Abstract:
An electrically conductive composite is disclosed that includes a dielectric material having a first side and a second side, conductive particles within the dielectric material layer, and a discontinuous layer of a conductive material on a first side of the dielectric layer. The conductive particles are aligned to form a plurality of conductive paths from the first side to the second side of the dielectric material, and each of the conductive paths is formed of at least a plurality of conductive particles. The discontinuous layer includes a plurality of non-mutually connected portions that cover portions of, but not all of, the first side of the dielectric material such that exposed portions of the underlying first side of the dielectric material remain exposed through the discontinuous layer, yet the discontinuous layer facilitates the electronic coupling together of a plurality of the conductive paths from the first side to the second side of the dielectric material.

Description:
BACKGROUND 
       [0001]    The invention generally relates to conductive polymeric and elastomeric materials for use in a wide variety of applications, including without limitation, conductive adhesives, conductive gaskets and conductive films. 
         [0002]    The design of an electrically conductive pressure sensitive adhesive (PSA), for example, has long presented challenges at least because adhesive strength and flexibility generally decrease with increased electrical conductivity. The materials that are typically used (added) to provide good electrical conductivity are generally less flexible and inhibit adhesion. A conventional way to prepare a conductive coating is to fill a polymeric material with conductive particles, e.g., graphite, silver, copper, etc., then coat, dry and cure the polymeric binder. In these cases the conductive particles are in such a concentration that there is a conductive network formed when the particles are each in physical contact with at least one other neighboring particle. In this way, a conductive path is provided through the composite. 
         [0003]    For pressure sensitive adhesives, however, if the particle concentration is high enough to form a network in which particle-to-particle contact is maintained then there is little chance that the polymer (e.g., elastomer) system of the PSA component is present in high enough concentrations to flow out to make surface-to-surface contact between the substrates and an electrode, i.e., act as an adhesive. Conversely, if the PSA component is in sufficient concentration to make sufficient surface contact to the substrate, the PSA would have to interrupt adjacent conductive particles such that particle-to-particle contact is disrupted. 
         [0004]    Another type of electrically conductive PSA includes conductive spherical particles with diameters equal to or greater than the thickness of the PSA. In this fashion the signal or current may be carried along the surface of the particles, thus providing current flow anisotropically in the z dimension of the adhesive. The continuity of the adhesive however, may be compromised. 
         [0005]    Salts, such as sodium or potassium chloride, readily dissolve when in an aqueous medium, and their ions dissociate (separate into positive and negative ions). The dissociated ions may then convey an electrical current or signal. For this reason, salts have long been added to water, which then may be added to polymeric and elastomeric materials, to provide good electrical conductivity. For example, U.S. Pat. No. 6,121,508 discloses a pressure sensitive adhesive hydrogel for use in a biomedical electrode. The gel material is disclosed to include at least water, potassium chloride and polyethylene glycol, and is disclosed to be electrically conductive. U.S. Pat. No. 5,800,685 also discloses an electrically conductive adhesive hydrogel that includes water, salt, an initiator or catalyst and a cross linking agent. The use of such hydrogels however, also generally requires the use of a conductive surface at one side of the hydrogel (away from the patient) that is capable of receiving the ionically conductive charge, such as silver/silver chloride, which is relatively expensive. 
         [0006]    While these hydrogel/adhesives can have good electrically conductive properties, they often have only fair adhesion properties. Another downside is that the electrical conductivity changes with changing water content, such as changes caused by evaporation, requiring that the hydrogels be maintained in a sealed environment prior to use, and then used for a limited period of time only due to evaporation. 
         [0007]    U.S. Pat. No. 7,651,638 discloses a water insensitive alternating current responsive composite that includes a polymeric material and a polar material (such as an organo salt) that is substantially dispersed within the polymeric material. The polymeric material and the polar material are chosen such that they each exhibit a mutual attraction that is substantially the same as the attraction to itself. Because of this, the polar material neither clumps together nor blooms to a surface of the polymeric material, but remains suspended within the polymeric material. This is in contrast to the use of salts in other applications wherein the salt is intended to bloom to the surface (to provide a conductive layer along a surface, e.g., for static discharge). 
         [0008]    U.S. Pat. No. 5,082,595 discloses an electrically conductive pressure sensitive adhesive that includes carbon particles, and the conductive adhesive is disclosed to be prepared by incorporating black filler (carbon) into the pressure sensitive adhesive in such a manner as to impart electrical conductivity, yet have a concentration low enough to avoid adversely affecting the physical properties (such as tack) of the adhesive. In particular, this patent states that a slurry of the carbon black in an organic solvent is formed under mild agitation or stirring in the absence of high shear, so that carbon structures are thereby formed. The mixture may then be introduced into an adhesive. Such a composite, however, may not provide sufficient adhesiveness and conductivity in certain applications. 
         [0009]    Such composites may also include areas with relatively greater or lesser concentrations of structures of carbon black. Certain conductive polymeric and elastomeric materials that include conductive particles in concentration within the polymeric or elastomeric material, may therefore exhibit inconsistent electrical properties over the surface of the material. 
         [0010]    There remains a need therefore, for a composite for use as a conductive polymeric material that provides electrical conductivity without compromising the desired properties of the polymeric material, and further, there is a need for conductive polymeric materials that provide consistent electrical characteristics. 
       SUMMARY 
       [0011]    In accordance with an embodiment, the invention provides an electrically conductive composite that includes a dielectric material layer having a first side and a second side; conductive particles within the dielectric material, and a discontinuous layer of a conductive material on a first side of the dielectric material. The conductive particles are aligned to form a plurality of conductive paths from the first side to the second side of the dielectric material, and each of the conductive paths is formed of at least a plurality of conductive particles. The discontinuous layer of a conductive material includes a plurality of non-mutually connected portions that cover portions of, but not all of, the first side of the dielectric material such that exposed portions of the underlying first side of the dielectric material remain exposed through the discontinuous layer, yet the discontinuous layer facilitates the electronic coupling together of a plurality of the conductive paths from the first side to the second side of the dielectric material. 
         [0012]    In accordance with another embodiment, the invention provides an electrically conductive composite including a dielectric material having a first side and a second side, conductive particles within the dielectric material, and a patterned layer of a conductive material on a first side of the dielectric material. The conductive particles are aligned to form a plurality of conductive paths from the first side to the second side of the dielectric material, and each of the conductive paths is formed of at least a plurality of conductive particles. The patterned layer includes a plurality of mutually connected portions that cover portions of, but not all of, the first side of the dielectric material such that exposed portions of the underlying first side of the dielectric material remain exposed through a plurality of portions of the patterned layer, yet the patterned layer facilitates the electronic coupling together of a plurality of the conductive paths from the first side to the second side of the dielectric material. 
         [0013]    In accordance with a further embodiment, the invention provides an electrically conductive composite that includes a dielectric material having a first side and a second side, conductive particles within the dielectric material, and a layer of a conductive material on a first side of the dielectric layer. The conductive particles are aligned to form a plurality of conductive paths from the first side to the second side of the dielectric material, and each of the conductive paths is formed of at least a plurality of conductive particles. The layer of conductive material has a thickness of less than about 10 mils and includes open portions through which exposed portions of the underlying first side of the dielectric material remain exposed through the open portions of the layer of conductive material, yet the layer of conductive material facilitates the electronic coupling together of a plurality of the conductive paths from the first side to the second side of the dielectric material. 
         [0014]    In accordance with a further embodiment, the invention provides a method of forming an electrically conductive composite. The method includes the steps of providing a dielectric material; dispersing conductive particles within the dielectric material; conducting electrophoresis or dielectrophoresis to cause said conductive particles to become aligned to form a plurality of conductive paths from a first side of the dielectric material to a second side of the dielectric material, each of said conductive paths being formed of at least a plurality of conductive particles; depositing a partial layer of conductive material on a first side of the dielectric layer, said frangible layer including a plurality of mutually conductive portions that cover portions of, but not all of, the first side of the dielectric material such that exposed portions of the underlying first side of the dielectric material remain exposed through a plurality of portions of the partial layer, yet the partial layer facilitates the electronic coupling together of a plurality of the conductive paths from the first side to the second side of the dielectric material; and applying a release layer to the first side of the dielectric material together with the partial layer of conductive material, to provide that the release layer may be later removed, permitting the first side of the dielectric material together with the partial layer of conductive material to be applied to a subject. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The following description may be further understood with reference to the accompanying drawings in which: 
           [0016]      FIG. 1  shows an illustrative diagrammatic view of a composite for use in accordance with an embodiment of the invention prior to electrophoresis; 
           [0017]      FIGS. 2 and 3  show illustrative diagrammatic views of the composite of  FIG. 1  during electrophoresis; 
           [0018]      FIG. 4  shows an illustrative diagrammatic view of the composite of  FIG. 1  following electrophoresis; 
           [0019]      FIG. 5  shows an illustrative diagrammatic view of the composite of  FIG. 4  during analysis of certain of its electrical properties; 
           [0020]      FIG. 6  shows an illustrative diagrammatic view of two composites of  FIG. 4  being combined face to face and include electrodes for coupling to impedance monitoring equipment; 
           [0021]      FIG. 7  shows an illustrative diagrammatic view of a portion of composite in accordance with an embodiment of the present invention; 
           [0022]      FIG. 8  shows an illustrative diagrammatic plan view of a portion of a composite in accordance with an embodiment of the present invention that includes a discontinuous layer of conductive material; 
           [0023]      FIG. 9  shows an illustrative diagrammatic plan view of a portion of a composite in accordance with another embodiment of the present invention that includes a continuous patterned layer of conductive material; 
           [0024]      FIG. 10  shows an illustrative diagrammatic plan view of a portion of a composite in accordance with an embodiment of the present invention that includes a discontinuous layer of conductive material in a striped pattern; 
           [0025]      FIG. 11  shows an illustrative diagrammatic plan view of a portion of a composite in accordance with an embodiment of the present invention that includes a continuous layer of conductive material in a spiral pattern; 
           [0026]      FIG. 12  shows an illustrative diagrammatic plan view of a portion of a composite in accordance with an embodiment of the present invention that includes a continuous layer of conductive material in a random pattern; 
           [0027]      FIG. 13  shows an illustrative diagrammatic plan view of a portion of a composite in accordance with an embodiment of the present invention that includes a layer of conductive material in a logo pattern; 
           [0028]      FIGS. 14A and 14B  show an illustrative diagrammatic view of a composite in accordance with an embodiment of the invention that show a thin film conductive layer being transferred from a carrier to the composite; and 
           [0029]      FIG. 15  shows an illustrative diagrammatic view of a composite in accordance with an embodiment of the invention with a release layer applied to the subject contact surface thereof. 
       
    
    
       [0030]    The drawings are shown for illustrative purposes only. 
       DETAILED DESCRIPTION 
       [0031]    Applicant has discovered that certain conductive materials, such as materials that include conductive paths that are formed by electrophoresis, may exhibit inconsistent electrical properties. For example, a conductive material may include conductive paths that are formed by an electrophoretic process that causes conductive particles (that are initially randomly dispersed within a continuous medium), to align in the presence of an electric field, thereby forming the conductive paths. The continuous medium may or may not include a polar material as described in U.S. Pat. No. 7,651,638, the disclosure of which is hereby incorporated by reference in it entirety. 
         [0032]    If the conductive composite is then employed in an application that requires certain electrical properties in specific areas of the composite, the electrical properties may be inconsistent. Because of this, the formed structure channel density may be less where it is needed and greater where it is not, and the polymeric adhesive density may be less where it is needed and greater where it is not. 
         [0033]    As shown in  FIG. 1 , a composite of the present invention prior to electrophoresis includes conductive materials  10  (e.g., 5% by weight carbon particles) within a dielectric material  12  (e.g., a pressure sensitive adhesive, a heat activatable adhesive or a radiation curable adhesive). As shown in  FIGS. 2 and 3 , when conductive plates  14 ,  16  are positioned on either side of the composite and an electric field (V 1 -V 2 ) is applied, the conductive material  10  naturally find areas where an initial conductive particle becomes attracted to a first inner surface  18  of the composite. As soon as this happens the distance between the initially attached conductive materials and an opposing second inner surface  20  is then shorter than the full distance between the inner surfaces  18 ,  20 . This causes more conductive particles to become attracted to the ends of the thus forming channels, and within a very short time, conductive channels  22   a ,  22   b ,  22   c  are formed across the composite as shown in  FIG. 4 . If an alternating electric field is applied, the particles will alternately build from opposing inner surfaces (which may be referred to as dielectrophoresis). 
         [0034]    When a plate conductors (e.g.,  14 ,  16 ) are employed the placement of the channels is believed to be largely a function of slight variations in the conductors  14 ,  16  that favor an initial particle becoming attracted to a first surface. Once this occurs, the channel locations are thus defined. The use of one or more point conductors, on the other hand, would provide certainty in the placement of the channels. 
         [0035]    Conductive particles, dispersed in a continuous polymeric layer and then acted upon by an electric field may, under certain conditions therefore, agglomerate to form “Z” directional conductive channels, providing electrophoretically formed conductive pathways through a polymeric composite. Once formed, these conductive channels are fairly durable and permit a biomedical electrode (such as an EKG electrode) to pass certain standards for such electrodes as discussed in more detail below. The conductive particles may include any of carbon or metal (including silver and silver nanoparticles). It is also believed that a large number of thermoplastic polymers may be employed in place of the pressure sensitive adhesive when subjected to a sufficiently high or long standing electromagnetic field. Thus heat activated adhesives and even thermally or radiation thermoset adhesive, may also be employed in certain applications. As used herein the term “activation” refers to both electrophoretic (DC voltage applied) and dielectrophoretic (AC voltage applied). 
         [0036]    When a composite that includes randomly located channels is tested for its electrical characteristics, these characteristics (e.g., impedance), may vary for different locations on the composite. For example,  FIG. 5  shows a composite that includes the pressure sensitive adhesive  12  and conductive channels  22   a ,  22   b ,  22   c  wherein impedance testing probes  28  and  30  placed on outer surfaces  24 ,  26  of the composite (and which happen to align with channel  22   a ) would measure a much lower impedance than would impedance testing probes  32 ,  34  (which do not align with any of the conductive channels  22   a ,  22   b ,  22   c ). The results of such impedance tests would be widely varying depending on whether the probes happened to be positioned in alignment with a conductive channel. 
         [0037]    Certain other tests may involve overlaying a first conductive composite on top of another conductive composite and measuring electrical characteristics across both composites combined. As shown in  FIG. 6 , if a first conductive composite including the pressure sensitive adhesive  12  and conductive channels  22   a ,  22   b ,  22   c  were place over a second conductive composite including a pressure sensitive adhesive  12 ′ and conductive channels  22   a ′ and  22   b ′, the channels  22   a ′ and  22   b ′ may not align with any of the channels  22   a ,  22   b  or  22   c . No conductive path would therefore, be provided between test plates  36 ,  38  that are coupled to impedance test sources  40 ,  42 . Even though each of the composites includes conductive channels, because their channels are not aligned, the test would report a high impedance. 
         [0038]    A test that involves combining two such composites (as discussed above with reference to  FIG. 6 ) is commonly used to test the impedance of biomedical electrodes. If the test results in a single impedance value of greater than 3,000 Ohms, the device has failed the test in accordance with AAMI EC12-2000—4.2.2.1. The results of the test may therefore vary considerably (between passing and failing) depending on the variability of whether conductive channels happen to become aligned between the test plates  36 ,  38 . 
         [0039]    In particular, an example of such a biomedical electrode is an EKG electrode where preformed conductive structures from two test electrodes being put through AAMI-EC-12-2000—4.2.2.1 protocol may not have a conductive structure from one of the test electrodes making contact with a conductive structure on the second test electrode. Thus each electrode individually may well work in detecting EKG signals, it may not pass AAMI-EC-12-2000—4.2.2.1, which is a major test in qualifying usable EKG electrodes. 
         [0040]    For example, when EKG electrodes are activated individually (i.e., not in contact with a second electrode during the activation process) and then put face to face to run AAMI-EC-12-2000—4.2.2.1 (test for impedance), the “Z” directional conductive structures formed on each electrode (conductive channels) may not line up to form connected pathways across the two electrodes. No contact between the “Z” directional conductive channels from one electrode and the “Z” directional conductive channels to the second electrode would result in a failure in this qualification test. 
         [0041]    A solution to this problem involves applying a conductive material over a portion of the surface of the preformed or preplaced conductive structure containing adhesive. This surface conductive material would make contact with the preformed conductive structures yielding a larger surface contact area, therefore allowing greater alignment of the conductive structures from the two electrodes to each other thus facilitating the passing of AAMI-EC-12-2000—4.2.2.1. 
         [0042]    As shown in  FIG. 7 , in accordance with an embodiment of the present invention, a conductive layer  50  is applied over a portion of, but not all of, an exposed surface  24  of the composite. As further shown in  FIG. 8 , the conductive layer  50  may be discontinuous, as long as many of the conductive portions each cover a plurality of conductive channels so that a probe positioned on the outer surface will contact some portion of the conductive layer  50   
         [0043]    As shown in  FIG. 9 , the conductive layer may be provided as a continuous pattern that forms a grid  52 , connecting a large number of conductive channels. As shown in  FIG. 10 , the conductive layer may be formed as a plurality of discontinuous patterned shapes such as stripes  54 . As shown in  FIG. 11 , the conductive layer may be formed as a continuous spiral pattern  56 . As shown in  FIG. 12 , the conductive layer may be formed as a continuous random line pattern  58 . As shown in  FIG. 13 , the conductive layer may be formed as a continuous or discontinuous logo pattern  60 . 
         [0044]    The conductive layers  50 ,  52 ,  54 ,  56 ,  58  and  60  may be formed in a variety of ways, for example including but not limited to, printing of a conductive ink, or the application of a frangible film by transfer as discussed below in more detail with regard to  FIGS. 14A and 14B . 
         [0045]    To insure that pre-activated ECG electrodes pass the AAMI Impedance Test therefore, a conductive layer is placed over the conductive channels contacting at least one or more “Z” directional conductive channels and further, making them of such a size so as to increase the probability that at least two such surface conductive layers make contact with one another. 
         [0046]    The composition and application of this conductive layer material may involve the application of a carbon dispersion based coatable or printable material such as FLEXcon&#39;s EXV-216 carbon dispersion sold by FLEXcon Company, Inc. of Spencer, Mass. at a thickness of between about 1 mil and about 10 mil, and preferably between about 2 mil and about 5 mil. This material may be coated either directly by screen, gravure, or flexographic printing etc., directly onto the continuous polymeric layer, or printed to a specific arrangement of either continuous or discrete elements on a carrier with limited adhesion to said coating or ink. The preprinted pattern may then be applied to activated electrode via a transfer mechanism. 
         [0047]    As shown for example, in  FIGS. 14A and 14B , such a conductive layer  62  may be coated or printed onto a carrier substrate  64 . The conductive layer  62  may be a frangible layer that is formed by printing or other deposition technique, or by thin film transfer, and may, for example, include conductive ink or a metal foil. When the carrier substrate is brought into contact with the polymer  12  ( FIG. 14A ), the conductive layer  62  is transferred to the polymer  12  as it has a greater adhesion to the polymer  12 . When the substrate  64  is then separated to from the polymer  12  ( FIG. 14B ), the conductive layer  62  remains with the polymer  12  as it has the greater adhesive to the polymer. In other embodiments, one may apply the conductive ink as a single continuous layer to a releasable substrate and then, using a patterned die, transfer the desired geometric form to the electrode. 
         [0048]    An alternative conductive material from which to construct the conductive layer may be from the numerously available metallic foils or metallic conductive particle inks, metal transfer made from about 2500 Å deposition of aluminum vacuum deposited on a silicone coated 2 mil (50μ) PET film, were also determined to be viable as a conductive bridge material. 
         [0049]    As shown in  FIG. 15 , a composite of the invention that includes a partial layer  70  of conductive material (e.g., a conductive layer  50 ,  52 ,  54 ,  56 ,  58  or  60 ), may be applied to a dielectric material  12  having formed conductive channels  22   a ,  22   b ,  22   c  as discussed above, and a release liner  72  may then be applied to the exposed surface of the dielectric material  12  that includes the partial layer  70 . During use, the release layer  72  may be removed, and the composite may be adhered to a subject (e.g., as a biomedical electrode applied to a patient). 
       Example 1 
       [0050]    To a 2 mil (50μ) polyethylene terephthalate (PET) film was coated a conductive carbon dispersion of FLEXcon&#39;s EXV-216 carbon dispersion at a surface resistance of ˜80 Ω/sq. Over this was coated 2.5 mil (63μ) dispersion of FLEXcon&#39;s V-12 HW with 10% conductive carbon (by weight on total solids). Using the AAMI impedance test protocol discussed above, the “Z” direction impedance was measured and averaged 1300 kΩ. When a ring with a 0.69″ (17.5 mm) O.D. and 0.25″ (6.36 mm) I.D. composed of FLEXcon&#39;s EXV-216, was placed on top of the V-12 HW/Carbon mixture on each of two electrodes and the two electrodes were then placed face to face with the rings making contact against each other and the V-12HW adhesive, the resulting Z direction impedance averaged 775Ω. 
         [0051]    The difference in impedance is believed to be the result of the conductive rings connecting a few, random, occasional, Z directional conductive channels formed naturally by just having conductive particles dispersed in the polymeric component, in this case a PSA. The conductive ring (bridge) makes contact with some of these random conductive channels and provides an X-Y direction component to the connectivity of the pathways resulting in lower net impedance. It should be noted that if the V-12 HW/Carbon mixture, as described above was subjected to electrophoretic aggregation, thus forming many more conductive channels the impedance would be at least 2× lower or around 300Ω or less. 
         [0052]    Another consideration of using a surface conductive layer as a conductive bridge is that it could act as a contact through which the activation process is accomplished. In this scenario the conductive layer would be placed on the surface of the dispersed conductive particle polymer mix, then electrical contact made to the conductive bridge and the “conductive channels” would form between the bridge layer and the second electrical contact. 
         [0053]    Of course, if either the bridge layer or the second electrical contact is constructed with multiple elements, it becomes possible to activate the conductive particle polymer mixture in a selective pattern. This allows unique electrical pathways to be created specific to desired patterns which would form the basis for an electrical “thumbprint”. 
         [0054]    There is a limitation as to the extent to which the area of the bridge material vs. the total area of the continuous polymer layer, at least less than 100%. This becomes quite apparent when the continuous polymer layer is to act as an adhesive. An example of this important limitation can be realized in the case of an EKG electrode. Here sufficient contact area of the PSA to maintain adequate bond to the patient&#39;s skin. Obviously the tackier the PSA, the less surface area would be needed to maintain adhesion, however a stiffer, less conformable device would require more surface area to establish and maintain above. Further other environmental conditions such as dryness temperature at which the bond must be maintained as well as other structural forces, attaching wires for example, must be considered when determining what is the minimum or preferable % bonding area, which in turn directs the maximum surface area of the surface conductive layer. Thus there is not generally applicable hard and fast rule as to the extent of conductive bridge material on the surface of an adhesive layer, the totality of the mechanical, environmental etc. requirements dictate the limit on a specific application. It has also been found that carbon is not the only conductive particulate, silver for one, can form conductive structures by electrophoresis. 
       Example 2 
       [0055]    Silver nanoparticle dispersion, obtained from Chasm Technology Inc., 480 Neponset St., Canton, Mass. 02021, designated, 120727-01, was blended into FLEXcon&#39;s V-95 at ˜0.75% silver by weight to the adhesive polymer dry. Samples were prepared as done for the carbon particle system already described, applied to a EXV-216 conductive carbon coating at 1.3 mil (32.5μ) dry deposition, with a resulting surface resistance of ˜80 Ω/sq. to a 2 mil white PET film. The silver nano-particles &amp; V-95 mix was then coated to produce a 2 mil dry thickness. The adhesive (V-95) nanoparticle dried mix was then applied to a stainless steel plate, which would serve as a ground for the activation stage. To activate, a 10 μF capacitor was charged to 200 VDC and discharged through a 20 KΩ resistor on through the V-95/Silver nanoparticle mixture to ground. 
       Results 
       [0056]    With an average starting impedance, measured at 10 Hz and 20 mV, of 5200 KΩ the samples then yielded an average of 1.4 KΩ after activation. As with the carbon particle, analog structures running through the thickness were observed. 
       Example 3 
       [0057]    A second carbon dispersion was prepared by mixing into FLEXcon&#39;s V-95 PSA˜5% “Black Pearls” carbon black from the Cabot Corporation, Boston, Mass. and ˜7% (on solids) Arquad HTL8-MS, being used here more as a dispersant aid. A 2 mil (50 μm) dry coating was applied to a substrate, 2 mil (50 μm) white PET coated with FLEXcon&#39;s EXV-216 to a surface resistance of 80 Ω/sq. inch. The pre-activation impedance averaged 2.5 mega Ω; post activation the impedance averaged 2.7 KΩ. Activation conditions were the same as for the previously used carbon dispersions (from the test apparatus used in AAMI EC 12-2000—4.2.2.4 Defibrillation Overload Recovery test). 
         [0058]    Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the present invention.