Note: Descriptions are shown in the official language in which they were submitted.
~ ~573~
CONDUCTIVE AD~lESIVE AND BIOMEDICAI, ELECTRODE
Field of the Invention
This invention relates to the fleld of conductive
adhesives, particularly those used in biomedical electrodes
to establish an electrlcal connection between the skln of
the human anatomy and an electromedical apparatus, such
as a high impedance electromyograph, electrocardiograph,
electrical neurostimulator for pain relief, and the llke.
More particularly, it relates to conductive adhesives for
use in so-called "dry" bioelectrodes which do not require
the use of messy creams or gels to enhance conductivity
between the skin and the electrode plate.
Background Art
Copending Canadian Patent Application Serial No.
367,329 filed December 22, 1980, inventor Michael R. Engel,
disclosed a conductive adhesive for biomedical electrode
applications made by an improved solventless process. The
adhesive is synthetic, dermally-nonirritating, conformable,
cohesive, ionic and hydrophillic. The process by which the
electrode is made involves the steps of: (1) forming an
adhesive precursor comprising (a) a water-soluble polyhydric
alcohol which is liquid at room temperature, (b) an ionic
unsaturated free-radically polymerizable material which
is soluble in the polydric alcohol, (c) a free radical initiator
which is soluble in the polyhydric alcohol, and (d) a multi-
funtional unsaturated free radically polymerizable cross-linking
agent; (2) coating the adhesive precursor on one side of
an electrode plate (conductive sensing element); and (3)
polymerizing the coated precursor in situ.
In the preferred embodiment of the conductive
adhesive of the aforementioned disclosure, the ionic monomer
is acrylic acid neutralized with an inorganic base such
as potassium hydroxide.
The conductive adhesives of my previous dis-
closure are especially useful in electrosurgical grounding
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plate electrodes. They offer significant advantages over
pr~or art conductive adhesives such as those described by
Berg in U.S. Patent No. 4,066,078. Berg discloses two classes
of conductive adhesives plasticlzed with a polyhydric alcohol,
viz., (1) polymers or copolymers derived from the polymerlzation
of an ester of an olefinically unsaturated carboxylic ester
and an alcohol havlng a quarternary ammonium group, and
(2) sulfated cellulose esters. The processes by which these
adhesives are formed into electrodes are much more tedlous
and expensive than those described in my previous disclosure
and do not result in as good overall adhesive properties.
The conductive adhesives of my previous disclosure are also
an improvement over those specifically described in U.S.
Patent No. 4,352,359 to Larimore et al. The Larimore conductive
lS adhesives may be formed from similar ionic monomers, but
a more expensive process is used and no crosslinked polymers
are disclosed. Crosslinking allows for higher amounts of
polyhydric alcohol without reducing viscosity below acceptable
levels. A higher polyhydric alcohol level enhances
hydrophilicity, thereby improving electrical conductivity.
Although the conductive adhesives of my previous
disclosure provide significant improvements over the prior
art, particularly when used in grounding plate electrodes,
one problem has been encountered with their use in ECG
electrodes. Electrodes utilizing my prior conductive adhesives
do not recover satisfactorily following a defibrillation
overload when used in disposable ECG electroces. Their polariza-
tion potential is too high to meet standards proposed by
the Association for the Advancement of Medical Instrumentation
(AAMI).
I have now discovered that by the addition of
ionic salts, preferably those con-taining a halide ion, to
the electrically-conductive adnesives of my prior disclosure,
I am able to produce non-polarizing electrodes.
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Since the added salts provide the conductivity needed for good
electrical performance, inclusion of an ionic monomer in the
adhesive precursor is no longer necessary.
Description of the Invention
Accordin~ to the present invention, there is provided an
essentially dry disposable biomedical electrode comprising an
electrode plate or sensing element having a top surface and a
bottom, skin-directed surface. The electrode plate has a means for
electrical connection to a lead wire of an electro-medical device.
The bottom surface of the electrode plate is coated with a swell-
able, non-water-soluble, eonformable, cohesive, dermally-non-
irritating, hydrophilie eonductive material for enhancing the
electrical connection with the skin. The conductive material is
formed from an essentially solventless process ln situ on the
electrode plate or a transfer sheet. The proeess involves first
forming an adhesive precursor comprising (1) a water-soluble
poiyhydric alcohol which is a liquid at about 20C; (2) at least
one non-ionic unsaturated free radically polymerizable material
soluble in said polyhydric alcohol; (3) a free radical initiator
soluble in said polyhydric alcohol; (4) a crosslinking agent of a
multifunctional unsaturated free radically polymerizable material
soluble in said polyhydric alcohol; and (5) a non-polymerizable
ionizable salt in an amount effective to render said adhesive pro-
duct electrically conductive.
The non-ionic polymerizable material may comprise one
non-ionic monomer or a mixture of non-ionic monomers. It is also
contemplated that ionic polymerizable materials which are soluble
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in the polyhydric alcohol may be included in the precursor without
departing from the spirit of the invention.
The adhesive precursor is coated directly onto
one surface of the electrode plate or onto a releasable
transfer surface. The precursor is polymerized ln situ,
preferably by ultraviolet radiation. The cured conductive
--4--
layer is then ready for use. If pol~merization occurs on a
transfer surface, the adhesive layer is stripped from the
transfer surface and applied to the e]ectrode plate. An
ECG electrode containing the conductive adhesive of tha
invention should exhibi~ a "Polarization Potential" (as
hereinafter defined~ which does not exceed 100 millivolts.
The term l'solventless" is used herein to mean
that there are substantially no materials present in the
adhesive precursor which are not present in the final
composition of the electrically conductive adhesive~
Stated another way, when the polymerization of the
precursor is complete, and the adhesive is ready for use,
at least 99% of the starting materials are still present.
The term "hydrophilic" is used herein to mean the
conductive adhesive will absorb some water.
The term l'conformable" as used herein refers
generally to the compliance of the conductive material. It
must be suficiently compliant to conform to the surface of
the skin beneath the electrode plate to provide a high
surEace area of contact between the skin and the electrode
plate.
The term "cohesive" refers to the internal
integrity of the conductive material. Generally, the
conductive material is film-forming and must be more
cohesive than adhesive to the skin so that, when the
electrode is removed from the skin, the conductive layer
remains intact and does not leave an objectionable residue.
The term "swellable" refers to the imbibing of
~ solvPnts by the polymer matrix wi-th a concomitant increase
in the volume of the polymer matrix.
The term "dermally non~irritating" means that the
conductive adhesive can be used safely on mammalian skin
without causiny an unacceptable amount of irritation or
other toxic side effects.
The electrically conductive material is darived
from the essentially solventless process of polymerizing
the adhesive pracursor of which one component is the
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water-soluble polyhydric alcohol. The term "polyhydric
alcohol" as used herein refers to a compound or polymer
having more than one hydroxyl yroup. rrhe polyhydric
alcohol is water soluble and a li~luid at room temperature,
e.y., approximately 20C. The polyhydric alcohol is
present in the precursor in amounts oE ~rom 10 to 9~ parts
per weight of the precursor, with 50 to about 70 be~ng
pre~erred. Examples o~ useful polyhydric alcohols are
propylene g:Lycol; 1,2,4 butane triol; polyethyleneoxide
(e.g., "Carbowax" 400); and glycerol, with the latter being
preferred. One skilled in the art will recognize that
polyhydric alcohols which are not normally liquid at room
temperature, may be mixed with those that are liquid at
room temperature to form a material which is use-ful
according to the present invention. One skilled in the art
would also recognize that the dihydric alcohol ethylene
glycol may be useful in the present invention, but may
cause dermal reactions which limit its utility.
As stated above, the precursor is also comprised
of at least one non-ionic unsaturated free radically
polymerizable material which is soluble in the polyhydric
alcohol. The total amount of polymerizable material
(including any ionic monomers, if present) in the precursor
will generally range from about 20 to 30, preferably about
25 to 28, parts by weight of the precursor. The type of
non-ionic polymerizable material used is not critical so
long as it provides the desired performance properties in
the cured and crosslinked state, e.g., conformability,
tackiness, cohesiveness, dermal nonirritability, etc.
Examples of useful non-ionic free radically polymerizable
monomers which are soluble in the polyhydric alcohol are
acrylic acid, methacrylic acid, hydroxyethyl methacrylate,
and N-vinyl pyrrolidone. The most preferred performance
properties for ECG electrodes are provided by acrylic acid
in an arnount between about 25 and 28 parts by weight oE the
precursor.
,-,,~,k
31~j
The precursor is further comprised of 0.1 to 5 parts by
weight per 100 parts of the unsaturated material of a crosslinking
agent of a multiEunctional unsaturated Eree radically polymerizable
material. Examples are triethylene-glycol-bis-methacrylate,
ethyleneglycol-bis-methacrylate, bisacrylamide, and triethylene-
glycol-bis-acrylate with the former heing preferred in amounts
about 0.15 to about 1.5 parts by weight per 100 parts of the
unsa-turated material.
The initiation of polymerization of the precursor is
facilitated by the presence of at least 0.1 part by weight per
100 parts of the unsaturated material of a Eree radical initiator
which is soluble in the polyhydric alcohol. The initiator may be
of the thermal or photo class. The actual selection is dependent
on the monomers and the polyhydric alcohol. An example of useful
thermal initiators are benzoyl peroxide, azobisisobutyronitrile,
di-t-butyl peroxide and cumyl peroxide. Examples of useful photo-
initiators are disclosed in the article Photoinitiators - An Over-
view by G. Berner et al in the Journal of Radiation Curing (April
1979), pp. 2 through 9. The preferred photoinitiator is benzil-
dimethylketal.
The electrical conductivity of the adhesives of theinvention is provided by the addition of an effective amount of a
non-polymerizable ionizable salt to the adhesive precursor. The
salt is preferrably presen-t in an amount ranging frorn about 0.1 to
5.5 percent by weigh-t of the precursor. Any organic or inorganic
ionizable salt may be used which provides the necessary electrical
conductivity, is dermally-nonirritating, does not interfere with
36
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the physical proper-ties of the adhesive and, in the case of ~CG
electrodes, allows the electrode to exhibit -the required Polariz-
ation Potential, i.e., recover rapidly followincJ defibrillation
overload. It has been found that ionizable salts containinc3
halide ions provide the best overall results, especially inorganic
halide salts containing chloride and bromide ions. Particularly
preferred are inorganic chloride salts such as
7~
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potassium chloride. The most preferred adhesive precursor
contains potassium chloride in a concentration between 3.5
and 4.5 percent by weiyht oE the precursor.
In some cases, it may be necessary to add a small
amount of water, e.g., 10 percent by weight or less, to the
precursor to ensure complete solubili~ation oE the salt.
The water becomes part oE the conductive adhesive coating
and may evaporate to some extent if the electrode is used
or stored in environments of low humidity and/or high
temperature. However, under normal conditions, the small
water content appears to have little or no negative effect
on the performance of the electrode.
It will be recognized by one skilled in the art
that other additives (e.g., tackifers, such as polyacrylic
acid) may be added to the precursorO In fact, the
preferred precursor contains about 4.0 pereent by weight of
polyaerylie aeid to inerease tackiness.
The essentially solventless precursor can be
coated on to the electrode plate or transfer sheet and,
depending on the free radieal initiator, exposed to either
heat or aetinie radiation whieh results in the Eormation of
an eleetrieally eonduetive pressure-sensitive adhesive.
The preeursor may also be exposed to eleetron beam
radiation to facilitate the crosslinking.
Description of the Drawings
A better understanding of the invention will be
facilitated by reference to the aeeompanying drawings
wherein:
Figure 1 is an exploded seetional view of a
disposable ECG electrode containing the conductive
adhesive of the invention;
Figure 2 is a top plan view of an alternative
embodiment of the ECG eleetrode of Figure l;
Figure 3 is an exploded seetional view of the
eleetrode of Figure 2; and
Figure 4 is a eircuit used in the defibrillation
3~i
overload recovery test (described in example 1 below).
Referring to Figure 1, a disposable ECG
electrode 10 is illustrated in which the electrode plate 12
is provided by a circular piece of nonwoven web 14 approxi-
mately 1 3/16 inches in dialneter which has ~)een vaporcoated with sllver 16 on its lower surface. Electrode
plate 12 is co~nected to an electrocardiograph (not shown)
by means of a standar-l stud/eyelet connector. In the
embodiment il~ustrated, stud 18 is made of stainless steel
and eyelet 18 is formed of plastic (injection molded ABS)
having a conventional silver/silver chloride coatin~.
Conductive adhesive layer 22, approximately 28 I-nils thick,
covers the lower, skin-directed surface of the electrode
plate 12. A release liner 24 protects the conductive
adhesive prior to use.
The electrode 26 of Figures 2 and 3 colnprises a
circular piece of standard pressure-sensitive adhesive tape
28 such as Micropore~ brand tape sold by the 3M Company,
Saint Paul, Minnesota. Adhesive tape 28 is laminated to a
disc of tin foil 30 approximately 1.7 mils thick and 1 1/4
inches in diameter. Tin foil disc 30 constitutes the
conductive electrode plate of the electrode. Tab 32
extends from tape 28 and tin foil disc 30 to provide a
means for connecting the electrode plate to an electro-
cardiograph by way of any alligator clamp (not shown) orother suitable connector. Tab 32 is reinforced with a
piece of polyethylene 34 (preferrably colored) so as to be
readily visible to the user. Conductive adhesive layer 36,
approximately 28 mils thick, is applied to the lower,
skin-directed surface of tin foil disc 30. Release liner
38 is used to protect the adhesive prior to use.
No elaborate packaging is required for electrodes
according to the invention since they are essentially "dry"
and moisture loss is generally not a problem.
The embodiments ill~strated in the drawings are
merely illustrative. The specific construction of the
electrode is not critical to the invention. The ECG elec-
~5~73~
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.
trodes illustrated are designed to have a low Polarization
Potential in accordance with the standards proposed by
AAMI. It is well ]cnown to those skilled in the art that to
achieve a low Polarization Potential the electrode plate
must ~e selected so dS to be non~polariziny. Silver-silver
chloride or tin electrode plates in combillation witll a
conductive adhesive conta1nillg chloride :ion are pre~erably
used. Other suitable materials include nobel metals, but
they are not practica] on account o~ cost.
The conductive adhesives of the present invention
may be used in biomedical electrodes other than
non-polarizing ECG electrodes, such as electrosurgical
grounding plate electrodes and electrodes Eor trans-
cutaneous electrical nerve stimulation (TENS)o However,
for such other applications, they offer no perceived
advantages over the conductive adhesives described in my
previous disclosure, Serial No. 367,329, filed December 22,
1980. In some cases they may be less desirable, particu-
larly since the presence of an ionizable salt in the
present adhesive may cause some corrosion problems when
used in combination with certain metal electrode plates
such as aluminum.
The invention is further illustrated by reference
to the following non-limiting examples.
Example 1
Powdered polyacrylic acid as the sodium salt (18
grams) (X 739 from B. F. Goodrich Chemical Division,
Cleveland, Ohio) is dissolved in warm distilled water (18
grams) by stirring for one hour, and added to a mixture of
potassium chloride (17.1 grams), distilled water (25.0
grams), acrylic acid (115.0 grams), glycerine (250.0
grams), triethyleneglycol-bis-methacrylic (0.3 grams) and
0.35 grams of Irgacure 651 (a benzildimethylketal from
Ciba-Geigy). The ingredients are mixed for ~ hours in a
glass jar to insure dissolution of all components. During
mixing, the jar is covered with aluminum foil to prevent
--10--
premature polymerization.
This adhesive precursor is knife-coated onto
8-pound tissue paper (from Crystal Tissue Company) which
is layered on 7G-poun-l silicone coated pa~er (froln the
H. P. Smith Company). The resu1ting coating adhesive
thickness is approxilnately 28 mi~s.
The coated substrate is then passed throucJh a
3-foot inert chamber (N2 atmosphere) under a bank of UV
lights consisting of thirty 18-inch "black liyht" tubes for
one minute which results in the polymerization of the
coating.
A 4 rllil web of nonwoven polyester number 760 from
3M Company, Industrial Electrical Products Division was
vaporcoated with 2000A silver. Stainless steel studs and
silver/silver chloride-plated plastic eyelets were crimped
through the silver vaporcoated film. This film was then
hand laminated to the polymerized conductive adhesive and 1
3/16 inch diameter electrodes as illustrated in Figure 1
were produced. These electrodes were allowed to equili-
brate for one day at 50~ relative humidity (R.H.) and 74F.
After equilibrating for one day the samples were tested for
conductivity and defibrillator recovery.
Impedence
Impedence in ohms of the electrode was measured
using a ~odel 4800A Vector Impedence Meter manufactured by
Hewlett Packard, Palo Alto, Calif. Measurements were
conducted in the conventional manner on electrode pairs
connected face-to-face (adhesive-to-adhesive) using a low
level signal suitable for measurements on ECG electrodes.
The impedence of the electrode at 10 Hz was found to be 185
Ohms.
Polarization Potential
The Polarization Potential of the electrode was
determined using the defibrillation overload recovery test
set forth in "AAMI Draft Standard for Pregelled Disposable
3~i
Electrodes", May, 1981, Section 2.2.2.4, Electrical Performance
Standards. The test was conducted as follows uslng the clrcuit
shown in Figure 4:
1. Two electrodes 40 and 42 are connected adheslve-
to-adhesive and connected to the test circuit (~`igure 4)
with switch 44 closed and switches 46 and 48 open.
2. At least 10 seconds are allowed for the capacitor
50 to fully charge to 200 V; switch 44 is then opened.
3. The capacitor 50 is discharged through the
electrode pair by holding switch 46 closed long enough to
discharge the capacitor 50 to less than 2 V. This time should
be no longer than 2 seconds.
4. Switch 48 is closed immediately, and the electrode
pair is connected to the offset measurement system (switch
46 is open).
5. The electrode offset is recorded to the nearest
1 mV, 5 seconds after the closure of switch 48 and every
10 seconds thereafter for the next 30 seconds. The overload
and measurement is repeated three times.
The test circuit of Figure 4 should have the following
characteristics: Resistor 52 has a resistance of 10 kilohms,
and resistor 54 is a 5 watt, 100 ohm resistor. Capacitor
50 has a capacitance of lO~ F. All capacitors and resistors
should be within 90 to 110 percent of the specified values.
The offset recorder input amplifier 56 has a resistance
of 10 megohms and must have an input impedance from 0 to
10 Hz od 10 M, + 10 percent, and a bias current of less
than 200 nA. The error of the voltage-recording equipment
should be no greater than ~ 5 percent of full scale of l00
mV. A 10 mV change must be measurable with an error no yreater
than + 1 mV. For this purpose, the full scale range and
resolution of the recording instrument may be adjusted as
needed.
The tes-t sequence (Steps 1-5) is repeated for
3 electrode pairs. The "Polarization Po-tential" (as used
3~
herein means the potential 15 second after the fourth
pulse) should not exceed 100 m~ volts. The electrode of
this example was found to have a Polarlzatlorl Potent~al
of l8.8 millivolts.
Eollowing the manufacturing and testlng procedures
set forth in Example~ 1, the following electrodes were made.
Example Amount Impedence Polarization
No._ Saltgrams) %(10 Hz) Potential
2 Cacl28.6 1.97500 ohms 19.5mv
3 ICBr17.1 3.851600 7.2
4 NH4CL8.6 1.97770 18.0
NaCl17.1 1.97425 17.8
6 SnC1417.1 1.972900 15.5
7 KCl0.43 0.10850 91.0
8 KCl23.5 5.22240 14.7
9 None -- -- 1150 255.0
When no salt was used and 8 percent potassium hydroxide
was present (according to the disclosure of Canadian Patent
Application Serial No. 367,329 filed December 22, 1980,
referred to on page 1), the impedance was 200 ohms and the
Polarization Potential was 350 millivolts. A combination
of potassium hydroxide and potassium chloride, 2% and 4~,
respectively, brought the impedance down to 130 ohms and
the Polarization Potential to 20 millivolts.
The following examples illustra-te adhesive precursors
containing different nonionic monomers and an assessment
of the adhesive properties (initial thumb tack) of the cured
adhesive.
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E~ample 10
_
~nount
Ingredient(grams) In_tlal Thumb Taek
KCl 5.7
5 Water distilled 8.3
Dissolved polyacrylic 12.0
acid (sodium salt) in
water (50% by weight)
Metllaerylic acid38.3 high taek
10 Irgaeure 651 .L2
Glycerine 83.3
TEGBM .1
Example 11
Amount
Inc3redient(grams) Initial Thumb Taek
-
KCl 5 7
Water distilled 8.3
Dissolved polyaerylie 12.0
acid (sodium salt) in
water (50% by weight)
Hydroxyethyl methacrylate 38.3 low-medium taek
Irgaeure 651 .12
Glyeerine 83.3
TEGBM .1
~xample 12
Amount
Ingredient(grams) Initial Thumb Taek
~Cl 17.1
Water 25.0
30 Dissolved poly aeylie36.0
aeid (sodium salt) in
~ater (50% by weight)
N-vinyl pyrrolidone115.0 high taek
Irgueure 651 .12
35 Glyeerine 250.0
TE~BM .3
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Example 13
Amount
_ Ingredient (grams) Initial Thumb_Tack
KCl 5.7
5 Water 8.3
Dissolved poly acylic acid 12.0
(sodium salt) in water
(50% by wei(3ht)
Acrylic acid 20.0 hiyh tack
Hydroxyethyl methacrylate 18.3
Irgucure 651 .12
Glycerine 83O3
TEGBM .1
_xample 14
Amount
Ingredient (grams) Initial Thumb Tack
KCl 17.1
Water 25.0
Acrylic acid 115.0 high tack
20 Irgacure 651 0.35
Glycerine 250.0
Triethylene glycol 0.3
dimethacrylate
The following examples illustra-te adhesive
precursors containing different polyhydric alcohols and an
assessment of the adhesive properties of the cured
adhesive.
-15-
Example 15
Amount
Ingredient (grams) Initial Thumb Tack
KCl 5.7
5 Water 8.3
Dissolved poly acrylic12.0 medium tack
acid (sodiwn salt) in
water (50~ by weiyht)
Acrylic acid 38.3
Irgacure 651 .12
Propylene glycol 83.3
TEGBM .1
Example 16
Amount
Ingredient (grams) Initial Thumb Tack
KCl 5 7
Water 8.3
Dissolved poly acrylic1200 high taclc
acid (sodium salt~ in
water (50~ by weight)
Acrylic acid 38.3
Irgacure 651 .12
1,2,4 Butanetriol 83~3
TEGBM .1
Example 17
~nount
_ Ingredient (grams? Initial Thumb Tack
KCl 5 7
Water 8.3
30 Dissolved poly acrylic12.0 high tack
acid (sodium salt) in
water (50% by weight)
Acrylic acid 38.3
Irgacure 651 .12
Carbowax~ ~400 poly-83.3
ethylene oxide
TEGBM .1
