA/S8 decreases by 15.45 %, 16.77 %, and 2.8 %, respectively, indicating that PVA/S8 possesses
an optimal recovery.
Acknowledgement
The authors would especially like to thank Chung Shan Medical University and Feng Chia
University, Taiwan, R.O.C., for financially supporting this research under Contract 12G27304.
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