Challenges & Solutions
In addition to cochlear implants, Cochlear also develops and manufactures Baha, a bone anchored hearing implant for conductive hearing loss, mixed hearing loss and single sided deafness. Recently, a third product family has been added to the portfolio, the so-called Hybrid CI which uses a combination of electrical and acoustical stimulation to address ski-slope hearing loss. Next to this, Cochlear is working on a Direct Acoustic Cochlear Stimulator to address mixed hearing loss with a high degree of sensori-neural hearing loss.
Along the journey to the technology used today, many challenges have arisen to which solutions have been found after years of research, innovation and technology.
How to reproduce the complex coding of sound in a cochlear implant
The cochlea is arranged so that the apex is sensitive to low frequencies, and the base to high frequencies. This property of the inner ear is exploited by placing a tiny electrode array consisting of 22 platinum contacts into the cochlea, and by implementing corresponding sound coding strategies in the external sound processor. In current systems, sound is captured via one or more microphones in a behind-the-ear processor, and divided by a filter bank into 22 frequency channels. The channel spacing and allocation of the filter bank approximates the frequency relationship in the cochlea. The signal amplitude in each channel is then analysed periodically (~ 1 ms) and controls the bi-phasic current pulses delivered by the electrode array. Research into future sound coding methods is hoped to improve speech perception in noise and music appreciation, which remain difficult for cochlear implant recipients.
Use materials that are resistant to body fluids and harmless to the recipient so that the implant can be used for a lifetime (~75 years)
Cochlear implants are often implanted in infants with the expectation that they will function for the life of the recipient. Materials must be selected which are both biocompatible (harmless to the recipient) and biostable (unaffected by body fluids). Cochlear achieves this in two ways: first, materials are selected which have proven biocompatability and biostability properties; secondly, the materials are subjected to rigorous testing. Most of the materials used are similar to those used in the Cochlear implants of 25 years ago and build on the experience of pacemakers going back a further 20 years. Any changes to materials only take place after extensive and rigorous testing which includes accelerated aging, chemical analysis and animal studies. Cochlear continually explores new materials for future products which might bring further quality or cost improvements.
Miniaturise the complex technology of the implant
Cochlear implants are implanted in very young children, typically around 12-18 months, but with a trend to younger implantation. This requires the cochlear implant to be very compact so it can be implanted safely in small skulls. Cochlear achieves this by a high degree of integration in the implant electronics through the use of custom integrated circuits, use of high-density feedthrough technology and smart design of the implant package.
Early implants were designed for adults and they were more than 12 mm thick. A bed was drilled in the mastoid bone to seat them, but even then there was a visible lump on the side of the head. More recent implants have been thinner, with the most recent model being less than 4 mm thick and virtually undetectable once implanted. Implants also need to be very robust, so that they are not damaged by impact, such as might result from a child falling and hitting their head. Combined with the need to reduce the thickness of the implant, this has been a demanding requirement, but the latest design is very rugged as well as thin.
Miniaturise the complex technology of the sound processor so that it is light and comfortable to wear every day
The sound processor converts sound into a data stream that is transmitted to the implant through a transcutaneous radio frequency (RF) link. This requires a large number of electronic components yet it needs to be small enough and light enough to be worn comfortably behind the ear of a child or adult. To achieve this, miniature electronic components including custom integrated circuits are assembled onto a multi-layer flexible printed circuit board that is then folded so that it fits inside the curved shape of the processor. To power the electronics and the implant, the processor uses high density energy sources using Zinc-Air or rechargeable Lithium Ion battery technologies to keep the size and weight to a minimum.
Develop a flexible implant platform to enable people to upgrade their sound processor and take advantage of today's technology
The implant stimulates the nerve cells by sending bi-phasic current pulses into the cochlea. The variables are electrode position, current amplitude, pulse width and rate. The sound processor to implant interface includes a dedicated communications protocol that encodes these variables in an efficient and flexible manner. The protocol is standardised to provide both backward- and forward-compatibility with sound processors. The implant is powered by the sound processor via a magnetic inductive link. It is the same link that communicates data. The implant is idle in between stimulations. Hence the sound processor power is minimised allowing future processor designs to utilise the latest battery technology in the smallest possible dimensions.