Biomedical Engineering Reference
In-Depth Information
lumbar-tapered fusion devices are examples of this direction. This combination
product approach is expected to continue producing more examples of
synergistic devices and biological engineering products.
Similar combination ideas have been used at all levels of device design. MIO
is another example mentioned previously. MIO resulted when metal ions from
conducting wire corrosion reacted with the lead insulation polymer. MIO was
successfully minimized by applying a barrier layer to the wire, which effectively
prevented metal corrosion and release of ions into the polymer insulation.
In a final example, recall the lead design used in DBS therapy. This particular
design offered a very simple approach to the design of a neurostimulation device
that meets the needs of the two mechanically different implant environments.
Here, the solution was not a single complex lead that met the two requirements,
but rather a two-lead solution: a fine lead for positioning in the brain, and a more
robust lead to handle the mechanically dynamic neck region.
4.6 Reliability and testing
Though materials and engineering principles used for implantable medical
devices are essentially the same as those for other products, strict requirements
exist for patient safety, demonstrated effectiveness and reliability. The scrutiny
required to meet these requirements is intense, and as expected this adds a very
heavy burden and risk to device manufacturers.
Even with the best of intentions, things can go awry. For example, prior to
Food and Drug Administration (FDA) regulations enacted in 1976, a pacemaker
called Xytron was introduced. To ensure the device reliability, extra engineering
measures were used. The device circuitry was first encased in an epoxy pocket,
then sealed in a metal case. Later, it was discovered that the seal of the soldered
case could fail, allowing moisture to enter the case, permeate the epoxy pocket,
and reach the circuitry imbedded inside. The moisture combined with the ionic
residual of the solder aid made a conductive path in the pocket. The leaking
current gradually caused the growth of dendrites (metal crystals) which shorted
out the circuitry. Clearly, systems that ensure product reliability and test
methods that capture failure modes are critical to prevent device complications
and failures.
Downsizing, adding more functions, and increasing longevity have been
constant driving forces in the device industry. These development goals must be
combined with the universal goal of improving device reliability. There may be
a conflict with this direction down the road.
Figure 4.15 shows the downsizing trends of some leads developed from 1980
to 2000. As mentioned, the defibrillation voltage is about 800 V. A typical
polymer insulation material has an electrical strength of 20 V/m. This high
voltage would require the insulation layer to be at least 40±50m thick. With
the effort of lead downsizing, we may soon approach this limit.
