ARTIFICIAL ORGANS

The sensorial awakening of the bionic hand

Tests on using the prosthesis prototype SmartHand at the Department of Electrical Measurements, Lund University (SE). © Lund University
© Lund University
© Lund University
© Lund University
© Lund University
© Lund University
© Lund University
© Lund University

Are artificial organs the solution to the shortage of transplant organs? In an age when the living and the man-made are coming together, the excitement is palpable. But what about cooperation between biological and electronic processes? The search for a new generation of bionic hand provides elements of a response.

To constitute an alternative to a donor organ, the artificial organ must do more than fulfil the physiological function of the natural organ. In the case of renal insufficiency, for example, a dialysis machine effectively replaces the kidney. But it usually requires patients to attend a specialist medical centre three times a week to be connected up to a machine for four hours at a time. The artificial organ would be implanted in the patient. For this it must be different in scale and must merge with the body, become biocompatible, be tissue engineered and use nano-compressors and intelligent sensors. In short, it must be at the leading edge of the most advanced technologies. To date, the fully artificial organ remains essentially experimental

Replacing a limb whose physiological function (walking, gripping, etc.) is activated by a conscious decision brings an additional challenge, that of manipulating the artificial limb as intuitively as possible. The brain issues the command and, despite the amputation, the cerebral nerve impulses continue to be transmitted to the remaining muscles. Muscles that, at least in the imagination, control the lost limb. In the latest generation prosthesis, electrodes fitted in the joint record the muscular electrical activity of the stump. These signals, known as electromyograms (EMGs), control the movement of an artificial hand, for example.

Did you say bionic?

‘The existing systems are in reality very primitive,’ explains Fredrik Sebelius, an electronics expert at Lund University (SE) and coordinator of the SmartHand project. Funded to the amount of EUR 1.8 million under the Sixth Framework Programme, by the end of 2009 the project will be ready with a prototype of the next generation of artificial hands. ‘Present systems,’ continues the researcher, ‘have two electrodes, each placed on a different muscle.

The strongest signal prevails and it can take as long as six months to succeed in controlling the only two movements possible, namely opening and closing the hand.’ The first imperative is to design a prosthesis that adapts to the user.' ‘The problem stems from the natural instability of the muscular signals,’ explains Fredrik Sebelius. ‘These vary from one individual to the next, with the degree of muscular training, and even from day to day depending on what you have drunk or eaten.’ The aim is to improve the EMG processing and widen the range of possible movements. ‘We have placed 16 electrodes on the forearm. On this basis, our method teaches the system to recognise the different combinations of EMG received.’ Incorporating a genuinely intelligent system, the solution involves a shape recognition algorithm based on artificial neuron networks. The system learns while the user executes a series of imposed movements: bending the fingers the one after the other, then all together, with the fingers spread wide, the thumb pointing outward, etc. ‘After carrying out this series of exercises the system is calibrated. Two hours is now enough for a user to master an initial set of movements.’

A sensory map of the lost hand

A functional hand is essential, but a hand is not just a perfected gripping tool. It is an inherent part of our identity, of our relationship with the world and with others. A hand is irreplaceable: at the clinical level alone transplantation still presents too many risks and disadvantages. Göran Lundborg, a hand surgeon at the Malmö University Hospital (SE) and member of the SmartHand project, highlights something of a paradox: ‘The amputee would like to have an artificial hand but one which he experiences as an inherent part of his own body.’

In principle, it is possible to induce a sensitivity in the artificial hand by connecting a sensor located on the prosthesis to an electrode implanted directly into the somato-sensory cortex or into the peripheral nervous system. But the specific nature of the relationship between a lost hand and the brain allows us to envisage another possibility. ‘The amputation of a hand causes considerable functional reorganisations of the sensorial cortex,’ explains Göran Lundborg. ‘One of the consequences of these reorganisations is the formation of a sensory map of the lost hand on the surface of the stump.’ Stimulation of a precise location on this map is felt by the amputee at the same place on his lost hand (see box). Is this then the way of giving the amputee the sensation of having an artificial hand that is very much his own? It is too soon to say. The first stage is to re-create the sensation of force exerted on the object gripped.

A false hand that feels real

‘Existing artificial skins were quickly discarded,’ Fredrik Sebelius. ‘Too many integrated sensors and that means wires with the constant risk of them breaking in a flexible structure.’ While awaiting more practical skins, researchers have come up with a clever trick. This involves measuring the tension of a cable that, inside each finger, is linked to a motor. This cable causes the joints to bend mechanically on each finger used in gripping an object. ‘As soon as one of the fingers makes contact with an object we have a signal,’ explains Fredrik Sebelius.

This signal is relayed to the user using an experimental device that commands five activators each positioned in contact with the ‘fingers’ of the stump’s sensory map. The user feels a pressure from 1 to 10 depending on the tension measured and is able to identify which of the fingers is at the origin of these sensations. This device is currently at the clinical evaluation stage. ‘We have demonstrated that the stimuli activate the corresponding areas of the brain, triggering genuine sensations from the artificial hand,’ announces Göran Lundborg.

It is difficult at this stage to assess the cost of such equipment, but it will be high. This raises the question of who will pay. The patient, assisted to a greater or lesser extent by his private insurance policy or the national public health system? ‘If the patient regains his autonomy in day-to-day life and is less dependent on medical and social assistance, the outlay could prove profitable in the long term for society,’ believes Freygardur Thorsteinsson, an engineer with the industrialist Össur (IS), a partner in the project. Hence the attempts to identify those patients who would benefit most. ‘We will be charging a university or an independent research body with the socio-economic analysis of the long-term cost effectiveness of our high-tech prostheses,’ he says. So what will finally decide the future for artificial organs? Urgency in the face of organ shortages, medical and scientific progress or economic considerations? Ultimately, it will be down to societal choices.

Sandrine Dewez

  1. H. Ehrsson, B. Rosén, A. Stockselius, C. Ragnö, P. Köhler & G. Lundborg, Upper limb amputees can be induced to experience a rubber hand as their own. Brain (2008) 131, 3443-3452, brain.oxfordjournals.org/cgi/content/full/131/12/3443
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The art of playing a good trick on the brain

You are seated at a table, with a rubber hand placed in front of you and your own hand concealed from view behind a screen. Simultaneously and at precisely the same spot, a brush strokes the hand placed in front of you as well as your own, which is hidden from view. You soon experience a strange illusion, the sensation of the brush strokes coming from the rubber hand that suddenly feels as if it is your own hand. What is happening? The brain has resolved the contradiction that it perceived between the visual and tactile information.

Carried out for the first time in 1998, this experiment known as the rubber hand illusion has just been repeated with 18 volunteer hand amputees at the Department of Neurosciences at the Karolinska Institutet (SE)(1) ‘For the first time an amputee has the feeling that an artificial hand is part of his own body,’ announces Göran Lundborg, a hand surgeon at Malmö University Hospital (SE). But how is this possible when the hand is not there? Because on the surface of the stump the lost hand is present in the form of sensations. Inducing tactile sensations in an artificial hand is therefore as simple as playing a trick on the brain.


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