Stanford University scientists have created a stretchy, sweat absorbing, wearable patch that measures how much cortisol the wearer is producing. Cortisol is released when a person feels stressed and measuring it can help doctors determine if a patient's pituitary or adrenal glands are functioning properly. The patch works by absorbing sweat through small holes, where it pools in a reservoir topped by a cortisol-sensitive membrane. The membrane allows charged ions such as potassium and sodium, also found in sweat, to pass through unless they are blocked by cortisol. The sensor measure these blocked ions, not the cortisol itself. Learn more about the wearable patch that detects cortisol levels in sweat at Stanford University.