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There is a need for high-quality implantable microphones for existing semiimplantable middle-ear hearing systems and cochlear prosthesis to make them totally implantable, thus overcoming discomfort, inconvenience, and social stigma. This paper summarizes and compares the results of an in-vitro study on three design approaches and the feasibility of using microelectromechanical system acoustic sensors as implantable microphones to convert the umbo vibration directly into a high-quality sound signal. The requirements of sensors were selected including the ability to withstand large body shocks or sudden changes of air pressure. Umbo vibration characteristics were extracted from literature and laboratory measurement data. A piezoelectric vibration source was built and calibrated to simulate the umbo vibration. Two laboratory models of the acoustic sensor were studied. The model-A device, using electrets-microphone as the sensor, was designed and tested in the laboratory and on temporal bones. The results verify that the laboratory measurement is consistent with the temporal bone characterization and achieves a near flat frequency response with a minimum detectable signal of a 65-dB sound-pressure-level (SPL) at 1 kHz. The model-B sensor was then designed to increase the sensitivity and provide an easy mounting on umbo. The model-B device can detect 40-dB SPL sound in the 1-2 kHz region, with 100-Hz channel bandwidth. The results of model-A and model-B displacement sensors and the acceleration sensor are summarized and compared. A preliminary design of the implantable displacement sensor for totally implantable hearing-aid systems is also presented.