WO2009023919A1

WO2009023919A1 - Improvements to implantable microphones

Info

Publication number: WO2009023919A1
Application number: PCT/AU2008/001216
Authority: WO
Inventors: John Chambers
Original assignee: Cochlear Limited
Priority date: 2007-08-20
Filing date: 2008-08-20
Publication date: 2009-02-26

Abstract

A structure is disclosed for attenuating tissue vibration in implanted microphones. The structure (20) includes in one form opposed faces defining a cavity, the structure providing poor impedance matching and hence good attenuation of tissue vibrations.

Description

IMPROVEMENTS TO IMPLANTABLE MICROPHONES

FIELD OF THE INVENTION

The present invention relates to implantable microphones, for example for hearing prostheses, and more particularly to devices and systems incorporating such devices. BACKGROUND TO THE INVENTION

For many forms of hearing prostheses, an important development is the introduction of implantable microphones. This has been proposed in relation to various such devices, including middle ear implants, hearing aids and cochlear implants.

Cochlear implant systems are routinely used to treat profound hearing loss. While current systems incorporate external microphones and other parts, fully implanted systems are more appealing to the user as they not only offer cosmetic invisibility, they allow the user to hear when bathing and engaged in water sports.

Unfortunately, sound reception from a microphone within the body is severely degraded, because differences in the density of human body tissue and air cause much of the acoustic energy arriving from everyday sources to reflect from the skin. Transmission loss in intervening body tissue further reduces the resulting signal that reaches the implanted microphone. As a consequence, the level of the received signal is comparable or lower in amplitude than sound arising from body noise. Body noise includes a variety of internal noises which are propagated through the body, including breathing, chewing, heart activity, head scratching, clothing movement and muscle induced hair and skin movement. The inherent pressure sensitive characteristics of any implanted microphone renders it sensitive to the inertial reaction force of adjacent body tissue. This may occur when either the microphone or the body tissue vibrate independently under the effect of the described body noise. The subsequent degradation to the user's hearing experience due to body noise has been reported by users as intrusive and has been shown to reduce listening and conversation abilities. At the usual location for an implanted microphone, body noise is conveyed to the implanted microphone via the cranial bone mass, causing the housing of the implant containing the microphone to vibrate.

Prior art figure 1 illustrates schematically a typical implanted cochlear implant device. Housing 1 is an hermetically sealed titanium metal implant housing covered with a thin layer of biocompatible elastomer material. Coil 2 can be seen extending from one side of housing 1 , and a cable 3 from the other side.

The cable is connected to an electrode array 4.

Prior art figure 2 illustrates the mechanical situation of the housing 1. The housing is disposed adjacent to the cranial bone mass 12, facing outward through a layer of soft body tissue 11. A diaphragm 8, typically less than 100 μm in thickness is provided to detect sounds, which are transmitted to the microphone

6. Microphone 6 may be of the piezo-electric or electret type as is commonly employed by modem ear worn hearing aids. Sounds 10 initially propagate through the air, induce vibrations in the soft tissue 11, and in turn In diaphragm 8. This is the desired process. However, body noise vibrations cause diaphragm 8 to be accelerated back and forth so as to manifest a force on diaphragm 8, partly due to its own mass, but more dominantly from the forces applied to it as it reacts against the mass of the body tissue 11 immediately adjacent to the diaphragm 8.

These vibrations are illustrated as arrows 7. US patent publication No. 2005-0197524 discloses an implantable microphone modified to have reduced sensitivity to undesired vibration such as body noise. It discloses that the noise be attenuated by placing a compliant component, formed for example from an elastomeric material, in the path of the undesired vibration. It is an object of the present invention to provide an effective structure for reducing the impact of body noise on implanted microphones.

SUMMARY OF THE INVENTION

In a broad form, the present invention provides a body noise attenuation device, formed from a generally rigid shell having a void therein, and having compliant members in the shell to allow relative movement of the upper and lower surfaces of the shell.

According to one aspect, the present invention provides a vibration attenuating structure for an implantable microphone, the microphone being mounted in a housing, the structure including relatively rigid opposed faces and a flexible coupling between the faces, so as to define a sealed cavity, the cavity containing a compressible medium, wherein the structure is sized and operatively adapted to be positioned between the housing and an body structure. This simple mechanical structure allows for an effective attenuation of the body noise transmitted via bone or other tissue, whilst having a simple and robust construction.

According to another aspect, the present invention provides an implantable microphone assembly, including an implantable microphone, a housing for the microphone, and a vibration attenuating structure, the structure including relatively rigid opposed faces and a flexible coupling between the faces, so as to define a sealed cavity, the cavity containing a compressible medium, wherein the structure is affixed to the housing so as to operatively be positioned between a body structure and the housing. According to a further aspect, the present invention provides a method of reducing body noise in a an implantable microphone, wherein a vibration attenuating structure is disposed between a housing for the microphone, the structure including relatively rigid opposed faces and a flexible coupling between the faces, so as to define a sealed cavity. According to yet another aspect, the present invention provides an implantable microphone assembly, including an Implantable microphone, a housing for the microphone, and a vibration attenuating structure, the structure including a relatively rigid face that is attached via a flexible coupling to the implant housing so as to define a sealed cavity, the cavity containing a vaccum or compressible medium as well the electronic and other internal components of the implant, wherein the structure is affixed to the housing so as to operatively be positioned between a body structure and the housing.

BRIEF DESCRIPTION OF THE DRAWINGS Implementations of the present invention will now be described with reference to the drawings, in which:

Figure 1 is an overall schematic view of a prior art device; Figure 2 is a schematic view, partly in section, illustrating certain mechanical issues of implanted microphones;

Figure 3 is a schematic view of one embodiment of the present invention;

Figure 4 is a view, partly in section, of the embodiment of figure 3; Figure 4A is an alternative configuration to that shown in Figure 4 whereby the implant housing is shown partially incorporated in or recessed within the periphery of an implementation of the present invention;

Figure 4B is another alternative configuration to that shown in Figure 4:

Figure 5 is a schematic view of another embodiment of the present invention;

Figure 6 is a view, partly in section, of the embodiment of figure 5;

Figure 7 is a view, partly in section, of another embodiment of the invention having a similar form to that shown in Figure 11 ;

Figure 8 illustrates movement of the embodiment of figure 7; and Figure 9 illustrates a further alternative embodiment of the present invention;

Figure 10 illustrates two embodiments of the invention that incorporate a perforated dividing member to dampen or control mechanical resonance;

Figure 11 Illustrates one simple implementation of the present invention; and

Figure 12 shows a graph of vibration sensitivity for the device shown in figure 11. DESCRIPTION OF PREFERRED EMBODIMENT

Specific implementations of the present invention will now be described with reference to an application of such a microphone, being for a totally implanted cochlear implant. It will be understood, however, that the inventive structure, and device incorporating or using it, may be employed for any application for an implanted or implantable microphone. The microphone may be part of an existing device, or a free standing component remote from the implanted device.

One implementation of the invention, as shown in figure 3, is a vibration attenuating structure 20 which is operatively deployed between the implant and cranial bone so as to reduce the effect of cranial bone vibrations on the housing of the implant, and more particularly on any microphone in the implant.

In a simple form, as shown in figure 4 for example, an implementation may consist of a hollow cavity member 20, having opposing faces 21, 22 connected via a flexible coupling 24. The cavity 23 may be filled with a compressible medium such as air, or an inert or low atomic weight gas, for example. To further improve effectiveness, it is preferred that the gas is at a lower than atmospheric pressure.

While the incorporation of a partial or complete vacuum within the cavity member 20 improves the effectiveness of the vibration shield and alleviates thermal expansion effects, such an improvement necessitates the use of increased material thickness to preserve the integrity of the structure against the compressive loading of normal atmospheric pressure. Since this adds undesirably to the total weight of the implant any optimisation of cavity pressure and material thickness must therefore take account of the actual form, size and weight of the implant as a whole.

Cranial bone vibration applied to one side of the shield is poorly conveyed across the gas filled cavity. This is because of poor mechanical impedance matching, similar to that which occurs when airborne sound is transmitted from air into skin, as previously described. The flexible join or connection 24 between the opposing faces of the shield reduces the vibration forces transmitted via cranial bone 12 and the lower face 22 to the upper face 21 nearest the implant 1. Whilst in vivo measurements of bone conducted vibration show that the flexible coupling must allow the opposing faces to move independently by more than 100 nm in order to accommodate lower frequency vibrations, compression and expansion travel distances of around 1 mm are necessary to accommodate other forces including thermal expansion and atmospheric pressure changes such as those encountered during air travel.

The joint can be a concertina type structure as shown, or any other mechanical structure which similarly exhibits the necessary degree of compliance such that the forces which produce a vibratory displacement of one face are substantially prevented from being applied to the opposing face. It is important that the structure be hermetically sealed, so that spaces are not created for infection to develop. The joint is also configured to provide sufficient freedom for opposing faces to move back and forth independently and without contact over the full range of displacement likely to be Imposed on the structure by the audio frequency vibrations of nearby cranial bone. Being gas filled, the structure according to this implementation must not only respond to cranial bone vibration in the manner described, it must do so while expanding and contracting in response to cyclic and abnormal body temperature and pressure changes. Thermal expansion and contraction must be accommodated in conjunction with pressure changes arising from, for example. locally applied force and acceleration, and air pressure changes resulting from meteorological conditions and environmental changes associated with air conditioning, air and high-speed rail transportation and water immersion.

This joint and the associated shield structure must remain functional and hermetic during the envisaged 5 to 70 year lifetime of the prosthesis. While a longer life span is of course desired, current rechargeable battery technology currently limits this to the life span quoted. Of course, the present invention is applicable whatever the lifetime.

The upper and lower faces at least should be formed from a relatively rigid material, for example titanium. The material will of course also need to be biocompatible. The cavity member 20 is preferably a separately constructed component, affixed to the upper face 21 by welding or the like, in a flush fashion without the creation of cavities or inclusions. Accordingly this implementation acts to shield the implant and its microphone from much of the reported cranial bone conducted body noise. Whilst several specific examples of joint structures will be discussed, it will be understood that the functional requirement of this structure is to provide the necessary movement between the upper and lower faces. Any structure which achieves this may be used. Whilst a structure formed from a single material is preferred, a different materials may be used for different parts of the structure, for example for the joint portion. Laser, electric arc or resistance welding of the titanium metal components of the vibration shield as well as its attachment to the titanium metal implant housing must be undertaken in the absence of air so as to avoid the weakening and damaging effects of metal oxidation which arises when chemically reactive titanium is heated in an oxygen rich atmosphere such as air.

Figure 4A is an alternative configuration to that shown in Figure 4, whereby the implant housing is shown partially incorporated in or recessed within the periphery of a vibration attenuating structure according an implementation of the present invention so as to provide improved volumetric efficiency of the combined implant and invention as a whole. This configuration also has the potential to improve the vibration shielding effect of the invention since it provides more space to accommodate a larger and more flexible concertina type coupling structure.

Figure 4B is a another alternative configuration to that shown in Figure 4, where the bulkhead or separating member 21 in Figure 4 has been partially or fully removed so as to increase the effective vibration shielding or de-coupling volume of the void 23 in Figure 4 to that indicated as 21 B in Figure 4B. In effect, the housing provides the faces, and has the joint structure included with it. This configuration is especially beneficial since it increases the effectiveness of the vibration shield while adding little to the overall volume of the entire implant

Various alternative configurations of vibration shield can be applied. One option is to extend the structure beyond the profile of the implant housing. Another option is to deploy the structure separately to the implant housing.

Anti-inflammatory drug or other substance elutiπg materials applied to the surfaces of the implant case or shield can be used to modify the fibrotic, fat and fluid content, or other characteristic of nearby body tissue to preferentially favour the propagation of wanted sounds and or to preferentially disfavour, the propagation of unwanted vibration. This is achieved for example; by taking advantage of the characteristic that shear wave propagation is less across adipose fatty tissue than it is across densely fibrous and inflexible, scar tissue.

Figures 5 and 6 depict another alternative implementation. In this arrangement, otherwise similar to that shown in figures 3 and 4, the width and breadth of structure 20 has been increased, in order to further increase the effectiveness of the shielding effect.

A further implementation is illustrated in figures 7 and 8. In this arrangement, an alternative structure is used to provide the required flexible joint between the upper and lower faces 21 , 22. Increased vibration shielding is achieved using this configuration as vibration forces applied to the lower surface 22 act upon the flexible joints 26, 27 in a more pivotal that compressive manner, thereby increasing the flexibility of the structure 20. Figure 9 illustrates an alternate configuration whereby the vibration shield is surgically deployed in such a manner as to allow a layer of compliant body tissue 29 to exist between the shield and the housing of the implant. Small flexible pedestals or spacing members 28, constructed from a biocompatible material such as silicone elastomer, maintain a minimum separation distance between the shield and implant so as to ensure the viability of the intervening body tissue.

Figure 10 illustrates two examples of a vibration shield 29 that incorporate a dividing member 30. One or more small holes or perforations 31 are incorporated into the dividing member so as to provide a frictional flow loss for the medium that fills the interior of the vibration shield, and which flows back and fourth from one side of this dividing member to the other due to applied vibration force. The incorporation of this structure or any other form of frictional loss acts to dampen mechanical resonances that would otherwise arise.

Figure 11 illustrates a simple implementation of the present invention. In structure, it is very similar to the implementation shown in figure 7. Figure 12 shows the change in the vibration sensitivity of an implantable microphone shown in figure 11 , when the microphone was immersed in 10 mm of water and the device shown in figure 11 is located between this microphone and a source of applied vibration. The plotted 20 to 40 dB reduction in vibration sensitivity represents a reduction factor of 10 to 100 times, demonstrating the effectiveness of an example of the invention within the audio frequency range of interest that is dominated by typical body noise.

It will be appreciated that many other variations and additions are possible within the general inventive concept, as will be apparent to those skilled in the art, and that many alternative constructions are possible. For example, whilst the examples have illustrated a structure having two faces, more face components could be used to achieve the same objectives, with differently configured joints.

Claims

CLAIMS:

1. A vibration attenuating structure for an implantable microphone, the microphone being mounted in a housing, the structure including relatively rigid faces and a flexible coupling between the faces, so as to define a sealed cavity, wherein the structure is sized and operatively adapted to be positioned between the housing and an body structure.

2. A structure according to claim 1 , wherein the structure is operatively affixed to the housing.

3. A structure according to claim 2, wherein the structure forms the enclosure for an implantable device.

4 A structure according to claim 1 , wherein the flexible coupling is a corrugated structure.

5. A structure according to claim 1, wherein the flexible coupling is a concertina structure.

6. A structure according to claim 1 , wherein the structure has a larger area than the corresponding surface of the housing.

7. A structure according to claim 1, wherein the cavity contains a vacuum or a compressible medium.

8. An implantable microphone assembly, including an implantable microphone, a housing for the microphone, and a vibration attenuating structure, the structure Including relatively rigid faces and a flexible coupling between the faces, so as to define a sealed cavity, the cavity containing a compressible medium, wherein the structure is affixed to the housing so as to operatively be positioned between a body structure and the housing.

9. An assembly according to claim 8, wherein the assembly forms the enclosure for an implantable device.

10. An implantable microphone assembly according to claim 8, wherein the flexible coupling is a concertina structure.

11. An implantable microphone assembly according to claim 9, wherein the flexible coupling is a concentric corrugated structure.

12. An implantable microphone assembly according to claim 6, wherein the structure has a larger area than the corresponding surface of the housing.

13 An implantable microphone assembly, including an implantable microphone, a housing for the microphone, and a vibration attenuating structure, the structure including a relatively rigid face that is attached via a flexible coupling to an enclosure for an implantable device, so as to define a sealed cavity including at least a part of the interior of the implantable device, the cavity being operatively positioned between a body structure and the housing.

14. A method of reducing body noise in a an implantable microphone, wherein a vibration attenuating structure is disposed between a housing for the microphone, the structure including relatively rigid faces and a flexible coupling between the faces, so as to define a sealed cavity.