Although no medical cure currently exists for spinal cord injury, paralyzed patients in the future could be able to walk again thanks to robotic exoskeleton technology, being developed all around the world. A team of Belgian researchers is working on a mind-controlled variant called Mindwalker, a system that converts electroencephalography (EEG) signals from the brain, or electromyography (EMG) signals from shoulder muscles, into electronic commands to control the exoskeleton.
The Mindwalker project (also known as: Mind-controlled orthosis and VR-training environment for walk empowering) is a three-year initiative supported by 2.75 million euros in funding from the European Commission. The ultimate goal of the project is to help paralyzed people who spend their lives in a wheelchair get back them on feet by bypassing the spinal cord entirely and routing brain signals to the robotic exoskeleton.
One approach for controlling the exoskeleton uses so-called ‘steady-state visually evoked potential’, a method that reads flickering visual stimuli produced at different frequencies to induce correlated EEG signals. Detection of these EEG signals is used to trigger commands such as ‘stand’, ‘walk’, ‘faster’ or ‘slower’.
A second approach is based on processing EMG signals generated by the user’s shoulders and exploits the natural arm-leg coordination in human walking: arm-swing patterns can be perceived in this way and converted into control signals commanding the exoskeleton’s legs.
A third approach, ‘ideation’, is also based on EEG-signal processing. It uses the identification and exploitation of EEG Theta cortical signals produced by the natural mental process associated with walking. The approach was investigated by the Mindwalker team but had to be dropped due to the difficulty, and time needed, in turning the results of early experiments into a fully exploitable system.
Regardless of which method is used, the brain signals have to be filtered and processed before they can be used to control the exoskeleton. To achieve this, the Mindwalker researchers fed the signals into a ‘Dynamic recurrent neural network'(DRNN), a processing technique capable of learning and exploiting the dynamic character of the BNCI (brain-neural-computer interface) signals.
The team adopted a practical approach for collecting EEG signals from the user’s scalp. Most BNCI systems are either invasive, requiring electrodes to be placed directly into brain tissue, or require users to wear a ‘wet’ capon their head, necessitating lengthy fitting procedures and the use of special gels to reduce the electrical resistance at the interface between the skin and the electrodes. While such systems deliver signals of very good quality and signal-to-noise ratio, they are impractical for everyday use.
The Mindwalker team therefore turned to a ‘dry’ technology developed by Berlin-based eemagine Medical Imaging Solutions: a cap covered in electrodes that the user can fit themselves, and which uses innovative electronic components to amplify and optimise signals before sending them to the neural network.
‘The dry EEG cap can be placed by the subject on their head by themselves in less than a minute, just like a swimming cap,’ Mr Ilzkovitz says.
The exoskeleton supports up to 100kg person and is powerful enough to recover balance from instability created by the user’s torso movements during walking or a gentle push from the back or side. The exoskeleton’s own weight is 30kg without batteries, and uses springs fitted inside the joints that are capable of absorbing and recovering some of the energy otherwise dissipated during walking, in order to make it more energy efficient.
Once the tests with able-bodied trial users will be completed the system will then be transferred to the Foundation Santa Lucia for conducting a clinical evaluation with five to 10 volunteers suffering from spinal cord injuries. These trials will help identify shortcomings and any areas of performance improvement, the project coordinator says.