Self-supporting terminal for implantable pulse generator and method of manufacturing the same

A terminal for an implantable pulse generator for connecting an electrode lead to the generator includes a base section formed of partially stabilized zirconia (PSZ) mounted on the exterior of the pulse generator housing and a conductive terminal member coupled to the base section. The base section, terminal member and pulse generator housing are joined by a biocompatible brazing alloy which is composed, by weight, of about 13% to about 17% copper, about 13% to about 17% nickel, and the balance being titanium.

BACKGROUND OF THE INVENTION 
The present invention relates to a self-supporting terminal for connecting 
electrode leads to an implantable pulse generator, such as a cardiac 
pacemaker. The invention further relates to coupling a ceramic to either 
another ceramic or to a metal via a biocompatible active alloy. 
Self-supporting terminals for implantable pulse generators, such as cardiac 
pacemakers, are generally disclosed in U.S. Pat. No. 4,445,511, which 
discloses a terminal comprising an alumina ceramic base brazed to a metal 
terminal piece such as titanium. The ceramic base has a generally 
cylindrical shape which surrounds the base of the terminal piece. A hole 
is provided in the upper section of the terminal piece to facilitate the 
insertion of an electrode lead, which can be fixed in place by means of a 
screw (such as a grub screw) which is located in the upper section of the 
terminal piece and disposed so as to project into the path of the lead 
once it is located in the hole. 
The afore-described terminal is subjected to torque from several sources, 
however. During tightening of the screw to hold the electrode lead in 
place, for example, a wrench is commonly used which exerts major torque on 
the terminal. Also, insertion or withdrawal of the electrode lead can 
result in the application of a sideways, i.e., horizonaal force to the 
terminal. Any of these forces may be sufficient to destroy the coupling 
between the ceramic base and terminal piece or pacemaker, thereby 
destroying the integrity of the terminal. 
An important consideration in implantable pulse generators is compactness 
of the device. As connection terminals are made smaller, alumina ceramics 
known in the current art of self-supporting terminals lack the necessary 
strength to withstand the sideways and flexural torque exerted during 
implantation and adjustment of the pacemaker. To ensure that sufficient 
strength is provided, severe limitations on the dimensions of the terminal 
are therefore present. A need is apparent in the art for terminals of 
sufficient strength yet which have dimensions smaller than the alumina 
ceramic terminals now in use. 
For example, the sideways bending strength S of a connector terminal has 
been found to be proportional to the cube of the diameter D of a terminal 
having a cylindrical configuration, i.e., S=D.sup.3. The following table 
shows the relationship between terminal diameter reduction and required 
increase in material strength in order to withstand the same sideways 
load: 
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Terminal Increase in Material 
Strength Using 
Diameter Strength Required 
Current Material 
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1.0 D 0% 1.0 S 
0.9 D 37% 0.73 S 
0.8 D 95% 0.51 S 
0.7 D 192% 0.34 S 
0.6 D 363% 0.22 S 
0.5 D 700% 0.13 S 
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Accordingly, presently-used ceramics cannot achieve a sufficient reduction 
in size or dimension of the connector terminal without the accompanying 
loss of sideways or flexural strength, as shown above. Specifically, known 
terminals utilizing alumina ceramics do not have sufficiently high 
sideways bending or flexural strengths, so suitable reductions in terminal 
dimensions cannot be achieved. 
A further problem associated with the alumina ceramic used in current 
devices is the possibility of cracking the ceramic during the brazing 
cycle. In the brazing process, such as shown in U.S. Pat. Nos. 4,426,033 
and 4,591,535, the terminal is heated to a brazing temperature followed by 
a cooling down period. If this cooling down phase is too fast, the alumina 
ceramic has a tendency to crack due to its low thermal shock resistance. A 
superior ceramic and brazing alloy is therefore needed in the art of 
self-supporting terminals. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a 
self-supporting terminal having increased flexural strength for connecting 
an electrode lead to an implantable pulse generator. 
A further object of the invention is to provide a self-supporting terminal 
of reduced dimensions for an implantable pulse generator characterized by 
reduced diameter and height over prior art devices, thereby allowing for a 
more compact implantable generator. 
Another object of the invention is to provide a self-supporting terminal 
comprising a ceramic having a high thermal shock resistance whereby 
cracking during a cool down phase is avoided. Accordingly, a faster 
brazing cycle can be employed, resulting in improved cost-efficiency and 
faster processing time in the manufacture of such terminals. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be obvious from 
the description or may be learned by practice of the invention. The 
objects and advantages of the invention may be realized and attained by 
means of the instrumentalities and combinations particularly pointed out 
in the appended claims. 
To achieve the objects and in accordance with the purpose of the invention, 
as embodied and broadly described herein, the invention includes a 
terminal for an implantable pulse generator for connecting an electrode 
lead to the generator, the terminal comprising: an exterior base section 
formed of partially stabilized zirconia, the base section being mounted on 
the generator; and a metallic terminal member including means for 
receiving the electrode lead, the member being supported by the base 
section. In particular, the base section, terminal member and pulse 
generator are coupled by means of a biocompatible active alloy braze. 
The accompanying drawing, which is incorporated in and constitutes part of 
this specification, illustrates one embodiment of the invention and, 
together with the following description, serves to explain the principles 
of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the presently preferred embodiment 
of the invention, an example of which is illustrated in the accompanying 
drawings. Throughout the drawings, like reference characters are used to 
designate like elements. 
FIG. 1 illustrates a cross section of a preferred embodiment of a 
self-supporting terminal 10 for an implantable pulse generator, such as a 
cardiac pacemaker (not shown in full). Terminal 10 is also shown in 
perspective view in FIG. 2. The terminal 10 is mounted externally on a 
wall 12 of the housing of the pulse generator. The terminal 10 may serve, 
for example, to connect an electrode lead (not shown) to electronics 
housed within the wall 12 of the pulse generator. 
In accordance with the present invention, the terminal comprises an 
exterior base section formed of partially stabilized zirconia, the base 
section being mounted on the pulse generator. As embodied herein, the base 
section comprises base 14 which is preferrably formed of partially 
stablized zirconia ("PSZ") ceramic using magnesium oxide or yttrium oxide 
as a stabilizer. The PSZ substrate can be processed to remove any 
irregularities from its surface, such processing being most easily 
achieved by grinding as the toughening behavior of PSZ permits formation 
of a defect-free surface. As shown in cross section in FIG. 1, PSZ 
substrate 14 is preferably formed as a cylinder having a central aperture 
16 and first and second end faces 18 and 20, respectively. 
Also in accordance with the present invention, a metallic terminal member 
is provided, including means for receiving the electrode lead, the 
terminal member being supported by the base section. As embodied herein, 
the terminal member is generally designated by reference character 22 in 
the drawing. In a preferred embodiment, terminal member 22 includes a 
shaft portion 24 and an electrode lead receiving portion 26. These two 
portions may be integrally formed of a conducting material, such as 
aluminum, titanium or other metals suitable for implantation in the body. 
As shown in FIG. 1, shaft portion 24 is disposed within central aperture 16 
of base member 14 so that second face 20 of the base member substantially 
abuts the corresponding face of electrode receiving portion 26. Terminal 
member 22 is thus seated on and supported by base member 14. To ensure 
sound placement of these two elements, terminal member 22 may include a 
flange 28 having an interior diameter substantially identical to the 
exterior diameter of base member 14, as shown in FIG. 1. 
The electrode lead receiving portion 26 of terminal member 22 includes a 
hole 30 having a diameter sufficient to receive the tip of an electrode 
lead (not shown). To secure the lead in place within hole 30, a grub screw 
32 may be provided which has a threaded portion and an end protruding into 
hole 30. Once the lead is in place in hole 30, grub screw 32 is tightened 
so as to firmly entrap the electrode lead and thereby ensure consistent 
electrical and mechanical interconnection. 
According to the present invention, the terminal member, base member and 
pulse generator are coupled together by a biocompatible alloy braze. As 
shown in the drawings, this braze is indicated by reference character 34 
and is applied at the junction of base member 14 and flange 28 of terminal 
member 22. Braze 34 may also be applied at the junction of base member 14 
and pulse generator housing 12 through the intermediary of a collar 36 
having a generally circular shape suitable to encapture first face 18 of 
base member 14. A disk 38 may also be provided abutting first face 18 and 
contacting, along its outer periphery, collar 36. 
Electrical interconnection between the electrode lead and circuitry (not 
shown) within the pacemaker can be accomplished by soldering or otherwise 
coupling a wire or other conductor to the end of shaft 24 that is disposed 
in the interior of pacemaker housing 12. In such a case, i.e., where no 
connection between terminal member 22 and housing 12 is desired, either 
one or both collar 36 and disk 38 are non-conductive and can be formed of 
ceramic or other insulating material. Alternately, these elements can be 
eliminated altogether so that base member 14 is brazed directly to housing 
12 during construction of the overall device. 
Where it is desired to electrically interconnect the electrode lead and 
housing 12, collar 36 and disk 38 can be made of metal or another 
conductive material. 
The particular interconnection between the terminal member and pulse 
generator (housing or internal circuitry) is considered obvious to 
practitioners of ordinary skill in light of this disclosure, and 
variations in such interconnection--including elimination and/or 
substitution of various elements for collar 36 and disk 38--can be made 
without departing from the spirit or scope of the present invention. The 
invention is more directly concerned with the employment of a PSZ base 
member and a biocompatible alloy braze for construction of the overall 
device, as it is these features which particularly contribute to the 
reductions in size of the inventive terminal while allowing the necessary 
strength and torque resistance to be maintained. 
In accordance with the preferred embodiment, braze 34 is biocompatible and 
corrosion-resistant. The approximate composition of braze 34 is, by 
weight, from about 13% to about 17% copper, from about 13% to about 17% 
nickel, with the balance being titanium. A suitable alloy braze meeting 
these requirements is available from the WESGO Division of GTE Products 
Corporation of Belmont, California and is sold under the tradename 
"Ticuni". 
To construct the terminal according to the present invention, the PSZ base 
element is first processed, such as by grinding, to remove irregularities 
and form a defect-free surface. The base element is then assembled in a 
close fitting fashion to the terminal member and collar/disk (if any) and 
the active brazing alloy is placed in a position as to enable the braze to 
flow between the PSZ and member interface when the overall assembly is 
heated to the flow temperature of the brazing alloy. This may be done, for 
example, by placing a foil of brazing alloy between the PSZ element and 
adjacent member; alternately, braze alloy in the form of powder can be 
adhered to the interface joining surfaces by a clear lacquer whereafter 
the braze can flow into the joint interface when the assembly is heated to 
the flow temperature of the braze alloy. 
A fast brazing cycle can be used due to the much higher thermal shock 
resistance exhibited by the PSZ ceramic in comparison with alumina 
ceramics. This particular advantage eliminates destruction of the terminal 
due to problems such as cracking during the brazing cycle. This leads to 
reductions in processing time and increases in cost effectiveness. 
As can be appreciated from the structure described above and shown in FIGS. 
1 and 2, a terminal constructed according to the present invention may 
present joints formed of either PSZ ceramic with metal or PSZ ceramic with 
other ceramics and insulating materials. Ideally, the member to be brazed 
to the PSZ ceramic has thermal expansion properties similar to that of the 
PSZ material. By way of example and without intending to limit the present 
invention, the following materials can be brazed to PSZ ceramic through 
use of the "Ticuni" brazing alloy and application of a flow temperature of 
1100.degree. C.: PSZ ceramic, titanium, stainless steel and aluminium 
oxide. Thus, a wide range of materials can be employed for the terminal 
member, collar, disk and pulse generator housing without departing from 
the spirit and scope of the present invention. 
The PSZ ceramic used in the aforedescribed embodiment of the invention has 
a sideways bending strength or flexural strength that is approximate 80% 
greater than that of alumina ceramic. Since compactness is a critical 
factor in the design of implantable pulse generators, such as pacemakers, 
the present invention allows for a terminal of reduced dimension without 
loss of bending strength or flexural strength. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. Thus, it is intended that the specification and drawing 
be considered as exemplary only, with the true scope and spirit of the 
invention being indicated by the following claims.