In many cases, damage to the spinal cord and strokes result
in various types of paralysis. Nowadays, thanks to neural prostheses,
electrical stimulation makes it possible to restore certain functions
- such as the activity of the bladder and respiratory organs. Researchers
in the Neuros network are studying the application of this technique
to the particularly complex field of mobility. This European scientific
collaboration has already resulted in the development of a system
likely to be of help to the partially disabled. It also opens up very
interesting prospects for the creation of autonomous light-weight
implant devices capable of restoring movement to patients suffering
from many types of paralysis.
The technologies available are likely
to be of benefit to the partially disabled. Implantable technology
should make it possible to help a much larger number of patients
suffering from various forms of paralysis.
There are several thousand cases of damaged
spinal cords each year, the majority of them affecting young people
(60 % between the ages of 18 and 30). Such damage often results in
paralysis, since the neuromuscular system - although intact - no longer
receives commands from the central nervous system. In addition to
the personal tragedies caused by such damage, important social costs
are incurred through the need to provide life-long care for the patients.
The restoration of some degree of mobility would considerably improve
the quality of life of disabled persons by offering them access to
numerous occupations - mainly in the professional sector. Furthermore,
the restoration of partial mobility is also essential in order to
avoid certain secondary effects such as demineralisation of the bones,
kidney or bladder stones, infections, contractures, etc.
"A neural prosthesis delivers electric impulses which produce an action
potential in the nerves linked to the paralysed muscles," explains
Iddo Bante, the coordinator of the Neuros project. While the principle
is simple, its translation into clinical practice encounters control
system difficulties. Sensors are needed to deliver the desired stimulation
to the paralysed limbs; information on the movement performed needs
to be fed back to the system; care also has to be taken to avoid muscular
fatigue entailed by this non-physiological electrical activity. Furthermore,
existing systems demand particularly high levels of user attention.
The Neuros network aims to develop methods and techniques designed
to make the neural prosthesis concept fully applicable to mobility.
This objective calls for a triple approach: the development of autonomous
control systems, the use of information obtained from natural sensors,
and the development of stimulation systems and electrodes that permit
highly selective muscular activation.
"The implementation of such a project requires research of a very
high standard, which brings us to another fundamental objective
of the Neuros network, namely to ensure the ongoing training of
young scientists. Since the mid-1980s, our network has attained
the critical mass necessary to conduct work of this quality. European
researchers working in this field have established some very useful
contacts, and their collaboration has been given a powerful boost
through the training and mobility programme."
From the intention to the reaction
The first difficulty encountered in creating a neural prosthesis
is how to identify the user's intention. "We are not seeking to
use direct signals from the central nervous system, since these
do not allow sufficiently clear interpretation for the control of
a neural prosthesis. Accordingly, we translate the person's intention
by measuring movements, forces or even neural electrical activity
in those parts of the body over which the patient is still capable
of exercising voluntary control."
Researchers at the University of Ålborg (Denmark) recently succeeded
in demonstrating the possibility of selectively recording signals
emitted by sensory nerves, going on to show that these signals are
perfectly usable for the creation of an autonomous functional electrical
stimulation (FES) system. Amplifiers for these nerve signals have
been developed by a London-based team along with permanently implantable
electrodes to record nerve signals and to stimulate the neuromuscular
However, a neural prosthesis can only be truly autonomous provided
that it receives feedback on the movement performed. This is why
the Danish researchers are concentrating on picking up signals on
the skin to feed to the control loops of the neural prostheses.
"The dynamic relationship between the signal measured by the electrodes
and the pressure applied on the skin has been determined. On this
basis, the signals have been used in two applications. The first
sought to determine foot contact in a walking-aid system and the
second to detect slippage in an FES-controlled prehensile hand system."
In order to ensure autonomous mobility, the neural prosthesis also
needs to be informed of the position of the body and the limbs,
without having to depend on external equipment. "At the University
of Twente we have developed a triaxial accelerometer - capable of
assessing acceleration in three dimensions - which measures the
movements and orientation of certain parts of the body. This instrument
does not need to be connected to any laboratory equipment or external
reference, as is the case with other systems. Furthermore, its energy
consumption is low, and its very small size (3 cubic millimetres)
allows it to be implanted."
The control strategies developed by the European researchers have
been successfully tested. "The methods which permit the interpretation
of the user's intentions, in terms of the motor skill to be exercised,
have proved perfectly effective and do not require the user to give
explicit commands for each movement which he or she wishes to perform.
Control strategies for certain actions - getting up, remaining in
an upright position - have also been developed and simulated on
biomechanical models. Trials are currently under way. As a next
step, the most promising of these strategies will be transferred
to clinical systems. Although not directly linked to the objectives
of Neuros, this supplementary stage reflects the aims of many other
European projects, notably Tide Crest, in which we are also actively
"We hope that European industry will become increasingly involved,"
Iddo Bante stresses. "The technologies already available are likely
to benefit the partially disabled who have difficulty in walking.
As for the implantable easy-to-use autonomous technology which is
now within our reach, this should enable us to provide effective
neural prostheses of benefit to a much larger number of patients
suffering from various forms of paralysis."