Biomedical Engineering Reference
In-Depth Information
long times for the detection of the signals and also for the ecient stimulation
in order to restore natural sensations with prosthetic limbs. An illustrative
example of a biocompatible neural microprobe is presented in Figure 6.6. The
microprobes have longitudinal gold electrodes recessed within grooves designed
to guide the growth of regenerating axons. A biocompatible polymer (SU8),
tested first with primary Schwann cells and explanted root ganglion neurons
and then implanted in vivo, showed no signs of tissue damage or inflammatory
reaction a year after implantation.
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Arrays of multisite microelectrodes can be chronically implanted in the brain
to collect the intention-driven neuronal activity and convert it into a controlled
signal that enables the movement in paralyzed persons who are unable to sense
or move their limbs. Figure 6.7 shows an array of 100 electrodes. Each
electrode is 1mm long, spaced 400 mm apart, in a 10
10 grid.
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A computer-brain interface decodes the signals and transfers the firing
patters into motor commands. Subsequently, a computer gateway engages
effectors. The applications are intended for people with tetraparesis from spinal
cord injury, brainstem stroke, muscular dystrophy or amyotrophic lateral
sclerosis. Scalp-based, electroencephalography (EEG) driven brain-computer
interfaces, transcranial and electrocorticographic interfaces are currently
explored in order to solve the issues related to the implantation surgery and the
problems associated with the transcutaneous connections, plus the need to link
the patient to bulky equipment that necessitates the constant assistance of a
technician.
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Ideally, a wireless and miniaturized system, completely
automatic, would be required for practical use, although the challenges to
achieve this remain dicult.
For amputated limbs, with intact spinal cord function, electrodes can be
implanted only in the residual stump, so the muscle movement modulated by
intent is harnessed and then converted into electrical pulses that move the
prosthesis. Moving prosthetics at the speed of thought is possible by targeted
muscle re-innervation, a technique that implies the cutting of the redundant
nerves serving nearby chest muscles that previously moved the missing arm, for
example. The motor nerves in the arm stump are then separated and connected
to the chest muscles. In four to six months the signals generated from the cortex
to move an arm or hand is transmitted to the patient's chest muscles.
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The latest advances in robotics technology are extremely promising for
partially paralyzed individuals, the disabled and elderly people who have
mobility problems due to weak muscles. The 'hybrid assistive limb' is a
computerized suit equipped with sensors that detect the brain signals produced
when a person attempts to move. The sensors relay these signals to the motors
and computer inside the suit, which work together to direct limb movement
through mechanical leg braces strapped to the thighs and knees. All this
happens with only fractions of a second delay. The system provides the indi-
vidual with motor assistance. The 10 kg computer is wireless, battery operated
and belted to the waist. The suit can not only help the disabled and elderly, but
could also be used to aid caregivers lift or move those who are ill or infirm, and
even help laborers to move heavy equipment. The system is an example of a
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