The second generation of bionic arms produced sensation in 2006 when Claudia Mitchell, 26, a former US marine, who lost both her arms after a motorcycle accident, could move her prosthetic arms, using the power of her thoughts.

Now, she experiences the third generation of bionic arms that move and feel like the real thing, due to a new surgical technique which permits the patient to intuitively control her limb and regain her sense of touch.

Physicians in Chicago, US, have attached the stumps of the motor nerves that once went to her arms into the muscles in her chest and side. The stumps of her sensory nerves, that once received impulses of heat and touch from her now-amputated arm, have been attached to the skin on her chest. With the second generation of bionic arms, Mitchell could accomplish astonishing tasks compared to previous prototypes: she could carry out simple tasks at her command, like cutting up food at four times the speed of conventional prosthesis. Now she has regained the sensation of having her arm touched when the patch of skin on her chest is touched.

The majority of the most advanced prosthetic arm techniques employ myoelectric motors that respond to commands of muscles in the chest and back. This technique is limited, because it permits only one movement at a time, like bending the elbow or opening the hand and the patient must learn how to make these movements using their chest and back muscles.

The new technique developed by Todd Kuiken and colleagues at the Rehabilitation Institute of Chicago is named "targeted muscle reinnervation". The stump of the motor nerves attached to nearby muscles are fitted with myoelectric sensors to detect contraction. "When the person imagines closing their hand, the signal goes down the nerve. Then we use that signal to control the prosthetic hand," explains prostheticist Laura Miller.

Three patients have received till now the new much more efficient bionic arms. The researchers have successfully reattached in the case of Mitchell the sensory nerves into the skin of her chest.
When she is touched in her upper chest, the patient feels sensations as if from her missing fingers, with a distinct range of pressures, temperatures and vibrations. “This should allow more coordinated movements,” Miller explains. "Anybody who's ever tried to button their shirt when their hands are cold will know how important sensory-motor integration is," says Greg Clark, a bioengineer at the University of Utah in Salt Lake City, US, involved in developing a similar system that uses electrodes to tap signals from nerves directly.

The researchers are developing a prosthetic arm able to pick up sensory input (for pressure, vibrations, and temperature) and transmit it to the chest zone that feels like the hand, by now only for pressure, using a plunger-like mechanism.

Photo credit: Todd Kuiken et al