Balanced-armature-based, electromagnetic actuator for round window stimulation of the inner ear with static pre-load

Roughly 5% of the population is affected by hearing loss. In case of a functional middle ear impairment with decreased transmission, different types of active middle ear implants may restore hearing. To drive the round window membrane (RWM) for example, bone anchored actuators or floating mass transducer (FMT) have been used in clinics. Bone anchored devices are usually too big for the round window niche and FMTs display low displacement and force at low frequencies. This paper focuses on the stimulation of the RWM by a stator-based actuator that combines small size and bone anchoring. The design features a rotational-symmetric balanced armature core with flexible membrane parts manufactured micro technologically. Membranes and assembled prototypes were evaluated on the bench in the range of 0.1 kHz - 10 kHz and prototypes tested in human temporal bones. We demonstrate the feasibility of a balanced armature actuator of appropriate size for RW placement. With 1.4 mm outer diameter and 4.9 mm length, the actuator fits the anatomical constraints. Acoustic stimulation of the RWM resulted in a maximum output of approximately 80 eq. dB SPL at 300 Hz increasing to > 100 eq. dB SPL above 5 kHz. The measured output is comparable to clinically used devices. The design lacked sufficient robustness leading to a low yield in manufacturing and failures in experiments, although the output level was insensitive to static force loading up to 30 mN. Future developments require an increased maximum output level and improved robustness.