Optimal fit of additively manufactured earmoulds: Characterisation of adhesion properties using oscillatory tribology

An essential criterion for the overall performance of hearing aids is a reliable fit of the earmoulds in the ear canal, thereby ensuring wearing comfort and patient safety. Insufficient adhesion may lead to acoustic leakage and even unintended loss of the device, particularly under the influence of jaw movements and biological secretions such as sweat and cerumen. Additive manufacturing enables the fabrication of polymer earmould surfaces with tailored microtopographies that improve frictional retention in the ear canal, thereby reducing the risk of unintentional device dislodgement. In this work, we present an approach to characterise the behaviour of additively manufactured, microstructured polymer surfaces against a silicone cylinder mimicking the surface of the human ear canal by using an oscillatory tribometer. The method reproduces the transition from static to dynamic friction under oscillatory motion that mimics jaw movement during everyday use. A 3D-printed, microstructured spherical counter body is pressed against the silicone cylinder with a constant normal force. The oscillation amplitude is increased stepwise. From the curves of the friction coefficient, we determine the cross-over from adhesion-dominated to sliding-dominated states. At small distances of the oscillating strokes, static friction is dominant. Beyond a certain value of the stroke, characteristic drops and fluctuations appear in the friction coefficient, corresponding to the onset of relative motion. By analysing the stroke-dependent coefficient of friction, we define a critical stroke as the intersection point of two linear fits describing different regimes. This critical stroke represents the threshold at which the contact changes from a securely adhering to a failing state. The proposed analysis offers a systematic way to compare the adhesion performance of different additively manufactured earmould surfaces. The results provide design guidelines for optimised surface structuring to achieve maximal adhesion while avoiding premature onset of sliding, thereby contributing to more reliable and patient-specific earmoulds.