By the end of the year, BodyMedia will start selling the first disposable health-monitoring patch. The company, which makes fitness-tracking armbands, took its small sensors and loaded them into a 0.5-ounce, 3.5-inch-long patch that sticks to the user's upper arm. BodyMedia found a breathable adhesive that makes the patch stick for up to seven days, even in water, while keeping it comfortable to peel off. The patch, which is less than half the weight of BodyMedia's smallest armband monitor, senses motion, sweatiness, skin temperature and the rate of heat flowing off the body. It wirelessly uploads the data to a website or app that charts steps taken, calories burned, activity levels and sleep patterns.