Abstract:
An electrode bundle for implantation in soft tissue comprises two or more electrodes aligned in parallel. Each electrode comprises an electrode element, an anchoring element joined to the electrode element at a portion intermediate between the front end and the rear end thereof and a means bundling the electrodes disposed between the anchoring element and the rear end. The anchoring element forms an angle a from 0° to 6° with the electrode element and extends in the direction of the rear end thereof. Also disclosed is a stack of electrode bundles, methods for insertion of the bundle and the stack into soft tissue, and their uses.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a medical electrode bundle for insertion into soft tissue such as the brain, the spinal cord, endocrine organs, muscles, and connective tissue.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electrodes that can be implanted for a long time into the central nervous system (CNS) have a wide application. In principle, all brain nuclei can be recorded from or stimulated by such electrodes and their functions monitored and controlled. Stimulation of the brain or spinal cord can be of particular value in situations when brain nuclei are degenerated or injured. Monitoring brain activity can be useful if linked to drug delivery or other measures such as electrical stimulation. Electrodes can also be used to lesion specific sites in tissue. To record and stimulate brain structures various forms of implanted electrodes have been developed and used in the past. For a long time implantable electrodes have been used for symptomatic treatment of Parkinson&#39;s disease (U.S. Pat. No. 6,647,296 B), chronic pain and control of spinal function.  
         [0003]     Single relatively stiff electrodes with one or multiple sites for recording and/or stimulation disposed along their shafts (US 2003083724 A), twisted wire electrodes, multi-channel electrodes consisting of a multitude of parallel wires (WO 03077988 A, U.S. Pat. No. 6,171,239 B), multi-array needle-like electrodes (U.S. Pat. No. 6,171,239 B) protruding from a base plate are known in the literature. Known multichannel electrode arrays with electrodes protruding from a base plate consist of relatively stiff wires or needle-like electrodes that allow recordings/stimulation only in superficial parts of the brain. Further electrodes of this kind are disclosed in US 20060178709 A, US 20050228249 A, U.S. Pat. No. 5,366,493 A, and U.S. Pat. No. 6,597,953 B.  
         [0004]     A particular problem with known electrodes of this kind is their retention at the desired site in tissue, in particular in the brain, over an extended period of time, such as from one week to one month and event for one year or more. Dislocation after insertion may make them entirely useless. For instance, implanted wire electrodes used in pain control are not equipped with anchoring means capable of retaining them in their original position close to the target cells. Consequently, they are often dislocated after insertion into brain areas that exhibit constant rhythmic movements due to breathing and heart activity, like the brain stem or the spinal cord resulting in therapy failure.  
         [0005]     Another problem with known electrodes is damage of brain or other soft tissue caused during insertion.  
         [0006]     Known multi-channel electrode arrays with stiff electrodes are vulnerable to tissue acceleration, for example when the patient&#39;s head is suddenly moving. The resulting shearing forces may irritate the tissue or even kill cells adjacent to the electrode. In addition, there has not been provided a solution to the problem of precisely positioning multiple ultra thin and flexible electrodes deep into the central nervous system.  
       OBJECTS OF THE INVENTION  
       [0007]     One object of the invention is to provide a multi-channel electrode of the aforementioned kind that can be placed at a desired location in soft tissue and be anchored there without causing excessive damage to the tissue.  
         [0008]     Another object of the invention is to provide a multi-channel electrode of the aforementioned kind that is retained in a chosen location in soft tissue over an extended period of time and that is not easily displaced by corporal movements of the person into which the electrode is implanted.  
         [0009]     An additional object of the invention is to provide an electrode of this kind which additionally is easy to manufacture.  
         [0010]     Further objects of the invention will become evident from a study of a short description of the invention, a number of preferred embodiments illustrated in a drawing, and of the appended claims.  
       SUMMARY OF THE INVENTION  
       [0011]     According to the present invention is disclosed an electrode bundle of the aforementioned kind comprising two or more electrodes each comprising an electrically conducting electrode element and an anchoring element. The electrode element has a rear end and a front end. The anchoring element functions in the manner of a barb and comprises a preferably pointed tip at its one end; at its other end it is attached to the electrode element from which it extends obliquely in the direction of the rear end thereof so as to form a sharp angle a of 0° to 6°, and also from about 5° to about 25°. In the electrode bundle, which has a distal end and a proximal end, the two or more electrodes are aligned with their front ends defining a distal end portion of the bundle and with their rear ends defining a proximal end portion of the bundle and their electrode elements disposed about in parallel so as to make the anchoring elements to extend in proximal directions from the bundle. This arrangement makes the anchoring elements to function as barbs once the electrode bundle has been inserted into tissue, and thus to anchor the electrode bundle therein so as to prevent accidental withdrawal. This traditional kind of anchoring effect is accompanied by the important capacity of the electrode of the invention to become even better anchored if submitted to a force seeking to withdraw it from tissue. Anchoring is facilitated by the anchoring element having being curved away from the electrode element, in particular at a portion extending from the tip of the anchoring element. An attempt to withdraw an electrode inserted into soft tissue forces the anchoring element away from the electrode element, making it penetrate deeper into surrounding tissue and thus to distribute the withdrawing load over a larger volume of tissue. It is preferred for the electrodes to be coated by a non-conductive material such as a polymer or lacquer, except for at the tips of their anchoring elements.  
         [0012]     According to an advantageous aspect of the invention the electrode bundle comprises two or more electrodes and at least one separate electrically non-conducting anchoring element comprising a rod and a barb extending from the rod in a proximal direction.  
         [0013]     The electrode bundle of the invention may have any desired cross-section in a transverse direction, that is, a direction perpendicular to an axis extending from its distal end to its proximal end; a circular cross-section is preferred. According to a preferred aspect of the invention the electrode bundle may however also be substantially flat and have a corresponding cross-section; in such case it is preferred for it to be arranged on a flat and non-conductive support. An electrode bundle that is not substantially flat is preferably bunched at or near its distal end by means of a collar, a sleeve, a ring, an adhesive or similar. In bunching consideration should be given to the disposal of the electrode elements and the anchoring elements or the electrode elements with integrated anchoring elements so as to obtain a desired electrode bundle geometry for insertion into tissue; preferably the anchoring elements are disposed so as to extend radially outwards from the bundle in the manner of supporting ribs of an umbrella. They are preferably flexible. If they are not flexible it is preferred for them to be attached to the respective electrode element in a hinge-like manner. This will allow their free ends to be displaced radially outwards when a force acting in a proximal direction is applied to the electrode bundle, that is, to act as barbs that retain or anchor the electrode bundle in tissue.  
         [0014]     The insulated electrical leads connected to rear portion of the electrodes by means of, for instance, a solder or electrically conducting glue are suitably housed in a common flexible tube.  
         [0015]     According to another preferred aspect of the invention the electrode bundle comprises two or more electrodes each having one or more integral anchoring elements. The integral anchoring element(s) are conductively connected with the electrode, electric contact with soft tissue being preferably provided exclusively via the anchoring element(s), most preferred at or near their preferably pointed tips(s). If the diameter of the anchoring elements at or in close proximity of the tips is very small, such as below 2 μm, the tips can have any shape. In such case the electrode element surface and most of the anchoring element surface is covered by an insulating layer, such as a thin layer of a substantially flexible polymer material such as polyethylene or Teflon or a lacquer. It is within the scope of the present invention to increase surface of the electrode tips by, for instance, roughening, grooving, etching, etc.. It is preferred to make the electrode of the invention from one piece of a thin wire with good conducting properties. The wire may be insulated from the start, and the insulation ablated at portions of the electrode that should be in electrical contact with tissue. According to an advantageous aspect of the invention the electrode wire is of a memory metal that of a transformation temperature at or slightly below body temperature; desired portions of the memory metal wire can be treated so as to change shape at or slightly below body temperature. A memory effect is particularly useful in unfolding the anchoring element of the electrode. The electrode of the invention can also have a non-conducting core of, for instance, glass fibre covered with a conducting metal layer; the metal layer can be insulated in a conventional manner by a lacquer or a polymer coat.  
         [0016]     According to a further preferred aspect of the invention the electrode elements with integral anchoring elements or separate electrode elements and anchoring elements are bonded to one or several oblong sheets of a non-conductive material each sheet having a front end and rear end and being preferably similar in form. The electrode elements and the anchoring elements or the electrode elements with integrated anchoring elements on a sheet are disposed in parallel and with their front and rear ends in proximity of the front and rear ends, respectively, of the sheet from which long sides the barb portions of the anchoring or combined elements extend in a proximal direction. Two or more such sheets may be superimposed in a congruent manner and bonded to each other to form a layered electrode bundle. The sheets may be also wrapped around a cylinder or cone core.  
         [0017]     The electrode bundle of the invention permits ultra thin electrodes to be inserted into and anchored in deep tissue. This substantially reduces the risk of dislocation of the electrodes due to tissue movements. To further improve anchoring properties, the barb element surface may be rough or have adhesive properties. The electrically insulating layer or coating on the electrodes may be given a rough or uneven surface. The coating should preferably be biocompatible and/or be covered by a biocompatible material such as a polysaccharide to facilitate insertion into tissue and/or improve anchoring properties by attached hydrophobic groups, for instance be silanized. The use of coating materials that reduce scar formation is also preferred.  
         [0018]     The electrode bundle of the invention provides a means for electrical stimulation of soft tissue over an extended period of time, such as a month or more and even one or several years. Electrical pulses of desired shape and length can be generated in a control unit to which the electrode leads are connected. Additionally the electrode bundle of the invention provides a means for sampling electric signals generated by the human or animal body, such as signals produced by neurones, or sampling electrochemical signals from intra- or extra cellular fluids. The electrode bundle can be used for a number of therapeutic purposes, such as pain control, treatment of Parkinson&#39;s disease and epilepsy, boosting of memory, and mood control.  
         [0019]     Particularly advantageous is a rotationally symmetric electrode bundle of the invention similar in form to the skeleton of an umbrella. For reasons of simplicity, this rotationally symmetric electrode bundle may be termed “umbrella electrode”. The umbrella electrode of the invention allows a large set of very small and delicate electrodes, such as electrodes with a diameter in the nanometer or lower micrometer range, to be inserted into a brain structure. The umbrella electrode is inserted into tissue in a folded state and anchored there by an unfolding movement. The umbrella electrode may comprise hundreds and even thousands of electrode elements and, once disposed and anchored in the brain, be capable of serving as a stimulating or recording device addressing numerous of neurons/small groups of neurons. The umbrella electrode may also be implanted in any other soft tissue.  
         [0020]     The invention will now be explained in more detail by reference to preferred embodiments of the invention illustrated in a rough drawing, which, for clarity reasons, is not to scale; most transverse dimensions are greatly exaggerated. 
     
    
     DESCRIPTION OF THE DRAWING  
       [0021]      FIG. 1  is an axial section A-A ( FIG. 2 ) of an electrode of the electrode bundle of the invention;  
         [0022]      FIG. 2  is a top view of an array of  12  electrodes of an electrode bundle of the invention, disposed in the same manner as in the bundle;  
         [0023]      FIGS. 3   a  and  3   b  are transverse sections B-B and C-C ( FIG. 4 ), respectively, of a first embodiment of the electrode bundle of the invention;  
         [0024]      FIG. 3   c  is a top view of the electrode bundle of  FIGS. 1 and 2 , in a folded state;  
         [0025]      FIG. 4  is an axial section D-D ( FIG. 3   c ) of the electrode bundle, in a folded state;  
         [0026]      FIG. 5  is an axial section corresponding to that of  FIG. 4  of the electrode bundle, in an unfolded state;  
         [0027]      FIG. 6  is an axial section E-E ( FIG. 7 ) of a second embodiment of the electrode bundle of the invention, in cooperation with a sensing electrode, in the same view and state as the electrode bundle in  FIG. 4 ;  
         [0028]      FIG. 7  is a top view of the electrode bundle of  FIG. 6 , in the same state;  
         [0029]      FIG. 7   a  is a top view of the electrodes of the electrode bundle of  FIGS. 6 and 7 , in the same view and disposition as in  FIG. 7 ;  
         [0030]      FIG. 8  is a top view of a flat third embodiment of the electrode bundle of the invention, in a folded state;  
         [0031]      FIG. 9  is the flat electrode bundle of  FIG. 8 , in the same view and in an unfolded state;  
         [0032]      FIG. 10  is an enlarged partial sectional view F-F ( FIG. 9 ) of the electrode bundle of  FIGS. 8 and 9 , seen in direction R ( FIG. 9 );  
         [0033]      FIG. 11  is a variation of the electrode bundle of  FIGS. 8-10 , in the same view and in an unfolded state;  
         [0034]      FIG. 12  is a partial sectional view of a variation of the embodiment of the flat electrode bundle of  FIGS. 8-10 , in a section corresponding to section E-E in  FIG. 9  and in a folded state;  
         [0035]      FIG. 13  is a stacked embodiment of the electrode bundle of FIGS.  8  to  10 , in a section corresponding to section F-F in  FIG. 9  and in a folded state;  
         [0036]      FIG. 14  is a further embodiment of an electrode of the electrode bundle of the invention, in a side view;  
         [0037]      FIGS. 15   a  and  15   b  are side views of another embodiment of the electrode bundle of the invention, in a folded state and an unfolded state, respectively;  
         [0038]      FIGS. 16   a - 16   c  are side views of still another embodiment of an electrode of the electrode bundle of the invention, in a folded state ( 16   a ) and an unfolded state ( 16   b  and  16   c ), comprising a further anchoring element shown in a folded state in  FIGS. 16   a  and  16   b,  and in an unfolded state in  FIG. 16   c;    
         [0039]      FIGS. 17   a  and  17   b  a side views of a further embodiment of an electrode of the electrode bundle of the invention, in a folded state ( 17   a ) and an unfolded state ( 17   b ) comprising an anchoring element of memory metal;  
         [0040]      FIGS. 18   a  and  18   b,  are side views and  FIG. 18   c  is a sectional view H-H of an additional embodiment of an electrode of the electrode bundle of the invention, which is shown in  FIGS. 18   d  and  18   e  in two phases of insertion into tissue and anchoring. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     EXAMPLE 1  
       [0041]     Preferred embodiments of the electrode bundle of the invention. FIGS.  1  to  5  illustrate a first embodiment of the electrode bundle of the invention in form of an umbrella electrode  1 . The umbrella electrode  1  comprises twelve like electrodes  2 ,  2 ′,  2 ″,  2 ′″,  2 ″″,  2 ′″″,  2 ″″″, etc.. An electrode  2  comprises a cylindrical electrically conducting electrode rod  3  having a tip  4  at its one (front) end. An integral anchoring element or barb  5  having a tip  6  at its free end extends in a skew rearward fashion from a section of the electrode rod  3  near the tip  4  in the direction of the blunt distal end of the electrode rod  3 , to which a thin insulated wire  11  is attached by a solder  10 . The electrode rod  3  and the barb  5  are made of a good conductor such as platinum, silver, gold, copper, their alloys and other suitable metals and alloys; carbon fibres and fibres of electrically conducting polymers may also be used. The electrode element  3  and the barb  5  can also be made of a core of non-conductive material, such as glass or polymer covered by a conductive material such as metal. If the electrode  3  and the barb  5  have a core of non-conductive material covered by a layer of conductive material such as a metal, their preferred diameter is the same as for electrodes made of electrically conductive wire. If the electrode element  3  and the barb  5  is made of metal wire their diameter is preferably from about 10 −4  m to 10 −7  m. A particularly preferred diameter is from 1×10 −6  m to 25×10 −6  m. Except for at their tips  4 ,  6  the electrode element  3  and the barb  5  are cylindrical. In an unfolded state (FIGS.  1  to  4 ) their axes (not shown) include a sharp angle α of about 100°. At the region of attachment of the barb  5  to the rod  3  the diameter of the former narrows, which makes the barb  5  more flexible in that region than otherwise, so as to provide a hinge function. Except for its tip  6  the barb  5  and the electrode rod  3  is electrically insulated by a thin layer of polymer such as Teflon or polyethylene or by a lacquer. This is not shown in the Figures; the non-insulated tip  6  portion is indicated by dotted lines S extending along it. The insulating coating can have a roughened surface (not shown) to improve the anchoring properties of the barb  5 .  
         [0042]     The umbrella electrode  1  is assembled by disposing the twelve electrodes  2 ′,  2 ″,  2 ′″,  2 ″″,  2 ′″″,  2 ″″″, etc. along the periphery of a circle of a size to allow the rods  3 ,  3 ′,  3 ″, etc. of neighbouring electrodes to abut each other and with the axes of the barbs  5 ,  5 ′,  5 ″, etc extending in an axial direction in respect of the circle. This disposition is shown in  FIG. 2 . For forming a physically stable electrode bundle, that is, the umbrella electrode  1 , the electrode rods  3 ,  3 ′,  3 ″, etc. are disposed along the periphery of a short cylinder  7  of same diameter ( FIG. 3   a ). In this disposition the electrodes  2 ′,  2 ″,  2 ′″,  2 ″″,  2 ′″″,  2 ″″″ are mounted by clamping them to the cylinder  7  by means of a sleeve  8 . The cylinder  7  is of stiff polymer material such as polycarbonate whereas the sleeve is of a thermosetting polymer such as polypropylene. It is within the ambit of the invention to use electrodes with electrode rods  3  and/or barbs  5  of different length in a single electrode bundle. A disposition of electrode rods  3  that is not rotationally symmetrical is also comprised by the invention. The correctly disposed electrodes  2  can also be mounted by gluing, in which case the mounting cylinder  7  and sleeve  8  can be dispensed with. The umbrella electrode  1  of  FIGS. 1-5  is inserted into tissue in the configuration of  FIG. 4 , in which the barbs  5 ,  5 ′,  5 ″, etc. form a sharp angle a, such as one of about 10°, with the respective electrode rods  3 ,  3 ′,  3 ″, etc.. After the desired depth of insertion is reached slight withdrawal of the umbrella electrode  1  in a proximal direction makes the barbs  5 ,  5 ′,  5 ″, etc. rotate at their restrictions  9  that function in a hinge-like manner. The umbrella electrode  1  thereby unfolds while the tips  6 ,  6 ′,  6 ″, etc. of the barbs  5 ,  5 ′,  5 ″, etc. move radially outwards and thereby are inserted into the tissue surrounding the electrodes  2 ′,  2 ″,  2 ′″,  2 ″″,  2 ′″″,  2 ″″″, etc. and anchored against further withdrawal. By the controlled withdrawal movement over a short distance the angle α is widened from about 10° to about 30° to about 60° and even up to 90°. Attempts to withdraw the electrode bundle  1  even more meets increased resistance. After point of maximum resistance is reached the electrodes  2 ′,  2 ″,  2 ′″,  2 ″″,  2 ′″″,  2 ″″″, etc. will eventually swing backwards to allow the electrode bundle  1  to be easily withdrawn from tissue.  
         [0043]     The umbrella electrode  20  of  FIGS. 6, 7 , and  7   a  corresponds substantially to the umbrella electrode  1  of  FIGS. 1-5  but cooperates with a central sensing electrode  21  with a non-insulated tip  22  for monitoring insertion into tissue to a desired depth guided by electrical signals emanating from nerve synapses and neurones. Elements of the umbrella electrode  20  corresponding to those of the electrode  1  retain the numbering of the later with an asterisk; the cylinder  23  along the periphery of which the electrodes  2 *,  2 **,  2 ***, etc. are mounted has a central bore  25  in which the sensing electrode  21  is snugly mounted displaceably in a distal/ proximal direction as indicated by double arrow L in  FIG. 6 . Insertion, anchoring, and withdrawal parallels that of electrode  1  except for that the sensing electrode  21  is inserted first to find the correct depth of insertion for the electrode bundle  20 . The electrode bundle  20  is then inserted to the correct depth by sliding it along the sensing electrode  21 , and anchored in that position by a slight displacement in the opposite direction. Insertion of the electrode bundle  20  to the correct depth is facilitated by distance marks (not shown) printed on the sensing electrode  21 , which can be completely withdrawn after insertion of the bundle  20  to the desired depth.  
         [0044]     In contrast to the umbrella electrodes  1  and  20  the electrode bundle  30  of the invention illustrated in  FIGS. 8-10  is substantially flat; it comprises six electrodes and retains important anchoring features of the umbrella electrodes  1  and  20 . Of the faces  31 ′,  31 ″ of a flat non-conductive polycarbonate support  31 , which may have smooth or a dimpled surface, one  31 ′ carries a pattern of six electrodes, comprising L-formed leads  32 ,  33 ,  34 ,  35 ,  36 ,  37  with short base portions  32 ′,  33 ′,  34 ′,  35 ′,  36 ′,  37 ′ prepared by micro lithographic etching from a thin metal layer bonded to face  31 ′ or by screen printing a thin conducting layer onto that face. The L-formed leads  32 ,  33 ,  34 ,  35 ,  36 ,  37  extend from the proximal or rear end  38  of the support  31  to close to its distal or front pointed end  39  and from there, in a mirroring manner, to either long side  58 ,  59  of the support  31  by their short bases  32 ′,  33 ′,  34 ′,  35 ′,  36 ′,  37 ′. The combination lead/base combination  32 / 32 ′,  33 / 33 ′,  34 / 34 ′, etc. forms an electrode. Close to their free ends the bases  32 ′,  33 ′,  34 ′,  35 ′,  36 ′,  37 ′ carry pointed cylindrical anchoring elements  46 ,  47 ,  48  and  49 ,  50 ,  51  extending in a skew proximal direction. The leads  32 ,  33 ,  34 ,  35 ,  36 ,  37  and, except for their pointed tips  40 ,  41 ,  42 ,  43 ,  44 ,  45 , the anchoring elements  46 ,  47 ,  48 ,  49 ,  50 ,  51  are covered with a thin layer of insulating polymer. At their proximal ends each of the leads  32 ,  33 ,  34 ,  35 ,  36 ,  37  has a thin insulated metal wire  52 ,  53 ,  54 ,  55 ,  56 ,  57  attached by soldering. Upon inserting the electrode bundle  30  of  FIG. 8  into soft tissue such as brain tissue to a desired depth withdrawal for a short distance makes the anchoring elements  46 ,  47 ,  48 ,  49 ,  50 ,  51  to slightly rotate in a plane parallel with the support  31 , thereby being inserted into and anchored in the tissue surrounding the electrode bundle  30 . Rotation of the anchoring elements is facilitated by constrictions  60 ,  61 ,  62 , etc. ( FIG. 10 ) in short end portions  46 ′,  47 ,  48 ′, etc. of the anchoring elements  46 ,  47 ,  48 , etc. set off by 90° by which end portions the anchoring elements  46 ,  47 ,  48 , etc. are joined to the respective leads  32 ,  33 ,  34 , etc.. To facilitate insertion into tissue the front end  39  of the support  31  is pointed.  
         [0045]     The electrode bundle  70  of the invention illustrated in  FIG. 11  differs from that of  FIGS. 8-10  in that leads  72 ,  73 ,  74 ,  75 ,  76 ,  77  do not have bases extending to either of the long sides  91 ,  92  of the support  90  but are extending straight from the rear end to the pointed front end. The anchoring elements  78 ,  79 ,  80 ,  81 ,  82 ,  83  are attached to the leads  72 ,  73 ,  74 ,  75 ,  76 ,  77  in a manner so as to space them equidistantly over the entire width of the oblong support  90 , one half  78 ,  79 ,  80  pointing, in a skew proximal direction, towards one long side  91 , the other half  81 ,  82 ,  83  in a mirroring manner to its other long side  92 . In this embodiment the flexible and insulated wires soldered to the proximal ends of the leads  72 ,  73 ,  74 ,  75 ,  76 ,  77  are numbered  84 ,  85 ,  86 ,  87 ,  88 ,  89 .  
         [0046]     The electrode bundle  100  of  FIG. 12  differs from the embodiment of FIGS.  8  to  10  in that the elements disposed on one face  31 ′ of the support  31  of the embodiment of FIGS.  8  to  10  have been duplicated on the other face in a mirroring manner (mirror plane G-G in  FIG. 10 ). The electrode bundle  100  thus is provided with electrodes on both faces  101 ′,  101 ″ of its support  101 . The elements copied from the embodiment of FIGS.  8  to  10  retain their reference numbers but provided with one asterisk (elements on the original face  101 ′) or two asterisks (elements on the mirrored face  101 ″).  
         [0047]     The stacked electrode bundle  110  of  FIG. 13  is obtained by superposition of three flat electrode bundles of the kind shown in  FIGS. 8-10 . The electrode bundle  110  has three layers of electrodes, a first layer comprising anchoring elements  46 +,  47 +,  48 +, etc. and corresponding leads on a support  111 +, a second layer comprising anchoring elements  46 ++,  47 ++,  48 ++, etc. and corresponding leads on support  111 ++, and a third layer comprising anchoring elements  46 +++,  47 +++,  48 +++, etc. and corresponding leads on support  111 +. The layers are kept apart by cylindrical spacers  113 . A cover  112  that protects the first layer is spaced from that layer by cylindrical spacers  114 . The insulating coat (not shown) of one or more electrode layers of the electrode bundle can be optionally covered by a shielding coat, such as a thin metal coat, to minimize crosstalk between electrodes of different layers. The shielding coats can be in electric contact with each other and grounded.  
         [0048]     The embodiment  120  of the electrode of the invention shown in  FIG. 14  is formed from one piece of metal wire. The barb  123  is joined to the electrode rod  121  by a 180° bend  122  forming the blunt tip of the electrode  120 . At its free end the barb  123  has a sharp point  124 . At its rear end an insulated flexible lead  126  is attached to the electrode rod  121  by a solder  125 . Near the bend  122  the barb  123  carries a notch  127 . When a force seeking to move the barb  123  away from the rod  121  is applied to the barb it will bend at the notch  127 , which thus has a hinge-like function. The angle of the notch may be used to control the angle of the hinge-like function. Seen in the direction of the electrode rod  121  the barb  123  is slightly bent in a convex manner, which facilitates insertion into surrounding tissue while keeping damage to the tissue at a minimum. Except for its sharp tip  124  the electrode  120  is covered by a thin layer of insulating polymer material; the free electrode surface at tip  124  is indicated by dotted lines S extending parallel to it.  
         [0049]     In the embodiment of an electrode  130  of the electrode bundle of the invention of  FIGS. 15   a  and  15   b  the hinge means allowing unfolding of the anchoring element  133  is accomplished by providing the anchoring element  133  with a section  137  of lower resistance against a bending force by, for instance, annealing the anchoring element  133  except for its section  137 . Elements  131 ,  132 ,  134 ,  135 , and  136  have the same meaning as elements  121 ,  122 ,  124 ,  125 , and  126  of the embodiment of  FIG. 14 ; S in combination with dotted lines again denotes that the surface of anchoring element point  134  is not insulated.  
         [0050]     In the embodiment of an electrode  140  of the electrode bundle of the invention of  FIGS. 16   a - 16   c  the anchoring element  143  is provided with a barb  148  near its point  144  that again lacks insulation, as indicated by S in combination with dotted lines. The electrode  140  is made from one piece of a memory metal wire. Sections of the wire can be mechanically and thermally treated so as to make them to change physical shape at certain temperatures. An anchoring element section  147  close to the point where the anchoring element  143  is joined to the bend  142  at the top end of the electrode rod  141  has been treated in a manner to make it, on insertion of the electrode into tissue of 37° C., change shape; the memory effect makes the anchoring element  143  bend away from the electrode rod  141  to assume the shape shown in  FIG. 16   b.  Close to its point  144  the anchoring element  143  has a barb  148 , which has been treated so as to provide it with a memory making it bend away from the anchoring element  143  on warming. The electrode  140  thereby assumes the conformation shown in  FIG. 16   c.  The barb  148  provides the electrode  148  with an additional anchoring effect. In the embodiment of  FIGS. 16   a - 16   c,  the memory effect of the anchoring element  143  and the barb  148  differ in that the former returns faster to its stable configuration at body temperature of the barb  148 . It is however within the scope of the invention and also preferred that both sections  147 ,  148  return to their stable configuration at the same rate. Elements  145 ,  146  are functionally identical to elements  125  and  126 , respectively, of the embodiment of  FIG. 14 . S in combination with dotted lines again denotes that the surface of anchoring element point  144  is not insulated.  
         [0051]     The anchoring element  153  of the electrode  150  of the electrode bundle of the invention of  FIGS. 17   a  and  17   b  is of a memory metal having a transition temperature of about 33° C. to 35° C. In the folded state ( FIG. 17   a ) the memory metal is in a strained, substantially straight state. Upon insertion into tissue the memory metal warms up to the transition temperature, at which the metal seeks to return to its unstrained curved state ( FIG. 17   b ). Movement of the pointed  154  end of the anchoring element  153  away from the electrode element  151  is however impeded by surrounding tissue. A slight withdrawal of the electrode  150  in a proximal direction allows the anchoring element to unfold and thus anchor the electrode  150  in tissue. Reference numbers  152 ,  155 , and  156  have the same meaning as reference numbers  142 ,  145 , and  146  in  FIGS. 16   a - 16   c.    
         [0052]     The anchoring element  163  of the electrode  160  of the invention of  FIGS. 17   a  and  17   b  is resiliently flexible whereas the electrode element  161  is stiff.  FIG. 18   a  shows the electrode  160  in an unrestrained state. In  FIGS. 18   b  and  18   c  the anchoring element  163  has been forced towards the electrode element  161  so as to abut it, and has been attached in this resiliently strained state to the electrode element  161  by means of a biocompatible adhesive  166 , which is water soluble or swellable; if swellable it substantially looses its adhesive properties when swollen. The adhesive  161  is of a kind that it does not dissolve or substantially swell immediately on contact with water. The time required for dissolution or swelling of the adhesive in an aqueous environment to a degree that the anchoring element  163  is released from abutment to the electrode element  161  by the resilient force of the anchoring element  163  can be suitably varied, such as from a few seconds to one or two minutes and even more. This allows the electrode  160  to be inserted into tissue to a desired depth before the anchoring element  163  comes off. Suitable biocompatible adhesives include saccharose, gelatine, gelatine derivatives, fibrin tissue glue, cellulose derivatives, modified starch, and collagen gel. The adhesive  166  is applied to the anchoring element  163  held in abutment with the electrode element  161  as an aqueous solution or gel. The solution or gel then is dried on the electrode  160 . Drying can be accelerated by gently heating the solution or gel up to the boiling temperature. The dissolution or swelling rate of the adhesive  166  can be set by varying the drying or heating time and/or the temperature. With saccharose adhesive dissolution times below and above 1 min can be easily achieved by, for instance, heating the solution or gel for a selected period of time. After insertion into tissue  167  by means of a micromanipulator  165  and dissolution or swelling of the adhesive  166  the state shown in  FIG. 18   d  is reached. The micromanipulator  167  comprises releasable means for coupling with the electrode  160 , which are however not shown in the Figures. A portion of the electrode  163  extending from the tip  164  has come off the electrode element  161 . Its further displacement away from the electrode element  161  is however barred by surrounding soft tissue  167 . Withdrawal of the electrode  160  for a short distance d in a direction opposite to the insertion direction makes the point  164  of the anchoring element  163  penetrate the surrounding tissue  167  so as to “unfold” the anchoring element  163  and make it adopt the substantially unstrained state in  FIG. 18   e.  For reasons of simplicity the principle of insertion into and anchoring in soft tissue  167  of a single electrode  160  is visualized in  FIGS. 18   d  and  18   e  rather than insertion and anchoring of an electrode bundle comprising two or more of such electrodes  160 ; electrodes  160  when bundled will behave in essentially the same manner. The embodiment of  FIGS. 18   a  to  18   e  can also be carried out with an electrode element that is not stiff, in particular with one that is resiliently flexible. In such case it is preferred that the portion of the electrode element of equal length as and facing the anchoring element has a bent configuration mirroring that of the anchoring element; if both elements are made of the same piece of wire the resulting electrode will have a straight configuration in the twofold strained state corresponding to that of  FIGS. 18   b  and  18   c.  In  FIGS. 18   a - 18   e  the flexible electrical lead attached to the rear end of the electrode element  161  is not shown.  
       EXAMPLE 2  
       [0053]     Positioning the electrode bundle in tissue. For optimal performance of the electrode it is critical to place it with high precision in the target tissue, in particular neuronal tissue. Since the size of the brain and spinal cord and the relative size and location of their various regions varies considerably between individuals, coordinate guided insertion is not sufficiently accurate. Preferably a ‘tracking’ procedure is used to locate the coordinates of the brain or spine region of interest. In this procedure the correct coordinates are determined by multiple recording/stimulation traces with single electrodes. They may also be determined by tracking by means of a guiding electrode like the guiding electrode  21  of the embodiment of  FIGS. 6 and 7 . Once the correct coordinates are determined they can be used to guide the insertion of the electrode bundle into the brain. The electrode bundle can be introduced into tissue by means of a micromanipulator capable of temporarily holding it at or near its rear or proximal end. The electrode bundle may be equipped with attachment means for co-operation with the micromanipulator. For example, the attachment means comprises one or several bores in the base or in the support of the bundle, respectively, into which the corresponding numbers of bars disposed at the front or distal end of the micromanipulator are inserted. Alternatively the sleeve or the support of the electrode bundle is clamped by arms of the micromanipulator during insertion and released upon deposition at the desired depth. The micromanipulator is then withdrawn.  
         [0054]     In case of erroneous positioning the electrode bundle of the invention can be simply withdrawn; the hinge-like connection of the barb portion(s) to the anchoring element base or electrode element allows the former to swing backwards on withdrawal with minimal tissue damage. Alternatively the electrode bundle is coated in a folded state by a substrate dissolvable in body fluids, such as a polysaccharide or gelatin. Thereby the barbs are prevented from unfolding until the polysaccharide or gelatine has dissolved, allowing the umbrella electrode to be withdrawn from an erroneous position during a short period of time from insertion. Withdrawal of the electrode bundle of the invention for an appropriate short distance after dissolution of the dissolvable substrate anchors it in the tissue. According to a preferred aspect of the invention the unfolded electrode bundle coated with a dissolvable substrate comprises a guide electrode disposed in the centre of the bundle and which is insulated except at its distal tip and not provided with a barb element. During insertion the guide electrode is exclusively used for stimulation and recording of electrical signals to determine the desired location in tissue.  
       EXAMPLE 3  
       [0055]     Electrical control of/recording of electrical signals by the electrode bundle. For an electrode of the invention intended for signal recording purposes, the electrode elements are electrically connected to a signal amplifying unit comprising an amplifying circuit and, optionally, a transmitter for wireless connection to a control unit. To avoid loss of signal strength the amplifying unit with the transmitter is disposed close to the electrode bundle. For transmission over a short distance, such as up to about  20  cm, a miniaturized first transmitter may be mounted at the rear portion of the bundle, that is, at a portion extending from the spinal cord or brain tissue, transmitting the signals to a transfer unit comprising a receiver and a second transmitter. The transfer unit may be implanted in soft tissue and comprise a wirelessly rechargeable battery.  
         [0056]     An electrode of the invention intended for stimulation is connected to a preferably telemetrically controlled stimulating unit. Telemetric control is exerted by a control unit comprising a control circuit and a transmitter. The recording, stimulating, and signal transfer units are preferably powered by a small rechargeable battery.  
       EXAMPLE 4  
       [0057]     Field of application. The electrode bundle of the invention is primarily intended for treatment of patients (but also animals) with pain, injuries or degeneration in the brain and/or the spinal cord; as a research tool in studies of neuronal network function, and plasticity, development and aging of the nervous system; as an interface in brain-computer communication enabling prosthesis control or control of skeletal muscle; and for control of endocrine and exocrine organ function.  
         [0058]     Clinical use. The electrode bundle of the invention may serve to aid patients with brain or spinal damage of various kind by recording signals from remaining neurons in case of e.g. stroke or degenerative disease and/or to stimulate neurons to compensate for lost functions. Similar uses are possible in animals. For instance, the electrode bundle can be used to relieve pain by stimulation of analgesic brain stem centres, such as nuclei in the periaqueductal grey substance; to relieve or decrease tremor in Parkinson&#39;s disease, choreatic and other involuntary movements by stimulation within the basal ganglia or associated nuclei; to boost memory by stimulation of cholinergic and/or monoaminergic nuclei in case of Alzheimer&#39;s disease or other degenerative diseases; to control mood, aggression, anxiety, phobia, affect, sexual over-activity, impotence, eating disturbances by stimulation of limbic centres or other brain areas; to rehabilitate patients after stroke or damage of the brain/spinal cord by stimulation of remaining connections in cortex cerebri or descending motor pathways; to re-establish control of spinal functions such as bladder and bowel emptying after spinal cord injury by stimulating relevant parts in the spinal cord; to control spasticity by stimulation of inhibitory supraspinal descending centres or appropriate cerebellar areas.  
         [0059]     The electrical bundle may be used for electrolytic leasioning of specific tissue sites by passing electrical currents through the tissue. In such case the intensity of the current administered via the electrode bundle is chosen to be adequate for accomplishing cell death in a tissue volume adjacent to the front end of the electrode bundle. For example, the electrode bundle can be used to lesion tumours or CNS sites that have developed abnormal activity after e.g. an insult or a degenerative disease.  
         [0060]     Examples of combined recording and stimulation: monitoring of epileptic attacks by electrodes implanted into the epileptic focus coupled with a system for administration of antiepileptic drugs and/or electrical pulses; compensating for lost connections in the motor system by recording central motor commands, and stimulating the executive parts of the motor system distal of the lesions; selecting a site producing abnormal electrical activity by recording neuronal activity at the site, followed by lesioning the tissue at the site by administration via the electrode bundle of a current of adequate strength for an adequate period of time.  
         [0061]     Use as a research tool. The electrode bundle of the invention may be used for studies of normal as well as abnormal functions of the brain and the spinal cord and/or the peripheral nervous system (PNS). In such studies it is necessary to record neuronal activity and to simultaneously interact with the undisturbed CNS or PNS. For this purpose, the electrode bundle(s) of the invention is implanted in the CNS and/or PNS for a long time.  
         [0062]     Use as interface for communication with computers and neuroprosthesis. In patients with damage to the peripheral nervous system, it can be useful to record command signals from the CNS. These signals can be interpreted by computer programs and used to control neuroprosthesis such as artificial hands or feet, and also to control stimulation of muscles and organs such as the bladder and the bowel.  
         [0063]     Control of the function of endocrine and exocrine organs. In patients with deficient hormone secretion or regulation, the electrode bundle of the invention may be used to control the secretion of hormones from exocrine or endocrine organs.