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Scientists hope to give artificial limbs “feelings”

Nov. 26, 2007
World Science staff

A ma­jor draw­back of ar­ti­fi­cial limbs is that they lack feel­ings. With­out the di­rect feed­back that comes from the tou­ch sensa­t­ion, the re­place­ment limb is harder to con­trol.

Sci­en­tists now say they may have par­tially solved the prob­lem.

In two arm am­putees, sur­geons re­rout­ed ma­jor nerves that nor­mally lead to the hand so that they in­stead go to pec­to­ral mus­cles in a patch of the chest. As a re­sult, when pa­tients were tou­ched in that part of the chest, they felt as though they were be­ing tou­ched both there and on the hand at the same time, the re­search­ers re­ported.

“This work of­fers the pos­si­bil­ity that an am­pu­tee may one day be able to feel with an ar­ti­fi­cial limb as al­though it was his own,” they wrote in a pa­per on their find­ings, to ap­pear in this week’s early on­line edi­tion of the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

To com­plete a sys­tem that would give feel­ing to an ar­ti­fi­cial hand, they wrote, “sen­sors could be placed in a pros­thetic hand to meas­ure con­tact forc­es and tem­per­a­ture.” Mean­while, a de­vice con­nect­ed to these sen­sors could stim­u­late the “re-innervated” skin on the chest or else­where. That could “pro­vide sen­so­ry feed­back that ap­pro­pri­ately cor­re­lates to hand per­cep­tion.”

What might lim­it such a pro­ce­dure’s ef­fec­tive­ness, they ac­knowl­edged, is that the new “hand” sensa­t­ion is im­pre­cise. When a sin­gle point on the chest was tou­ched, pa­tients felt as though a large part of their hand, or even more than one part, was be­ing tou­ched.

This con­ceivably could change as pa­tients adapt to con­sist­ent use of a de­vice, the sci­en­tists added hope­ful­ly. The feel­ing of be­ing tou­ched in two places at once did­n’t con­fuse the pa­tients, ac­cord­ing to the re­search­ers, who were able to cre­ate maps of which chest ar­eas cor­re­sponded to which zone of the “hand” in terms of feel­ings.

The team, Todd Kuiken of the Re­ha­bilita­t­ion In­sti­tute of Chi­ca­go and col­leagues, said the pe­cu­liar du­al sensa­t­ions have been “highly sta­ble” so far in both pa­tients, a 54-year-old man and a 24-year-old woma­n. In both, the situa­t­ion has per­sisted to the pre­sen­t—more than five years in the ma­n’s case, they wrote.

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A major drawback of artificial limbs is that they lack feelings. Without the direct feedback that comes from the touch sensation, the replacement limb is harder to control. Scientists now say they may have partially solved the problem. In two arm amputees, surgeons rerouted major nerves that normally lead to the hand so that they instead go to pectoral muscles in a patch of the chest. As a result, when patients were touched in that part of the chest, they felt as though they were being touched both there and on the hand at the same time, the researchers reported. “This work offers the possibility that an amputee may one day be able to feel with an artificial limb as although it was his own,” they wrote in a paper on their findings, to appear in this week’s early online edition of the research journal pnas. To complete a system that would give feeling to an artificial hand, they wrote, “sensors could be placed in a prosthetic hand to measure contact forces and temperature.” Meanwhile, a device connected to these sensors could stimulate the “re-innervated” skin on the chest or elsewhere. That could “provide sensory feedback that appropriately correlates to hand perception.” What might limit such a procedure’s effectiveness, they acknowledged, is that the new “hand” sensation is imprecise. When a single point on the chest was touched, patients felt as though a large part of their hand, or even more than part, was being touched. This conceivably could change as patients adapt to consistent use of a device, the scientists added hopefully. The feeling of being touched in two places at once didn’t confuse the patients, according to the researchers, who were able to create maps of which chest areas corresponded to which zone of the “hand” in terms of feelings. The team, Todd Kuiken of the Rehabilitation Institute of Chicago and colleagues, said the peculiar dual sensations have been “highly stable” so far in both patients, a 54-year-old man and a 24-year-old woman. In both, the situation has persisted to the present—more than five years in the man’s case, they wrote.