Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2002-09-20
2003-05-20
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S306100, C361S303000
Reexamination Certificate
active
06567259
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to feedthrough capacitor terminal pin subassemblies and related methods of design and construction, for protecting implantable medical devices from electromagnetic interference commonly found in the environment. More specifically, the present invention relates to improved performance feedthrough capacitor terminal pin subassemblies which offer attenuation to EMI at lower frequencies and also at higher attenuation levels, particularly in medical implant applications.
Feedthrough terminal assemblies are generally well known for connecting electrical signals through the housing or case of an electronic instrument. For example, in an implantable medical device, such as a cardiac pacemaker, defibrillator or the like, the terminal pin assembly comprises of one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which are connected to cardiac tissue or other tissue to be stimulated which effectively act as an antennae and tend to collect stray electromagnetic interference (EMI) signals for transmission into the interior of the medical device. The hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device.
Most implantable medical devices in the United States today incorporate feedthrough capacitor EMI filters at their input terminals or in conjunction with the hermetic terminal. However, due to size constraints and mechanical constraints, the capacitance value of these filters has been relatively low (in the 490 to 4000 picofarad range). These capacitance values have been very effective for attenuation of cell phones and other high frequency emitters, however, they do very little to protect the implantable medical device against lower frequency EMI.
It has been well-documented in in-vivo and in-vitro studies that certain kinds of electromagnetic interference can cause disruption of the implantable medical device. For example, in cardiac pacemakers it has been shown that digitally modulated EMI can cause pacemaker inhibition, asynchronous pacing or missed beats. All of these conditions would be highly undesirable and potentially life threatening in a pacer-dependent patient. It has also been shown that EMI can cause an implantable cardioverter defibrillator to inadvertently deliver its high voltage shock therapy. This is very uncomfortable to the patient and is equivalent to a very hard blow to the chest. In prior art devices, such as those shown in U.S. Pat. Nos. 5,333,095; 4,424,551; 5,905,627; 5,751,539 and 6,008,980 (the contents of which are incorporated herein), the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple and shield electromagnetic interference (EMI) signals into the housing of the medical device.
For example,
FIG. 1
is a cut away view of a typical cardiac pacemaker
30
showing an internal circuit board
32
and a broadband EMI filter
34
. In order for the broadband EMI filter
34
to work properly, it must be mounted directly at the point of lead
36
ingress and egress.
The broadband EMI filer
34
is typically of coaxial construction also known as a feedthrough capacitor EMI filter. The feedthrough capacitor
34
is optimally bonded directly to the hermetic terminal
38
(
FIG. 2
) of the implantable medical device that is used to exclude entry of body fluid. The location of the broadband EMI filter
34
at the point of lead ingress and egress is essential so that undesirable incoming EMI signals can be decoupled and shunted directly to the titanium or stainless steel pacer or can or housing
40
and dissipated as harmless energy (heat).
With reference to
FIG. 2
, in a typical prior art unipolar construction (as described in U.S. Pat. No. 5,333,095), a round/discoidal (or rectangular) ceramic feedthrough filter capacitor
42
is combined with a hermetic terminal pin assembly
38
to suppress and decouple undesired interference or noise transmission along a terminal pin or lead
36
. The feedthrough capacitor
42
is coaxial having two sets of electrode plates
44
,
46
embedded in spaced relation within an insulative dielectric substrate or base
48
, formed typically as a ceramic monolithic structure. The dielectric substrate or base
48
is generally constructed of barium titinate dielectrics that have been built doped with suitable materials to form the desired dielectric properties. One set of the electrode plates
44
is electrically connected at an inner diameter cylindrical surface of the coaxial capacitor structure
42
to the conductive terminal pin
36
utilized to pass the desired electrical signal or signals. The other or second set of electrode plates
46
is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule
50
of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing
40
of the electronic device
30
. The number and dielectric thickness spacing of the electrode plate sets
44
,
46
varies in accordance with the capacitance value and the voltage rating of the coaxial capacitor
42
. The outer feedthrough capacitor electrode plates
46
(or “ground” plates) are coupled in parallel together by a metallized layer
52
which is either fired, sputtered or plated onto the ceramic capacitor
42
. This metallized band, in turn, is coupled to the ferrule
50
by conductive adhesive, soldering, brazing, welding, or the like. The inner feedthrough capacitor electrode plates
44
(or “active” plates) are coupled in parallel together by a metallized layer
54
which is either glass frit fired or plated onto the ceramic capacitor
42
. This metallized band
54
, in turn, is mechanically and electrically coupled to the lead wire
36
by conductive adhesive or soldering, or the like. In operation, the coaxial capacitor
42
permits passage of relatively low frequency electrical signals along the terminal pin
36
, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing
40
. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (6) and additional lead configurations. The feedthrough capacitors (in both discoidal and rectangular configurations) of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing
40
is constructed from a biocompatible metal such as titanium alloy, which is electrically and mechanically coupled to the hermetic terminal pin assembly
38
which is in turn electrically coupled to the coaxial feedthrough filter capacitor
42
. Alternatively, the feedthrough capacitor can be grounded to one or more terminal pins as described in U.S. Pat. No. 5,905,627. As a result, the filter capacitor
42
and terminal pin assembly
38
prevents entrance of high frequency interference signals to the interior of the pacemaker housing
40
, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
Feedthrough filter capacitors for cardiac pacemakers and the like, have typically been constructed by preassembly of the coaxial capacitor
42
onto or within a cylindrical or rectangular hermetically sealed terminal pin subassembly
38
which includes the conductive pin
36
and ferrule
50
. More specifically, the terminal pin subassembly
38
is prefabricated to include one or more conductive terminal p
Haskell Donald K.
Roberts John E.
Stevenson Robert A.
Dinkins Anthony
Greatbatch-Sierra, Inc.
Kelly Bauersfeld Lowry & Kelley LLP
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