Biomedical Engineering Reference
In-Depth Information
FIGURE 7-31
Piezoceramic
actuator types.
(a) Monomorph
element. (b) Bimorph
element.
(c) Monomorph
element stack.
One of the more interesting mechanisms uses magneto-rheological fluids (MRFs),
which respond to a magnetic field with a dramatic change in rheological behavior. They
can reversibly and instantaneously change from a free-flowing liquid to a semisolid with
controllable yield strength, which allows them to be used for high-bandwidth vibrotactile
devices.
7.7.2.4 Electrotactile Displays
Most researchers believe that the electric currents passing through the skin from surface
electrodes directly stimulate afferent nerve fibers or whole nerve bundles if electrodes are
sufficiently large. Depending on the applied voltage, current, waveform electrode size,
force and hydration, among other things, subjects describe the sensation as a tingle, itch,
vibration, buzz, touch, pressure, pinch, sharp, and ultimately burning pain (Kaczmarek,
Webster et al., 1991).
As with the electrodes discussed in Chapter 2, the object is to convert electron flow in
the lead wire to ionic flow within the tissue. Electrode requirements are therefore similar
to those used for monitoring electrical signals. Ideally, the current density should be kept
as uniform and as low as possible, but small variations in the contact pressure and sweat
concentration make this impractical.
The impedance of electrodes can be modeled as a capacitor and a resistor in paral-
lel as shown in Figure 7-32. For a monophasic 10 mA pulse with a duration of 10
μ
s
FIGURE 7-32
Simplified electrical
model of a 12 mm2
electrode. [Adapted
from (Kaczmarek,
Webster et al.,
1991).]
