Biomedical Engineering Reference
In-Depth Information
electrodes (a dull charcoal color) is a microporous structure that has a very rough texture
rather than a smooth one.
Our experience with electrodes coated with titanium nitride and IROX is that they per-
form similarly. Typical capacitances are in the range 10 to 20
F/mm
2
. Platinum black and
activated glassy carbon are other electrode materials that have high capacitance and exhibit
excellent biocompatibility. Other interesting possibilities are to coat the electrode with lay-
ers of microspheres to increase the wetted surface or to create porous metallic electrodes
and then “plug” the pores with carbon.
µ
Neuromuscular Electrical Stimulation
Neuromuscular electrical stimulation (NMES) is the use of electrical stimulation of the
intact peripheral nervous system to contract a muscle, either through direct activation of
the motor neurons in the mixed peripheral nerve, or indirectly through re
ex recruitment.
When the stimuli are used to activate muscles directly, without activation of the peripheral
nerve, the modality is known as electrical muscle stimulation (EMS).
NMES may be used for therapeutic or functional purposes. Therapeutic use is directed
toward lessening impairments, prevention of secondary complications, or halting progres-
sion of a disabling condition. Therapeutic NMES includes strengthening muscles, lessen-
ing of spasticity,
fl
ow.
Functional NMES is more commonly known as functional neuromuscular stimulation
(FNS), which is a replacement for lost or impaired motor control. Today, transcutaneous
FNS is widely used in the rehabilitation of paralyzed patients in whom natural nervous
control of muscular contraction has been lost due to a spinal cord injury or a central nerv-
ous system disorder. In its best known applications, FNS has been used to restore function
to a
preventing muscle atrophy,
and improving regional blood
fl
cial electrical stimulation patterns that enable the sub-
ject, for instance, to use upper extremity functions, to stand up, or to walk [Kralj and Bajd,
1989].
One or more pairs of surface electrodes along with conductive creams or gels are used
to activate the excitable tissues. Surface electrical stimulation typically consists of a train
of regular monophasic or biphasic pulses. The rate at which the nerve
ff
ected limbs by providing arti
fi
re depends
on the frequency of pulse repetition. A single pulse produces a short-lived muscle twitch
of not more than 250 ms. If pulses are repeated more frequently than this, the muscle does
not have time to relax between stimuli, and at some point tetanic (continuous) contraction
occurs.
Regardless of what mechanism is used to evoke muscle contractions, nerve and muscle
stimulators are FDA class II prescription devices. The international standard that speci
fi
bers
fi
cally
covers the design, performance, and testing of these devices is IEC-60601-2-10,
Medical
Electrical Equipment—Part 2: Particular Requirements for the Safety of Nerve and Muscle
Stimulators
.
Transcutaneous NMES devices are relatively simple. Figures 7.17 and 7.18 show the
circuit diagram of a battery-powered stimulator that can be used in NMES therapy and as
an output channel for transcutaneous FNS. Whenever a stimulus is required, Q1 discharges
C1 into step-up transformer T1, which yields a high-voltage pulse on its secondary. The
transformers that we have used successfully are the type built for the small inverters
designed for
fi
150 V peak) charges
capacitor C2 to a voltage regulated by zener D1. The setting of R5 selects the current that
fl
fl
fluorescent lamp lanterns. The high-voltage pulse (
flows through the electrodes. With switch SW1 open, the pulse output is monophasic
(although the capacitance of the electrodes will generate a discharge phase through R7).
With the switch open, the charge accumulated in the coupling capacitor (C2 and C3 back-
to-back in series
flows through the electrodes after the stimulus to yield
net-zero charge transfer through the tissue.
5
µ
F nonpolar)
fl
Search WWH ::
Custom Search