Implantable Microdevices - Introduction to BioMEMS - page 402

8.2.3 Microfabricated Cochlear Implants
Kensall Wise's group has worked on the development of implantable electrodes for improved
cochlear implants ( Figure 8.7 ). From the point of view of microfabrication, the cochlea is a chal-
lenging site because of its spiral anatomical architecture ( Figure 8.7a ), but on the other hand,
from the point of view of bioaccessibility of the signal, the cochlea is an engineer's dream: the hair
cells are on the surface, and even as the hair cells start dying in adult individuals (the cause for
hearing loss as we age), the nerve terminals stay intact (so the nerve can be stimulated through
a thin layer of skin without need for penetrating electrodes). he Wise group has designed a lin-
ear array of extracellular electrodes patterned on the outside of an inlatable helix-shaped plastic
structure ( Figure 8.7b through d ). Upon application of air pressure, the plastic support chamber
causes the electrodes to straighten up, facilitating insertion and electrode-tissue contact ( Figure
8.7e through g ). Improved electrode-tissue contact is beneicial for reducing current density at the
electrode surface, which otherwise can produce harmful biproducts through electrochemical pro-
cesses. he ability to fold and unfold should prove essential to reach the apical tip of the cochlea,
where the cells that sense low-frequency sounds reside and should be less damaging to the cochlea
than present implants, which bend by friction against the walls and reach only half-way into the
cochlea (indeed, cochlear implants are known to cause irreversible hearing loss).
It has not been determined yet whether these microfabricated, actuatable cochlear implants
featuring many electrodes will bring additional beneits to the deaf who presently enjoy the

Chamber/array assembly (top view)

a c

Seal

(side view)

Silicon array UV core adhesive Chambers (PET)

Winding mandrel

d

Chamber/array
assembly

Solid model of the guinea pig
scala tympani

Al mandrel

Fluidic actuating chambers

b

Deactivated
tapered-helical
shape
(modiolus
hugging)

e f g

0 kPa 103 kPa 206 kPa

Monolithic silicon
electrode array

Electrode face normal
is perpendicular to
modiolus axis

1.5 mm

FIGURE 8.7 A.microfabricated.cochlear.implant.with.pneumatic.actuation..(From.B..Y..Arcand,.P. T..
Bhatti,.N..V..Butala,.J..Wang,.C..R..Friedrich,.and.K..D..Wise,.“Active.positioning.device.for.a.peri-
modiolar.cochlear.electrode.array,”. Microsyst. Technol .,.10,.478-483,.2004..Figure.contributed.
by.Ken.Wise.)