Patent Publication Number: US-2022218986-A1

Title: A tissue stimulation device with distal and proximal return electrode

Description:
FIELD 
     The present disclosure relates to a tissue stimulation system for providing electrical stimulation. 
     BACKGROUND 
     Implantable electrical stimulation systems may be used to deliver electrical stimulation therapy to patients to treat a variety of symptoms or conditions such as headaches, lower back pain and incontinence. 
     In many electrical stimulation applications, it is desirable for a stimulation system, typically comprising a therapeutic lead (a lead comprises electrodes and electrical connections), to provide electrical stimulation to one or more precise locations within a body. Typically, stimulation is provided using one or more stimulation electrodes that are configured to transfer electrical pulses to tissue with respect to one or more electrical returns. 
     US application US 2010/0057165 describes a neurostimulation paddle lead carrying a plurality of electrodes comprising at least four columns of electrodes having a spacing between two inner electrode columns less than a spacing between the inner electrode columns and adjacent outer electrode columns. The inner electrode columns may also be longitudinally offset from the outer electrode columns. The methods and neurostimulation systems steer current between the electrodes to modify a medial-lateral electrical field created adjacent spinal cord tissue. 
     US application US 2015/0099959 describes an implantable electrode array, including an organic substrate material configured to be implanted into an in vivo environment and to optionally dissolve after implantation and be absorbed, and an electrode mounted to the organic substrate material and configured to acquire signals generated by the in vivo environment. The electrode array includes a connection pad mounted to the organic substrate, and an MRI-compatible conductive trace formed between the electrode and the connection pad. 
     Anatomy and treatment protocols can vary greatly it is therefore advantageous to provide a stimulation system that can be configured to a high degree. Systems and devices are known which comprise a plurality of electrodes proximate to each other in the lead, allowing one or more electrodes to be selected for use. Some electrodes may be selectable as either stimulation or return electrodes, allowing some tuning of the region of stimulation. The degree of electrical energy, such as voltage, current and/or power may also be varied, to provide some degree of control over the level of stimulation. 
     It is an object of the invention to provide an improved tissue stimulation system that provides additional configuration possibilities and adjustments. 
     GENERAL STATEMENTS 
     According to a first aspect of the present disclosure, there is provided a tissue stimulation system comprising an implantable end and a stimulation energy source, the implantable end comprising: an elongated substrate, disposed along a longitudinal axis, the substrate having a first and second surface disposed along substantially parallel transverse planes; one or more stimulation electrodes, comprised in the second surface and configured to transmit energy, in use, to human or animal tissue; and one or more proximal return electrodes, comprised in the first surface or second surface, disposed proximate the one or more stimulation electrodes; the stimulation energy source further comprising: one or more distal return electrodes, disposed distantly from the one or more stimulation electrodes; and a pulse energy controller; the tissue stimulation system further comprising: one or more interconnections between the implantable end and a stimulation energy source, configured and arranged to connect the output of the pulse generator to the one or more stimulation electrodes whereby electrical energy may be transferred, during use, as one or more electrical stimulation pulses to the one or more stimulation electrodes with respect to an electrical return; wherein: the one or more proximal return electrodes are configured as a first part of the electrical return for the one or more stimulation electrodes; and the one or more distal return electrodes are configured as a second part of the electrical return for the one or more stimulation electrodes; the pulse energy controller further comprising a ratio controller, configured and arranged to modify the electrical potential and/or current ratio of the first part to the second part. 
     By providing an implantable end with one or more proximal return electrodes proximate the one or more stimulation electrodes, a substantially transverse electric field may be provided. It may then provide a more concentrated current density and distribution in directions approximately perpendicular to the longitudinal axis (transversely-oriented electric field). This provides an additional degree of configuration for tissue stimulation systems and devices. 
     In addition, a potential and/or current ratio controller provides a convenient way to concentrate or diffuse this substantially transverse field in combination with one or more distal return electrodes comprised in the stimulation energy source: when the electrical return is mainly provided by the one or more proximal return electrodes in the implantable end, the electric field is stronger (more local). Alternatively, when the electrical return is mainly provided by the one or more distal return electrodes in the stimulation energy source, the electric field is weaker (more global). 
     It may be advantageous to configure and arrange the one or more distal return electrodes to be disposed more than 18 mm, preferably more than 24 mm, from the one or more corresponding stimulation electrodes. Additionally or alternatively, it may be advantageous to configure and arrange the one or more proximal return electrodes ( 400 ) to be disposed within less than 8 mm, preferably less than 6 mm, from the one or more corresponding stimulation electrodes. 
     In this context, corresponding means the one or more stimulation electrodes that are configured, in use, to transmit energy to tissue with respect to at least one distal and at least one proximal return electrode. 
     According to a further aspect of the present disclosure, there is provided a tissue stimulation system wherein the one or more proximal return electrodes are elongated along the longitudinal axis. 
     Having elongated electrodes provides stimulation over extended longitudinal dispositions. 
     According to another aspect of the present disclosure, there is provided a tissue stimulation system wherein the one or more stimulation electrodes have a first extent along the longitudinal axis, and the one or more proximal return electrodes have a second extent along the longitudinal axis, the second extent being substantially the same or greater than the first extent. 
     The control over the electric field is preferably provided over at least substantially the same length (extent along the longitudinal axis) as the one or more stimulation electrodes. If the proximal return electrodes are substantially longer (greater extent), they may be operated as a proximal return electrode for more than one stimulation electrode so if another stimulation electrode is selected, a further reconfiguration of the electrodes may be avoided. In other words, if a different active stimulation electrode is selected, this typically results in stimulation energy being applied at a different longitudinal disposition. A substantially longer proximal return electrode may provide an electrical return for the currently selected stimulation electrode and the previously selected stimulation electrode. 
     According to a further aspect of the present disclosure, there is provided a tissue stimulation system wherein the one or more stimulation electrodes are elongated along the longitudinal axis. 
     By being elongated, stimulation energy may be applied over a plurality of dispositions along the longitudinal axis. This greatly reduces the chance that, after implantation, the electrode is not proximate enough to a stimulation target. Particularly, when the target is a nerve. 
     According to yet another aspect of the present disclosure, there is provided a tissue stimulation system wherein the one or more proximal return electrode comprises two proximal return electrode regions, electrically connected to each other, the two proximal return electrode regions being disposed on opposing sides of the one or more stimulation electrode. 
     By providing suitably configured return electrodes with two regions, treatment current density and the position where the highest current densities occur may be predetermined and/or controlled. In some cases, a very high degree of control may not be possible due to, for example, the nature of the treatment, the system and/or device, the implant site and the individual patient in such cases, the configuration of the return electrodes may exert a degree of influence on the current density and its distribution. 
     According to a further aspect of the present disclosure, there is provided a tissue stimulation system wherein the two proximal return electrode regions are two non-contiguous electrode regions, electrically connected to each other; and each return electrode region is separated at least partially along the first transverse axis from the one or more stimulation electrodes by an electrical insulator. 
     As surface area and relative disposition are among the factors that influence treatment current density and distribution, providing at least two non-contiguous regions may provide a high degree of control. In addition, the shape and proximity to the corresponding stimulation electrode may also provide a degree of control. 
     According to a further aspect of the present disclosure, there is provided a tissue stimulation system wherein the two proximal return electrode regions are comprised in a substantially contiguous proximal return electrode. 
     In general, it is advantageous to maximize the tissue contact-area of the one or more return electrodes as this increase the efficiency of energy transfer to the tissue through the one or more stimulation electrodes. In addition, when using a substrate, such as LCP (Liquid Crystal Polymer), which may be easily manipulated using semiconductor processes known to the skilled person, such as deposition, etching and lithography this means that a high degree of control may be exerted on the shape, dimensions (extent), disposition and electrical, physical &amp; mechanical properties of the return electrode. 
     According to a still further aspect of the present disclosure, there is provided a tissue stimulation system wherein one or more of the two proximal return electrode regions extend along the first transverse axis ( 700 ) between an edge of the one or more corresponding stimulation electrodes and an edge of the substrate. 
     In general, it is advantageous to maximize the tissue contact-area of the one or more return electrodes as this increase the efficiency of energy transfer to the tissue through the one or more stimulation electrodes. 
     According to a still further aspect of the present disclosure, a tissue stimulation system is provided wherein a further proximal return electrode is comprised in the second surface. 
     The second surface also comprises the one or more stimulation electrodes, so the space available for one or more return electrodes may be limited. There are fewer restrictions regarding the space available on the first surface for the one or more return electrodes. Having one or more proximal return electrode on both surfaces provides a high degree of flexibility for configuration. 
     According to another aspect of the present disclosure, a tissue stimulation system is provided wherein a plurality of stimulation electrodes is provided, each having one or more corresponding proximal return electrodes. 
     This allows stimulation to be provided at different positions along the longitudinal axis by predetermining and/or controlling the energy passing through one or more of the electrodes. 
     According to another further aspect of the present disclosure, there is provided a tissue stimulation system wherein the combined tissue contact-area of the one or more proximal return electrodes and the one or more distal return electrodes is no less than the tissue contact-area of the one or more stimulation electrodes. 
     In general, it is advantageous to maximize the tissue contact-area of the one or more return electrodes as this increase the efficiency of energy transfer to the tissue through the one or more stimulation electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of some embodiments of the present invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and which are not necessarily drawn to scale, wherein: 
         FIGS. 1A, 1B &amp; 1C  depict longitudinal cross-sections through a first embodiment of an implantable end (lead) of a stimulation system; 
         FIGS. 2A, 2B &amp; 2C  depict an example of a stimulation energy source; 
         FIGS. 3A, 3B &amp; 3C  depict longitudinal cross-sections through a second embodiment of an implantable end (lead) of a stimulation system; 
         FIGS. 4A &amp; 4B  depict examples of how the electric field may be configured to vary the strength of the field close to the electrodes; 
         FIG. 5  and  FIG. 6  depict examples of nerves that may be stimulated to treat headaches; 
         FIG. 7  depicts examples of nerves that may be stimulated for other treatments; 
         FIGS. 8A, 8B &amp; 8C  depict longitudinal cross-sections through a second embodiment of an implantable end (lead) of a stimulation system; 
         FIGS. 9A, 9B &amp; 9C  depict longitudinal cross-sections through a second embodiment of an implantable end (lead) of a stimulation system; and 
         FIGS. 10A and 10B  depict example of electrical paths through the patient body. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, numerous non-limiting specific details are given to assist in understanding this disclosure. 
     In general, stimulation systems described herein may comprise a stimulation energy source and an implantable end the implantable end comprises one or more stimulation electrodes. “Implantable end” means that at least this section of the stimulation system is configured and arranged to be implanted. Optionally, one or more of the remaining sections of the stimulation systems may also be configured and arranged to be implanted. 
       FIGS. 1A, 1B &amp; 1C  depict longitudinal cross-sections through a first embodiment  100  of an implantable end (lead) of a stimulation system  100 ,  150  comprising:
         an elongated substrate  300 , disposed along a longitudinal axis  600 , the substrate having a first  310  and second  320  surface disposed along substantially parallel transverse planes  600 ,  700 . For substrates  300  with a degree of flexibility, the degree to which the first  310  and second  320  surface are along substantially parallel transverse planes  600 ,  700  may be determined by laying the substrate  300  on a substantially flat surface. As depicted, the first surface  310  lies in a plane comprising the longitudinal axis  600  and a first transverse axis  700  the first transverse axis  700  is substantially perpendicular to the longitudinal axis  600 . As depicted, the plane of the first surface  310  is substantially perpendicular to the plane of the cross-section drawing (substantially perpendicular to the surface of the paper). The substrate  300  has a thickness or extent along a second transverse axis  750  this second transverse axis  750  is substantially perpendicular to both the longitudinal axis  600  and the first transverse axis  700  it lies in the plane of the drawing (along the surface of the paper) as depicted. The first surface  310  is depicted as an upper surface and the second surface  320  is depicted as a lower surface.       

     To clarify the different views, the axes are given nominal directions:
         the longitudinal axis  600  extends from the end comprising a stimulation energy source  150  on the left, to the end of the implantable end (the lead), depicted on the right of the page;   the first transverse axis  700  extends into the page as depicted; and   the second transverse axis  750  extends from bottom to top as depicted.       

     For example, the elongated substrate  300  may comprise an elastomeric implantable end composed of silicone rubber, or another biocompatible, durable polymer such as siloxane polymers, polydimethylsiloxanes, polyurethane, polyether urethane, polyetherurethane urea, polyesterurethane, polyamide, polycarbonate, polyester, polypropylene, polyethylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polysulfone, cellulose acetate, polymethylmethacrylate, polyethylene, and polyvinylacetate. Suitable examples of polymers, including LCP (Liquid Crystal Polymer), are described in “Polymers for Neural Implants”, Hassler, Boretius, Stieglitz, Journal of Polymer Science: Part B Polymer Physics, 2011, 49, 18-33 (DOI 10.1002/polb.22169), In particular, Table 1 is included here as reference, depicting the properties of Polyimide (UBE U-Varnish-S), Parylene C (PCS Parylene C), PDMS (NuSil MED-1000), SU-8 (MicroChem SU-8 2000 &amp; 3000 Series), and LCP (Vectra MT1300). 
     Flexible substrates  300  are also preferred as they follow the contours of the underlying anatomical features very closely. Very thin substrates  300  have the additional advantage that they have increased flexibility. 
     Preferably, the flexible substrate  300  comprises an LCP, Parylene and/or a Polyimide. LCPs are chemically and biologically stable thermoplastic polymers which allow for hermetic sensor modules having a small size and low moisture penetration. 
     Advantageously, an LCP may be thermoformed allowing complex shapes to be provided. Very thin and very flat sections of an LCP may be provided. For fine tuning of shapes, a suitable laser may also be used for cutting. For example, LCP substrates  300  with thicknesses (extent along the second transverse axis  750 ) in the range 50 microns (um) to 720 microns (um) may be used, preferably 100 microns (um) to 300 microns (um). For example, values of 150 um (micron), 100 um, 50 um, or 25 um may be provided. Similarly, substrate widths (extent along the first transverse axis  600 ) of 2 mm to 20 mm may be provided using LCP, for example. 
     At room temperature, thin LCP films have mechanical properties similar to steel. This is important as implantable substrates  300  must be strong enough to be implanted, strong enough to be removed (explanted) and strong enough to follow any movement of the anatomical feature and/or structure against which it is implanted. 
     LCP belongs to the polymer materials with the lowest permeability for gases and water. LCPs can be bonded to themselves, allowing multilayer constructions with a homogenous structure. 
     In contrast to LCPs, polyimides are thermoset polymers, which require adhesives for the construction of multilayer substrates. Polyimides are a thermoset polymer material with high temperature and flexural endurance. 
     An LCP may be used, for example, to provide a substrate having multilayers (not depicted) in other words, several layers of 25 um (micron) thickness. Electrical interconnections and/or interconnect layers may also be provided by metallization using techniques from the PCB (Printed Circuit Board) industry, such as metallization with a bio-compatible metal such as gold, silver or platinum. Electro-plating may be used. These electrical interconnections and/or interconnect layers may be used to provide electrical energy to any electrodes. 
     The implantable end of the system  100  depicted in  FIG. 1  further comprises:
         one or more stimulation electrodes  200 , comprised in the second surface  320  and configured to transmit energy, in use, to human or animal tissue (after implantation). In this example, it is electrical energy, and two stimulation electrodes  200  are depicted. Each stimulation electrode  200  has a longitudinal extent along the longitudinal axis  600  and a transverse extent along a first transverse axis  700 , the first transverse axis  700  being substantially perpendicular to the longitudinal axis  600  and substantially parallel to the second surface  320 .       

     “Comprised in the second surface” means that stimulation electrode  200  is relatively thin, and attached to the second surface  320 . The electrode  200  may also be embedded in the second surface  320 . 
     In general, one or more stimulation electrodes  200  may be provided. The number, dimensions and/or spacings of the stimulating electrodes  200  provided in the implantable end  100  may be selected and optimized depending on the treatment for example, if more than one electrode  200  is provided, each electrode  200  may provide a separate stimulation effect, a similar stimulation effect or a selection may be made of one or two electrodes  200  proximate the tissues where the effect is to be created. The electrodes  200  may comprise a conductive material such as gold, silver or platinum, iridium, and/or platinum/iridium alloys and/or oxides. 
       FIG. 1B  depicts two stimulation electrodes  200 , each elongated along the longitudinal axis  200 . Although an oval cross-section is suggested in  FIGS. 1A and 1B , any shape may be used, such a square, rectangular, triangular, polygonal, circular, elliptical, oval, and round. An elongated electrode (or strip electrode) as depicted in  FIG. 9  may also be used. 
     The implantable end  100  of the system  100 ,  150  depicted in  FIG. 1  further comprises:
         one or more interconnections  250 , configured and arranged to provide the one or more stimulation electrodes  200  with electrical energy from a stimulation energy source  150 ; and   one or more interconnections  250 , configured and arranged to provide an electrical return for the one or more stimulation electrodes ( 200 ,  220 ) using one or more proximal return electrodes  400 .       

     The one or more interconnections  250  may be disposed between the first  310  and second surface  320 , comprised in the first surface  310 , comprised in the second surface  320 , or any combination thereof. In this case, they are depicted as being disposed between the first  310  and second surface  320 . 
     They may comprise one or more conductors, such as a metal, formed as required for example, in one or more conductive: wire, strand, foil, lamina, plate, and/or sheet. They may be a substantially contiguous (one conductor) or comprise a plurality of conductors. In this, they are depicted as three wire-like conductors, encapsulated in the substrate  300 , disposed along the longitudinal axis  600 , two of which are connected to the two stimulation electrodes  200 . The third wire-like conductor is connected to the one proximal return electrode  400  (described below in more detail) via a further wire-like conductor, disposed along the second transverse axis  750 . 
     An interconnection  250  in the context of this disclosure is not configured or arranged to be, in use, in contact with human or animal tissue. For example, by embedding the one or more interconnections  250  in a low conductance or insulating substrate  300 , such as LCP. Note that an interconnection  250  may be comprised in the first  310  or second surface  320  if it rendered low conductance and/or insulating by including one or more layers between the interconnection  250  and any human or animal tissue. 
     Additionally or alternatively, the substrate  300  may be a multilayer, comprising one or more electrical interconnection layers to provide the electrode  200  with electrical energy. In use, the electrical interconnections are connected to a source of electrical power (not depicted). If an LCP multilayer is used, the thickness (extent of the substrate  300  along the second transverse axis  750  or the perpendicular distance between the first surface  310  and the second surface  320 ) may be typically approximately 150 um (micron) in the sections with no electrodes  200  or interconnections, 250 um in the sections with an electrode  200 , and 180 um in the sections with an electrical interconnection  250 . If multilayers are used, electrical interconnection layers of 25 um (micron) may be used, for example. 
     The implantable end  100  of the system  100 ,  150  depicted in  FIG. 1  further comprises:
         one or more proximal return electrodes  400  (in this case, one proximal return electrode  400 ), comprised in the first surface  310 , disposed proximate the one or more stimulation electrodes  200  at substantially the same longitudinal disposition  600 .       

     In the context of this disclosure, proximal is used to describe proximity to the one or more stimulation electrodes  200 , comprised in the implantable end  100 . 
     The one or more proximal return (or ground) electrodes  400  are configured to provide, in use, a corresponding electrical return for one or more stimulation electrodes  200 . In other words, the electrical return  400  closes the electrical circuit. Any suitable configuration and arrangement may be provided. Additionally or alternatively, one or more return (ground) electrodes may be provided proximate the one or more stimulation electrodes  200 , at the implantable end  100  of the system  100 ,  150 . 
     In some descriptions of conventional stimulation devices, the return electrode may be referred to as an anode. Traditionally, this has been provided via the housing of an IPG (Implantable Pulse Generator). Stimulation electrodes may similarly be referred to as cathodes. 
     The one or more proximal return electrodes  400  may comprise a conductive material such as gold, silver, platinum, iridium, and/or platinum/iridium alloys and/or oxides. 
     Alternatively, the depicted proximal return electrode  400  may be functionally described as follows: it comprises two proximal return electrode regions  400   a ,  400   b , electrically connected to each other, the two proximal return electrode regions  400   a ,  400   b  being disposed on opposing sides of the one or more stimulation electrode  200 . In other words, if the device  100  was viewed in a transverse cross-section  600 ,  700  (substantially parallel to the first  310  and second  320  substantially planar transverse surfaces), the main regions  400   a ,  400   b  of the proximal return electrode  400  that influence the stimulation current density are disposed directly “above” the one or more stimulation electrodes  200  (but further along the second transverse axis  750 ). In other words, at substantially the same disposition along the first transverse axis  700  as the one or more stimulation electrodes  200 . The regions  400   a ,  400   b  are disposed along the first transverse axis  700  approximately proximate (and approximately parallel to) opposing edges of the substrate  300 . 
     The two proximal return electrode  400  regions a, b are comprised in a substantially contiguous proximal return electrode  400 . 
     An end (or lead)  100  suitable for implanting may comprise, for example, 12 stimulation electrodes over a length of 15 cm. Each stimulation electrode may have dimensions in the order of 6 to 8 mm along the longitudinal axis  600  and 3 to 5 mm along the first transverse axis  700 , so approximately 18 to 40 square mm (mm 2 ). If a strip of 4 mm wide (extent along the first transverse axis  700 ) is provided as a return electrode, then a length (extent along the longitudinal axis  600 ) of 4.5 to 10 mm also provides a tissue contact-area of 18 to 40 square mm (mm 2 ). 
       FIG. 1B  depicts a view of the second surface  320  of the implantable end  100  depicted in  FIG. 1A . In other words, the second surface  320  is depicted in the plane of the paper, lying along the longitudinal axis  600  (depicted from bottom to top) and in the first transverse axis  700  (depicted from left to right). The second transverse axis  750  extends into the page. This is the view facing the animal or human tissue which is stimulated (in use). The first surface  310  is not depicted in  FIG. 1B , but lies at a higher position along the second transverse axis  750  (into the page), and is also substantially parallel to the plane of the drawing. 
     The substrate  300  extends along the first transverse axis  700  (considered the width of the implantable end  100  of the stimulation device) between two extents. 
     The implantable end  100  of the device may be implanted by first creating a tunnel and/or using an implantation tool. 
     The one or more proximal return electrodes  400  are depicted in  FIGS. 1A and 1C , but not in  FIG. 1B . 
     After implantation of the implantable end  100  of the system  100 ,  150 , a source of stimulation energy may be configured and arranged to provide, in use, electrical energy to the one or more stimulation electrodes  200  with respect to the electrical return applied to the one or more return electrodes  400 . 
     By providing one or more proximal return electrodes proximate the one or more stimulation electrodes at substantially the same longitudinal disposition, a substantially transverse electric field may be provided. It may then provide a more concentrated current density and distribution in directions approximately perpendicular to the longitudinal axis (transversely-oriented electric field). This provides an additional degree of configuration for stimulation systems and devices. 
       FIGS. 2A, 2B &amp; 2C  depict longitudinal cross-sections through an example of a stimulation energy source  150 , suitable for use with any of the implantable ends described in this disclosure, including the first example  100  depicted in  FIG. 1 . Optionally, the stimulation energy source  150  may be configured and arranged to be implantable. 
     In this example, they are assumed to be comprised in the same substrate  300 . The stimulation energy source  150  comprises analogous features to those depicted in  FIG. 1 : 
     the implantable end  100  comprises the same features as depicted in  FIG. 1 :
         the elongated substrate  300 , disposed along the longitudinal axis  600 , with the first  310  and second  320  surfaces;   the first transverse axis  700  extending into the page as depicted, and the second transverse axis  750  extending from bottom to top as depicted;   the longitudinal axis  600  extending from the stimulation energy source  150  end on the left, to the implantable end  100 , towards the right of the page.       

     The dimensions of the substrate  300  (extent along the first transverse axis  700  or width, extent along the second transverse axis  750  or thickness) at the implantable  100  and energy source  150  ends are depicted as approximately the same. It is convenient if the device  100 ,  150  comprises the same substrate  300 , allowing it to be made from a single piece of material this is advantageous if the implantable end  100  of the device is substantially completely implanted as this may reduce the risk of fluid ingress into the device. 
     The stimulation energy source  150  further comprises:
         a pulse energy controller  550 , configured and arranged to provide stimulation energy through the one or more stimulation electrodes  200  as one or more electrical pulses. This changes the electrical potential (or voltage) and/or current applied to the one or more stimulation electrodes  200 . The pulse energy controller  550  may be connected to the one or more electrodes  200  through one or more interconnections  250 .   one or more distal return (or ground) electrodes  450 , configured and arranged to provide, in use, a corresponding electrical return for the one or more stimulation electrodes  200  which receive stimulation energy from the pulse energy controller  550 . In other words, the electrical return  450  closes the electrical circuit. Any suitable configuration and arrangement may be provided for example, as depicted, one distal return electrode  450  is comprised in the second surface  320 , proximate the pulse energy controller  550 . Additionally or alternatively, one or more distal return (ground) electrodes  450  may be comprised in the first surface  310 . Additionally or alternatively, a further distal return electrode  450  may be provided between the energy source  150  and the implantable end  100 , but closer to the energy source end  150 . In this case, an additional substrate protrusion  350  is provided for the further distal ground electrode  450 .       

     In the context of this disclosure, distal is used to describe proximity to the pulse energy controller  550 , comprised in the stimulation energy source  150  and/or close to the energy source  150 . 
     Preferably, the one or more distal return electrodes  450  that are to be actively used are comprised in a section of the device that is configured and arranged to be implantable. In addition, during use, the active one or more distal return electrodes  450  are preferably implanted so that they may provide a corresponding electrical return for the implanted one or more stimulation electrodes  200  that are active. 
     The one or more distal return electrodes  450  may comprise a conductive material such as gold, silver, platinum, iridium, and/or platinum/iridium alloys and/or oxides. 
     The stimulation energy source  150  further comprises:
         one or more interconnections  250 , configured and arranged to connect the output of the pulse generator  550  to the one or more stimulation electrodes  200  such that electrical energy may be transferred. In  FIG. 2B , three dashed longitudinal lines are used to represent interconnections  250  between the pulse energy controller  550  and the implantable end  100  one for each stimulation electrode  200  and one for the proximal return electrode  400 .   one or more interconnections  250 , configured and arranged to provide an electrical return for the one or more stimulation electrodes  200 ,  220  using one or more distal return electrodes  450 . In  FIG. 2B , one dashed longitudinal line is used to represent interconnections  250  between the pulse energy controller  550  and the further distal return electrode  450  comprised in the substrate protrusion  350 .       

     The functions comprised in the pulse energy controller  550 , the separation into functional units, and the components used in each functional unit may be any suitable mix to perform the required functions. In terms of the invention, during stimulation operation, the pulse energy controller  550  transfers electrical energy to one or more stimulation electrodes  200  as one or more electrical stimulation pulses. 
     The one or more proximal return electrodes  400  are configured as a first part of the electrical return  400 ,  450  for the one or more stimulation electrodes  200 ; and the one or more distal return electrodes  450  are configured as a second part of the electrical return  400 ,  450  for the one or more stimulation electrodes  200 . 
     So the electrical return comprises the first part, proximate the one or more stimulation electrodes  200  and the second part, distant from (distal) one or more stimulation electrodes. 
     The pulse energy controller  550  further comprises a ratio controller, configured and arranged to modify the electrical ratio of the first part (proximal) of the electrical return to the second part (distal). The ratio controller provides a convenient way to concentrate or diffuse the substantially transverse  700 ,  750  electric field. 
     The proximal:distal ratio may vary between 0:1 and 1:0. Expressed in percentages, this is 0%: 100% to 100%: 0%. When one of the parts of the electrical returns is approximately 0, it is substantially disabled and very similar to the situation where those types of electrodes are not connected (or are disconnected). 
     At 1:0, the electric field is stronger (more localized in the regions close to the stimulation electrodes). At 0:1, the electric field is weaker (more global, and distributed through the tissue between the proximal return electrodes  400  and the distal electrodes  450 ). 
     Stimulation electrodes  200 , such as those depicted in  FIG. 1C , may have, for example, a dimension along the longitudinal axis  600  (a longitudinal extent) on the order of 6 to 8 mm, with a pitch of 10 to 12 mm along the longitudinal axis. 
     An active proximal return electrode  400  is most preferably disposed at substantially the same longitudinal disposition as the corresponding active one or more stimulation electrodes  200 . 
     Although less preferred, an active return electrode  400  may also be considered proximal if it is disposed within a distance of one stimulation electrode longitudinal extent (for example, 6 to 8 mm) from the corresponding active one or more stimulation electrodes  200 . 
     Although even less preferred, an active return electrode  400  may also be considered proximal if it is disposed within a distance of two stimulation electrode longitudinal extents (for example, 12 to 16 mm) from the corresponding active one or more stimulation electrodes  200 . 
     An active return electrode  450  may be considered distal if it is more than three stimulation electrode longitudinal extent (for example, 18 to 24 mm) from the corresponding one or more active stimulation electrodes  200 . 
     US 2010/0057165 describes more than one return electrodes at different distances from one or more corresponding stimulation electrode. However, in the terms of the invention, these are proximal return electrodes comprised in the implantable end or paddle. It does not describe the combined use of a distal return electrode, comprised in the stimulation energy source, together with a proximal electrode using a predetermined ratio. 
       FIG. 10A  depicts a simplified electrical diagram of the electrical energy paths through the patient body. 
     The electrical energy is provided as stimulation pulses from an energy source  1550  comprised in the pulse energy controller  550 . The stimulation pulses are provided to one or more stimulation electrodes  200 . Two main paths of stimulation current are created through the patient body:
         a proximal (local) path, from the one or more stimulation electrodes  200 , through proximal resistive tissue  1200  to the one or more proximal return electrode  400 , and returning back to the pulse energy controller  550 ;   a distal (global) path, from the one or more stimulation electrodes  200 , through distal resistive tissue  1250  to the one or more distal return electrode  450 , and returning back to the pulse energy controller  550 .       

     As depicted in  FIG. 10A , the ratio controller may be implemented using:
         a proximal variable resistor  1400  connected between the one or more proximal return electrodes  400  and the electrical return of the pulse energy source  1550 ;       

     and
         a distal variable resistor  1450  connected between the one or more distal return electrodes  450  and the electrical return of the pulse energy source  1550 .       

     By coupling together the variable adjustments of the proximal variable resistor  1400  and the distal variable resistor  1450 , an increase in one of the resistance values may cause a decrease in resistance of the other, and vice-versa. 
     Additionally or alternatively, a fixed resistor may be used in one of the paths to adjust the ranges of ratios that may be controlled in this way. 
     One proximal path  1200 ,  400 ,  1400  is depicted in  FIG. 10A  optionally, if more than one active proximal return electrode  400  is provided, more than one proximal path  1200 ,  400 ,  1400  may also be provided. They may be configured for operation together (in other words, more than one proximal return electrodes  400  connected electrically to the same proximal variable resistor  1400 ) or they may be configured to be operated separately (in other words, more than one proximal return electrode  400  connected electrically to more than one proximal variable resistor  1400 ). If configured to be operated separately, the ratios between different proximal paths may be adjusted this may be advantageous as it may create a more complex distribution of field densities. 
     One distal path  1250 ,  450 ,  1450  is depicted in  FIG. 10A  optionally, if more than one active distal return electrode  450  is provided, more than one distal path  1250 ,  450 ,  1450  may also be provided. They may be configured for operation together (in other words, more than one distal return electrodes  450  connected electrically to the same distal variable resistor  1450 ) or they may be configured to be operated separately (in other words, more than one distal return electrode  450  connected electrically to more than one distal variable resistor  1450 ). If configured to be operated separately, the ratios between different distal paths may be adjusted this may be advantageous as it may create a more complex distribution of field densities. 
     The variable resistors  1400 ,  1450  may be linear. One or both may be non-linear. One or both may be logarithmic Additionally or alternatively, any non-linear or logarithmic behavior may be modified by adapting a user interface to make the adjustments seem more linear to the user. If a digital control system is used, then any required behavior may be provided. 
     The resistance of human tissue is typically in the range 0.8 kOhm to 1.2 kOhm. Human tissue is highly conductive. The resistance of the paths is less affected by the length of the path through tissue, and affected to a higher degree by the tissue contact area of the electrodes  200 ,  400 . 
     With pulses of 50 Hz (250 microseconds), typical values are:
         a proximal resistive tissue  1200  of 0.8 kOhm to 1.2 kOhm   a proximal variable resistance  1400  of 0 to 2 kOhm   a distal resistive tissue  1250  of 0.8 kOhm to 1.2 kOhm   a distal variable resistance  1450  of 0 to 2 kOhm       

       FIG. 10B  depicts an alternative simplified electrical diagram of the electrical energy paths through the patient body. 
     It is the same as the circuit in  FIG. 10A , except for the use of a rheostat  1425  instead of the variable resistors  1400  and  1450 . One end of the rheostat  1425  is electrically connected to the proximal return electrode  400 , and the other side of the rheostat  1425  is connected to the distal return electrode  450 . The tap (or slider) is connected to the electrical return of the pulse energy source  1550 . 
     By moving the tap (or slider), an increase in one of the path resistance values is provided at the same time as a decrease in resistance of the other path, and vice-versa. 
     Additionally or alternatively, a fixed resistor may be used in one of the paths to adjust the ranges of ratios that may be controlled in this way. 
     Variable resistors and/or rheostats  1400 ,  1425 ,  1450  may result in unwanted heat generation in the pulse energy controller  550 . 
     Alternatively or additionally, a time multiplexer may be used to control the amount of time that each return path  1200 ,  400  and  1250 ,  450  is connected to the energy source. Such a configuration and arrangement reduces heat generation in the pulse energy controller  550 . The longer that a path is connected within a particular period of time, the larger its contribution to electrical return. For example:
         connecting one of the return paths for a plurality (X) of stimulation pulses, then connecting the other return paths for a further plurality (Y) of stimulation pulses. The ratio is then determined by X:Y for the period X+Y. For example, a 1:1 ratio is possible by switching between pulses.   connecting one of the return paths for a percentage (P) of a pulse width, then connecting the other return paths for a further percentage (Q) of the pulse width. The ratio is then determined by P:Q for the pulse width P+Q. For example, a 1:1 ratio is possible by switching halfway through each pulse.       

     Variable resistors  1400 ,  1450  may also be used in combination with a time multiplexer. 
     Alternatively or additionally, a plurality of (more than one) distal return paths  450 ,  1250  may be provided by providing more than one distal return electrode  450 . When the tissue contact area of the return electrodes is maximised, the resistance  1250  through the patient tissue is mainly determined by the tissue contact surface area of the one or more distal return electrodes  450 . 
     Alternatively or additionally, a plurality of (more than one) proximal return paths  400 ,  1200  may be provided by providing more than one proximal return electrode  400 . When the tissue contact area of the return electrodes is maximised, the resistance  1200  through the patient tissue is mainly determined by the tissue contact surface area of the one or more proximal return electrodes  400 . 
     Alternatively or additionally, the pulse energy controller  550  may comprise switches which may switch one or more of the plurality of return paths  400 ,  1200  and/or  450 ,  1250  into or out of the electrical return circuit to the energy source  1550 . The time multiplexer described above may be further configured and arranged to switch between the plurality of return paths. 
     Additionally or alternatively, the one or more return electrodes  400 ,  450  may be increase or reduced in tissue contact area to predetermine the tissue contact area, which may influence the resistance of the return path  1200 ,  1250  through the body. 
     Additionally or alternatively, resistive elements may be comprised in the pulse energy controller  550 , the one or more interconnections  250 , and/or the one or more return electrode  400 ,  450  to predetermine differences in resistance between the return paths  400 ,  1200  and  450 ,  1250   
       FIGS. 4A &amp; 4B  depict examples of how the electric field may be configured to vary the strength of the field close to the electrodes.  FIG. 4A  &amp;  FIG. 4B  depict transverse cross-sections in the plane comprising the first transverse axis  700  and the second transverse axis  750  through a modified version of the electrodes depicted in  FIGS. 1A, 1B and 1C . As viewed, the longitudinal axis  600  is perpendicular to the plane of the drawing (or the plane of the paper), and the direction is emerging. In other words, the transverse cross-section is viewed from an implantable end  100  looking towards an energy source  150  end. 
     One or more proximal return electrodes  400  are provided, comprised in the first surface  310 . One or more stimulation electrodes  200  are provided, comprised in the second surface  320 . 
     A corresponding proximal return electrode  400  and a stimulation electrode  200  are depicted. The proximal return electrode  400  is configured as a return (for example, a ground or 0V) for the stimulation electrode  200  if a positive voltage is applied to stimulation electrode  200 , an electric field may be provided, in use, in the region between the stimulation electrode  200  and the proximal return electrode  400 . Examples of lines of equipotential  570   a  to  570   f  are also depicted the first equipotential  570   a  approximately coincides with the edges of the transverse extent  700  of the proximal return electrode  400 . This is approximately the same potential as the proximal return electrode  400 , here (for example) ground or 0V. 
     The last equipotential  570   f  approximately coincides with the edges of the transverse extent  700  of the stimulation electrode  200 . This is approximately the same potential as the stimulation electrode  200 . 
     Between the first  570   a  and last  570   f  equipotential lines, intermediate equipotential lines  570   b  to  570   e  are depicted the distance between the equipotential lines  570  increases linearly over the distance “around” the substrate from the transverse edge  700  of the proximal return electrode  400  to the transverse edge  700  of the stimulation electrode  200 . 
     For example, if 5V is applied to the stimulation electrode  200 , and the proximal return electrode  400  is configured as ground (0V), then the approximate potential at each equipotential line  570  is  570   a  at 0V,  570   b  at 1V,  570   c  at 2V,  570   d  at 3V,  570   e  at 4V and  570   f  at 5V. 
     The transverse disposition  700  of the stimulation electrode  200  and the proximal return electrode  400  are approximately the same, providing a substantially symmetrical electrical field. 
     The extent along the first transverse axis  700  (width) of the corresponding return  400  and stimulation  200  electrodes is larger in  FIG. 4A  than in  FIG. 4B . The first equipotential  570   a  is substantially disposed at the transverse edge  700  of the proximal return electrode  400  and the last equipotential  570   f  is substantially disposed at the transverse edge  700  of the stimulation electrode  200 . The electrical field is provided between these two edges, “around” the substrate the disposition difference along the first transverse axis  700  and/or the second transverse axis  750  determine the derivative of the potential over the distance the closer the edges, the more local (the stronger) the electric field. 
     In addition, although not depicted in  FIGS. 4A and 4B , the relative disposition along the longitudinal axis  600  of the edges also determines the longitudinal disposition of the electric field. 
     Although depicted as substantially symmetrical, the transverse positions of the proximal return  400  and stimulation  200  electrodes may be asymmetrical to provide a more asymmetrical electric field. 
     As depicted, the transverse extent  700  of the proximal return electrode  400  is less than the transverse extent  700  of the substrate. By making the transverse extents  700  more similar and optionally equal, the first equipotential  570   a  then approximately coincides with the edges of the transverse extent  700  of the substrate. 
     The devices  100 ,  101 ,  102 ,  103 ,  104 ,  105 ,  106  may further comprise one or more (conventional) stimulation electrodes not having a corresponding proximal return electrode. 
     Any of the proximal return electrode configurations  400 ,  401 ,  402 ,  403  disclosed herein may be combined with any of the stimulation electrode configurations  200 ,  220  disclosed. 
     From US 2011/0093043 A1, it is known to operate a stimulation device using a combination of return electrodes at different separations from the one or more stimulation electrodes. Although, substantially transverse fields may be created, as depicted in  FIG. 20  and explained in the corresponding part of the description, the device requires two implantable electrode ends to create a substantially transverse field. This is because the proximal return electrode is not at substantially the same longitudinal disposition in the implantable end as the corresponding stimulation electrode the lead comprises a sequence of electrodes, some of which may be configured as return electrodes, which creates a substantially longitudinal field. In addition, the return electrode (or anode) may be provided via the housing of the Implantable Pulse Generator (IPG). 
     In the embodiments described in this disclosure, the use of an elongated substrate  300  with stimulation electrodes at substantially the same longitudinal disposition as the one or more proximal return electrodes means that a substantially transverse field may be created using only one lead. When using implanted leads (implantable ends comprising one or more electrodes), a reduction in the number of leads is advantageous. 
     This may be provided by a tissue stimulation system  100 ,  101 ,  102 ,  103 ,  150  comprising:
         an elongated substrate  300 ,  350 , disposed along a longitudinal axis  600 , the substrate having a first  310  and second  320  surface disposed along substantially parallel transverse planes  600 ,  700 , the substrate  300 ,  350  further comprising:
           one or more stimulation electrodes  200 ,  220 , comprised in the second surface  320  and configured to transmit energy, in use, to human or animal tissue; and   one or more proximal return electrodes  400 ,  401 ,  402 ,  403 , comprised in the first surface  310  or second surface  320 , disposed proximate the one or more stimulation electrodes  200 ,  220  at substantially the same longitudinal disposition  600 ;   
           the stimulation system further comprising:   a pulse energy controller  550 , configured and arranged to transfer electrical energy, during use, as one or more electrical stimulation pulses to the one or more stimulation electrodes  200 ,  220  with respect to an electrical return  400 ;       

     wherein:
         the one or more proximal return electrodes  400 ,  401 ,  402 ,  403  are configured as the electrical return  400  for the one or more stimulation electrodes  200 ,  220 .       

     In addition, in the embodiments in this disclosure, the geometric relationships between the one or more stimulation electrodes may be predetermined to a high degree, and are less dependent on the dispositions after implantation. Instead of relying on a distal return electrode that is at least tens of centimeters away (in US 2011/0093043 A1, use is made of the IPG housing), the embodiments in this disclosure use one or more distal return electrodes  450  more than three stimulation electrode longitudinal extents (for example, 18 to 24 mm) from the corresponding one or more active stimulation electrodes  200 . This may improve control of the field density. Additionally, it may also reduce energy loss through the return paths. 
     As the resistance is mainly dependent on the tissue contact areas of the return electrodes  400 ,  450 , the embodiments in this disclosure may provide return path resistances which may be predetermined to a high degree. 
       FIGS. 3A, 3B and 3C  depict a second embodiment of an implantable end  101 . It is the same as the first embodiment  100 , depicted in  FIG. 1  except:
         the proximal return electrode  401  similarly comprises two electrode regions  401   a ,  401   b , electrically connected to each other, where each region comprises an electrode region. In  FIG. 1 , the two electrode regions  400   a ,  400   b  are comprised in a substantially contiguous proximal return electrode  400 . Here, in  FIG. 3 , they are two non-contiguous electrode regions a, b, electrically connected to each other; and each return electrode region a, b is separated at least partially along the first transverse axis  700  from the one or more stimulation electrodes  200  by an electrical insulator, in this case the substrate material  300 .   the electrical connection between the two non-contiguous proximal return electrode regions  401   a ,  401   b  is made through one or more interconnections  250 .       

     Similar to the return electrode  400  depicted in  FIG. 1 , the longitudinal  600  extent of the electrode regions  401   a ,  401   b  in  FIG. 3  are approximately the same as the longitudinal  600  extent of the corresponding one or more stimulation electrodes  200 . 
     The electrode regions  401   a ,  401   b  are least partially disposed along the first transverse axis  700  on opposite sides of the lower stimulation electrode  200 . In other words, if the device  101  was viewed in a transverse cross-section  600 ,  700  (substantially parallel to the first  310  and second  320  substantially planar transverse surfaces), the regions  401   a ,  401   b  of the proximal return electrode  401  that influence the stimulation current density are disposed directly “above” (further along the second transverse axis  750 ) the transverse edges of the one or more stimulation electrode  200 . 
       FIGS. 8A, 8B and 8C  depict a third embodiment of an implantable end  102 . It is the same as the second embodiment  101 , depicted in  FIG. 3  except:
         as depicted in  FIG. 8C , no proximal electrode is comprised in the first surface  310     a proximal return electrode  402  is provided, comprised in the second surface  320  and comprising two electrode regions  402   a ,  402   b  as two non-contiguous electrode regions  402   a ,  402   b , electrically connected to each other through one or more interconnections  250 , and configured to provide, in use, a corresponding proximal electrical return  402  for the one or more stimulation electrodes  200 .       

     As in  FIG. 3 , this proximal return electrode  402  has a longitudinal  600  extent which approximates the longitudinal  600  extent of the one or more stimulation electrodes  200 . 
     The regions  402   a ,  402   b  providing the corresponding proximal electrical return are at approximately the same longitudinal disposition  600  as their corresponding one or more stimulation electrodes  200 . 
     Similar to proximal return electrode  401  in  FIG. 3 , the two electrode regions  402   a ,  402   b  of the proximal return electrode  402  are disposed on opposing sides of the one or more stimulation electrode  200 . Also each return electrode region  402   a ,  402   b  is separated at least partially along the first transverse axis  700  from the one or more stimulation electrodes  200  by an electrical insulator. 
       FIGS. 9A, 9B &amp; 9C  depict a longitudinal cross-section through an implantable end of a fourth embodiment  103 . It is the same as the third embodiment  102 , depicted in  FIG. 8  except:
         a stimulation electrode  220  is provided which is elongated along the longitudinal axis  600 .       

     As in  FIG. 8 , the proximal return electrode  403  comprises comprising two electrode regions  403   a ,  403   b  electrically connected to each other, through one or more interconnections  250 , and configured to provide, in use, a corresponding electrical return for the stimulation electrode  220 . 
     The two electrode regions  403   a ,  403   b  are elongated along the longitudinal axis  600 , and disposed on opposing sides of the stimulation electrode  220 , each electrode region  403   a ,  403   b  being separated from the stimulation electrode  220  by an electrical insulator (in this case, a separation between the conducting return electrode  403   ab  and the conducting stimulation electrode  220  which have been applied to a very low conducting (and/or very high resistant) substrate  300 ). Typically, the separation will be in the range 1 to 2 mm. Less than 1 mm may also be used, although it may be necessary to compensate for parasitic capacitance. 
     In general, one or more stimulation electrodes  220  may be provided. The number, dimensions and/or spacings of the stimulating electrodes  220  may be selected and optimized depending on the treatment for example, if more than one electrode  220  is provided, each electrode  220  may provide a separate stimulation effect, a similar stimulation effect or a selection may be made of one or two electrodes  220  proximate the tissues where the effect is to be created. The electrodes  220  may comprise a conductive material such as gold, silver, platinum, iridium, and/or platinum/iridium alloys and/or oxides. 
     Although a rectangular cross-section is suggested in  FIGS. 9A and 9B , any shape may be used, such a square, rectangular, triangular, polygonal, circular, elliptical, oval, and round. Typically, an elongated electrode  220  is used to provide stimulation energy along the entire extent this is advantageous if the position of nerves to be stimulated is difficult to determine precisely. 
     In practice, the disposition and path of the nerve pathways vary from person-to-person, and it can happen that after implantation, a stimulation device may not function correctly due to misalignment. However, by using an elongated electrode  220 , implanted at a significant angle (in some cases, approximately perpendicular), alignment becomes less critical there is an increased chance that the elongated electrode  220  crosses a point in one or more nerve pathways, and the device  106  may be used to stimulate that nerve pathway. 
     In general, the combined active tissue contact-area of the return electrodes  400 ,  401 ,  402 ,  403 ,  450  is preferably equal to or more than the active tissue contact area contact-area of the one or more stimulation electrodes  200 ,  220 . The tissue contact-areas to be considered are not the total contact areas, but the contact areas configured to be active during use typically, this will be the whole (or a large proportion) of the return electrodes  400 ,  401 ,  402 ,  403 ,  450 , and one or more stimulation electrodes  200 ,  220 . The stimulation electrode  220  may be selected to provide tissue stimulation at a particular disposition two or more stimulation electrodes  220  may be made active if stimulation over a larger area is required and/or at a disposition between the active electrodes  220 . 
     In general, the ratio between the tissue contact areas does not need to be determined exactly they are preferably of a similar order of magnitude. For example, it may be sufficient if the combined active tissue contact area of the one or more return electrodes is equal to or more than 70% to 100% of the active tissue contact area of the one or more stimulation electrodes  200 ,  220 . 
     An implantable device with an end (or lead) suitable for implant may comprise, for example, 12 stimulation electrodes over a length of 15 cm. A stimulation electrode may have dimensions on the order of 6 to 8 mm along the longitudinal axis  600  and 3 to 5 mm along the first transverse axis  700 , so approximately 18 to 40 square mm (mm2) If a strip of 4 mm wide (extent along the first transverse axis  700 ) is provided as a return electrode, then a length (extent along the longitudinal axis  600 ) of 4.5 to 10 mm also provides a tissue contact-area of 18 to 40 square mm (mm2) The electric field is more concentrated between the strip (elongated electrode) and the corresponding stimulation electrode  200 ,  220 . 
       FIG. 5  and  FIG. 6  depict examples of nerves that may be stimulated using a suitably configured implantable ends  100 ,  101 ,  102 ,  103  to provide neurostimulation to treat, for example, headaches or primary headaches. Providing suitably configured proximal return electrodes  400  means that the stimulation current density in substantially transverse directions  700 ,  750  may be increased, providing an improved stimulation along a longitudinal axis of one or more nerves or nerve branches. 
       FIG. 5  depicts the left supraorbital nerve  910  and right supraorbital nerve  920  which may be electrically stimulated using a suitably configured device.  FIG. 6  depicts the left greater occipital nerve  930  and right greater occipital nerve  940  which may also be electrically stimulated using a suitably configured device. 
     Depending on the size of the region to be stimulated and the dimensions of the part of the device to be implanted, a suitable location is determined to provide the electrical stimulation required for the treatment. Approximate implant locations for the distal part of the stimulation device comprising stimulation devices  100 ,  101 ,  102 ,  103  are depicted as regions:
         location  810  for left supraorbital stimulation and location  820  for right supraorbital stimulation for treating chronic headache such as migraine and cluster.   location  830  for left occipital stimulation and location  840  for right occipital stimulation for treating chronic headache such as migraine, cluster, and occipital neuralgia.       

     In many cases, these will be the approximate locations  810 ,  820 ,  830 ,  840  for the implantable device  100 ,  101 ,  102 ,  103 . 
     For each implant location,  810 ,  820 ,  830 ,  840  a separate stimulation system may be used. Where implant locations  810 ,  820 ,  830 ,  840  are close together, or even overlapping, a single stimulation system may be configured to stimulate at more than one implant location  810 ,  820 ,  830 ,  840 . 
     A plurality of stimulation devices  100 ,  101 ,  102 ,  103  may be operated separately, simultaneously, sequentially or any combination thereof to provide the required treatment. 
       FIG. 7  depicts further examples of nerves that may be stimulated using a suitably configured improved implantable device  100 ,  101 ,  102 ,  103  to provide neurostimulation to treat other conditions. As in  FIGS. 5 and 6 , the ability to increase the stimulation current density in transverse directions  700  improves the stimulation along a longitudinal axis of the nerve or nerve branches. The locations depicted in  FIG. 5  and  FIG. 6  ( 810 ,  820 ,  830 ,  840 ) are also depicted in  FIG. 7 . 
     Depending on the size of the region to be stimulated and the dimensions of the part of the device to be implanted, a suitable location is determined to provide the electrical stimulation required for the treatment. Approximate implant locations for the part of the stimulation device comprising stimulation electrodes are depicted as regions:
         location  810  for cortical stimulation for treating epilepsy;   location  850  for deep brain stimulation for tremor control treatment in Parkinson&#39;s disease patients; treating dystonia, obesity, essential tremor, depression, epilepsy, obsessive compulsive disorder, Alzheimer&#39;s, anxiety, bulimia, tinnitus, traumatic brain injury, Tourette&#39;s, sleep disorders, autism, bipolar; and stroke recovery;   location  860  for vagus nerve stimulation for treating epilepsy, depression, anxiety, bulimia, obesity, tinnitus, obsessive compulsive disorder and heart failure;   location  860  for carotid artery or carotid sinus stimulation for treating hypertension;   location  860  for hypoglossal &amp; phrenic nerve stimulation for treating sleep apnea;   location  865  for cerebral spinal cord stimulation for treating chronic neck pain;   location  870  for peripheral nerve stimulation for treating limb pain, migraines, extremity pain;   location  875  for spinal cord stimulation for treating chronic lower back pain, angina, asthma, pain in general;   location  880  for gastric stimulation for treatment of obesity, bulimia, interstitial cystitis;   location  885  for sacral &amp; pudendal nerve stimulation for treatment of interstitial cystitis;   location  885  for sacral nerve stimulation for treatment of urinary incontinence, fecal incontinence;   location  890  for sacral neuromodulation for bladder control treatment;       

     and
         location  895  for fibular nerve stimulation for treating gait or footdrop.       

     Other condition that may be treated include gastro-esophageal reflux disease and inflammatory diseases. 
     The descriptions thereof herein should not be understood to prescribe a fixed order of performing the method steps described therein. Rather the method steps may be performed in any order that is practicable. Similarly, the examples are used to explain the algorithm, and are not intended to represent the only implementations of these algorithms—the person skilled in the art will be able to conceive many different ways to achieve the same functionality as provided by the embodiments described herein. 
     In general, for any of the configurations described and depicted in this disclosure, any electrode  200 ,  400  may be connected as either a stimulating  200  or return electrode  400 . This may be advantageous if it is uncertain whether the implantable end is above or below the targeted tissue for example, above or below a nerve. 
     Although the present invention has been described in connection with specific exemplary embodiments, it should be understood that various changes, substitutions, and alterations apparent to those skilled in the art can be made to the disclosed embodiments without departing from the spirit and scope of the invention as set forth in the appended claims. 
     For example, a method of controlling pulse energy of a tissue stimulation system  100 ,  101 ,  102 ,  103 ,  150  may be provided, the system comprising an implantable end  100 ,  101 ,  102 ,  103  and a stimulation energy source  150 ,
         the implantable end  100 ,  101 ,  102 ,  103  comprising:
           an elongated substrate  300 ,  350 , disposed along a longitudinal axis  600 , the substrate having a first  310  and second  320  surface disposed along substantially parallel transverse planes  600 ,  700 ;   one or more stimulation electrodes  200 ,  220 , comprised in the second surface  320 ; and   one or more proximal return electrodes  400 ,  401 ,  402 ,  403 , comprised in the first surface  310  or second surface  320 , disposed proximate the one or more stimulation electrodes  200 ,  220 ;   
           the stimulation energy source  150  comprising:
           one or more distal return electrodes  450 , disposed distantly from the one or more stimulation electrodes  200 ,  220 ; and   a pulse energy controller  550  comprising a ratio controller;   
           the tissue stimulation system  100 ,  101 ,  102 ,  103 ,  150  further comprising:
           one or more interconnections  250  between the implantable end  100 ,  101 ,  102 ,  103  and a stimulation energy source  150 ;   
           the method comprising:
           configuring the one or more stimulation electrodes  200 ,  220  to transmit energy, in use, to human or animal tissue;   configuring and arranging the output of the pulse energy controller  550  to connect to the one or more stimulation electrodes  200  whereby electrical energy may be transferred, during use, as one or more electrical stimulation pulses to the one or more stimulation electrodes  200 ,  220  with respect to an electrical return  400 ,  450 ;   configuring the one or more proximal return electrodes  400 ,  401 ,  402 ,  403  as a first part of the electrical return  400 ,  450  for the one or more stimulation electrodes  200 ,  220 ;   configuring the one or more distal return electrodes  450  as a second part of the electrical return  400 ,  450  for the one or more stimulation electrodes  200 ,  220 ; and   configuring and arranging the ratio controller to modify the electrical potential and/or current ratio of the first part to the second part.   
               

     For example, the return electrode embodiments may be implemented using one or more stimulation electrodes described in this disclosure. Examples of the implementation include F1, F2, F3, F4 or F5: 
     F.1 An implantable stimulation device comprising:
         an elongated substrate  300 , disposed along a longitudinal axis  600 , the substrate having a first  310  and second  320  surface disposed along substantially parallel transverse planes  600 ,  700 , the substrate  300  further comprising:   a stimulation electrode  200 ,  220 , comprised in the second surface  320  and configured to transmit energy, in use, to human or animal tissue, the stimulation electrode  200 ,  220  having a longitudinal extent along the longitudinal axis  600  and a transverse extent along a first transverse axis  700 , the transverse axis  700  being substantially perpendicular to the longitudinal axis  600  and substantially parallel to the second surface  320 ; and   a return electrode  400 ,  401 ,  402 ,  403 , comprised in the first surface  310 , proximate the stimulation electrode  200 ,  220 , configured to provide, in use, a corresponding electrical return for the stimulation electrode  200 ,  220 ;   wherein:   the return electrode  400 ,  401 ,  402 ,  403  is elongated along the longitudinal axis  600 ; and   the return electrode  400 ,  401 ,  402 ,  403  has a longitudinal extent substantially greater than the transverse extent of the return electrode  400 ,  401 ,  402 ,  403 .
 
F.2 The implantable stimulation device according to F1, wherein:
   the return electrode  400 ,  401 ,  402 ,  403  has a longitudinal extent greater than or approximately equal to the longitudinal extent of the stimulation electrode  200 ,  220 .
 
F. 3 The implantable stimulation device according to F1 or F2, wherein:
   the return electrode  400 ,  401 ,  402 ,  403  has a transverse extent greater than or approximately equal to the transverse extent of the stimulation electrode  200 ,  220 .
 
F.4 The implantable stimulation device according to F1, F2 or F3, wherein:
   an active tissue contact-area of the return electrode  400 ,  401 ,  402 ,  403  is equal to or more than the active tissue contact-area of the one or more stimulation electrodes  200 ,  220  configured to be active during use.
 
F.5 The implantable stimulation device according to F1, F2, F3 or F4, wherein:
   the number of non-contiguous return electrode regions a,b,c,d is less than or equal to the number of non-contiguous stimulation electrodes  200 ,  220 .       

     For example: 
     An implantable stimulation device  100 ,  101 ,  102 ,  103  comprising:
         an elongated substrate  300 , disposed along a longitudinal axis  600 , the substrate having a first  310  and second  320  surface disposed along substantially parallel transverse planes  600 ,  700 , the substrate  300  further comprising:   a stimulation electrode  200 ,  220 , comprised in the second surface  320  and configured to transmit energy, in use, to human or animal tissue, the stimulation electrode  200 ,  220  having a longitudinal extent along the longitudinal axis  600  and a transverse extent along a first transverse axis  700 , the transverse axis  700  being substantially perpendicular to the longitudinal axis  600  and substantially parallel to the second surface  320 ; and   a return electrode  401 ,  402 ,  403 , comprised in the first surface  310  or second surface  320 , proximate the stimulation electrode  200 ,  220 , configured to provide, in use, a corresponding electrical return for the stimulation electrode  200 ,  220 ;   wherein:   the return electrode  401 ,  402 ,  403  comprises two transversely-separated electrode regions a, b, c, d elongated along the longitudinal axis  600 , electrically connected to each other, the two electrode regions a, b, c, d being disposed on opposing transversal  700  sides of the stimulation electrode  200 ,  220 ;   the two electrode regions a, b, c, d have a longitudinal extent greater than or approximately equal to the longitudinal extent of the stimulation electrode  200 ,  220 ; and   each electrode region a, b, c, d is transversely separated from the stimulation electrode  200 ,  220  by an electrical insulator.       

     REFERENCE NUMBERS USED IN DRAWINGS 
     
         
           100  a first type of implantable end of a tissue stimulation system 
           101  a second type of implantable end of a tissue stimulation system 
           102  a third type of implantable end of a tissue stimulation system 
           103  a fourth type of implantable end of a tissue stimulation system 
           150  a stimulation energy source 
           200  one or more stimulation electrodes 
           220  an elongated stimulation electrode 
           250  one or more electrical interconnections 
           300  an elongated substrate 
           310  a first substantially planar transverse surface 
           320  a second substantially planar transverse surface 
           330  a first transverse extent 
           340  a second transverse extent 
           350  substrate protrusion for a return electrode 
           400  a first type of one or more proximal return electrodes 
           401  a second type of one or more proximal return electrodes 
           401   ab  a first and second region of a second type of proximal return electrode 
           402  a third type of one or more proximal return electrodes 
           402   ab  a first and second region of a third type of proximal return electrode 
           403  a fourth type of one or more proximal return electrodes 
           403   ab  a first and second region of a fourth type of proximal return electrode 
           450  one or more distal return electrodes 
           550  a pulse energy controller 
           570  electric potential 
           600  a longitudinal axis 
           700  a first transverse axis 
           750  a second transverse axis 
           810  location for left supraorbital nerve or cortical stimulation 
           820  location for right supraorbital stimulation 
           830  location for left occipital nerve stimulation 
           840  location for right occipital nerve stimulation 
           850  location for deep brain stimulation 
           860  location for vagus nerve, carotid artery, carotid sinus, phrenic nerve or 
         hypoglossal stimulation 
           865  location for cerebral spinal cord stimulation 
           870  location for peripheral nerve stimulation 
           875  location for spinal cord stimulation 
           880  location for gastric stimulation 
           885  location for sacral &amp; pudendal nerve stimulation 
           890  location for sacral neuromodulation 
           895  location for fibular nerve stimulation 
           910  left supraorbital nerve 
           920  right supraorbital nerve 
           930  left greater occipital nerve 
           940  right greater occipital nerve 
           1200  proximal resistive tissue 
           1250  distal resistive tissue 
           1400  proximal variable resistor 
           1425  rheostat 
           1450  distal variable resistor 
           1550  energy source