X-ray transparent medical electrode

The signal conducting element of a medical electrode of the type bridged to skin by electrolyte and which is required to be X-ray transparent comprises a thin layer of a conductive paint adhered to a relatively thin supporting substrate, said substrate with adhered paint extending remotely from the electrolyte-to-skin interface for circuit connection and protected by a barrier which excludes electrolyte from the remote circuit connection.

BRIEF SUMMARY OF THE INVENTION 
A compactly assembled X-ray transparent medical electrode of the type 
contacted to the skin for electrocardiograph and like forms of monitoring 
or for stimulation utilizes a conductor member comprising a thin strip of 
nonconductive material having a thin layer of electrically conductive 
paintable material adhered to one face thereof. A first area of the 
adhered conductive material is adapted for contact by an electrolyte 
loaded sponge which is used to bridge the conductor member to the skin. A 
second area of the conductive material which is remote from the 
aforementioned first area is provided for connection of the electrode to 
external equipment. The conductive paintable material provides a 
continuously conductive layer extending from the electrolyte engaging 
first area to the remotely disposed second area of the conductive 
material. The conductor member is provided with barrier means covering a 
portion of the paintable conductive material between the first and second 
areas to limit the area of the paintable material to which the electrolyte 
has access and thereby minimize artifact noise attributable to migration 
of electrolyte to surface areas of the paintable material which have not 
been passivated with respect to the electrolyte. In one embodiment, the 
electrolyte loaded sponge is retained in position by sandwiching of the 
conductor member along with the electrolyte sponge between a flexible 
cover disc and a flexible skin contacting pad having an aperture receiving 
the electrolyte sponge for presenting such sponge to the surface of live 
skin for stimulation or monitoring purposes. In another embodiment, the 
electrolyte sponge is supported in contact with the paintable conductive 
material at the area thereof provided for electrolyte contact by adhesion 
of the sponge to a flexible cover member having an adhesive surface, said 
electrolyte sponge being encircled by an annulus of a flexible and 
breathable sheet material adhered on one face thereof to the flexible 
cover member and provided at the opposite face thereof with a pressure 
sensitive adhesive layer effective for attaching the electrode to the skin 
of a patient and in so doing effective to press the electrolyte loaded 
sponge firmly against the skin of a patient. By way of illustration, the 
second embodiment includes a snap fastener type conductor arrangement for 
convenient connection of the electrode to remote monitoring or stimulating 
equipment.

DETAILED DESCRIPTION 
Having reference to FIGS. 1 and 2, the preferred embodiment includes a 
signal conductor 10 which comprises a flexible, dimensionally stable, 
substrate 12 of a nonconductive plastic sheet material such as 
polyethylene terphthalate. The substrate is shaped as by die stamping to 
have an elongated stick portion 14 terminating at one end of the stick 
portion with an enlarged circular island 16. Applied to this nonconductive 
substrate is an electrically conductive surface layer 18, the boundaries 
of which are spaced inwardly from the side edges of the nonconductive 
substrate. The surface layer 18 thus has a circular terminal portion 20 
smaller than the island 16 and, extending outwardly from the terminal 
portion 20 along the stick portion 14, a longitudinally disposed 
conductive finger 22. 
The surface layer 18 comprises a layer of silver paint which is preferably 
applied by silk screening. As an example, the silver paint may be DuPont 
Conductor Composition No. 9793, a conductive paint tailored for screen 
printing electrical circuits onto plastic substrates. This Composition is 
commercially available from the DuPont Chemical Corporation. While silk 
screening of a commercially available compound is preferred for the 
formation of the conductive surface layer 18, it is to be understood that 
other procedures for preparation of the surface layer 18, such as brush 
painting, or printing, or spraying through a suitable mask may also be 
utilized in the practice of the present invention. 
For reasons to be described, a nonconductive moisture impermeable barrier 
layer 26 comprising an acrylic plastic or the like is applied over the 
conductive surface layer 18 at that portion of the conductive finger 22 
which is adjacent the terminal portion 20. 
After the described preparation, the face of the signal conductor 10 which 
is opposite the face bearing the conductive layer 18 is pressed against an 
adhesive layer 30 supported by a circular plastic disc 28. The disc 28 
which is preferably relatively impermeable to moisture may be a pliant 
plastic, such as a vinyl plastic. The exposed face of the circular island 
16, together with the exposed face of the circular terminal portion 20 are 
then contacted by a circularly formed sponge 32. The sponge 32 is larger 
in diameter than the island 16 and smaller in diameter than the disc 28. 
The sponge 32 is retained in position by a circularly formed flexible, 
resilient and nonconductive foam plastic sheet or pad 34 pressed against 
the adhesive layer 30 so as to sandwich the sponge 32 and the signal 
conductor 10 between the disc 28 and the pad 34. 
The pad 34 can be seen to have an adhesive layer 36 on one face thereof and 
a centrally located aperture 38 which is smaller in diameter than the 
sponge 32. Accordingly, as the pad 34 is pressed against the disc 28, the 
outer margin of the sponge 32 is compressed while the central portion of 
the sponge 32 is displaced outwardly through the aperture 38 so as to 
produce a pad portion 40 projecting outwardly from the adhesive coated 
face of the foam plastic pad 34. 
It will be noted that as the pad 34 is pressed firmly against the adhesive 
layer 30 contained on the disc 28, the signal conductor 10 is securely 
positioned by reason of the back surface of the substrate 12 being 
adhesively engaged to the adhesive layer 30 of the disc 28 and by reason 
of the retention of the underlying pad 34 by such adhesive layer. 
The thus mounted sponge 32 is next saturated with any suitable electrolyte, 
it being convenient for this purpose to introduce the electrolyte into the 
sponge 28 with aid of an injection device, such as a hypodermic needle. 
The electrode device as thus far described is illustrated in full assembly 
in FIG. 2 where the assembled device has been given the reference number 
42. While not illustrated, the assembled device may be protected for 
purposes of storage and shipment by a protective cover of a type sized to 
underlie the entire adhesive coated area of the pad 34 and of the type 
containing a recessed central portion sized to accommodate the outward 
projection of the pad portion 40. Such protective cover would, of course, 
be provided with a surface engaging the adhesive layer 36 which is of a 
type readily stripped from the adhesive layer 36. 
The electrode device as described contains a circular terminal portion 20 
which comprises a silver paint in intimate contact with the electrolyte 
loaded sponge 32. As understood by those skilled in the art the silver 
located at the surface of the silver paint will intially have a reaction 
to the electrolyte contacted thereto such that should the electrode then 
be connected to monitoring equipment an erratic signal will appear. In 
time, however, the silver contacted by the electrolyte will become 
passivated with respect to that electrolyte and a stable signal will 
thereafter appear on the monitor. If, however, the electrolyte should 
migrate to a new and thus unpassivated area of the silver paint, the 
signal then being monitored would again become erratic until passivation 
was again accomplished. In order to protect the electrode device from such 
signal aberration, the present invention contemplates the provision of 
means restricting the areas of the silver paint which are permitted to be 
contacted by electrolyte. Thus FIG. 1 displays a protective barrier layer 
26 overlying a portion of the finger 22 adjacent the terminal portion 20. 
The preferred layer 26 comprises a nonconductive plastic such as an 
acrylate plastic painted over the finger 22 for an appreciable area as 
shown in FIG. 1. Alternatively, the indicated area may be covered with a 
pressure sensitive adhesive capable of adhering to the confronting face of 
the pad 34 so as to retard electrolyte migration. As a further alternative 
any nonconductive material capable of forming a dam across the finger 22 
adjacent the terminal portion 20 may be used to effectively retard 
electrolyte migration. 
The barrier layer 26 can be recognized as a device which divides the silver 
painted layer 18 into a first exposed portion for receiving a signal to be 
monitored by ionic conduction through the electrolyte and a second exposed 
portion for relaying the signal by electronic conduction for application 
to an external circuit, not shown. The first exposed portion comprises 
primarily the island 16, and, while it should be understood that all areas 
of the layer 18 participate in electronic conduction, the second exposed 
portion comprises the outward end 24 of the finger 22 where isolated from 
contact with the electrolyte. This outward end 24 provides a surface 
engageable by any electronic conduction means, not shown, which may be 
used to relay the signal remotely to the external circuit without regard 
to the nature of such electronic conduction means. Thus, precautions have 
been taken to retard access of the electrolyte to the outer end of the 
finger 22 and accordingly little opportunity exists for the development of 
signal artifacts of the type that could result from contact of the 
electrolyte to dissimilar metals. 
Since whatever metal that might be used for remote transmission of the 
signal being monitored is protected from contact with electrolyte the 
signal relay means may be reusable and thus may be a permanent component 
of the external circuit, whereas the electrode device 42 is conveniently 
disposable. 
An advantage to the construction of the present electrode is that the outer 
end of the flexible stick portion 14 can be attached to whatever signal 
relay means is to be employed for remote transmission without inducing any 
pressure on the patient to whom the electrode may have been attached. 
Furthermore, the stick portion 14, because flexible, will not effectively 
transmit external forces to which the stick portion may be exposed to the 
terminal portion 20 and thus forces acting on the outer end of the stick 
portion have only a minimal affect on the interface between the 
electrolyte sponge 32 and the electrically conductive surface layer 18. 
An important attribute of the present invention is that the layer 18, which 
has been described as produced by silk screening silver paint, together 
with the substrate 12 can be rendered sufficiently transparent to X-ray 
radiation that the presence of the electrode 42 on a portion of the body, 
such as the chest, will not be discernable when the subject being 
monitored is also exposed to the X-ray radiation normally used to produce 
a chest X-ray photograph. In the practice of the present invention, chest 
X-ray transparency is attained if the substrate 12 comprises a layer of 
polyethylene terphthalate, such as sold under the Trademark Mylar, which 
is in the range of 1/2 mil (0.00127 cm) to 20 mils (0.0508 cm) in 
thickness and the silver paint layer applied to such substrate is in the 
range of 1/2 mil (0.00127 cm) to 6 mils (0.0152 cm) thick. Accordingly the 
combined thickness of the substrate 12 and the conductive surface layer 18 
can range between 1 mil (0.00254 cm) and 26 mils (0.0660 cm). 
The above indicated thickness range for Mylar represents the range of 
thicknesses in which Mylar is thought to be commercially available. As a 
practical matter, however, the thickness of the Mylar layer appears to be 
unimportant because the Mylar thickness is always small in relation to the 
amount of tissue also interposed in the path of the chest X-ray radiation 
even in the case of neonatal chest X-rays. Likewise, the lower thickness 
in the range recited for the silver paint represents the minimum thickness 
of silver paint that can be silk screened with commercially available 
equipment, and the largest thickness in the recited range is merely the 
maximum thickness of silver paint that can be silk screened with 
commercially available equipment. Thus it is thought that thicker layers 
of Mylar and/or silver paint may be employed in an X-ray procedure without 
interference to the intended use of the resultant X-ray photograph. 
It has been reported that X-ray transparency obtained through use of a 
vacuum deposition of silver proves to be undesirable because vacuum 
deposited silver is not securely retained by a substrate such as Mylar. In 
contrast to such report, a silver paint, such as the aforementioned DuPont 
Conductor Composition No. 9793, is found to provide ample adhesion between 
the silver paint and the described Mylar substrate. Additionally, the 
silver paint provides the obvious convenience and economy of a painting 
operation as compared to a vacuum deposition technique. 
While silver paint has been described as the preferred signal conducting 
means for use in the present invention, those skilled in the art will 
appreciate that other paints such as, for example, a conductive gold 
paint, may be employed in lieu of the silver paint. However, the 
experience to date with gold paint as well as paints produced with other 
conductive metals is that an electrocardiograph trace recorded while using 
silver as the conductive metal adhered by painting to the signal conductor 
element 10 produces a trace of greater fidelity. 
The modified electrode 50 illustrated in FIGS. 3, 4 and 5 is superficially 
quite similar to the electrode 42, but a number of improvements embodied 
in the electrode 50 warrant a thorough description of this modification. 
The modification includes a signal conductor 52 comprising a substrate 54 
supporting a paintable layer 60 of a conductive paint which is adhered to 
one face of the substrate. The substrate 54 which may be similar in shape 
to the previously described substrate 12 is again a dimensionally stable 
plastic sheet such as a sheet of polyethylene terphthalate. The substrate 
is shaped as by stamping to have a circular island 56 located at one end 
of a relatively narrow and elongate stick portion 58. The electrically 
conductive paintable layer 60 is shaped to have a circular terminal 
portion 62 adhered to the aforementioned island 56 and is shaped to have 
an elongated finger portion 64 extending integrally outward from the 
terminal portion 62. The finger portion 64 extends substantially the 
entire length of the stick portion 58 and it can be noted that the 
outermost end of the stick portion has been folded back on itself to 
produce an exposed conductive tab 68. In particular it can be noted in 
FIG. 4 that the conductive paint is located on the face of the major 
length of the stick portion 58 which is visible in FIG. 4 and when the 
stick portion is folded to form the tab 68, the conductive paint surface 
70 on the tab 68 is placed behind the stick portion 58 so as to be exposed 
at the opposite side of the stick portion 58. The conductive tab 68 
together with that part of the finger portion 64 over which the conductive 
tab has been folded are through perforated to form an aperture 76 for 
receipt of an eyelet 74 which is pressed into a stud 72. Since, as will be 
explained, the stud 72 is protected from electrolyte contact, this stud 
may be a conventional metallic snap fastener part. It is preferred, 
however, that the eyelet 74 be a non-metallic piece which may be molded 
from a plastic material such as polyethylene or polypropylene. Thus the 
stud 72 which is electrically conductive bears against the exposed 
electrically conductive paint surface 70 residing on the tab 68. On the 
other hand the eyelet 74, being a nonconductive plastic, lays harmlessly 
adjacent the conductive paint extending along the finger portion 64. 
The face of the substrate 54 which is opposite the conductive paint layer 
60 is pressed against an adhesive layer 80 applied to a pliant and 
relatively impermeable protective cover 78. The cover 78 which may be a 
vinyl plastic is generally circular in shape but departs from true 
circularity by reason of the presence of an outward projection 82 at the 
margin thereof. When the conductor member 52 is pressed against the 
adhesive layer 80 present on the cover 78, the conductor member is aligned 
lengthwise with the outward projection 82. As is evident the length of the 
outward projection 82 allows the conductor member 52 to be pressed into 
position against the cover 78 without the adhesive present on the 
projection 82 contacting the exposed surface 70 of the tab 68. 
As was the case with the electrode 42, it is preferred that the paintable 
layer 60 comprise a silver paint and, to minimize signal artifacts, it is 
preferable that the finger portion 64 be so protected as to prevent a 
migration of electrolyte to unpassivated areas of the layer 60 capable of 
reacting chemically with the electrolyte. Thus as illustrated in FIG. 4 a 
substantial length of the finger portion 64 has been protected with a 
barrier layer 66. This barrier layer may comprise a coating of a 
nonconductive plastic material such as acrylate plastic or simply an 
adhesive layer capable of adhering to an annulus 86 to be applied over the 
conductor member 52 as will be more fully described. It is preferred that, 
whatever device may be used to form the barrier layer 66, the barrier 
layer extend from substantially the juncture between the finger portion 64 
and the terminal portion 62 to substantially the fold which places the tab 
68 under the stud 72. The nonconductivity of the barrier layer 66 is 
important for two purposes. One purpose, as already described, is to 
preclude a migration of electrolyte to unprotected areas of the conductive 
paint layer 60. The second purpose is to avoid artifact signals 
attributable to contact of the conductive paint layer 60 by the skin of a 
patient. Accordingly, the exposed face of the finger portion 64 appearing 
in FIG. 4 is coated with an insulating layer including the barrier layer 
66 and the nonconductive eyelet 74, such insulation extending from 
substantially the terminal portion 62 to the fold at the tab 68. Of course 
the conductive stud 72 remains exposed but has minimal access to the 
patient's skin as will be more fully explained. 
Upon completion of the barrier layer 66 an initially dry sponge 84 whose 
diameter noticeably exceeds the diameter of the island 56 is pressed 
against the island in such a fashion that the outer peripheral portions of 
the sponge adhere to the adhesive layer 80 present on the cover 78. The 
sponge 84 is then saturated with an electrolyte. Surrounding the sponge 84 
is an annulus 86 comprising a surgical tape formed of any suitable porous 
and thus breathable material. The annulus 86 has a central aperture 88 of 
a diameter adequate to surround without contacting the circular sponge 84. 
The annulus 86 is adhered at one face thereof to the adhesive layer 80 
present on the cover 78 and is provided at the opposite face thereof with 
a pressure sensitive adhesive layer 90 for attachment to the skin of a 
patient. FIG. 3 illustrates the side of the resulting electrode 50 which 
will face away from the skin when attached to a patient, and as evident 
the stud 72 is positioned to project away from the skin of the patient 
with minimal possibility of contact to the skin. 
Those skilled in the art will appreciate that during periods of storage and 
use of the electrode 50, electrolyte applied to the sponge 84 may migrate 
so as to reach the interface between the annulus 86 and the barrier layer 
66 with the consequence that capillary action may advance the electrolyte 
outwardly in the general direction of the stud 72. Those skilled in the 
art will further appreciate, however, that any such capillary action will 
be of limited effect since any outward movement of the electrolyte will 
terminate when the electrolyte reaches the outer periphery of the annulus 
86. Thus, even though a minor amount of capillary action might occur, the 
extent of action is limited by the dimensions of the annulus 86 with the 
consequence that only a minor proportion of electrolyte will be able to 
migrate away from the electrolyte sponge 84. 
As suggested in FIG. 3, connection of the electrode 50 to signal monitoring 
equipment may be conveniently made by attaching a conventional snap 
fastener connector 92 to the snap fastener stud 72. 
Although the preferred embodiments of this invention have been described, 
it will be understood that various changes may be made within the scope of 
the appended claims.