Ultrasonic Cleaning-Induced Failures in Medical ... - ASM International

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
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
industry 1, 2 . However, only limited, anecdotal evidence has
been provided for ultrasonic cleaning-induced fatigue in small
medical devices 3 . 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 structure 4
. 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
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
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microscopy (SEM) examination of the fracture surfaces
clearly indicated fatigue was the cause of the breaks, shown in
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.

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.
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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.

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.
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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 prestress),
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.

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.
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 during
crimping).
Figure 13: Close up of stent shape when resonance occurs at
a natural frequency of 140.0 kHz.

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 rinsing 5
. 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.
Conclusions
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 wellestablished
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
45
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
12 th 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, 4 th Edition, Wiley and Sons, 1974.
5 Handbook of Silicon Wafer Cleaning Technology, 2 nd
Edition, Editors K.A. Reinhardt, W. Kern, William Andrews
Publishing, 2008.

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