The bionic hand closes slowly. Its slender metal digits whirr as they jitter into a loose fist, as though they are wrapping around an invisible baton. "OK, closed," says the test subject. The test subject is Amanda Kitts. In 2006, a Ford F350 hit her Mercedes sedan head-on. The collision rent the truck's tire from its chassis and shoved the axle into Kitts' car, where it nearly severed her arm. "It wasn't completely off, but it was mincemeat," she says. "There was no saving it.

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