Vestibular system is a motion sensing organ vital to human balance system. Patients suffering from vestibular disorders usually experience symptoms including blurred vision, vertigo and increased risks to fall. It is reported that more than 1 billion dollars are spent each year on treating such diseases. In order to accomplish balance prosthesis, an engineering motion sensing system that possesses a comparable function as the natural vestibular system is needed. One unique characteristic of human body motion is the low frequency, which is different from the resonance frequency range of conventional solid state accelerometers and gyroscopes. This makes it difficult for solid state sensors to achieve large response by the body motion. In this work, we developed a liquid state motion sensing system, whose resonant frequency is within the range of that of body motion. The sensor consists of a curved microchannel, a liquid droplet that can move freely on the channel, and microelectrodes that detect the relative movement of the droplet along the channel. The simple fabrication approach and surface treatment techniques are described in the paper. Droplet dynamics revealing the unique frequency response is theoretically investigated followed by experimental validation. Binary electrical signals are used to determine the acceleration, which minimizes the influence of external electromagnetic noises. Characterization of a single sensor for 1D measurement and an assembled sensor set for 2D measurement demonstrates that low-frequency motion can be precisely determined by the liquid state sensing system. The study of liquid state inertial sensor paves a new route of body motion detection. Considering its concise configuration, simple fabrication, miniaturized size and low cost, the sensor provides a promising candidate for balance prosthesis. It also enables the in-depth investigation of motion detection methodology.

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