Ultrasonic Cleaning-Induced Failures in Medical Devices
B.A. James, C. McVeigh, S.N. Rosenbloom, E. P. Guyer, S.I. Lieberman Exponent Failure Analysis Associates, Menlo Park, CA, USA
Abstract
Ultrasonic cleaning is often used as part of the manufacturing process of small medical devices such as guide wires and vascular implants. Ultrasonic cleaning at frequencies close to the natural frequency of the device can result in resonance, resulting in significant mechanical damage and possibly premature failure. This paper provides case studies of ultrasonic cleaning-induced fatigue and corresponding failures in small medical devices. Preventative measures, including analytical tools such as finite element analysis (FEA), to ensure that ultrasonic cleaning frequencies do not result in resonance and stresses sufficient to cause fatigue damage are also discussed.
Background
Ultrasonic cleaning has been known for years to have the potential to induce harmonic oscillation and corresponding fatigue damage in small structures used in the electronics industry1, 2. However, only limited, anecdotal evidence has been provided for ultrasonic cleaning-induced fatigue in small medical devices3. While the dangers of ultrasonic cleaning may be well known within specific medical device companies, judging by the number of problems observed by the authors, this issue does not appear to be common knowledge across the entire medical device industry.
The natural frequency of a given structure is a function of its elastic moduli, geometry, and mass. Vibration amplitudes increase dramatically as the frequency of an impressed force approaches the natural frequency of the structure4
A medical device manufacturer noted occasional strut fractures during balloon expansion tests associated with the validation of a new stent geometry. These stents had been subjected to typical manufacturing processes, including ultrasonic cleaning for several minutes. Scanning electron
microscopy (SEM) examination of the fracture surfaces clearly indicated fatigue was the cause of the breaks, shown in
. A condition of resonance occurs when the impressed force frequency equals a structure’s natural frequency. At and near resonance, relatively small energy input can result in large vibration amplitudes. These large deflection amplitudes at near-resonance conditions can result in fatigue crack initiation and growth.
Case Study 1 – 316L Stainless Steel Stent
Figure 1 and Figure 2. The stents had not been subjected to any source of cyclic loading other than ultrasonic cleaning. Stents that had not been subjected to balloon expansion were also examined for any evidence of cracks. Several cracks were observed in these undeployed stents, an example of which is shown in Figure 3.
Figure 1: Lower magnification SEM image of 316L stent fracture surface
Figure 2: Higher magnification SEM image of 316L stent fracture surface.
Medical Device Materials V Proceedings from the Materials & Processes for Medical Devices Conference 2009 August 10–12, 2009, Minneapolis, Minnesota, USA, J. Gilbert, Ed., p 41-45
Figure 3: SEM image of ultrasonic crack in stent, prior to deployment.
Case Study 2 – Catheter Wire Fracture
A catheter guide wire fractured during service, breaking within the patient. A non-destructive examination of the fractured wire using SEM was conducted. Evidence preservation issues required that the guide wire fracture surface could not be cleaned prior to the SEM examination. Clear evidence of fatigue crack initiation and growth was observed between the debris on the wire fracture surface, as shown in Figure 4 and Figure 5. The subject guide wire had not knowingly been subjected to any cyclic loading prior to service. However, the manufacturer did report that ultrasonic cleaning was used during manufacturing. Based on our analysis, it appears that ultrasonic cleaning was the only process that could produce the thousands of cycles needed to result in the observed fatigue crack initiation and propagation.
Figure 4: SEM image of guide wire fracture surface. For evidence preservation, the fracture surface was not cleaned.
Figure 5: Higher magnification SEM image of guide wire fracture showing beach marks consistent with fatigue crack growth.
Case Study 3 – Nitinol Stent Fracture
A nitinol stent subjected to an unintentionally long ultrasonic cleaning treatment fractured during bend testing. SEM analysis showed that striations were present on the fracture surface, indicating that a fatigue crack preceded the overload fracture as shown in Figure 6 and Figure 7. Several other cracks were observed at multiple locations along the stent, shown in Figure 8.
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Figure 6: SEM image of fractured nitinol stent.
Figure 7: SEM image of striations on fractured nitinol stent.
Figure 8: SEM image of cracks from other locations on the subject nitinol stent.
Finite Element Analysis
FEA is a numerical analysis technique which is commonly used to compute stresses and strains arising in various structures as a function of the structure’s geometry, material constitutive response, applied loads, and boundary conditions. FEA can also be used to solve for the normal modes (eigenmodes) of an oscillating structure; i.e., the patterns of motion in which each part of the structure moves sinusoidally with the same frequency. The excitation frequencies associated with these normal modes are known as the natural or resonant frequencies of the structure. All physical structures have a set of normal modes (and associated resonant frequencies) that depend on the structural stiffness (mechanical properties and geometry), density, applied loading (or existence of a residual or pre-stress), and boundary conditions. Although a structure may have normal modes spread over a wide frequency range, FEA mode extractions are used to scan an appropriate range of forcing frequencies (e.g., the frequency of the ultrasonic cleaner). This is a useful exercise to determine if resonance is likely to occur at or near a proposed cleaning frequency. Alternatively, in the event that failure has already occurred, the extracted modes can be used to elucidate the cause of failure.
Case Study 4 – FEA of Stainless Steel Stent Failure A 316L stainless steel stent (model shown in Figure 9) exhibited evidence of fatigue originating at the intrados of a crown-strut interface after a crimping operation during manufacture, as shown in Figure 10. Figure 11 is an SEM image of the fracture surface and revealed a fatigue-related failure. However, the source of the cyclic stress was unclear. It was noted that prior to crimping, an ultrasonic cleaning procedure had been employed. The ultrasonic cleaner was known to operate at approximately 140 kHz. An FEA eigenmode analysis was performed to extract all of the key resonant frequencies near 140 kHz; the associated modes are shown in Figure 12. The deformed shapes clearly show localized motion at the stent ends where failure was observed during subsequent crimping. The natural mode which occurs at exactly 140 kHz is shown in Figure 13 in more detail. The displacement of the end crowns at this natural frequency would drive a significant cyclic stress amplitude in the failure region. This data indicates that ultrasonic cleaning was the likely candidate for initiating the fatigue crack which subsequently propagated and failed during crimping.
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Figure 9: Finite Element model of subject stainless steel stent.
Figure 10: Fracture observed at crown-strut interface.
Figure 11: SEM of fracture surface in stainless steel stent.
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 during crimping).
Figure 13: Close up of stent shape when resonance occurs at a natural frequency of 140.0 kHz.
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Discussion Ultrasonic cleaning is a seemingly benign process that is prevalent throughout the medical device industry. It has been demonstrated in this paper that ultrasonic cleaning can induce 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 cleaning frequency is susceptible to this failure mechanism. The obvious concern is one of reliability. If small cracks are induced in these devices, it is challenging at best to detect these defects unless the devices are subjected to stresses and thorough inspection prior to the final manufacturing step. As a result, incipient ultrasonic cleaning-induced cracks can remain undetected in finished devices. There are virtually no practical non-destructive techniques for detecting minute cracks in small devices that can be used in a large manufacturing process. However, FEA can be a useful tool in characterizing the susceptibility of small-scale devices to ultrasonic-induced 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 manufacturing industry has long recognized the potential harmful effects of ultrasonic cleaning-induced fatigue on small parts. However, the ability to produce surfaces free of particulates and contaminates is important to the electronics manufacturing industry. Additionally, these large-scale electronics cleaning processes are also subjected to stricter environmental controls. Due to cleaning-induced damage concerns, many electronics industry cleaning procedures do not utilize ultrasound. These cleaning procedures include overflow and cascading rinse-baths, as well as spin and spray rinsing5
Conclusions
. Any new cleaning 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.
The potential for ultrasonic cleaning to induce harmonic oscillation and subsequent fatigue damage in small structures is an issue that is not yet common knowledge throughout the medical device industry. The problem has been well-established in the electronics industry, and has been proven to affect a variety of medical devices made from different alloys. FEA is capable of accurately predicting the natural frequency of a device, allowing 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-induced damage have been used successfully in the electronics manufacturing
industries, and could be employed in the medical device industry to avoid the problems associated with ultrasonic cleaning-induced resonance and fatigue.
References 1 L.A. Mallette, I. Chen, T.W. Johnson, “A Case Study in Ultrasonic Cleaning 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 Ultrasonic Cleaning on the Bond Wires”, Proceedings of the 12th IPFA, 2005, Singapore, 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 in Engineering, 4th Edition, Wiley and Sons, 1974. 5 Handbook of Silicon Wafer Cleaning Technology, 2nd Edition, Editors K.A. Reinhardt, W. Kern, William Andrews Publishing, 2008.
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