Setting a target on implantable medical devices such as respiration-supporting pacemaker for amyotrophic lateral sclerosis (ALS), we develop an energy harvester that could earn electrical power from the mechanical motion of liquid droplets on an electrical charged plate called “electret.” A PDMS sheet with micro fluidic channels is laminated onto a silicon substrate with built-in permanent electrical charges. A pair of capacitive electrodes is formed in the sealed fluidic channels, in which water droplets are displaced back and forth due to the applied pressure that simulates the motion of heartbeat. Typical output power of 0.17 µW/cm² is obtained at 0.47 Hz. Analytical model suggests that the extension of the electret plate to ~ 6 cm² delivers 1 mW power, which is sufficient to drive the implantable medical devices.