Even though cochlear implants have restored basic hearing to about 220,000 deaf people worldwide, they do require the persons wears a microphone and associated electronics behind the ear, which not only creates a social stigma, but it also raises issues in terms of reliability and prevents patients from swimming and some other activities. These problems can now be avoided thanks to a tiny prototype microphone that can be implanted in the middle ear.
A study published online in the Institute of Electrical and Electronics Engineers journal Transactions on Biomedical Engineering reports that the proof-of-concept device, developed by an engineer from Utah University and his team in Ohio, has been successfully tested in the ear canals of four cadavers.
About one-third of the 220,000 people worldwide with profound deafness or severe hearing impairment with cochlear implants are from the U.S., with two-fifths of the recipients being children. Conventional cochlear implants consist of three key parts, i.e. a microphone that picks up sound, a signal processor and a radio transmitter coil, which are worn externally behind the person’s ear that transmit signals to the internal receiver-stimulator that is implanted under the skin behind the ear that converts sound signals into electric impulses, which are then transmitted via cable to between 4 and 16 electrodes that wind through the cochlea of the inner ear, bypassing the ear canal, eardrum and hearing bones and stimulate auditory nerves that enable the patient to hear.
Senior author, Darrin J. Young, an associate professor of electrical and computer engineering at the University of Utah and USTAR, the Utah Science Technology and Research initiative, says:
“It’s a disadvantage having all these things attached to the outside. Imagine a child wearing a microphone behind the ear. It causes problems for a lot of activities. Swimming is the main issue. And it’s not convenient to wear these things if they have to wear a helmet.”
He continues:
“for adults, it’s social perception. Wearing this thing indicates you are somewhat handicapped and that actually prevents quite a percentage of candidates from getting the implant. They worry about the negative image,” and in terms of reliability, Young adds, “if you have wires connected from the microphone to the coil, those wires can break.”
Normally, when sound enters into the ear canal, the eardrum vibrates. The eardrum connects at the umbo to a three connected tiny bones, i.e. the malleus, incus and stapes, generally known as the hammer, anvil and stirrup, which vibrate. The stirrup touches the cochlea, the inner ear’s fluid-filled chamber, which moves hair cells (sensory receptors) on the cochlea’s inner membrane move and triggers the release of a neurotransmitter chemical that carries the sound signals to the brain. These hair cells, or sensory receptors do not function in profoundly deaf people who qualify as cochlear implant candidates. The reason for this could be birth defects, adverse effects from drugs, exposure to excessively loud sounds or certain virus infections.
The device, developed by Young, implants all the external components, so that sounds are transmitted through the ear canal into the eardrum, which vibrates as it does normally. At the umbo, however, Young has attached a sensor with a chip known as an accelerometer to detect the vibration, both of which act as a microphone that detects sound vibrations and converts them into electrical signals sent to the electrodes in the cochlea. Even though patients still need to wear a charger behind the ear, the implanted battery can be recharged at night whilst sleeping and as Young states, the battery life would last from one to several days.
He says that the prototype, which is approximately the size of an eraser on a pencil, has to be made smaller and needs improvement in being able to detect quieter, low-pitched sounds, therefore tests in people should be possible in about three years time. The aim of the current prototype of the packaged, middle-ear microphone, which is 2.5 mm by 6.2 mm, i.e. about one-tenth by one-quarter inch and weighs 25 mg, or less than a thousandth of an ounce is to reduce the package to 2 x 2 mm in size.
According to the study, the most efficient incoming transmitted sound to the microphone is to first surgically remove the anvil, one of three, small bones located in the middle ear. Young states the microphone may also be part of an implanted hearing aid that could replace conventional hearing aids for a certain group of patients whose hearing bones have degraded and are unable to adequately convey sounds from conventional hearing aids. The surgical implant would require the approval from the U.S. Food and Drug Administration.
Traditional microphones include a membrane or diaphragm that moves and produces an electrical signal change in response to sound, however, they require a hole for the sound to enter, which would get clogged by growing tissue if implanted. In contrast, Young’s middle-ear microphone uses an accelerometer, a 2.5-microgram mass attached to a spring that would be placed in a sealed package containing a low-power silicon chip to convert sound vibrations to outgoing electrical signals. The package is glued to the umbo, enabling the accelerometer to vibrate when it received vibrations from the eardrum. The moving mass generates an electrical signal, amplified by the chip, which in turn connects to a speech processor and stimulator wired to the electrodes in the cochlea.
Young explains:
“Everything is the same as a conventional cochlear implant, except we use an implantable microphone that uses the vibration of the bone.”
The team tested the new microphone by using the temporal bones and ear canal, eardrum and hearing bones from four cadaver donors. They inserted tubing into the ear canal that contained a small loudspeaker, and generated tones of various frequencies and loudness. The sounds the implanted microphone picked up were measured by using a laser device to measure the vibration of the tiny ear bones. They discovered that the umbo, i.e. the location where the eardrum connects to the hammer or malleus, produced the greatest sound vibration, especially if the anvil bone was surgically removed first.
When the prototype microphone unit was attached to the umbo, the team discovered that whilst it was able to pick up medium pitches at conversational volumes, it did, however, have difficulties in detecting quieter, low frequency sounds, which is something Young plans to improve.
Whilst the output of the microphone was made to speakers in the study, in a real person, the microphone would transmit the sound to the implanted speech processor. Young demonstrated the microphone by using it to record the start of Beethoven’s Ninth Symphony whilst implanted in a cadaver ear and found that it was easily recognizable, even though it sounded a little bit fuzzy and muffled, yet Young says:
“The muffling can be filtered out.”
Written By Petra Rattue