Regaining proper hearing with electroacoustic transducer

Researchers from the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart have developed a new device that will improve patients’ hearing and can be implanted during outpatient surgery.

The new solution is composed of three parts: a case with a microphone and battery; wireless, optical signal and energy transmission between the outer and middle ear; and an electroacoustic transducer - the centrepiece and loudspeaker of the implant. Researchers are developing the electroacoustic transducer, which will be round in shape and measure approximately 1.2 mm. The IPA’s partners in the project, which is sponsored by the Federal Ministry of Education and Research, are the University Department of Otolaryngology, Head and Neck Surgery Tübingen, the Natural and Medical Sciences Institute (NMI) at the University of Tübingen, and the hearing aid specialist auric Hörsysteme GmbH. “Our goal is to take the better sound quality of implantable hearing aids and combine it with a much simplified operation,” said Dominik Kaltenbacher, engineer at IPA. “To implant our system, all surgeons have to do is make a small incision at the side of the eardrum and then fold it forward. This can be done in outpatient surgery.”

The electroacoustic transducer, which takes the form of a piezoelectric microactuator, is then placed directly at the connection between the middle and inner ear known as the ‘round window’. From there it transmits acoustic signals to the inner ear in the form of amplified mechanical vibrations, thereby enhancing the hearing capacity of patients. “The electroacoustic transducer works on the same principle as bending actuators,” explains Kaltenbacher. “The bending elements, which are arranged in the shape of a pie, consist of a laminated composite made from piezoceramics and silicon. If voltage is applied, the elements bend upwards and generate a mechanical vibration. This spreads to the membrane of the round window and the inner ear, stimulating the auditory nerve.”

The effect: although the round window implant is no larger than a pinhead, it can output volumes of up to 120 decibels, which is roughly the noise a jackhammer makes. “This high performance is necessary for very good speech comprehension, particularly for high-pitched sounds, which people who are severely hard of hearing find especially difficult to pick up,” says the IPA researcher.

Experts are currently testing a first working prototype in the laboratory. Results have been positive to date. “The individual components of the hearing aid have all been developed. The next step is to optimise and assemble them,” said Kaltenbacher. The implant must measure up to high requirements: the material must be encased so the body tolerates it and it has to remain stable over long periods - after all, hearing aid implants should last at least 10 years. The optimised individual components should be ready by June of this year; testing of the overall system is planned for 2014.