Once more, with feel­ing

Pros­thetic hand al­lows am­putee to feel and make fine distinctions be­tween ob­jects in his grasp.

The Star Malaysia - Star2 - - SCIENCE - By MELISSA HEALY

THE hu­man hand is a won­der of strength, sen­si­tiv­ity and dis­crim­i­na­tion – not only be­cause of those four fin­gers and the op­pos­able thumb, but be­cause of the hu­man brain that con­trols it. No won­der, then, that for those who de­sign hand pros­the­ses, recre­at­ing the nat­u­ral dex­ter­ity of the brain-pow­ered hand is a daunt­ing chal­lenge.

But a new study demon­strates that, with the aid of some ar­ti­fi­cial sen­sors and elec­trodes sunk into a user’s arm, a pros­thetic hand can be made to de­tect the need for a firm grasp or a light touch, to make fine distinctions be­tween an ob­ject’s tex­ture, weight and size, and to re­spond ac­cord­ingly with no de­tectable de­lay.

The lat­est re­port, pub­lished last Wed­nes­day in Sci­ence Trans­la­tional Medicine, marks another key step in the ef­fort to en­hance the func­tion of pros­thetic limbs by de­vis­ing “brain­ma­chine in­ter­faces”.

In bioengi­neer­ing labs across the world, engi­neers and brain sci­en­tists are work­ing to re-cre­ate the con­nec­tion that once ex­isted be­tween the brain and the lost limb and trans­fer it to the new pros­thetic limb.

By a va­ri­ety of dif­fer­ent meth­ods, the re­searchers are try­ing to make pros­thetic limbs that step, bend, reach or grasp in re­sponse to a user’s thoughts. Here, how­ever, a team of engi­neers from France, Italy, Bri­tain and Den­mark worked to­gether to make the com­mu­ni­ca­tion be­tween brain and pros­the­sis two-way.

The pros­thetic hand could be di­rected to open and close at the con­scious com­mand of the user. But it could also send back to the user’s brain in­for­ma­tion about the touched ob­ject – de­tails, for in­stance, about its size, weight, tex­ture and den­sity.

Real-time con­trol

The aim of such “real-time bidi­rec­tional con­trol”, as the au­thors of the cur­rent study put it, would al­low pros­thetic hand move­ments that are more nat­u­ral, more dex­ter­ous and more re­spon­sive to a user’s needs.

Al­low­ing sen­sory in­put to tem­per mo­tor com­mands would some­day al­low a pros­thetic user to em­ploy a dif­fer­ent grasp to pluck an egg from a nest than she would to wield a ham­mer.

With Den­nis Aabo Sørensen, a 36-year-old man who had his in­jured left arm am­pu­tated be­low the el­bow 10 years be­fore, the re­searchers tested their abil­ity to re­store the “user con­trol loop”. Mo­tor com­mands for a va­ri­ety of dif­fer­ent grasps, as well as for an open hand, made their way down the arm and were de­tected by sen­sors on the skin of the sub­ject’s stump, then digi­tised and con­veyed to the pros­thetic hand. This ca­pa­bil­ity is al­ready mak­ing its way into broader use in pros­thet­ics.

But re­turn­ing sen­sory in­put from an ar­ti­fi­cial hand is a newer trick. To do so, the re­searchers im­planted a suite of elec­trodes into two nerves em­bed­ded in the mus­cles of the sub­ject’s up­per arm.

Those elec­trodes car­ried elec­tri­cal charges in three dif­fer­ent strengths, cor­re­spond­ing to sen­sors em­bed­ded in the pros­thetic hand it­self. The light­est con­tact with an ob­ject would set off the weak­est charge de­tectable to the sub­ject.

A grasp that ex­erted in­creas­ing force on the sides of an ob­ject would set off a charge of in­creas­ing power, stop­ping just short of in­duc­ing a sen­sa­tion of pain.

Dex­ter­ity

Four months and 700 tri­als later, the sub­ject, who wore a blind­fold and sound-can­celling ear­phones dur­ing his test runs, was not only able to close his hand around a va­ri­ety of ob­jects with a high de­gree of dex­ter­ity; he was also able to dis­tin­guish be­tween a pack of cot­ton, a stack of plas­tic glasses and a piece of wood, and with­out any dis­cernible de­lay, re­spond to those very dif­fer­ent tex­tures with dif­fer­ent grips.

The sub­ject be­gan re­fin­ing his use of the pros­thetic hand al­most im­me­di­ately, and the re­searchers ob­served clear signs of grow­ing “sen­si­tiv­ity” within a week of his try­ing the user con­trol loop for the first time.

In time, the 36-year-old am­putee who served as the group’s sub­ject be­came able to dis­tin­guish quickly – by pros­thetic touch alone – be­tween such tricky paired ob­jects as a man­darin orange and a base­ball.

His re­stored sen­sa­tion ap­peared to in­duce an “ar­ti­fi­cial al­beit close to nat­u­ral neu­ral cod­ing” that al­lowed him to learn quickly on his own and to use his grow­ing in­tu­ition to fill in un­known prop­er­ties of the ob­jects his pros­the­sis touched. – Los An­ge­les Times / McClatchyTri­bune In­for­ma­tion Ser­vices

Sen­sory re­train­ing: den­nis aabo Sørensen, of aal­borg, den­mark, who lost his left hand in a fire­works ac­ci­dent a decade ago, show­ing the sen­sory feed­back en­abled pros­the­sis he is test­ing in rome. It’s far from the bion­ics of sci­ence fic­tion movies but is part of a ma­jor ef­fort to cre­ate more life­like, and us­able, pros­thet­ics. — aP Photo/Paolo San­talu­cia.

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