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Using the latest technology, it is possible to invent an arm that attached on the body and move this arm with brain, the same mechanism as a real arm. It has been developed at the University of Pittsburgh, The arm has a fully mobile shoulder and elbow and a gripper that works like a hand.
They use monkeys for their experiment. Here’s how it works. They inserted a number of tiny probes into the brains of monkeys which can interpret signals from individual nerve cells in the motor cortex.
Given that even the smallest of movements involve thousands of nerve signals too numerous to track, the scientists used a gadget to work out the most important signals. Then the computer algorithm averages out the dominant signals to obtain the overall movement. The algorithm then sends this dominant movement message to the robotic arm, which carries out the actions the monkey intended to perform with its own limbs. The algorithm also used to accurately predict the velocity and direction of normal arm movement.
However, the concern of this robo-arm, it becomes more difficult to generate more complex movements, such as those of the human hand. The biggest problem is durability of the probes. Typically they last for about six months.

Sources:
http://news.bbc.co.uk/2/hi/health/4275245.stm