Most hearing-impaired people can be provided with acoustic hearing aids that provide reasonable degrees of benefit in everyday life. Some, however, have so little hearing that even the most powerful of hearing aids is of little practical use. In these cases, it is possible to bypass the mis-functioning inner ear by electrically stimulating the auditory nerve directly with a cochlear implant - a kind of 'bionic ear'. Such devices, only laboratory curiosities in the 1960's, have now become routine clinical options for both adults and children.
Cochlear implants are, of course, not appropriate for most people with a hearing impairment. For patients with a conductive hearing loss, which affects the transmission of sounds through the middle ear, surgical techniques are often of benefit. Also, because the implant relies on a stimulation of any remaining parts of the auditory nerve, patients with severe or complete damage to the nerve are unlikely to benefit. Perhaps the most common reason not to implant a hearing-impaired person is that s/he has too much hearing! Cochlear implants are only meant for patients with profound to total hearing losses - those who get very little help from ordinary hearing aids.
Although there are several types of commercial cochlear implants
currently available they all possess similar components, namely
a microphone, a speech processor, a transmitter link and a fully
implantable device containing an electronics package and the electrodes
(arranged in an array). As the implantable device is buried under
the skin a radio-frequency transmitter link is required to drive
the electrodes, hence they are termed transcutaneous (across the
skin) devices. The radio-frequency link is held in place with
an implanted magnet. The microphone is worn on an ear hook and
the speech processor is body worn (although the first behind-the-ear device has recently been introduced). The electrode array is usually
a multi-channel electrode placed inside the cochlea to stimulate
different areas of the auditory nerve.
Sound is picked up by the microphone and sent to the speech processor.
There it is analysed and coded for the individual and transmitted
across the radio-frequency link to the buried electronics in the
implant, where it is converted into electrical stimuli for the
The UCLID Cochlear Implant System
The UCLID (University College London Implantable Device) Cochlear
Implant System is being developed between UCL (the Department
of Medical Physics and Bioengineering ,
the Department of Phonetics and Linguistics ,
and the Adult Cochlear Implant Programme and the Institute of
Laryngology and Otology at the Royal National Throat, Nose and
Ear Hospital) and Nobel Biocare AB.
It is an experimental device and differs in some ways from the
current commercial systems.
The main difference is the use of a percutaneous, or skin-penetrating,
connector to link the speech processor directly to the electrode
array. This removes the need for the radio-frequency (rf) link
and provides us with an electrically transparent link, allowing
unparalleled access to the auditory nerve. The rf link used in
current systems places restrictions on the processing strategies
that can be used and the buried electronics may not allow future
strategies to be implemented. This link is also responsible for
up to half of the power consumption of the system, a fact responsible
for the slow development of a behind-the-ear processor. Magnetic
resonance imaging (MRI) is fast becoming an important diagnostic
tool but current systems that rely on an implanted magnet to fix
the rf link cannot be scanned without minor surgery.
To take advantage of the percutaneous link a speech processor
has been specially designed to provide a highly flexible processor
that can implement all the current processing strategies and investigate
the potential of future ones. The UCLID speech processor is currently
being used for research into electrical stimulation parameters
in patients already implanted with INERAID devices. Initial tests
have established the effectiveness of the CIS implementation.
Data has been collected from studies into the effects of between-channel
inter-pulse intervals and CIS cycle rate.
The Percutaneous Connector:
The percutaneous connector consists of a titanium pedestal, which
houses the electrode array and an internal connector. The titanium
pedestal has been developed between UCL and Nobel Biocare AB,
of Sweden, and is based on an earlier experimental device developed
at UCL (the EPI Bioglass® implant) and the medical
titanium abutments manufactured by Nobel Biocare AB, employing
the Brånemark System®. (These abutments
are primarily used in dentistry and maxillofacial applications
but also as a conduction route for the Bone Anchored Hearing Aid,
The obvious concern in the use of a percutaneous system is the
permanent break in the skin, offering a possible pathway for ingress
of infection and leak of body fluids. Titanium has been used
for over 25 years as an implant material and titanium abutments
have very good clinical results for the reaction of the surrounding
skin and osseointegration (fixing) into the underlying bone.
Extensive testing of the pedestal design has been carried out
and the incidence of complications is likely to be small.
The titanium pedestal is partially implanted and
fixes into the bone through a break in the skin smaller than the size of a pencil
top eraser. The surrounding skin will be thinned to prevent movement and reduce
the likelihood of infection. The height of the pedestal out of the bone has
been kept low to reduce trauma damage and several safety features have been
incorporated. The pedestal will be implanted behind the ear.
As the pedestal breaks the skin it is important that the area
is regularly cleaned by the user. This requires a simple daily
task of cleaning around the base of the pedestal with a soft baby
toothbrush or similar with warm water. Once the skin has healed
and the pedestal is fixed into the bone there are no restrictions
on activities such as bathing and swimming.
The internal connector, sealed in the bottom of the
pedestal, offers a flat electrical contact surface to the external connector
that leads to the speech processor. This internal contact surface has been designed
to be extremely hard wearing and easy to clean. At present the internal connector
ofers 11 contacts but future designs will provide more than twice this number.
The electrode array is connected to the underside
of the internal connector and exits the base of the pedestal and is thus wholly
implanted. The electrode array is specially designed and consists of 9 active
channel electrodes, for intracochlear insertion, and 2 indifferent earth channels,
for placement under the temporal muscle.Present transcutaneous systems have
between 8 and 22 active channels.
The external connector, linked to the speech processor
by a cable, has been specially manufactured by Cinch Connectors Ltd., of England,
and is based on a state-of-the-art connecting system originally developed for
military use. This connector offers a low profile with a high density of electrical
contacts - there are 11 active channels available, together with an earthing
channel to disperse any static charge. The design of the sprung plunger contacts
in the external connector allows the mating contacts in the pedestal to be flat
contact pads. A spring-latching mechanism is included, for fixation to the pedestal,
as well as positive locating tongues. The external connector links the electrodes
directly to the speech processor. The length of the cable to the speech processor
can be made to suit individual requirements. The contact materials have been
chosen so that all appreciable wear occurs on the external connector which can
be replaced when required.
The UCLID Speech Processor:
The speech processor is based upon a digital signal
processing (DSP) chip which is able to carry out operations such as filtering
and compressing signals at very high speeds while running from batteries. The
device is completely programmable and contains reprogrammable "flash"
memory to store user information. (This kind of technology is used in digital
mobile phones and audio CD's). The DSP is combined with microphone output circuits
and eight constant current generators whose outputs can be changed very rapidly,
either independently or simultaneously. This gives the processor the ability
to implement all of the speech processing strategies (algorithms) that are currently
in use, as well as great flexibility in developing new strategies.
The UCLID processor at present can use three processing strategies,
namely: a two-channel algorithm which presents "speech pattern
elements"; a multi-channel "compressed analogue"
(CA) algorithm; and a multi-channel implementation of "continuous
interleaved sampling" (CIS). The CA and CIS processing options
are currently offered by several of the commercial systems in
The processor can be programmed to use various different processing
methods, so that comparisons can be made and new processing methods
developed. Each individual user will be able to chose the processing
method that is best suited to them in a particular situation,
for example one processing strategy may be preferred for listening
to speech, another for identifying environmental sounds.
At present the UCLID speech processor is larger than those available
commercially, although it can be worn easily on a belt or in a
pocket. However, as development continues the size of the device
is expected to reduce significantly. It is hoped that commercial
cochlear implant companies will take on the processing methods
developed and, due to the low power requirements, a truly behind-the-ear
device is feasible in the future.
The Fitting System:
A dedicated software package which will run from
a pc has been developed for the fitting procedure and programming of the speech
processor. For safety reasons an interface box will be used to isolate the patient.
Using this system the patient fitting data and processing strategy can be easily
and quickly programmed into the speech processor.
Speech Perception Studies:
Studies carried out to date have shown a replication
of previous comparisons of the CIS method with the CA processing of the standard
INERAID speech processor, confirming that the UCLID processor is operating effectively.
Three users of the INERAID implant showed significantly higher consonant and
sentence recognition scores with a five-channel CIS processor running on the
UCLID compared to the INERAID processor. A study of the effects of between-channel
inter-pulse intervals has shown that for a 500 Hz CIS cycle rate processor,
10µs inter-pulse intervals between adjacent electrodes give significantly
better consonant identification than 260µs inter-pulse intervals. Using
10µs interpulse intervals between adjacent electrodes, an increase of the
CIS cycle rate from 500 to 1000 Hz leads to a significant improvement in consonant
What Are The Aims Of The Work?
The percutaneous connector is designed to overcome the inherent limitations of the current commercial systems by providing direct electrical access to the auditory nerve. With this unique access the individual parameters of speech strategies, both current and future, can be fully explored and their influence on the percieved sounds noted. This will lead to a greater understanding of how the auditory system 'uncodes' the speech strategy presented to the auditory nerve and hence to the development of strategies that provide the user with more 'realistic' and useful sound information.
This work is supported by grants from The Clothworkers' Foundation and The Hearing Research Trust.
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