Piezoelectric-film-based acoustic emission sensor array with thermoactuator for monitoring knee joint conditions

This paper presents a novel piezoelectric lead zirconium titanate (PZT) thin film acoustic emission (AE) sensor array equipped with thermoresponsive polymer actuators that enhance its sensitivity for monitoring knee joint conditions. A hydrothermal PZT film was grown on a micromachined titanium-film four-beam array for use as an acoustic sensing material. The thickness of the grown film was 10 μm and its grain size was less than 1 μm. Moreover, it had a perovskite structure and demonstrated strong piezoelectric properties. A thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPA) was formed on the beam array to electrically control the pressure on the sensing beam array and the underlying micropins. Such control was required for achieving an excellent couple for detecting a target, thereby increasing the sensitivity to AE signals. The fabricated PNIPA actuator was observed to generate more than 0.5% strain under a load six times its weight. Furthermore, the array configuration rendered the AE source detection more accurate compared with a regular single-detection-head sensor. The developed AE sensor was used for noninvasively examining the knee joint condition of a porcine leg when executing continuous back-and-forth motion. The result indicated that a joint under moderate extensive motion yielded higher AE signal amplitude and a wider bandwidth at major dominant frequencies compared with the joint before its extensive motion. An over-exercised joint with obvious cartilage degeneration yielded considerably high AE amplitude and a change in the pattern of dominant frequency peaks.