| August 13, 1997 |
Dear Medical Device Manufacturer:
This is to alert you to a failure mode we recently
became aware of involving vacuum loss in electronic resonating
components. This concerns us, because electronic resonating components
are frequently used in medical devices, and their failure could
have serious adverse effects on device function. To our knowledge,
this problem has not been previously documented in the scientific
literature.
The oscillator circuits of medical devices often
employ a crystal resonating component to maintain strict control
of timing functions and electrical signal synchronization. A
resonating component typically consists of a piezoelectric element,
housed in a metal or ceramic case. The interior of the case is
evacuated in order to decrease motional resistance. This improves
the resonating quality of the component. Slow degradation of
the interior vacuum can occur in two ways: gas leakage through
case joints and diffusion of helium through the glass that surrounds
the electrical pins of some resonating components. The loss of
vacuum significantly increases the component parameter known as
motional or series resistance, and the slow rise in resistance
can result in device malfunction years after the device is manufactured.
The duty cycle of the timing signal has been found to change
significantly as a result of rising motional resistance. In some
cases, oscillation has ceased. Loss of oscillation is generally
recognized as a catastrophic failure mode of these components,
but changes in timing signal duty may be equally catastrophic,
more insidious, and less widely recognized as a failure mechanism.
Microprocessors, microcontrollers, and memory circuits are examples
of digital circuits that may be affected by changing duty cycle.
One way to prevent vacuum loss problems is to immunize
devices against changing motional resistance. This can be accomplished
by design, making the oscillator circuitry tolerate the expected
change in motional resistance. Another way to prevent problems
is to eliminate the vacuum. Resonating components can be manufactured
with any fill gas and pressure desired. If the gas composition
and pressure inside the component matches the expected exterior
gas environment, there will be no influx of gas to cause motional
resistance change. However, since this will reduce the resonating
quality of the component, such reduction must be accommodated
in the final circuit design.
If an evacuated resonating component is deemed necessary,
then maintenance of the vacuum over the expected life of the device
is mandatory. Ensuring vacuum longevity is not easy. If one
assumes that gas infiltration is an exponential function of the
pressure enveloping the component, then the maximum allowable
gas leakage rate for the typical resonating component will be
less than ten to the minus tenth atmospheres cubic
centimeters per second. This quality of component
seal is impossible to verify during production. Thus the only
way to ensure vacuum longevity is to pay meticulous attention
to the component's material composition, design validation, acceptance
procedures, and manufacturing quality.
In many cases, vacuum loss cannot be completely prevented
but can be limited and expected to rarely occur. In such cases,
redundant or backup circuits can be used to mitigate vacuum loss
problems. However, manufacturers must ensure that the switching
mechanism from primary to redundant or backup function occurs
reliably and in advance of any malfunction that could result from
slowly rising motional resistance. Checking for backup circuit
engagement at the extreme of expected motional resistance or only
when oscillating function is lost has proven to be an ineffective
deterrent to vacuum loss problems.
Resonating component vacuum loss represents a failure
mode to which some medical devices are particularly susceptible.
This alert is intended to provide notice of this failure mode
to the designers and manufacturers of implantable medical products.
However, any device that incorporates an electronic resonating
component is potentially susceptible. Therefore, we encourage
all designers and manufacturers to consider this failure mode
during design risk assessment and when devices are analyzed for
failure. In addition, we recommend that manufacturers assess
the susceptibility of their marketed devices to malfunction caused
by slowly changing resonating component resistance and consider
remedial action if a device is susceptible.
If you have any questions regarding this letter,
please contact Eugene O'Bryan, Electronics Engineer (HFZ-141),
FDA, 9200 Corporate Boulevard, Rockville, Maryland 20850; or fax
to (301) 443-9101; or E-mail to ero@cdrh.fda.gov.
|
Sincerely yours,
D. Bruce Burlington, M.D.
|
Updated August 14, 1997
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