Liquid Behavior Control in Hearing Aids via 3D Printed Surface Microstructures

This thesis aims to experimentally investigate the behavior of liquids on unique surface structures created through additive manufacturing. This research paves the way for developing functional surfaces capable of repelling liquids, which have broad applications across various fields. While simple models have been used for over 70 years to predict liquid wetting behavior, they still exhibit certain limitations. Therefore, there is a continuing need for experimental studies exploring the interaction of liquids with more complex structures made possible using additive manufacturing. The insights gained from such research can be applied to a wide range of applications, including hearing aid devices.

Literature demonstrates that reentrant structures are the optimal surface configurations for repelling liquids. To apply these functional surface microstructures using soft polymers, various 3D printing techniques were explored. Direct bottom-up DLP printing of a simple reentrant feature using a hydrophilic soft biocompatible polymer showed superior hydrophobic properties, reaching a water static contact angle of 149°. However, more can be achieved if the material is made of hydrophobic material such as silicone. Hence, an alternative method was then explored using indirect FDM printing, where a mold was printed with FDM and subsequently cast with silicone. Reentrant structures, having a reentrant angle of ∼120°, produced through this method exhibited amphiphobic properties, capable of repelling both water and a lower surface tension liquid, ethylene glycol. This was evaluated through static and dynamic contact angle tests. While repellency to liquids with much lower surface tension, such as oil, would be more advantageous in real-world applications, the current structures were not sufficient to achieve this. Increasing the reentrant angle could potentially enhance the repellency to oils.

A higher reentrant angle can be attained using the indirect DLP printing with silicone as cast material. Multiple reentrant structures with reentrant angle ≥ 180° were incorporated by designing surface features into a mold printed using DLP. This mold formed the holes in a mesh resembling the mesh in the hearing aid dome, where sound passes through and earwax often enters. This placement is the most strategic for preventing intrusion of earwax. This mold included pins with surface structures along their sides forming the series of reentrant structures when negatively replicated in silicone. This improved the breakthrough pressure for oil infiltration by 70%.

Finally, the full potential of double reentrant structures, with a reentrant angle of 270°, can be harnessed to make hydrophilic materials perform comparably to hydrophobic ones. These structures were integrated into the mesh of hearing aid wax guards using DLP printing, forming wall structures around the perforations that face the electronic components of the device. The results showed that wax guards with double reentrant structures significantly improved hydrostatic breakthrough pressure, reaching approximately 80% of the performance of conventional hydrophobic-coated guards and about 570% higher than uncoated, unstructured guards. This demonstrates that coating-free, structured wax guards are possible, providing sustainable effective protection against earwax.

In conclusion, this thesis explored the characterization and understanding of liquid behavior on reentrant structures fabricated through additive manufacturing. These structures, with their exceptional liquid-repellent properties, hold significant potential for various applications, including hearing aid devices. The study successfully demonstrated the feasibility of integrating coating-free, oleophobic surface structures into hearing aid components, achieving comparable or even superior liquid repellency without the use of harmful coatings. The insights gained from this research contribute to the advancement of functionalized surfaces in medical devices and other liquidrepellent applications.