Patent Publication Number: US-2023141622-A1

Title: Minimally invasive neurostimulation device with sensing capability

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 63/277,448, filed Nov. 9, 2021, the disclosure of which is hereby incorporated by reference. 
    
    
     FIELD 
     The present application relates to implantable neurostimulation systems, and more specifically to minimally invasive implantable neurostimulation systems with sensing for closed loop control. 
     BACKGROUND 
     Implantable medical devices may be configured to deliver electrical stimulation therapy or monitor physiological signals. Electrical stimulation of nerve tissue, for example, may provide relief for a variety of disorders thereby improving the quality of life for many patients. 
     Some implantable medical devices (IMDs) may employ electrical leads that carry electrodes. For example, electrodes may be located at a distal portion of an elongate lead. Other examples of electrical leads may be relatively short, having one or more electrodes located along a body of the lead. Such electrical leads are provided separate from the housing or body of the IMD and coupled to the IMD during implantation to provide stimulation via the electrode at a location separated from the housing of the IMD. 
     Simulation of different nerve branches and clusters have been explored for treating various ailments. One avenue that has shown promising development has been the stimulation of the tibial nerve for the treatment of certain ailments such as incontinence or over-active bladder. 
     SUMMARY 
     Some embodiments of the present disclosure are directed to minimally invasive, leadless neurostimulation devices. Leadless devices do not require the use of a separate lead to deliver stimulation therapy and instead provide a unitary structured device that may be more robust and less invasive than lead-based counterpart devices. The leadless devices further include one or more sensors or electrodes configured to sense nerve activity or muscle activity, and provide closed loop feedback for adjustment of the stimulation therapy regime. 
     Embodiments of devices disclosed herein may include a housing containing at least one controller and processing circuitry therein configured to deliver neurostimulation therapy, and an attached header unit. The header unit includes one or more primary electrodes that form a portion of the exterior and side of the header unit. The one or more primary electrodes are electrically insulated from other portions of the exterior surface of the neurostimulation device. The housing of the neurostimulation device includes a secondary electrode that operates in conjunction with the one or more primary electrodes to provide electrical simulation therapy or neuro sensing capabilities. The secondary electrode is positioned on the same side of the device as the one or more primary electrodes positioned in the header unit. The device may further include one or more sensors arranged on the housing or footer of the device, the one or more sensors configured to sense stimulated nerve activity. At least one controller (e.g., processor and processing circuitry) configured to receive sensed nerve activity and adjust one or more parameters of the neurostimulation therapy delivered by the primary electrode. The size, shape, and separation distance between the primary electrode(s) and the secondary electrode are discussed and may contribute to more effective and efficient stimulation of the tibial nerve. In some embodiments, the size, shape, and separation distance between the primary electrode(s) and the secondary electrode may be configured to produce an impedance of less than about 2,000 Ohms. 
     In an embodiment, the disclosure describes a leadless neurostimulation device including a header unit having at least one primary electrode having a contact surface that defines an external surface of the leadless neurostimulation device, and a housing. The housing includes a secondary electrode positioned on the same side of the leadless neurostimulation device as the at least one primary electrode, a footer coupled to the housing opposite of the header unit, and a controller. The controller is configured to operate in a closed-loop to transmit an electrical stimulation signal between the primary electrode to the secondary electrode to provide electrical stimulation therapy to a tibial nerve of a patient, measure a physiologic parameter in response to transmission of the electrical stimulation therapy, and adjust one or more parameters of the electrical stimulation signal based on the measured physiologic parameter. 
     In another embodiment, the disclosure describes a neurostimulation device that includes a header unit having at least one primary electrode having a contact surface that defines an external surface of the leadless neurostimulation device, and a housing that includes a secondary electrode positioned on the same side of the leadless neurostimulation device as the at least one primary electrode, a footer coupled to the housing opposite of the header unit, a tail lead extending from the footer and comprising one or more sensors, and a controller configured to operate in a closed-loop to: transmit an electrical stimulation signal between the primary electrode to the secondary electrode to provide electrical stimulation therapy to a tibial nerve of a patient, measure a physiologic parameter with the one or more sensors in response to transmission of the electrical stimulation therapy, and adjust one or more parameters of the electrical stimulation signal based on the measured physiologic parameter. 
     The above summary is not intended to describe each illustrated embodiment or every implementation of the subject matter hereof. The figures and the detailed description that follow more particularly exemplify various embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The disclosure can be more completely understood in consideration of the following detailed description of various embodiments of the disclosure, in connection with the accompanying drawings, in which: 
         FIG.  1    is schematic view showing an example leadless neurostimulation device as described herein. 
         FIGS.  2 A- 2 D  are schematic side views of example header units that may be used with the device of  FIG.  1   . 
         FIGS.  3 A- 3 E  are schematic views of example header units that include a plurality of primary electrodes that may be used with the leadless neurostimulation device of  FIG.  1    or with the header unit and electrode arrangements of  FIGS.  2 A- 2 D . 
         FIGS.  4 A- 4 D  are schematic side views of example sensor arrangements that may be incorporated into device  10 . 
         FIG.  5    is a schematic side view of a neurostimulation device with a sensor tail. 
         FIG.  6 A  is a side view of a patient&#39;s leg showing the leadless neurostimulation device of  FIG.  1    implanted in a patient&#39;s leg near the tibial nerve. 
         FIG.  6 B  is a cross-sectional view of a patient&#39;s leg showing the leadless neurostimulation device of  FIG.  1    implanted near the patient&#39;s tibial nerve. 
         FIG.  7    is a plot showing examples of the minimum threshold level of current needed to observe a tibial nerve response based on a function of return offset in modeling studies using the disclosed leadless neurostimulation devices. 
         FIGS.  8 A and  8 B  are plots showing modeling results of the effect of secondary electrode radius and the offset of the secondary electrode from the primary electrode in an example header on the stimulation threshold of a model of the tibial nerve located above the Y=0 axis. 
         FIG.  8 C  is a plot showing modeling results of stimulation threshold limit for the disclosed device compared to a disc-shaped stimulation device for depth and anterior/posterior relationship. 
         FIG.  9    is a plot showing the threshold current as a function of stimulation depth for both the 10 mm and 20 mm electrode offsets using a model of the disclosed leadless neurostimulation devices. 
         FIG.  10    is a comparative plot of models showing the threshold stimulation current as a function of stimulation depth for a comparative disc stimulation device and the disclosed leadless neurostimulation device. 
     
    
    
     While various embodiments are amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the claimed inventions to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the subject matter as defined by the claims. 
     DETAILED DESCRIPTION 
     Embodiments of the neurostimulation devices described herein may be useful for numerous types of neurostimulation therapies, such as for pain control, autonomic nervous system modulation, functional electrical stimulation, tremors, and more. In preferred embodiments, the neurostimulation devices described herein may be useful for both sensing and stimulating one or more nerves to control symptoms of overactive bladder, urge urinary incontinence, urgency frequency, urinary incontinence, stress incontinence, nocturia, painful bladder syndrome, chronic pelvic pain, incontinence, retention, sexual dysfunction, fecal incontinence, intractable constipation, irritable bowel syndrome, inflammatory bowel disease, sexual dysfunction, obesity, gastroparesis, interstitial cystitis, neurogenic bowel or bladder (Parkinson&#39;s disease, multiple sclerosis, stroke, spinal cord injury, neuropathy), or other pelvic health conditions. These embodiments may also be useful for both sensing and stimulating one or more peripheral nerves to control pain in one or more areas of the body, such as a foot, ankle, leg, groin, shoulder, arm, wrist, or the back, for example. In one example, embodiments of the disclosed neurostimulation devices may be used to both sense and stimulate a tibial nerve of a patient. 
       FIG.  1    is schematic view showing an example leadless neurostimulation device  10 . Leadless neurostimulation device  10  includes a housing  12  containing components therein configured for delivering neurostimulation therapy, a header unit  14  that includes one or more primary electrodes  18 , and a mounting plate  16  that couples housing  12  to header unit  14 . Header unit  14  includes at least one primary electrode  18  that forms part of an exterior surface of header unit  14 . Housing  12  includes a secondary electrode  20  that forms part of an exterior surface of housing  12  and is positioned on the same side of device  10  as primary electrode  18 . In an alternate embodiment not depicted, primary electrode  18  and secondary electrode  20  may be arranged on opposite sides of device  10 . Device  10  further includes one or more sensors  50  positioned along housing  12 . 
     Primary electrode  18  and secondary electrode  20  operate in conjunction with one another, e.g., via a controller, to provide stimulation therapy to a target treatment site (e.g., a tibial nerve). Secondary electrode  20  may also be referred to as a case electrode, can electrode, or reference electrode. In an embodiment, primary electrode  18  may comprise a cathode and secondary electrode  20  may comprise an anode. In some embodiments, primary and secondary electrodes  18  and  20  may be characterized as a bipolar system or pair of electrodes. 
     The terms “primary” and “secondary” are used to differentiate two or more electrodes that are configured to transmit an electrical signal therebetween. The terms are not used to imply a hierarchy among the electrodes, positive and negative terminal, a total number of electrodes, or a directionality by which a signal is transmitted between the electrodes. In some embodiments, electrode  18  may be configured as the secondary electrode and electrode  20  configured as the primary electrode. In some embodiments, electrodes  18  and  20  may be switched between primary and secondary as desired. 
     One or more sensors  50  may be operated by a controller and processing circuitry within device  10  for sensing a physiologic parameter indicative of nerve or muscle activity measured in response to stimulation of the targeted nerve by primary electrode  18 . For example, sensors  50  may be configured to sense nerve activity based on measuring or monitoring an electromyogram (EMG) signal, e.g., a neuromuscular response, evoked compound action potential (ECAP), e.g., nerve capture but without the neuromuscular response, or combinations thereof, to provide real-time indication regarding the effectiveness of the stimulation therapy. Such signals may include M-wave, H-wave, F-wave, and combinations thereof. The EMG signal or ECAP response may occur shortly after the delivered electrical stimulation therapy, e.g., less than a millisecond (ms) allowing for a direct correlation between applied stimulation and measured response. 
     A response signal obtained by sensors  50  (or selected electrode/sensor combination) may be used by the controller and processing circuitry of device  10  to control and adjust the stimulation parameters of the stimulation therapy being delivered by primary electrode  18 . The information may be used in a closed-loop feedback system to optimize or adjust one or more of the stimulation parameters for better or more effective therapy. Such adjustments to the simulation therapy may include a change in an amplitude or pulse width of the stimulation signal, change in the selected primary electrode  18  or electrode pair (e.g., if a plurality of primary electrodes are present) to ensure better capture of the targeted nerve, change in frequency, change in sensor or stimulation configurations, and the like. 
     Header unit  14  includes outer housing  24 , primary electrode  18 , and dielectric mount  26 . Outer housing  24  is coupled to mounting plate  16  and may define a partially recessed cavity that receives dielectric mount  26  and primary electrode  18 . Outer housing  24  and mounting plate  16  may be made of metal or metal alloy (e.g., titanium or titanium alloy) to allow for easy coupling there between (e.g., laser welding) as well as allow for the coupling of mounting plate  16  to housing  12 . Additionally, or alternatively, outer housing  24  or mounting plate  16  may be composed of a ceramic material, or non-conductive plastic material (e.g., polypropylene) including appropriate mechanisms (e.g., metal inserts) for coupling outer housing  24  to mounting plate  16 . 
     In some embodiments, the seam between mounting plate  16  and outer housing  24  may form at least a partial hermetic seal. In an alternate embodiment not depicted, header unit  14  may be configured so as to be coupled directly to housing  12 , without the need for a separate mounting plate element. 
     Primary electrode  18  defines exterior contact surface  30  configured to be brought into direct contact with tissue of the patient. Contact surface  30  may also form a portion of a side of header unit  14 , which is preferably on the same side of device  10  as secondary electrode  20 . 
     The exterior perimeter of contact surface  30  is at least partially bordered by dielectric mount  26 , which may also form a portion of the exterior surface of header unit  14  absent of the dielectric coating disclosed below. Dielectric mount  26  electrically insulates and physically separates primary electrode  18  from outer housing  24 , mounting plate  16 , and other portions of device  10 . Additionally, or alternatively, dielectric mount  26  may be molded around primary electrode  18  using silicone or liquid silicone rubber LSR, for example, to help physically retain primary electrode  18  within header unit  14  of device  10 . In some embodiments, dielectric mount  26  may be formed integrally with outer housing  24  provided the components are formed of a non-conductive material. 
     Outer housing  24  may form the majority of the body of header unit  14 . In particular, outer housing  24  forms the side of header unit  14  opposite of contract surface  30 , the perimeter edges (apart from the contact surface provided with mounting plate  16 ), and a portion of the same side of header unit  14  as contact surface  30  of primary electrode  18 . In some embodiments, outer housing  24  may have a rounded, semi-circular, or D-shaped perimeter edge that provides a relatively smooth surface without any abrupt or sharp edges or lines than may present an irritation to the patient after implantation. In some embodiments, outer housing  24  is configured to receive and form a partial shell around dielectric mount  26 . As such, outer housing  24  may define a concave interior surface (not shown) that receives a portion of dielectric mount  26 . Dielectric mount  26  may be secured to outer housing  26  using a suitable adhesive material (e.g., non-conductive medical adhesive, epoxy volcanized silicone, or the like). 
     Primary electrode  18  may be of suitable shape to provide electrical stimulation to the tibial nerve through the fascia layer of a patient. In some embodiments, contact surface  30  of primary electrode may be substantially flat (e.g., flat or nearly flat) as shown in  FIG.  1   . Alternatively, primary electrode  18  may define a curved surface (e.g., a semi-cylindrical shape or other 2D or 3D curved plane) that helps primary electrode  18  follow the curvatures of the fascia layer of a patient when implanted to provide better contact and focusing of the electrical signal directed to the tibial nerve. The curved surface may extend over the entirety of contact surface  30 , or only over a portion of the surface. Additionally, or alternatively, the curvature may be confined to only contact surface  30  of primary electrode  18 , or may extend over other portions of device  10  such as other parts of header unit  14 , mounting plate  16 , or housing  12 . By including the curvature over other portions of device  10 , the device may provide a more ergonomic fit when implanted while also helping to direct the stimulation signal to the tibial nerve. 
     In some embodiments, contact surface  30  of primary electrode  18  may also protrude from the plane defined by housing  12 . Such a protrusion may help apply additional pressure to the fascia of the patient and help guide the electrical stimulation signal deeper into the tissue of the patient. Primary electrode  18  may also define one or more interlocking features, carveouts, recesses, or other structures that reduce the overall volume of primary electrode  18  without interfering with contact surface area  30 . The reduced volume and interlocking features may also help reduce manufacturing costs as well as help fix primary electrode  18  relative to dielectric mount  26 . 
       FIGS.  2 A- 2 D  are schematic side views (top-down) of example header units  40 A- 40 D that may be used with device  10  of  FIG.  1   . Each header unit  40 A- 40 D includes one or more primary electrodes  42 A- 42 D that may be curved, protrude away from the plane defined by housing  12 , or both. The curvatures shown in  FIGS.  2 A- 2 C  generally curve relative to the centerline defined by device  10  (e.g., into the page in  FIGS.  2 A- 2 C  through the center of the device) to help focus the electrical stimulation, sensing, or both to a line substantially parallel (e.g., parallel or nearly parallel) to the center line of device  10 . Additionally, or alternatively, by protruding primary electrodes  42 A- 42 D, the electrode may lie in closer proximity to the tibial nerve compared to secondary electrode  20  which can help guide or steer the electrical stimulation to the nerve allowing for deeper nerve stimulation (e.g., stimulation of tibial nerve with deep or anterior/posterior tracks). 
     The material of primary electrode  18  (or sensors  50  described below) may depend on the type of signals to be measured and the electrical stimulation therapy to be delivered. In some examples, primary electrode  18  or sensors  50  of device  10  may be made from, but are not limited to titanium, titanium alloy, platinum iridium, or the like. In some embodiments, at least contact surface  30  is formed of platinum iridium, which provides low impedance to bodily tissue. The body of primary electrode  18  may be made of the same or different material than contact surface  30 . For example, primary electrode  18  may be formed of titanium with contact surface  30  formed of platinum iridium. Using platinum iridium or titanium may be beneficial in reducing or eliminating the potential for charge buildup on the external surface of device  10  during operation. Alternatively, primary electrode  18  may be formed out of titanium (e.g., commercially pure titanium) or a titanium alloy, which may provide cost or assembly benefits in the construction of device  10  compared to forming primary electrode out of platinum iridium. In some examples, forming rounded edges on the electrodes or sensors may also help avoid edge defects. 
     Header unit  14  is coupled to mounting plate  16  and likewise mounting plate  16  is coupled to housing  12 . Housing  12  includes secondary electrode  20 . In some embodiments, secondary electrode  20  may be defined by an area of the body of housing  12 . For example, housing  12  may be formed of a metallic material (e.g., titanium) and electrically coupled to the processing circuitry of leadless neurostimulation device  10  so that housing  12  forms part of the electrical circuit. The outer surface of housing  12 , including portions of mounting plate  16 , header unit  14 , and footer  54 , may be coated with a dielectric material apart from the surface area that defines secondary electrode  20 , primary electrode  18 , and sensors  50 . The dielectric material may at least partially encapsulate device  10  such that the boundary created by the dielectric material in turn defines the area of secondary electrode  20 , contact surface  30 , or both. 
     Alternatively, housing  12  may include an aperture through the body of housing  12 , and secondary electrode  20  may be positioned within the aperture. In such examples, secondary electrode  20  would be a standalone component separate and independent of housing  12  such as secondary electrode  20  is electrically isolated from housing  12 . While this configuration is recognized as a possible arrangement, the below description assumes that housing  12  forms secondary electrode  20  such that the two components are one and the same with secondary electrode  20  being defined based on the application of the dielectric coating to the exterior surface of housing  12 . 
     The dielectric coating may be applied using any suitable technique. In some such examples, the areas defining contact surface  30  and secondary electrode  20  (and other areas of device  10  not intending to receive the application of the dielectric coating) may be masked with a suitable material such as tape. Leadless neurostimulation device  10  may be then coated using vapor deposition, dip coating, spray coating of similar technique with an adherent dielectric material followed by subsequent removal of the mask material to expose the surfaces of contact surface  30  and secondary electrode  20 . 
     Suitable dielectric materials for coating leadless neurostimulation device  10  may include, but are not limited to, parylene, LSR, or silicone. Additionally, or alternatively, the outer surface of neurostimulation device  10  or portions thereof, may include a surface treatment such as an anodization treatment to modify portions of the surface to make the surface non-conductive. For example, portions of housing  12 , outer housing  24 , or both, if made of metal (e.g., titanium) may be treated through anodization to make select surfaces non-electrically conductive. In such examples, for purposes of this disclosure the exterior surface of the components may still be characterized as being metal (e.g., titanium) although the component has received such surface treatment. 
     In preferred examples, the outer surface of leadless neurostimulation device  10  may be formed primarily of parylene. Formation of the desired electrode profiles may utilize dielectric blocking methods (e.g., use of a masking material during manufacture) or dielectric removal methods (e.g., removal via laser or soda blast) without damaging the dielectric coating. 
     In some embodiments, the dielectric coating may also contribute to creating a hermetic seal around leadless neurostimulation device  10 . The general configuration of attaching header unit  14  and housing  12  respectively to mounting plate  16  may also produce a hermetic seal within device  10 . Coating device  10  with a dielectric material possessing sealing properties such as parylene, LSR, or silicone may either provide additional robustness to the hermetic seal of device. Providing leadless neurostimulation device  10  in a hermetically sealed form may contribute to the device&#39;s long-term functionality thereby providing advantages over other non-hermetically sealed devices. 
     The controller, processing circuitry, and other components of neurostimulation device  10  are contained within housing  12 . Examples of such components may include one or more electronic circuits for delivering electrical stimulation therapy, telemetry hardware, power supply, memory, processor(s). The one or more processors (e.g., controller) may be used to control one or more parameters of the stimulation therapy and receive sensor information from the one or more sensors  50 . As discussed further below, the received data from the sensors may be used in a controlled-loop feedback system to adjust a parameter of the stimulation therapy to improve or optimize the therapy regime. Housing  12  can also include communication circuitry disposed therein for receiving programming communication from an external programmer (e.g., external programing device), or providing feedback to a programmer or other external devices. 
     In one example, housing  12  can include an energy source enclosed therein, e.g., a rechargeable or non-rechargeable battery. In another example, leadless neurostimulator  10  can also be configured to receive energy signals from an external device and transduce the received energy signals into electrical power that is used to recharge a battery of the device, an energy source e.g., a battery, processing circuitry, and other necessary components enclosed therein. In some embodiments, device  10  can be configured to receive energy signals from an external device and transduce the received energy signals into electrical power that is used to recharge a battery of device  10 . Additionally, or alternatively device  10  may include a non-rechargeable primary cell battery. 
     In some embodiments, housing  12  of leadless neurostimulation device  10 , and its various processing components may be substantially similar to the housing portion of the InterStim Micro Neurostimulator available from Medtronic. The InterStim Micro Neurostimulator may be modified to receive header unit  14  described herein along with modifications to provide secondary electrode  20  and sensors  50 . The total volume of neurostimulation device  10  may be relatively small as well: 0.5 cubic centimeters (cc) to about 6 cc, about 1.5 cc to about 3.5 cc, or about 2 cc to about 3 cc. 
     The size, shape, and physical separation distance between primary electrode  18  and secondary electrode  20  can affect the functionality and effectiveness of leadless neurostimulation device. In some embodiments, primary electrode  18  may define a contact surface area of about 5 mm 2  to about 90 mm 2 . In preferred embodiments that include only a single primary electrode  18 , the contact surface area may be greater than about 10 mm 2 , greater than about 15 mm 2 , greater than about 18 mm 2 , greater than about 20 mm 2 , less than 35 mm 2 , less than 30 mm 2 , and less than 25 mm 2 . In embodiments, primary electrode  18  may define a contact surface area between about 12 mm 2  and 22 mm 2 . Secondary electrode  20  may define a contact surface area of about 40 mm 2  to about 120 mm 2 . In embodiments, secondary electrode  20  may define a contact surface area between about 60 mm 2  and 71 mm 2 . However, devices having larger sized secondary electrodes may increase the minimal current needed to create a therapeutic response. The separation distance between primary electrode  18  and secondary electrode  20  may be about 5 mm to about 20 mm. 
     In some embodiments, the size, shape, and physical separation distance between primary electrode  18  and secondary electrode  20  may be configured to produce an impedance of less than 2,000 ohms (e.g., between about 100 ohms and 1,000 ohms). In other embodiments, the size, shape, or physical separation distance between electrodes may be configured to produce an impedance greater than 2,000 ohms. Additionally, or alternatively, primary and secondary electrodes  18  and  20  may be arranged in a non-concentric arrangement such that one electrode does not substantially encircle the other. 
     In some embodiments, one or more of primary and secondary electrodes  18  and  20  may be configured to operate in one or more modes including one or more sensing modes where, for example, the electrode is used to detect measurable feedback from the tibial nerve (e.g., sensed activity or the nerve prior to or after stimulation) or sense the relative location of the tibial nerve to optimize stimulation and a delivery mode where the electrode delivers stimulation therapy to the tibial nerve. The processing circuitry may select one or more optimal electrodes based on proximity to the tibial nerve for the delivery of stimulation therapy so as to steer the stimulation field. Additionally, or alternatively, in a sensing mode, one or more of primary and secondary electrodes  18  and  20  may be configured to monitor the activity of the tibial nerve or adjacent tissue prior to or during the delivery of simulation therapy to determine if sufficient therapy has been delivered. The sensory mode may be actuated by processing circuitry contained in the body of housing  12 . 
     In some embodiments, header unit  14  may include a plurality of primary electrodes  18 .  FIGS.  3 A- 3 E  are schematic views or example header units  40 E- 401  that each include a plurality of primary electrodes  44 . In some embodiments primary electrodes  44  may be similarly sized and shaped or include a collection of differently shaped and sized electrodes. Having relatively rounded edges along primary electrodes  44  can help reduce charge buildup defects. 
     The inclusion of more than one primary electrode  44  in device  10  may increase functionality and precision of device  10 . For example, one or more of primary electrodes  44  may be configured to operate in one or more modes and allow for selection of an appropriate electrode  44  via appropriate switching circuitry to deliver therapy. Having multiple primary electrodes  44  provides the option of selecting a particular electrode or electrode pair that is best positioned for the delivery of stimulation therapy. This improved stimulation targeting could limit possible side effects from stimulating unintended areas. Additionally, improved targeting through the use of sensory or neural feedback using for example sensors  50  as discussed further below may also help improve stimulation therapy by allowing for customizable stimulation parameters such as modification to the stimulation signal (e.g., adjustment to the stimulation voltage, amplitude, duration, and the like) or provide options for unique therapy applications (e.g., providing stimulation to two sides of the nerve simultaneously or interpretation of different types of sensory signals) using one or multiple wave forms. For example, in response to sensory data received from one or more sensors  50 , the processing circuitry may select one or more optimally placed primary electrodes  44  based on proximity to the tibial nerve for the delivery of stimulation therapy to help steer the stimulation field. 
     In some examples, primary electrodes  44  may be used for both stimulation and sensing purposes. In such arrangements device  10  may include a controller and processing circuitry configured to selectively switch between stimulation and sensing operational modes using preselected or user selected electrode pair combination. For example, one of primary electrodes  44  may be used for delivering stimulation therapy to a target site in conjunction with secondary electrode  20 , while the same or a different primary electrode  44  may be used in a sensing operational mode to determine if sufficient electrical stimulation therapy has been delivered to the target treatment site. In such examples, the primary electrode  44  being used for sensing purposes may be used in conjunction with one of the other primary electrodes  44  or secondary electrode  20  to complete the sensing circuit. Switching circuitry within device  10  may alternate between stimulation and sensing modes to allow for use of the selected electrode pair combination in the desired operational mode, and may also alternate between selections of electrodes. 
     In some examples, one or more of primary electrodes  44  may be configured to operate in one or more modes including one or more sensing modes where, for example, the electrode is used to detect measurable feedback from the tibial nerve (e.g., sensed activity or the nerve prior to or after stimulation) or sense the relative location of the tibial nerve to optimize stimulation and a delivery mode where the electrode delivers stimulation therapy to the tibial nerve. The processing circuitry may select one or more optimal primary electrodes  44  based on proximity to the tibial nerve for the delivery of stimulation therapy so as to steer the stimulation field. Additionally, or alternatively, in a sensing mode, one or more of primary electrodes  44  may be configured to monitor the activity of the tibial nerve or adjacent tissue prior to or during the delivery of simulation therapy to determine if sufficient therapy has been delivered. The sensory mode may be actuated by processing circuitry contained in the body of housing  12 . 
     In some examples, one or more of the primary electrodes  44 , not being used in the active stimulation mode, may be used with one or more of sensors  50  as an independent circuit to the stimulation operation. In such examples, device  10  may be configured to both deliver stimulation therapy and sense information in response to the delivery of stimulation therapy based on the two different electrical pathways. The stimulation and sensing operations may be performed simultaneously to provide real-time feedback, or may be performed in sequence where device  10  alternates between stimulation and sensing operational modes. 
     In some examples, selection of the desired primary electrode  44 , secondary electrode  20 , and sensor  50  pair combination for a desired operational mode (e.g., stimulation or sensing modes) may be done by a clinician or a programmer through a separate programming device containing a user interface. For example, a clinician or programmer can use a separate programming device (not shown) that allows the clinician to select which electrode-pair combination in device  10  that should be designated as the stimulation electrodes, and which electrode or sensor pair combination should be designated for the sensing configuration. The programming device may further provide the clinician or programmer the option of having the stimulation and sensing modes performed either simultaneously, or in alternating sequence, depending on which electrode pairs are selected for the different operational modes and whether simultaneous operation is permissible with the selected pairs. 
     In some examples, the programming device may also have a calibration mode that provides feedback information to the programmer or clinician to help optimize which electrode or sensor pair is best positioned for performing the different operational modes. For example, during such calibration, device  10  may provide a test stimulation using a given electrode combination and obtain sensor information using different electrode/sensor pairs in response to the electrical stimulation. This sensor information may be displayed to the clinician or programmer through the programming device so that the user can determine which electrode pair combination is best positioned for sensing a particular type of signal in response to the stimulation therapy. The user may then be given the option of selecting that particular electrode/sensor pair to conduct the sensory measurements in the closed loop feedback system. 
       FIGS.  4 A- 4 D  show side views of device  10  and examples of possible combinations and placements for the disclosed sensors. As shown in  FIG.  4 A , housing  12  may include one or more sensors  50  contained thereon for sensing various parameters of nerve traffic or muscle activity. For example, the electrode sensors  50  may be configured to sense nerve activity based on, or in response to, the stimulation provided by primary electrode  18 . Through such sensing, sensors  50  may measure nerve activity (e.g., activity of the tibial nerve) to provide real-time indication regarding the effectiveness of the stimulation therapy that may be used by the controller, including processing circuitry, to adjust one or more parameters associated with the stimulation therapy. 
     In embodiments, sensors  50  are generally described in this disclosure as being present in quantities of two or more and operating independent of the stimulation electrodes (e.g., primary and secondary electrodes  18  and  20 ). Having sensors  50  operate independent of the stimulation electrodes allows sensors  50  to be operated simultaneously with the stimulation electrodes. This in turn allows for sensing to be coordinated with stimulation therapy to assess and determine whether the parameters of the stimulation therapy need to be adjusted, in real time, to optimize the stimulation. In an alternative embodiment, device  10  may include a single sensor  50  configured to function in conjunction with stimulation electrodes such that device can both stimulate and sense nerve traffic or muscle response at the same time to provide real time analysis of the stimulation therapy parameters. 
     In other examples, as indicated above, one or more of sensors  50  may be provided by one of the stimulation electrodes. For example, device  10  may include a single sensor  50  that operates with secondary electrode  20  as the reference electrode, and/or one of primary electrodes  44  may be operated in a sensing capacity. In such examples, device  10  may include switching circuitry to oscillate between stimulation and sensing modes. In the stimulation mode, primary electrode  18  and secondary electrode  20  may function as described above to provide stimulation therapy. Device  10  may then switch to a sensing mode where sensor  50  and secondary electrode  20 , or other components, operate to determine muscle or nerve activity in response to the stimulation therapy. Device  10  would then oscillate between stimulation and sensing modes to optimize stimulation therapy, while there would be a partial delay between delivering stimulation therapy and sensing nerve or muscle fiber response activity. In such embodiments, device  10  may conserve space by having a reduced overall volume due to the reduced number of electrodes needed to be included with device  10 . In some examples device  10  may include up to four sensors/electrodes distributed in a combination primary electrode  18  and sensor  50  in any desirable combination (e.g., one primary electrode  18  and three sensors  50 , two primary electrodes  18  and to sensors  50 , or the like). 
     In some examples, sensors  50  may be exposed through one or more apertures within housing  12 . In embodiments, housing  12  may be used as the conductive material forming secondary electrode  12 , and sensors  50  may be electrically isolated from housing  12  by dielectric material  52  that electrically isolates sensors  50  from one another and from the substrate material of housing  12 . Dielectric material  52  may be the same material as dielectric mount  26  or constructed from another suitable material. Sensors  50  may include any suitable type of electrode sensor device that is electrically coupled to electronic circuitry of device  10  by, for example, conductive traces or the like. Further, sensors  50  may be constructed of any suitable materials including the same materials described above with respect to primary electrode  18 . 
     Sensors  50  or the electrode/sensor pair selected for operation in the sensory mode, may be configured to measure various signals designed to assess the efficacy of stimulation therapy provided by the stimulation electrodes or to avoid side effects detected by the sensing electrodes. Useful signals may include monitoring an electromyogram (EMG) signal or evoked compound action potential (ECAP), or combinations thereof. The signals may be obtained and analyzed by processing circuitry of device  10  to determine, for example, whether the proper nerve(s) is being stimulated by the stimulation therapy or the proper muscle response is evoked, whether the secondary or unintended nerve or muscle fibers are being stimulated by the stimulation therapy, whether the amplitude of the stimulation electrodes is sufficient to trigger a threshold therapeutic response or to evoke a response to the correct nerve fibers, whether the stimulation therapy is excessively high for the desired therapeutic effect, or the like. 
     For example, some applications of electrical stimulation can trigger an electromyography (EMG) signal. A measured EMG, within a specific time window related to nerve conduction up and down the leg at specific locations (e.g., great toe, lesser toes), may indicate that the medical device adequately stimulated the nerve. The EMG signal may be used to determine if the amplitude of the stimulation therapy is sufficient to trigger a desired response. As an example, H-wave or F-wave responses analyzed as part of the EMG signal acquired for tibial nerve stimulation may be used to as an indicator that motor neurons in the sacral plexus or spinal cord have been sufficiently activated. Additionally, the strength of the EMG signal response over time may indicate the plasticity that repeated stimulation may induce and thereby provide an indication relating to overall effectiveness of a given stimulation therapy. Device  10  may operate in a closed loop system, configured to alter the parameters of the stimulation therapy, e.g., stimulation level, pulse width, pulse pattern or other parameters dependent on the amplitude or other characteristics of the EMG signal. In some embodiments, inspection of the EMG waveform, e.g., the peak arrival time or a zero-crossing arrival time, can assist in determining if the proper nerve tissue is being stimulated versus non-targeted fiber tissue. For example, a latency in the arrival time may indicate that nearby muscle or other muscle innervated by the target nerve is being stimulated, rather than the target nerve itself. Such information can be used by device  10  to adjust one or more sensing or stimulation settings to improve device operation. 
     Another example of the electrical signal generated by the patient in response to an electrical stimulation greater than or equal to the sensory threshold is an evoked compound action potential (ECAP), which is synchronous firing of a population of neurons which occurs in response to the application of a stimulus including, in some cases, an electrical stimulus by device  10 . Inspection of the ECAP waveform, e.g., the peak arrival time or a zero-crossing arrival time, can assist in determining if the proper nerve tissue is being stimulated versus non-targeted fiber tissue. Further, threshold detection of the signals can indicate a minimum amplitude of electrical stimulation is being provided. However, measurement of the ECAP may be useful to determine if a minimum threshold electrical stimulation has been delivered, and may also be useful in determining number of fibers stimulated, a percentage of the fibers being captured within a targeted group, or the type of fibers being stimulated (e.g., alpha fibers, beta fibers, c-fibers, and so forth). 
     The sensory data collected by the selected sensors/electrode pair (e.g., detection of EMG signal or ECAP) may be used by the controller in a closed-loop feedback system configured to monitor and optimize the electrical stimulation therapy provided by device  10 . For example, the collected sensor data may be used to determine whether a minimum or maximum desired electrical stimulation therapy is being delivered to the target treatment site, whether an appropriate target nerve fiber is being captured by the electrical stimulation, may help mitigate device migration concerns or reduce the amplitude of stimulation provided if an otherwise non-optimally placed electrode were selected for the delivery of electrical stimulation, or the like. Additionally, sensory data can help to “steer” where the stimulation therapy is delivered after device  10  has been implanted thereby reducing the time needed during implantation otherwise used to assess proper placement. 
     Whether sensors  50  are configured to operate in an EMG, ECAP, simultaneous EMG/ECAP, or other sensor mode may be chosen by the controller and processing circuitry or by the clinician (e.g., through an external programming device) without modification to the hardware used in device  10 . However, whether sensors  50  are intended to work in one sensory mode or another may affect the general orientation and placement of sensors  50  on device  10 . For example, in an ECAP sensory mode, sensors should be oriented on device  10  so as to be aligned along and parallel to a length of the tibial nerve, or other target nerve or muscle tissue, when device  10  is implanted. Placing sensors  50  in such a linear and parallel fashion allows for the ECAP signal to be measured based on the readings at proximal and distal sections of the target nerve or muscle fiber. Thus, in some embodiments, it may be beneficial for sensors  50 , and primary and secondary electrodes  18  and  10 , to be arranged in a substantially linear (e.g., linear or nearly linear) fashion along a longitudinal length or axis of device  10  as depicted in  FIGS.  1  and  4 A  for example. Such an arrangement may increase the overall capabilities of device  10 . The general alignment of sensors  50  may be less important for detecting EMG signals, thus allowing greater options for placement locations of sensors  50 . 
     Sensors  50  may be positioned along device  10  so that sensors are sufficiently separated from primary electrode  18  to reduce the amount of signal interference/artifact generated during electrical stimulation. In some examples, sensors  50  and primary electrode  18  may be generally positioned at opposite ends of device  10 , which may reduce the amount of signal interference generated by the stimulation therapy. Additionally, because sensors  50  are designed to assess responsive nerve or muscle activity due to the delivery of stimulation therapy, placing sensors  50  along the lower edge of device  10  opposite of primary electrode  18 , sensors  50  will be positioned closer to the patient&#39;s knee (see  FIG.  6 A ) compared to primary electrode  18  thus capturing responsive traffic generated by primary electrode  18 . Additionally, it may be beneficial to maximize the separation distance between sensors  50  and primary electrode  18  and secondary electrode  20  to reduce the amount of noise generated by the stimulation therapy. In some examples, sensors  50  may be separated from primary electrode  18  (e.g., edge-to-edge) by a distance of at least 3 mm away, more preferably at least 5 mm of edge-to-edge separation. 
     While the example in  FIG.  4 A  shows only two sensors  50 , aligned linearly with primary and secondary electrodes  18  and  20  and being on the same surface (e.g., within the same plane) as secondary electrode  20 , in another example, sensors  50  may be positioned at other positions along device  10 , such as positioned along one or more perimeter surfaces of device  10  opposite of secondary electrode  20  (e.g., on the backside of device  10 ). For example,  FIG.  4 B  shows a side view of housing  12 A with sensors  50 A positioned along the perimeter edges of housing  12 A. alternatively, sensors may be positioned on the opposite side of device  10  compared to secondary electrode  20 . Such a configuration may be useful for EMG sensing to capture a larger area of signal detection while ECAP sensing will typically occur on the same face as a targeted nerve (e.g., the same side as secondary electrode  20 ). 
     In some embodiments, one or more of the sensors may be formed within the footer section of device  10 . For example,  FIG.  4 C  shows a side view of device  10  of housing  12 B with sensors  50 B positioned along the perimeter edge of footer  54  opposite of header  14 . In this manner, the material forming footer  54  of device  10  may be used to electrically isolate sensors  50 B from housing  12 B and secondary electrode  12 . Sensors  50 B may be positioned in footer  54  on the same side as secondary electrode  20  or on other sides or perimeters of footer  54 . Such a construction may help increase the separation distance between primary electrode  18  and sensors  50 B as well as eliminate the need to modify the construction of housing  12 B and make the assembly of device  10  more convenient. In some embodiments, one of primary electrode  18  or secondary electrode  20  may be arranged on footer  54 . 
     In some embodiments, device  10  may include a plurality of sensors  50 C arranged in a grid style fashion.  FIG.  4 D  shows a side view of device  10  of housing  12 C with sensors  50 C arranged in two rows (i.e., an upper and lower row), each with multiple sensors  50 C per row. A sensor  50 C within each upper and lower row may form a sensor pair  56  configured to operate in the manner described above. By having two or more sensor pairs  56 , the processing circuitry of device  10  may analyze the sensed signal acquired by each pair  56  to determine which pair  56  is best positioned for sensing the stimulation signal. The interrogation and analysis of sensors  50 C may be performed periodically to determine if device migration has occurred with time. For example, during routine operation of device  10 , the switching circuitry of device  10  may be configured to, at select intervals, switch between various sensor pairs  56  to determine which sensor pair is best positioned for sensory signal capture. The controller may then select the optimally placed sensor pair  56  to obtain sensory signals in response to stimulation therapy and use the obtained sensor information in the closed-loop feedback system to maintain optimization of the stimulation therapy parameters. 
     Additionally, or alternatively, by having plurality of sensors  50 C within device  10 , multiple sensory signals may be assessed at a given time. For example, one sensor pair  56  may be used to examine the ECAP of the target nerve or muscle tissue, while another set of sensors  50 C may be used for EMG detection. 
     In another example, device  10  may include one or more sensors  50  positioned along the side opposite (not shown) of the side containing primary electrode  18  and secondary electrode  20 . In such examples, sensors  50  will be facing away from the tibial nerve rather than facing towards the nerve. The sensors may be positioned along housing  12  and similar manner to that described above with respect to  FIG.  4 A . Additionally, or alternatively, such sensors  50  may be incorporated into header unit  14  or footer  54  using the techniques described above. 
     In yet another embodiment, device  10  may include an elongated sensor tail configured to be coupled to device  10  and coupled to the processing circuitry of device  10  to provide the closed loop functionality.  FIG.  5    shows device  10 A with footer  54 A including a sensor tail  58  extending therefrom. Sensor tail  58  includes a plurality of sensors  60 , substantially similar in terms of functionality to sensors  50 . Sensor tail  58  allow for sensors  60  to be positioned further away from primary and secondary electrodes  18  and  20  to further reduce possible noise generated by the stimulation therapy. Sensor tail  58  modifies device  10 A such that the device may no longer be considered “leadless,” but still operates in a leadless design for purposes of delivery of stimulation therapy. Thus, despite the presence of sensor tail  58 , device  10 A may still be considered a leadless neurostimulation device. 
     Referring now to implantation,  FIG.  6 A  is a side view of a patient&#39;s leg  100  showing the leadless neurostimulation device  10  of  FIG.  1    implanted, and  FIG.  6 B  shows a cross-sectional schematic view of leadless neurostimulation device  10  implanted in leg  100  of a patient near the ankle adjacent to the tibial nerve  102 . The cross section of leg  100  illustrates tibia  104 , fibula  106 , fibularis  tertius    108 , flexor digitorum longus  110 , flexor hallucis longus  112 , fibularis  brevis    114 , soleus  116 , posterior tibial artery  118 , posterior tibial vein  120 , skin  122 , cutaneous fat layer  124 , and fascia layer  128 . In some examples, device  10  may be implanted outside of fascia layer  128  near tibial nerve  102 . Device  10  can be implanted through skin  122  and cutaneous fat layer  124  via a small incision  101  (e.g., about one to three cm) above the tibial nerve on a medial aspect of the patient&#39;s ankle. In other words, device  10  may be implanted in a pocket between skin and the fascia. Implantation outside fascia layer  128  may improve patient comfort and recovery because fascia layer  128  is not cut and does not need time to heal after implantation of device  10 . While incision  101  is shown approximately horizontal to the length of the tibial nerve, other incisions or implantation techniques could be used according to physician preference. 
     Device  10  may be positioned adjacent to the region defined by flexor digitorum longus  110 , flexor hallucis longus  112 , and soleus  116  in which tibial nerve  102  is contained and implanted adjacent and proximal to fascia layer  128  with primary electrode  18  and secondary electrode  20  facing toward tibial nerve  102 . Incision  101  preferably does not cross fascia layer  128  thereby reducing the risk of complications with the surgical procedure. In an embodiment, leadless neurostimulation device  10  may be implanted such that primary electrode  18  is oriented inferiorly relative to secondary electrode  20 . 
     Optional testing of leadless neurostimulation device  10  may be performed to determine if device  10  has been properly positioned in proximity to tibial nerve  102  to elicit a desired response from an applied electrical stimulation. Such testing may be performed in conjunction with sensors  50  to ensure proper function of the device. In an example, device  10  may controlled by an external programmer to deliver test stimulation, and one or more indicative responses are monitored, such as toe flexion from simulation of the tibial motor neurons controlling the flexor hallucis  brevis  or flexor digitorum  brevis , or a tingling sensation in the heel or sole of the foot excluding the medial arch. This can be associated with signal detection via sensor  50  to ensure proper functioning and establish a baseline. If such testing does not elicit appropriate motor or sensory responses, the practitioner may reposition device  10  and retest. 
     Once a practitioner has determined device  10  is properly positioned to provide an appropriate patient response to delivered stimulation therapy, housing  12  can be secured in place if needed. Such anchoring means may be optional as the natural shape of the region in which device  10  is implanted, and the shape of device  10  itself has shown good compatibility with the surrounding tissue thus preventing device  10  from shifting or rolling after implantation, in some embodiments, leadless neurostimulation device  10  may further include one or more suture points to help secure device  10  to fascia  102  or other parts of leg  100 . In some embodiments, a suture anchor  130  may be included at the distal end of housing  12 , opposite of the end attached to mounting plate  16 . 
     The closed-loop protocol of device  10  may periodically or continually collect information from sensors  50  during routine operation of device  10  to assess proper placement and stimulation values. The controller can update the stimulation regimen as needed to maintain the delivery of proper stimulation therapy. 
     An advantage of the devices and methods described herein can be improved patient safety and satisfaction after implant. In contrast to other approaches, leadless neurostimulation device  10  does not require fascia layer  128  to be disturbed which may reduce risks affiliated with alternative procedures. Further, as device  10  is a unitary structure and can be hermetically sealed, the device is more robust than other lead-based stimulation units. 
     During operation, an electrical stimulation signal may be transmitted between primary electrode  18  and secondary electrode  20  through fascia layer  128 . The electrical signal may be used to stimulate tibial nerve  102  which may be useful in the treatment of overactive bladder (OAB) symptoms of urinary urgency, retention, sexual dysfunction, urinary frequency and/or urge incontinence, or fecal incontinence. Sensors  50  (or others described above) may sense for the stimulation signal generated by primary and secondary electrodes  18  and  20 , by measuring the EMG or ECAP of the surrounding nerve or muscle fibers. Based on the sensed information, collected in real time, the processing circuitry of device  10  may adjust the stimulation parameters of device  10  to optimize the stimulation therapy provided. 
     EXAMPLES 
     Example 1— Minimum Threshold Current 
       FIG.  7    is a plot showing examples of the minimum threshold level of electrical current needed to observe a tibial nerve response based on a function of the return offset (e.g., separation distance between primary electrode  18  and secondary electrode  20 ) in modeling studies. The studies also examined the minimal level of current needed to induce a simulated stimulation to a tibial nerve a select distance away as a function of secondary electrode size (e.g., circular radius). The minimum threshold was evaluated as the current required to stimulate a model of a single axon at the center of a tibial nerve (above the Y=0 axis) and a saphenous nerve (below the Y=0 axis) models. 
     Exemplary leadless neurostimulation devices were modeled based on the device of  FIG.  1    and the power componentry of an InterStim Micro implantable system for Sacral Neuromodulation from Medtronic. The size of the contact surface of the primary electrodes was approximately 21.3 mm 2 . The size (radius) and positioning of the secondary electrode was modified for the study. The leadless neurostimulation devices were placed in computer models approximately 0.5 mm from a simulated fascia layer with approximately 6 mm separation to the tibial nerve. 
     As shown in  FIG.  7   , the minimum threshold current needed to obverse stimulation response to the tibial nerve occurred within the range of about 6 mm to about 13 mm of a return offset for the tested radii. For a secondary electrode size of about 4 mm (50 mm 2 ) a minimum threshold current of about 1.4 mA was observed at about a 9 mm offset. For a secondary electrode size of about 5 mm (79 mm 2 ) a minimum threshold current of about 1.55 mA was observed at about 8 mm offset. For a secondary electrode size of about 6 mm (113 mm 2 ) a minimum threshold current of about 1.7 mA was observed at about 7 mm offset. The smallest radii tested (4 mm) resulted in the lowest minimum threshold current (1.4 mA) but the largest return offset (9 mm). 
     Example 2— Offset and Depth Comparison 
     Simulations were conducted to examine the simulation depth as a function of the electrode offset (e.g., separation distance between primary and secondary electrodes) and stimulation voltage using modeling similar to Example 1. The size of the contact surface of the primary electrodes were approximately 21.3 mm 2  and the size of the secondary electrode was approximately 71 mm 2  (4.75 mm radius). The devices were tested at 10 mm and 20 mm electrode offsets. The leadless neurostimulation devices set in computer models approximately 0.5 mm from the fascia layer.  FIGS.  8 A and  8 B  are plots showing the threshold stimulation current in a cross-sectional view of the leg to capture the tibial nerve in the region above Y=0, and a cutaneous sensory nerve in the region below Y=0, for both 10 mm ( FIG.  8 A ) and 20 mm ( FIG.  8 B ) separation distances between the primary and secondary electrodes. The minimum threshold was evaluated as the current required to stimulate a model of a single axon at the center of a tibial nerve (above the Y=0 axis) and a saphenous nerve (below the Y=0 axis) models. The modeling demonstrated simulation obtainable within a radius of about 15 mm from the central axis of the device indicating that the disclosed device  12  may be useful in stimulating tibial nerves with deep or anterior tracks. 
     The modeling was compared to stimulation modeling for a disc-shaped stimulation device of 23 mm diameter and 2.2 mm thick. The disc stimulation device active electrode was modeled at about 12.5 mm 2  positioned at the center of the disc-shape and the return electrode was about 72.3 mm 2  and positioned at the perimeter edge of the side of the device.  FIG.  8 C  is a plot showing modeling results  FIG.  8 C  is a plot showing modeling results of stimulation threshold limit for the disclosed device compared to a disc-shaped stimulation device for depth and anterior/posterior relationship. The modeling demonstrated a notably reduced stimulation range (e.g., less than about 10 mm, e.g., 30% reduction in range) compared to the modeling of the present disclosed devices. It is believed that the reduction in operable range of the disc-shaped stimulation device may be due to the placement of the return electrode along the side of the device (e.g., not on the same side as the active electrode) as well as having the return electrode encircle the active electrode which negatively affect the possible pathway for the electrical stimulation. The modeling demonstrated that the disclosed device  12  may be useful in stimulating tibial nerves with deep or anterior tracks, particularly in comparison to disc-shaped stimulation devices. 
       FIG.  9    shows the threshold current as a function of stimulation depth for both the 10 mm and 20 mm electrode offsets. The simulation depth was measured along the normal of the device midline. The results showed relatively similar results for both the 10 mm and 20 mm offset samples with slightly lower threshold values being determined for the 10 mm offset device at stimulation depths less than 12 mm. 
     Example 3: Impedance and Depth Examination 
     The impedance and stimulation depth associated with the disclosed leadless neurostimulation devices were compared to a disc-shaped stimulation device using modeling similar to Example 1. The size of the contact surface of the primary electrode was approximately 21.3 mm 2  and the size of the secondary electrode was approximately 71 mm 2  (4.75 mm radius). The disc stimulation device was modeled to include a 23 mm diameter and 2.2 mm thick disc. The disc stimulation device active electrode was about 12.5 mm 2  positioned at the center of the coin-shape and the return electrode was about 72.3 mm 2  and positioned at the outer perimeter side of the device. Both devices were modeled approximately 0.5 mm from the fascia layer. The impedance of the disclosed leadless neurostimulation devices were found to be significantly lower than that of the disc stimulation device (e.g., modeled at about 1500 ohms or less compared to about 2100 ohms for the disc stimulation device). The comparatively lower impedance can allow for a higher current amplitude to be achieved for the same voltage, as well as better depth penetration. The comparatively lower impedance for the disclosed leadless electrodes may contributes to the device&#39;s ability to stimulate nerves over a larger area (laterally and depth) compared to the modeled disc-shaped device using comparable stimulation output. 
     Animal tests were also conducted to assess the practical impedance for representative neurostimulation devices of the disclose invention. Exemplary leadless neurostimulation devices were prepared by using an InterStim Micro implantable system for Sacral Neuromodulation from Medtronic that was modified to include the disclosed header unit  14  and secondary electrode  20 . The device was implanted in ovine models approximately 0.5 mm from the fascia layer with approximately 6 mm separation to the tibial nerve. The observed impedance was surprisingly low at values of about 300 ohms. (e.g., approximately 316±130 ohms for the 10 mm separation and approximately 282±85 ohms for the 20 mm separation). 
     The threshold stimulation current as a function of stimulation depth was also modeled and compared between the disc stimulation device and the disclosed leadless neurostimulation device, which are plotted in  FIG.  10   . The disclosed leadless neurostimulation devices demonstrated a significant improvement in reducing the minimum threshold current to obtain tibial stimulation with increasing stimulation depth. 
     Various embodiments of systems, devices, and methods have been described herein. These embodiments are given only by way of example and are not intended to limit the scope of the claimed inventions. It should be appreciated, moreover, that the various features of the embodiments that have been described may be combined in various ways to produce numerous additional embodiments. Moreover, while various materials, dimensions, shapes, configurations and locations, etc. have been described for use with disclosed embodiments, others besides those disclosed may be utilized without exceeding the scope of the claimed inventions. 
     It should be understood that various aspects disclosed herein may be combined in different combinations than the combinations specifically presented in the description and accompanying drawings. It should also be understood that, depending on the example, certain acts or events of any of the processes or methods described herein may be performed in a different sequence, may be added, merged, or left out altogether (e.g., all described acts or events may not be necessary to carry out the techniques). In addition, while certain aspects of this disclosure are described as being performed by a single module or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or modules associated with, for example, a medical device. 
     In one or more examples, the described techniques may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include non-transitory computer-readable media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer). Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor” as used herein may refer to any of the foregoing structure or any other physical structure suitable for implementation of the described techniques. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     Persons of ordinary skill in the relevant arts will recognize that the subject matter hereof may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the subject matter hereof may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the various embodiments can comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. Moreover, elements described with respect to one embodiment can be implemented in other embodiments even when not described in such embodiments unless otherwise noted. 
     Although a dependent claim may refer in the claims to a specific combination with one or more other claims, other embodiments can also include a combination of the dependent claim with the subject matter of each other dependent claim or a combination of one or more features with other dependent or independent claims. Such combinations are proposed herein unless it is stated that a specific combination is not intended.