Patent Application
20120165597
The present invention relates to methods and apparatuses for implantable microphones in particular microphones using piezoelectric polymer film technology, which may be used as part of hearing aid systems.
In many implantable hearing aid systems, much of, if not all of, the components of the system are positioned subcutaneously on, within or adjacent to a patient's skull, such as proximate to the mastoid process. Depending on whether some or all of the components are implanted, implantable hearing augmentation systems may be classified as either semi-implantable or fully implantable. In a semi-implantable hearing augmentation device system, one or more components of the system such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully-implantable hearing aid system, typically all of the components, e.g., the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient's auditory system (e.g., tympanic membrane, ossicles and/or cochlea).
A fully implantable hearing aid system, such as those used to stimulate the tympanic membrane, the ossicles or the cochlea have inherent advantages over traditional hearing aid systems and semi-implantable hearing aid systems because a fully implantable system is completely unobservable, eliminating the appearance of a handicap; it does not occlude the ear canal, eliminating comfort/incompatibility issues and improving low frequency sound perception for those with partial hearing loss; and it allows use in environments or activities incompatible with traditional hearing aids. Enablement of a fully implantable hearing aid system requires an implantable microphone with suitable performance.
Implantable microphones described in the art for use with implantable hearing aid systems generally employ an air-conduction type electret microphone encapsulated in a biocompatible housing with a membrane that defines an air chamber. These microphones are installed subcutaneously just above and behind the ear (U.S. Pat. No. 6,626,822), within the bony wall of the auditory canal (U.S. Pat. No. 6,516,228) or at other locations in the soft tissue separated from skull-borne vibrations (U.S. Pat. No. 7,354,394). A thin layer of tissue covering the microphone acts as an extension of the microphone diaphragm and couples vibrations induced by external air pressure disturbances to the embedded microphone sensor. Signals detected by the microphone may be processed, amplified and sent to an implanted transducer for stimulation of the middle ear, tympanic membrane or to electrodes for stimulation of the auditory nerve.
implantable microphones that rely on conversion of air-pressure changes within a sealed cavity to stimulate an encapsulated electret-type microphone are concerned with cavity dimensions, enclosed air pressure and membrane stiffness to provide an acceptable tradeoff between resonance frequency and sensitivity of the device. Since an implantable microphone must necessarily be hermetically sealed, with an implantable electret-type microphone, internal pressure cannot be equalized to atmosphere, so the size of the cavity affects the restoring force on the diaphragm and therefore the microphone sensitivity. Similarly, a stiff diaphragm causes a higher resonance frequency, but lower sensitivity due to the forces needed to move the membrane.
An implantable microphone using piezoelectric polymer film such as polyvinylidene fluoride (“PVDF”) may overcome the limitations of electret-type implantable microphones because it is well suited for detecting sound-induced vibration in tissue (whether vibration of a thin diaphragm or vibrational waves propagating through tissue) due to its high piezoelectric voltage constant, g, which relates voltage to induced strain, its low mechanical impedance, which is well matched to tissue and its general robustness and mechanical stability. Additionally, with piezoelectric polymer film, vibration is directly converted to an electrical signal by the piezoelectric effect, in contrast to existing electret-type implantable microphones that rely on conversion of mechanical vibration to pressure changes in an enclosed air cavity for subsequent detection by an air-conduction microphone.
The present invention seeks to address the limitations of electret-type (air-conducting) microphones for use in implantable hearing aids systems by providing a piezoelectric polymer film microphone that serves as an integral part of a fully implantable hearing aid system, such as a middle ear implant or cochlear implant. The piezoelectric polymer film design allows for a small package size, relative ease of construction, high durability and improved signal to noise ratio compared to implantable electret-based microphones.
The present invention comprises an implantable piezoelectric polymer film tissue conduction microphone for use with an implantable hearing aid system further comprising, a biocompatible housing, a piezoelectric polymer film mechanically coupled to tissue, signal conditioning electronics contained within the housing, and multiple electrically insulated leads disposed through the housing for connection to a separate implanted battery and control unit for the hearing device. In one embodiment, the piezoelectric polymer film may comprise polyvinylidene fluoride (“PVDF”). In another embodiment, the piezoelectric polymer film may comprise co-polymers of PVDF such as PVDF-TrFE; PVDF-TrFE-PZT; ferroelectric polymers; piezoelectric ceramic precursors; terpolymers of vinylidene fluoride; trifluoroethylene; ch lorofluoroethylene; silicon carbide (SiC)/PVDF composites. In yet another embodiment, the housing is cylindrical in shape to facilitate the anchoring of the microphone into the bone of the patient. In yet another embodiment, the piezoelectric polymer film is attached to a curved open frame structure such that the film serves as a diaphragm and seals one end of the housing. A thin biocompatible protective layer is disposed on the surface of the film and is in contact with the tissue.
In another embodiment, a piezoelectric polymer film microphone uses a non-curved (i.e., flat) open frame structure. In this case, the spherically pre-formed piezoelectric polymer film is self supported and is attached around its perimeter to the frame. The curvature may be directed toward the tissue to present a convex surface, or preferably (due to mechanical stability when loaded with tissue) a concave surface. In the case of a concave surface, the depression is filled with a cast silicone rubber contact layer to provide a flat or slightly convex tissue-contact surface. A self supported cylindrical sensor may alternatively be created by clamping/bonding the edges of the film (in the I-direction) but leaving the sides free. Curvature in the edge-supported cylindrical film may be induced by pre-forming the film or by casting/bonding a cylindrically-curved silicone rubber layer onto its surface to present a flat or slightly convex tissue-contact surface.
In yet another embodiment, a piezoelectric polymer film microphone incorporates a film wrapped around a silicone rubber contact pad in which a normal force on the pad generates a tension in the film axis due to the radial expansion of the rubber pad. The rubber contact pad incorporates a cylindrical section that is clamped against a stiff platform incorporated into the housing. The piezoelectric polymer film is wrapped around the cylinder and bonded to itself with an epoxy or cyanoacrylate or other adhesive.
In yet another embodiment, a curved piezoelectric polymer film surface is created using a solid curved frame with ridges that support the film and create thin air gaps between the film and frame. Small holes in the frame couple the air gaps with the air cavity behind the plate to reduce stiffness of the system. This arrangement may provide improved mechanical stability and reduce the effect of low frequency vibrations traveling within the tissue, such as those caused by user movements or breathing. It also provides additional microphone design flexibility, in that hole sizes and spacing and size of supporting ridges can be adjusted to fine tune the response.
The implantable piezoelectric polymer film microphone of the present invention (including, but not limited to, the PVDF microphone) may be subcutaneously implanted in the bony or cartilaginous wall of the ear canal, disposed on the surface or implanted in the temporal bone on the posterior or anterior side of the ear (mastoid region), or in any soft tissue in a region that facilitates the reception of acoustic signals. The implantable piezoelectric polymer film microphone of the present invention may be anchored into the posterior bony wall of the ear canal. This allows the microphone to take advantage of the natural sound amplification provided by the ear geometry, and makes implantation easier because of the thin dermis layer in this anatomical region. Additionally, this mounting may protect the piezoelectric polymer film microphone from mechanical damage. If mounted to the bone of the skull, the implantable piezoelectric polymer film microphone of the present invention may incorporate a rubber spacer to reduce the effect of bone-conducted vibrations caused by the user's speech.
All patents and patent applications cited herein are incorporated by reference in their entirety.
The piezoelectric polymer film microphone of the present invention is implanted in suitable sites of the body by surgical techniques that are used for the implantation of electret-type microphones, which are well known to those of skill in the art. The piezoelectric polymer microphone of the present invention may be subcutaneously implanted in the bony or cartilaginous wall of the ear canal (i.e., the bony wall of the ear canal), disposed on the surface or mounted to the temporal bone on the posterior or anterior side of the ear (mastoid region), or in any soft tissue in a region that facilitates the reception of acoustic signals such as in the soft tissue of the neck, or in other locations as described in U.S. Pat. Nos. 6,626,822, 6,516,228 and 7,354,394. The microphone may be anchored into the posterior bony wall of the ear canal to take advantage of the natural sound amplification provided by the ear geometry, and because of the thin dermis layer in this area making implantation easier. Additionally, this mounting may protect the microphone from mechanical damage. If mounted to the bone of the skull (i.e., the mastoid bone) the piezoelectric polymer film microphone of the present invention may incorporate a rubber spacer to reduce the effect of bone-conducted vibrations caused by user speech.
To reduce size and enable a secure attachment to the bony wall of the ear canal or to the mastoid by osteointegration, a housing (such as a cylindrical housing) having screw-type threads or groove features for engagement with the bone and to facilitate insertion therein may be employed. The threads may extend over the majority of the housing length (such as for full insertion into the bony wall), or on a distal portion of the housing (such as for partial insertion into the bone of the skull). The housing may be machined or manufactured from titanium or other biocompatible metals known in the art, such as stainless steel or gold, or from any type of implant-grade plastic known in the art such as PEEK. A plastic housing incorporating a conductive paint or metal plating on its interior may be used to reduce susceptibility of the microphone to electromagnetic interference. The housing may incorporate a distal flange that improves mechanical positioning and anchoring in place, especially in those areas of the skull in which the housing is positioned adjacent to an air void or cavity, such as that shown in
In one embodiment, the piezoelectric polymer film microphone sensor of the present invention is constructed by bonding (e.g., with cyanoacrylate, epoxy or double-sided adhesive) or, in the case of PVDF film, by mechanically clamping a pre-formed spherically shaped PVDF film (e.g., 5 mm diameter) to a spherically curved and open titanium frame. In the case in which the piezoelectric polymer film microphone of the present invention is comprised of PVDF film, the PVDF film is pre-formed by stretching it over a steel sphere at elevated temperature (e.g., 80° C.), under a poling field of 40V/micron (R. Lerch, “Electroacoustic transducers using piezoelectric polyvinylidene fluoride films”, J. Acoust. Soc. Amer., vol. 66, no. 4, pp. 952-954, 1979. Alternatively, pre-forming can be avoided by attaching a rectangular layer of PVDF film (e.g., 5×5 mm, 100 micron thick) to a curved and open frame such that the stretch direction (known as the “1” direction) of the film is along the radius of curvature of the frame (U.S. Pat. No. 6,937,736). Other piezoelectric polymer films such as copolymers of PVDF (e.g., PVDF-TrFE; PVDF-TrFE-PZT; ferroelectric polymers; piezoelectric ceramic precursors; terpolymers of vinylidene fluoride; trifluoroethylene; chlorofluoroethylene; silicon carbide (SiC)/PVDF composites; etc.) also may be used and are contemplated as part of the present invention.
In one embodiment, the frame is machined from a biocompatible metal, such as 304 or 316 stainless steel or titanium. To minimize the amount of inactive film material (which adds to parasitic capacitance), the width of the frame edge is maintained at a practical minimum to effectively clamp the film and resist deflection. A width of about 1 mm may be used. Radius of curvature directly impacts microphone sensitivity and resonance frequency (due to the effect on film compliance). A frame radius of, e.g., 10 mm-25 mm, may be used to provide a resonance frequency above the primary speech frequency band (e.g., 300-4 kHz) while maintaining sufficient device sensitivity.
In one embodiment, signal conditioning circuitry is positioned as close as possible to the sensor to drive further electrical stages or electrical leads. The pre-amplifier incorporates a high input impedance (e.g., >10M Ohm) JFET transistor for impedance conversion and signal gain and is packaged with the sensor in the microphone housing. The JFET amplifier has lower electronic noise than typical MOSFET amplifiers used for electret-based microphones. High pass filtering may be employed after signal amplification to reduce electronic noise below, e.g., 100 Hz. Depending on the distance between the microphone sensor and the hearing device control unit, and at the expense of sensitivity, the pre-amplifier may alternatively be located in the control unit, further simplifying the microphone design and reducing overall size.
In one embodiment, the PVDF film sensor includes a termination board or termination pads that allow attachment to the enclosed pre-amplifier by mechanical means or by conductive epoxy (e.g. E-Solder®, Von Roll Isola). The conditioned output signals are connected to the exterior of the housing by means of small lead through connector hermetically sealed into the housing. A thin, flexible shielded cable or individual (twisted) insulated wires connect the microphone to the battery/control unit. The electrical termination scheme may alternately utilize lithographically formed wires in a thin laminate for connection to hermetically sealed lead-throughs as described in U.S. Pat. No. 6,516,228 incorporated herein by reference.
In another embodiment, the frame with attached PVDF diaphragm is integrated into the microphone housing by mechanical fasteners or adhesives creating a hermetic seal. To protect the exposed PVDF electrode surface, a conformal layer of biocompatible polymer (e.g., 50 microns of parylene C) is vapor deposited onto the sensor to create a contact layer. The polymer provides a good match between the PVDF and the tissue. Alternate contact layer materials include polyimide or polyester laminates that may be incorporated into the film during its fabrication or applied by adhesives during microphone construction, or a thin layer of implant grade silicone rubber (e.g., Applied Silicone LSR30) cast onto the microphone diaphragm surface. To minimize mechanical loading effects and to reduce the microphone profile, the contact layer may be limited to 0.5 mm thickness.
In an alternate embodiment, a piezoelectric polymer film sensor uses a non-curved (i.e., flat) frame structure, e.g., an open frame structure. In this particular embodiment, the spherically pre-formed PVDF film is self supported and is attached around its perimeter to the frame. The curvature may be directed toward the tissue to present a convex surface, or preferably (due to mechanical stability when loaded with tissue) a concave surface. In the case of a concave surface, the depression is filled with a cast silicone rubber contact layer to provide a flat or slightly convex tissue-contact surface. In an alternate embodiment, a self supported cylindrical sensor may be created by clamping/bonding the edges of the film (in the 1-direction) but leaving the sides free. Curvature in the edge-supported cylindrical film may be induced by pre-forming the film or by casting/bonding a cylindrically-curved silicone rubber layer onto its surface to present a flat or slightly convex tissue-contact surface.
In yet a further embodiment, a curved piezoelectric polymer film surface is created using a solid curved frame with ridges that support the film and create thin air gaps between the film and frame. Small holes in the frame couple the air gaps with the air cavity behind the plate (R. Lerch, G. M. Sessler, “Microphones with rigidly supported piezopolymer membranes”, J. Acoust. Soc. Amer., vol. 67, no. 4, pp. 1379-1381, 1980) to reduce stiffness of the system. This embodiment is designed to provide improved mechanical stability and reduce the effect of low frequency vibrations traveling within the tissue, such as those caused by user movements or breathing. It also provides additional microphone design flexibility, in that hole sizes and spacing and size of supporting ridges can be adjusted to fine tune the response.
In an alternate embodiment, a piezoelectric polymer film tissue contact microphone incorporates a film wrapped around a silicone rubber contact pad in which a normal force on the pad generates a tension in the film axis due to the radial expansion of the rubber pad. The rubber contact pad incorporates a cylindrical section that is clamped against a stiff platform incorporated into the housing. The piezoelectric polymer film is wrapped around the cylinder and bonded to itself with an epoxy or cyanoacrylate or other adhesive. A small exposed tab allows access to the bottom electrode. Electrical leads are attached to both top and bottom electrodes and routed through holes in the platform to the microphone enclosure for signal conditioning and amplification.
Piezoelectric film such as PVDF is well suited for use as an implantable tissue contact sensor due to its high piezoelectric voltage constant, g, which relates voltage to induced strain, its low mechanical impedance, which is well matched to tissue and its general robustness and mechanical stability. Additionally, with piezoelectric film, tissue vibration is directly converted to an electrical signal by the piezoelectric effect, in contrast to contact sensors that rely on conversion of mechanical vibration to pressure changes in an enclosed air cavity for subsequent detection by an air-conduction microphone (such as those described in U.S. Pat. Nos. 6,516,228 and 7,433,484).
When clamped to a curved open frame structure, a piezoelectric polymer film 10, such as a PVDF film, provides very high sensitivity to normally directed mechanical displacement and its frequency response is flat when operated below resonance. The curvature translates a normally directed pressure or force F into tensile stresses along the film axis that can be much larger than the applied stress (
The microphone sensor can be constructed by bonding (e.g., with cyanoacrylate, epoxy or double-sided adhesive) or mechanically clamping a layer of piezoelectric polymer film such as PVDF film (e.g. 10 mm×20 mm, 52 micron thick) to a curved and open metal frame such that the stretch direction (known as the “1” direction) of the film is along the radius of curvature of the frame (U.S. Pat. No. 6,937,736 incorporated herein by reference). Other piezoelectric films such as copolymers of PVDF (e.g., PVDF-TrFE) may also be used.
The frame may be constructed of a biocompatible metal, such as 304 or 316 stainless steel or titanium. To minimize the amount of inactive film material (which adds to parasitic capacitance), the width of the frame edge is maintained at a practical minimum to effectively clamp the film and resist deflection. A width of, e.g., 1-2 mm, may be used in one example. Radius of curvature directly impacts microphone sensitivity and resonance frequency (due to the effect on film compliance). A frame radius of, e.g., 5 mm-20 mm, may be used to provide a resonance frequency above the primary speech frequency band (e.g., 300-4 kHz) while maintaining sufficient device sensitivity. The frame is integrated into the microphone housing, e.g., by mechanical fasteners or adhesives. Moreover, the frame may be configured in a number of different shapes, elliptical, circular, etc. depending upon the desired characteristics. Additionally, in alternative variations, the frame may be omitted from the enclosure and/or the piezoelectric polymer film may be secured directly to the housing and unsupported by the frame while the piezoelectric polymer film remains adhered to and in vibrational contact with the contact surface of the enclosure.
A contact layer (lens) of silicone RTV or polyurethane rubber (e.g., NuSil Med-6015 or Dow Corning X3-6121) is cast in place on the piezoelectric polymer film 10. The lens casting process ensures intimate mechanical contact between the lens and piezoelectric polymer film (such as PVDF film) over the entire surface and acts to seal the front surface of the microphone assembly from liquid intrusion. An alternate approach is to attach a piezoelectric polymer film to a pre-molded rubber contact layer using a flexible adhesive. This requires care to ensure intimate contact over the active film surface and a water-tight seal at the lens/housing interface. To minimize mechanical loading effects and to reduce the microphone profile, the contact lens may be limited, e.g., to 1-2 mm in thickness.
With this architecture, the amount of film curvature may be alternatively adjusted/controlled electronically by applying a DC electric field by means of a DC boost converter circuit connected via leads to first and second electrodes.
Alternately, the desired piezoelectric film curvature may be achieved by adhering the film to a rubber contact layer having a pre-defined curvature using a flexible adhesive and clamping the edges (in the 1-direction) between the frame and housing.
As with the curved/clamped film arrangement described earlier, the tensile force acts on the edge of the film; the small effective area of the film edge causes a much higher stress than that measured at the surface of the film, resulting in higher voltage for the same incoming pressure.
The high capacitance of the piezoelectric polymer film (such as the PVDF film) or electret microphone sensor calls for signal conditioning circuitry positioned as close as possible to the sensor in order to effectively drive further electrical stages. The pre-amplifier may incorporate a high input impedance (e.g., >10M Ohm) low noise JFET transistor or commercial electret amplifier chip for impedance conversion and signal gain and may be packaged with the sensor in the microphone housing. Band pass filtering may be employed after signal amplification to emphasize the speech frequency range, such as 300 Hz-4000 Hz.
An example of how the piezoelectric polymer microphone may be placed is illustrated in
As described above, the frame and piezoelectric polymer film (e.g., PVDF film) contained within the housing of the microphone may be configured in a number of different shapes.
Each of the microphone assemblies disclosed herein and as shown in FIGS. 2 and 5-9 may be incorporated with any of the housings disclosed herein and as shown in
Due to size constraints of the microphone itself, the components of the microphone assembly may be separated from one another while remaining in electrical communication. A first assembly, e.g., the microphone, may be separated from a second assembly such as an opposing side of the assembly which may incorporate additional digital signal processing electronics, transmitter or receiver circuitry (or both), an antenna and battery (e.g., lithium ion), depending on the application. Charging may be accomplished using inductive means (in which an induction coil is required in the appliance package) or by direct coupling of exposed electrical contacts.
The implantable piezoelectric polymer film microphone of the present invention may be used as an integral part of a hearing system, such as a middle-ear or cochlear implant. The signals detected by the implantable microphone may be processed/filtered, amplified and wirelessly transmitted using, e.g., near field magnetic induction (NFMI) or low-power radiofrequency (RF) link to an implanted receiving coil and sent to the implanted hearing device control module for further signal processing and stimulation of the middle ear or auditory nerve.
Modification of the above-described assemblies and methods for carrying out the invention, combinations between different variations as practicable, and variations of aspects of the invention that are obvious to those of skill in the art are intended to be within the scope of the claims.
This application claims the benefit of priority to U.S. Prov. App. 61/370,411 filed Aug. 3, 2010, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61370411 | Aug 2010 | US |
