High Frequency Testing of Nitinol Stent Material

Stent Testing

by Dynatek Labs | SFB 1999 | Publications, Stent Testing

High Frequency Testing of Nitinol Stent Material

James C. Conti1, Elaine R. Strope, Ray Gregory2

25th Annual Meeting of the Society for Biomaterials, Transactions, 398, (1999)

Dynatek Dalta Scientific Instruments, Fourth and Main St., Galena, MO  65656

1Southwest Missouri State University, Dept. of Physics

2 Applied Mathematics

SFB 1999

Introduction:

The use of Nitinol in implantable medical products has been increasing at a rapid rate over the last several years. There are two reasons for this: The shape memory properties and the superelasticity which make it highly convenient for the deployment of several medical products, including stents and heart valves. Because of the need for reliable and well-designed durability/fatigue experiments on these products, frequency response determinations of the materials are important so that appropriate protocols can be used.

In most cases the shape memory properties of the Nitinol are trained into it after the fabrication of the final product. As a result, it would be highly beneficial to develop raw material tests for incoming material that are predictory of the performance and properties of the materials in their final state. It is the purpose of this study to carry out initial investigations into the engineering properties of Nitinol wire that predict the performance of the material when fabricated into the final devised shape, as well as the speeds with which to test these final products.

Methods I:

Samples of Nitinol wire of varying diameters were subjected to four point bend testing at a frequency of 1Hz. After that, the largest wire was subjected to four point bend testing at 1, 20, and 40Hz, and three point bend testing at 1, 10, 20, and 40Hz. The testing was carried out at room temperature and in air with an electromagnetically actuated micromechanical tester.

Results I:

Table 1 is a summary of the results obtained from the three point bend test. The corrected simple modulus represents the slope of the stress-strain curve corrected for the cross sectional area of the wire. This simplifies the calculations and allows for the observation of the comparative properties of the material at different frequencies, rather than the absolute properties. The hysteresis column represents the difference in the integrated area under the forward portion minus the return portion of the cyclic loading curve.

As can be seen from the data, the apparent modulus of the material increases measurably when the speed of the test changed from 20 to 40Hz. Interestingly, the largest change in hysteresis occurred from 1Hz to 10Hz, Also, the increase in corrected simple modulus, when increasing the speed from 20Hz to 40Hz, is not accompanied by a measurable change in the hysteresis of the test.

Table 1

Nitinol – Three Point Bend Test

DIA.

µ  (in)

Freq.

 

Simple Modulus

(mN/µ/mm2)

Hysteresis

 

483 (0.019) Hz 0.3873 < 1%
483 (0.019) 10Hz 0.4480 6%
483 (0.019) 20Hz 0.4540 8%
483 (0.019) 40Hz 1.0751 8%

Table 2 is a summary of the four point bend testing. Again, this is a simple modulus test which reflects the deflection of the contact points, rather than actual deflection of the bottom part of the wire. As before, this was done in order to simplify the analysis, allowing comparison at the different frequencies. In this instance, a large increase in corrected simple modulus is seen with increase in wire diameter for testing at 1Hz.

These changes could, however, be a reflection of the fact that we are not measuring the deflection of the lowest point in the wire. Also, the frequency change from 1Hz through 40Hz is accompanied by an increase in the simple modulus and an increase in the percent of the input energy lost through hysteresis.

Table 2

Nitinol – Four Point Bend Test

DIA.

 µ  (in)

Freq.

 

Simple Modulus

(mN/µ/mm2)

Hysteresis

 

229 (0.009) 1Hz 0.2146 < 1%
330 (0.013) 1Hz 0.7243 < 1%
483 (0.019) 1Hz 0.8031 < 1%
483 (0.019) 20Hz 1.2729 ~ 3%
483 (0.019) 40Hz 1.4907 ~ 6%

Methods II:

Samples of Nitinol wires incorporated into actual stent/grafts were subjected to three point bend testing at point spacings of 1 inch and 2 inches. Compression was delivered at a deflection of 0.1 or 0.2mm at frequencies up to 100 Hz. Similar samples were subjected to four point bend testing.

Results II:

Tables 3-7 show the results of these experiments. Similar to the first series, a change in the simple modulus occurred at frequencies above 20 Hz. Figures 1-7 show how increasing the frequency of the testing results just in increases in hysteresis and then erratic traces. These erratic results could be the result of harmonics in the material.

Discussion:

Our attempt in this study was to compare three point and four point bend testing of incoming raw material Nitinol wire to be used eventually for the fabrication of stents or other implantable cardiovascular products, as well as Nitinol wire in the form of a stent. It was our intention to evaluate how the apparent stress-strain relationships varied when testing at different frequencies. Our loading was chosen to approximately replicate the type of loading that one might expect in extreme conditions for a stent implantation, or individual barb loading on a stent graft.

The data seem to suggest that the properties of these materials change measurably at testing frequencies above approximately 20Hz. It is apparent that a modification of the four point bend test displacement measuring might be appropriate when generating the modulus numbers. However, it is unlikely that the percentage hysteresis will change significantly in these tests with that modification of displacement measuring.

The results indicate the need to be particularly careful when testing fatigue or durability properties of incoming Nitinol material and final Nitinol products at frequencies above approximately 20Hz. It is possible that modification of testing parameters might allow more confidence in high speed testing; however, the complexity of Nitinol materials seem to render it somewhat different than standard metal wire testing.

Table 3 

Three point bend test – one inch separation – loading phase

 
f average stand dev average stand dev avg S mod
(Hz) (mm) (mm) (mN) (mN) (mN/mm)
0.1 -0.0767 0.0003 -47.0858 0.2700 614.1622
0.5 -0.0809 0.0004 -48.4530 0.5851 598.6779
1 -0.0814 0.0003 -48.9949 0.7697 602.1499
5 -0.0760 0.0008 -46.2622 0.1525 608.4459
10 -0.0797 0.0025 -48.3630 0.9915 607.0665
15 -0.0867 0.0011 -52.7823 1.3337 609.0262
20 -0.0779 0.0008 -49.9333 0.8011 641.2663
25 -0.0722 0.0049 -43.5215 2.9085 603.0688
30 -0.0581 0.0047 -36.4066 3.3118 626.2597
35 -0.0749 0.0087 -48.4003 6.2603 645.9110
40 -0.0786 0.0098 -45.4620 3.7892 578.1513
45 -0.0653 0.0009 -33.2954 0.2930 510.1445
50 -0.0614 0.0004 -33.3696 1.0095 543.1845
60 -0.0744 0.0016 -32.6593 1.9606 439.1658
70 -0.0739 0.0007 -23.9961 3.1533 324.7109
80 -0.0725 0.0017 -20.5915 7.2957 283.8897
90 -0.0634 0.0025 -22.4093 0.5458 353.6449
100 -0.0384 0.0016 -21.2401 6.7653 553.1267
Table 4         

Three point bend test – one inch separation – unloading phase

 
f average stand dev average stand dev avg S mod
(Hz) (mm) (mm) (mN) (mN) (mN/mm)
0.1 0.0768 0.0005 47.1726 0.3320 614.4933
0.5 0.0810 0.0001 48.9147 0.3362 603.8106
1 0.0810 0.0003 49.3580 0.4042 609.6085
5 0.0756 0.0010 46.1238 0.4163 609.8338
10 0.0798 0.0028 48.7602 1.4759 611.2858
15 0.0878 0.0028 54.0337 1.1420 615.6517
20 0.0781 0.0006 50.1868 0.8350 642.5962
25 0.0652 0.0017 40.9322 2.5635 628.1151
30 0.0656 0.0039 40.7898 4.8100 622.1124
35 0.0770 0.0104 46.7420 7.0980 607.3023
40 0.0785 0.0099 45.4802 4.9886 579.3652
45 0.0655 0.0005 33.4067 0.5465 509.7660
50 0.0615 0.0009 33.5632 0.5284 545.7425
60 0.0732 0.0008 30.8707 1.3555 421.9235
70 0.0747 0.0013 24.8947 1.9286 333.2624
80 0.0732 0.0026 21.9791 7.1251 300.3982
90 0.0632 0.0030 22.4198 0.9708 354.7437
100 0.0406 0.0053 18.3573 3.1585 452.1511
Table 5 

Three point bend test – two inch separation – loading phase

 
f average stand dev average stand dev avg S mod
(Hz) (mm) (mm) (mN) (mN) (mN/mm)
0.1 -0.2003 0.0002 -19.3702 0.4452 96.7222
0.5 -0.2019 0.0005 -19.8923 0.0511 98.5091
1 -0.2025 0.0002 -19.4134 0.1973 95.8529
5 -0.1934 0.0010 -17.9569 0.4090 92.8645
10 -0.1914 0.0026 -16.5510 1.0389 86.4732
15 -0.1950 0.0010 -19.1697 0.5602 98.3062
20 -0.1933 0.0011 -22.7251 0.7716 117.5637
25 -0.2293 0.0027 -26.9705 0.1308 117.6380
30 -0.1932 0.0003 -28.3780 1.4227 146.9092
35 -0.1986 0.0098 -42.5641 0.8832 214.2848
40 -0.1883 0.0017 -22.6495 0.9968 120.2630
45 -0.1888 0.0054 -23.5740 2.9542 124.8843
50 -0.1953 0.0005 -19.6530 0.1273 100.6468
60 -0.1816 0.0008 -33.9532 2.5015 187.0015
70 -0.1931 0.0036 -29.6986 1.3017 153.7992
80 -0.1857 0.0006 -58.3245 6.1137 314.1355
90 -0.0985 0.0191 -54.7739 22.2026 556.2688
100 -0.0550 0.0391 -29.2436 4.4126 531.3792
Table 6         
Three point bend test – two inch separation – unloading phase
 
f average stand dev average stand dev avg S mod
(Hz) (mm) (mm) (mN) (mN) (mN/mm)
0.1 0.2001 0.0003 18.4412 1.5567 92.1444
0.5 0.2016 0.0006 20.0123 0.2269 99.2838
1 0.2020 0.0003 19.6407 0.4230 97.2151
5 0.1933 0.0007 17.7972 0.8040 92.0864
10 0.1923 0.0023 16.4193 1.3269 85.3988
15 0.1946 0.0010 20.1090 0.4656 103.3529
20 0.1949 0.0010 22.2205 1.7310 114.0097
25 0.2295 0.0013 26.1323 0.9795 113.8661
30 0.1944 0.0028 28.6573 1.4395 147.3890
35 0.1998 0.0107 42.4954 1.1933 212.6899
40 0.1889 0.0008 22.9813 1.2810 121.6369
45 0.1852 0.0022 23.4308 3.6852 126.5162
50 0.1964 0.0012 19.8441 0.4693 101.0222
60 0.1829 0.0015 33.9479 2.5118 185.5751
70 0.1953 0.0048 29.6930 1.7734 152.0640
80 0.1864 0.0021 60.3540 4.7246 323.8453
90 0.1358 0.0080 55.7985 22.7939 410.7867
100 0.0637 0.0341 29.9346 4.8459 469.9314
Table 7         
Four point bend test – two inch separation – Simple Modulus
 
f S. mod.
(Hz) (mN/mm)
0.1 100.4430
0.5 96.8411
1 96.6315
5 94.8309
10 102.2390
15 94.1252
20 78.5111
25 102.2859
30 120.0522
35 141.0999
40 71.6953
45 -7.4292
50 18.7877
60 -95.7093
70 -154.1283
80 -45.0895
90 -286.0128
100 -609.3379

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