Abstract:
A total implantable hearing aid system is described. The system includes a single transformer that acts as an insulator between simulation circuitry and associated electrodes, and other system elements residing in the tissue. These other system elements include an RF receiver coil, a microphone system, a battery, and a digital signal processor. The transformer also increases the battery output voltage to a level needed by the simulation circuitry with electrodes.

Description:
BACKGROUND 
     Various types of hearing prostheses provide persons with different types of hearing loss with the ability to perceive sound. Hearing loss may be conductive, sensorineural, or some combination of both conductive and sensorineural hearing loss. Conductive hearing loss typically results from a dysfunction in any of the mechanisms that ordinarily conduct sound waves through the outer ear, the eardrum, or the bones of the middle ear. Sensorineural hearing loss typically results from a dysfunction in the inner ear, including the cochlea where sound vibrations are converted into neural signals, or any other part of the ear, auditory nerve, or brain that may process the neural signals. 
     Persons with certain forms of conductive hearing loss may benefit from hearing prostheses, such as acoustic hearing aids or vibration-based hearing aids. An acoustic hearing aid typically includes a small microphone to detect sound, an amplifier to amplify certain portions of the detected sound, and a small speaker to transmit the amplified sounds into the person&#39;s ear. Vibration-based hearing aids typically include a small microphone to detect sound, and a vibration mechanism to apply vibrations corresponding to the detected sound to a person&#39;s bone, thereby causing vibrations in the person&#39;s inner ear, thus bypassing the person&#39;s auditory canal and middle ear. 
     Persons with certain forms of sensorineural hearing loss may benefit from cochlear implants and/or auditory brainstem implants. For example, cochlear implants provide a person having sensorineural hearing loss with the ability to perceive sound by stimulating the person&#39;s auditory nerve via an electrode array implanted in the person&#39;s cochlea. In traditional cochlear implant systems, an external component of the cochlear implant detects sound waves, which are converted into a series of electrical stimulation signals delivered to the implant recipient&#39;s cochlea via the electrode array. Electrically stimulating auditory nerves in a cochlea with a cochlear implant enables persons with sensorineural hearing loss to perceive sound. 
     A traditional cochlear implant system includes an external speech processor unit worn on the body of a prosthesis recipient and a stimulator unit implanted in the mastoid bone of the recipient. In this traditional configuration, the external speech processor unit detects external sound and converts the detected sound into a coded signal through a suitable speech processing strategy. The coded signal is sent to the implanted stimulator unit via a transcutaneous link. The stimulator unit (i) processes the coded signal, (ii) generates a series of stimulation signals based on the coded signal, and (iii) applies the stimulation signals to the recipient&#39;s auditory nerve via electrodes. 
     In another example cochlear implant, the functionality of the external speech processor unit and the implanted stimulator unit are combined to create a totally implantable cochlear implant (TICI). The TICI system can be either a monolithic system containing all of the components within a single implant housing or a collection of implant housings coupled together. In operation, detected sound is processed by a speech processor in the TICI system, and stimulation signals are delivered to the recipient via the electrodes without the need for a transcutaneous transmission of signals between an external speech processor unit and an implanted stimulator unit as in the traditional cochlear implant configuration described previously. 
     SUMMARY 
     A prosthesis implanted in a body is described. The prosthesis includes a rechargeable energy source, an implant coil that recharges the energy source, a stimulation decoder that provides an output to a hearing stimulator, and a single transformer. The single transformer electrically isolates the implant coil from the stimulation decoder and the hearing stimulator, and modifies an output of the rechargeable energy source for use by the stimulation decoder and the hearing stimulator. 
     A totally implantable prosthesis is also described. The totally implantable prosthesis includes a first circuit block containing a power source and a converter circuit, a second circuit block containing a stimulation decoder circuit for stimulating electrodes, and a single transformer. The single transformer provides electrical isolation between the first circuit block and the second circuit block. The single transformer also modifies the voltage from the power source to the second circuit block. 
     An active medical implant device (AIMD) is also described. The AIMD includes a main implant component. The main implant component includes a first circuit separated from a second circuit with a single transformer. The AIMD also includes an implant coil connected to the main implant component, a microphone system connected to the first circuit and a cochlear electrode connected to the second circuit. The transformer prevents AC and DC leakage from the cochlear electrode to the implant coil, the microphone system and the first circuit. The transformer also modifies a voltage from the first circuit to the second circuit. The main implant component, the implant coil, and the microphone reside inside a recipient&#39;s tissue after implantation. 
     These as well as other aspects and advantages will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it is understood that this summary is merely an example and is not intended to limit the scope of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Presently preferred embodiments are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein: 
         FIG. 1  is a block diagram of a totally implantable cochlear implant (TICI) system, according to an example; 
         FIG. 2  is a block diagram of a main implantable component depicted in  FIG. 1 , according to an example; 
         FIG. 3  is a circuit diagram of a Class D driver, according to an example; and 
         FIG. 4  is a circuit diagram of an inverted Class D Driver, according to an example. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a totally implantable cochlear implant (TICI) system  100 , which is totally implantable; that is, all of the components of the TICI system  100  are configured to be implanted under skin/tissue  116  of a recipient. Because all of the components of the TICI system  100  are implantable, the TICI system  100  operates, for at least a finite period of time, without the need of an external device. 
     An external device  118  can be used to charge an internal energy source and to supplement the performance of the TICI system  100 . The external device  118  may be a dedicated charger, a conventional cochlear implant sound processor, a remote control, or other device. In one example, the external device  118  is a Behind the Ear (BTE) headpiece coil, including an external microphone. Various types of energy transfer, such as infrared, electromagnetic, capacitive, and inductive transfer, may be used to transfer power and/or data from the external device  118  to the TICI system  100 . 
     The TICI system  100  includes a microphone  110 . The microphone  110  is configured to sense a sound signal  120 . The microphone  110  may also include one or more components to pre-process the microphone output. An electrical signal  122  representing the sound signal  120  detected by the microphone  110  is provided to the main implantable component  102 . 
     The TICI system  100  also includes an implant coil  112 . The implant coil  112  transcutaneously receives power and data signals from the external device  118  using one or more types of wireless transmission. For example, radio frequency (RF) links may be used to transmit power and data to the implant coil  112 . The implant coil  112  may also transmit data signals to the external device  118 . The implant coil  112  receives power in a recharging mode of operation. 
     The implant coil  112  is typically a wire antenna coil comprised of multiple turns of electrically insulated single-strand or multi-strand platinum or gold wire. The implant coil  112  may be produced using a silicone molding process, which can provide additional electrical insulation. Other coil designs may be used. 
     The TICI system  100  also includes a main implantable component  102  having a hermetically sealed, biocompatible housing. For example, the biocompatible housing can be constructed of a metal, metal alloy, ceramic, peek polymers, or other suitable material. The housing protects the recipient of the TICI system  100  both chemically and electrically. 
     The main implantable component  102  performs sound detection, speech processing, and stimulation functions. As seen in  FIG. 1 , the main implantable component  102  includes a first circuit block  104  isolated from a second circuit block  106  via a single transformer  108 . If the housing material is magnetically (H-field) or electromagnetically (EM-field) transparent at the operating radio frequency, the implant coil  112  may also be located within the main implantable component  102 . For example, a housing material of ceramic or peek polymer may be suitable for containing the implant coil  112  within the main implantable component  102 . 
     The first circuit block  104  includes a rechargeable energy source. In one example, the rechargeable energy source is a battery, such as a lithium ion battery. The rechargeable energy source receives power from the implant coil  112  and stores the power. The output of the rechargeable energy source may be a DC voltage source or a DC current source. The power may then be distributed to the other components of the TICI system  100  as needed for operation. 
     The first circuit block  104  also includes a converter circuit. The converter circuit implements one or more speech processing and/or coding strategies to convert the pre-processed microphone output into data signals for use by a stimulation decoder circuit in the second circuit block  106 . Speech coding strategies include, but are not limited to, Continuous Interleaved Sampling (CIS), Spectral PEAK Extraction (SPEAK), Advanced Combination Encoders (ACE), and Fundamental Asynchronous Stimulus Timing (FAST). 
     The second circuit block  106  includes the stimulation decoder circuit that generates stimulation signals based on the coded signal received from the converter circuit in the first circuit block  104  and provides these signals to a hearing stimulator. The hearing stimulator delivers electrical stimulation signals to the cochlea of the recipient. In one example, the hearing stimulator is an electrode array  114 . 
     The electrode array  114  includes a plurality of intra-cochlear electrode pads or terminals  114   b  configured to be positioned within the implant recipient&#39;s cochlea, and one or more extra-cochlear electrodes  114   a . The intra-cochlear electrode pads or terminals  114   b  may include optical contacts and/or electrical contacts. The extra-cochlear electrode  114   a  has an extra-cochlear electrode lead terminating in an electrode tip (sometimes referred to as a “ball” electrode) at the distal end of the electrode lead. The electrode tip is configured to be positioned beneath muscle tissue near the implant recipient&#39;s cochlea. The intra-cochlear electrode pads or terminals  114   b  are typically configured to function as “active” (current source) electrodes, and the one or more extra-cochlear electrodes  114   a  are typically configured to function as “reference” (current sink) electrodes. 
     The transformer  108  is a radio frequency (RF) transformer. The transformer  108  acts as an insulator between the electrodes  114  and other system elements residing in the tissue, such as the implantable microphone system  110 , the implant coil  112 , and components within the first circuit block  104  (e.g., the rechargeable energy source). In this role, the transformer  108  prevents electrical (e.g., AC and DC) leakage between the electrodes  114  and the other implantable system elements. 
     Stimulation of tissues and nerves using alternating electrical currents passing through tissue can cause problems for the recipient. For example, excess DC currents can cause electrolysis, redox reactions, and chemical reactions. The transformer  108  reduces electrical leakage, which minimizes negative side effects caused by electrical leakage. 
     The transformer  108  also acts as a voltage boost element providing a boost to the energy source output voltage. The transformer  108  modifies a compliance voltage as necessary to provide the correct stimulation current on each electrode  114  based upon an auditory stimulation algorithm or scheme that controls the timing and intensity of auditory stimulation pulses applied to the electrodes  114 . The compliance voltage is the voltage available at the electrode  114  that can force current to flow while still maintaining control of the working electrode voltage. 
     In this role, the transformer  108  is a part of a step-up DC/DC converter. The energy source output voltages are often lower than the electrode compliance voltage needed for correct operation of the electrode current sources. The compliance voltage of the electrode current sources depends on the impedance between the intra-cochlear electrodes  114   b  and the extra-cochlear electrodes  114   a.    
     For example, if the impedance is 10 Kohms and the current source is 0.5 mA, the compliance voltage needs to be slightly greater than 5V (i.e., 10 Kohm*0.5 mA=5V). If the energy source is a lithium ion battery, the output voltage is approximately 3.7 volts. Thus, the transformer  108  boosts the lithium ion battery output of 3.7 volts to a value greater than 5 volts. 
     Using the transformer  108  in this manner provides tight coupling with minimal efficiency losses. By placing the energy source on the primary side of the transformer  108  and the stimulation decoder circuit on the secondary side of the transformer  108 , the transformer  108  provides step-up voltage control. Moreover, the transformer  108  prevents or reduces leakage between the electrode array  114  and other TICI system  100  components, such as the microphone  110 , the implant coil  112 , and components within the first circuit block  104 . 
       FIG. 2  is a block diagram of the main implantable component  102 , according to an example. As previously seen in  FIG. 1 , the main implantable component  102  includes the first circuit block  104  isolated from the second circuit block  106  via the single transformer  108 . The implant coil  112  is connected to the primary side of the transformer  108 . 
     In this example, the first circuit block  104  includes a converter circuit  202 , a battery  204 , and additional power circuitry. The converter circuit  202  is an audio to radio frequency stimulation converter. The additional power circuitry includes battery protection  206 , a battery manager  208 , a rectifier  210 , a modulator  212 , a power control block  214 , and a driver  216 . The second circuit block  106  includes a capacitor  218 , a power and data extractor  220 , and a stimulation decoder  222 . 
     The driver  216  operates in Class-D or inverted Class-D (Class D −1 ) mode delivering power and stimulation data from the microphone  110  and implant battery  204  to the transformer  108 .  FIG. 3  depicts a Class-D driver  300 , while  FIG. 4  depicts an inverted Class-D driver  400 . 
     Operation Mode 1: Charging 
     During charging, a signal from the external device  118  delivers stimulation data to the stimulation decoder  222 , and power to the implant battery  204  and the stimulation decoder  222 . The power delivered to the battery  204  is used to recharge the battery  204 . 
     With the Class-D driver  300 , the N-MOSFETS of the Class-D (Class D1 and Class D2 driver) H-bridge driver are both closed, and parallel resonance is obtained by Cres_1 and Cres_2 (e.g., 5 MHz). The battery  204  can be charged using the rectifier  210  and the battery manager  208 . From the perspective of the stimulation decoder  222 , the transformer  108  is part of a parallel resonance tank formed by Cres_1 and Cres_2 in series and the inductance of the implant coil  112 . The implant coil  112  is scaled by the primary-secondary ratio of the transformer  108 . 
     With the inverted Class-D driver  400 , both MOSFETs are open and parallel resonance is obtained with Cres  218  (e.g., at 5 MHz). The battery  204  can be charged using the rectifier  210  and the battery manager  208 . From the perspective of the stimulation decoder  222 , the transformer  108  is part of a parallel resonance tank formed by Cres  218  and the inductance of the implant coil  112 . The implant coil  112  is scaled by the primary-secondary ratio of the transformer  108 . 
     Operation Mode 2: Normal Operations (Charged) 
     During normal operation when the battery is charged, the converter circuit  202  converts the microphone signal  122  to stimulation data. The stimulation data is then transferred to the modulator  212 . Preferably, the modulator  212  is an on-off keying (OOK) modulator. The modulator  212  may use other modulations schemes, such as Amplitude Shift Keying (ASK), Continuous Phase Frequency Shift Keying (CPFSK), Binary Phase Shift Keying (BPSK), and Quadrature Phase-Shift Keying (QPSK). 
     The power control block  214  controls power to the stimulation decoder  222  and the stimulation compliance voltage. The power control block  214  controls power and the stimulation compliance voltage by adjusting the duty cycle of RF frames and RF cycles. For example, the power control block  214  may use pulse width modulation (PWM) to adjust the duty cycle of the RF frames and RF cycles. 
     For the Class-D driver  300 , the impedance seen from the Class-D driver side is a series resonance circuit formed by Cres_1 and Cres_2 in series and the inductance of the implant coil  112  scaled by the primary-secondary ratio of the transformer  108 . The impedance seen from the stimulation decoder  222  is a parallel resonance tank formed by Cres_1 and Cres_2 in series and the inductance of the implant coil  112  scaled by the primary-secondary ratio of the RF transformer  108 . 
     The inverted Class-D driver  400  is powered through the transformer  108  center tap via the rfc coil (RF choke) forming a current source. From the perspective of the stimulation decoder  222 , the transformer  108  is part of a parallel resonance tank formed by Cres  218  and the inductance of the implant coil  112 . The implant coil  112  is scaled by the primary-secondary ratio of the transformer  108 . 
     As described, the single transformer  108  is used as both an AC/DC barrier and a step-up converter. More than one transformer would increase the volume needed for the implant housing. The transformer  108  is part of a resonant tank circuit built by the implant coil  112  and one or more capacitors. 
     Another benefit of this single transformer design is the ability to monitor the quality of the microphone  110 . During normal operating mode, the external device  118  can monitor the quality of the microphone  110  by using the implant coil  112  to transfer stimulation data from the microphone  110  to the external device  118  through the implant coil  112  once the Class-D driver  300  or the inverted Class-D driver  400  is activated. In this scenario, the external device  118  is connected to an external coil that is magnetically coupled to the implant coil  112 . 
     As yet another benefit of this single transformer design is the ability to use a conventional RF link when the rechargeable energy source is faulty or dead. 
     In an alternative design, the transformer  108  is used as an insulator between the implant coil  112  and the electrodes  114 , and other circuitry is used for the step-up converter function. For example, this additional circuitry can include a boost converter to increase voltage at the current sources of the electrodes  114 . The boost converter is an active circuit containing at least an inductor and two capacitors. 
     In another alternative design, the RF transformer  108  provides insulation between the implant coil  112  and the electrodes  114 , and a two-wire transformer provides insulation between the rechargeable energy source and the microphone  110 , and the electrodes  114 . While this alternative includes two transformers, a single RF transformer reduces the implant housing volume as compared to a design containing two RF transformers. 
     It is intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it is understood that the following claims including all equivalents are intended to define the scope of the invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope of the following claims and equivalents thereto are claimed as the invention.