Abstract:
An implant comprising an electric power consuming device electrically coupled to an electric power source including an anode, a cathode and at least one potential probe which is independent of the anode and the cathode. By providing at least one potential probe which is independent of the anode and the cathode, implants can be monitored and controlled to avoid process which can damage the electrodes and shorten the service life of the electrodes and the service life of the implant itself.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an implant including an electric power consuming device connected to an electric power source with an anode and a cathode. This invention also relates to such an implant which consumes electric power itself and/or which is coupled to a separate, implanted device which consumes electric power. 
     2. Description of Related Art 
     In currently available implants which require an electric power source for operation, for example, cardiac pacemakers, hearing aids, stimulation devices and the like, either primary cells or secondary cells are used as the electric power source. A drop in the efficiency of the electric power source which endangers implant operation can be prevented by replacing or recharging the cell before the expected service life of the electric power source expires. However, because any replacement of the electric power source requires surgery on the implant wearer, achieving a long service life of the electric power source is very important and is of the highest priority in the field of implant technology. 
     In order to be able to predict the efficiency of the electric power source provided in the implant, whether it be a primary or secondary source, and to also prevent processes that damage the individual electrodes which can occur especially in charging processes of an electric power source made as a secondary cell, the electrodes should be monitored with respect to certain characteristics such as current and voltage. 
     In an implant equipped with a conventional electric power source, the electrodes of the electric power source cannot be observed and monitored independently from the other electrode. Rather, the characteristics of current and voltage which can be measured outside the power source, are always referenced to the entire combination of the electrodes provided in the electric power source. When these characteristics are measured, they are generally dependent on the fact that these electrodes have predictable properties during discharge, at rest and optionally, during charging. However, this measurement can be adulterated by simultaneous processes which polarize the electrodes differently. Thus, this measurement allows conclusions regarding the instantaneous state of the electric power source only under current conditions and only with accurate knowledge of the simultaneous processes under the boundary conditions prevailing at the time. 
     For example, when charging a secondary electrochemical cell, the equilibrium potentials of the two active electrodes are shifted to more negative (negative electrode) and more positive (positive electrode) potentials due to the existing internal resistances. The internal resistances are thus composed of ohmic and non-ohmic portions. The ohmic portions generally relate to contact and electrolytic resistors. The non-ohmic portions are dictated by the electrode composition and geometry and the electrochemical processes which take place on the electrodes. 
     Overall, there is a very complex network of resistive, capacitive and inductive components which can no longer be broken down especially when there is loading, i.e. when the electric power source supplies the implant with electrical energy. Therefore, a simple current/voltage measurement cannot provide the basis for concluding which of the electrodes involved behaves as desired and which does not. 
     Only by extensive experience with a given system under clearly defined boundary conditions (for example, “discharging at C/2 rate to an end discharge voltage of 1.5 V”; “charging at C/10 rate for 14 h”) can one skilled in the art assess whether the electric power source being tested is “good” or “bad” from simply measuring current and voltage values. In addition, even if the discharging behavior is known for a certain current load with a certain cut off criterion for a given electric power source, one skilled in the art still cannot exactly predict the behavior of the electric power source under different conditions, for example, at {fraction (1/10)} or {fraction (1/100)} of the current load at the known boundary conditions. At best, one skilled in the art can only give an estimate. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the primary object of the present invention is to devise an implant which allows more accurate and more reliable measurement of the electrode characteristics. 
     Another object of the present invention is to devise an implant which allows more accurate and more reliable monitoring of the electrode characteristics. 
     These objects are achieved by providing an implant of the initially mentioned type in which the electric power source has at least one potential probe which is independent of an anode or a cathode. In this manner, a reference potential is provided which is independent of the anode and cathode of the electric power source and by which unwanted secondary reactions or undesirably intense secondary reactions on the electrodes under consideration can be detected and prevented by controlled monitoring and/or control of individual electrode potentials relative to the reference potential. 
     Thus, when the respective electrode properties are known, the electrodes can be prevented from being irreversibly damaged, which can lead to premature failure of the electric power source. In an implant in accordance with the present invention, it is no longer necessary to combine extensive technical knowledge based on years of experience with tedious series of tests as required in the present implant designs. Rather, with the present invention, definitive and generally valid conclusions are possible with respect to the pertinent electrodes after performing a few, relatively non-time critical, measurements. Processes which damage electrodes can thus be easily avoided without the need for an analysis of the entire respective current/voltage curves based on numerous assumptions. By practicing the present invention, longer service lives of the electrodes used in the electric power source will result and premature access to the implant which would require surgery on the implant wearer is thereby avoided. 
     More specifically, in one embodiment of the present invention, the electric power source may be an electrochemical power source or a super-capacitor. Such an electrochemical power source may be made as a galvanic element, especially as a primary element, secondary element or as a fuel cell. The electric power source of the implant can be provided with an electrically conductive housing which has a tap which is used as the potential probe. For reasons of production engineering, this embodiment is the simplest to build since a tap from the outside may be attached to the housing of the electric power source, for example, by soldering, without requiring penetration into the housing. In this embodiment, the housing can have several sections electrically insulated from one another, at least two of the housing sections having a tap which are used as potential probes. For example, the housing of the electric power source provided in the implant can have a first housing section which surrounds the anode and a second housing section which surrounds the cathode, the second housing section being electrically insulated relative to the first housing section and the first and the second housing section each having a tap used as potential probes. In this embodiment of the present invention, the taps serve another function in addition to providing reference potentials for measurements of the anode and the cathode in that the taps also provide information on the state of the interior of the electric power source of the implant on various areas within the housing of the electric power source. 
     In yet another embodiment of the present invention, a third housing section may be provided between the first housing section and the second housing section which is electrically insulated relative to the first and the second housing sections. The first, second, and third housing sections may each include a tap which are used as potential probes. These potential probes allow the measurement of the potentials of the respective housing sections. Thus, information about the state of the individual areas of the electric power source of the implant can be thereby obtained. Of course, the present invention may also be modified for use in housings which are electrically insulated relative to the housing interior such that the housing is electrically neutral to the outside. 
     In another preferred embodiment of the present invention, there may also be provided, at least one more electrode which may be used as a potential probe for measuring the potential difference between an electrolyte and the anode or the cathode. 
     The implant may also be provided with a telemetry means in order to transmit data between the implant and an external measurement and/or control device. The telemetry means in which data signals are transmitted by magnetic induction or via infrared transmission are known in the prior art and are already being used in numerous implants. 
     These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments when viewed in conjunction with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic sectional view of an implant in accordance with the present invention. 
     FIGS. 2 through 6 each show a sectional view of a respective embodiment of an electric power source in accordance with different embodiments of the present invention which may be used in the implant of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As illustrated in FIG. 1, the implant  10  has a control unit  12 , an electric power source  14  and a telemetry means  16  which are all accommodated in a common implant housing  18  and which are connected to one another by appropriate wires. Furthermore, the control unit  12  is connected to wire  20  which is routed out of the implant housing  18  through an opening  22  and leads to an active element  23  which executes the desired implant function. For example, the active element  23  can be an actuator of a fully implanted hearing aid, stimulation electrodes, drug dispensing devices, or the like. The implant can receive via the telemetry means  16  interrogation signals or control signals from an external measurement and/or control device  25  and cab transmit data signals to the control device  25 . If the electric power source  14  is a rechargeable power source, the telemetry means  16  may also be used for receiving current signals sent from the control device  25  for recharging of the electric power source  16 . 
     FIG. 2 illustrates the details of an electric power source  14  in accordance with one embodiment of the present invention as applied to the implant  10  described above. In this embodiment, the electric power source  14  includes two electrodes  26  and  28  located in an electrically conductive, preferably hermetically sealed housing  24  which is made, for example, of metal. Although it is irrelevant for the operation of the implant described here which of the two electrodes  26  and  28  is the anode and which the cathode, for reasons of simplicity in this description, the electrode  26  is referred to as the anode  26  and the electrode  28  is referred to as the cathode  28 . The interior of the housing  24  may be filled with an electrolyte  30  and the anode  26  and the cathode  28  are separated from one another by a diaphragm  32 . The diaphragm  32  is an electrical insulator, but allows ion migration between the two electrodes  26  and  28 . In this embodiment, the diaphragm  32  may be made as a microporous plastic separator. The anode  26  and cathode  28  are electrically insulated relative to the electrically conductive housing  24 , for example, by means of an insulating layer  45  applied to the inside of the housing wall. Furthermore, the anode  26  and the cathode  28  are connected to the control unit  12  via wires  34  and  36  respectively, which in turn, are routed through penetrations  38  and  40 , respectively, out of the housing  24  as shown in FIG.  1 . On the housing  24 , there is a tap  42  on which a reference potential can be measured. If the housing is metal, the tap  42  can be made as a wire probe  44  which is conductively connected to the outside of the housing  24 , for example, by soldering, as is illustrated in FIG.  2 . In the illustrated embodiment, the housing is potential-free so that a zero potential can be tapped on the wire probe  44  as a reference to the potential of anode  26  and the potential of cathode  28 . 
     It should be understood that the electric power source  14  in the present application is used as a general term encompassing all types of commonly used power sources. For example, the electric power source  14  may be an electrochemical power source or a primary electrochemical cell which uses any of the ordinary electrode/electrolyte systems. For example, Zn/AgO, Zn/MnO 2 , lithium-based cells, organic systems, and those with liquid low-viscosity or high-viscosity electrolytes and solid electrolyte systems may all be used. Alternatively, if the electric power source  14  is made as a secondary electrochemical cell, metal/air batteries can be used, such as zinc/air systems, Zn/MnO 2  systems, nickel-cadmium cells, nickel/metal hydride systems, or lithium cells. In the present application, the term lithium cells is used in reference to cells in which a solid state cathode of interstitial compounds together with an anode of metallic lithium is used in combination with a liquid, organic electrolyte or electrolyte of a solid polymer or other solid or liquid electrolyte, as well as to lithium-ion cells with a liquid or solid polymer electrolyte, lithium alloy cells and the like. 
     If a polymer or solid electrolyte is used as the electrolyte, the polymer or solid electrolyte will perform a separator function in addition to its function as an ion conductor. Thus, in this situation, the diaphragm  32  shown in FIG. 2 can be eliminated. These polymer or solid electrolytes can be present in the form of true polymer or solid electrolytes or in the form of a microporous polymer with the electrolyte solution placed in its pores, or in the form of a gelled or solution-absorbing polymer or solid electrolyte. 
     In the “three-electrode device” shown in FIG. 2, the current-carrying electrodes  26  and  28  can be observed independently of one another by measuring and comparing the potentials on wire  34  and probe  44  an on wire  36  and probe  44 . 
     An alternative embodiment of an electric power source  14  in accordance with the present invention used in the implant  10  is illustrated in FIG.  3  and includes several potential probes which can be used depending on the measurement requirement. In this embodiment, the electrically conductive housing  24  is divided into two housing sections  46  and  48  which surround the electrodes  26  and  28  respectively. The electrodes  26  and  28  are located in an electrolyte  30  and are separated from one another by a diaphragm  32 . An insulator  50  is provided between the housing sections  46  and  48 . Taps  52  and  54  are provided on the housing sections  46  and  48  respectively so that the potential of the respective housing sections  46  and  48  can be measured via wires  56  or  58  in order to provide a reference potential in the evaluation of the electric power source  14 . 
     Instead of using a diaphragm  32  as shown in FIG. 3 to divide the housing interior into two areas which house the electrodes, the anode  26  and the cathode  28  of the electric power source  14  may each be surrounded by a diaphragm  60  and  62  respectively as illustrated in an alternative embodiment of FIG.  4 . This design allows ion migration to and from the anode or the cathode, but also acts as an electrical insulator thereby preventing electron migration. If the housing  24  of the electric power source  14  is made of metal or another conductive material, the housing  24  may then be divided by a peripheral insulator  50  into two housing sections  46  and  48  thereby preventing a short circuit between the anode or the cathode and the housing. 
     An alternative embodiment of the present invention is illustrated in FIG. 5 including an electric power source  14  equipped with three potential probes independent of the anode  26  and the cathode  28 . Two of these potential probes are formed by wires  56  and  58  which are attached to taps  52  and  54  respectively and provide a means for measuring the potentials of the housing sections  46  and  48 . The third potential probe  64  is located in the electrolyte  30  between the two electrodes  26  and  28 . If the housing  24  is a conductive housing, provisions must be made for insulating the third potential probe  64  and the housing  24 . As shown in FIG. 5, the third potential probe  64  can be routed through the housing  24  anywhere as long as provisions are made for suitable insulation. For example, a penetration through an electrically conductive housing can be provided by the component to be insulated such as through the feed line of the potential probe or by one of the wires which lead to the electrodes  26  and  28 , these wires passing through the electric insulator in the opening of the housing. Furthermore two or more of these lines can be combined in a common penetration instead of having each of the lines routed out of the housing  24  through its own penetration. 
     FIG. 6 shows another embodiment of an electric power source  14  as is used in the implant  10  described here. The housing  24  here is divided into three housing sections, a first housing section  46  which surrounds the anode  26 , a second housing section  48  which surrounds the cathode  28 , and a third housing section  66  located between the first and second housing sections  46  and  48 . Provided between each of the housing sections is an insulator  50 . In the space between the anode  26  and the cathode  28 , there are two potential probes  68  and  70  which are electrically insulated from one another by a diaphragm  72 . If the housing  24  is electrically conductive, the potential probes  68  and  70  can be routed through the insulators  50  in order to provide for insulation between the potential probes and the housing  24  in a manner analogous to the embodiment shown in FIG.  5 . In addition, in the present embodiment electric power source  14  shown in FIG. 6, the third housing section  66  may also be provided with a tap  74  on which the potential of the third housing section  66  can be measured via wire  76 . 
     It goes without saying that the embodiments described above may be combined with one another in various ways if provisions are made for suitable electrical insulation between the individual electrically conductive components, especially the electrodes, the potential probes and optionally, the housing. Thus, FIG. 6 shows an insulator  78  for shielding the potential probe  70  against the electrically conductive housing  24 . An insulator of this type is not actually needed in the embodiment shown in FIG. 6 since its function is already being performed by the insulating connecting piece  50  located between the first and the third housing section. However, such an insulator  78  would be necessary if the potential probe is inserted at a point where no such insulator has been already integrated into the wall of housing  24 . 
     Since the measurement probes are provided at the time of production in the above described implants, the measurement probes will be available for use long before the normally scheduled use. Thus, check measurements can be taken to monitor the implant and depending on the technology used in the implant, may provide an opportunity for improvements to the implant. Check measurements can be simple measurements of the potential differences between the reference electrode and an active electrode. However, more complex measurement processes may be carried out with the above described embodiments of the electric power source by using commercially available measurement equipment. For example, (cyclo)voltammetric studies with DC or combined DC/AC excitation signals and impedance spectroscopic measurements as well as other general electro-analytical methods commonly known in the art may be carried out depending on the objectives of the study or test. These measurements can also be carried out during the production of the electric power source  14  and be taken to monitor stability and utility until the implant is used. 
     In addition to these benefits prior to actual use, detectable electrode potentials acquire special importance and benefits during use of the implant. For example, it now becomes possible to interrupt the discharging process when the electrode enters an undesirable or harmful potential region by monitoring the behavior of the electrode. Subsequent processes can then be initiated to address the particular circumstance. Likewise, charging of the electric power source can also be interrupted in a controlled manner if one electrode enters an undesirable or even harmful potential region. It should be emphasized once again that this type of measurement and monitoring is not readily feasible in conventional two-electrode devices. And without these types of measurement and monitoring, information regarding the electric power source cannot be deemed reliable, thereby (giving rise to the possibility of irreversibly damaging the electric power source and adversely effecting the total service life of the implant. 
     From the foregoing, it should be apparent how the present invention provides an implant including an electric power source with at least one potential probe which is independent of the anode and the cathode. This allows one or more potential measurements in addition to the measurement of the electrode potentials for determining and monitoring of the condition of the electric power source. It should also be evident how the present invention may also be implemented with super-capacitors, especially with double layer capacitors, redox capacitors or pseudo-capacitors, and with fuel cells as noted previously. 
     While various embodiments in accordance with the present invention have been shown and described, it is to be understood that the invention is not limited thereto, and may be changed, modified and further applied by those skilled in the art. Therefore, this invention is not limited to the details shown and described previously but also includes all such changes and modifications which are encompassed by the appended claims.