Fewer and fewer soldiers are being killed in combat today, due to the increased use of body armor. An unfortunate side effect, however, is that body armor does not protect extremities. Consequently, a larger percentage of those surviving attacks become amputees. Now the Department of Defense is funding efforts to improve prosthetic hands by directly interfacing them to the nervous system. If successful, future bionic hands could even restore a sense of touch.

Recently this cooperative effort between medical and bioengineering researchers was outlined at the 95th annual Clinical Congress of the American College of Surgeons, where they described an effort to produce a bionic hand that moves like a normal hand. Recounting their work at the congress was University of Michigan surgeons Paul Cederna and William Kuzon, who worked on the project with bioengineers David Martin and Daryl Kipke, as well as research investigator Melanie Urbancheck and surgical resident Brent Egeland.

Key to providing sensory feedback to the bionic hand wearer is directly interfacing to his nervous system. Luckily, after an amputation the nerve endings at the site start growing and branching out as if to try to reconnect with the severed nerves of the missing limb. When the sprouting nerve endings cannot find the missing limb today, they grow into a jumbled mass that can send false signals to the brain, eliciting the feeling of a "phantom" limb in the amputee.

When the medical researchers described this phenomenon to their bioengineering collaborators, they together designed an "artificial neuromuscular junction," which gives the growing sprouting nerves in an amputees stump somewhere to grow. Created with living muscle cells and a nano-sized polymer placed on a biological scaffold, the growing nerve cells readily reconnected up to it when placed over the stump. As a result, the severed nerves hooked up to the muscle cells, thereby enabling the signals that used to move the fingers of the hand to instead control the contraction of the muscle cells on the scaffold.

So far only rats have had the artificial neuromuscular junction installed, but tests showed that both motor signals from the brain and sensory signals going to the brain were reconnected. Just as important is the fact that the reconnected nerves stop growing afterward, instead of forming a jumbled mass. As a result, the nerves started exchanging signals with the artificial neuromuscular junction—sending signals to it that could eventually be used to control motors that move a bionic hand. The nerves that lead from the hand to the brain also seemed to automatically reconnect, holding out the potential of restoring a sense of touch and temperature by routing those signals to sensors installed on the bionic hand.

Now the researchers are designing an artificial neuromuscular junction that they hope to start using in human trials within three years.