Abstract:
An improved arrangement for detecting DC current leakage in implanted devices is disclosed. Currents are monitored, so that any unexpected imbalance indicates that a fault condition has occurred. The principle of the system is that if everything working correctly, the same current should be flowing into the tissue from VDDH-ELEC as is passing out of the tissue into VSS_ELEC. Any difference is indicative of current flowing into or out of the tissue from unknown paths, which is indicative of some fault. This replaces routine monitoring for specific faults.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a current leakage detection device and method for implanted devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    Implanted devices, such as heart pacers and cochlear implants, have been widely deployed to assist with the management of various indications. The present invention is particularly concerned with devices which are electrically powered. 
         [0003]    Engineers and technicians have, with improvements in technology and knowledge, been making the devices smaller and therefore more readily implantable. Improvements to functions and the increased complexity of devices and functions are an important part of the progressive development of implantable devices. However, as implantable devices become increasingly complex, the potential for electrical failures increases. 
         [0004]    Such failures can result in DC current leakage, with the excess DC current passing through tissue of the implantee in ways which are not related to therapy. Such DC currents flows could result in electrolysis, or otherwise cause injury to the user. DC currents can also cause irreversible redox reactions at the electrodes of the implanted device. 
         [0005]    By way of example, current cochlear implants are capable of detecting fault conditions in only a very limited way, usually by regularly checking for particular faults. The faults being checked for are programmed into the implant based on the failure modes determined by the design team. For example, electrodes may short to ground. As devices become more complex, the number of failure modes that can lead to DC current leakage increases dramatically. As such, the present methods of checking for faults will take an increasing amount of time and power, and be increasingly complex to design and operate. Further, it becomes increasingly difficult to determine all possible failure modes, and to try to detect each specific failure mode. 
         [0006]    It is accordingly an object of the present invention to provide a simplified method and apparatus for detecting current leakage in implantable devices. 
       SUMMARY OF THE INVENTION 
       [0007]    In a broad form, the present invention provides a method of determining when a DC current leakage has occurred in an implanted device irrespective of the cause of the DC current leakage, wherein current flows are compared to detect an unexpected imbalance. 
         [0008]    In one aspect, the present invention provides a method of detecting DC current leakage in an electrical circuit of an implanted device, said method including at least the steps of: 
         [0009]    a) measuring, for each intended current path connected to one or more predetermined points in the circuit, the current flow relative to the direction of the point; 
         [0010]    b) summing each measured current flow to provide a total current; 
         [0011]    c) determining whether the total current is within a predetermined margin of zero; 
         [0012]    wherein if the total current is not within said predetermined margin of zero, determining that leakage current is present. 
         [0013]    In a second aspect, the present invention provides a method of detecting DC current leakage in an implanted device, said device including electrical output driving circuits powered by a respective power and ground rail, said. method including at least the steps of: 
         [0014]    a) measuring the current passing through the power rail; 
         [0015]    b) measuring the current passing through the ground rail; 
         [0016]    c) comparing the first and second measurements; and 
         [0017]    if there is a absolute difference greater than a predetermined margin between first and second measurements, determining that current leakage is present. 
         [0018]    According to another aspect, the present invention provides an implantable device including electrical output driving circuits powered by a respective power and ground rail, a first current monitoring device for said power rail, a second current monitoring device for the ground rail, and a comparator for comparing the first and second current monitoring device, such that if the comparator detects a difference greater than a predetermined margin, an indication is provided that current leakage is occurring. 
         [0019]    A determination that there is current leakage indicates that a fault of some sort appears to be present, and may lead to various actions, for example the activation of specific diagnostic software. It provides predominantly an overall indication that a fault is occurring, and existing diagnostic software can be used to identify and rectify or isolate the fault, or in an extreme situation, shut down the implant. 
         [0020]    The predetermined margin is required to allow for small errors and variations in the respective devices, and may be determined by the device designer on the basis of the accuracy and tolerances of the device. A time aspect may also be included to account for transient events. 
         [0021]    An advantage of the inventive approach is that it minimizes the need for identification in advance of all possible faults, and for monitoring software to continuously check for all these faults. Maintenance processes can be run at suitable times, with consequently less degradation of the performance of the implant due to the operation of checking and verification processes running in parallel with the intended function of the implant. 
         [0022]    Whilst the present invention may be applied to any implanted device, it is particularly envisaged as applicable to implantable devices which are intended to deliver electrical signals, for example cochlear implants. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    An illustrative embodiment of the present invention will be described with reference to the accompanying figures, in which: 
           [0024]      FIG. 1  shows a schematic overview of one embodiment of the invention in a broad form; 
           [0025]      FIG. 2  shows a block diagram of an embodiment of the invention; and 
           [0026]      FIG. 3  is a schematic illustration of another implementation. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0027]      FIGS. 1 and 2  show an example of a system  10  used to monitor the current flowing in an implanted device. In the example shown, the implanted device is a cochlear implant, and the implementation of the leakage current detector is a low-power form suitable for integration into a cochlear implant. 
         [0028]    It will be appreciated that the present invention can be applied to a wide variety of implantable devices, in which DC leakage current is a potential issue. The example used below is a cochlear implant, but the present invention is applicable to many other devices, for example other implanted hearing aids or hearing prostheses, neural stimulators, retinal prostheses, cardiac related devices such as pacers or defibrillators, implanted drug pumps, or other implanted electrical devices. 
         [0029]      FIG. 1  is a schematic overview of this implementation of the invention, within an integrated circuit which provides the electrical circuitry for operation of the implanted device. Box  5  represents the stimulation control and other internal circuitry, which sends control signals to the electrode stimulation circuitry  7 , which in turn delivers electrical stimuli to selected electrodes  9  in accordance with the control signals. This is a well known arrangement in cochlear implants, and as the present invention is not concerned with the details of stimuli presented or stimulation generation as such, this aspect will not be described in detail. Cochlear implants of this general type are disclosed, for example, in U.S. Pat. No. 4,532,930 to Crosby et al. The stimuli delivered by the electrodes  9  constitute the desired electrical output from the system: any DC current otherwise delivered by the implant is indicative of a fault. 
         [0030]    The stimulation power rail VDD-HI is the power rail for the system as a whole, and VSS is the corresponding ground rail. According to this implementation of the present invention, an additional stimulation power rail VDDH_ELEC, and an additional stimulation ground VSS-ELEC, are provided internal to the integrated circuit. The additional rails provide the power for the electrode stimulation circuitry  7 . The currents in VDDH_ELEC and VSS-ELEC are monitored by suitable measuring arrangements, shown here as A 1  and A 2   25 ′ respectively. 
         [0031]    The principle of the system is that if everything is working correctly, the same current should be flowing into the tissue from VDDH_ELEC as is passing out of the tissue into VSS-ELEC. This is the known current path. Any difference is indicative of current flowing into or out of the tissue from unknown paths, which is indicative of a fault of some kind. 
         [0032]    This system takes advantage of Kirchoff&#39;s current law in which the sum of currents flowing towards a point in a circuit is equal to the sum of currents flowing away from that point. Due to Kirchoff&#39;s current law, the total sum of currents entering the tissue in an implanted device is zero. If the sum of current entering along known paths is not zero, then the left-over current must be entering the tissue along an unknown or fault path. Therefore, should a comparison of A 1  and A 2  find a difference between the current flow there is a DC current leakage and hence a fault of some kind. 
         [0033]      FIG. 2  illustrates in more detail a practical implementation of this approach. Circuit  40  acts as a current mirror for the current through M 1 , which is effectively the current flowing into VDDH_ELEC. Similarly, circuit  50  acts as a mirror of the current in M 2 , which is effectively the current in VSS-ELEC. Circuit  80  is a comparator, which detects any current difference between M 1  and M 2 . 
         [0034]    A current mirror is a circuit which accurately tracks the current in another circuit, but in this case scaled down by a factor of 100. It is designed to copy a current flowing through an active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being ‘copied’ can be, and sometimes is, a varying signal current. Any current mirror, such as bipolar or MOSFET, may be used; the one shown is a MOSFET current mirror. The comparator working on the output of the current mirrors in an optimally operating circuit according to the present embodiment will detect no difference in current. The mirror circuits used should be very sensitive to differences in current, and accurately match the currents in the monitored circuits. In this application, it is desirable to be able to detect a difference of about 10 μA between M 1  and M 2 , on top of a common mode current of about 10 mA. This requires 0.1% accuracy in the ratio of M 1 /M 3  to M 2 /M 4 , which is close to the limit of what is achievable with this arrangement. 
         [0035]    The resolution can be increased by decreasing the mirroring ratio. However, this occurs at the expense of increased power consumption of the monitoring system. 
         [0036]    The comparison of the currents mirrored at monitors  40 ,  50  is accomplished by comparator  80 . The resistor  82  senses the current difference. The capacitor  84  suppresses or filters out temporary mirroring imbalances due to current switching spikes such as onset of stimulation pulses. The voltage across resistor  82  is compared at a bi-directional voltage comparator  86 . The comparator is capable of detecting a 10 μA difference between VDDH_ELEC and VSS-ELEC. 
         [0037]      FIG. 3  illustrates another implementation of the present invention. Instead of using current mirrors to sense the supply and ground currents, the currents get set directly by a current DAC (digital-to-analog converter)  90 , 91 . Digital feedback circuits  95 ,  96  responsive to respective comparators  93 , 94  ensure that VDDH_ELEC and VSS_ELEC are maintained at a desired voltage by adjusting the current DAC codes. Thus, at any time, the digital codes for the VDD and VSS current, Vdd_code&lt;n:1&gt; and Vss_code&lt;n:1&gt; represent the actual VDD and VSS currents (since we are setting them directly). A separate digital filtering and comparison circuit compares the VDD and VSS currents that are being set and detects any persistent difference between them, with allowance for differences in timing. A suitable time constant may be, for example,  1  ms or more. There is a key difference between this and the current mirror approach used in the example shown in  FIG. 2 . The current mirror approach takes a small (and imperfect) copy of the VDD and VSS currents and compares them. The current DAC approach illustrated in  FIG. 3 . actually sets the VDD and VSS currents at all times via digitally controlled current DACs. This means that the actual currents going in are known, not just copies. It shifts the filtering and comparison operations into the digital domain. Again, the current DAC approach may use MOSFETs, bipolar transistors etc. as discussed above. 
         [0038]    Another way of measuring the current difference is by using the Hall effect. The Hall effect refers to the potential difference (Hall voltage) on opposite sides of a thin sheet of conducting or semiconducting material in the form of a ‘Hall bar’ (or a van der Pauw element), in this case the power and ground rails VDDH_ELEC and VSS-ELEC through which an electric current is flowing. By measuring the Hall voltage across the element, one can determine the current flowing through the element. A Hall effect device could be used to measure the current difference in an alternative implementation. 
         [0039]    Alternatively, the current between VDDH_ELEC and VSS-ELEC could be measured directly. This could then be compared to the expected current. Whilst it is theoretically possible to determine the tissue current, this is very difficult in practice. The benefit of taking the difference between the two rails VDDH_ELEC and VSS-ELEC is that it then does not matter what the current should be and any difference between the currents is due to a fault condition. 
         [0040]    Alternatively, the current could be measured at each of the monitors  40 ,  50  independently before subtraction of the two currents occurs. Any method that can accurately measure the current difference between the power and ground rails VDDH_ELEC and VSS-ELEC can be used. 
         [0041]    The implementation described will detect leakage caused by any single failure external to the chip, for example an electrode shorted to some potential, or leakage to the coil. It will also detect many single failures internal to the chip. Some double failures may not be directly detected, but as the first failure to occur will be detected, this is not a significant issue in practice. 
         [0042]    A significant advantage of the present invention is that the occurrence of even a fault which had not been anticipated as possible can be detected. Further, it greatly reduces the need for diagnostic and testing processes to run in parallel with the functional operation of the device, thereby enhancing performance by not competing for processor time with the delivery of therapy. 
         [0043]    Once a fault condition is detected, the action taken will depend on the nature of the device, and potentially the nature or scale of the current difference. In some cases, it may provide an indication to the user through an existing feedback mechanism that a fault is detected, and that they should contact their clinician to determine how to proceed. In some cases the device may be automatically disabled. In a preferred arrangement, the detection of a fault triggers a software diagnostic process. This can more accurately trace the problem, and depending upon the outcome rectify the fault, indicate to the user that service is required, or shut the device down. 
         [0044]    The system  10  has been shown as placed within the implanted portion of a cochlear device. However, the entire circuit or parts of it could be brought outside the chip if desired in a suitable implementation. 
         [0045]    Whilst the invention has been described in relation to a cochlear implant, it will be readily appreciated that the invention is equally applicable to other implanted devices. For example, it may be applied to other implanted hearing devices such as implanted hearing aids, middle ear prostheses, brain stem implants and electro-acoustic devices. It may also be used in other implanted medical or other devices. It will further be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.