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Existing mechanical relays have reliability-related issues including unstable contact resistance and limited cycle life. These problems are caused by mechanical fatigue and wearing of the contact surfaces. In the liquid metal micro switch (LiMMS), we employ a liquid-liquid contact to address these issues. The switching operation is achieved by forming a gap in the liquid metal using gas expansion. Our prototype has a microchannel on a glass substrate to hold a tiny amount of mercury, 0.15 mm in width and 0.1 mm in height, and TaN thin-film heaters on a ceramic substrate to expand the operating gas by heating. The substrates are bonded together and the total size of the device is 5×5×1.4 mm. We successfully demonstrated 70-mΩ on-resistance, 1 ms-switching speed, the ability to handle up to 1 A of dc current and over 1×108 cycle operation. LiMMS also has good RF performance, better than 1 dB insertion loss, and better than 20 dB isolation up to 18 GHz.