Biomedical Engineering Reference
In-Depth Information
body. Current is driven into the tissue
first in one direction and then, after a brief interval,
in the other. Thus, ions moving in the tissue would
fi
rst be pushed one way and then
quickly the other way, stimulating the tissue and leaving the ions in their former positions
within the electrodes, interstitial
fi
fluids, and cells. This waveform is known as a
balanced
bidirectional pulse pair
. Neurons are especially sensitive to poisoning by metallic ions
released from the electrodes as well as by the products of electrolytic decomposition of
salts and water. Microscopic studies of brains stimulated with this balanced pulse pair at
low current densities showed that it causes no electrolytic damage to the neurons.
As shown in Figure 7.6
b
, true monophasic pulses are produced only when the energy
source is switched along the way to the tissue and there is no way for the electrode-tissue
interface capacitance to discharge. True monophasic waveforms are seldom, if ever, used
to stimulate tissue because they introduce net charge through the tissue that can cause elec-
trolysis and tissue damage. Monophasic waveforms really tend to be asymmetric biphasic,
as the net charge built up in the electrode-tissue interface or in a dedicated dc-blocking
capacitor discharges as shown in Figure 7.6
c
-
e
.
fl
DIRECT STIMULATION OF NERVE AND MUSCLE
Implantable cardiac pacemakers have been around since the late 1950s. More recently, the
same basic techniques have been applied to stimulate the vagus nerve for the control of
epilepsy, to stimulate the sacral roots to control the bladder and correct erectile dysfunction,
and to stimulate nerves in the spine for the control of pain and angina. In addition, interest in
functional electrical stimulation (FES) has grown rapidly during recent years, due primarily
to progress made in miniaturized hardware that makes multichannel stimulators possible.
New surgical techniques enable the use of chronically implanted stimulators to stimulate
speci
c nerves and brain sections directly within the body, making it possible to restore func-
tion lost due to disease or trauma. Advances are being made rapidly in the development of
implants for restoring limbs, sight (e.g., through arti
fi
cial retinas or by direct stimulation of
the visual cortex), and hearing (e.g., through cochlear implants) [Loeb, 1989].
As shown in Table 7.1, and with the exception of cardiac de
fi
brillation, all other applica-
tions in which the electrodes are placed in close contact with target tissue require the deliv-
ery of relatively narrow pulses (
fi
35 mA).
These can easily be produced with miniature circuits that use standard bipolar or MOSFET
transistors (discrete transistors or as part of an IC), tantalum capacitors, and implantable-
grade lithium batteries. Implantable stimulators typically use either a constant-current source
or a capacitor discharge circuit as output stages to generate stimulation pulses.
2 ms) of low voltage (
12 V) at low current (
Capacitor-Discharge Stimulators
In a capacitor-discharge output stage, an energy-storage capacitor, usually called a
tank
capacitor
, is charged to the desired peak voltage and then delivered to the target tissue.
Figure 7.7 shows the circuit of a simple capacitor-discharge pulse generator circuit that
can generate stimulation pulses with an amplitude of either 3 or 6 V from a 3-V source
(e.g., a single lithium battery). When inactive, the control logic for the stimulator sets the
HIGH AMPLITUDE STIMULUS line low, which charges tank capacitor C2 to VDD.
The STIMULUS signal is maintained low to keep transistor Q2 open, and line ACTIVE
DISCHARGE is maintained low to keep switch Q1 open. Coupling capacitor C1 slowly
discharges by way of resistor R1 (100 k
Ω
) through the tissue and electrodes connected to
terminals V
.
When a stimulus is to be generated, and if the amplitude selected is 6 V, the HIGH
AMPLITUDE STIMULUS line is set high, which closes Q4 and opens Q3. This causes
and V
Search WWH ::
Custom Search