Abstract:
An implanted electrolytic current injection device, comprising a reservoir of KCl in electrolytic contact with the interior of the scala media and including a charge injection electrode and a reservoir of saline solution in electrolytic contact with a part of the body that is saline. Also, a current source supplies current to a support electrode, which is moveable between the reservoir of KCl and the reservoir of saline solution. Accordingly, the support electrode may be alternately placed in the reservoir of KCl, for refreshing the charge injection electrode, and in the saline solution, for providing a source of electrons for driving the charge injection electrode. A driver moves the support electrode between the reservoirs.

Description:
RELATED APPLICATIONS  
       [0001]     The present patent application claims priority from U.S. provisional application No. 60/496,298 filed Aug. 19, 2003, and from U.S. application Ser. No. 10/780,544 filed Feb. 17, 2004, which is a divisional of U.S. application Ser. No. 10/287,989 filed Nov. 5, 2002, now U.S. Pat. No. 6,694,190. 
     
    
     STATEMENT OF GOVERNMENT SUPPORT  
       [0002]     The invention was made with government support under grant numbers R43DC005531-01 ZRG01 and 2R44DC005531-02. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention is generally related to devices and methods for correcting hearing loss.  
       BACKGROUND OF THE INVENTION  
       [0004]     As many as seven million Americans suffer from a form of hearing loss known as strial presbycusis, which is marked by a loss of hearing in all registers and, as the name indicates, is associated with the aging process. In a healthy ear there is a voltage difference across the basilar membrane, the organ that hosts the hair cells. This voltage difference, referred to as “endocochlear potential,” causes current to flow through the hair cells. Sound waves cause the hair cells to bend, thereby changing their electrical conductivity and the amount of current that flows through them. This process results in the electrical nerve impulses that are sent to the brain by the auditory nerve.  
         [0005]     It appears that the most frequent immediate cause of strial presbycusis is the deterioration of the stria vascularis, a structure that extends along the basilar membrane and produces the ions that create the endocochlear potential. The loss of endocochlear potential appears to result in both an immediate decline in hearing acuity and a gradual deterioration of the structure of the scala media. One potential method of restoring the enodocochlear potential is to inject additional charge by means of an electrode. This is difficult, however, because it requires the production of a DC current within the body. The body&#39;s interstitial fluid tends to foul and eventually destroy any implanted electrode producing a DC current. Further, metal electrodes either dissolve or become fouled with new material when they are driven with DC currents.  
         [0006]     Because of the tendency for DC electrodes to be fouled, existing therapeutic devices which produce electrical currents within the body, including pacemakers and neural stimulation systems, are driven by charge balanced, biphasic electrical pulses.  
       SUMMARY OF THE INVENTION  
       [0007]     In a first separate aspect, the present invention is an implanted electrolytic current injection device, comprising a reservoir of KCl in electrolytic contact with the interior of the scala media and including a charge injection electrode and a reservoir of saline solution in electrolytic contact with a part of the body that is saline. Also, a current source supplies current to a support electrode, which is moveable between the reservoir of KCl and the reservoir of saline solution. Accordingly, the support electrode may be alternatingly placed in the reservoir of KCl, for refreshing the charge injection electrode, and in the saline solution, for providing a source of electrons for driving the charge injection electrode. A driver moves the support electrode between the reservoirs.  
         [0008]     In a second separate aspect, the present invention is an electrolytic current injection device, implanted in a living body and comprising a reservoir of KCl controllably in electrolytic contact with the interior of the scala media and including an active electrode, the reservoir of KCL also being controllably in electrical contact with a saline portion of the body by way of a structure that does not permit a harmful level of ion transport between the KCl reservoir and the saline portion of the body. Also, a reservoir of saline solution is in electrolytic contact with a part of the body that is saline and including a refresh electrode. Additionally, a current source is electrically interposed between the active electrode and the refresh electrode. A controller places the current injection device into a current injection mode in which the current source creates electric current flow from the refresh electrode to the active electrode and simultaneously places the KCl reservoir into electrolytic contact to the scala media, thereby causing charge to be electrolytically injected into the scala media. Alternately, the controller places the current injection device into a refresh mode in which electric current flows from the active electrode to the refresh electrode and the KCl reservoir is removed from electrolytic contact to the scala media and into electrical contact to the NaCl portion of the body, thereby causing a refreshing electrolytic current into the refresh electrode.  
         [0009]     The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is an illustration of an implantable charge injection assembly and driver, according to the present invention, shown implanted in the skull.  
         [0011]      FIG. 2  is an illustration of the implantable charge injection assembly and driver of  FIG. 1 , shown in relation to the structure of the inner ear.  
         [0012]      FIG. 3  is an illustration of the implantable charge injection assembly of  FIG. 1 , shown in greater detail.  
         [0013]      FIG. 4  is a greatly expanded illustration of an electrostatically actuated micro machined gate, in its closed state, as utilized in the present invention.  
         [0014]      FIG. 5  is a greatly expanded illustration of an electrostatically actuated micro machined gate in its open state, as utilized in the present invention.  
         [0015]      FIG. 6  is an illustration of an alternative embodiment of an implantable charge injection assembly, which includes membranes that controllably and selectively permit the passage of electrolytes.  
         [0016]      FIG. 7  is an illustration of an additional alternative embodiment of an implantable charge injection assembly, which uses electromagnetic current steering.  
         [0017]      FIG. 8  is an illustration of an additional alternative embodiment of an implantable charge injection assembly, which has a rotatable electrode.  
         [0018]      FIG. 9  is an illustration of an additional alternative embodiment of an implantable charge injection assembly, which has two charge injection units.  
         [0019]      FIG. 10  is an illustration of an additional alternative embodiment of an implantable charge injection assembly, which has two charge injection units, but having a different construction from that of  FIG. 9 .  
         [0020]      FIG. 11  is a timing diagram for the assembly of  FIG. 9 , but that would apply equally as well (with analogous labeling) to the embodiment of  FIG. 10 , and the embodiment of  FIGS. 12 and 13 .  
         [0021]      FIG. 12  is a schematic diagram of an additional alternative embodiment of an implantable charge injection assembly, showing the assembly in a first state.  
         [0022]      FIG. 13  is a schematic diagram of an additional alternative embodiment of an implantable charge injection assembly, showing the assembly in a second state.  
         [0023]      FIG. 14  is a schematic diagram of yet another alternative embodiment of an implantable charge injection assembly.  
         [0024]      FIG. 15A  is a schematic diagram of a half wave rectification charge injection device according to the present invention, in charge injection mode.  
         [0025]      FIG. 15B  is a schematic diagram of a half wave rectification charge injection device according to the present invention, in electrode refresh mode.  
         [0026]      FIG. 16A  is a schematic diagram of an alternative embodiment of a half wave rectification charge injection device according to the present invention, in charge injection mode.  
         [0027]      FIG. 16B  is a schematic diagram of the charge injection device of claim  16 A, in active electrode refresh mode. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Referring to  FIGS. 1 and 2 , an implantable charge injection assembly  10  according to the present invention, is designed to be implanted in the human skull. A charge injection unit  12  will be placed so that it contacts the scala media of the subject. In one preferred embodiment, the structure of charge injection unit  12  includes an electrolytic fluid-filled liquid crystal polymer (LCP) housing  18  ( FIG. 3 ). The electrolytic fluid is an aqueous solution of  — 0.17_M KCl to match the potassium concentration of human scala media tissue. Referring to  FIG. 3 , a primary electrode  20  located in the housing  18  is made of conductive metal plated with IrOx and has a surface area of 1.6×10 9  μm 2 . Injection unit  12  includes a tip  22  that contacts the scala media and has an interior area that is less than one hundred thousandth that of electrode  20 , being between 100 μm 2  and 10,000 μm 2 . The length of the tip  22  is 0.2 mm to 0.5 mm.  
         [0029]     The dimensions of charge injection unit  12  determine the bulk of the DC resistance of unit  12 , which equals about 0.1 to 1 megohms, based on a resistivity of 36.7 ohm-cm for 0.17 M KCl at 37° C.  
         [0030]     Charge injection assembly  10  includes a tube  16  that extends from unit  12  to a refresh electrode  14  that is embedded in the temporalis muscle, or that may be located in a closed side chamber of the electrode assembly. Tube  16  has an inside diameter of 25 μm or more and is filled with KCl liquid of appropriate molarity.  
         [0031]     An electrode driver and switch control assembly  28  controls a micro machined gate  30  assembly with flap  32 (FIGS.  3   4  and  5 ), which exposes electrode  20  to either tip  22  or refresh electrode  14 . When the gate assembly  30  is positioned to connect electrode  20  to tip  22 , assembly  28  drives electrode  20  to cause it to inject charge into the scala media by way of tip  12 . When the gate assembly  30  is positioned to connect electrode  20  to the refresh electrode  14 , electrodes  20  and  14  will be driven so that electrolytic current flows into and thereby refreshes primary electrode  20 , analogous to half-wave rectification. The single bi-state gate could also be replaced by two separate single-state gates operating in opposite phase from one another.  
         [0032]     Referring to  FIGS. 4 and 5 , in one preferred embodiment gate  30  is electrostatically actuated. Gate  30  is made by the photolithographic conductive structures on thin sheets of liquid crystal polymer (LCP) combined with the laser micromachining of a small flap  32 . The flap  32  is kept closed by maintaining a small opposite charge on electrodes placed on the surfaces of flap  32 . The facing electrodes are electrically separated by a surface dielectric. To open the switch, like polarity is applied to both electrodes. By utilizing LCP material, which is thermoplastic, material can be selectively adhered by spot “welding” using an IR laser, or selectively removed using a UV laser, allowing a variety of designs to be implemented. In an alternative approach, the gate is mechanically pre-biased to remain closed. The bias is then overcome electrostatically to actuate the gate.  
         [0033]     Referring to  FIG. 6 , in an alternative preferred embodiment, a pair of ion-selective membranes  36  and  38  that permit the flow of positive ions from electrode surface  20  in a direction toward the tip of the electrode  22 , while simultaneously allowing the flow of negative ions from electrode  14  and surrounding tissue. In an additional alternative preferred embodiment, shown in  FIG. 7 , a magnet steers the electrolytic current to selectively connect electrode  20  with electrode  14  or tip  22 . When the electrolytic current changes its direction from the electrode, it is steered by the magnetic field so that positive current flows into the scala media and negative current flows to the refresh electrode. The interaction of DC currents with DC magnetic fields causes this effect. In yet another preferred embodiment, shown in  FIG. 8 , a primary electrode  20 ′ is rotatable, so that a first face  62  can be refreshed while a second face  64  is actively injecting current into the scala media.  
         [0034]     Electrode  20  (or  20 ′) is capable of passing a current of 10 μA for a duration of 3-6 sec through tip  22  and into the scala media. Scientific investigation has indicated that during the 3-6 second refresh periods for electrode  20 , the potential across the basilar membrane will persist. Referring to  FIG. 9 , an additional preferred embodiment of a charge injection assembly  90  permits a continuous injection of charge into the scala media, analogous to full-wave rectification. Patients that have a damaged scala media, which is less capable of storing charge, may prefer this embodiment. Assembly  90  includes a pair of charge injection units  106  and  108 , which are toggled in their active states by an electrode driver and switch control assembly  28  controlling ion selective membranes  36  and  38  to maintain a continuous charge injection. Units  106  and  108  include a pair of driving electrodes  120  and  122  respectively, and a pair of tips  124  and  126  respectively. One or more refresh electrodes  130  are used to maintain electrodes  120  and  122 , so that an injection of charge into the scala media can be continuously maintained, by switching between tips  124  and  126 . In an alternative embodiment, the duty factor of the charge injection is increased, but is still not continuous.  
         [0035]     Referring to  FIG. 10 , an alternative embodiment of an assembly  104  is conceptually the same as assembly  90  except for that instead of ion selective membranes  36  and  38  a pair of MEMS switches  130  and  132  are used for alternately occluding unit  106  and  108 .  
         [0036]     For any of the above described embodiments, the current driver and switch control assembly  28  is sized to drive a maximum current of 5-30 μA in either direction. In one preferred embodiment, in which the resistance of unit  12  is 1 MΩ, the driver is designed to remain linear over a range of at least ±30 volts. In another preferred embodiment, the dimensions of unit  12  are altered so as to reduce the resistance of unit  12 . In another preferred embodiment the voltage level of the fluid of the scala media is measured and used to regulate the amount of current injected. It is noted that a large peak voltage has the potential for causing damage to body tissue and should generally be avoided.  
         [0037]      FIG. 11  shows the logic of assemblies  90 ,  104  and  210  (see below), where i(t) is the current applied from the current generator, and the other graphs in the sketch of the logic show the positions of the MEMS switches. Note that the current drive is discontinuous and that the time that the drive is applied during each half cycle is less than the total time of a half cycle. Current is delayed at the beginning of each half cycle to ensure that the MEMS gates are properly opened and closed before current flows through the system. Current is shut off prior to the end of each half cycle to ensure that no current will be driven during the time that the MEMS gates close. In summary, while current is unidirectional (injected) into the scala media, it is not true DC, but is interrupted.  
         [0038]     One problem encountered with the use of the systems described above is that they may permit sodium ions from the body tissue outside the scala media to corrupt the scala media fluid, which is rich in potassium ions. Likewise, potassium ions from the scala media may migrate into and damage body tissue.  
         [0039]      FIGS. 12 and 13  show a charge injection assembly  210  designed to overcome the problem that is outlined in the paragraph above. The assembly  210  is modified to be fully closed and isolated from the tissue, save through a pair of valves  236  leading into the scala media. KCl is confined to the assembly  210  and to the scala media, where it is found naturally. A third metallic electrode  230  is contained in the KCl-filled electrode assembly. That third electrode is connected by a metallic conductor  240  to a fourth electrode  250 , which is embedded in the sodium-rich tissues that are external to the scala media via a fourth. This design contains the potassium-rich solutions in tissues where potassium is the normally the dominant ion. It provides a return path for the two active electrodes  220  and  222 , by way of valves  238 .  
         [0040]      FIG. 12  shows the implementation of assembly  210  with current flowing from electrode  220 , via the scala media and external tissue, through the external electrode  230  and thence to the right-hand assembly electrode  222 , which is negatively charged.  FIG. 13  reverses the process.  
         [0041]     Since current is not driven with a 100% duty cycle, as is described in the text associated with  FIG. 11 . The absence of current for a portion of the time, permits the internal electrode  230  and external electrode  250  to depolarize relative to each other.  
         [0042]     An alternative embodiment is shown in  FIG. 14 . As shown, current source  312  is injecting current into the scala media by way of electrode  314  and micropipette  316 . At the same time, electrode  318  is being refreshed by drawing electrolytic current in from an electrode  320 , which is electrically connected to a temporalis muscle-implanted electrode  324 . Alternating with the phase shown is a phase in which all of the switches are moved to their other polarities, electrode  314  is refreshed by electrolytic current originating at electrode  322  and electrode  318  injects current into the scala media. MEMS valves  326  and  328  are alternatively opened and closed, placing electrode  312  and then electrode  318  into electrolytic contact with the scala media in alternating sequence.  
         [0043]      FIGS. 15A and 15B  show a half wave rectifying charge injector  410 , in which an electrode  412  placed on a slidable boom  414  is slid into a reservoir  416  of saline solution in order to drive a charge injector electrode  418 . On alternating phases, electrode  412  is slid into a reservoir of KCl that is in fluid communication with charge injector electrode  418 , for the purpose of refreshing electrode  418 . During both phases, current source  420  drives electrodes  412  and  418 . Boom  414  may be moved by a nitinol wire, cilliary actuator arrays or gas actuation using either heated gases or electrolytically generated gases.  
         [0044]     Referring to  FIGS. 16   a  and  16   b , an alternative preferred embodiment of a current injection device  510 , similar to the embodiment of  FIGS. 15   a  and  15   b , has a NaCl reservoir  512  and a KCl reservoir  514 . The KCl reservoir  514  is connected to the scala media  515  by a passageway  516  also filled with water bearing KCl ions. Passageway  516  is selectively closeable by way of a valve  526 . The KCl reservoir is also electrically connected to NaCl bearing body tissue  518  by way of a passageway  520  filled with water bearing KCl ions, but that is blocked to fluid movement by way of a frit  522 , which is electrically conductive. In an alternative embodiment, passageway  520  is so long and thin as to prevent a harmful level of ion transfer.  
         [0045]     A valve  528  controls the electrolytic connection between KCl reservoir  514  and passageway  520 . A natural barrier  530  of body tissue prevents any harmful level of ion transfer between NaCl bearing tissue  518  and the KCl fluid fed in NaCl reservoir  512 . A current source  540  may be controlled to create current from refresh electrode  536  to active electrode  534  or vice versa.  
         [0046]     A controller (not shown) either places device  510  into a current injection mode ( FIG. 16A ), in which current is injected into the scala media or an active electrode refresh mode. In injection mode, the current source  560  sends electric current from refresh electrode  536  to the active electrode  534 . The circuit is completed by opening valve  526  thereby placing KCl reservoir  514  into contact with the scala media. Consequently the electric current flow from refresh electrode  536  to active electrode  534  is balanced by electrolytic current flows from KCl reservoir  514  to the scala media  515  and from NaCl tissue  518  to NaCl reservoir  512 . The circuit is completed by a movement of electrical charge through barrier  530 , which is somewhat electrically conductive.  
         [0047]     In refresh mode the current source  560  is reversed so that electric current flows from active electrode  534  to refresh electrode  536 . In this mode, also, valve  526  is closed and valve  528  is opened so that electrolytic current flows from glass frit  522  to active electrode  534 , thereby refreshing electrode  534 . Electrolytic current flows from NaCl reservoir  512  to NaCl tissue  518  and through a portion of passageway  522  to glass frit  522 . Electric current passes through glass frit  522 , completing the circuit.  
         [0048]     An alternative preferred embodiment is schematically very similar to the embodiment of  FIG. 6  but without tube  16  or valve  36 , and having two further innovations. First, the active electrode  20  and the counter or refresh electrode  16  are both expanded in surface area, to have a surface area of greater than 1 cm 2  and in one preferred embodiment in the range 10-100 cm 2  or greater. This can be accomplished using technology similar to that employed in the production of batteries and/or capacitors, in which foil is wrapped about itself or a set of conductive plates are joined together in close proximity to one another.  
         [0049]     In this alternative embodiment, also, the frequency of charge injection and refresh could be greatly slowed down, with the object of starting to inject charge slightly before the patient awakens and for the subsequent ten hours, so that during the waking day the patient has a proper voltage gradient across the hair cells. Then, at night time the refresh cycle could occur, when the patient is not in as great need of keen hearing. For this to work properly it is desirable to form electrodes  14  and  20  from a material that has a high (&gt;25 mC/cm 2 ) charge storage capacity, such as iridium oxide film, known in the industry as “IROF.” 
         [0050]     The terms and expressions which have been employed in the foregoing specification are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.