Self-powered flexible piezoelectric motion sensor with spatially aligned InN nanowires

The power output of piezoelectric sensors based on nanowires (NWs) is governed by various factors, but largely depends on the alignment of the NWs. However, aligning the NWs has proved to be difficult. We successfully developed self-powered and flexible piezoelectric motion sensors (PMSs) with InN NWs as a response medium. The device performance was maximized by spatially controlling the alignment of the InN NWs by applying a magnetic field, which was the first step in the development of the NW-PMSs. The output voltage of the PMS with the InN NWs aligned along the bending direction was measured to be 3.05 V, which was a significant improvement of 2.44 times compared to that with randomly-distributed NWs. A systematic analysis of the extent to which the performance of the PMSs depend on the device parameters, such as the length of the NWs, bending frequency, operation time (up to 30 days), relative humidity, and bending cycle, indicates that the device performance is sufficient for real-life applications. For example, attachment of the self-powered PMSs to human joints such as the finger, wrist, elbow, and knee revealed that these motion sensors are highly effective, thereby indicating the possibility of detecting the various motions of the human body. The self-powered PMSs developed in this work are expected to contribute to rehabilitation, disease prevention, and human–machine interaction.