Abstract:
Methods for filling transducers of a fully implantable hearing aid system with liquids having either a high or a low vapor pressure are described. Methods are also described for avoiding damage to transducers during their testing and shipment.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This patent application is a continuation-in-part of U.S. patent application Ser. No. 09/688,292 filed Oct. 13, 2000. 
     CLAIM OF PROVISIONAL APPLICATION RIGHTS 
     The patent application of which this is a continuation-in-part that is identified in the immediately preceding paragraph, and this continuation-in-part patent application both claim the benefit of U.S. Provisional Patent Application No. 60/159,154 filed on Oct. 13, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods employed for successfully fabricating, testing and transporting a minute transducer that includes a liquid filled chamber which has thin membranes forming portions of the chamber&#39;s wall. 
     2. Description of the Prior Art 
     U.S. Pat. No. 5,772,575 (“the &#39;575 patent”) that issued Jun. 30, 1998, on a patent application filed by S. George Lesinski and Armand P. Neukermans describes transducers used for a fully implantable hearing aid system. As disclosed in the &#39;575 patent, the fully implantable hearing aid system&#39;s transducer cannot exceed 1.2 mm in diameter, and includes a hollow chamber having two (2) walls that are formed by thin, flexible membranes. 
     The hollow chamber within these implantable transducers must be completely filled with a liquid material while concurrently excluding gas bubbles (except as such bubbles may be deliberately placed within the hollow chamber to provide a special low pass acoustic filter). In general, the presence of even a small bubble inside the hollow chamber may render the transducer ineffective because it prevents efficient liquid displacement transfer from a first thin membrane, i.e. an input membrane, to a second thin membrane, i.e. an output membrane. Because small orifices and complicated shapes are involved in fabricating an implantable transducer that cannot have a diameter greater than 1.2 mm, and because the surface to volume ratio within the hollow chamber is large, extremely small bubbles may be easily trapped within the hollow chamber during filling. 
     In addition to technical difficulties associated with bubble free filling of implantable transducers as summarized above, the thin membranes combined with the liquid within the transducer&#39;s hollow chamber presents additional technical difficulties while testing the transducers, and while they are being shipped. For example, the United States Food and Drug Administration (“FDA”) requires that class III type implantable devices be tested at an elevated temperature to accelerate failure of defective devices. Heating a liquid filled implantable transducer may cause the liquid within the hollow chamber to expand more rapidly than the rest of the transducer. Consequently, heating an implantable transducer that is filled with water surely increases the pressure of liquid within the hollow chamber, and may rupture or plastically deform one or both of the membranes. Similarly, a possibility exists that unprotected implantable transducers may be damaged during shipping if they are exposed to extreme temperatures. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide methods for filling transducers of a fully implantable hearing aid system with liquid regardless of the liquid&#39;s vapor pressure. 
     Another object of the present invention is to provide a method that prevents damaging liquid filled transducers of a fully implantable hearing aid system while testing them at elevated temperatures. 
     Another object of the present invention is to provide methods that prevent damaging liquid filled transducers of a fully implantable hearing aid system while they are being transported. 
     The transducer of the fully implantable hearing aid system has a body: 
     1. which surrounds the hollow chamber and includes at least one thin membrane that forms at least a portion of a wall of the hollow chamber; and 
     2. at least one passage having: 
     a. an exit that couples the passage to the hollow chamber; and 
     b. an entrance that is located distal from the hollow chamber for communicating via the passage with the hollow chamber from outside the implantable transducer, 
     Bubble free filling of the hollow chamber with a high vapor pressure liquid is effected by first establishing a seal around the entrance of the passage. Then, while protecting the thin membrane from damage, evacuating the hollow chamber through the passage by applying vacuum to the entrance thereof. After the hollow chamber has been evacuated, bubble free filling of the hollow chamber with a high vapor pressure liquid is effected by: 
     1. cooling the body of the implantable transducer to establish a temperature gradient therealong at least a portion of which has a temperature that is below a dew point of the high vapor pressure liquid; while concurrently 
     2. introducing into the entrance of the passage a vapor of the liquid. 
     Thus, upon condensation of the vapor within the hollow chamber of the implantable transducer, the hollow chamber becomes filled with the liquid without creating a bubble within the hollow chamber. 
     These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are schematic diagrams that respectively illustrate an arrangement for bubble-free filling with low vapor pressure liquid a hollow chamber within an implantable transducer. 
     FIGS. 2A and 2B are schematic diagrams that respectively illustrate an arrangement for individually filling chambers with a high vapor pressure liquid which uses vapor condensation with alternative techniques for protecting a thin membrane during evacuation of the implantable transducer&#39;s chamber. 
     FIG. 3 is a schematic diagram that illustrates an implantable transducer which includes supports located within the hollow chamber within the implantable transducer for supporting the thin membrane during evacuation of the chamber. 
     FIG. 4 is a schematic diagram that illustrates a controlled pressure oven that encloses an implantable transducer. 
     FIG. 5 is a schematic diagram that illustrates an arrangement suitable for safely transporting implantable transducers. 
     FIG. 6 is a schematic diagram that illustrates an alternative arrangement for protecting the implantable transducer&#39;s thin membrane during transportation. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1A illustrates a vacuum chamber  10  which encloses several implantable transducers  12 . Each of these implantable transducers  12  respectively includes a body  14  that has a thin input membrane  16  and output membrane  18  which are located on opposite sides of a hollow chamber  22 . A hollow filling tube  24  depends beneath each of the bodies  14 , and a hollow appendage  26 , which is long and closed at an end  28  furthest from the body  14 , project upward above each of the bodies  14 . The filling tubes  24  and the appendages  26  may be formed from metal tubes which are readily swaged to hermetically seal the chamber  22 . The ends  28  of the appendages  26  are at the highest points within the vacuum chamber  10 . The vacuum chamber  10  also encloses a container  32  that holds a low vapor pressure liquid  34  (e.g. silicone oil) with which the chambers  22  of each of the implantable transducers  12  will be filled. 
     Initially, gas is evacuated from the vacuum chamber  10 . If necessary, the implantable transducers  12  may be heated with infrared red lamps to purge moisture from the chambers  22 , the filling tubes  24  and the appendages  26 . This procedure also outgasses the liquid  34  in the container  32 , such that it can, in principle, absorb small amounts of gas later upon cooling. The elongated filling tubes  24  and appendages  26  may require a substantial out-gassing interval to remove all adsorbed gases. The pressure within the vacuum chamber  10  should preferably approach 10 −5  Torr. 
     Preferably the chamber  22  within the implantable transducer  12  is configured to avoid places that will trap gas bubbles. That is, interior surfaces of the implantable transducers  12  are configured such that entering liquid  34  entirely sweeps any gas remaining within the implantable transducer  12  in front of the liquid  34 , without trapping gas in any cavities. Interior surfaces of the implantable transducers  12  are polished to the maximum extent possible in an attempt to eliminate bubble formation. Before filling the chamber  22  with the liquid  34 , vapor of a surfactant material may be introduced into the vacuum chamber  10  to coat interior surfaces of the implantable transducers  12  everywhere thereby lowering the surface tension of the liquid  34  when it enters the implantable transducers  12 . 
     The container  32  is then tipped so the liquid  34  spills out to submerge open ends  38  of the filling tubes  24 . The vacuum within the vacuum chamber  10  is then very slowly reduced preferably by admitting into the vacuum chamber  10  a gas that is poorly absorbed by the liquid  34 . A difference thus established between the pressure within the vacuum chamber  10  and that within the implantable transducers  12  drives the liquid  34  up the filling tubes  24  to fill the chambers  22  of the implantable transducers  12 . The pressure within each implantable transducer  12  is initially extremely low, and gradually increases as the liquid  34  intrudes thereinto. The liquid drives any residual gas within the implantable transducer  12  into the top of the appendage  26 . Therefore it is advantageous to raise the pressure within the vacuum chamber  10  very slowly. 
     During filling of the chamber  22  with the liquid  34 , any entrapped bubbles, in areas not swept clean by the entering liquid, will be markedly compressed in the ratio of the pressure within the vacuum chamber  10  when filling begins to the pressure when filling ends. The maximum pressure that needs to be applied to fill the appendages  26  to their top approaches the hydrostatic pressure due to the height of the liquid  34  within the implantable transducers  12  above the surface of the liquid  34  at the bottom of the vacuum chamber  10 . Since for the minute implantable transducers  12  this height is approximately 1 cm, any bubbles will be compressed volumetrically in the ratio of about 760, and will be very small when the final pressure within the vacuum chamber  10  reaches 1 atmosphere. Any residual gas remaining within the implantable transducers  12  that does not form a bubble should collect at the top of the appendages  26  as illustrated in FIG.  1 B. This section of each of the appendages  26  may be swaged off and also the filling tubes  24 . In this way each implantable transducer  12  becomes hermetically sealed and virtually gas free. 
     If the liquid  34  for filling the implantable transducer  12  has a high vapor pressure, e.g. water, alcohol, etc., the method described in connection with FIGS. 1A and 1B is unsuitable for filling the implantable transducer  12 . FIG. 2A illustrates schematically an arrangement that is suitable for filling the implantable transducer  12  with a high vapor pressure liquid  34 . Similar to FIGS. 1A and 1B, the implantable transducer  12  illustrated in FIG. 2A includes a thin input membrane  16  and output membrane  18 , and may also include a transducer not illustrated in FIGS. 2A and 2B. The filling tube  24  of the implantable transducer  12 , which connects to a vacuum system  44 , includes an exit  42  located at the chamber  22  of the implantable transducer  12 . 
     A wall  46  surrounds the implantable transducer  12  together with an encircling O-ring  48  which seals with and establishes a vacuum tight enclosure around the implantable transducer  12 . A cooler  52 , also enclosed within the wall  46 , makes intimate thermal contact with the implantable transducer  12 . The cooler  52  may employ thermoelectric cooling or any other suitable technique. Preferably, the wall  46 , the implantable transducer  12  and the cooler  52  are arranged to establish a substantial temperature gradient along the implantable transducer  12 , with the portion of the implantable transducer  12  furthest from the exit  42  that contacts the cooler  52  being the coldest. 
     To fill the chamber  22  with high vapor pressure liquid  34 , first an entrance  54  of the filling tube  24  is inserted into a port  56  of the vacuum system  44  with an O-ring  58  sealing between the filling tube  24  and the port  56  so the chamber  22  can be evacuated. Concurrently, an approximately equal vacuum is applied to a chamber  59  that is located between the implantable transducer  12  and the surrounding wall  46  so pressures on opposite sides of the input membrane  16  and the output membrane  18  are equal while the chamber  22  is evacuated. Evacuation of the chamber  22  continues in this way until the pressure within the chamber  22  reaches approximately 10 −5  Torr. Upon reaching that low pressure, the composition of gases within the vacuum system  44  changes slightly by introducing into the vacuum system  44  a partial vapor pressure of a surfactant material. Because of the very low pressure, atoms of the surfactant diffuse into the chamber  22  quickly. In this way interior surfaces of the implantable transducer  12  become coated everywhere with surfactant to thereby lower the surface tension of the liquid  34  when it enters the implantable transducer  12  thus permitting the liquid  34  to readily wet interior surfaces of the implantable transducer  12 . 
     After the surfactant has diffused into the chamber  22 , the composition of the gases within the vacuum system  44  changes again upon introduction of a vapor of the high vapor pressure liquid  34  thereinto, and the pressure both within the vacuum system  44  and the wall  46  increases slightly, for example to a few Torr. The cooler  52  then lowers the temperature of the chamber  22 , preferably establishing the temperature gradient with the portion of the implantable transducer  12  furthest from the exit  42  that contacts the cooler  52  being the coldest. When the temperature within the chamber  22  drops below the dew point for the vapor pressure of the liquid  34  within the vacuum system  44 , the liquid  34  condenses inside of the implantable transducer  12 , preferably at the bottom. By gradually reducing the temperature of the implantable transducer  12  while maintaining it above the freezing temperature of the liquid  34 , and/or by increasing the vapor pressure of the liquid  34  within the vacuum system  44 , any vapor bubbles which may form within the chamber  22  condense into the liquid  34 . During filling of the implantable transducer  12 , temperatures everywhere else are maintained above the dew point established for vapor pressure of the liquid  34  within the vacuum system  44 . 
     FIG. 2B schematically illustrates a similar concept for filling the implantable transducer  12  with the liquid  34 . However, the configuration depicted in FIG. 2B omits the wall  46 . Instead, outside surfaces of the input membrane  16  and the output membrane  18  are covered with a quantity of a material  62  that adheres to them and that is easily removed, e.g. wax. The material  62  supports the input membrane  16  and the output membrane  18  during evacuation of the chamber  22 . Other than for omitting the wall  46  and adding the material  62 , the remainder of the procedure for filling the implantable transducer  12  illustrated in FIG. 2B is similar to that described above in connection with FIG.  2 A. 
     As illustrated in FIG. 3, some configurations for the implantable transducer  12  permit supporting the input membrane  16  during evacuation of the chamber  22  by disposing supports  72  adjacent thereto. The implantable transducer  12  illustrated in FIG. 3 is preferably fabricated from a silicon wafer  74  with etched supports  72  formed on an interior surface of the body  14  at numerous small points to support the input membrane  16  during evacuation. In the assembled implantable transducer  12 , the supports  72  are located near to but not contacting the input membrane  16 . In the configuration of the implantable transducer  12  depicted in FIG. 3, the input membrane  16  carries a transducer  76  on a surface thereof which is furthest from the supports  72 . A tube  78 , included in the implantable transducer  12  and preferably made from Ti, projects outward from the wafer  74 . The output membrane  18  seals the end of the tube  78  furthest from the input membrane  16 . 
     During evacuation of the implantable transducer  12 , the supports  72  restrict inward deflection of input membrane  16  to a few microns. Restricting inward deflection of the input membrane  16  to this small amount provides sufficient deflection of the input membrane  16  for normal operation of the implantable transducer  12 , while also adequately protecting the input membrane  16  and the transducer  76  from damage during vacuum filling of the implantable transducer  12  with the liquid  34 . 
     Accelerated life-testing mandated by the FDA for class III type implantable devices requires raising the device&#39;s temperature to stimulate accelerated device failure. For the implantable transducer  12  filled with the liquid  34 , elevated temperature presents a problem since volumetric expansion of a liquid  34  such as water is 200 PPM versus 25-30 PPM for a body made of titanium such as the tube  78  illustrated in FIG.  3 . Consequently, raising the temperature of a water filled implantable transducer  12  increases the pressure within the chamber  22 , and may rupture or plastically deform the input membrane  16  or the output membrane  18 . To prevent damaging the implantable transducers  12  in this way, as illustrated in FIG. 4 the implantable transducer  12  may be enclosed in a pressure oven  82  so the pressure outside the implantable transducer  12  increases in the same manner as pressure within the implantable transducer  12  rises. 
     Likewise, during transportation the implantable transducer  12  may be exposed to elevated storage temperatures that might damage it due to the expansion of the liquid  34  within the implantable transducer  12 . As illustrated in FIG. 5, to prevent damage during transportation the implantable transducers  12  may be enclosed in a can  86  which is made from a material (e.g. titanium) that has the same relative expansion as the implantable transducer  12 . The can  86  is completely filled with liquid (possibly the same liquid as that filling the implantable transducer  12 ). Hence, as the temperature of the can  86  changes pressures inside and outside of the implantable transducers  12  remain in equilibrium thereby preventing damage to the implantable transducers  12  within the can  86 . 
     Alternatively, as illustrated in FIG. 6 an end of the implantable transducer  12  carrying the output membrane  18  may be covered by a protective cover  92 . An O-ring  94  seals the cover  92  to the tube  78 , and sealed space between the tube  78  and the cover  92  is pressurized to prevent inadvertently damaging the implantable transducer  12  during transportation. 
     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is purely illustrative and is not to be interpreted as limiting. Consequently, without departing from the spirit and scope of the invention, various alterations, modifications, and/or alternative applications of the invention will, no doubt, be suggested to those skilled in the art after having read the preceding disclosure. Accordingly, it is intended that the following claims be interpreted as encompassing all alterations, modifications, or alternative applications as fall within the true spirit and scope of the invention.