The UCSD team printed electrodes onto standard temporary tattoo paper and paired it with a sensor. After each meal, the electrodes generate a current for about 10 minutes. The current draws the glucose—a type of sugar that diabetics have trouble breaking down—up near the skin's surface, allowing the device to read the glucose levels. The glucose is carried by sodium ions, which have a positive charge. By measuring how strong the charge is just under the skin, the sensor estimates how much glucose is in the bloodstream.