Their new biofuel cell array consists of a water-repellent paper substrate onto which multiple biofuel cells are laid out in series and in parallel; the number of cells depends on the output voltage and power required. In each cell, electrochemical reactions between lactate – a chemical found in sweat – and an enzyme present in the electrodes produce an electric current. Unlike previous designs, the entire device can be fabricated via screen printing, a technique generally suitable for cost-effective mass production. Similarly, previous cells used silver wires as conducting paths, but the new biofuel cells employ porous carbon ink. Paper layers collect sweat and transport it to all cells simultaneously through the capillary effect.