Digital twin Exoskeleton assists after stroke

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Researchers have developed a system that allows patients to relearn how to move arms and hands affected by paralysis shortly after a stroke. Targeted stimulation of the forearm muscles, an exoskeleton, and the digital twin make it possible.

Researchers at the Technical University of Munich (TUM) specifically stimulate muscles in the forearm through so-called functional electrical stimulation (FES). This is necessary, for example, to move fingers, grasp objects, or catch balls. Since in cases of hemiplegia following a stroke, not only the hand is usually affected but the entire side of the body, a framework additionally supports the entire arm up to the shoulder.

24 stroke patients have already used the overall system consisting of an exoskeleton for the arm and shoulder in combination with FES as part of the ReHyb research project—half of them at the Schön Klinik Bad Aibling Harthausen, Germany, which took over the management of the study.

Modular system with computer game: Train independently

To train arm gripping and movement very quickly after a stroke, the researchers also use a computer game that automatically adjusts its difficulty level to the abilities of the users: On a screen, balls of different colors and speeds fly towards the patients. The task is to catch the balls and assign them to the corresponding different-colored compartments.

The secret to success: Digital twin and exoskeleton

At the core of the development by TUM Professor Sandra Hirche is a digital twin that captures the individual conditions of each patient and incorporates them into a control loop. Among other things, the researchers need to determine how well each patient can move their arm and hand. In a stroke, paralysis can occur, for example, due to damage to the motor area responsible for movement in the brain. However, it is not possible to predict how severely the signals that are transmitted from the brain to the forearm muscles will be affected after a stroke. "Individual muscle strands in the forearm muscles can be stimulated to the right extent to move the hand and fingers," says the researcher from the Chair of Information-Oriented Control.

Additionally, the researchers need to know how strongly the muscles should be stimulated and how much the exoskeleton should assist. "With the help of algorithms, we bring these individual pieces of information together in a control loop," says control systems expert Hirche. This digital twin is necessary to individually support the movement of the arm and hand in affected individuals.

What is part of the system's digital twin

• Capture muscle activity: The musculoskeletal system, motor control, and the muscle nerve system are each affected to varying degrees by a stroke. By measuring the electrical voltage in the muscle, it is possible to determine how severely the nerves are affected, which are supposed to send signals from the brain to the muscles and are ultimately responsible for the movement of the fingers and hands.

• Stimulate forearm muscles: For functional electrical stimulation (FES), a sheet with 32 electrodes is applied to the forearm of the affected individuals. Depending on which electrodes are activated, individual fingers move, the hand stretches or contracts. The threshold at which the fingers and hand move can be individually adjusted.

• Support with exoskeleton: An exoskeleton makes it easier for affected individuals to move and rotate their arm or shoulder, for example, through a spring mechanism or specific motors. This support is necessary because the arm muscles are already weakened by the condition. Additionally, it would be difficult to place electrodes on the shoulder, for instance, in a way that targets the correct muscles. The exoskeleton helps patients relearn how to coordinate the movement of the hand, arm, and shoulder together.