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Considerations for Devising a Totally Implantable Cochlear Implant
Noel L. Cohen
Cochlear implants (CIs) have become extremely useful tools in the treatment of severe to profound hearing loss. Not only are modern implants able to provide environmental sounds to even the profoundly deaf, but also in the large majority of cases they are able to provide a significant degree of speech understanding without lip-reading. Although the modern multichannel CI has also become smaller in size, one remaining problem is that it requires external visible equipment during use. This external equipment brings with it problems that are both very concrete, as well as psychological.
External hardware, since it is exposed to the atmosphere as well as to the effects of head movement and gravity, tends to be somewhat unstable and prone to damage. This can take the form of actual damage to the transmitter coil, the speech processor, or whatever cables are included in the device, or there can be direct trauma to the hardware while it is being worn. Broken wires, cables, and transmitter coils are a constant source of annoyance and expense, both for the patient and for the manufacturer. When there is external hardware failure, much work is generated for the implant team, and the patient may be “off the air” for a distressing period of time. Much of the above would be minimized were the device totally implantable.
The necessity of wearing obvious and, in some cases, bulky external equipment calls attention to the patient’s hearing impairment. Although many of the implant recipients were accustomed to wearing visible hearing aids prior to implantation, the CI, even with a behind-the-ear (BTE) speech processor, is at least as visible as the largest modern hearing aid. In addition to the cosmetic aspect, some recipients (especially adolescents and those in the public eye) would prefer to be able to conceal their deafness by not having to wear any external hardware to hear. Finally, the CI is typically not worn during sleep, and cannot operate in water or during some types of physical activity.
♦ Fundamental Requirements for Design of Totally Implantable Cochlear Implant
In designing a totally implantable cochlear implant (TICI) several requirements are fundamental. First, the performance of the TICI must be equal to that of a conventional CI. Second, there must, by definition, be no visible hardware during use of the device. Third, there must be no additional risk to the patient, either during the implantation of the device or during extended use of it. Finally, there must be a plan for the long-term use, because as we will see later, battery failure is an inevitable occurrence.
♦ Totally Implantable Cochlear Implant Design
Although the device itself will be concealed beneath the scalp and within the mastoid cavity, middle ear and cochlea, there will be additions to the current components of the contemporary CI. In addition to the magnet and antenna coil, the electronics, and the electrode array, it will be necessary that there be an implanted power supply, rechargeable battery, and microphone and speech processor.
The external hardware would then be used primarily for recharging the battery, but there must also be components to allow remote control of the device, an on/off switch, diagnostics, and, if necessary, a power supply backup. In essence, the TICI will consist of several components: the microphone, the electronics, the electrode array, and the power supply. These can all be integrated into a single titanium and Silastic case, the so-called monobody design. Or the various components may be attached to one another by connectors that would potentially allow replacement of each of the several modules, should that fail or otherwise require replacement. The diagnostics and remote control would have to include an on/off switch (although that could also be incorporated into the implanted device); a battery level indicator; an alarm, should there be impending battery failure or some other problem a volume control and a sensitivity adjustment. This external control must also allow the audiologist to create and change programs, allow the patient and/or parent, to change programs, and permit the use of external devices such as FM and telephone coils. In addition to the above, there might be an audio output from the implanted device that allows the parent or audiologist to check on the sound quality received by the patient.
♦ Challenges for the Totally Implantable Cochlear Implant
Of paramount importance is the necessity for safety. The surgery will necessarily be more complex than conventional CI surgery, but it must not be so difficult that it becomes hazardous. There must also be certainty that the device itself is not going to cause any harm to the patient. Two of the great challenges are the power supply and the design and location of the microphone. The design of the power supply offers a very significant challenge. Currently, there is no infinitely rechargeable battery. It is inevitable that, given even the most advanced current technology, every rechargeable battery will have a finite life. In addition to this fundamental problem, it is important that the battery be as small as possible, that it maintain a charge to allow at least a full day’s uninterrupted use between charges, that it has no “memory effect” that would over time diminish the interval between charges, and that it not generate excessive heat during operation. The demands on the power supply for the TICI emphasize the need for effective power management by the entire device, and make it highly desirable that the power requirements of the system keep the drain on the battery as low as possible. In addition to these considerations, the battery must be incapable of permitting an injurious discharge of current. Because battery failure is inevitable, the manufacturer must have a clear strategy for dealing with the consequences. This will be the first instance in which a CI device will be guaranteed to fail rather than being warrantied against failure.
Battery failure options consist of adopting a modular design that would allow the surgical removal of the battery module and connection of a fresh battery module. Although this has been done with cardiac pacemakers, creating a sufficiently small, safe, and surgically feasible connector poses a major challenge. Two other options remain following battery failure: (1) use the TICI as a conventional CI with a conventional external speech processor, microphone, and power supply; perhaps in the future it may be possible to drive the entire device with just an external power source, simplifying the amount of external hardware; (2) surgically remove the TICI and place another TICI. All three of these strategies bring with them advantages and disadvantages. Replacing the first TICI with a second will give the patient the use and enjoyment of all the advantages of the TICI. It also brings with it the possibility of implanting an upgraded model, just as has been done with the present conventional CI. Against this strategy of replacement with a new TICI is the fact that the patient will need a second operation and perhaps several operations as batteries sequentially fail. This strategy also exposes the patient to repeated endocochlear trauma in removing the one electrode and replacing it. Finally, the cost of replacing these highly sophisticated and expensive devices every few years may be prohibitive.
The strategy of replacing the battery pack only brings with it the advantage of a much smaller operation with a lower cost and no additional endocochlear trauma. Against this strategy are the facts that repeated use of a connector brings with it an increased risk of failure. It is well known that connectors exposed to body fluids are notoriously liable to leak. This strategy still requires multiple surgeries, although each of them is less than replacing the entire TICI.
The third option would be to use the failed TICI as a conventional CI. This brings with it the advantages of no additional surgery, no further endocochlear trauma, and no additional cost. Against this strategy is the fact that the patient will have to revert to the use of a conventional CI after having had all of the advantages of the TICI.
It is to be hoped that advances in rechargeable battery and/or connector technology will mitigate these problems, but they continue to pose a serious weakness in the concept of the TICI.
The second major challenge for the design of a TICI is that of the microphone. An attractive site for the location of the microphone would be the external auditory canal. A totally implantable middle ear implant was designed by Zenner et al (2000), in which the titanium microphone was placed through an opening in the posterior wall of the external auditory canal, beneath the intact skin. This would allow it to function much like the tympanic membrane, receiving sound waves through the external auditory canal, and it would be safe from external trauma. Unfortunately, there is a long history of placing hardware deep into the skin of the external auditory canal, with unfortunate results that there has been a large incidence of erosion of these metallic elements through the very thin skin of the external auditory canal. This concept has been expanded by the work of Huttenbrink et al (Huttenbrink, 1997; Huttenbrink et al, 2001).
Other potential sites for the microphone are above and behind the auricle, yet these generally suffer from the disadvantage that they are likely to be deep to hair-bearing skin and exposed to the noise of head wear, hair brushing, or even the sound of hair moving external to the microphone. On the other hand, this location would allow the microphone to be incorporated within the device, rather than having to be a separate module. It is likely that the first experimental uses of the TICI will include such an integrated microphone. Maniglia et al (1999) and Ko et al (2001) devised an ingenious method of using the tympanic membrane itself as the membrane of a microphone by connecting it to an electromechanical system, which in turn could be connected to a CI electrode. This would eliminate the need for a separate microphone with the above-mentioned problems. Unfortunately, this system may be so difficult to place surgically and tune properly that it appears to have been largely abandoned.
♦ Conclusion
The critical requirements of the TICI include the battery, microphone, and safety. These three issues will determine the ultimate utility of what, if successful, will constitute a very real major step in the evolution of the CI. TICI devices are currently in various stages of development at all three major CI manufacturing companies. It remains to be seen how soon these devices will supplant current designs.
References
Huttenbrink KB. (1997). Implantable hearing aids for severe hearing loss. Basic considerations on their use and attempt at technical development (Dresden/Bochum model). HNO 45:742–744
Huttenbrink KB, Zahnert TH, Bornitz M, Hofmann G. (2001). Biomechanical aspects in implantable microphones and hearing aids and development of a concept with a hydroacoustical transmission. Acta Otolaryngol 121:185–189
Ko WH, Zhu WL, Kane M, Maniglia AJ. (2001). Engineering principles applied to implantable otologic devices. Otolaryngol Clin North Am 34:299–314
Maniglia AJ, Abbass H, Azar T, et al. (1999). The middle ear bioelectronic microphone for a totally implantable cochlear hearing device for profound and total hearing loss. Am J Otol 20:602–611
Zenner HP, Leysieffer H, Maassen M, et al. (2000). Human studies of a piezoelectric transducer microphone for a totally implantable electronic hearing device. Am J Otol 21:196–204
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