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This letter reports a novel ultrasonic-based method to power biosensors. Compared with commonly used radio-frequency radiation methods, ultrasonic power transmission is relatively safe for the human body and does not cause electronic interference. To extract ambient kinetic energy with arbitrary motion directions, a novel 2-D microelectromechanical systems energy converter is designed with resonance frequencies of 38520 and 38725 Hz. Working in the diagonal direction, the device has a bandwidth of 302 Hz, which is twice wider than that of a comparable 1-D resonator device. A storage capacitor is charged up to 0.95 V in 15 s, when the converter is driven by an ultrasonic transducer at a distance of 0.5 cm, indicating the energy-conversion capability of 21.4 nW.