First thought-controlled prosthetic arm to be implanted in patients

12 December 2012

The world’s first robotic arm controlled by thoughts is due to be implanted in the first patient this winter by Chalmers University of Technology in Sweden.

The prosthetic arm will be anchored directly to the skeleton and electrodes connected to nerves and muscles will transfer signals from the brain to the arm and also provide feedback to the brain.

Problems with current systems

Ever since the 1960s, amputees have been able to use prostheses controlled by electrical impulses in the muscles. Unfortunately, however, the technology for controlling these prostheses has not evolved to any great extent since then.

For example, very advanced electric hand prostheses are available, but their functionality is limited because they are difficult to control. "All movements must by pre-programmed," said researcher Max Ortiz Catalan. "It’s like having a Ferrari without a steering wheel. Therefore, we have developed a new bidirectional interface with the human body, together with a natural and intuitive control system."

Today’s standard socket prostheses, which are attached to the body using a socket tightly fitted on the amputated stump, are so uncomfortable and limiting that only 50% of arm amputees are willing to use one at all.

Robotic arm controlled by skin electrodes

Currently, in order to pick up the electrical signals to control the prosthesis, electrodes are placed over the skin. The problem is that the signals change when the skin moves, since the electrodes are moved to a different position. Additionally, the signals are also affected when we sweat, since the resistance on the interface changes.

In existing prostheses, amputees use only visual or auditory feedback. This means, for example, that you have to look at or hear the motors in the prosthesis in order to estimate the grip force applied to a cup if you want to move it around.

The new technique

This research project is using a Brånemark titanium implant (OPRA Implant System), which anchors the prosthesis directly to the skeleton through what is known as osseointegration. Osseointegration was developed in the 1960s by Professor Per-Ingvar Brånemark. He discovered that titanium is not rejected by the body, but is integrated into the surrounding bone tissue. The method is used for tooth implants and for leg, arm and face prostheses as well as for anchoring hearing aids.

The researchers are planning to implant the electrodes directly on the nerves and remaining muscles. Since the electrodes are closer to the source and the body acts as protection, the bio-electric signals become much more stable.

Osseointegration enables the signals inside the body to reach the prosthesis. The electrical impulses from the nerves in the arm stump are captured by a neural interface, which sends them to the prostheses through the titanium implant. These are then decoded by sophisticated algorithms that allow the patient to control the prosthesis using his or her own thoughts.

How the new system will work

With the new method, patients receive feedback too, as the electrodes stimulate the neural pathways to the patient’s brain, in the same way as the physiological system. This means that the patient can control their prosthesis in a more natural and intuitive way. This has not been possible previously.

Giving more control to amputees

Max Ortiz Catalan, doctoral student at Chalmers University of Technology said, “Our technology helps amputees to control an artificial limb, in much the same way as their own biological hand or arm, via the person's own nerves and remaining muscles. This is a huge benefit for both the individual and to society. Osseointegration is vital to our success. We are now using the technology to gain permanent access to the electrodes that we will attach directly to nerves and muscles.

"Many of the patients that we work with have been amputees for more than 10 years, and have almost never thought about moving their missing hand during this time. When they arrived here, they got to test our virtual-reality environment or our more advanced prostheses in order to evaluate the decoding algorithms. We placed electrodes on their amputation stumps, and after a few minutes, they were able to control the artificial limbs in ways that they didn’t know they could, most of the times. This made the patients very excited and enthusiastic.

“By testing the method on a few patients, we can show that the technology works and then hopefully get more grants to continue clinical studies and develop the technology further. This technology can then become a reality for lots of people. We want to leave the lab and become part of the patients’ everyday life. If the first operations this winter are successful, we will be the first research group in the world to make ‘thought-controlled prostheses’ a reality for patients to use in their daily activities, and not only inside research labs.”