Use of a Shape-Memory Rotary Actuator In Neurological Research

This work presents a shape-memory rotary actuator, which was designed to passively mobilize body joints. The design of the actuator was specially conceived so that one and the same NiTi wire describes two co-axial counter-rotating coils along a spiralling sequence of pulleys. With this special arrangement, the magnetic field induced by the injected current is thus automatically counterbalanced and cancelled. In the presented embodiment, two rotary actuators, each mounting a 0.25mm-diameter wire, were attached to an exoskeleton that can be worn parallel to the ankle joint. By simultaneously heating the wire in each actuator by a 7s-pulse of 30V-dc, the rotation imparted to the ankle is approximately 30°, which is large enough to stimulate cerebral neural circuitry. Due to its unique electromagnetic compatibility, this actuator was utilised for a neurological research campaign on movement perception with functional magnetic resonance (fMRI) and magnetoencephalography (MEG). These techniques have tight electromagnetic noise constraints that could never be met by electric motors. The exoskeleton successfully provided repetitive passive mobilisation of the ankle in 15 healthy volunteers while either fMRI or MEG signals from their brains were recorded. No significant noise affected the acquisitions with either technique and interesting neural features could be revealed: fMRI showed a synchronisation between the passive movement and metabolic consumption in cortical areas devoted to somatosensory and motor control of the lower limb; MEG was able to contrast rest against movement, proving that passive mobilisation stimulates similar brain areas as voluntary motion of the ankle joint.