A preliminary study on detection of muscle activity using a flexible AlN piezoelectric thin film sensor

Muscle activity can be used to generate control signals for prosthetic devices and human-assisting manipulators. Recently, a novel method has been developed to measure muscle movement using a flexible piezoelectric thin film sensor, which is made of oriented aluminum nitride (AIN) film. However, rigorous relationship between muscle activity and the sensor's output has not been clarified. As a preliminary study, this paper proposes a simple model to illustrate the sensor output according to limb's cross-sectional dimensional change. Then, experiments have been carried out for comparison between muscle movement measurement methods using the AIN film sensor and ultrasound imaging. In the experiments, simultaneous measurement of signals of the AIN film sensor and ultrasonography was conducted during muscle actions. Both isokinetic and isometric muscle contractions were considered in the experiments. Since Young's modulus of the AIN film sensor is significantly larger than those of skin, panniculus adiposus, and muscle layer, precise dimensional changes in muscles cannot be obtained from the proposed AIN film sensor based on a direct sensor attachment method. From the experimental results, however, it is found that times of contraction and relaxation indicated by the AIN film sensor agree well with the results obtained from ultrasonography. It is considered that the timing information of muscle activity can be detected with the AIN film sensor.