„Further Optimising the Natural Motion Sequence“ -- COMPAMED Trade Fair
Photo: Prof. Klaus-Peter Hoffmann
Prof. Klaus-Peter Hoffmann;
© Fraunhofer IBMT

Klaus-Peter Hoffmann works at Fraunhofer Institute for Biomedical Engineering (IBMT) in Germany. There he is head of the department of Medical Engineering and Neuroprosthethics. COMPAMED.de spoke with the expert about artificial arms controlled by muscles, driving a car with hand replacement and prostheses of the future.

COMPAMED.de: Mr. Hoffmann, in former times, prostheses were made of wood or iron. Today they are complicated high tech products which replace a lost body part better and better. Are there any differences between people with and without prostheses yet?

Klaus-Peter Hoffmann: Yes, there are. In spite of giving back patients the biggest part of their independence, prostheses cannot replace a missing part of the body completely.

COMPAMED.de: Can you name some weak points?

Hoffmann: If the patient takes an object with his hand prosthesis he cannot feel the structure of the surface yet. With the temperature it is the same. He lacks the sensory feedback. However, there is also another big disadvantage. With the latest arm prostheses one can implement movements only successively and not at the same time. The patient can open his hand or turn it around, but cannot execute both movements together.

COMPAMED.de: The motion of prostheses become possible at least through the muscular strength of the patient. Can you explain this?

Hoffmann: In the amputation stump there are often still muscles leftover. If they are contracted, an electric potential originates and is captured via electrodes on the skin. The impulses are amplified, converted into control signals and transferred to small electric motors of the prosthesis. In this way the patient can move his prosthesis.

COMPAMED.de: However, there are also researchers who go even further. In some cases, the prosthesis is not moved by muscle power, but by thoughts of the patient. How is this possible?

Hoffmann: It does not matter if a patient thinks of a movement or if he executes the movement in reality. In both cases there originates an electrical activity which is captured by electrodes on the scalp directly.

COMPAMED.de: How can the electrodes be fixed on the head?

Hoffmann: There are caps which the patient can put on his head. On those caps the electrodes are fixed. But there is also the possibility to implant the electrodes surgically in the brain. With both possibilities the movement command of the brain is sent to a computer which manages the prosthesis. However, this is only one possibility to move prostheses by thought. On a young Austrian man doctors have linked the peripheral nerve with the musculature in the breast area. The peripheral nerve is supplied by the brain area that releases the movements of hand and arm. If the patient now thinks of a movement, the brain impulses are diverted to the breast muscles, captured there with small sensors, strengthened and passed on to the prosthesis.

COMPAMED.de: This example recently was topic in all media. In this case, the principle of prostheses which are controlled by muscles and thoughts were combined. The young man now can even drive a car with his arm prosthesis. Do you think that is really safety on the road?

Hoffmann:: I think this technique is as safe as any other technical system in a car too. Since the patient can control gas and brake pedal normally with his feet I don´t see any real danger. Moreover, he has passed his driver licence test. If there had been any doubts to the technology, I am sure that he would not have been admitted to the test.

Photo: arm prosthesis and a cup
The development of arm prostheses is a big challenge for medical technology; © Hemera

COMPAMED.de: What future characteristics prostheses can provide in your opinion?

Hoffmann: We just develop a system to control hand prostheses even better. By this we try further optimising the natural motion sequence. Besides, the electrodes for capturing the muscle activity are not fixed anymore on, but also under the skin, directly onto the muscle. Then a measuring implant should take up several muscle signals, so that different movements of the hand can be released at the same time. We hope that people with arm prostheses are able to move in future all fingers individually, distinguish cold and warm temperature and feel the structure of surfaces. The development of such prostheses is one of the biggest challenges to medical technology.