Patent Publication Number: US-8996132-B2

Title: Leads with tip electrode for electrical stimulation systems and methods of making and using

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/906,776 filed May 31, 2013 which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/654,579 filed on Jun. 1, 2012, both of which are incorporated herein by reference. 
    
    
     FIELD 
     The invention is directed to the area of electrical stimulation systems and methods of making and using the systems. The present invention is also directed to electrical stimulation leads with a tip electrode designed to facilitate retention of the tip electrode on the distal end of the lead, as well as methods of making and using the leads and electrical stimulation systems. 
     BACKGROUND 
     Electrical stimulation can be useful for treating a variety of conditions. Deep brain stimulation can be useful for treating, for example, Parkinson&#39;s disease, dystonia, essential tremor, chronic pain, Huntington&#39;s Disease, levodopa-induced dyskinesias and rigidity, bradykinesia, epilepsy and seizures, eating disorders, and mood disorders. Typically, a lead with a stimulating electrode at or near a tip of the lead provides the stimulation to target neurons in the brain. Magnetic resonance imaging (“MRI”) or computerized tomography (“CT”) scans can provide a starting point for determining where the stimulating electrode should be positioned to provide the desired stimulus to the target neurons. 
     After the lead is implanted into a patient&#39;s brain, electrical stimulus current can be delivered through selected electrodes on the lead to stimulate target neurons in the brain. Typically, the electrodes are formed into rings disposed on a distal portion of the lead. The stimulus current projects from the ring electrodes equally in every direction. Because of the ring shape of these electrodes, the stimulus current cannot be directed to one or more specific positions around the ring electrode (e.g., on one or more sides, or points, around the lead). Consequently, undirected stimulation may result in unwanted stimulation of neighboring neural tissue, potentially resulting in undesired side effects. 
     One embodiment is an implantable electrical stimulation lead including a lead body having a distal portion, a distal tip, and a proximal portion; a plurality of electrodes disposed along the distal portion of the lead body; a plurality of terminals disposed along the proximal portion of the lead; and a plurality of conductors, each conductor electrically coupling at least one of the electrodes to at least one of the terminals. The plurality of electrodes includes a tip electrode disposed on the distal tip of the lead body. The tip electrode has a base and a separate plug attached to the base. The base defines an interior lumen closed at one end by the plug. A portion of the lead body extends into the interior lumen of the base. 
     Another embodiment is an implantable electrical stimulation lead including a lead body having a distal portion, a distal tip, and a proximal portion; a plurality of electrodes disposed along the distal portion of the lead body; a plurality of terminals disposed along the proximal portion of the lead; and a plurality of conductors, each conductor electrically coupling at least one of the electrodes to at least one of the terminals. The plurality of electrodes includes a tip electrode disposed on the distal tip of the lead body. The tip electrode has an electrode body, a stem extending from the electrode body, and a plurality of shaped retention features extending from the stem. A portion of the lead body extends around the stem and shaped retention features. The shaped retention features facilitate retention of the tip electrode on the lead body. 
     Yet another embodiment is an implantable electrical stimulation lead including a lead body having a distal portion, a distal tip, and a proximal portion; a plurality of electrodes disposed along the distal portion of the lead body; a plurality of terminals disposed along the proximal portion of the lead; and a plurality of conductors, each conductor electrically coupling at least one of the electrodes to at least one of the terminals. The plurality of electrodes includes a tip electrode disposed on the distal tip of the lead body. The tip electrode has an electrode body, a stem extending from the electrode body, and a flange attached to the stem opposite the electrode body. A portion of the lead body extends around the stem and flange. The flange facilitates retention of the tip electrode on the lead body. 
     A further embodiment is an implantable electrical stimulation lead including a lead body having a distal portion, a distal tip, and a proximal portion; a plurality of electrodes disposed along the distal portion of the lead body; a plurality of terminals disposed along the proximal portion of the lead; and a plurality of conductors, each conductor electrically coupling at least one of the electrodes to at least one of the terminals. The plurality of electrodes includes a tip electrode disposed on the distal tip of the lead body. The tip electrode has an electrode body and the electrode body defines an interior lumen and a plurality of protrusions extending into the interior lumen. A portion of the lead body extends into the interior lumen of the electrode body. The plurality of protrusions in the interior lumen facilitates retention of the tip electrode on the lead body and hinders rotation of the tip electrode around the distal tip of the lead body. 
     Another embodiment is an implantable electrical stimulation lead including a lead body comprising a distal portion, a distal tip, and a proximal portion; a plurality of electrodes disposed along the distal portion of the lead body; a plurality of terminals disposed along the proximal portion of the lead; and a plurality of conductors, each conductor electrically coupling at least one of the electrodes to at least one of the terminals. The plurality of electrodes includes a tip electrode disposed on the distal tip of the lead body. The tip electrode has an electrode body and a plurality of arms extending from the electrode body. The electrode body defines an interior lumen and an opening to the interior lumen. The plurality of arms extends over the opening to the interior lumen. A portion of the lead body extends into the interior lumen of the electrode body and around the plurality of arms. The plurality of arms facilitates retention of the tip electrode on the lead body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. 
       For a better understanding of the present invention, reference will be made to the following Detailed Description, which is to be read in association with the accompanying drawings, wherein: 
         FIG. 1  is a schematic side view of one embodiment of a device for brain stimulation, according to the invention; 
         FIG. 2  is a schematic perspective view of one embodiment of a portion of a lead having a plurality of segmented electrodes and a tip electrode, according to the invention; 
         FIG. 3  is a schematic perspective view of a second embodiment of a portion of a lead having a plurality of segmented electrodes and a tip electrode, according to the invention; 
         FIG. 4  is a schematic diagram of radial current steering along various electrode levels along the length of a lead, according to the invention; 
         FIG. 5  is a schematic cross-sectional view of one embodiment of a two piece tip electrode, according to the invention; 
         FIG. 6A  is a schematic side view of an embodiment of a portion of a lead with a two-piece tip electrode prior to coupling of the two pieces together, according to the invention; 
         FIG. 6B  is a schematic side view of the portion of the lead of  FIG. 6A  with the two pieces of the tip electrode coupled together, according to the invention; 
         FIG. 7  is a schematic cross-sectional view of one embodiment of a tip electrode with a stem, according to the invention; 
         FIG. 8A  is a schematic perspective view of one embodiment of a tip electrode with a stem and flange, according to the invention; 
         FIG. 8B  is a schematic perspective view of one embodiment of a pre-electrode that can be ground down to form the tip electrode of  FIG. 8A , according to the invention; 
         FIG. 9A  is a schematic perspective view of one embodiment of a tip electrode with a shaped interior lumen, according to the invention; 
         FIG. 9B  is a schematic perspective view of one embodiment of a pre-electrode that can be ground down to form the tip electrode of  FIG. 9A , according to the invention; 
         FIG. 10A  is a schematic perspective view of one embodiment of a tip electrode with an interior lumen and multiple arms extending from an edge of the electrode over the opening of the interior lumen, according to the invention; and 
         FIG. 10B  is a schematic perspective clew of one embodiment of a pre-electrode that can be ground down to form the tip electrode of  FIG. 10A , according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention is directed to the area of electrical stimulation systems and methods of making and using the systems. The present invention is also directed to electrical stimulation leads with a tip electrode designed to facilitate retention of the tip electrode on the distal end of the lead, as well as methods of making and using the leads and electrical stimulation systems. 
     A lead for deep brain stimulation may include stimulation electrodes, recording electrodes, or a combination of both. In at least some embodiments, the stimulation or recording electrodes of the lead can include a tip electrode and one or more ring electrodes or segmented electrodes or any combination thereof. In at least some embodiments, at least some of the stimulation electrodes, recording electrodes, or both are provided in the form of segmented electrodes that extend only partially around the circumference of the lead. In some embodiments, these segmented electrodes may be provided in sets of electrodes, with each set having electrodes radially distributed about the lead at a particular longitudinal position. In some embodiments, the segmented electrodes can be provided in any other suitable arrangement including, for example, arranging segmented electrodes in one or more helices around the circumference of the lead or arranging segmented electrodes along only one side of the lead. 
     A practitioner may determine the position of the target neurons using the recording electrode(s) and then position the stimulation electrode(s) accordingly without removal of a recording lead and insertion of a stimulation lead. In some embodiments, the same electrodes can be used for both recording and stimulation. In some embodiments, separate leads can be used; one with recording electrodes which identify target neurons, and a second lead with stimulation electrodes that replaces the first after target neuron identification. A lead may include recording electrodes spaced around the circumference of the lead to more precisely determine the position of the target neurons. In at least some embodiments, the lead is rotatable so that the stimulation electrodes can be aligned with the target neurons after the neurons have been located using the recording electrodes. For illustrative purposes, the leads are described herein relative to use for deep brain stimulation, but it will be understood that any of the leads can be used for applications other than deep brain stimulation including, but not limited to. spinal cord stimulation, dorsal root ganglion stimulation, and stimulation of other nerves, muscle tissue, or organs. 
     Deep brain stimulation devices and leads are described in, for example, U.S. Pat. No. 7,809,446; and U.S. Patent Application Publications Nos. 2007/0150036; 2009/0187222; 2009/0276021; 2010/0076535; 2010/0268298; and 2011/0078900; and U.S. Pat. applications Ser. Nos. 12/177,823; 61/022,953; and 61/316,759. Each of these references is incorporated herein by reference. 
       FIG. 1  illustrates one embodiment of a device  100  for brain stimulation. The device includes a lead  110 , a plurality of electrodes  125  disposed at least partially about a circumference of the lead  110 , a plurality of terminals  135 , a connector  130  for connection of the electrodes to a control unit, and a stylet  140  for assisting in insertion and positioning of the lead in the patient&#39;s brain. The stylet  140  can be made of a rigid material. Examples of suitable materials for the stylet include, but are not limited to, tungsten, stainless steel, and plastic. The stylet  140  may have a handle  150  to assist insertion into the lead  110 , as well as rotation of the stylet  140  and lead  110 . The connector  130  fits over a proximal end of the lead  110 , preferably after removal of the stylet  140 . 
     The control unit (not shown) is typically an implantable pulse generator that can be implanted into a patient&#39;s body, tor example, below the patient&#39;s clavicle area. The pulse generator can have eight stimulation channels which may be independently programmable to control the magnitude of the current stimulus from each channel. In some cases the pulse generator may have more than eight stimulation channels (e.g., 16-, 32-, or more stimulation channels). The control unit may have one, two, three, four, or more connector ports, for receiving the plurality of terminals  135  at the proximal end of the lead  110 . 
     In one example of operation, access to the desired position in the brain can be accomplished by drilling a hole in the patient&#39;s skull or cranium with a cranial drill (commonly referred to as a burr), and coagulating and incising the dura mater, or brain covering. The lead  110  can be inserted into the cranium and brain tissue with the assistance of the stylet  140 . The lead  110  can be guided to the target location within the brain using, for example, a stereotactic frame and a microdrive motor system. In some embodiments, the micro-drive motor system can be fully or partially automatic. The microdrive motor system may be configured to perform one or more the following actions (alone or in combination): insert the lead  110 , retract the lead  110 , or rotate the lead  110 . 
     In some embodiments, measurement devices coupled to the muscles or other tissues stimulated by the target neurons, or a unit responsive to the patient or clinician, can be coupled to the control unit or microdrive motor system. The measurement, device, user, or clinician can indicate a response by the target muscles or other tissues to the stimulation or recording electrode(s) to further identify the target neurons and facilitate positioning of the stimulation electrode(s). For example, if the target neurons are directed to a muscle experiencing tremors, a measurement device can be used to observe the muscle and indicate changes in tremor frequency or amplitude in response to stimulation of neurons. Alternatively, the patient or clinician may observe the muscle and provide feedback. 
     The lead  110  for deep brain stimulation can include stimulation electrodes, recording electrodes, or both. In at least some embodiments, the lead  110  is rotatable so that the stimulation electrodes can be aligned with the target neurons after the neurons have been located using the recording electrodes. 
     Stimulation electrodes may be disposed on the circumference of the lead  110  to stimulate the target neurons. Stimulation electrodes may be ring-shaped so that current projects from each electrode equally in every direction from the position of the electrode along a length of the lead  110 . Ring electrodes, however, typically do not enable stimulus current to be directed to only one side of the lead. Segmented electrodes, however, can be used to direct stimulus current to one side, or even a portion of one side, of the lead. When segmented electrodes are used in conjunction with an implantable pulse generator that delivers constant current stimulus, current steering can be achieved to more precisely deliver the stimulus to a position around an axis of the lead (i.e., radial positioning around the axis of the lead). 
     To achieve current steering, segmented electrodes can be utilized in addition to, or as an alternative to, ring electrodes. Though the following description discusses stimulation electrodes, it will be understood that all configurations of the stimulation electrodes discussed may be utilized in arranging recording electrodes as well. 
       FIGS. 2 and 3  illustrate embodiments of a distal portion of a lead  200  for brain stimulation. The lead  200  includes a lead body  210 , one or more optional ring electrodes  220 , a plurality of segmented electrodes  230 , and a tip electrode  240 . It will be understood that other lead embodiments can include only a tip electrode and one or more ring electrodes or only a tip electrode and one or more segmented electrodes. Other embodiments can include a tip electrode and a combination of ring and segmented electrodes in arrangements other than those illustrated in  FIGS. 2 and 3 . 
     The lead body  210  can be formed of a biocompatible, non-conducting material such as, for example, a polymeric material. Suitable polymeric materials include, but are not limited to, silicone, polyurethane, polyurea, polyurefhane-urea, polyethylene, or the like. Once implanted in the body, the lead  200  may be in contact with body tissue for extended periods of time. In at least some embodiments, the lead  200  has a cross-sectional diameter of no more than 1.5 mm and may be in the range of 1 to 1.5 mm. In at least some embodiments, the lead  200  has a length of at least 10 cm and the length of the lead  200  may be in the range of 25 to 70 cm. 
     The electrodes  220 ,  230 ,  240  may be made using a metal, alloy, conductive oxide, or any other suitable conductive biocompatible material. Examples of suitable materials include, but are not limited to, platinum, platinum iridium alloy, iridium, titanium, tungsten, palladium, palladium rhodium, or the like. Preferably, the electrodes are made of a material that is biocompatible and does not substantially corrode under expected operating conditions in the operating environment for the expected duration of use. 
     Each of the electrodes  220 ,  230 ,  240  can either be used or unused (OFF). When the electrode is used, the electrode can be used as an anode or cathode and carry anodic or cathodic current. In some instances, an electrode might be an anode for a period of time and a cathode for a period of time. 
     Stimulation electrodes in the form of ring electrodes  220  may be disposed on any part of the lead body  210 , usually near a distal end of the lead  200 . A stimulation electrode in the form of tip electrode  240  is disposed at the distal end of the lead. In  FIG. 2 , the lead  200  includes one ring electrode  220  and one tip electrode  240 . Any number offing electrodes  220  may be disposed along the length of the lead body  210  including, for example, one, two three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen or more ring electrodes  220 . It will be understood that any number of ring electrodes may be disposed along the length of the lead body  210 . 
     In some embodiments, the ring electrode(s)  220  and tip electrode  240  are substantially cylindrical and wrap around the entire circumference of the lead body  210 . The tip electrode  240  also extends over the tip of the lead. In some embodiments, the outer diameters of the ring electrode(s)  220  and tip electrode  240  are independently substantially equal to the outer diameter of the lead body  210 . The length of the ring electrode(s)  220  and tip electrode  240  may independently vary according to the desired treatment and the location of the target neurons or other tissue. In some embodiments the length of one or more of the ring electrode(s)  220  and tip electrode  240  are less than or equal to the corresponding diameters of the ring electrode(s)  220  and tip electrode  240 . In other embodiments, the lengths of one or more of the ring electrode(s)  220  and tip electrode are greater than the corresponding diameters of the ring electrode(s)  220  and tip electrode  240 . In at least some embodiments, the surface area of the tip electrode  240  and one of the ring electrode(s)  220  may be equal of substantially equal (e.g., within 10% or 5% of each other). 
     Deep brain stimulation leads and other leads may include one or more sets of segmented electrodes. Segmented electrodes may provide for superior current steering than ring electrodes because target structures in deep brain stimulation are not typically symmetric about the axis of the distal electrode array. Instead, a target may be located on one side of a plane running through the axis of the lead. Through the use of a radially segmented electrode array (“RSEA”), current steering can be performed not only along a length of the lead, but also around a circumference of the lead. This provides precise three-dimensional targeting and delivery of the current stimulus to neural target tissue, while potentially avoiding stimulation of other tissue. Examples of leads with segmented electrodes include U.S. Pat. Nos. 8,295,944; and 8,391,985; and U.S. Patent Applications Publication Nos. 2010/0268298; 2011/0005069; 2011/0078900; 2011/0130817; 2011/0130818; 2011/0238129; 2011/0313500; 2012/0016378; 2012/0046710; 2012/01.65911; 2012/0197375; 2012/0203316; 2012/0203320; and 2012/0203321, all of which are incorporated herein by reference. 
     In  FIG. 2 , the lead  200  is shown having a plurality of segmented electrodes  230 . Any number of segmented electrodes  230  may be disposed on the lead body  210  including, for example, one, two three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen or more segmented electrodes  230 . It will be understood that any number of segmented electrodes  230  may be disposed along the length of the lead body  210 . 
     The segmented electrodes  230  may be grouped into sets of segmented electrodes, where each set is disposed around a circumference of the lead  200  at a particular longitudinal portion of the lead  200 . The lead  200  may have any number of segmented electrodes  230  in a given set of segmented electrodes. The lead  200  may have one, two, three, four, five, six, seven, eight, or more segmented electrodes  230  in a given set. In at least some embodiments, each set of segmented electrodes  230  of the lead  200  contains the same number of segmented electrodes  230 . The segmented electrodes  230  disposed on the lead  200  may include a different number of electrodes than at least one other set of segmented electrodes  230  disposed on the lead  200 . 
     The segmented electrodes  230  may vary in size and shape. In some embodiments, the segmented electrodes  230  are all of the same size, shape, diameter, width or area or any combination thereof. In some embodiments, the segmented electrodes  230  of each circumferential set (or even all segmented electrodes disposed on the lead  200 ) may be identical in size and shape. 
     Each set of segmented electrodes  230  may be disposed around the circumference of the lead body  210  to form a substantially cylindrical shape around the lead body  210 . The spacing between individual electrodes of a given set of the segmented electrodes may be the same, or different from, the spacing between individual electrodes of another set of segmented electrodes on the lead  200 . In at least some embodiments, equal spaces, gaps or cutouts are disposed between each segmented electrode  230  around the circumference of the lead body  210 . In other embodiments, the spaces, gaps or cutouts between the segmented electrodes  230  may differ in size or shape. In other embodiments, the spaces, gaps, or cutouts between segmented electrodes  230  may be uniform for a particular set of the segmented electrodes  230 , or for all sets of the segmented electrodes  230 . The sets of segmented electrodes  230  may be positioned in irregular or regular intervals along a length the lead body  210 . 
     Conductor wires that attach to the tip electrode  240 , ring electrode(s)  220 , and segmented electrodes  230  extend along the lead body  210 . These conductor wires may extend through the material of the lead  200  or along one or more lumens defined by the lead  200 , or both. The conductor wires are presented at a connector (via terminals) for coupling of the electrodes  220 ,  230 ,  240  to a control unit (not shown). 
     When the lead  200  includes a tip electrode  240 , ring electrode(s)  220  and segmented electrodes  230 , the ring electrodes  220  and the segmented electrodes  230  may be arranged in any suitable configuration. The tip electrode  240  will generally be at the distal tip of any arrangement containing a tip electrode  240 . For example, when the lead  200  includes a tip electrode  240 , a ring electrode  220  and two sets of segmented electrodes  230 , the tip electrode  240  and ring electrode  220  can flank the two sets of segmented electrodes  230  (see e.g.,  FIG. 2 ). Alternately, the tip electrode  240  and ring electrode  220  can be disposed distal to the two sets of segmented electrodes  230  (see e.g.,  FIG. 3 ). It will be understood that other configurations are possible as well (e.g., alternating ring and segmented electrodes, or the like). 
     Any combination of tip electrode  240 , ring electrodes  220 , and segmented electrodes  230  may be disposed on the lead  200 . For example, the lead may include a ring electrode, two sets of segmented electrodes, each set formed of three segmented electrodes  230 , and a tip electrode at the end of the lead. This configuration may simply be referred to as a 1-3-3-1 configuration as illustrated in  FIG. 2 . It may be useful to refer to the electrodes with this shorthand notation.  FIG. 3  illustrates a lead with a 3-3-1-1 configuration. Possible configurations fur a 16-electrode lead with a tip electrode include, but are not limited to, 3-3-3-3-3-1 and 1-3-3-2-3-3-1. 
     Markers or other indicia may be provided sot that the practitioner can determine the orientation of the segmented electrodes when implanted. Examples of suitable markers and indicia can be found in, for example, U.S. Patent Application Publications Nos. 2012/0016378 and 2012/0203321 and U.S. patent Applications Ser. Nos. 13/750,725 and 13/787,171, all of winch are incorporated herein by reference. 
       FIG. 4  is a schematic diagram, to illustrate radial current steering along various electrode levels along the length of the lead  200 . While conventional lead configurations with ring electrodes are only able to steer current along the length of the lead (the z-axis). the segmented electrode configuration is capable of steering current in the x-axis, y-axis as well as the z-axis. Thus, the centroid of stimulation may be steered in any direction in the three-dimensional space surrounding the lead  200 . In some embodiments, the radial distance, r, and the angle θ around the circumference of the lead  200  may be dictated by the percentage of anodic current (recognizing that stimulation predominantly occurs near the cathode, although strong anodes may cause stimulation as well) introduced to each electrode. In at least some embodiments, the configuration of anodes and cathodes along the segmented electrodes allows the centroid of stimulation to be shifted to a variety of different locations along the lead  200 . 
     As can be appreciated from  FIG. 4 , the centroid of stimulation can be shifted at each level along the length of the lead  200 . The use of multiple sets of segmented electrodes at different levels along the length of the lead allows for three-dimensional current steering. In some embodiments, the sets of segmented electrodes are shifted collectively (i.e., the centroid of simulation is similar at each level along the length of the lead). In at least some other embodiments, each set of segmented electrodes is controlled independently. Each set of segmented electrodes may contain two, three, four, five, six, seven, eight or more segmented electrodes. It will be understood that different stimulation profiles may be produced by varying the number of segmented electrodes at each level. For example, when each set of segmented electrodes includes only two segmented electrodes, uniformly distributed gaps (inability to stimulate selectively) may be formed in the stimulation profile. In some embodiments, at least three segmented electrodes  230  in a set are utilized to allow for true 360° selectivity. 
     As previously indicated, the foregoing configurations may also be used while utilizing recording electrodes. In some embodiments, measurement devices coupled to the muscles or other tissues stimulated by the target neurons or a unit responsive to the patient or clinician can be coupled to tire control unit or microdrive motor system. The measurement device, user, or clinician can Indicate a response by the target muscles or other tissues to the stimulation or recording electrodes to further identify the target neurons and facilitate positioning of the stimulation electrodes. For example, if the target neurons are directed to a muscle experiencing tremors, a measurement device can be used to observe the muscle and indicate changes in tremor frequency or amplitude in response to stimulation of neurons. Alternatively, the patient or clinician may observe the muscle and provide feedback. 
     A tip electrode can be used in combination with one or more ring electrodes, one or more segmented electrodes, or any combination of ring and segmented electrodes. In at least some embodiments, a tip electrode may be selected to have the same, or substantially the same, surface area as one or more ring electrodes of the lead. 
     A tip electrode can be designed to improve retention of the tip electrode on the lead. For example, a tip electrode may have a hollow cylindrical base and a separate plug that can be attached to the base.  FIG. 5  illustrates a tip electrode  540  having a base  542  and a separate plug  544 . The base  542  and plug  544  are typically formed of a suitable metal, alloy, or other conductor. The base  542  is a hollow tubular structure with an interior lumen  541  with openings at opposing ends of the tubular structure that will allow material to flow through the base. The open interior lumen  541  facilitates retention of the base on the lead. The base  542  includes a distal opening  543  that is shaped to receive the plug  544 . In some embodiments, the base  542  will have sloped edges  545  at the distal opening  543  that correspond to sloped edges  547  on the plug  544  to facilitate mating of the base and plug. It will be understood that other configurations of the distal opening and corresponding surface on the plug can be used to facilitate mating of the base and plug. 
       FIGS. 6A and 6B  illustrate a distal portion of one embodiment of a lead  500  having a tip electrode  540  with a base  542  and a separate plug  544 . The lead also includes a lead body  510 ,  510 ′ and one or more additional electrodes  520 . In at least some embodiments the lead body  510  is formed by molding the lead body  510 ′ between the electrodes  520  (and, at least in some embodiments, between the electrodes and the terminals at the proximal end of the lead—see,  FIG. 1 ). The material of the lead body  510  can also be molded between the distal-most electrode  520  and the base  542  of the tip electrode  540 . During the molding process, the material that will form the lead body can flow into the interior lumen  541  (see,  FIG. 5 ) of the base  542 . Any molding process can be used including, but not limited to, injection molding. The lead body  510 ,  510 ′ can be formed of any material that can be molded by flowing the material around the other components and then solidify the material to form the lead body. Any suitable process can be used to solidify the material including, but not limited to, cooling the material, photocuring, heat curing, crosslinking, and the like. Examples of suitable materials can include silicone, polyurethane, polyetheretherketone, and the like. As an example, the methods for forming a lead with segmented electrodes disclosed in U.S. Patent Application Publication No. 2011/0078900, incorporated herein by reference, can be modified to include a tip electrode (by, for example, replacing the distal-most ring electrode in  FIGS. 7A-7E  with a tip electrode). 
     When the lead body  510  is formed, the lead body will extend into the interior lumen of the base  542  and facilitate retention of the tip electrode  540  on the lead  500 . After the lead body  510  is formed, the plug  544  can be attached (preferably, permanently) to the base  542  by, for example, welding, soldering, adhesive (preferably, conductive adhesive), press-fit, crimping, threading on the base and plug, or any combination thereof or any other suitable fastening arrangement. Preferably, the plug  544  and base  542  are also in electrical communication with each through the fastening arrangement. In at least some embodiments, a portion of the lead body may be removed from the distal opening  543  of the base  542  to allow attachment of the plug  544  to the base. 
     A tip electrode conductor (not shown) is attached, welded, soldered, or otherwise electrically coupled to the tip electrode  540 . The coupling of the tip electrode conductor may occur prior to forming the lead body  510 . The tip electrode conductor, like other conductors in the lead, extends along the lead and is electrically coupled to one of the terminals disposed along the proximal end of the lead. In some embodiments, the tip electrode conductor is coupled to the base  542  at, for example, the surface of the interior lumen  541  or the proximal end of the base. In some embodiments, the tip electrode conductor is attached to the plug  544 . 
     A tip electrode may include a stem with one or more features to facilitate retention of the tip electrode at the distal end of the lead.  FIG. 7  illustrates in cross-section one embodiment of a tip electrode  740  having an electrode body  742  and a stem  746  with shaped retention features  748  formed on the stem. The electrode body  742  includes at least a portion of the surface that is exposed to tissue, when the lead is implanted, for providing stimulation to the tissue. When the lead body is formed, the material of the lead body forms around the stem of the tip electrode and facilitates retention of the tip electrodes within the lead body of the resulting lead. The shaped retention features  748  hinder extraction of the tip electrode from the lead body. The shaped retention features  748  on the stem typically extend away from the adjacent portions of the stem  746  to interact with material at the distal end of the lead, such as the lead body, to resist withdrawal of the tip electrode  740  from the distal end of the lead. Examples of suitable shaped retention features include, but are not limited to, one or more stepped features, sloped protrusions, flanges, teeth, protruding threads, or the like formed on the stem or a roughened surface formed on the stem. In the embodiment illustrated in  FIG. 7 , the shaped retention features have a sloping surface  760  on one side and a stepped surface  762  on the other side to resist withdrawal from the distal end of the lead. 
     A tip electrode conductor (not shown) is attached, welded, soldered, or otherwise electrically coupled to the tip electrode  740 . In some embodiments, the tip electrode conductor is coupled to the stem  746 . For example, the tip electrode conductor could be coupled to the side of the stem  746  or the proximal end of the stem. In other embodiments, the tip electrode conductor can be attached to the main portion of the tip electrode (i.e., the non-stem portion of the tip electrode). The tip electrode conductor extends along the lead and is electrically coupled to one of the terminals disposed along the proximal end of the lead. 
       FIG. 8A  illustrates another embodiment of a tip electrode  840  that includes an electrode body  842  and a stem  846  with a flange  850  attached to the end of the stem. The electrode body  842  includes at least a portion of the surface that is exposed to tissue, when the lead is implanted, for providing stimulation to the tissue. The flange may have any suitable shape including, but not limited to, disc-shaped, square-shaped, hexagonal-shaped, octagonal-shaped, triangular-shaped, and die like. In the illustrated embodiment, the flange is gear-shaped with a disc having regular indentations  852  formed in the sides of the disc leaving regular protrusions  854  around the edge of the disc. The tip electrode conductor (not shown) can be attached to any portion of the tip electrode including, but not limited to, the flange  850 , the protrusions  854  of the flange, or the indentations  852  within the flange. 
     When the lead body is formed, the material of the lead body forms around the stem  846  and flange  850  of the tip electrode  840  which facilitates retention of the tip electrode within the lead body of the resulting lead. The flange  850  hinders extraction of the tip electrode  840  from the lead body. Portions of the lead body formed within the indentations  852  and around the protrusions  854  of the flange may facilitate both retention of the tip electrode on the lead and reduce the likelihood of rotation of the tip electrode. Typically, a non-circular flange will provide some resistance to rotation of the tip electrode. 
     In at least some embodiments, a pre-electrode  840 ′, as illustrated in  FIG. 8B , is provided during manufacture. The lead body is formed around this pre-electrode  840 ′. Then the lead body and pre-electrode  840 ′ are ground down to obtain the tip electrode  840  of  FIG. 8A . This grinding process also removes excess lead body material leaving the lead body at the desired diameter. 
       FIG. 9A  illustrates an embodiment of a tip electrode  940  with an electrode body  942  and an interior lumen  941 . The electrode body  942  includes at least a portion of the surface that is exposed to tissue, when the lead is implanted, for providing stimulation to the tissue. When the lead body is formed, the material of the lead body flows into the Interior lumen  941  to facilitate retention of the tip electrode on the lead body of the resulting lead. The tip electrode  940  defines protrusions  956  on the surface that defines the interior lumen  941 . These protrusions  956  facilitate retaining the tip electrode on the lead body and also resist rotation of the tip electrode around the lead body. In the illustrated embodiment, the pattern of protrusions  956  form a star shape in cross-section, but it will be recognized that outer regular and irregular shapes generated by protrusions into the interior lumen can be used to resist rotation of the tip electrode around the lead body. 
     In addition, the proximal end of the tip electrode  940  defines a non-circular opening  958  with protrusions  956 ′ that form a flange with respect to an adjacent portion  960  of the interior surface of tip electrode  940  defining the interior lumen  941 . These protrusions  956 ′, and the resulting flange-like arrangement, resist rotation of the tip electrode around the lead body and also resist removal of the tip electrode from the distal end of the lead. 
     A tip electrode conductor (not shown) can be attached to any portion of the tip electrode including, but not limited to, the proximal end of the tip electrode, the protrusions  956 ′, or the surface defining the interior lumen  941 . In at least some embodiments, a pre-electrode  940 ′, as illustrated in  FIG. 9B , is provided during manufacture. The lead body is formed around, and within, this pre-electrode  940 ′. Then the lead body and pre-electrode  940 ′ are ground down to obtain the tip electrode  940  of  FIG. 9A . This grinding process also removes excess lead body material leaving the lead body at the desired diameter. It will be understood that this embodiment can be modified to include a base with the interior lumen and a separate plug as illustrated in  FIGS. 5-6B . 
       FIG. 10A  illustrates an embodiment of a tip electrode  1040  with an electrode body  1042 , interior lumen  1041 , and multiple arms  1060  extending from the electrode body  1042  and over the entrance  1064  to the interior lumen  1041 . The electrode body  1042  includes at least a portion of the surface that is exposed to tissue, when the lead is implanted, for providing stimulation to the tissue. When the lead body is formed, the material of the lead body flows into the interior lumen  941  to facilitates retention of the tip electrode within the lead body of the resulting lead. These arms  1060  facilitate retaining the tip electrode on the lead body and also resist rotation of the tip electrode around the lead body. In the illustrated embodiment, tip electrode  1040  has three arms  1060 , but it will be recognized that any number of arms can be used including, but not limited to, one, two, thee, four, six, or more arms. The arms  1060  can be distributed around the edge of the electrode body  1042  in any regular or irregular arrangement. 
     The tip electrode conductor (not shown) can he attached to any portion of the tip electrode including, but not limited to, the proximal end of the tip electrode, the arms  1060 , or the surface defining the interior lumen  1041 . In at least some embodiments, a pre-electrode  1040 ′, as illustrated in  FIG. 10B , is provided during manufacture. The lead body is formed around, and within, this pre-electrode  1040 ′. Then the lead body and pre-electrode  1040 ′ are ground down to obtain the tip electrode  1040  of  FIG. 10A . This grinding process also removes excess lead body material leaving the lead body at the desired diameter. It will be understood that this embodiment can be modified to include a base with the interior lumen and arms and a separate plug as illustrated in  FIGS. 5-6B . 
     The above specification, examples, and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.