WO2010085838A1

WO2010085838A1 - Electrical neurostimulator package

Info

Publication number: WO2010085838A1
Application number: PCT/AU2010/000029
Authority: WO
Inventors: Janusz A. Kuzma
Original assignee: Neurostimulation Devices And Technology Pty Ltd
Priority date: 2009-01-27
Filing date: 2010-01-14
Publication date: 2010-08-05
Also published as: EP2382007A1; CN102481449A

Abstract

Described herein is technology relevant to the field of implantable electrical neurostimulators. For example, embodiments include but are not limited to cochlear implants, implantable hearing aids, deep brain stimulators and pain control devices, as well as constituent components of such devices, methods for their manufacture, and methods for their use. For the present purposes, embodiments are described particularly by reference to exemplary cochlear implants, although that should not be regarded as necessarily limiting on the underlying concepts.

Description

ELECTRICAL NEUROSTIMULATOR PACKAGE

Field of the Invention

The present invention relates to generally to the field of medical devices, with embodiments directed towards the field of implantable electrical neuro stimulators. For example, embodiments include but are not limited to cochlear implants, implantable hearing aids, deep brain stimulators and pain control devices, as well as constituent components of such devices, methods for their manufacture, and methods for their use.

Background

The following discussion of the background art is intended to place the invention in an appropriate context and to allow the unique characteristics and advantages of it to be more fully understood. However, any discussion of the background art throughout the specification should in no way be considered as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.

In terms of neurostimulator design (for example in the context of cochlear implants), reliability requirements push the limits of manufacturing technologies. Examples of such devices are described in US Patent No. 5,105,811 and US Patent No. 5,562,7101. Most neurostimulator designs are based on the concept of creating a hermetic metal or ceramic capsule equipped with a number of hermetic electrical feedthroughs for transferring electrical signals in and out between the internal space of the capsule and the surrounding environment.

In the last 20 years, an industry standard for multi-output neuro stimulators has emerged, particularly for cochlear implants. In particular, a main capsule is made from biocompatible metal like titanium, and a multi-contact feedthrough provides connection between internal electronic circuitry, a stimulating lead, and connection to an external antenna loop (or loops) for data and power transfer.

To guarantee required reliability of such devices, which in some cases are designed to function for a lifetime, and to provide a device of very small dimensions and ergonomic shape, the design and manufacturing technology is particularly complex and challenging. Summary

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

One embodiment provides an implantable device including: a first portion formed of a flexible non-conductive material, the first portion containing one or more conductive coils; a second portion coupled to the first portion, the second portion including a rigid container that defines a cavity therein for maintaining processing circuitry; and an electrode assembly coupled to the second portion, the electrode assembly including an elongate body that extends away from the first and second portions, wherein the electrode assembly is coupled to the processing circuitry.

One embodiment provides a device wherein the second portion includes a base and a top separated by a sidewall, and wherein a plurality of feedthroughs are formed through the sidewall for allowing the passage of wires between the first and second portions. One embodiment provides a device wherein the first portion contacts with outer portions of the feedthroughs.

One embodiment provides a device including a rigid cage that is fixedly mounted to the second portion, and that includes formations for mechanically anchoring the first portion to the second portion. One embodiment provides a device wherein the cage is welded to the second portion.

One embodiment provides a device wherein the cage is integrally formed with the second portion.

One embodiment provides a device wherein the cage includes an array of cage members positioned to define windows that provide for the anchoring of the first portion to the cage.

One embodiment provides a device wherein the first portion is formed by a molding process whereby a moldable material that substantially defines the first portion flows through the windows and into the cage thereby to anchor the first portion with respect to the second portion.

One embodiment provides a device wherein the electrode assembly is coupled to the processing circuitry via the feedthroughs. One embodiment provides a device wherein the rigid container is configured for protecting a point of connection, on the underside of the device, between the electrode assembly and the second portion, from downward impact.

One embodiment provides a device wherein the electrode assembly is mounted to the second portion at a location at or adjacent a lower surface of the second portion, wherein the location is inwardly spaced apart from the periphery of the second portion.

One embodiment provides a device wherein the location is substantially central with respect to the lower surface of the second portion.

One embodiment provides a device wherein the rigid container is metallic, and wherein the electrode assembly is electrically insulated with respect to the rigid container.

One embodiment provides a device including a power transfer loop including: a multi- stranded wire coil; and a lumen surrounding the coil, wherein the lumen is formed from a resilient plastic tube. One embodiment provides a device including a ground electrode including a round metal disk, wherein a top surface and connecting strip of the disk are exposed, whereas edges and a bottom surface of the disk are insulated.

One embodiment provides a device wherein the rigid container is metallic, and wherein the ground electrode is mounted in a recess formed in an external surface of the rigid container.

One embodiment provides a method for forming an implantable device, the method including: providing a rigid container that defines a cavity therein for maintaining processing circuitry, wherein the rigid container includes a feedthrough assembly for allowing the passage of wires between the processing circuitry and components external of the container, wherein the feedthrough assembly is protected by a cage; connecting the processing components to components external of the container, including an electrode assembly and a coil, by way of the feedthrough assembly; using a moulding process to define a flexible flap for containing the coil, wherein the flexible flap extends into the cage thereby to mechanically connect the flap to the container.

One embodiment provides an implantable device including: a rigid container that defines a cavity therein for maintaining processing circuitry, wherein the rigid container includes a feedthrough assembly for allowing the passage of wires between the processing circuitry and components external of the container, wherein the feedthrough assembly is protected by a cage; components external of the container that are coupled to the processing components by way of the feedthrough assembly, wherein the components external of the container include including an electrode assembly and a coil,; flexible flap for containing the coil, wherein the flexible flap is formed by a mounding process and extends into the cage thereby to mechanically connect the flap to the container.

One embodiment provides a power transfer loop for an implantable device, the power transfer loop including: a multi- stranded wire coil; and a lumen surrounding the coil, wherein the lumen is formed from a resilient plastic tube.

One embodiment provides a method for forming a power transfer loop for an implantable device, the method including: providing a multi- stranded wire coil; and encasing the coil within a resilient plastic lumen thereby to define an encased coil; heating the encased coil to above a predetermined temperature; forming the encased coil to a desired shape; and allowing the encased coil to cool, such that it sets in the desired shape.

Reference throughout this specification to "one embodiment", "some embodiments" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment", some embodiments" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may.

Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination. Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

In the claims below and the description herein, any one of the terms "comprising", "comprised of, or "which comprises" is an open term that means including at least the elements/features that follow, but not excluding others. Thus, the term "comprising", when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B. Any one of the terms "including", "which includes" or "that includes" as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, "including" is synonymous with and means the same as "comprising".

Similarly, the term "coupled", when used herein, should not be interpreted as being limitative to direct connections only. The terms "coupled" and "connected," along with their derivatives, may be used. The scope of the expression a "device A coupled to a device B" should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

Brief Description of the Drawings Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. IA provides a front perspective view of a device according to one embodiment.

FIG. IB provides a rear perspective view of the device of FIG. IA. FIG. 2 provides a partially cut-away perspective view of a shell component of the device of FIG. IA, showing an array of hermetic electrical feedthroughs.

FIG. 3A provides an exploded front perspective view of the device of FIG. IA. FIG. 3B provides an exploded front perspective view of the device of FIG. IA. FIG. 4 shows the device of FIG. IA in a partially constructed state. FIG. 5A provides a top view of the device of FIG. IA.

FIG. 5B provides a cross-sectional view along the line B-B of FIG. 5A. FIG. 5C provides a more detailed view of a region of FIG. 5B, showing connection Ia wires to feedthroughs.

FIG. 6A provides a perspective view of an electrode assembly according to one embodiment. FIG. 6B provides a perspective sectional view of the electrode assembly shown in

FIG. 6A.

FIGS. 7A & 7B demonstrate construction of a low impedance coil according to one embodiment.

FIG. 8 provides a perspective view of a component according to one embodiment, this component providing a coil flap for the device of FIG. IA.

Detailed Description of Preferred Embodiments

General Context

Embodiments of the present invention are directed to improving design, reliability and other such factors, in a manner especially suitable for a stimulator that is to be implanted on the surface of the human skull. The best examples of such devices are cochlear implants, implantable hearing aids, deep brain stimulators and peripheral nerve stimulators like migraine suppression devices. Taking into account that devices placed on the surface of the skull bone are exposed to significant mechanical stresses, the present inventor has observed various issues which still present a significant challenge and have not necessarily been sufficiently addressed by existing designs or technologies. These include: • Creating an acceptable, low profile device (but still with sufficient internal space to accommodate electronics), retained in a stable position and resilient to loss of hermeticity if exposed to a mechanical impact, providing sufficient protection to internal circuitry.

• Providing effective and reliable connection to stimulating leads or transducers.

• Providing mechanically robust protection of external power and communication antenna coils, maintaining their best possible electrical characteristics.

Design and manufacturing technology described below provides, for some embodiments, an effective solution in the context of these challenges, providing a highly reliable device with shape suitable for placement at head level of adults and small children.

Device Overview Embodiments described herein described herein are directed to a design and manufacturing technique of implantable, hermetic package for electrical neuro stimulators with RF power and data communication functionalities. Embodiments are described by reference to a hermetic package-type device in the form of a device 10 shown in the various figures. Device 10 is particularly suitable for cochlear implants, implantable hearing aids and other head- implanted neuro stimulators.

In overview, device 10 is a substantially planar device visually/notionally defined by three segments: a rigid hermetic container 11, which contains processing components; a flexible flap 70, which contains embedded telemetry coils; and an electrode assembly 20, which is configured for neuro stimulation. Various significant aspects of device 10 are discussed further mellow, including:

• General construction and manner of manufacture.

• Reliable feedthrough assembly.

• Under-device electrode connection.

• Construction of the ground electrode. • Construction/formation of coils.

It will be appreciated that each of these provides respective advantages to device 10. However, it will be similarly appreciated that not all of these aspects are present in all embodiments; various embodiments include some of the significant features of device 10, but not others. General Construction and Manner of Manufacture

The following section details an exemplary method for forming device 10. This is provided substantially as an introduction to the various components of device 10, and should not be necessarily regarded as limiting from a construction method perspective. Furthermore, various steps may be performed in sequences other than described herein. Figs 3A and 3B show exploded view of the device 10. Both figures illustrate reference positioning of all main components and illustrate key assembly processes. Container 11 defines a cavity for containing processing equipment (for example one or more printed circuit boards) and the like for providing neuro stimulation functionalities to device 10. The nature of this processing equipment is generally ignored for the purpose of the present disclosure, and varies depending on the specific application. Container 11 presently includes an upper shell 100 and a lower shell 120 which are welded together to define a cavity therebetween. In the present embodiment the upper shell is acts substantially as an open container having sidewalls, whilst the lower shell is substantially planar and acts as a lid for sealing the container of the upper shell. However, in other embodiments alternate constructions are used to define the metal container. This hermetic metal container performs the role of protecting sensitive electronic circuitry contained within hermetically sealed internal space from the destructive effect of bodily fluids, and mechanically guarding fragile wiring connections to the feedthrough outputs.

Shell 100 is preferably machined from solid piece of metal like titanium or its alloy. Tapered edges of shell 100 give the device an ergonomic profile by blending its contour underneath the skin, and at the same time make the construction stiff and resilient to deformation by an external blow to the head.

Container 11 includes a sidewall equipped with an array of hermetic electrical feedthroughs 101, as best shown in FIG. 2, collectively referred to as feedthrough assembly 15. The feedthrough assembly is defined by a plurality of apertures, in the form of feedthroughs 101, which extend through a sidewall of container 11 and into flap 70. Positioning of the feedthroughs 101 on the side wall of the shell 100 makes it possible to create a large open space 105 within the metal shell to accommodate internal circuitry, whilst containing the overall thickness of the device. Feedthrough assembly 15 allows for the wiring of components within the cavity to components outside of the cavity (for example to telemetry coils and the electrode assembly). This construction allows the side section containing sensitive feedthroughs 100 to be much thicker and more resilient to damage without increasing the overall thickness of the device. Assembly begins with placement and connection of the electronic circuits to the internal feedthrough outputs of feedthroughs 101. Container 11 is sealed by hermetically welding upper shell 100 to lower shell 120 about their respective edges.

A recess 103 is machined in the top of shell 100 to accommodate, in a stable position, ground electrode assembly 30. Electrode assembly 30 is adhesively fixed to the bottom of the recess and optionally held in place by a separate metal ring welded to the edge of the recess. This is advantageous in the sense that the ground electrode assembly 30 in use directly faces the skin overlaying the implant which provides limited protection from external forces. During manufacture, ground electrode 30 is fixed in recess 103, and connected to its respective feedthrough (or feedthroughs).

In terms of manufacture, after connection of the wires, the ground electrode assembly 30 is fixed in the shell recess 103 and connecting strip 33 is welded to a corresponding one of feedthroughs 101. Next, power coil 50 and data coil 60 are connected to a corresponding one of feedthroughs 101. Feedthrough cage 90 then placed over feedthrough array 101 (and components connected to it), and spot welded to the edge of front recess of shell 100, as shown in the figures.

Following this, wires of electrode assembly 20 and telemetry coils 50 and 60 are connected to their corresponding external feedthrough outputs. At this point, the wiring of device 10 is complete. The next stage in construction of device 10 is to place a cage 90 over feedthrough assembly 15 (although, in some embodiments the positioning of the cage occurs prior to wiring, for example where wires pass through apertures in the cage). This cage is spot welded to container 11, to either or both of shell 100 and shell 120. In some embodiments cage 90 is integrally formed with either or both of shell 110 and shell 120. Cage 90 is configured to provide two key functionalities:

• Protection of feedthrough assembly 15. Cage 90 includes a protective upper surface 91, which protects the delicate wire exit points of feedthrough assembly 15 by providing a rigid overlying cover.

• Connection of flap 70 to container 11. Cage 90 includes an array of cage members, presently being a lower horizontal member connected to upper surface 91 by a plurality of vertical members, thereby to define windows 92. These allow anchoring of flap 70, as discussed further below. Following the mounting of cage 90, a bottom cover 110 is placed over electrode assembly 20 and spot welded to the shell 120. The nature of cover 110 provides various advantageous functionalities, which are discussed further below.

Finally, the whole assembly shown in Fig. 4, including a magnet 80, is placed inside a moulding die, and liquid polymer (such as silastic or the like) is injected to fill mould cavity to form flap 70. The flexible flap 70 keeps telemetry coils 50, 60 and magnet assembly 80. in stable defined position. A very important detail of this process is that the liquid polymer flows through the windows at the front of cage 90 to create a strong mechanical bond between the flap and the hermetic case assembly of container 11. That is, the liquid polymer forming flap 70 flows into cage 90, thereby to anchor flap 70 to container 11. Additionally, the polymer flows around feedthrough assembly 15, thereby protecting wiring, and partially into the feedthroughs 101, thereby assisting in hermetic sealing.

Figs 5A, 5B and 5C show in more detail the final positioning of all components at final assembly stage and forming of the antenna coil 70. Specifically illustrated in Fig 5C is electrode assembly wiring 21 connection to feedthroughs 101 and mechanical engagement of flap 70 with cage 90. An important element of this embodiment is that cover 110 overhangs edge of shell 100, creating free passage for wires 21 to feedthroughs 101. This forms effective mechanical protection for wires 21 which are inherently fragile. At the same time, well pronounced cylindrical shape of cover 110 performs a fixation role for the implant when placed in skull drillout.

Connection of Flexible Component to Rigid Component

In this type of stimulator construction, a significant challenge is to provide a reliable mechanical connection between flexible flap 70 and hard metal container 11. This connection is responsible for the integrity of the connection of telemetry coils 50 and 60 entering the space under cage 90 and connecting to the corresponding feedthroughs. It is common practice to extend flap material over the hard metal case as a substantial over- moulding layer. While this method improves bonding between the two segments, it increases the overall thickness and footprint of the device which is a real drawback. In the technology disclosed herein, as described in the preceding section, this connection is accomplished by the transition of polymer used to form flap 70 into the space under cage 90 through array of windows in the front face. This provides superior mechanical bonding without increasing the dimensions of the implant. As noted above, a particularly important step in the assembly of device 10 is a selective overmoulding of the complete assembly to form coil flap 70 as shown in FIG. 8. This process involves the entire partially constructed implant assembly being placed inside cavity of moulding tool. Liquid polymer is then injected into the tool. The tool is shaped such that the polymer forms antenna flap 70; fills space within cage 90 through windows 92; and through overhang of cover 110, fills that cavity, fixing lead within.

The role of windows 92 in the cage 90 is particularly significant in the context of this embodiment. These perforations provide a very efficient anchorage of antenna flap 70 to the case assembly, and thereby eliminate a need for traditional bulky external overmoulding of shell 100 to provide sufficient mechanical retention of the antenna flap. Such overmoulding would also significantly increase dimensions of the implant.

FIG. 5 shows in detail design principles of connecting wires 21 to feedthroughs 101. The end of the lead assembly is placed close to the edge of the shell containing array of feedthroughs 101. The wires 21 are transferred to the feedthroughs and fused to corresponding feedthrough pins. After wire connection, a protective metal cover 110 is placed over the end of the lead and spot welded to the bottom cover 120.

Under-Device Electrode Connection

As noted above, electrode assembly 20 egresses from container 11 via cover 110, which is positioned on the underside of device 10 away from the device's periphery. In this manner, the point of electrode connection and egression is protected from impact damage by way of rigid container 11. That is, the positioning of cover 110 provides an electrode connection position protected from mechanical stresses transmitted through overlaying layer of skin by overhanging portions of container 11. This differs from (and significantly improves upon) known devices, where the electrode assembly typically egresses at the periphery.

Bottom cover 110 is presently configured for use as device fixation stump embedded securely within a drillout of skull bone. This allows for reliably positioning of device 10 with respect to the skull. Additionally, cover 110 guides the lead of electrode assembly 20 into the middle ear cavity. A further important element of this embodiment is that cover 110 overhangs the edge of shell 100, creating free passage for the wires 21 to the feedthroughs 101. This forms effective mechanical protection for the wires 21, which are inherently fragile.

Construction of the ground electrode. An approach for construction of the ground electrode assembly 30 according to one embodiment is illustrated in FIGS. 6 A & 6B. In overview round metal disk 31 preferably made form platinum foil is moulded within plastic form 32 preferably "Peek". Peek is a preferred material due to its superior mechanical strength (much higher than the gold used for coil construction), excellent insulation characteristics, and biocompatibility. As shown in the figures, the top surface of the platinum disk 31 remains exposed, as well as connecting strip 33. The edges and bottom of the disk are insulated to prevent shorting to the shell 100 upon assembly of device 10. This results in a particularly advantageous electrode construction compared to that which is known in the art.

Construction/formation of coils. FIGS. 7A & 7B demonstrate an approach for construction of a low impedance coil according to one embodiment. This is particularly suitable for power transfer coil 50. As context, in the case of passive implants without their own battery or implants with rechargeable power source, it is important to have very efficient power transfer. The best performance can be achieved with single or multi-turn antenna coils made from low impedance biocompatible metal like pure gold. Unfortunately, while gold has excellent electrical characteristics, its mechanical strength is very poor. Under mechanical stress due to movement or impact, it is very easily fractured even in a flexible, multi- stranded configuration. Embodiments of the present invention propose a solution including the placement of multi- stranded coil wire within lumen of mechanically resilient plastic tube 51. The preferred choice of material for tube 51 is "Peek" which has superior mechanical properties that provide effective protection for the contained wire from impact or tensile stress. However, other materials may be used.

The wire 52 and tube 51 assembly shown in FIG. 7 A is formed into a desired shape, for example that shown in FIG. 7B. This is preferable accomplished within the cavity of a holding tool by heating the assembly to about 300°C, followed by cooling. This process substantially permanently remoulds the shape of whole coil assembly 50. The data coil 60 is optionally constructed by similar means, or alternately be formed from a more resilient metal such as stainless steel. In the case of such more resilient materials, the use of a polymer lumen may not be necessary.

Conclusions It will be appreciated that the embodiments described above provide significant advantages in the field of neurostimulation. Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. While there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as falling within the scope of the invention. For example, any formulae given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added to or deleted from methods described herein whilst remaining within the scope of the present invention.

Claims

1. An implantable device including: a first portion formed of a flexible non-conductive material, the first portion containing one or more conductive coils; a second portion coupled to the first portion, the second portion including a rigid container that defines a cavity therein for maintaining processing circuitry; and an electrode assembly coupled to the second portion, the electrode assembly including an elongate body that extends away from the first and second portions, wherein the electrode assembly is coupled to the processing circuitry.

2. A device according to claim 1 wherein the second portion includes a base and a top separated by a sidewall, and wherein a plurality of feedthroughs are formed through the sidewall for allowing the passage of wires between the first and second portions.

3. A device according to claim 2 wherein the first portion contacts with outer portions of the feedthroughs.

4. A device according to claim 2 including a rigid cage that is fixedly mounted to the second portion, and that includes formations for mechanically anchoring the first portion to the second portion.

5. A device according to claim 4 wherein the cage is welded to the second portion.

6. A device according to claim 4 wherein the cage is integrally formed with the second portion.

7. A device according to claim 4 wherein the cage includes an array of cage members positioned to define windows that provide for the anchoring of the first portion to the cage.

8. A device according to claim 7 wherein the first portion is formed by a molding process whereby a moldable material that substantially defines the first portion flows through the windows and into the cage thereby to anchor the first portion with respect to the second portion.

9. A device according to claim 2 wherein the electrode assembly is coupled to the processing circuitry via the feedthroughs.

10. A device according to claim 1 wherein the rigid container is configured for protecting a point of connection, on the underside of the device, between the electrode assembly and the second portion, from downward impact.

11. A device according to claim 1 wherein the electrode assembly is mounted to the second portion at a location at or adjacent a lower surface of the second portion, wherein the location is inwardly spaced apart from the periphery of the second portion.

12. A device according to claim 11 wherein the location is substantially central with respect to the lower surface of the second portion.

13. A device according to claim 1 wherein the rigid container is metallic, and wherein the electrode assembly is electrically insulated with respect to the rigid container.

14. A device according to claim 1 including a power transfer loop including: a multi-stranded wire coil; and a lumen surrounding the coil, wherein the lumen is formed from a resilient plastic tube.

15. A device according to claim 1 including a ground electrode including a round metal disk, wherein a top surface and connecting strip of the disk are exposed, whereas edges and a bottom surface of the disk are insulated.

16. A device according to claim 15 wherein the rigid container is metallic, and wherein the ground electrode is mounted in a recess formed in an external surface of the rigid container.

17. A method for forming an implantable device, the method including: providing a rigid container that defines a cavity therein for maintaining processing circuitry, wherein the rigid container includes a feedthrough assembly for allowing the passage of wires between the processing circuitry and components external of the container, wherein the feedthrough assembly is protected by a cage; connecting the processing components to components external of the container, including an electrode assembly and a coil, by way of the feedthrough assembly; using a moulding process to define a flexible flap for containing the coil, wherein the flexible flap extends into the cage thereby to mechanically connect the flap to the container.

18. An implantable device including: a rigid container that defines a cavity therein for maintaining processing circuitry, wherein the rigid container includes a feedthrough assembly for allowing the passage of wires between the processing circuitry and components external of the container, wherein the feedthrough assembly is protected by a cage; components external of the container that are coupled to the processing components by way of the feedthrough assembly, wherein the components external of the container include including an electrode assembly and a coil,; flexible flap for containing the coil, wherein the flexible flap is formed by a mounding process and extends into the cage thereby to mechanically connect the flap to the container.

19. A power transfer loop for an implantable device, the power transfer loop including: a multi-stranded wire coil; and a lumen surrounding the coil, wherein the lumen is formed from a resilient plastic tube.

20. A method for forming a power transfer loop for an implantable device, the method including: providing a multi-stranded wire coil; and encasing the coil within a resilient plastic lumen thereby to define an encased coil; heating the encased coil to above a predetermined temperature; forming the encased coil to a desired shape; and allowing the encased coil to cool, such that it sets in the desired shape.