Biomedical Engineering Reference
In-Depth Information
In addition, most implantable middle ear devices almost completely eliminate feedback,
one of the most annoying adverse effects of conventional aids. Finally, some devices may
allow patients to continue receiving amplification while swimming or bathing.
Implantable middle ear devices are generally available in two types: piezoelectric and
electromagnetic. Electromagnetic devices are characterized by high efficiency and variable
placement of the implant magnets, while piezoelectric MEIHDs are characteristically small
and simple from an electronic perspective but are less efficient than their electromagnetic
counterparts (Shohet, 2008).
When attached to the stapes, a driving force of between 0.16 and 16
μ
N is required
to produce a displacement of between 0.1 and 10 nm in the oval window. If the system
is modeled as a spring-mass-damper as described in Chapter 4, the relationship between
force,
F
(N), and the steady-state displacement,
x
(m), is
F
=
kx
(6.20)
For the numbers quoted, the spring constant
k
=
1
.
6 kN/m.
6.7.1 Piezoelectric Devices
As discussed in Chapter 2, piezoelectric devices operate by applying an oscillating voltage
to a piezoceramic crystal that changes its length and thereby produces a vibratory signal.
The major disadvantage is that output power is directly proportional to the size of the crystal
and there is very little space in the middle ear. Two configurations of actuators are available:
the monomorph and the bimorph. The monomorph uses expansion and contraction to
provide displacement directly, and the bimorph uses two sheets of piezoelectric material
bonded together with opposite polarities to cause the structure to bend.
WORKED EXAMPLE
Consider a monomorph lead zirconate titanate (PZT) crystal 4 mm in diameter with a thick-
ness
t
25 mm driven by 0.5 V. What is the maximum force available, and what is the
displacement?
First calculate the area,
A
(m
2
=
0
.
)
, or the piezoelectric element
d
2
4
10
−
3
2
=
π
=
π
×
(
4
×
)
10
−
6
m
2
A
=
12
.
6
×
4
Now calculate the force per unit volt from equation (3.50). Note that because the piezoelectric
material operates as a capacitor, it takes on the same form, with the addition of a piezoelectric
constant
d
33
=
10
−
12
110
×
C/N. In this equation
ε
is the dielectric constant of the material
10
−
12
C
2
/Nm
2
(1700 for PZT) and
ε
o
=
8
.
8542
×
is the permittivity of free space.
εε
o
A
d
33
t
=
F
x
10
−
12
10
−
6
8
.
85
×
×
1700
×
12
.
6
×
=
=
6
.
9N
/
V
110
×
10
−
12
×
250
×
10
−
6
So, for an applied voltage of 0.5 V, the force is 3.4 N.
