Patent Publication Number: US-2023158299-A1

Title: Hybrid multi-phasic stimulation

Description:
BACKGROUND 
     Field of the Invention 
     The present invention relates generally to the electrical stimulation in implantable medical device. 
     Related Art 
     Medical devices have provided a wide range of therapeutic benefits to recipients over recent decades. Medical devices can include internal or implantable components/devices, external or wearable components/devices, or combinations thereof (e.g., a device having an external component communicating with an implantable component). Medical devices, such as traditional hearing aids, partially or fully-implantable hearing prostheses (e.g., bone conduction devices, mechanical stimulators, cochlear implants, etc.), pacemakers, defibrillators, functional electrical stimulation devices, and other medical devices, have been successful in performing lifesaving and/or lifestyle enhancement functions and/or recipient monitoring for a number of years. 
     The types of medical devices and the ranges of functions performed thereby have increased over the years. For example, many medical devices, sometimes referred to as “implantable medical devices,” now often include one or more instruments, apparatus, sensors, processors, controllers or other functional mechanical or electrical components that are permanently or temporarily implanted in a recipient. These functional devices are typically used to diagnose, prevent, monitor, treat, or manage a disease/injury or symptom thereof, or to investigate, replace or modify the anatomy or a physiological process. Many of these functional devices utilize power and/or data received from external devices that are part of, or operate in conjunction with, implantable components. 
     SUMMARY 
     In one aspect, a method is provided. The method comprises: delivering, via at least one of a plurality of implantable electrodes of an implantable medical device, a stimulation current pulse to inject charge into tissue of a recipient of the implantable medical device; shorting the at least one of the plurality of implantable electrodes with one or more other of the plurality of implantable electrodes to remove a first portion of the charge injected into the tissue by the stimulation current pulse; and delivering at least one discharging current pulse to the tissue of the recipient to remove a second portion of the charge injected into the tissue by the stimulation current pulse. 
     In another aspect, an implantable medical device is provided. The implantable medical device comprises: a plurality of electrodes configured to be implanted adjacent to tissue of a recipient; and a stimulation subsystem configured to generate a plurality of hybrid multi-phasic stimulation signals for delivery via one or more of the plurality of electrodes, wherein each hybrid multi-phasic stimulation signal comprises at least one cathodic current pulse and at least one anodic current pulse, and wherein the at least one cathodic current pulse and at least one anodic current pulse are separated from one another by at least one shorting period. 
     In another aspect, a method is provided. The method comprises: delivering at least a first current pulse to tissue of a recipient of an implantable medical device via at least one of a plurality of implantable electrodes, wherein the first pulse has a first current polarity and is configured to stimulate the tissue; delivering at least a second current pulse having a second current polarity to the tissue of the recipient, wherein the second current polarity is an opposite current polarity to the first current polarity; and prior to delivering the at least one discharging current pulse, shorting the at least one of the plurality of implantable electrodes with one or more other of the plurality of implantable electrodes. 
     In another aspect, a method is provided. The method comprises: delivering at least one stimulation current pulse to stimulate tissue of a recipient of an implantable medical device, wherein the at least one stimulation current pulse injects charge into the tissue of the recipient; and removing the charge injected into the tissue by the least one stimulation current pulse through a combination of at least one discharging current pulse and a period of electrode shorting that precedes the at least one discharging current pulse. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are described herein in conjunction with the accompanying drawings, in which: 
         FIG.  1 A  is a schematic diagram illustrating a cochlear implant system, in accordance with certain embodiments presented herein; 
         FIG.  1 B  is a side view of a recipient wearing a sound processing unit of the cochlear implant system of  FIG.  1 A ; 
         FIG.  1 C  is a schematic view of components of the cochlear implant system of  FIG.  1 A ; 
         FIGS.  1 D  is a block diagram of the cochlear implant system of  FIG.  1 A ; 
         FIG.  2    is a schematic diagram illustrating an example biphasic stimulation signal; 
         FIG.  3    is a schematic diagram illustrating an example monophasic stimulation signal; 
         FIGS.  4 A and  4 B  are schematic diagrams illustrating an example hybrid multi-phasic stimulation signal, in accordance with certain embodiments presented herein; 
         FIG.  4 C  is a schematic diagram illustrating another example hybrid multi-phasic stimulation signal, in accordance with certain embodiments presented herein; 
         FIG.  5    is a schematic diagram illustrating another example hybrid multi-phasic stimulation signal, in accordance with certain embodiments presented herein; 
         FIG.  6    is a schematic diagram illustrating another example hybrid multi-phasic stimulation signal, in accordance with certain embodiments presented herein; 
         FIG.  7    is a schematic diagram illustrating another example hybrid multi-phasic stimulation signal, in accordance with certain embodiments presented herein; 
         FIG.  8    is a schematic diagram illustrating electrode shorting, in accordance with certain embodiments presented herein; 
         FIG.  9    is another schematic diagram illustrating electrode shorting, in accordance with certain embodiments presented herein; 
         FIG.  10    is a schematic diagram illustrating an example hybrid monophasic stimulation signal, in accordance with certain embodiments presented herein; 
         FIG.  11    is a schematic diagram illustrating a vestibular implant, in accordance with certain embodiments presented herein; 
         FIG.  12    is a flowchart of a method, in accordance with certain embodiments presented herein; 
         FIG.  13    is a flowchart of another method, in accordance with certain embodiments presented herein; and 
         FIG.  14    is a flowchart of another method, in accordance with certain embodiments presented herein. 
     
    
    
     DETAILED DESCRIPTION 
     Presented herein are hybrid multi-phasic stimulation techniques for stimulation of tissue of a recipient of an implantable medical device. In particular, in accordance with the hybrid multi-phasic stimulation techniques, at least one stimulation current pulse is delivered to the tissue of the recipient in order to stimulate the tissue. The at least one stimulation current pulse injects charge into the tissue that is then removed through the combination of at least one discharging current pulse and a period of electrode shorting. 
     Merely for ease of description, the hybrid multi-phasic stimulation techniques presented herein are primarily described with reference to a specific implantable medical device system, namely a cochlear implant system. However, it is to be appreciated that the hybrid multi-phasic electrical stimulation techniques presented herein may also be implemented by other types of implantable medical devices or implantable medical device systems. For example, the hybrid multi-phasic electrical stimulation techniques may be implemented by other auditory prostheses or systems includes other auditory prostheses, such as middle ear auditory prostheses, bone conduction devices, direct acoustic stimulators, electro-acoustic prostheses, auditory brain stimulators, etc. The techniques presented herein may also be used with tinnitus therapy devices, vestibular devices (e.g., vestibular implants), visual devices (i.e., bionic eyes), sensors, pacemakers, drug delivery systems, defibrillators, functional electrical stimulation devices, catheters, seizure devices (e.g., devices for monitoring and/or treating epileptic events), sleep apnea devices, electroporation devices, etc. 
       FIGS.  1 A- 1 D  are diagrams illustrating an example cochlear implant system  102  configured to implement certain embodiments of the techniques presented herein. The cochlear implant system  102  comprises an external component  104  and an implantable component  112 . In the examples of  FIGS.  1 A- 1 D , the implantable component is sometimes referred to as a “cochlear implant.”  FIG.  1 A  is a schematic diagram illustrating the implantable component  112  implanted in the head  141  of a recipient, while  FIG.  1 B  is schematic drawing of the external component  104  worn on the head  141  of the recipient.  FIG.  1 C  is another schematic view of the cochlear implant system  102 , while  FIG.  1 D  is a block diagram illustrating further details of the cochlear implant system  102 . For ease of description,  FIGS.  1 A- 1 D  will generally be described together. 
     As noted, cochlear implant system  102  includes an external component  104  that is configured to be directly or indirectly attached to the body of the recipient and an implantable component  112  configured to be implanted in the recipient. In the examples of  FIGS.  1 A- 1 D , the external component  104  comprises a sound processing unit  106 , while the implantable component  112  includes an internal coil  114 , a stimulator unit  142 , and an elongate stimulating assembly  116  configured to be implanted in the recipient&#39;s cochlea. 
     In the example of  FIGS.  1 A- 1 D , the sound processing unit  106  is an off-the-ear (OTE) sound processing unit, sometimes referred to herein as an OTE component, that is configured to send data and power to the implantable component  112 . In general, an OTE sound processing unit is a component having a generally cylindrically shaped housing  105  and which is configured to be magnetically coupled to the recipient&#39;s head (e.g., includes an integrated magnet configured to be magnetically coupled to a magnet in the implantable component  112 ). The OTE sound processing unit  106  also includes an integrated external coil  108  that is configured to be inductively coupled to the implantable coil  114 . 
     It is to be appreciated that the OTE sound processing unit  106  is merely illustrative of the external devices that could operate with implantable component  112 . For example, in alternative examples, the external component may comprise a behind-the-ear (BTE) sound processing unit or a micro-BTE sound processing unit and a separate external. In general, a BTE sound processing unit comprises a housing that is shaped to be worn on the outer ear of the recipient and is connected to the separate external coil via a cable assembly (cable), where the external coil is configured to be inductively coupled to the implantable coil  114 . It is also to be appreciated that alternative external components could be located in the recipient&#39;s ear canal, worn on the body, etc. 
       FIGS.  1 A- 1 D  illustrate an arrangement in which the cochlear implant system  102  includes an external component. However, it is to be appreciated that embodiments of the present invention may be implemented in cochlear implant systems having alternative arrangements. For example, embodiments presented herein can be implemented by a totally implantable cochlear implant or other totally implantable medical device. A totally implantable medical device is a device in which all components of the device are configured to be implanted under skin/tissue of a recipient. Because all components are implantable, a totally implantable medical device operates, for at least a finite period of time, without the need of an external device. An external device can be used to, for example, charge the internal power source (battery). 
     Returning to the specific example of  FIGS.  1 A- 1 D ,  FIG.  1 D  illustrates that the OTE sound processing unit  106  comprises one or more input devices  113  that are configured to receive input signals (e.g., sound or data signals). The one or more input devices  113  include one or more sound input devices  118  (e.g., microphones, audio input ports, telecoils, etc.), one or more auxiliary input devices  119  (e.g., audio ports, such as a Direct Audio Input (DAI), data ports, such as a Universal Serial Bus (USB) port, cable port, etc.), and a wireless transmitter/receiver (transceiver)  120 . However, it is to be appreciated that one or more input devices  113  may include additional types of input devices and/or less input devices (e.g., the wireless transceiver  120  and/or one or more auxiliary input devices  119  could be omitted). 
     The OTE sound processing unit  106  also comprises the external coil  108 , a charging coil  121 , a closely-coupled transmitter/receiver (transceiver)  122 , sometimes referred to as or radio-frequency (RF) transceiver  122 , at least one rechargeable battery  123 , and a processing module  124 . The processing module  124  comprises one or more processors  125  and a memory device (memory)  126  that includes hybrid multi-phasic processing logic  128 . The memory device  126  may comprise any one or more of: Non-Volatile Memory (NVM), Ferroelectric Random Access Memory (FRAM), read only memory (ROM), random access memory (RAM), magnetic disk storage media devices, optical storage media devices, flash memory devices, electrical, optical, or other physical/tangible memory storage devices. The one or more processors  125  are, for example, microprocessors or microcontrollers that execute instructions for the hybrid multi-phasic processing logic  128  stored in memory device  126  (e.g., execute instructions for implementation of the hybrid multi-phasic stimulation techniques presented herein). 
     The implantable component  112  comprises an implant body (main module)  134 , a lead region  136 , and the intra-cochlear stimulating assembly  116 , all configured to be implanted under the skin/tissue (tissue)  115  of the recipient. The implant body  134  generally comprises a hermetically-sealed housing  138  in which RF interface circuitry  140  and a stimulator unit  142  are disposed. The implant body  134  also includes the internal/implantable coil  114  that is generally external to the housing  138 , but which is connected to the transceiver  140  via a hermetic feedthrough (not shown in  FIG.  1 D ). 
     As noted, stimulating assembly  116  is configured to be at least partially implanted in the recipient&#39;s cochlea. Stimulating assembly  116  includes a plurality of longitudinally spaced intra-cochlear electrical stimulating contacts (electrodes)  144  that collectively form a contact or electrode array  146  for delivery of electrical stimulation (current) to the recipient&#39;s cochlea. 
     Stimulating assembly  116  extends through an opening in the recipient&#39;s cochlea (e.g., cochleostomy, the round window, etc.) and has a proximal end connected to stimulator unit  142  via lead region  136  and a hermetic feedthrough (not shown in  FIG.  1 D ). Lead region  136  includes a plurality of conductors (wires) that electrically couple the electrodes  144  to the stimulator unit  142 . The implantable component  112  also includes an electrode outside of the cochlea, sometimes referred to as the extra-cochlear electrode (ECE)  139 . 
     As noted, the cochlear implant system  102  includes the external coil  108  and the implantable coil  114 . Generally, a magnet is fixed relative to each of the external coil  108  and the implantable coil  114 . The magnets fixed relative to the external coil  108  and the implantable coil  114  facilitate the operational alignment of the external coil  108  with the implantable coil  114 . This operational alignment of the coils enables the external component  104  to transmit data and power to the implantable component  112  via a closely-coupled wireless link formed between the external coil  108  and the implantable coil  114 . In certain examples, the closely-coupled wireless link is a radio frequency (RF) link. However, various other types of energy transfer, such as infrared (IR), electromagnetic, capacitive and inductive transfer, may be used to transfer the power and/or data from an external component to an implantable component and, as such,  FIG.  1 D  illustrates only one example arrangement. 
     As noted above, sound processing unit  106  includes the processing module  124 . The processing module  124  is configured to convert received input signals (received at one or more of the input devices  113 ) into output signals for use in stimulating a first ear of a recipient (i.e., the processing module  124  is configured to perform sound processing on input signals received at the sound processing unit  106 ). Stated differently, the one or more processors  125  are configured to execute hybrid multi-phasic processing logic  128  in memory  126  to convert the received input signals into output signals  145  that represent electrical stimulation for delivery to the recipient. 
     As described further below, electrical stimulation signals in accordance with embodiments presented can comprise hybrid multi-phasic stimulation signals. As such, the output signals  145  generated by the sound processing unit  106  represent the hybrid multi-phasic stimulation signals (e.g., comprise commands/data for use by the stimulator unit  142  to form hybrid multi-phasic stimulation signals). 
     As noted,  FIG.  1 D  illustrates an embodiment in which the processing module  124  in the sound processing unit  106  generates the output signals. In an alternative embodiment, the sound processing unit  106  can send less processed information (e.g., audio data) to the implantable component  112  and the sound processing operations (e.g., conversion of sounds to output signals  145 ) can be performed by a processor within the implantable component  112 . That is, the implantable component  112 , rather than the sound processing unit  106 , could include a processing module that is similar to processing module  124  of  FIG.  1 D . 
     Returning to the specific example of  FIG.  1 D , the output signals  145  are provided to the RF transceiver  122 , which transcutaneously transfers the output signals (e.g., in an encoded manner) to the implantable component  112  via external coil  108  and implantable coil  114 . That is, the output signals are received at the RF interface circuitry  140  via implantable coil  114  and provided to the stimulator unit  142 . The stimulator unit  142  is configured to utilize the output signals to generate electrical stimulation signals (e.g., current signals) for delivery to the recipient&#39;s cochlea via “stimulation channels,” where each stimulating channel comprises one or more of the stimulating contacts  144 . In this way, cochlear implant system  102  electrically stimulates the recipient&#39;s auditory nerve cells, bypassing absent or defective hair cells that normally transduce acoustic vibrations into neural activity, in a manner that causes the recipient to perceive one or more components of the received sound signals. 
     As noted, the processing module  124  generates the output signals  145  which, in turn, are used by the stimulator unit  142  to generate current pulses and/or electrode shorting periods that, as described below, form hybrid multi-phasic stimulation signals presented herein. As such, the processing module  124  and the stimulator unit  142  are sometimes collectively referred to herein as a “stimulation subsystem”  143  that, in general, is configured to generate a plurality of hybrid multi-phasic stimulation signals for delivery via one or more of the plurality of electrodes  144 . 
     In general, with electrical stimulation, the flow of anions (−) and cations (+) is controlled by the mechanics of the circuitry within the stimulator unit. In a stimulator unit, the cathode is considered the negative pole (−) because it discharges anions (−), while the anode is the positive pole (+) because it discharges cations (+). Therefore, depending on the configuration of the polarity of a stimulator at a given time, the stimulator will discharge either cations or anions into the body part being stimulated. In cathodic stimulation, anions (−) are discharged into the body as current flows from the cathode (−), through the tissue, and back to the anode (+). In anodal stimulation, cations (+) are discharged into the body as current flows from the anode (+), through the tissue, and back to the cathode (−). 
     In addition, electrical stimulation of the tissue (e.g., nerve cells) requires “charge balancing.” That is, any charge delivered to the recipient&#39;s tissue must also be removed/withdrawn from the tissue, at least to a level such that there is a net average DC current flow below a predetermined threshold (e.g., approximately less than 100 nA). 
     As shown in  FIG.  2   , certain implantable medical devices use a biphasic stimulation signals  247  (biphasic stimulation) to ensure charge balancing. In conventional biphasic stimulation, the device&#39;s stimulation circuit (stimulator) delivers a first cathodic (−) current pulse  250  followed by a second anodic (+) current pulse  252  (or vice versa in alternative embodiments), where the first and second current pulses are generally “balanced.” That is, the first and second current pulses are generally configured to inject substantially the same amount of charge, but with opposing polarities, into the tissue. 
     More specifically, the implantable medical device generates and injects the cathodic current pulse  250  into the electrode-tissue interface. The cathodic pulse  250  depolarizes axons in the recipient&#39;s tissue and, accordingly, triggers an action potential (e.g., with an auditory prosthesis such as cochlear implant system  102 , the cathodic current pulse is the stimulating part that evokes a hearing perception). 
     As shown, the cathodic current pulse  250  is followed by an Inter Phase Gap (IPG)  251  (e.g., a time period in which no stimulation signals are delivered). After the IPG  251 , the implantable medical device then generates and injects the anodic current pulse  252 . The anodic pulse  252  injects a charge into the electrode-tissue interface to reverse the potentially damaging electrochemical processes that can occur at the electrode-tissue interface during delivery of the cathodic current pulse  250 . That is, the cathodic pulse and the anodic current pulse are generally “balanced” in that they each inject similar charges (e.g., within about 5 percent of one another), but with opposite polarities, into the electrode-tissue interface. The end result is that the tissue is generally charge balanced. 
     As noted, the anodic current pulse  252  may be followed by a period  254  of shorting (shorting period) in which all the implantable electrodes are shorted together. In general, biphasic stimulation pulses (i.e., two sequential current pulses of opposite polarity, such as pulses  250  and  252 ) produce stimulation with close to zero net charge at the electrode-tissue interface (e.g., generally remove charge imbalance to a point to ensure a net average DC current flow below a predetermined threshold). As a result, the shorting period  254  is relatively short and may function as a safety mechanism. In conventional biphasic stimulation, the shorting period  254  has a predetermined length (no feedback loop). 
     Since the anodic current pulse  252  is configured to balance the charge injected by the cathodic current pulse  250 , the anodic current pulse  252  is generated with a current source and generally requires a similar amount of energy from the implant power supply as the cathodic current pulse. That is, nearly half of the power consumed through biphasic stimulation is consumed through generation of the second polarity current pulse (e.g., the anodic current pulse  252 ) which merely removes charge from the tissue. Since, in biphasic stimulation the shorting period  254  is relatively short, biphasic stimulation is well suited for use with higher stimulation rate stimulation strategies/paradigms. 
     Almost half of the stimulation power used on biphasic stimulation can be saved by the use of monophasic stimulation in which the anodic pulse is replaced by a long electrode shorting period. That is, shown in  FIG.  3    is a monophasic stimulation signals  349  where the implantable medical device (e.g., cochlear implant system  102 ) only generates a cathodic current pulse  350  (or only an anodic pulse in alternative embodiments) and replaces the anodic current pulse with a longer period  354  of shorting (shorting period) that immediately follows the cathodic current pulse  350 . In conventional monophasic stimulation, the shorting period  354  has a predetermined length (no feedback loop). 
     The advantage of monophasic stimulation is that the shorting period  354  requires no stimulation energy from the implantable medical device and, as such, stimulation power is almost halved relative to biphasic stimulation. As is the case with biphasic stimulation, the length of the shorting period  354  needs to be sufficiently long (e.g., have a time length) so as to extract the injected charge and keep the average net DC current to below a predetermined threshold, such as approximately less than 100 nA. Stated differently, the shorting period  354  has a time length that is sufficient to ensure removal of remaining charge imbalance, at least to a point to ensure a net average DC current flow below a predetermined threshold. A disadvantage of monophasic stimulation is that the time length of the shorting period  354  can be very long in order for the charge to decay to an acceptable level which, in turn limits the use of monophasic stimulation with higher stimulation rate stimulation strategies. 
     As described above, biphasic stimulation and monophasic stimulation each have associated advantages and disadvantages. In particular, whereas the brief shorting period makes biphasic stimulation well suited can be used with higher stimulation rate stimulation strategies/paradigms, biphasic stimulation also consumes a significant amount of power simply for the purposes of charging balancing. In addition, monophasic stimulation does not consume power for the purposes of charging balancing, the long shorting period monophasic stimulation limits the use of the monophasic stimulation with higher stimulation rate stimulation strategies. 
     Presented herein are hybrid multi-phasic stimulation techniques that provide the reduced power benefits of monophasic stimulation with a shorter stimulation signal time duration that enables the use of higher stimulation rate stimulation strategies (e.g., a time duration that is shorter than monophasic stimulation). More specifically, referring specifically to the arrangement of  FIGS.  1 A- 1 D , the cochlear implant system  102  is configured to generate one or more “hybrid multi-phasic stimulation signals” based on one or more received sound signals. As used herein, each “hybrid multi-phasic stimulation signal” comprises at least a first current pulse having a first current polarity followed by at least a second current pulse having a second (opposing) current polarity and a shorting period. In addition, the first and second current pulses in the hybrid multi-phasic stimulation signal are “unbalanced” or “non-charge balancing,” meaning that the charge injected by the second current pulse is intentionally less than the charge injected by the first current pulse. As such, the energy/power required to generate the second current pulse is lower than the energy required to generate the first current pulse. As described further below, in certain embodiments a hybrid multi-phasic stimulation signal may also include one or more additional shorting periods and/or one or more additional current pulses. 
     As described further below, the hybrid multi-phasic stimulation techniques presented herein may be particularly advantageous with multipolar stimulation strategies where multiple stimulation signals are simultaneously delivered via multiple different electrodes (e.g., multiple electrodes stimulating at the same time). The power requirements associated with known and future multipolar stimulation strategies appear to be many times greater than monopolar stimulation. 
       FIGS.  4 A and  4 B  are schematic diagrams illustrating an example hybrid multi-phasic stimulation signal  460  in accordance with embodiments presented herein. For ease of description, the examples of  FIGS.  4 A and  4 B  will generally be described with reference to the cochlear implant system  102  of  FIGS.  1 A- 1 D . However, as explained elsewhere herein, the hybrid multi-phasic stimulation techniques may be applied in a variety of contexts and may be implemented by a number of different implantable medical devices and/or different implantable medical device systems. 
       FIGS.  4 A and  4 B  each include a first curve/trace  462  illustrating the current associated with the hybrid multi-phasic stimulation signal  460  at the interface between the implanted electrodes  144  and the recipient&#39;s cochlea (e.g., the electrode-tissue interface). Stated differently, curve  462  illustrates the instantaneous current injected or withdrawn from the electrode-tissue interface, over time.  FIGS.  4 A and  4 B  each also include a second curve/trace  464  illustrating the voltage associated with the with the hybrid multi-phasic stimulation signal  460  at the electrode-tissue interface. That is, curve  464  illustrates the instantaneous voltage at the electrode-tissue interface, over time. 
     In  FIG.  4 A , the voltage curve  464  is shown in a simplified form that ignores current times resistance (IR) offset.  FIG.  4 B  illustrates the voltage curve  464  so as to include the IR offset. For ease of illustration and understanding, the embodiments of  FIGS.  4 A- 4 B  will generally be described with reference to the simplified illustration of  FIG.  4 A . In addition, the other embodiments presented herein will also be illustrated in the simplified form that ignores IR offset on the voltages. 
     Returning to the specific example of  FIGS.  4 A and  4 B , the hybrid multi-phasic stimulation signal  460  begins with a first current pulse  450  having a first polarity. In this example, the first current pulse  450  is a cathodic (−) current pulse (e.g., stimulator unit  142  is configured to discharge anions (−) into the electrode-tissue interface). The first current pulse  450  is the portion of the hybrid multi-phasic stimulation signal  460  that evokes a percept from the recipient (i.e., the portion of the signal that depolarizes axons and triggers an action potential). As such, in this example, the first current pulse  450  is sometimes referred to herein as a “stimulation pulse” and the hybrid multi-phasic stimulation signal  460  is sometimes referred to herein as a “cathodic hybrid multi-phasic stimulation signal.” 
     As shown in  FIGS.  4 A and  4 B , the first current pulse  450  is followed by an Inter Phase Gap (IPG)  451 , which is a time period in which no current is delivered to, or withdrawn from, the electrode-tissue interface. The IPG  451  may have a predetermined, dynamically determined or variable duration as the longer the duration of the IPG, the lower the critical/firing threshold (at least for biphasic pulses) of the stimulated nerve cells. In general, the critical threshold of a nerve cell is the current level that causes the nerve cell to become depolarized beyond the cell&#39;s critical threshold, thereby causing the nerve cell to undergo an action potential. This action potential is sometimes referred to as the “firing” of the nerve cell. 
     In accordance with the hybrid multi-phasic stimulation techniques presented herein, the IPG  451  is followed by a first period  470  of shorting (shorting period) during which all the implantable electrodes  144  are “shorted” together. As used herein, reference to “shorting” of implantable electrodes, such as electrodes  144 , means that all of the electrodes are connected together to a same low impedance (e.g., connected together internally within the implantable medical device). Since the electrodes are all connected to the same low impedance, the shorting, if enabled for a sufficient period of time, dissipates any charge at the electrode-tissue interface. That is, the shorting of the electrodes withdraws residual charge from the electrode-tissue interface (e.g., brings all of the electrodes to the same potential, if a shorting period is sufficiently long). 
     As noted above, monophasic stimulation relies upon a long shorting period to withdraw residual charge from the electrode-tissue interface. However, also as noted above, although a long shorting period is energy efficient (e.g., does not require any power to remove the charge), a long shorting period is also not useable with higher rate stimulation strategies. As such, the shorting period  470  has a short time length that is sufficient to withdraw only part of the residual charge from the tissue-electrode interface. That is, as shown in  FIGS.  4 A and  4 B , at the end  471  of the shorting period  470 , a residual charge remains at the electrode-tissue interface. 
     To remove or substantially reduce the residual charge remaining at the electrode-tissue interface, the hybrid multi-phasic stimulation signal  460  comprises a second current pulse  452  (current pulse) having a second polarity (e.g., an opposing polarity to the first polarity of the first current pulse  450 ). As noted, in the example of  FIGS.  4 A and  4 B , the first current pulse  450  is a cathodic pulse. As such, in this example, the second current pulse  452  is an anodic pulse (e.g., the stimulator unit  142  is configured to discharge cations (+) into the electrode-tissue interface). Since, in this example, the second current pulse  452  is the portion of the hybrid multi-phasic stimulation signal  460  that balances residual charge, the anodic current pulse  452  is sometimes referred to herein as a “discharging pulse.” 
     As noted above, the shorting period  470  is provided between the first current pulse  450  and second current pulse  452  and removes some of the charge introduced into the electrode-tissue interface by the first current pulse  450 . That is, at the beginning of the second current pulse  452 , some charge has already been removed from the electrode-tissue interface. As a result, since the second current pulse  452  does not have to discharge all of the charge introduced by the cathodic pulse  450 , the second current pulse  452  has a time length (and/or amplitude) that is less than the first current pulse  450 . That is, the first current pulse  450  and the second current pulse  452  are “unbalanced,” meaning that the charge discharged by the second current pulse  452  is not the same as the charge introduced by the first current pulse  450 . Instead, the charge removed by the combination of the shorting period  470  and the second current pulse  452  is generally equivalent to the charge introduced by the cathodic pulse  450 . Stated differently, the shorting period  470  and the anodic pulse  452  are collectively configured to “balance” the charge at the electrode-tissue interface. 
     In general, the techniques presented herein allow for control of the percentage of charge removed by the shorting period by varying the length of the shorting period. There is a tradeoff between overall shorting length (and stimulation duration/rate) and power saved where a brief shorting duration reduces overall stimulation duration, but also reduces power savings. In accordance with certain embodiments presented herein, the shorting period  470  removes approximately at least 10 percent (%) of the charge introduced by the first current pulse  450 . In accordance with other embodiments presented herein, the shorting period  470  removes approximately at least 25 percent (%) of the charge introduced by the first current pulse  450 . In other embodiments, the shorting period  470  removes approximately at least 50 percent (%) of the charge introduced by the first current pulse  450 . In further embodiments, the shorting period  470  removes approximately at least 70 percent (%) of the charge introduced by the first current pulse  450 . In still further embodiments, the shorting period  470  removes approximately at least 80 percent (%) of the charge introduced by the first current pulse  450 . In still further embodiments, the shorting period  470  removes approximately at least 90 percent (%) of the charge introduced by the first current pulse  450 . Since the charge discharged by the second current pulse  452  is not the same as the charge introduced by the first current pulse  450 , the second current pulse  452  is sometimes referred to as an “abbreviated” or “condensed” discharging pulse. Similarly, since the charge withdrawn by the shorting period  470  is not the same as the charge introduced by the cathodic pulse  450 , the shorting period  470  is sometimes referred to as an “abbreviated” or “condensed” shorting period. 
     In the example of  FIGS.  4 A and  4 B , the hybrid multi-phasic stimulation signal  460  also includes a second period of shorting  472  (second shorting period) after the second current pulse  452 . The second shorting period  472  may operate as a safety mechanism to, in certain examples, ensure that the residual charge at the electrode-tissue interface is reduced at least to a level such that there is a net average DC current flow below a predetermined threshold. 
     The presence of the second shorting period  472  increases the time length of the hybrid multi-phasic stimulation signal  460 . As such, it may be advantageous to adaptively eliminate the second shorting period  472 . To this end, in certain examples, the cochlear implant system  102  is configured to measure the voltage  464  at the electrode-tissue interface after the second current pulse  452 . By performing a plurality of such measurements over time (e.g., during a plurality of different hybrid multi-phasic stimulation signals), the cochlear implant system  102  can adapt future stimulation signals (e.g., adapt the attributes of future discharging pulses) in a manner that ensures future shorting period  470  and the second current pulses  452  can ensure the residual charge at the electrode-tissue interface is reduced at least to a level such that there is a net average DC current flow below a predetermined threshold, without the need for the second shorting period. 
     Stated differently, any error in the size of the discharging pulse must be accommodated by a subsequent shorting period that adds time to the stimulation and potentially limits the stimulation rate. In certain examples, the stimulation subsystem  143  includes a feedback mechanism (feedback system) that can capture voltage measurements during certain hybrid multi-phasic signals (e.g., following delivery of stimulation current pulses) and use the voltage measurements to adaptively adjust the size of subsequent discharging pulses (amplitude and or duration) to ensure the average charge delivery is zero (e.g., feedback system that can alter the charge distribution in a following stimulation to, on average, account for imbalances) This will reduce the need for, and potentially eliminate, the final shorting period. 
       FIGS.  4 A and  4 B  include a curve  453  illustrating removal of the charge in accordance with standard monophasic stimulation (e.g., without the abbreviated discharging pulse  452  and in accordance with monophasic stimulation signal  349  of  FIG.  3   ). As can be seen from curve  453 , the hybrid multi-phasic stimulation signal  460  has a total time length  473  that is shorter than a time length  475  of a standard monophasic stimulation signal (e.g., monophasic stimulation signal  349 ). 
       FIGS.  4 A and  4 B  have been primarily described with reference to a time length difference between the first current pulse  450  and the second current pulse  452  that makes the two current pulses unbalanced. However, it is to be appreciated that the charge imbalance between the first current pulse  450  and the second current pulse  452  may be a result of other signal attributes, such as different amplitudes, a combination of different amplitudes and different time lengths, etc. In addition, the tissue slowly dissipates charge by itself, which leads to a very small imbalance, the effect of which is very small in short time frames. 
     In the example of  FIGS.  4 A and  4 B , cochlear implant system  102  generates the first current pulse  450  to trigger action potentials in the recipient&#39;s auditory neurons (e.g., based on received sound signals). After the IPG  451 , the cochlear implant system  102  initiates the initial shorting period  470  to remove an amount of the charge at the electrode-tissue interface. Once the voltage  464  drops to a predetermined level (e.g., approximately 10% of its peak value, approximately 25% of its peak value, approximately 50% its peak value, approximately 75% of its peak value, approximately 90%, of its peak value, etc.), the cochlear implant system  102  generates the abbreviated (low energy) second pulse  452  to accelerate the removal of the remaining stored charge. 
     Although the abbreviated anodic pulse  452  does require consume some power from the cochlear implant system  102 , the power consumed is significantly less than a full anodic pulse used in a typical biphasic stimulation strategy. When the electrode-tissue interface is approximated as a capacitor, then the energy stored at the electrode-interface (E) is given as shown below in Equation 1. 
       E=½CV 2 ,   Equation 1:
 
     where C is the charge at the electrode-tissue interface and V is the voltage at the electrode-tissue interface. 
     As noted, this energy is injected during the first current pulse  450  and has to be extracted by the combination of the shorting period  470  and the second current pulse  452 . For example, if the initial shorting period  470  were to reduce the voltage across the electrode-tissue interface (acting as a capacitor) to a half of the peak value, then the energy required to fully discharge the electrode-tissue interface is only one quarter of the energy required by the anodic pulse in typical biphasic stimulation. 
     As noted, at the end of the hybrid multi-phasic stimulation signal  460 , the remaining/residual charge must be at least at level such that there is a net average DC current flow below a predetermined threshold (e.g., approximately less than 100 nA). As such, the charge to be injected with the second current pulse  452  must be coordinated with the charge withdrawn via the shorting period  470  as any error (positive or negative) in the anodic charge injection will need to be dissipated in the second shorting period  472 . Therefore, a larger error in the second current pulse  452  requires a longer second shorting period  472 , which in turn makes the hybrid multi-phasic stimulation signal  460  less suited for higher stimulation rate strategies. Similarly, the second shorting period  472  can be totally avoided if the anodic charge injection of the discharging pulse  452  is accurate enough to ensure an average DC current below the predetermined threshold (e.g., less than 100 nA). 
     For a rectangular current pulse, the injected charge is equal to the current amplitude times the duration. Stated more generally, the injected charge is equal to the time integral of the current. The current amplitude and the pulse duration can be adjusted (one up, the other down), while still ensuring that the correct total charge is delivered. Given the above, several different techniques can be applied to determine the charge to be injected via the abbreviated discharging pulse. 
     In certain examples, the charge to be injected via the abbreviated discharging pulse can be a pre-calculated/predetermined amount of charge determined based on measured impedance. For example, during fitting of the cochlear implant system  102 , the impedances of the electrodes (electrode impedances) are calculated. The charge that the abbreviated discharging pulse needs to provide to bring the total stimulation charge to zero can be calculated (estimated) based on the electrode impedance and pre-determined values that are the result of, for example, testing during design of the system. A lookup table or equation with inputs of impedance, stimulation amplitude, stimulation pulse duration, and shorting duration can be used to determine the required charge for the abbreviated discharging pulse, where the needed charge is given as shown below in Equation 2. 
       Charge=f(impedance, stimulation amplitude, stimulation pulse duration, shorting duration).   Equation 2:
 
     In certain examples, the charge to be injected via the abbreviated discharging pulse can be dynamically determined based on the instantaneous voltage of the stimulating electrode, after the first stimulation pulse of the stimulation signal. That is, in the example of  FIGS.  4 A and  4 B , the cochlear implant system  102  is configured to dynamically measure the electrode voltage  464  after the cathodic pulse  450 . In addition, after the shorting period  470  (i.e., at  471 ), the cochlear implant system  102  is configured to measure the electrode voltage  464  and these voltages are used to calculate the required charge to be removed via the abbreviated discharging pulse  452  (e.g., calculate the charge needed to bring the total stimulation charge to zero), where the needed charge is given as shown below in Equation 3. 
       Charge=f(electrode voltage).   Equation 3:
 
     As noted above an implantable medical device in accordance with embodiments presented herein is configured to sequentially deliver a plurality of hybrid multi-phasic stimulation signals to a recipient. In certain embodiments, the implantable medical device is configured to measure the voltages during each hybrid multi-phasic stimulation signal (e.g., after each cathodic pulse and shorting period). However, in other embodiments, to simplify the design or save power, the voltage could be measured in every n th  hybrid multi-phasic stimulation signal (e.g. only measure the voltage in a subset of the plurality of hybrid multi-phasic stimulation signals). For hybrid multi-phasic stimulation signals during which the voltage is not measured, the charge to be removed via the abbreviated discharging pulse could be estimated based on the stimulation pulse charge (known) and knowledge of the required discharging pulse charge for similar previous stimulation pulses. 
     Each of the above methods have associated errors. However, these errors could be reduced over time through the use of dynamic feedback techniques. For example, the errors could be reduced over time by measuring the electrode voltage after the abbreviated discharging pulse(s) and using these voltages to adjust the charge delivered by the brief discharge pulse by a small delta. Therefore, over time, the measured electrode voltage tends towards zero and the residual charge tends towards zero. 
     In the example of  FIGS.  4 A and  4 B , the shorting period  470  occurs between the first current pulse  450  and the second current pulse  452 . As such, in this example, the shorting period  470  is sometimes referred to as an “intra-pulse” shorting period. In other embodiments, the second current pulse  452  could precede the shorting period  470 . Such an example is shown in  FIG.  4 C . 
     More specifically,  FIG.  4 C  is a schematic diagram illustrating an example hybrid multi-phasic stimulation signal  460 (C) in accordance with embodiments presented herein. Hybrid multi-phasic stimulation signal  460 (C) is similar to hybrid multi-phasic stimulation signal  460  except that the abbreviated discharging pulse  452  precedes the shorting period  470 . In this example, the shorting periods  472  is omitted (e.g., combined with the shorting period  470 ). 
       FIGS.  4 A,  4 B, and  4 C  have generally been described above with reference to rectangular shaped current pulses. Is to be appreciated that the use of rectangular shaped current pulses in the embodiments of  FIGS.  4 A,  4 B, and  4 C , as well as in other embodiments, is merely illustrate and that the techniques presented herein may be implemented with current pulses having other shapes (e.g., triangular current pulses, ramped current pulses, etc.). 
     As noted,  FIGS.  4 A,  4 B, and  4 C  generally illustrate “cathodic” hybrid multi-phasic stimulation signals  460  and  460 (C) where the stimulation pulse  450  is a cathodic pulse and the abbreviated discharging pulse  452  is an anodic pulse. It is to be appreciated that the techniques presented herein may also be implemented with so-called “anodic” stimulation where the stimulation pulse is an anodic pulse and the abbreviated discharging pulse is a cathodic pulse. 
     For example,  FIG.  5    is a schematic diagram illustrating an example hybrid multi-phasic stimulation signal  560  in accordance with such embodiments presented herein. For ease of description, the example of  FIG.  5    will generally be described with reference to the cochlear implant system  102  of  FIGS.  1 A- 1 D . However, as explained elsewhere herein, the hybrid multi-phasic stimulation techniques may be applied in a variety of contexts and may be implemented by a number of different types of implantable medical devices. 
       FIG.  5    includes a first curve/trace  562  illustrating the current associated with the hybrid multi-phasic stimulation signal  560  at the interface between the implanted electrodes  144  and the recipient&#39;s cochlea (e.g., the electrode-tissue interface). Stated differently, curve  562  illustrates the instantaneous current injected or withdrawn from the electrode-tissue interface, over time.  FIG.  5    also includes a second curve/trace  564  illustrating the voltage associated with the with the hybrid multi-phasic stimulation signal  560  at the electrode-tissue interface. That is, curve  564  illustrates the instantaneous voltage at the electrode-tissue interface, over time. In  FIG.  5   , the voltage curve  564  is shown in a simplified form that ignores IR offset. 
     The hybrid multi-phasic stimulation signal  560  begins with a first current pulse  550  (current pulse) having a first polarity. In this example, the first current pulse  550  is an anodic (+) current pulse and is the portion of the hybrid multi-phasic stimulation signal  560  that evokes a percept from the recipient (i.e., the portion of the signal that depolarizes axons and triggers an action potential). As such, in this example, the anodic current pulse  550  is sometimes referred to herein as a “stimulation pulse” and the hybrid multi-phasic stimulation signal  560  is sometimes referred to herein as an “anodic hybrid multi-phasic stimulation signal.” 
     As shown in  FIG.  5   , the first current pulse  550  is followed by an Inter Phase Gap (IPG)  551 , which is a time period in which no current is delivered to, or withdrawn from, the electrode-tissue interface. In accordance with the hybrid multi-phasic stimulation techniques presented herein, the IPG  551  is followed by a first period  570  of shorting (shorting period) during which all the implantable electrodes  144  are “shorted” together. The shorting period  570  has a time length that is sufficient to withdraw only part of the residual charge from the tissue-electrode interface. That is, as shown in  FIG.  5   , at the end  571  of the shorting period  570 , a residual charge remains at the electrode-tissue interface. 
     To remove or substantially reduce the residual charge remaining at the electrode-tissue interface, the hybrid multi-phasic stimulation signal  560  comprises a second current pulse  552  (current pulse) having a second polarity. As noted, in the example of  FIG.  5   , the first current pulse  550  is an anodic pulse. As such, in this example, the second current pulse  552  is a cathodic current pulse (e.g., the stimulator unit  142  is configured to discharge anions (−) into the electrode-tissue interface). Since the second current pulse  552  is the portion of the hybrid multi-phasic stimulation signal  560  that balances residual charge, the second current pulse  552  is sometimes referred to herein as a “discharging pulse.” 
     As noted above, the shorting period  570  is provided between the first current pulse  550  and the second current pulse  552  and removes some of the charge introduced into the electrode-tissue interface by the first current pulse  550 . That is, at the beginning of the second current pulse  552 , some charge has already been removed from the electrode-tissue interface. As a result, since the second current pulse  552  does not have to discharge all of the charge introduced by the anodic pulse  550 , the second pulse  552  has a time length (and/or amplitude) that is less than the first current pulse  550 . That is, the first current pulse  550  and the second current pulse  552  are “unbalanced,” meaning that the charge discharged by the second current pulse  552  is not the same as the charge introduced by the first current pulse  550 . Instead, the charge discharged by the combination of the shorting period  570  and the second current pulse  552  is generally equivalent to the charge introduced by the first current pulse  550 . Stated differently, the shorting period  570  and the second current pulse  552  are collectively configured to “balance” the charge at the electrode-tissue interface (e.g., ensure that the residual charge at the electrode-tissue interface is reduced at least to a level such that there is a net average DC current flow below a predetermined threshold). 
     Since the charge discharged by the second current pulse  552  is not the same as the charge introduced by the first current pulse  550 , the second current pulse  552  is sometimes referred to as an “abbreviated” or “condensed” discharging pulse. Similarly, since the charge withdrawn by the shorting period  570  is not the same as the charge introduced by the first current pulse  550 , the shorting period  570  is sometimes referred to as an “abbreviated” or “condensed” shorting period. 
     In the example of  FIG.  5   , the hybrid multi-phasic stimulation signal  560  also includes a second period of shorting  572  (second shorting period) after the anodic pulse  552 . The second shorting period  572  may operate as a safety mechanism to, in certain examples, ensure that the residual charge at the electrode-tissue interface is reduced at least to a level such that there is a net average DC current flow below a predetermined threshold. 
     The presence of the second shorting period  572  increases the time length of the hybrid multi-phasic stimulation signal  560 . As such, it may be advantageous to adaptively eliminate the second shorting period  572 . To this end, in certain examples, the cochlear implant system  102  is configured to measure the voltage  564  at the electrode-tissue interface after the second current pulse  552 . By performing a plurality of such measurements over time (e.g., during a plurality of different hybrid multi-phasic stimulation signals), the cochlear implant system  102  can adapt future stimulation signals (e.g., adapt the attributes of future discharging pulses) in a manner that ensures future shorting period  570  and the second current pulses  552  can ensure the residual charge at the electrode-tissue interface is reduced at least to a level such that there is a net average DC current flow below a predetermined threshold, without the need for the second shorting period. 
     In the example of  FIG.  5   , the shorting period  570  occurs between the anodic pulse  550  and the cathodic pulse  552 . As such, in this example, the shorting period  570  is sometimes referred to as an “intra-pulse” shorting period. In other embodiments, the cathodic pulse  552  could precede the shorting period  570  (e.g., similar to signal  460 (C) of  FIG.  4 C ). 
       FIG.  5    include a curve  553  illustrating removal of the charge in accordance with standard monophasic stimulation (e.g., without the abbreviated discharging pulse  552 , such as monophasic stimulation signal  349  of  FIG.  3   ). As can be seen from curve  553 , the hybrid multi-phasic stimulation signal  560  has a total time length  573  that is shorter than a time length  575  of a standard monophasic stimulation signal (e.g., monophasic stimulation signal  349 ). 
       FIG.  5    has been primarily described with reference to a time length difference between the anodic pulse  550  and the cathodic pulse  552  that makes the two current pulses unbalanced. However, it is to be appreciated that the charge imbalance between the cathodic pulse  550  and the anodic pulse  552  may be a result of other signal attributes, such as different amplitudes, a combination of different amplitudes and different time lengths, etc. 
     In the example of  FIG.  5   , cochlear implant system  102  generates the anodic current pulse  550  to trigger action potentials in the recipient&#39;s auditory neurons (e.g., based on received sound signals). After the IPG  551 , the cochlear implant system  102  initiates the initial shorting period  570  to remove an amount of the charge at the electrode-tissue interface. Once the voltage  564  drops to a predetermined level (e.g., approximately half of its peak value), the cochlear implant system  102  generates the abbreviated (low energy) discharging pulse  552  to accelerate the removal of the remaining stored charge. 
     Although the abbreviated discharging  552  does require some consume some power from the cochlear implant system  102 , the power consumed is significantly less than a full cathodic pulse used in a typical biphasic stimulation strategy. As noted, the energy injected during the anodic pulse  550  has to be extracted by the combination of the shorting period  570  and the cathodic pulse  552 . For example, if the initial shorting period  570  were to reduce the voltage across the electrode-tissue interface (acting as a capacitor) to a half of the peak value, then the energy required to fully discharge the electrode-tissue interface is only one quarter of the energy required by the anodic pulse in typical biphasic stimulation. 
     As noted, at the end of the hybrid multi-phasic stimulation signal  560 , the remaining/residual charge must be at least at level such that there is a net average DC current flow below a predetermined threshold (e.g., approximately less than 100 nA). As such, the charge to be injected with the cathodic pulse  552  must be coordinated with the charge withdrawn via the shorting period  570  as any error (positive or negative) in the cathodic charge injection will need to be dissipated in the second shorting period  572 . Therefore, a larger error in the cathodic pulse  552  requires a longer second shorting period  572 , which in turn makes the hybrid multi-phasic stimulation signal  560  less suited for higher stimulation rate strategies. Similarly, as noted above, the second shorting period  572  can be totally avoided if the cathodic charge injection of the discharging pulse  552  is accurate enough to ensure an average DC current below the predetermined threshold (e.g., less than 100 nA). The charge that needs to be withdrawn via the cathodic pulse  552 , and hence the parameters (e.g., amplitude, time length, etc.) can be determined as described above with reference to the embodiments of  FIGS.  4 A and  4 B . 
       FIG.  6    illustrates another example hybrid multi-phasic stimulation signal  660  in accordance with certain embodiments presented herein. The hybrid multi-phasic stimulation signal  660  is generally a hybrid biphasic stimulation signal that comprises two discharging pulses, rather than a single discharging pulse as in the examples of  FIGS.  4 A,  4 B, and  5   . 
     More specifically,  FIG.  6    includes a first curve/trace  662  illustrating the current associated with the hybrid multi-phasic stimulation signal  660  at the interface between the implanted electrodes  144  and the recipient&#39;s cochlea (e.g., the electrode-tissue interface). Stated differently, curve  662  illustrates the instantaneous current injected or withdrawn from the electrode-tissue interface, over time.  FIG.  6    also include a second curve/trace  664  illustrating the voltage associated with the hybrid multi-phasic stimulation signal  660  at the electrode-tissue interface. That is, curve  664  illustrates the instantaneous voltage at the electrode-tissue interface, over time. In  FIG.  6   , the voltage curve  664  is shown in a simplified form that ignores IR offset. 
     The hybrid multi-phasic stimulation signal  660  begins with a first current pulse  650  (current pulse) having a first polarity. In this example, the first current pulse  650  is a cathodic (−) current pulse. The first current pulse  650  is the portion of the hybrid multi-phasic stimulation signal  660  that evokes a percept from the recipient. As such, the first current pulse  650  is sometimes referred to herein as a “stimulation pulse” and the hybrid multi-phasic stimulation signal  660  is sometimes referred to herein as a “cathodic hybrid multi-phasic stimulation signal.” 
     As shown in  FIG.  6   , the first current pulse  650  is followed by an Inter Phase Gap (IPG)  651 , which is a time period in which no current is delivered to, or withdrawn from, the electrode-tissue interface. In accordance with the hybrid multi-phasic stimulation techniques presented herein, the IPG  651  is followed by a second current pulse  652 ( 1 ), which in turn is followed by a first period  670  of shorting (shorting period) during which all the implantable electrodes  144  are shorted together. The shorting period  670  is followed by a third current pulse  652 ( 2 ). The second current pulse  652 ( 1 ), shorting period  670 , and third current pulse  652 ( 2 ) are collectively configured to “balance” the first current pulse  650 . That is, the second current pulse  652 ( 1 ), shorting period  670 , and third current pulse  652 ( 2 ) function to remove or substantially reduce the residual charge remaining at the electrode-tissue interface. 
     As noted, in the example of  FIG.  6   , the first current pulse  650  is a cathodic pulse. As such, in this example, the second current pulse  652 ( 1 ) and the third current pulse  652 ( 2 ) are each anodic current pulses. The second current pulse  652 ( 1 ) and the third current pulse  652 ( 2 ) collectively comprise the portion of the hybrid multi-phasic stimulation signal  660  that discharges residual charge and, as such, are sometimes referred to herein as “discharging pulses.” 
     In general, the example of  FIG.  6    is similar to that of  FIGS.  4 A and  4 B , except that the single discharging pulse used in the examples of  FIGS.  4 A and  4 B  is split into two discharging pulses that are separated by the shorting period  670 , which has a time length that is sufficient to withdraw only part of the residual charge from the tissue-electrode interface. The second current pulse  652 ( 1 ) is a brief pulse configured to reduce the peak current at the beginning of the shorting period  670 . 
     As noted above, the shorting period  670  is provided between the second current pulse  652 ( 1 ) and the third current pulse  652 ( 2 ) and removes some of the charge introduced into the electrode-tissue interface by the cathodic pulse  650 . As a result, since the second current pulse  652 ( 1 ) and the third current pulse  652 ( 2 ) do not have to discharge all of the charge introduced by the cathodic pulse  650 , the second current pulse  652 ( 1 ) and the third current pulse  652 ( 2 ) collectively have a time length (and/or amplitude) that is less than the cathodic pulse  650 . That is, the cathodic pulse  650  and the combination of the two anodic pulses  652 ( 1 ) and  652 ( 2 ) are “unbalanced,” meaning that the charge discharged by the combination of the two anodic pulses  652 ( 1 ) and  652 ( 2 ) is not the same as the charge introduced by the cathodic pulse  650 . Instead, the charge discharged by the combination of the shorting period  670  and the two anodic pulses  652 ( 1 ) and  652 ( 2 ) is generally equivalent to the charge introduced by the cathodic pulse  650 . Stated differently, the shorting period  670  and the two anodic pulses  652 ( 1 ) and  652 ( 2 ) are collectively configured to “balance” the charge at the electrode-tissue interface (e.g., ensure that the residual charge at the electrode-tissue interface is reduced at least to a level such that there is a net average DC current flow below a predetermined threshold). 
     Since the charge discharged by the two anodic pulses  652 ( 1 ) and  652 ( 2 ) is not the same as the charge introduced by the cathodic pulse  650 , the two anodic pulses  652 ( 1 ) and  652 ( 2 ) are sometimes referred to as “abbreviated” or “condensed” discharging pulses. Similarly, since the charge withdrawn by the shorting period  670  is not the same as the charge introduced by the cathodic pulse  650 , the shorting period  670  is sometimes referred to as an “abbreviated” or “condensed” shorting period. 
     In the example of  FIG.  6   , the hybrid multi-phasic stimulation signal  660  also includes a second period of shorting  672  (second shorting period) after the anodic pulse  652 . The second shorting period  672 . However, as noted elsewhere herein, the shorting period  672  may, in certain embodiments, be omitted or adaptively eliminated. 
     In accordance with the techniques presented herein, the amplitude of the current during shorting is not controlled by the implantable medical device and, instead, the current is the residual voltage divided by the tissue impedance. As such, the current may be higher than desirable at the start of the shorting period.  FIG.  6    illustrates an example in which the peak current can be reduced through the use of a small/brief discharging pulse (where the current is controlled) prior to the shorting period (e.g., signal  660  has reduced power requirements, shorter time, and a limit to the initial peak shorting current). 
       FIGS.  4 A,  4 B,  5 , and  6    have been described with reference to hybrid multi-phasic stimulation signals that are generally biphasic in nature. It is to be appreciated that hybrid multi-phasic stimulation signals may include different numbers of current pulses, such as triphasic stimulation signals. Triphasic stimulation, for example, three (3) current pulses with alternating polarity (e.g., either a cathodic-anodic-cathodic pulse sequence or an anodic-cathodic-anodic pulse sequence). In a conventional triphasic stimulation signal, the current injected by the second current pulse generally “balances” the current injected by the first and third current pulses. That is, in general, the charge injected by the second current pulse is approximately equal to the current injected by the combination of the first and third current pulses, but with an opposite polarity. In certain conventional triphasic stimulation signals, the first and third pulses each inject substantially the same amount of charge into the electrode-tissue interface. Due to the charge balancing via equal and opposite polarities, conventional triphasic stimulation may consume substantial amounts of power. 
       FIG.  7    illustrates an example hybrid multi-phasic stimulation signal  760  that is generally triphasic in nature, but which consumes less power than a standard/conventional triphasic stimulation signal. More specifically,  FIG.  7    includes a first curve/trace  762  illustrating the current associated with the hybrid multi-phasic stimulation signal  760  at the interface between the implanted electrodes  144  and the recipient&#39;s cochlea (e.g., the electrode-tissue interface). Stated differently, curve  762  illustrates the instantaneous current injected or withdrawn from the electrode-tissue interface, over time.  FIG.  7    also includes a second curve/trace  764  illustrating the voltage associated with the with the hybrid multi-phasic stimulation signal  760  at the electrode-tissue interface. That is, curve  764  illustrates the instantaneous voltage at the electrode-tissue interface, over time. In  FIG.  7   , the voltage curve  764  is shown in a simplified form that ignores IR offset. 
     The hybrid multi-phasic stimulation signal  760  begins with a first current pulse  750  (current pulse) having a first polarity and ends with a third current pulse  776  also having the same first polarity (either both cathodic or both anodic). Between the first pulse  750  and the third current pulse  776  is a second current pulse  752  having the opposite polarity. That is, the first and third current pulses have the same polarity, and the second current pulse has the opposite polarity. 
     In the example of  FIG.  7   , the first current pulse  750  and the third current pulse  776  are both cathodic (−) current pulses and the second current pulse  752  is an anodic (+) current pulse. However, as noted elsewhere herein, in alternative embodiments the polarities may be inverted (e.g., the first and third current pulses may be anodic and the second phase pulse may be cathodic). 
     As shown in  FIG.  7   , the cathodic current pulse  750  is followed by an Inter Phase Gap (IPG)  751 , which is a time period in which no current is delivered to, or withdrawn from, the electrode-tissue interface. In accordance with the hybrid multi-phasic stimulation techniques presented herein, the IPG  751  is followed by a first period  770  of shorting (shorting period) during which all the implantable electrodes  144  are shorted together. 
     The shorting period  770  is followed by the second current pulse  752  which, as noted above, has an opposing polarity to the first current pulse  750 . The second current pulse  752  is followed by another period of shorting  772  (shorting period) during which during which all the implantable electrodes  144  are shorted together. The shorting period  772  is followed by the third current pulse  776  which, as noted above, has the same polarity to the first current pulse  750 . 
     Each of the shorting periods  770  and  772  are configured to remove some of the charge from the tissue. In particular, shorting period  770  removes some of the charge introduced into the electrode-tissue interface by the cathodic pulse  750 , while the shorting period  772  removes some of the charge introduced into the electrode-tissue interface by the anodic pulse  752 . Due to the use of the shorting periods  770  and  772 , each of the anodic pulse  752  and the cathodic pulse  776  can have a time length (and/or amplitude) that is less than the second and third stimulation pulses in conventional triphasic stimulation signals. That is, the pulses  750 ,  752 , and  776  are generally “unbalanced,” meaning that the charge discharged by the anodic pulse  752  is not the same as the charge introduced by the cathodic pulses  750  and  776 . Instead, the charge discharged by the combination of the shorting period  770  and the anodic pulse  752  is generally equivalent to the charge introduced by the cathodic pulse  750 , cathodic pulse  776 , and the current withdrawn via the shorting period  772 . Stated differently, the shorting period  770  and the anodic pulse  752  are collectively configured to “balance” the charge at the electrode-tissue interface (e.g., ensure that the residual charge at the electrode-tissue interface is reduced at least to a level such that there is a net average DC current flow below a predetermined threshold). 
       FIG.  7    have been primarily described with reference to a time length difference between the cathodic pulse  750 , cathodic pulse  776 , and the anodic pulse  752  that makes the current pulses unbalanced. However, it is to be appreciated that the charge imbalance between the cathodic pulse  750 , cathodic pulse  776 , and the anodic pulse  752  may be a result of other signal attributes, such as different amplitudes, different amplitudes and different time lengths, etc. 
     As noted elsewhere herein, reference to “shorting” of implantable electrodes means that all of the electrodes are connected to a same low impedance (e.g., connected together internally within the implantable medical device). Since the electrodes are all connected to the same low impedance, the shorting, if enabled for a sufficient period of time, dissipates any charge at the electrode-tissue interface, which results in lower or zero potential. That is, the shorting of the electrodes withdraws residual charge from the electrode-tissue interface (e.g., brings all of the electrodes to the same potential, if a shorting period is sufficiently long). 
       FIG.  8    illustrates an example embodiment arrangement for shorting a plurality of electrodes. In particular, shown in  FIG.  8    is a plurality of electrodes  844  forming an electrode or contact array  846 . Each of the electrodes  844  is selectively electrically connectable to a common point  880  via low impedance path comprising a corresponding switch  881 . The switches  881  are controlled by the implantable medical device to selectively connect all of the electrodes together, via the low impedance path, to the common point  880  (e.g., dissipates any charge at the electrode-tissue interface). That is, during shorting, all of the switches  881  are closed, thereby forming a low impedance path (short) between all electrodes. 
     As noted, in the arrangement of  FIG.  8   , the electrodes  844  are each connected to the same common point  880  via a corresponding switch  881 . However, in the example of  FIG.  8   , when the shorting begins, the implantable medical device is unable to control of the amount of current that flows from the electrodes  844  to the common point  880 . That is, as noted, the electrode-tissue interface is capacitive and, when a charge is present, there is a voltage present. The voltage divided by the resistance of the system determines the current that will flow when the shorting begins (e.g., amount of current is controlled by the amount of charge left at that electrode-tissue interface and the impedance of the system, including that of the tissue, wires, etc.). If the resistance of the system is too low, a large initial peak shorting current could be present, where such a large current could, for example, result in an undesirable second percept. Therefore, in accordance with certain embodiments presented herein, the start of the shorting period could include a controllable impedance to limit the initial shorting current, potentially without requiring any power to be supplied by the implantable medical device. 
     More specifically, shown in  FIG.  9    is a plurality of electrodes  944  forming an electrode or contact array  946 . Each of the electrodes  944  is selectively electrically connectable to a common point  980  via a low impedance path that includes a corresponding switch  981  and a corresponding variable resistor  982 . The switches  981 , and the variable resistors  982 , are controlled by the implantable medical device to selectively connect all of the electrodes to the common point  980  (e.g., dissipates any charge at the electrode-tissue interface). However, in contrast to the embodiment of  FIG.  8   , the addition of the variable resistors  982  enables the implantable medical device to control (e.g., limit) the initial shorting current to a predetermined or dynamically determined (e.g., lower) value. 
     In addition, during shorting, the voltage at the electrode-tissue interface will decrease. As such, in certain embodiments, the resistance values of the variable resistors  982  can also be dynamically/adaptively decreased to minimize the time needed to remove the charge from the electrodes (e.g., the resistance values of the variable resistors  982  change over time in relation to the voltage at the electrode-tissue interface). 
       FIG.  10    illustrates a hybrid monophasic pulse  1060 , in accordance with embodiments presented herein. In the example of  FIG.  10   , the hybrid monophasic pulse  1060  comprises a single current pulse  1050  (either cathodic or anodic pulse) followed by a period of shorting  1054  (shorting period) to balance the charge. In the example of  FIG.  10   , an IPG  1051  is provided between the current pulse  1050  and the shorting period  1054 . Adding the IPG  1051  makes the hybrid monophasic pulse  1060  more effective than conventional monophasic pulses where, for the same level of current and charge, increasing the IPG 1051  will cause the resulting percept to increase. Stated differently, the same percept can be produced by lowering the current and increasing the IPG  1051 , thereby saving power. In general, the tradeoff is stimulation duration and therefore maximum stimulation rate. 
     Embodiments presented herein have been primarily described with reference to an example auditory prosthesis system, namely a cochlear implant system. However, as noted above, it is to be appreciated that the techniques presented herein may be implemented by a variety of other types of implantable medical devices (or systems that include other types of implantable medical devices) that provide a wide range of therapeutic benefits to recipients, patients, or other users. For example, the techniques presented herein may be implemented by other auditory prostheses, such as acoustic hearing aids, middle ear auditory prostheses, bone conduction devices, direct acoustic stimulators, electro-acoustic prostheses, other electrically simulating auditory prostheses (e.g., auditory brain stimulators), etc. The techniques presented herein may also be implemented by tinnitus therapy devices, vestibular devices (e.g., vestibular implants), visual devices (i.e., bionic eyes), sensors, pacemakers, drug delivery systems, defibrillators, functional electrical stimulation devices, catheters, seizure devices (e.g., devices for monitoring and/or treating epileptic events), sleep apnea devices, electroporation devices, etc. 
       FIG.  11    illustrates an example vestibular stimulator system  1102  in accordance with embodiments presented herein. In this example, the vestibular stimulator system  1102  comprises an implantable component (vestibular stimulator)  1112  and an external device/component  1104  (e.g., external processing device, battery charger, remote control, etc.). 
     The vestibular stimulator  1112  comprises an implant body (main module)  1134 , a lead region  1136 , and a stimulating assembly  1116 , all configured to be implanted under the skin/tissue (tissue)  1115  of the recipient. The implant body  1134  generally comprises a hermetically-sealed housing  1138  in which RF interface circuitry, one or more rechargeable batteries, one or more processors, and a stimulator unit are disposed. The implant body  134  also includes an internal/implantable coil  1114  that is generally external to the housing  1138 , but which is connected to the transceiver via a hermetic feedthrough (not shown). 
     The stimulating assembly  1116  comprises a plurality of electrodes  1144  disposed in a carrier member (e.g., a flexible silicone body). In this specific example, the stimulating assembly  1116  comprises three ( 3 ) stimulation electrodes, referred to as stimulation electrodes  1144 ( 1 ),  1144 ( 2 ), and  1144 ( 3 ). The stimulation electrodes  1144 ( 1 ),  1144 ( 2 ), and  1144 ( 3 ) function as an electrical interface for delivery of electrical stimulation signals to the recipient&#39;s vestibular system. In accordance with embodiments presented herein, the vestibular nerve stimulator  1102  is configured to stimulate the recipient&#39;s vestibular system using hybrid multi-phasic stimulation signals or hybrid monophasic stimulation signals, such as the signals described above with reference to  FIGS.  4 A,  4 B,  5 ,  6 ,  7 , and  10   . That is, the vestibular nerve stimulator system  1102  is configured to generate and deliver hybrid multi-phasic stimulation signals or hybrid monophasic stimulation signals to receive via one or more of the stimulation electrodes  1144 ( 1 ),  1144 ( 2 ), and  1144 ( 3 ). 
     The stimulating assembly  1116  is configured such that a surgeon can implant the stimulating assembly adjacent the recipient&#39;s otolith organs via, for example, the recipient&#39;s oval window. It is to be appreciated that this specific embodiment with three stimulation electrodes is merely illustrative and that the techniques presented herein may be used with stimulating assemblies having different numbers of stimulation electrodes, stimulating assemblies having different lengths, etc. 
       FIG.  12    is a flowchart of a method  1290  in accordance with embodiments presented herein. Method  1290  begins at  1292  where an implantable medical device delivers via at least one of a plurality of implantable electrodes, a stimulation current pulse to inject charge into tissue of a recipient of the implantable medical device. At  1294 , the implantable medical device shorts the at least one of the plurality of implantable electrodes with one or more other of the plurality of implantable electrodes to remove a first portion of the charge injected into the tissue by the stimulation current pulse. At  1296 , the implantable medical device delivers at least one discharging current pulse to the tissue of the recipient to remove a second portion of the charge injected into the tissue by the stimulation current pulse. 
       FIG.  13    is a flowchart of a method  1390  in accordance with embodiments presented herein. Method  1390  begins at  1392  where an implantable medical device delivers at least a first current pulse to tissue of a recipient of an implantable medical device via at least one of a plurality of implantable electrodes, wherein the first pulse has a first current polarity and is configured to stimulate the tissue. At  1394 , the implantable medical device delivers at least a second current pulse having a second current polarity to the tissue of the recipient, wherein the second current polarity is an opposite current polarity to the first current polarity. At  1396 , prior to delivering the at least one discharging current pulse, the implantable medical device shorts the at least one of the plurality of implantable electrodes with one or more other of the plurality of implantable electrodes. 
       FIG.  14    is a flowchart of a method  1490  in accordance with embodiments presented herein. Method  1490  begins at  1492  where an implantable medical device delivers at least one stimulation current pulse to stimulate tissue of a recipient of an implantable medical device, wherein the at least one stimulation current pulse injects charge into the tissue of the recipient. At  1494 , the implantable medical device removes the charge injected into the tissue by the least one stimulation current pulse through a combination of at least one discharging current pulse and a period of electrode shorting that precedes the at least one discharging current pulse. 
     It is to be appreciated that the embodiments presented herein are not mutually exclusive and that the various embodiments may be combined with another in any of a number of different manners. 
     The invention described and claimed herein is not to be limited in scope by the specific preferred embodiments herein disclosed, since these embodiments are intended as illustrations, and not limitations, of several aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.