Abstract:
Several embodiments of body-compatible biomedical electrodes suitable for long-term implantation, and substantially free of detection of unwanted artifact signals. The electrodes are useful both for sensing body potentials, and for delivery of stimulating electrical signals. The electrodes can be coupled to implanted telemetry circuits, or connected to external electronic devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority benefit of U.S. Provisional Application 60/516,694 filed Nov. 3, 2003, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     There exists a need for biomedical electrodes which are suitable for long-term implantation, and for either stimulation of brain, cardiac or muscle function, or for signal sensing to enable monitoring or recording of neurological electrical signals and the like. For example, monitoring of brain electrical activity (electroencephalogram or “EEG”), or muscles (electromyogram or “EMG,” and electrocardiogram or “ECG”). An immediate application relates to such studies in animal experimentation.  
         [0003]     The several electrode configurations of this invention are significant improvements with respect both to materials chosen for long-term implantation without tissue erosion, inflammation, or infection, and to elimination of spurious electrical signals by isolation of the electrode from interfering biopotential signals. The electrodes are bidirectional in that they are useful for either sensing biopotentials, or for delivering stimulating signals. The electrodes can be used with implanted electronics and telemetry transmitters, or by connection (through a transcutaneous skin exit) to external signal-conditioning and recording equipment.  
       SUMMARY OF THE INVENTION  
       [0004]     A chronically implantable biomedical electrode assembly, useful for delivering stimulating electrical signals, or for detecting tissue or muscle potentials. The assembly is constructed of body-compatible materials, and is substantially free of detection of unwanted artifact signals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a side sectional view of a first electrode assembly for monitoring in contact with dura mater tissue of the brain;  
         [0006]      FIG. 2  is a top view of the first assembly;  
         [0007]      FIG. 3  is a bottom view of the first assembly;  
         [0008]      FIG. 4  is a side sectional elevation of a second electrode assembly for deep-brain positioning of the electrode;  
         [0009]      FIG. 5  is a sectional side elevation of a third electrode assembly similar to the second assembly, but having multiple electrodes, and an optional flexible electrode-supporting shaft;  
         [0010]      FIG. 6  is a partial sectional elevation of a skull and brain with implanted first and third electrode assemblies;  
         [0011]      FIG. 7  is a side sectional elevation of a fourth electrode assembly for muscle implantation;  
         [0012]      FIG. 8  is a bottom view of the fourth assembly;  
         [0013]      FIG. 9  is a side sectional elevation of a fifth electrode assembly similar to the fifth assembly, but having multiple electrodes;  
         [0014]      FIG. 10  is a bottom view of the fifth assembly; and  
         [0015]      FIG. 11  is a side sectional view of the fifth electrode assembly as implanted between two muscle layers.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     A first electrode assembly  10  is shown in  FIGS. 1-3 , and is especially suitable for implantation beneath the skull for sensing electrical EEG activity in a specific area of the brain&#39;s dura mater, or for delivering electrical signals to such area. The assembly has a circular button-like base  11  with a downwardly extending central tubular section  12  with a central opening  13 . An undersurface  14  of the base is flat, and the base upper surface has a central flat section  15 , and a downwardly tapered side section  16 . A central opening  18  extends downwardly, and is tapered outwardly to a flat bottom surface  19  surrounding an upper end of opening  13 .  
         [0017]     A pair of recessed bone-screw openings  20  extend through the base, and are spaced apart 180 degrees on opposite sides of central opening  18 . A tubular opening  21  extends from a side edge of the base into central opening  18 , and a pair of circular passages  22  are formed through the base on opposite sides of and adjacent the outer end of tubular opening  21 . Base  11  is made of a nonconductive tissue-compatible rigid plastic such as an acetal-resin polymer marketed under the trademark Delrin®.  
         [0018]     A conductive electrode  24  has an enlarged circular head  25 , with a downwardly extending pin  26  making a press fit in base central opening  13 . A rounded lower end  27  of the pin extends beneath the lower end of tubular section  12 . The electrode is made of a conductive and tissue-compatible nontarnishing metal such as type Ti6A14V titanium.  
         [0019]     A lead wire  29  with biocompatible shielding, and for either external connection, or to connection with implanted circuitry, is fitted into tubular opening  21  to extend into central opening  18 . A short folded section of annealed nickel ribbon  30  is welded to the top of electrode head  25 , and soldered to a stripped inner end of the lead wire. Opening  18  is then filled with an epoxy material  31  (type 6203FF is suitable) to be level with flat upper surface  15  of the base. The junction of the lead wire at the inlet of opening  21  is stabilized and sealed with a layer of RTV sealant  32  (available from Dow Corning) applied over slight recesses of the upper and lower edges of side section  16  adjacent the inlet. The RTV sealant penetrates and fills passages  22  to form a secure bond.  
         [0020]     Assembly  10  is compact, and base  11  typically has an outside diameter of about one-half inch, and electrode  24  has an overall axial length of about 0.2 inch. The diameter of electrode pin  26  is about 0.04 inch. Lead wire  29  preferably uses a stainless-steel conductor, and biocompatible insulated wires of this type are available from Cooner Wire in Chatsworth, Calif.  
         [0021]      FIG. 6  illustrates implanted electrode assembly  10  as positioned beneath scalp  33  and underlying muscle layer  34 , and with underside  14  of base  11  fitted against skull  35 . Tubular section  12  is fitted into a drilled passage  36  through the skull to place electrode lower end  27  against dura mater  37  of brain  38 . The electrode assembly is secured against the skull by a pair of self-tapping titanium (type TiGAL7Nb is suitable) bone screws  39 .  
         [0022]      FIG. 4  shows a second embodiment of an electrode assembly  42  for deep-brain implantation. Assembly  42  has a base  43  which corresponds to base  11  described above. An elongated rigid plastic tube  44  is fitted into the base central tubular section, and has a plastic collar  45  (Delrino plastic is again suitable) secured at its upper end, the collar resting on the flat bottom surface of the base upper-central opening. Tube  44  is made of a biocompatible material such as polysulfone, polyetheretherketone, or Delrin® plastic.  
         [0023]     A tapered titanium electrode  46  is press fitted into the bottom of tube  44 , and is secured (by a welded and soldered nickel ribbon as described above) to the stripped end of a shielded lead wire  47  extending through base  43  and downwardly through tube  44 . The upper and lower ends of the tube are filled with RTV sealant  48 .  
         [0024]      FIG. 5  shows a third electrode assembly  50  which is similar to assembly  42 , but features multiple electrodes, and an optional flexible plastic tube  51  which may be favored for certain types of deep-brain implantations. Three coiled and shielded independent lead wires  52  surrounded by silicone tubing  53  are fed through a base  54  (corresponding to bases  11  and  43  as described above), and to extend downwardly into tube  51 .  
         [0025]     One of the lead wires is stripped, and welded/soldered as already described through an opening  56  in the sidewall of tube  51  to a titanium ring electrode  57  press fitted over the tube. A second lead wire is similarly secured to a second titanium ring electrode  58  spaced further down the tube. A third lead wire extends to the bottom of the tube for welded/soldered attachment to a tapered titanium tip electrode  59  corresponding to electrode  46  of assembly  42 . The tube interior spaces adjacent the lead wire and electrode interfaces are again filled with an RTV sealant.  
         [0026]     Referring again to  FIG. 6 , electrode assembly  50  (this time with a straight and rigid plastic tube supporting the electrodes) is secured at its base to skull  35  by a pair of titanium bone screws  60 . The base tubular section and electrode-supporting tube extend through a drilled skull passage  61  to position the ring and tip electrodes at various levels of the brain.  
         [0027]      FIGS. 7 and 8  show a fourth electrode assembly  64  for muscle stimulation, or to detect electromyogram signals. The assembly has a base  65  similar to those described above, but having an oval shape in plan view ( FIG. 8 ). A shielded lead wire  66  extends through a tubular passage  67  in the base to a base upper-central opening  68 . A titanium electrode  59  is seated in opening  68 , and a rounded electrode tip extends slightly below the undersurface of the base. A stripped inner end of the lead wire is soldered to a nickel ribbon which is welded to the electrode head as already described. The upper part of opening  68  is filled above the electrode head with epoxy material  70 , and the lead wire is paired into the housing by RTV sealant  71 , again as described above. A pair of holes  72  through the base on opposite sides of the electrode are provided to enable sutured attachment of the assembly to muscle.  
         [0028]      FIGS. 9 and 10  show a fifth electrode assembly  74  which is similar to assembly  64 , but which accommodates two spaced-apart titanium electrodes  75  mounted in a base  76 . Two coiled lead wires  77  extend through a silicone tube  78  for attachment to the electrode heads as already described.  
         [0029]      FIG. 11  shows electrode assembly  74  as implanted between upper and lower muscle layers  80  and  81 . The dual electrodes are in contact with the lower muscle layer, and electrically isolated from the upper muscle layer. Again, these electrode assemblies are bidirectional, and can be used for sensing muscle potentials, or for delivery of stimulating signals.  
         [0030]     There have been described several embodiments of bidirectional medical electrode assemblies made of materials which are body compatible, and suitable for long-term implantation without adverse tissue reaction. The electrodes are “site specific” in that they are isolated from and insensitive to adjacent non-target tissue potentials. As compared to prior-art conductor wires secured to bone screws, and fine wire electrodes implanted in the brain, the electrodes of this invention are substantially free of signal attenuation, interference or cross talk from overlying muscles, and noise and induced lead-whips potentials.