Patent Publication Number: US-11647960-B2

Title: Flexible neural electrode array

Description:
FIELD OF INVENTION 
     The present invention relates to a flexible neural electrode array. The invention also relates to a technology of designing and producing mechanically very flexible neural electrode arrays. Neural electrode arrays are used for electrically interfacing with the nervous system in order to read biological signals or to electrically activate nervous tissue e.g., for diagnostic and/or therapeutic purposes. 
     BACKGROUND 
     State of the Art 
     Commonly, flexible neural electrode arrays consist of a flexible polymeric sheet in which metal electrode contacts are embedded. Above the electrode contacts, the polymeric sheet is spared so that metal contacts that provide the electrical interface to the nervous system are exposed. 
     While the polymeric sheet is usually made from soft silicone rubber, polyurethane or other elastomers with a thickness of some 10 μm up to 2 mm, the metal contacts are usually made from hard noble metal (alloy) foil of a few 10 μm thickness. 
     Problem Related to State of the Art 
     The stiffness of the metal contacts locally corrupts the mechanical flexibility of the polymeric sheet. Either it inhibits e.g. a tight bending radius or it is plastically deformed so that it would not be able to get back to its original shape once the bending is reversed. In case of combined bending and stretching, the metal contacts can actually angle off the sheet, exposing a metal edge that can potentially injure neural tissue. 
     SUMMARY 
     Embodiments of the invention relate to a flexible neural electrode array, comprising a layer of metal which is arranged on a first layer of polymeric material and which forms a number of contact pads, the first layer of polymeric material being flexible along a predefined direction, each contact pad of the number of contact pads having a sequence of cuts through the metal, each cut extending in a straight line across the predefined direction, wherein each cut having an inner end and an outer end, the inner end being within the contact pad, the outer end being at an edge of the contact pad, and each second cut of the sequence of cuts having its outer end at the same edge of the contact pad. 
     Advantageous embodiments of the invention may comprise the following features. 
     Each contact pad with the sequence of cuts forms at least one meander pattern portion when stretched in the predefined direction. 
     The electrode array may further comprise a second layer of polymeric material, the second layer of polymeric material being arranged at least partially on the layer of metal, and being flexible along the predefined direction, the second layer of polymeric material having at least one opening defining each contact of the number of contact pads. 
     This further layer may be applied for better fixating the metal layer. 
     The at least one meander pattern portion may be stretchable and/or compressible along the predefined direction. 
     The electrode array may further comprise a number of cut-aways arranged in the metal layer at the inner ends of the cuts. 
     The metal layer may further comprise a number of anchoring elements, the anchoring elements serving for fixating the metal layer with at least one of the layers of polymeric material. 
     The anchoring elements may be embodied as through holes in the metal layer, the holes being arranged in at least one predetermined region along the predefined direction, and holes allowing the polymeric layers to be connected to each other through the holes. 
     In one embodiment, at least one of the openings may be rectangular. 
     In one embodiment, at least one opening may be circular. 
     In one embodiment, at least one opening may have annular form. 
     Each cut of the sequence of cuts may extend radially with respect to the circular or annular form. 
     The electrode may be formed as cuff electrode, the at least one contact pads being ring-shaped. 
     The electrode may be formed as a cable, such as typical deep brain stimulation leads, with electrode contacts at an outer surface in shapes of rings or segmented rings. 
     The invention further provides a method of fabricating a flexible neural electrode array, the method comprising the following steps:
         a) depositing a layer of polymer  40  on a mechanical carrier  130 ;   b) removing the polymer layer, using a laser at location  70 , from locations where weld contacts are to be formed;   c) laminating a metal foil  10  on the polymer layer  40 ;   d) cutting the metal foil  10  to shape with a laser, thus forming anchoring holes  30  and meander structures  80 ,  50 ;   e) depositing a covering polymer layer;   f) forming the weld contacts by ablating the polymer layer from the locations  70 , and forming at least one electrode contact  20  by ablating the polymer  40  from at least one area of the polymer covering the metal layer.       

     The method may further comprise at least one of the following steps:
         g) removing the mechanical carrier  130 ;   h) attaching a wire  110  to the weld pad  70 ; and   i) sealing the weld  70  pad with polymer.       

     The layer of polymer may be deposited to a thickness of some 10 μm. 
     The metal foil may have a thickness of 5 to 50 μm. 
     In step e), the covering polymer layer may have a thickness of some 10 μm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and embodiments thereof are described in connection with the drawing. In the drawing, 
         FIG.  1    is a top view on a flexible neural electrode according to an embodiment of the invention; 
         FIG.  2    is a detailed view on a sub section of the flexible neural electrode, the left portion being a top view while extended by pull-forces, the right portion being cross section A-A′ of the left portion; 
         FIG.  3    is a detailed view on a sub section of flexible neural electrode; the left portion being relaxed (compact) and the right portion being extended by pull-forces; 
         FIG.  4    is a detailed view of circular cut at the end of the meander cut lines, reducing mechanical stress peaks in the metal during contact stretching, as shown in the right illustration; 
         FIG.  5    is an illustration of a two-contact nerve cuff electrode to be wrapped around a peripheral nerve with two meander contacts embedded in a self-spiraling polymeric sheet forming  3 -dimensional ring-shaped contacts, with the right portion being shown rolled in a cuff and the right portion being shown flattened; 
         FIG.  6    illustrates a production process for the flexible neural electrode of the invention; 
         FIG.  7    illustrates in A) a paddle electrode with two strip-shaped meander contacts, and in B) a paddle electrode with four disk-shaped meander contacts; 
         FIG.  8    illustrates how two-dimensional ring-shaped (annular) electrodes (example to the left: three concentric rings) can be realized as stretchable in two planar directions (including simultaneous stretching in x- and y-direction); 
         FIG.  9    illustrates a grid electrode with  12  ring-shaped (annular) electrode contacts e.g., of  FIG.  8   ; and 
         FIG.  10    illustrates a further embodiment of the invention in two views. 
     
    
    
     DETAILED DESCRIPTION 
     The flexible neural electrode array according to the first embodiment of the invention comprises a layer of at least one piece of metal foil  10 , made from e.g., stainless steel, platinum, iridium, or platinum-iridium alloy, that is patterned to a meander, which extends in a longitudinal direction A-A′ of the metal foil, refer to  FIGS.  1  and  2   . The basic shape of the piece of metal is rectangular. The piece of metal foil is sandwiched between a first (bottom) layer  40  and a second (top) layer  60 , both of polymeric material, e.g., silicone rubber. The electrical contact intended for interfacing with the neural tissue is established by opening the second (top) polymer layer  60 , thus exposing at least one part  20  of the metal foil  10 . The at least one part  20  defines the one or more contact pads for interfacing with the neural tissue. 
     The outer edges along the direction A-A′ of the meander are perforated, thus forming anchoring elements (through holes)  30 ,  30  so that first (bottom) polymer layer  40  and second (top) polymer layer  60  can reach through these holes  30  and mechanically fixate the metal  10 , refer to  FIG.  2   . The resulting arrangement can follow stretching (refer to  FIG.  3   ) and bending movements in longitudinal direction A-A′. 
     The creation of the meander can be done by straight cut lines  50  across direction A-A′ (refer again to  FIGS.  2 ,  3   ). These straight cut lines are also denoted as the sequence of cuts. Each cut goes through the metal layer. Each cut has its inner end within the contact pad, i.e., within the area of the contact pad, and its outer end at an outer edge of the contact pad, i.e., at the border of the area of the contact pad. Each second cut of the sequence of cuts has its outer end at the same edge of the contact pad, i.e., the each second cut end on the right edge of the contact pad, while the other cuts of the sequence of cuts end on the left edge of the contact pad. In this embodiment, the cuts are parallel to each other, and orthogonal to the edges of the contact pad where they end. 
     Additional to the cuts, circular cut-aways  120  at the ends of the cuts (refer to  FIG.  4   ) can be done, allowing the mechanical stress on the metal during stretching to be more evenly distributed, thus lowering the risk of crack initiation. 
     As can be seen from  FIG.  1   , the at least one contact pad  20  is on the at least one meander pattern portion. The meander portion forms the contact pad, and provides the electrical interconnection between the contact pads as well. There is no need for a separate interconnection structure on the device. 
     For electrically connecting the electrode contact to a wire, the metal foil comprises a weld pad area  70  at a longitudinal end of the metal foil. The wire is attached to the metal contact at the weld pad area  70  e.g., by resistance welding, laser welding, ultrasonic welding, brazing or soldering. After attachment, the weld area is electrically sealed with a polymer. 
     Alternatively, the welding area may be located remote to the meander contact. The weld pad is electrically connected to the meander contact using a conductive path made from the same metal foil as the meander contact and the contact pad. 
       FIG.  2    is a detailed view on a sub section of the flexible neural electrode array, left: top view while extended by pull-forces; right: cross section A-A′. The meander pattern is formed by stretching the electrode array along predefined direction  140  (refer to  FIG.  3   ). A-A′ corresponds to the predefined direction  140 . 
     As can be seen in  FIG.  3   , the electrode (i.e., the polymeric layers, and the patterned metal layer) is flexible along one predefined direction  140 . The cuts  50  are across that predefined direction  140 , refer to the left Figure. The cuts  50  enable the metal layer to be flexible along the predefined direction  140  too. 
     An embodiment of a flexible electrode array is shown in  FIG.  5   . The flexible electrode array is a cuff electrode, which can be wrapped around a peripheral nerve. 
     Hereto, it defines a volume  100  inside the cuff The wrapping direction is along the direction  140 . The cuff electrode has two meander contacts  80  embedded in a self-spiraling polymeric sheet  90 . The cable  150  extends axially from the cuff 
     The electrode array with the meander contact can be part of numerous applications. It is of particular use when a soft and flexible polymer body carries electrode contacts that experience stretching, bending, twisting. 
     The metal layer formed as a meander does not dominate the device&#39;s mechanical properties over that of the polymer layer(s). 
       FIG.  7    illustrates further embodiments of the invention. In  FIG.  7 A , a paddle electrode with two strip-shaped meander contacts is shown, and in  FIG.  7 B , a paddle electrode with four disk-shaped meander contacts  20  is shown. In the same way as in the first embodiment, the cuts in the metal layer are across the predefined direction  140  along which the electrode is flexible. 
       FIG.  8    illustrates a third embodiment of the invention. It illustrates how two-dimensional ring-shaped (annular) electrodes. In the example to the left: three concentric rings  20  can be realized as stretchable in two planar directions (including simultaneous stretching in planar x- and y-directions). The cuts in the pads  20  are not illustrated. The right sketch shows a single stretchable ring electrode can be designed applying meander contacts. Weld contacts for wires are not shown for clearness. Here, the cuts extend on lines which go out radially from the center. Each second cut of the sequence of cuts  50  ends on the outer perimeter of the pad  20 . The other cuts end on the inner perimeter of the pad (i.e., the ring). As in the previously described embodiments, anchoring elements  30 , i.e., through holes the metal along the edges of the pads. 
       FIG.  9    illustrates a grid electrode with  12  ring-shaped (annular) electrode contacts. Each contact pad  20  is formed as described with reference to  FIG.  8   . A possible way of making meander ring-contacts is illustrated in  FIG.  8    (right). The grid electrode of  FIG.  9    is flexible along two planar directions  140  (x, y-directions), i.e., stretch forces  140  may be applied in the Figure left/right, and up/down. 
       FIG.  10    illustrates a further embodiment of the invention. In this embodiment, the electrode is formed as a cable, such as typical deep brain stimulation leads, with electrode contacts  20  at an outer surface in shapes of e.g., three rings  20  (Fig above) or e.g., three segmented rings  20  (Fig bottom). Thus, the electrode (as illustrated in the right sketch of  FIG.  5   ) is wrapped such that the pads  20  are at the outside surface of the thus formed cable. Since the metal is shaped as meanders, the electrode array (i.e., polymer layers  90  and metal layers  20 ) can be wrapped in the way as described. The predefined direction  140  is the direction of the wrapping. 
     Typical applications for the electrode of the invention include—but are not limited to—the contacts of
         nerve cuff electrodes with ring-shaped contacts, refer to  FIG.  5   ;   nerve cuff electrode with dot-shaped contacts;   paddle or grid electrodes with areas of high mechanical bending/stretching/twisting forces, refer to  FIG.  7   .   ring-shaped contacts, refer to  FIG.  8 ,  9   ;   ring-shaped contacts, on the outside of a cable-style electrode (such as a deep brain stimulation lead);   segmented ring-shaped contacts, on the outside of a cable-style electrode (such as a segmented deep brain stimulation lead).       

       FIG.  6   a   ) to  i ) illustrates a fabrication process suitable for producing the flexible electrode array of the invention. 
     In step a), a layer of polymer  40  is deposited to a thickness of typically some 10 μm on a mechanical carrier  130 . 
     In step b) the polymer layer  40  is removed using a laser at location  70  where a weld contact is to be formed. 
     In step c), a metal foil  10  of a typical thickness of 5 to 50 μm is laminated on the polymer  40 . 
     In step d), the metal foil  10  is cut to shape with a laser, forming anchoring holes  30  and the meander  80 ,  50 . 
     In step e), a covering polymer layer  60  is deposited at a thickness of typically some 10 μm. 
     In step f) the polymer layer ablated at the locations of the weld contact  70  and the electrode contact  20 . 
     In step g), the mechanical carrier  130  is removed. 
     In step h), a wire  110  is attached to the weld pad  70 . 
     In step i), the weld  70  pad is sealed with polymer. 
     REFERENCE NUMERALS 
       10  Metal layer (foil) 
       20  Metal layer (contact pad(s)) 
       30  Anchoring element (through hole) 
       40  First (bottom) layer of polymeric material 
       50  Cut (cutting edge of meander structure 
       60  Second (top) layer of polymeric material 
       70  Weld pad 
       80  Meander contact integrated in sheet 
       90  Polymeric sheet 
       100  Volume inside cuff for nerve placement 
       110  Wire 
       120  Circular cut-away as stress relieve 
       130  Mechanical carrier (fabrication aid) 
       140  direction of stretch force 
       150  Cable (wire bundle) 
     A-A′ Longitudinal direction