Figure 9: F<strong>in</strong>ite Element model of subject sta<strong>in</strong>less steel stent. Figure 10: Fracture observed at crown-strut <strong>in</strong>terface. Figure 11: SEM of fracture surface <strong>in</strong> sta<strong>in</strong>less steel stent. 44 Figure 12: Shape assumed by stent when resonance occurs at a natural frequency of 139.6 kHz, 139.7 kHz, 140.0 kHz and 140.5 kHz. At each frequency the maximum amplitude occurs at the end crowns (where failure was observed dur<strong>in</strong>g crimp<strong>in</strong>g). Figure 13: Close up of stent shape when resonance occurs at a natural frequency of 140.0 kHz.
Discussion <strong>Ultrasonic</strong> clean<strong>in</strong>g is a seem<strong>in</strong>gly benign process that is prevalent throughout the medical device <strong>in</strong>dustry. It has been demonstrated <strong>in</strong> this paper that ultrasonic clean<strong>in</strong>g can <strong>in</strong>duce significant damage to small medical devices subjected to this process. It is noted that any small device with a resonant frequency near that of the ultrasonic clean<strong>in</strong>g frequency is susceptible to this failure mechanism. The obvious concern is one of reliability. If small cracks are <strong>in</strong>duced <strong>in</strong> these devices, it is challeng<strong>in</strong>g at best to detect these defects unless the devices are subjected to stresses and thorough <strong>in</strong>spection prior to the f<strong>in</strong>al manufactur<strong>in</strong>g step. As a result, <strong>in</strong>cipient ultrasonic clean<strong>in</strong>g-<strong>in</strong>duced cracks can rema<strong>in</strong> undetected <strong>in</strong> f<strong>in</strong>ished devices. There are virtually no practical non-destructive techniques for detect<strong>in</strong>g m<strong>in</strong>ute cracks <strong>in</strong> small devices that can be used <strong>in</strong> a large manufactur<strong>in</strong>g process. However, FEA can be a useful tool <strong>in</strong> characteriz<strong>in</strong>g the susceptibility of small-scale devices to ultrasonic-<strong>in</strong>duced fatigue. The example given here was performed after the failure, but this approach can be conducted as a proactive (rather than reactive) step to help mitigate possible failures. As stated above, the electronics manufactur<strong>in</strong>g <strong>in</strong>dustry has long recognized the potential harmful effects of ultrasonic clean<strong>in</strong>g-<strong>in</strong>duced fatigue on small parts. However, the ability to produce surfaces free of particulates and contam<strong>in</strong>ates is important to the electronics manufactur<strong>in</strong>g <strong>in</strong>dustry. Additionally, these large-scale electronics clean<strong>in</strong>g processes are also subjected to stricter environmental controls. Due to clean<strong>in</strong>g-<strong>in</strong>duced damage concerns, many electronics <strong>in</strong>dustry clean<strong>in</strong>g procedures do not utilize ultrasound. These clean<strong>in</strong>g procedures <strong>in</strong>clude overflow and cascad<strong>in</strong>g r<strong>in</strong>se-baths, as well as sp<strong>in</strong> and spray r<strong>in</strong>s<strong>in</strong>g 5 . Any new clean<strong>in</strong>g process applied to small medical devices should be analyzed for each specific circumstance to ensure that it does not adversely affect the performance of the device. Conclusions The potential for ultrasonic clean<strong>in</strong>g to <strong>in</strong>duce harmonic oscillation and subsequent fatigue damage <strong>in</strong> small structures is an issue that is not yet common knowledge throughout the medical device <strong>in</strong>dustry. The problem has been wellestablished <strong>in</strong> the electronics <strong>in</strong>dustry, and has been proven to affect a variety of medical devices made from different alloys. FEA is capable of accurately predict<strong>in</strong>g the natural frequency of a device, allow<strong>in</strong>g a manufacturer to make process or even design changes that avoid resonant conditions that could lead to significant mechanical damage and premature failure. Alternative techniques that avoid ultrasonic-<strong>in</strong>duced damage have been used successfully <strong>in</strong> the electronics manufactur<strong>in</strong>g 45 <strong>in</strong>dustries, and could be employed <strong>in</strong> the medical device <strong>in</strong>dustry to avoid the problems associated with ultrasonic clean<strong>in</strong>g-<strong>in</strong>duced resonance and fatigue. References 1 L.A. Mallette, I. Chen, T.W. Johnson, “A Case Study <strong>in</strong> <strong>Ultrasonic</strong> <strong>Clean<strong>in</strong>g</strong> Damage”, Aerospace Conference, IEEE, Volume 3, Issue 21-28, 1998, pp. 77-80. 2 J. Yuqi, T. Dong, S. Xianzhong, L. Fai, “The Effect of <strong>Ultrasonic</strong> <strong>Clean<strong>in</strong>g</strong> on the Bond Wires”, Proceed<strong>in</strong>gs of the 12 th IPFA, 2005, S<strong>in</strong>gapore, IEEE, pp. 237-241. 3 R.V. Marrey, R. Burgermeister, R.B. Grishaber, R.O. Ritchie, “Fatigue Life and Prediction for Cobalt-Chromium Stents: A Fracture Mechanics Analysis”, Biomaterials 27, 2006, pp. 1988-2000. 4 S. Timoshenko, D.H. Young, W. Weaver, Vibration Problems <strong>in</strong> Eng<strong>in</strong>eer<strong>in</strong>g, 4 th Edition, Wiley and Sons, 1974. 5 Handbook of Silicon Wafer <strong>Clean<strong>in</strong>g</strong> Technology, 2 nd Edition, Editors K.A. Re<strong>in</strong>hardt, W. Kern, William Andrews Publish<strong>in</strong>g, 2008.
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