resistance to degradation and oxidation. Many of them also have surfactant properties and functions that make them ideal as water and grease repellents.  So...

https://www.apple.com/jp/environment/pdf/Apple_PFAS_Commitment_November-2022.pdf

assessment of eardrum mobility, audiometric bone-conduction testing to verify the absence of conductive hearing loss, and audiometric air-conduction testing...

https://www.apple.com/jp/health/pdf/Hearing_Health_Features_on_AirPods_Pro_2_October_2024.pdf