Patent Publication Number: US-2021170184-A1

Title: Manufacturing implantable tissue stimulators

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application No. 62/934,849, filed on Nov. 13, 2019. The entire content of this application is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to manufacturing implantable tissue stimulators using various overmolding techniques. 
     BACKGROUND 
     Modulation of tissue within the body by electrical stimulation has become an important type of therapy for treating chronic, disabling conditions, such as chronic pain, problems of movement initiation and control, involuntary movements, dystonia, urinary and fecal incontinence, sexual difficulties, vascular insufficiency, and heart arrhythmia. For example, an external antenna can be used to send electrical energy to electrodes on an implanted tissue stimulator that can pass pulsatile electrical currents of controllable frequency, pulse width, and amplitudes to a tissue. 
     SUMMARY 
     In general, this disclosure relates to methods of manufacturing implantable tissue stimulators, such as methods that incorporate injection molding or dip coating techniques. 
     In one aspect, a method of manufacturing an implantable stimulation device includes providing a circuit board of the implantable stimulation device, the circuit board being equipped with circuit components and an antenna, adhering one or more electrodes to the circuit board, and applying an insulation material to the circuit board such that the insulation material forms a housing that surrounds the circuit board, the circuit components, and the antenna, while leaving the one or more electrodes exposed for stimulating a tissue. 
     Embodiments may include one or more of the following features. 
     In some embodiments, the method further includes attaching one or more joints to the circuit board and, after attaching the one or more joints, adhering the one or more electrodes to the circuit board respectively at the one or more joints. 
     In some embodiments, the method further includes attaching the one or more joints to the circuit board automatically by soldering, laser welding, or applying a conductive epoxy. 
     In some embodiments, the method further includes forming the one or more joints such that each of the one or more joints has a cube-like shape. 
     In some embodiments, the method further includes manufacturing the implantable stimulation device without any cables. 
     In some embodiments, the method further includes positioning the circuit board atop a support component, the support component defining a through opening sized to allow passage of a surgical tool, and covering the circuit board with a protective component, the protective component defining a channel that accommodates the circuit components on the circuit board. 
     In some embodiments, the method further includes alternately placing one or more spacers and the one or more electrodes over an assembly of the support component, the circuit board, and the protective component, placing an extended housing component over the assembly, and adhering the one or more spacers, the one or more electrodes, and the extended housing component to the circuit board. 
     In some embodiments, the one or more spacers and the extended housing component includes carbothane or other flexible polymer. 
     In some embodiments, the assembly is a first assembly, wherein the first assembly, the one or more spacers, the one or more electrodes, and the extended housing component together form a second assembly, and wherein the method further includes surrounding the second assembly with a heat shrink tube. 
     In some embodiments, the method further includes flowing the flexible polymer around the second assembly in a reflow tower and, after flowing the flexible polymer, removing the heat shrink tube from the second assembly. 
     In some embodiments, the insulation material of the implantable stimulation device is provided by at least one of the extended housing component and the one or more spacers. 
     In some embodiments, the method further includes securing the second assembly, equipped with the heat shrink tube, to a clamp of the reflow tower, and translating a heating element shuttle of the reflow tower along the second assembly to flow the flexible polymer into the second assembly. 
     In some embodiments, the method further includes placing an assembly of the circuit board, equipped with the circuit components, the antenna, and the one or more electrodes, within an injection mold. 
     In some embodiments, the method further includes filling the injection mold with the insulation material to form a housing of the implantable stimulation device. 
     In some embodiments, the injection mold defines one or more cavities that extend perpendicular to the implantable stimulation device, and the method further includes forming one or more tissue fixation devices respectively at the one or more cavities. 
     In some embodiments, the insulation material includes liquid silicone rubber. 
     In some embodiments, the method further includes dip coating an assembly of the circuit board, equipped with the circuit components, the antenna, and the one or more electrodes, to form a housing of the implantable stimulation device. 
     In some embodiments, the housing includes the insulation material. 
     In another aspect, an implantable stimulation device includes a circuit board equipped with circuit components, an antenna, and one or more electrodes. The implantable stimulation device further includes a housing formed of an insulation material that surrounds the circuit board, the circuit components, and the antenna, the insulation material leaving the one or more electrodes exposed for stimulating a tissue. 
     In some embodiments, the implantable stimulation device further includes one or more joints by which the one or more electrodes are adhered to the circuit board, each of the one or more joints having a cube-like shape. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  is a perspective view of a tissue stimulator manufactured in part using an injection molding technique with extrusion components. 
         FIG. 2  is a top view of an electronic assembly of the tissue stimulator of  FIG. 1 . 
         FIG. 3  is a side view of the electronic assembly of  FIG. 2 . 
         FIG. 4A  is a side cross-sectional view of a circuit board of the electronic assembly of  FIG. 2  with electrodes attached thereto. 
         FIG. 4B  is a perspective view of a joint of the electronic assembly of  FIG. 2 . 
         FIG. 5  illustrates various techniques by which the electrodes of  FIG. 4  can be attached to the circuit board of  FIG. 4 , including laser welding, soldering, and conductive epoxy application. 
         FIGS. 6-9  illustrate a series of steps involved in manufacturing the tissue stimulator of  FIG. 1 . 
         FIG. 10A  illustrates a reflow tower that can be used to manufacture the tissue stimulator of  FIG. 1 . 
         FIG. 10B  illustrates an enlarged view of a portion of the reflow tower of  FIG. 10A . 
         FIG. 11A  is a perspective view of a tissue stimulator manufactured in part using an injection molding technique with a silicone material. 
         FIG. 11B  illustrates an injection mold that can be used to manufacture the tissue stimulator of  FIG. 11A . 
         FIG. 12  is a perspective view of a tissue stimulator manufactured in part by dip coating. 
         FIG. 13  is a subassembly of the tissue stimulator of  FIG. 12  that is dipped in a solution to form the tissue stimulator of  FIG. 12 . 
         FIG. 14  is a top view of an end of a circuit board of the tissue stimulators of  FIGS. 1, 11, and 12 . 
         FIG. 15  is an enlarged perspective view of an end the tissue stimulators of  FIGS. 1, 11, and 12  at intermediate and final manufacturing steps. 
         FIG. 16  is a system block diagram of the neural stimulation system of the therapy delivery system of  FIG. 1 . 
         FIG. 17  is a detailed block diagram of the neural stimulation system of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a tissue stimulator  100  designed to be implanted within a patient&#39;s body for delivering electrical therapy to tissues within the body. The tissue stimulator  100  has an overmolded exterior design that provides strength and a smooth profile for optimal insertion and performance within the patient. For example, the tissue stimulator  100  includes a housing  130  that is molded (e.g., overmolded or injection molded) of a transparent (e.g., clear), opaque, or translucent material around various internal components of the tissue stimulator  100 . Accordingly, the tissue stimulator  100  is a monolithic device for which electronic components are secured to a small, flat substrate and that can be delivered to the body through an introducer needle. Referring to  FIGS. 1-3 , the tissue stimulator  100  further includes a circuit board  102 , various circuit components  104 , an antenna  106 , and electrodes  108  that are secured to the circuit board  102 , as well as spacers  150  that are arranged alternately with the electrodes  108 . The tissue stimulator  100  further includes multiple pads  110  at which the electrodes  108  are respectively attached to the one or more circuit boards  102 . 
     In some embodiments, a tissue stimulator that is substantially similar in construction and function to the tissue stimulator  100  may alternatively include more than one of any of the above-mentioned components. For example, in addition to the electrodes  108 , the pads  110 , and the spacers  150 , such a tissue stimulator may include one or more antennas  106 , as well as one or more circuit boards  102  that are each provided as one or more small, flat substrates. 
     Referring still to the example tissue stimulator  100  of  FIGS. 1-3 , the circuit board  102  is provided as one or more flexible substrates, including multiple layers  112  in which at least one antenna  106  is interposed. The circuit board  102  defines contact sites  114  that locate the pads  110 . Each pad  110  typically has a length of about 0.5 mm to about 4 mm, a width of about 0.05 mm to about 0.5 mm, and a thickness of about 0.0125 mm to about 0.5 mm. The circuit board  102  is typically made of a dielectric substrate, such as polyimide. In some embodiments, additional dielectric materials may be applied to the circuit board  102  along certain regions for stiffening. 
     The circuit components  104  are distributed along the length of the circuit board  102  and may be secured to the circuit board  102  via solder, solder paste, or conductive epoxy. Example circuit components  104  include diodes, capacitors, resistors, semiconductors, and other electromechanical components. The antenna  106  is integrated directly into one of the layers  112  of the circuit board  102  and is designed to receive an input signal carrying electrical energy that can be used by the circuit components  104  and relayed to the electrodes  108  so that the electrodes  108  can apply one or more electrical pulses to adjacent tissue. Arrangement of the antenna  106  along a layer  112  contributes to a compact and simplified structure of the electronic device  100  in that such configuration avoids the need for additional cables or attachment features to communicate the antenna  106  with the circuit components  104 . In some embodiments, the tissue stimulator  100  may include additional trace pathways to serialize the circuit components  104  and render the electronic device  100  viewable with standard imaging equipment (e.g., X-ray equipment). For example, the circuit board  102  may include one or more built-in coupling traces that can extend a transmission zone of the tissue stimulator  100 . Such coupling traces may or may not be directly connected to the primary circuit components  104  (e.g., as in the case of near field RF coupling). An electronic assembly  132  including a circuit board  102  that is equipped with circuit components  104 , an antenna  106 , and layers  112  may be manufactured individually or in an array of individual electronic assemblies  132  as part of a mass production process. The electrodes  108  are embodied as generally cylindrical structures that can be secured to the pads  110  at the contact sites  114 . The electrodes  108  typically have a length of about 0.5 mm to about 6 mm and an internal diameter of about 0.9 mm to about 1.5 mm. Referring to  FIGS. 2-4 , the electrodes  108  are attached to the contact sites  114  and around the circuit board  102  at joints  118 . The joints  118  provide additional surface area at which the electrodes  108  can be attached to the circuit board  102 . The electrodes  108  and the joints  118  are typically made of one or more biocompatible materials (e.g., noble metals or other metals) that have good conductivity characteristics and that therefore result in a good tissue response, such as stainless steel, platinum, platinum-iridium, gallium-nitride, titanium-nitride, iridium-oxide, or other materials. The joints  118  may have cross-sectional shape that is substantially circular (e.g., as shown in  FIG. 4A ), triangular, square, square-round, rectangular, or similar to any of these shapes. The joints  118  may have a three-dimensional shape that is substantially spherical (e.g., corresponding to the circular cross-sectional shape shown in  FIG. 4 ), substantially cubic or cube-like (e.g., as shown in  FIG. 4B ), or that is of another shape. A shape of the joints  118  provides an outer surface at which the electrodes  108  can be sufficiently attached to the contact sites  114 . Furthermore, the joints  118  can serve as fiducial markers (e.g., radio-opaque markers or other types of visual markers). 
     In some embodiments, the joints  118  are attached to the circuit board  102  at the contact sites  114  in an automatic manner (e.g., via surface mount techniques that utilize tape and reel machine mechanisms) at a high production rate with reduced labor. In some embodiments, the joints  118  are soldered to the circuit board  102  by hand. The joints  118 , embodied as any of the shapes described above, typically have a thickness of about 0.05 mm to about 0.5 mm and typically have a length that is a bit shorter than the respective electrodes  108 . The circuit board  102  and the joints  118  are sized, dimensioned, and arranged to promote filing of cavities with insulation material that forms the housing  130  during manufacturing of the tissue stimulator  100 , as will be discussed in more detail below. 
     Referring to  FIG. 5 , the electrodes  108  may then be attached to the circuit board at the joints  118  using various attachment techniques, such as laser welding, soldering, and conductive epoxy application (e.g., chemical bonding). Such techniques can be carried out automatically using computer controlled processing heads (e.g., laser heads  122 , soldering tips  124 , and syringes  126  applying epoxy  140 ) that can be controlled to attach multiple electrodes  108  to the joints  118  on the circuit board  102  in one pass or in multiple passes. In this manner, the electrodes  108  can be attached to the circuit board  102  in a uniform manner within specified tolerances and without cables (e.g., stainless steel wires, braided wires, or other wires) extending along the circuit board  102  and between the electrodes  108  that would otherwise need to be manually assembled with the electrodes  108  and the circuit board  102 . In some embodiments, the electrodes  108  may be slid over the circuit board  102  and positioned at the joints  118  as part of the laser welding, soldering, or epoxy techniques discussed above. For example, a microscope with optical tweezers or other specialty tooling and equipment may be used to position the electrodes  108  along the circuit board  102 . 
     As compared to conventional implantable electronic devices for which electrodes are secured to a circuit board via multiple cables, the tissue stimulator  100  is more easily assembled (e.g., automatically and more quickly at a lower cost), more flexible, can withstand more bending forces (e.g., avoiding the problem of cables popping off of electrodes), is more mechanically robust within a moving body, and is therefore less likely to fail mechanically. Additionally, the electrodes  108  may be assembled more uniformly with respect to positional accuracy and mechanical integrity, as compared to electrodes that are manually secured to a circuit board with multiple cables. 
     In some embodiments, an overall footprint and shape of the tissue stimulator  100  are selected to provide optimized electrical and mechanical performance of the circuit components  104  and the electrodes  108 , provide a minimal overall size of the tissue stimulator  100 , and provide an anchoring structure that prevents or reduces movement of the tissue stimulator  100  within the body. 
       FIGS. 6-9  illustrate a series of steps involved in manufacturing the tissue stimulator  100 . Referring to  FIG. 6 , the electronic assembly  132  is placed atop a lower component  134  (e.g., a support component). The lower component  134  is an elongate component that extends a length at least as long as the length of the circuit board  102 . The lower component  134  has a generally semi-circular outer cross-sectional shape and defines a flat recessed surface  136  for supporting the circuit board  102 . The lower component  134  defines an interior through channel  138  that is sized to allow passage of ancillary surgical equipment, such as a steering stylet, a rigidity stylet, or an implantable receiver, etc. The lower component  134  typically has a length of about 1 cm to about 38 cm or more and a maximum width (e.g., a diameter) of about 0.5 mm to about 2.0 mm. The interior through channel  138  typically has a diameter of about 0.2 mm to about 1.0 mm. 
     Referring to  FIG. 7 , an upper component  142  (e.g., a protective component) is subsequently placed atop the electronic assembly  132  (e.g., over the circuit components  104 ) while the electronic assembly  132  is supported on the lower extrusion component  134 . The upper component  142  is an elongate component that extends a length of about 1 cm to about 30 cm or more to protect the circuit components  104 . The upper component  142  has a generally circular outer cross-sectional shape and defines a generally rectangular channel  144  that is sized to fit over the circuit components  104  and the width of the lower component  134 . Both the lower and upper components  134 ,  142  are typically made of polyurethane or other flexible polymers such as carbothane, pellethane, silicone, or thermoplastic polyurethane (TPU) and may be formed via extrusion, injection molding or other suitable technique. 
     In some embodiments, a tissue stimulator that is similar in construction and function to the tissue stimulator  100  may not be formed using the upper component  142  and may instead be formed with a cylindrical tube that has an inner diameter fitting around the outer diameter of the electronic assembly  132 . 
     Referring to  FIG. 8 , the electrodes  108  and the spacers  150  are slid over and positioned along the electronic assembly  132 . For example, an electrode  108  may be attached to a joint  118  in an automated manner via any of techniques discussed above with respect to  FIG. 5 . A spacer  150  then may be slid over and positioned adjacent to the attached electrode  108  in an automated manner. For example, the spacers  150  may be placed on the electronic assembly  132  using an automated fixture that slides the spacers  150  into position and then welds, melds, or melts the spacers  150  in place. The spacers  150  have a generally circular outer cross-sectional shape and an inner cross-sectional shape that is formed to pass over the lower component  134  and the joints  118  attached thereto. The spacers  150  typically have a length of about 0.5 mm to about 6 mm and an internal diameter of about 0.2 mm to about 1.5 mm. The spacers  150  are typically made of materials that can reflow when heat is applied to create a water-tight seal against surrounding components, such as the components of the electronic assembly  132 . For example, the spacers  150  may be made of flexible biocompatible polymers, such as polyurethane, pellethane, carbothane, or silicone. Another electrode  108  is positioned against a free end of the spacer  150  and attached to a respective joint  118 , and the remaining spacers  150  and electrodes  108  are assembled to the circuit board  102  in a like manner in an alternating arrangement. 
     Referring to  FIG. 9 , an extended housing component  146  is placed (e.g., slid) over an end of the circuit board  102  that is opposite the electrodes  108 . The component  146  typically has a length of about 1 mm to about 450 mm and an internal diameter of about 0.2 mm to about 1.5 mm. The component  146  may be made of the same material from which the spacers  150  are made. When all of the spacers  150  and the component  146  are made of the same polymer material, the spacers  150  and the component  146  are fused together molecularly during a heating process to provide the strong, durable bonds. The lower and upper components  134 ,  142  may also or instead be formed of the same polymer material as spacers  150 . Furthermore, components  134  and  142  may be lengthened, making component  146  unnecessary. The assembly  158  as shown in  FIG. 9  is then placed in a reflow tower, where a piece of heat shrink tube (not shown) is placed (e.g., automatically placed) around the entire assembly  158 . In some implementations, the piece of heat shrink tube is placed around the assembly  158  before the assembly  158  is placed in the reflow tower. Once the entire assembly  158  is reflowed (as described below), the heat shrink tube is cut and peeled off of the assembly  158 , revealing the tissue stimulator  100  as illustrated in  FIG. 1 . Upon completion of the manufacturing process, the interior channel  138  extends nearly the full length of the tissue stimulator  100 . 
       FIG. 10A  illustrates an example reflow tower  200  that can be used to perform the manufacturing step shown in  FIG. 9 . The reflow tower  200  includes a support frame  202 , a heating element shuttle  204  that is translatable vertically along the support frame  202 , and multiple (e.g., four) clamps  206  that are designed to grasp components, such as mandrels, tissue stimulators  100 , catheters, or other products. The grasped components remain stationary while the heating element shuttle  204  moves along (e.g., around and without touching) the length of the grasped components. The reflow tower  200  also includes a control panel  208  by which several parameters (e.g., speed, timing, and temperature) can be controlled, potentially for each of the grasped components. 
       FIG. 10B  is an enlarged view of the heating element shuttle  204 . Shown in  FIG. 10B  is a plastic stabilizing component  210 , a heater band  214 , heating support components  216 , and the tissue stimulators  100 . The tissue stimulators  100  are stationary, and the heating element shuttle  204  moves along the length of the tissue stimulators  100  as governed at the control panel  208 . 
     A coated mandrel (e.g., coated with polytetrafluoroethylene) is placed inside of assemblies  158  that have been wrapped with heat shrink tube and that have been clamped into the reflow tower  200 , which may be hanging vertically. When the reflow process is initiated, the precise, temperature-controlled heating element shuttle  204  traverses the length of an assembly  158  to reflow the polymer material, e.g., of the spacers  150  and the component  146 , to molecularly join them together, thereby unifying the assembly  158  section by section. 
     In some embodiments, a tissue stimulator that is similar in construction and function to the tissue stimulator  100  may be manufactured via overmolding with a different material, such as silicone. For example,  FIG. 11A  illustrates such a tissue stimulator  300  that is substantially similar in construction and function to the tissue stimulator  100 , except that a housing  330  of the tissue stimulator  300  is formed from a different insulation material, such as liquid silicone rubber. Accordingly, the tissue stimulator  300  further includes a circuit board  102 , various circuit components  104 , an antenna  106 , and electrodes  108  that are secured to the circuit board  102 . Spaces between the electrodes  108  are filled during the overmold process with the liquid silicone rubber. Accordingly, the tissue stimulator  300  does not include the spacer components  150 . The tissue stimulator  300  also includes multiple pads  110  at which the electrodes  108  are respectively attached to the circuit board  102 , as discussed above with respect to the tissue stimulator  100 . 
     In some embodiments, a tissue stimulator that is substantially similar in construction and function to the tissue stimulator  300  may alternatively include more than one of any of the above-mentioned components. For example, in addition to the electrodes  108  and the pads  110 , such a tissue stimulator may include one or more antennas  106  and one or more circuit boards  102 . 
     Referring still to the example tissue stimulator  300 ,  FIG. 11B  illustrates an injection mold  350  (e.g., a cavity mold) that can be used to manufacture the tissue stimulator  300 . In some embodiments, the tissue stimulator  300  may be manufactured by placing an assembly of the circuit board  102 , equipped with the circuit components  104 , the antenna  106 , and the electrodes  108  inside of the injection mold  350  and fixtured in such a way that the assembly is encased (e.g., entirely encased) in the insulation material to form a cylindrical shape of the housing  330 . For example, the insulation material is injection molded under high pressure or low pressure or gravity poured to fill a mold with the assembly in place. The insulation material flows and fills all cavities and curves of the assembly, but does not overflow the electrodes  108 , which are intended to contact a patient&#39;s tissue. In some embodiments, the mold may have cavities that extend perpendicular to the tissue stimulator  300  to form fixation features (e.g., tines) on the housing  130  that provide a tissue anchoring capability to the tissue stimulator  300 . 
     In some embodiments, a tissue stimulator that is similar in construction and function to the tissue stimulator  100  may be manufactured using a dip coating process. For example,  FIG. 12  illustrates such a tissue stimulator  400  that is substantially similar in construction and function to the tissue stimulator  100 , except that a housing  430  of the tissue stimulator  400  and spaces between electrodes  108  are formed by dip coating. Accordingly, the tissue stimulator  400  further includes a circuit board  102 , various circuit components  104 , an antenna  106 , electrodes  108  that are secured to the circuit board  102 , and multiple pads  110  at which the electrodes  108  are respectively attached to the circuit board  102 , as discussed above with respect to the tissue stimulator  100 . 
     For example, an insulation material may be formed by dissolving polyurethane in a solvent to form a liquid solution. Referring to  FIG. 13 , an assembly  432  that includes the circuit board  102  equipped with the circuit components  104 , the antenna  106 , and the electrodes  108  is dipped into the liquid solution to coat the assembly  432  with the liquid solution. Iterative dips may be performed to achieve a desired cylindrical shape and diameter of the tissue stimulator  400 . For example, dip coating applies the liquid solution layer by layer. After an initial dip, the assembly is air dried for a period of time to evaporate a liquifying chemical component of the liquid solution, and then the process is repeated to iteratively increase a diameter of the assembly. 
     Referring to  FIG. 14 , either or both ends  152  of the circuit board  102  may be formed with a circular opening  154  that can be used for securing the circuit board  102  to a fixture during any of the above-discussed manufacturing processes. Referring to  FIG. 15 , in some embodiments, either or both ends  152  of the circuit board  102  may be clipped off and replaced with smooth, hemispherical caps  156  on the tissue stimulator  100 ,  300 ,  400 . 
     Referring to  FIG. 16 , any of the tissue stimulators  100 ,  300 ,  400  may be embodied as a tissue stimulator  814  of a neural stimulation system  800 . The neural stimulation system further includes a pulse generator  804  that is located exterior to the patient (e.g., handheld by the patient), a transmit (TX) antenna  810  that is connected to the pulse generator  804  and positioned against a skin surface of the patient, and a programmer module  802  that runs a software application. The neural stimulation system  800  is designed to send electrical pulses to a nearby (e.g., adjacent or surrounding) target nerve tissue to stimulate the target nerve tissue by using remote radio frequency (RF) energy without cables and without inductive coupling to power the tissue stimulator  814 . Accordingly, the tissue stimulator  814  is provided as a passive tissue stimulator in the neural stimulation system  800 . In some examples, the target nerve tissue is in the spinal column and may include one or more of the spinothalamic tracts, the dorsal horn, the dorsal root ganglia, the dorsal roots, the dorsal column fibers, and the peripheral nerves bundles leaving the dorsal column or the brainstem. In some examples, the target nerve tissue may include one or more of cranial nerves, abdominal nerves, thoracic nerves, trigeminal ganglia nerves, nerve bundles of the cerebral cortex, deep brain, sensory nerves, and motor nerves. 
     In some embodiments, the software application supports a wireless connection  804  (e.g., via Bluetooth®). The software application can enable the user to view a system status and system diagnostics, change various parameters, increase and decrease a desired stimulus amplitude of the electrical pulses, and adjust a feedback sensitivity of the RF pulse generator module  806 , among other functions. 
     The RF pulse generator module  806  includes stimulation circuitry, a battery to power generator electronics, and communication electronics that support the wireless connection  804 . In some embodiments, the RF pulse generator module  806  is designed to be worn external to the body, and the TX antenna  810  (e.g., located external to the body) is connected to the RF pulse generator module  806  by a wired connection  808 . Accordingly, the RF pulse generator module  806  and the TX antenna  810  may be incorporated into a wearable accessory (e.g., a belt or a harness design) or a clothing article such that electric radiative coupling can occur through the skin and underlying tissue to transfer power and/or control parameters to the tissue stimulator  814 . 
     The TX antenna  810  can be coupled directly to tissues within the body to create an electric field that powers the implanted tissue stimulator  814 . The TX antenna  810  communicates with the tissue stimulator  814  through an RF interface. For instance, the TX antenna  810  radiates an RF transmission signal that is modulated and encoded by the RF pulse generator module  806 . The tissue stimulator  814  includes one or more antennas (e.g., dipole antennas) that can receive and transmit through an RF interface  812 . In particular, the coupling mechanism between the TX antenna  810  and the one or more antennas on the tissue stimulator  814  is electrical radiative coupling and not inductive coupling. In other words, the coupling is through an electric field rather than through a magnetic field. Through this electrical radiative coupling, the TX antenna  810  can provide an input signal to the tissue stimulator  814 . 
     In addition to the one or more antennas, the tissue stimulator  814  further includes internal receiver circuit components that can capture the energy carried by the input signal sent from the TX antenna  804  and demodulate the input signal to convert the energy to an electrical waveform. The receiver circuit components can further modify the waveform to create electrical pulses suitable for stimulating the target neural tissue. The tissue stimulator  814  further includes electrodes that can deliver the electrical pulses to the target neural tissue. For example, the circuit components may include wave conditioning circuitry that rectifies the received RF signal (e.g., using a diode rectifier), transforms the RF energy to a low frequency signal suitable for the stimulation of neural tissue, and presents the resulting waveform to an array of the electrodes. In some implementations, the power level of the input signal directly determines an amplitude (e.g., a power, a current, and/or a voltage) of the electrical pulses applied to the target neural tissue by the electrodes. For example, the input signal may include information encoding stimulus waveforms to be applied at the electrodes. 
     In some implementations, the RF pulse generator module  806  can remotely control stimulus parameters of the electrical pulses applied to the target neural tissue by the electrodes and monitor feedback from the tissue stimulator  814  based on RF signals received from the tissue stimulator  814 . For example, a feedback detection algorithm implemented by the RF pulse generator module  806  can monitor data sent wirelessly from the tissue stimulator  814 , including information about the energy that the tissue stimulator  814  is receiving from the RF pulse generator  806  and information about the stimulus waveform being delivered to the electrodes. Accordingly, the circuit components internal to the tissue stimulator  814  may also include circuitry for communicating information back to the RF pulse generator module  806  to facilitate the feedback control mechanism. For example, the tissue stimulator  814  may send to the RF pulse generator module  806  a stimulus feedback signal that is indicative of parameters of the electrical pulses, and the RF pulse generator module  806  may employ the stimulus feedback signal to adjust parameters of the signal sent to the tissue stimulator  814 . 
     In order to provide an effective therapy for a given medical condition, the neural stimulation system  800  can be tuned to provide the optimal amount of excitation or inhibition to the nerve fibers by electrical stimulation. A closed loop feedback control method can be used in which the output signals from the tissue stimulator  814  are monitored and used to determine the appropriate level of neural stimulation current for maintaining effective neuronal activation. Alternatively, in some cases, the patient can manually adjust the output signals in an open loop control method. 
       FIG. 17  depicts a detailed diagram of the neural stimulation system  800 . The programmer module  802  may be used as a vehicle to handle touchscreen input on a graphical user interface (GUI)  904  and may include a central processing unit (CPU)  906  for processing and storing data. The programmer module  802  includes a user input system  921  and a communication subsystem  908 . The user input system  921  can allow a user to input or adjust instruction sets in order to adjust various parameter settings (e.g., in some cases, in an open loop fashion). The communication subsystem  908  can transmit these instruction sets (e.g., and other information) via the wireless connection  804  (e.g., via a Bluetooth or Wi-Fi connection) to the RF pulse generator module  806 . The communication subsystem  908  can also receive data from RF pulse generator module  806 . 
     The programmer module  802  can be utilized by multiple types of users (e.g., patients and others), such that the programmer module  802  may serve as a patient&#39;s control unit or a clinician&#39;s programmer unit. The programmer module  802  can be used to send stimulation parameters to the RF pulse generator module  806 . The stimulation parameters that can be controlled may include a pulse amplitude in a range of 0 mA to 20 mA, a pulse frequency in a range of 0 Hz to 2000 Hz, and a pulse width in a range of 0 ms to 2 ms. In this context, the term pulse refers to the phase of the waveform that directly produces stimulation of the tissue. Parameters of a charge-balancing phase (described below) of the waveform can similarly be controlled. The user can also optionally control an overall duration and a pattern of a treatment. 
     The tissue stimulator  814  or the RF pulse generator module  806  may be initially programmed to meet specific parameter settings for each individual patient during an initial implantation procedure. Because medical conditions or the body itself can change over time, the ability to readjust the parameter settings may be beneficial to ensure ongoing efficacy of the neural modulation therapy. 
     Signals sent by the RF pulse generator module  806  to the tissue stimulator  814  may include both power and parameter attributes related to the stimulus waveform, amplitude, pulse width, and frequency. The RF pulse generator module  806  can also function as a wireless receiving unit that receives feedback signals from the tissue stimulator  814 . To that end, the RF pulse generator module  806  includes microelectronics or other circuitry to handle the generation of the signals transmitted to the tissue stimulator  814 , as well as feedback signals received from tissue stimulator  814 . For example, the RF pulse generator module  806  includes a controller subsystem  914 , a high-frequency oscillator  918 , an RF amplifier  916 , an RF switch, and a feedback subsystem  912 . 
     The controller subsystem  914  includes a CPU  930  to handle data processing, a memory subsystem  928  (e.g., a local memory), a communication subsystem  934  to communicate with the programmer module  802  (e.g., including receiving stimulation parameters from the programmer module  802 ), pulse generator circuitry  936 , and digital/analog (D/A) converters  932 . 
     The controller subsystem  914  may be used by the user to control the stimulation parameter settings (e.g., by controlling the parameters of the signal sent from RF pulse generator module  806  to tissue stimulator  814 ). These parameter settings can affect the power, current level, or shape of the electrical pulses that will be applied by the electrodes. The programming of the stimulation parameters can be performed using the programming module  802  as described above to set a repetition rate, pulse width, amplitude, and waveform that will be transmitted by RF energy to a receive (RX) antenna  938  (e.g., or multiple RX antennas  938 ) within the tissue stimulator  814 . The RX antenna  938  may be a dipole antenna or another type of antenna. A clinician user may have the option of locking and/or hiding certain settings within a programmer interface to limit an ability of a patient user to view or adjust certain parameters since adjustment of certain parameters may require detailed medical knowledge of neurophysiology, neuroanatomy, protocols for neural modulation, and safety limits of electrical stimulation. 
     The controller subsystem  914  may store received parameter settings in the local memory subsystem  928  until the parameter settings are modified by new input data received from the programmer module  802 . The CPU  906  may use the parameters stored in the local memory to control the pulse generator circuitry  936  to generate a stimulus waveform that is modulated by the high frequency oscillator  918  in a range of 300 MHz to 8 GHz. The resulting RF signal may then be amplified by an RF amplifier  926  and sent through an RF switch  923  to the TX antenna  810  to reach the RX antenna  938  through a depth of tissue. 
     In some implementations, the RF signal sent by the TX antenna  810  may simply be a power transmission signal used by tissue stimulator  814  to generate electric pulses. In other implementations, the RF signal sent by the TX antenna  810  may be a telemetry signal that provides instructions about various operations of the tissue stimulator  814 . The telemetry signal may be sent by the modulation of the carrier signal through the skin. The telemetry signal is used to modulate the carrier signal (e.g., a high frequency signal) that is coupled to the antenna  938  and does not interfere with the input received on the same lead to power the tissue stimulator  814 . In some embodiments, the telemetry signal and the powering signal are combined into one signal, where the RF telemetry signal is used to modulate the RF powering signal such that the tissue stimulator  814  is powered directly by the received telemetry signal. Separate subsystems in the tissue stimulator  814  harness the power contained in the signal and interpret the data content of the signal. 
     The RF switch  923  may be a multipurpose device (e.g., a dual directional coupler) that passes the relatively high amplitude, extremely short duration RF pulse to the TX antenna  810  with minimal insertion loss, while simultaneously providing two low-level outputs to the feedback subsystem  912 . One output delivers a forward power signal to the feedback subsystem  912 , where the forward power signal is an attenuated version of the RF pulse sent to the TX antenna  810 , and the other output delivers a reverse power signal to a different port of the feedback subsystem  912 , where reverse power is an attenuated version of the reflected RF energy from the TX Antenna  810 . 
     During the on-cycle time (e.g., while an RF signal is being transmitted to tissue stimulator  814 ), the RF switch  923  is set to send the forward power signal to feedback subsystem  912 . During the off-cycle time (e.g., while an RF signal is not being transmitted to the tissue stimulator  814 ), the RF switch  923  can change to a receiving mode in which the reflected RF energy and/or RF signals from the tissue stimulator  814  are received to be analyzed in the feedback subsystem  912 . 
     The feedback subsystem  912  of the RF pulse generator module  806  may include reception circuitry to receive and extract telemetry or other feedback signals from tissue stimulator  814  and/or reflected RF energy from the signal sent by TX antenna  810 . The feedback subsystem  912  may include an amplifier  926 , a filter  924 , a demodulator  922 , and an A/D converter  920 . The feedback subsystem  912  receives the forward power signal and converts this high-frequency AC signal to a DC level that can be sampled and sent to the controller subsystem  914 . In this way, the characteristics of the generated RF pulse can be compared to a reference signal within the controller subsystem  914 . If a disparity (e.g., an error) exists in any parameter, the controller subsystem  914  can adjust the output to the RF pulse generator  806 . The nature of the adjustment can be proportional to the computed error. The controller subsystem  914  can incorporate additional inputs and limits on its adjustment scheme, such as the signal amplitude of the reverse power and any predetermined maximum or minimum values for various pulse parameters. 
     The reverse power signal can be used to detect fault conditions in the RF-power delivery system. In an ideal condition, when TX antenna  810  has perfectly matched impedance to the tissue that it contacts, the electromagnetic waves generated from the RF pulse generator module  806  pass unimpeded from the TX antenna  810  into the body tissue. However, in real-world applications, a large degree of variability exists in the body types of users, types of clothing worn, and positioning of the antenna  810  relative to the body surface. Since the impedance of the antenna  810  depends on the relative permittivity of the underlying tissue and any intervening materials and on an overall separation distance of the antenna  810  from the skin, there can be an impedance mismatch at the interface of the TX antenna  810  with the body surface in any given application. When such a mismatch occurs, the electromagnetic waves sent from the RF pulse generator module  806  are partially reflected at this interface, and this reflected energy propagates backward through the antenna feed. 
     The dual directional coupler RF switch  923  may prevent the reflected RF energy propagating back into the amplifier  926 , and may attenuate this reflected RF signal and send the attenuated signal as the reverse power signal to the feedback subsystem  912 . The feedback subsystem  912  can convert this high-frequency AC signal to a DC level that can be sampled and sent to the controller subsystem  914 . The controller subsystem  914  can then calculate the ratio of the amplitude of the reverse power signal to the amplitude of the forward power signal. The ratio of the amplitude of reverse power signal to the amplitude level of forward power may indicate severity of the impedance mismatch. 
     In order to sense impedance mismatch conditions, the controller subsystem  914  can measure the reflected-power ratio in real time, and according to preset thresholds for this measurement, the controller subsystem  914  can modify the level of RF power generated by the RF pulse generator module  806 . For example, for a moderate degree of reflected power the course of action can be for the controller subsystem  914  to increase the amplitude of RF power sent to the TX antenna  810 , as would be needed to compensate for slightly non-optimum but acceptable TX antenna coupling to the body. For higher ratios of reflected power, the course of action can be to prevent operation of the RF pulse generator module  806  and set a fault code to indicate that the TX antenna  810  has little or no coupling with the body. This type of reflected power fault condition can also be generated by a poor or broken connection to the TX antenna  810 . In either case, it may be desirable to stop RF transmission when the reflected power ratio is above a defined threshold, because internally reflected power can lead to unwanted heating of internal components, and this fault condition means that the system cannot deliver sufficient power to the tissue stimulator  814  and thus cannot deliver therapy to the user. 
     The controller  942  of the tissue stimulator  814  may transmit informational signals, such as a telemetry signal, through the RX antenna  538  to communicate with the RF pulse generator module  806  during its receive cycle. For example, the telemetry signal from the tissue stimulator  814  may be coupled to the modulated signal on the RX antenna  938 , during the on and off state of the transistor circuit to enable or disable a waveform that produces the corresponding RF bursts necessary to transmit to the external (or remotely implanted) pulse generator module  806 . The RX antenna  938  may be connected to electrodes  954  in contact with tissue to provide a return path for the transmitted signal. An A/D converter can be used to transfer stored data to a serialized pattern that can be transmitted on the pulse modulated signal from the RX antenna  938  of the tissue stimulator  814 . 
     A telemetry signal from the tissue stimulator  814  may include stimulus parameters, such as the power or the amplitude of the current that is delivered to the tissue from the electrodes  954 . The feedback signal can be transmitted to the RF pulse generator module  806  to indicate the strength of the stimulus at the target nerve tissue by means of coupling the signal to the RX antenna  938 , which radiates the telemetry signal to the RF pulse generator module  806 . The feedback signal can include either or both an analog and digital telemetry pulse modulated carrier signal. Data such as stimulation pulse parameters and measured characteristics of stimulator performance can be stored in an internal memory device within the tissue stimulator  814  and sent on the telemetry signal. The frequency of the carrier signal may be in a range of 300 MHz to 8 GHz. 
     In the feedback subsystem  912 , the telemetry signal can be down modulated using the demodulator  922  and digitized by being processed through the analog to digital (A/D) converter  920 . The digital telemetry signal may then be routed to the CPU  930  with embedded code, with the option to reprogram, to translate the signal into a corresponding current measurement in the tissue based on the amplitude of the received signal. The CPU  930  of the controller subsystem  914  can compare the reported stimulus parameters to those held in local memory  928  to verify that the tissue stimulator  814  delivered the specified stimuli to target nerve tissue. For example, if the tissue stimulator  814  reports a lower current than was specified, the power level from the RF pulse generator module  806  can be increased so that the tissue stimulator  814  will have more available power for stimulation. The tissue stimulator  814  can generate telemetry data in real time (e.g., at a rate of 8 kbits per second). All feedback data received from the tissue stimulator  814  can be logged against time and sampled to be stored for retrieval to a remote monitoring system accessible by a health care professional for trending and statistical correlations. 
     The sequence of remotely programmable RF signals received by the RX antenna  938  may be conditioned into waveforms that are controlled within the tissue stimulator  814  by the control subsystem  942  and routed to the appropriate electrodes  954  that are located in proximity to the target nerve tissue. For instance, the RF signal transmitted from the RF pulse generator module  806  may be received by RX antenna  938  and processed by circuitry, such as waveform conditioning circuitry  940 , within the tissue stimulator  814  to be converted into electrical pulses applied to the electrodes  954  through an electrode interface  952 . In some implementations, the tissue stimulator  814  includes between two to sixteen electrodes  954 . 
     The waveform conditioning circuitry  940  may include a rectifier  944 , which rectifies the signal received by the RX antenna  938 . The rectified signal may be fed to the controller  942  for receiving encoded instructions from the RF pulse generator module  806 . The rectifier signal may also be fed to a charge balance component  946  that is configured to create one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge at the one or more electrodes  954  (that is, the pulses are charge balanced). The charge balanced pulses are passed through the current limiter  948  to the electrode interface  952 , which applies the pulses to the electrodes  954  as appropriate. 
     The current limiter  948  insures the current level of the pulses applied to the electrodes  954  is not above a threshold current level. In some implementations, an amplitude (for example, a current level, a voltage level, or a power level) of the received RF pulse directly determines the amplitude of the stimulus. In this case, it may be particularly beneficial to include current limiter  948  to prevent excessive current or charge being delivered through the electrodes  954 , although the current limiter  548  may be used in other implementations where this is not the case. Generally, for a given electrode  954  having several square millimeters of surface area, it is the charge per phase that should be limited for safety (where the charge delivered by a stimulus phase is the integral of the current). But, in some cases, the limit can instead be placed on the current, where the maximum current multiplied by the maximum possible pulse duration is less than or equal to the maximum safe charge. More generally, the current limiter  948  acts as a charge limiter that limits a characteristic (for example, a current or duration) of the electrical pulses so that the charge per phase remains below a threshold level (typically, a safe-charge limit). 
     In the event the tissue stimulator  814  receives a “strong” pulse of RF power sufficient to generate a stimulus that would exceed the predetermined safe-charge limit, the current limiter  948  can automatically limit or “clip” the stimulus phase to maintain the total charge of the phase within the safety limit. The current limiter  948  may be a passive current limiting component that cuts the signal to the electrodes  954  once the safe current limit (the threshold current level) is reached. Alternatively, or additionally, the current limiter  948  may communicate with the electrode interface  952  to turn off all electrodes  954  to prevent tissue damaging current levels. 
     A clipping event may trigger a current limiter feedback control mode. The action of clipping may cause the controller to send a threshold power data signal to the RF pulse generator module  806 . The feedback subsystem  912  detects the threshold power signal and demodulates the signal into data that is communicated to the controller subsystem  914 . The controller subsystem  914  algorithms may act on this current-limiting condition by specifically reducing the RF power generated by the RF pulse generator module  806 , or cutting the power completely. In this way, the RF pulse generator module  806  can reduce the RF power delivered to the body if the tissue stimulator  814  reports that it is receiving excess RF power. 
     The controller  950  may communicate with the electrode interface  952  to control various aspects of the electrode setup and pulses applied to the electrodes  954 . The electrode interface  952  may act as a multiplex and control the polarity and switching of each of the electrodes  954 . For instance, in some implementations, the tissue stimulator  814  has multiple electrodes  954  in contact with the target neural tissue, and for a given stimulus, the RF pulse generator module  806  can arbitrarily assign one or more electrodes to act as a stimulating electrode, to act as a return electrode, or to be inactive by communication of assignment sent wirelessly with the parameter instructions, which the controller  950  uses to set electrode interface  952  as appropriate. It may be physiologically advantageous to assign, for example, one or two electrodes  954  as stimulating electrodes and to assign all remaining electrodes  954  as return electrodes. 
     Also, in some implementations, for a given stimulus pulse, the controller  950  may control the electrode interface  952  to divide the current arbitrarily (or according to instructions from the RF pulse generator module  806 ) among the designated stimulating electrodes. This control over electrode assignment and current control can be advantageous because in practice the electrodes  954  may be spatially distributed along various neural structures, and through strategic selection of the stimulating electrode location and the proportion of current specified for each location, the aggregate current distribution on the target neural tissue can be modified to selectively activate specific neural targets. This strategy of current steering can improve the therapeutic effect for the patient. 
     In another implementation, the time course of stimuli may be arbitrarily manipulated. A given stimulus waveform may be initiated at a time T_start and terminated at a time T final, and this time course may be synchronized across all stimulating and return electrodes. Furthermore, the frequency of repetition of this stimulus cycle may be synchronous for all of the electrodes  954 . However, the controller  950 , on its own or in response to instructions from the RF pulse generator module  806 , can control electrode interface  952  to designate one or more subsets of electrodes to deliver stimulus waveforms with non-synchronous start and stop times, and the frequency of repetition of each stimulus cycle can be arbitrarily and independently specified. 
     For example, a tissue stimulator  814  having eight electrodes  954  may be configured to have a subset of five electrodes, called set A, and a subset of three electrodes, called set B. Set A may be configured to use two of its electrodes as stimulating electrodes, with the remainder being return electrodes. Set B may be configured to have just one stimulating electrode. The controller  950  could then specify that set A deliver a stimulus phase with 3 mA current for a duration of 200 us, followed by a 400 us charge-balancing phase. This stimulus cycle could be specified to repeat at a rate of 60 cycles per second. Then, for set B, the controller  950  could specify a stimulus phase with 1 mA current for duration of 500 us, followed by a 800 us charge-balancing phase. The repetition rate for the set B stimulus cycle can be set independently of set A (e.g., at 25 cycles per second). Or, if the controller  950  was configured to match the repetition rate for set B to that of set A, for such a case the controller  950  can specify the relative start times of the stimulus cycles to be coincident in time or to be arbitrarily offset from one another by some delay interval. 
     In some implementations, the controller  950  can arbitrarily shape the stimulus waveform amplitude, and may do so in response to instructions from the RF pulse generator module  806 . The stimulus phase may be delivered by a constant-current source or a constant-voltage source, and this type of control may generate characteristic waveforms that are static. For example, a constant current source generates a characteristic rectangular pulse in which the current waveform has a very steep rise, a constant amplitude for the duration of the stimulus, and then a very steep return to baseline. Alternatively, or additionally, the controller  950  can increase or decrease the level of current at any time during the stimulus phase and/or during the charge-balancing phase. Thus, in some implementations, the controller  950  can deliver arbitrarily shaped stimulus waveforms such as a triangular pulse, sinusoidal pulse, or Gaussian pulse for example. Similarly, the charge-balancing phase can be arbitrarily amplitude-shaped, and similarly a leading anodic pulse (prior to the stimulus phase) may also be amplitude-shaped. 
     As described above, the tissue stimulator  814  may include a charge balancing component  946 . Generally, for constant current stimulation pulses, pulses should be charge balanced by having the amount of cathodic current should equal the amount of anodic current, which is typically called biphasic stimulation. Charge density is the amount of current times the duration it is applied, and is typically expressed in the units uC/cm 2 . In order to avoid the irreversible electrochemical reactions such as pH change, electrode dissolution as well as tissue destruction, no net charge should appear at the electrode-electrolyte interface, and it is generally acceptable to have a charge density less than 30 uC/cm 2 . Biphasic stimulating current pulses ensure that no net charge appears at the electrode  954  after each stimulation cycle and that the electrochemical processes are balanced to prevent net dc currents. The tissue stimulator  814  may be designed to ensure that the resulting stimulus waveform has a net zero charge. Charge balanced stimuli are thought to have minimal damaging effects on tissue by reducing or eliminating electrochemical reaction products created at the electrode-tissue interface. 
     A stimulus pulse may have a negative-voltage or current, called the cathodic phase of the waveform. Stimulating electrodes may have both cathodic and anodic phases at different times during the stimulus cycle. An electrode  954  that delivers a negative current with sufficient amplitude to stimulate adjacent neural tissue is called a “stimulating electrode.” During the stimulus phase, the stimulating electrode acts as a current sink. One or more additional electrodes act as a current source and these electrodes are called “return electrodes.” Return electrodes are placed elsewhere in the tissue at some distance from the stimulating electrodes. When a typical negative stimulus phase is delivered to tissue at the stimulating electrode, the return electrode has a positive stimulus phase. During the subsequent charge-balancing phase, the polarities of each electrode are reversed. 
     In some implementations, the charge balance component  946  uses one or more blocking capacitors placed electrically in series with the stimulating electrodes and body tissue, between the point of stimulus generation within the stimulator circuitry and the point of stimulus delivery to tissue. In this manner, a resistor-capacitor (RC) network may be formed. In a multi-electrode stimulator, one charge-balance capacitors may be used for each electrode, or a centralized capacitors may be used within the stimulator circuitry prior to the point of electrode selection. The RC network can block direct current (DC). However, the RC network can also prevent low-frequency alternating current (AC) from passing to the tissue. The frequency below which the series RC network essentially blocks signals is commonly referred to as the cutoff frequency, and in some embodiments, the design of the stimulator system may ensure that the cutoff frequency is not above the fundamental frequency of the stimulus waveform. In the example embodiment  800 , the tissue stimulator  814  may have a charge-balance capacitor with a value chosen according to the measured series resistance of the electrodes and the tissue environment in which the stimulator is implanted. By selecting a specific capacitance value, the cutoff frequency of the RC network in this embodiment is at or below the fundamental frequency of the stimulus pulse. 
     In other implementations, the cutoff frequency may be chosen to be at or above the fundamental frequency of the stimulus, and in this scenario the stimulus waveform created prior to the charge-balance capacitor, called the drive waveform, may be designed to be non-stationary, where the envelope of the drive waveform is varied during the duration of the drive pulse. For example, in one embodiment, the initial amplitude of the drive waveform is set at an initial amplitude Vi, and the amplitude is increased during the duration of the pulse until it reaches a final value k*Vi. By changing the amplitude of the drive waveform over time, the shape of the stimulus waveform passed through the charge-balance capacitor is also modified. The shape of the stimulus waveform may be modified in this fashion to create a physiologically advantageous stimulus. 
     In some implementations, the tissue stimulator  814  may create a drive-waveform envelope that follows the envelope of the RF pulse received by the RX antenna  938 . In this case, the RF pulse generator module  806  can directly control the envelope of the drive waveform within the tissue stimulator  814 , and thus no energy storage may be required inside of the tissue stimulator  814 , itself. In this implementation, the stimulator circuitry may modify the envelope of the drive waveform or may pass it directly to the charge-balance capacitor and/or electrode-selection stage. 
     In some implementations, the tissue stimulator  814  may deliver a single-phase drive waveform to the charge balance capacitor or it may deliver multiphase drive waveforms. In the case of a single-phase drive waveform (e.g., a negative-going rectangular pulse), this pulse comprises the physiological stimulus phase, and the charge-balance capacitor is polarized (charged) during this phase. After the drive pulse is completed, the charge balancing function is performed solely by the passive discharge of the charge-balance capacitor, where is dissipates its charge through the tissue in an opposite polarity relative to the preceding stimulus. In one implementation, a resistor within the tissue stimulator  814  facilitates the discharge of the charge-balance capacitor. In some implementations, using a passive discharge phase, the capacitor may allow virtually complete discharge prior to the onset of the subsequent stimulus pulse. 
     In the case of multiphase drive waveforms, the tissue stimulator  814  may perform internal switching to pass negative-going or positive-going pulses (phases) to the charge-balance capacitor. These pulses may be delivered in any sequence and with varying amplitudes and waveform shapes to achieve a desired physiological effect. For example, the stimulus phase may be followed by an actively driven charge-balancing phase, and/or the stimulus phase may be preceded by an opposite phase. Preceding the stimulus with an opposite-polarity phase, for example, can have the advantage of reducing the amplitude of the stimulus phase required to excite tissue. 
     In some implementations, the amplitude and timing of stimulus and charge-balancing phases is controlled by the amplitude and timing of RF pulses from the RF pulse generator module  806 , and in other implementations, this control may be administered internally by circuitry onboard the tissue stimulator  814 , such as controller  550 . In the case of onboard control, the amplitude and timing may be specified or modified by data commands delivered from the pulse generator module  806 . 
     While the RF pulse generator module  806  and the TX antenna  810  have been described and illustrated as separate components, in some embodiments, the RF pulse generator module  806  and the TX antenna  810  may be physically located in the same housing or other packaging. Furthermore, while the RF pulse generator module  806  and the TX antenna  810  have been described and illustrated as located external to the body, in some embodiments, either or both of the RF pulse generator module  806  and the TX antenna  810  may be designed to be implanted subcutaneously. While the RF pulse generator module  806  and the TX antenna  810  have been described and illustrated as coupled via a wired connection  808 , in some embodiments (e.g., where the RF pulse generator module  806  is either located externally or implanted subcutaneously), the RF pulse generator module  806  and the TX antenna  810  may be coupled via a wireless connection. 
     Other embodiments of tissue stimulation systems, tissue stimulators, leads, and methods of manufacturing tissue stimulators and leads are within the scope of the following claims. For example, the improved techniques described above with respect to manufacturing the tissue stimulators  100 ,  300 ,  400  can also provide an improved lead, where a substantially simplified circuit board replaces the above-described stimulator circuit board, circuit components, and antenna. In an embodiment of such an improved lead, the circuit board acts as the electrical conduit, replacing standard cables (e.g., wires) that are commonly attached to electrodes. This alternative design results in a substantially stronger lead that is more easily assembled, more flexible, that can withstand more bending forces, that is more mechanically robust within a moving body, and that allows for more uniform assembly of electrodes, as discussed above with respect to the tissue stimulators  100 ,  300 ,  400 .