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In hip arthroplasty, relative movements between the femoral stem and bone can lead to implant loosening, resulting in extensive bone loss. Acoustic emission (AE) analysis is a promising technique for a nondestructive and noninvasive detection of these relative movements. To develop such a detection method, in vitro investigations using piezoelectric AE sensors on implant stems in artificial or human femora are required to characterize the AE signals induced by loosening. This study aims to identify suitable coupling materials to bridge the gap between the planar AE-sensor surface and the exposed freeform surface of the femur. Four coupling materials, both synthetic and natural, with acoustic properties similar to human soft tissue were investigated for signal attenuation and repeatability between tests. The synthetic materials demonstrated better inter-sample repeatability. One synthetic material exhibited higher flexibility, enabling better adaptation to the sensor and resulting in significantly lower signal attenuation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025. Published by Elsevier Ltd.)