Patent Application
20100069997
The present invention relates to neurostimulation, and more particularly to an output stage for signal generation componentry of a neurostimulation implant device. The invention is particularly apt for auditory neurostimulation applications, and reduces the power and volume requirements in such applications.
The utilization of neurostimulation implant devices is ever-increasing. Such devices utilize a plurality of implanted electrodes that are selectively activated to affect a desired neuro-response, including sound sensation, pain/tremor management, and urinary/anal incontinence. By way of primary interest, auditory neurostimulation implant devices include auditory brainstem implant (ABI) and cochlear implant (CI) devices.
In the case of CI devices, an electrode array is inserted into the cochlea of a patient, e.g. typically into the scala tympani so as to access and follow the spiral curvature of the cochlea. The array electrodes are selectively driven to stimulate the patient's auditory nerve endings to generate sound sensation. In this regard, a CI electrode array works by utilizing the tonotopic organization, or frequency-to-location mapping, of the basilar membrane of the inner ear. In a normal ear, sound vibrations in the air are transduced to physical vibrations of the basilar membrane inside the cochlea. High frequency sounds do not travel very far along the membrane, while lower frequency sounds pass further along. The movement of hair cells, located along the basilar membrane, creates an electrical disturbance, or potential, that can be picked up by auditory nerve endings that generate electrical action pulses that travel along the auditory nerve to the brainstem. In turn, the brain is able to interpret the nerve activity to determine which area of the basilar membrane is resonating, and therefore what sound frequency is being sensed. By directing which electrodes of a CI electrode array are activated, cochlear implants can selectively stimulate different parts of the cochlea and thereby convey different acoustic frequencies corresponding with a given audio input signal.
With ABI systems a plurality of electrodes may be implanted at a location that bypasses the cochlea. More particularly, an array of electrodes may be implanted at the cochlea nucleus, or auditory cortex, at the base of the brain to directly stimulate the brainstem of a patient. Again, the electrode array may be driven in relation to the tonotopic organization of a recipient's auditory cortex to obtain the desired sound sensation.
As may be appreciated, in the case of either ABI electrodes or CI electrodes, audio signals (e.g. from a microphone) may be processed, typically utilizing what is referred to as a speech processor, to generating stimulation signals utilized to selectively drive the electrodes for stimulated sound sensation. Further, in both implant approaches, a source of power may be included to power the stimulation signal generator.
Neurostimulation generally provides a system that recovers any charges that are injected into a patient's body through the electrodes (i.e., “equilibrating charges”), so that accumulated charges do not remain in the tissue of a patient. To accomplish this, subsequent to each stimulation interval with a predetermined level of electrical current for a predetermined time period, the same level of electrical current for the same time period may be applied in the opposite direction. That is, a plurality of biphasic pulses (i.e., stimulation pulses and discharge pulses) may be delivered to a patient's tissue through the electrode array. Any difference between the ideal discharge and the actual discharge results in a disruptive leakage current.
In view of the foregoing, a primary objective of the present invention is to provide an output stage for an auditory neurostimulation electrode that receives power from a single power supply.
An additional objective of the present invention is to provide an output stage for an auditory neurostimulation electrode that has a relatively small volume and low power requirements.
A further objective of the present invention is to provide an output stage for an auditory neurostimulation electrode that is intrinsically capable of maintaining an equilibrium of charges without requiring complex control and monitoring means.
One or more of the above-noted objectives and additional advantages are realized by an output stage of the present invention. The output stage for an auditory neurostimulation electrode that is operable to effect a plurality of stimulation and discharge intervals may include a stimulation channel and a discharge channel, each coupled to the electrode, wherein a stimulation current may flow through the stimulation channel during a stimulation interval, and a discharge current may flow through the discharge channel during a discharge interval. The output stage may further include a controller that is operable to selectively control the flow of current through the stimulation channel and the discharge channel during the stimulation and discharge intervals, respectively. Additionally, one of the stimulation channel and the discharge channel may couple the electrode to a voltage supply, and the other of the stimulation channel and the discharge channel may couple the electrode to a reference potential node. In this regard, the present invention provides an output stage that is intrinsically capable of maintaining an equilibrium of charges, operates using a single power supply, and does not require complex control or monitoring means.
In one aspect, the controller may be operable to control the timing of the stimulation and discharge intervals such that the intervals are successive. Furthermore, the controller may be operable to selectively adjust the magnitude of the stimulation and discharge currents. As can be appreciated, these features may be advantageous as they provide the ability to selectively adapt a neurostimulation system to the needs of a particular patient.
In one aspect, the amount of charge transferred during a discharge interval is greater than the amount of charge transferred during a stimulation interval. In this regard, the output stage may be intrinsically capable of maintaining an equilibrium of charges and may operate to remove the charges from the tissue of a patient each stimulation/discharge cycle. In one embodiment, this is accomplished by providing components in the stimulation and discharge channels that are sized to possess certain desirable conductive properties. For example, the stimulation channel and the discharge channel may each include one or more transistors (e.g., a MOSFET, a bipolar junction transistor, or the like) whose relative physical dimensions (e.g., channel length, channel width, etc. . . . ) are chosen so that the charges transferred during the discharge interval are slightly greater than the charges transferred during the stimulation interval.
In a related aspect, the amount of charges that are transferred during a stimulation interval and a discharge interval may be determined by corresponding stimulation and discharge current mirrors. In this regard, physical properties of the various components (e.g., transistors) of the current mirrors may be chosen to provide suitable stimulation and discharge currents.
In another aspect, the output stage may include a charge recovery mechanism that is operable to recover accumulated charges from an electrode. For example, in one embodiment, a resistor is provided that is selectively interconnectable between an electrode and a reference potential node, such that the controller may selectively cause the accumulated charges to be removed from the electrode at a desirable time (e.g., when a patient turns the neurostimulation apparatus off at night).
In yet another aspect, the output stage may be interconnected with an electrode interface that is operable to selectively interconnect an output of the output stage to one or more of a plurality of auditory neurostimulation electrodes. In one embodiment, the electrode interface is operable to selectively interconnect the output of the output stage to a first and second set of the plurality of auditory neurostimulation electrodes to effect a plurality of successive stimulation and discharge intervals on the first and second sets of electrodes. Further, the first and second sets of electrodes may not be identical. For example, the first set of electrodes may include the electrodes e1, e2, and e3, while the second set may include the electrodes e3, e4, e5, and e6.
In another embodiment, a method for driving an electrode for auditory neurostimulation is provided. The method may include first transferring a stimulation current between an electrode and one of a voltage supply and a reference potential node. Further, the method may include second transferring a discharge current between the electrode and the other of the voltage supply and the reference potential node. In this regard, a method for driving an auditory neurostimulation electrode that utilizes a single power supply is provided.
Various features and refinements to the above-noted method may also be provided. For example, in one embodiment, the amount of charge transferred in the first transferring step may be less than the amount of charge transferred in the second transferring step. Further, the method may also include limiting the amount of charge transferred in the second transferring step dependent upon the voltage potential on the electrode.
In another aspect, the method may include selectively alternating between the first and second transferring steps to provide auditory neurostimulation to a patient. Additionally, the amount of charge transferred and the duration of each transferring step may be selectively varied. This may be accomplished by providing a controller, or by providing components (e.g., transistors) whose conductive properties are dependent upon their respective physical dimensions. In one embodiment, current mirrors that are coupled to the electrode may provide the current for each transferring step.
In a related aspect, the method may include removing accumulated charges from the electrode. This step may be performed at any desirable time. In one embodiment, the accumulated charges are removed when a patient turns an implant device off.
Additional aspects and corresponding advantages will be apparent to those skilled in the art upon consideration of the further description that follows.
a is a schematic illustration of the present invention during a stimulation interval.
b is a schematic illustration of the present invention during a discharge interval.
As shown in
As further shown in
The embodiment shown in
Further, the embodiment may provide for simultaneous stimulation or pulsatile (e.g. non-simultaneous) stimulation. For example, under the control of the I/O processor and circuitry 28, two of the electrodes 12 may be selected to provide stimulation current such that unequal amounts of stimulation current are provided by the two electrodes (e.g., the current magnitudes are different). This bias in stimulation current will create an intermediate pitch perception for the patient between the two electrodes. The tonotopic location of the pitch perception can be controlled by the bias in the current between the two electrodes.
Reference is now made to
In particular,
The operation of the stimulation and discharge current mirrors is now described. In this embodiment, the core of the stimulation current mirror is the transistor 48 whose drain is shorted to its gate (i.e., diode connected) and thus operates in the saturation region. The current through the transistor 48 is provided by a connection between its source and the voltage VDD of the single power supply and a variable amplitude current source 48 (or current sink). When the switch 42 is in the position shown in
From Equation (1), it should be appreciated that the relative magnitude of the output current may be designed by sizing the dimensions of the transistors 48 and 50 accordingly. Further, the absolute magnitude of the currents may be controlled by the variable amplitude current source 46.
Subsequent to the stimulation current mirror is utilized to deliver a stimulation interval to the electrode 12, the discharge current mirror may be utilized to equilibrate the charges on the electrode 12 by applying a current in the opposite direction. This discharge interval is graphically illustrated in
As indicated by the presence of εE, the gain of the discharge current mirror may be designed to be slightly larger than the gain of the stimulation current mirror, such that the discharge current is slightly larger than the stimulation current. As discussed further below, this is to ensure system stability.
The control logic 40 may be operable to control the timing of the stimulation intervals and discharge intervals by selectively toggling the switches 42 and 44. In this regard, the control logic 40 may include any combination of software and hardware. Further, the control logic 40 may be hard coded or programmable by a patient or a technician. For example, it may be desirable to selectively adjust the duration of each stimulation-discharge cycle or the period between cycles to provide the best performance to a patient. Similarly, the variable amplitude current source 46 may be controllable by a patient or a technician. In this regard, it may be desirable to increase or decrease the magnitude of the neurostimulation to provide the optimum performance. In the case where the control logic 40 or the current source 46 is programmable, a suitable user interface may be provided.
This initial difference between the magnitudes of the stimulation pulses and the discharge pulses will cause the rest voltage, the voltage potential that the electrode returns to after the completion of a stimulation-discharge cycle, to increase slowly due to the accumulation of charges that are not discharged from the electrode. The portion of the graph 63 indicated by an arrow 65 illustrates this effect of accumulating charges. It should be noted that as the rest voltage on the electrode increases, the discharge pulses will also increase due to in the increased voltage at the drain (i.e., “headroom”) of the transistor 54. As the system reaches stead-state (e.g., a few tens of stimulation-discharge cycles and typically much less than one second), the stimulation pulses 60N and discharge pulses 62N will both be at their desired magnitudes, and the rest voltage will have reached a stable level that permits both transistors 50 and 54 to operate in saturation mode, as shown in the portion of the graph 63 indicated by the arrow 66.
As discussed above, the output stage 20 may be designed such that the discharge current is slightly larger than the stimulation current when the system is operating in steady-state. This feature may be achieved by sizing the transistors of the aforementioned current mirrors accordingly. The primary purpose for this design is to provide a simple solution for ensuring system stability. As can be appreciated, when the discharge current is slightly greater than the stimulation current, the rest voltage on the electrode will tend to decrease since the charges removed from the electrode each cycle are greater than the charges delivered to the electrode. However, if the rest voltage is decreased to a point where the transistor 54 does not have enough headroom to fully operate in saturation mode, then the discharge current will automatically be reduced to a level that is below the stimulation current, which causes the rest voltage on the electrode to increase. Thus, the present design provides for a simple automatic feedback mechanism to ensure that the system remains intrinsically stable. Notably, this design does not require any intricate monitoring and control means to ensure that the charges are equilibrated, which reduces the hardware required, the power consumed, and the complexity of the design.
In one embodiment of the present invention, a charge recovery mechanism is provided to recover the charges on an electrode that are present due to the initial transient leakage current. The charge recovery mechanism may be operable to remove the accumulated charges periodically, when the apparatus is turned off, or any other desirable time. The effect of the charge recovery mechanism on the equivalent leakage current is shown in the graph 70 at the time indicated by the dashed line 76. As can be seen, substantially all of the charges that accumulated when the apparatus was turned on at time 72 are then recovered at time 76 so that virtually no disruptive charges remain in the tissue of a patient.
For the purpose of controlling the electrode interface 38, the I/O processor and circuitry 28 may comprise an electrode interface controller 36 that is interconnected to the electrode interface 38, and is operable to control the routing operation of the electrode interface 38. In this regard, a control signal may comprise a digital signal and the electrode interface controller 38 may include digital logic. Further, the power source 32 may be interconnected to the electrode interface 38 to provide power to various digital and analog componentry therein.
It should be appreciated that numerous variations to the embodiments described above may be provided to achieve an output stage for a neurostimulation system that is powered by a single power supply. For example, the description above is directed to a system that utilizes a positive stimulation current and a negative discharge current, but the currents may also be in the opposite direction. Further, although MOS current sources were described, the present invention will also work well using other technologies (e.g., bipolar junction transistors) or other combinations of components. Additionally, although a single output stage 20 was illustrated driving one or more electrodes in an electrode array, it should be appreciated that multiple, independent output stages may be used to drive one or more electrodes in one or more electrode arrays to suit a particular application.
