Biomedical Engineering Reference
In-Depth Information
The programming system coil is placed over the implant site and held in place mag-
netically. The coil couples with the implant by means of a radio frequency signal used to
program the device in the same manner as a traditional digital hearing aid. The program-
ming system also allows for extensive testing and diagnostics of the stimulator.
The charger system consists of the base station, charging coil, and charger body. To
charge the implant, the wearer removes the charger body from the base station and places
the coil on the skin over the implant site. The charger body contains a clip that allows the
charger to be attached to the belt of the wearer during charging. Typically, charging time
will be about 1 hour if performed daily. While recharging the implant, the wearer can
perform normal daily activities, turn the implant on and off, and adjust the volume.
A remote is used to control the stimulator when the device is not being charged. It
allows the wearer to turn the implant on and off and to adjust the volume. To use this
facility, the wearer holds the remote against the skin over the implant.
The U.S. Phase I trial results yielded a 15-20 dB functional gain across audiometric
frequencies in 20 patients. The pure-tone averages and monaural word recognition scores
were better with the hearing aid in the same ear preoperatively, whereas the patients
generally perceived more benefit in the postoperatively implant-aided conditions (Shohet,
2008; Traynor and Fredrickson, 2008).
6.7.1.4 Envoy System
Another totally implantable piezoelectric device is the Esteem by Envoy Medical (orig-
inally St. Croix Medical), shown in Figure 6-24. This device uses the eardrum as the
microphone, taking advantage of the natural acoustics of the ear canal without obstruc-
tion, interference, or any external devices. Therefore, the input signals are identical to
those received by a person with normal hearing. This mechanical signal is converted to
an electrical signal by a piezoelectric transducer (the sensor) at the head of the malleus or
incus. The electrical signal is amplified and filtered by a programmable digital processor
before being converted back to a vibratory signal using another piezoelectric transducer
(the driver) attached to the head of the stapes. The incus lenticular process is removed to
prevent feedback to the sensor, and this is one of the main disadvantages of the system.
FIGURE 6-24
Envoy medical
system. [Adapted
from (Shohet 2008).]
