Smart hydrogels are sensitive to stimuli such as changes in temperature or pH. Changes in the environment alter the swelling properties of the hydrogels and can cause them to increase or decrease the release of the drug impregnated into the fibers. An example of this would be hydrogels that release insulin in the presence of high glucose levels in the bloodstream. These glucose sensitive hydrogels are modified with the enzyme glucose oxidase. In the presence of glucose, the glucose oxidase will catalyze a reaction that ends in increased levels of H. These H ions raise the pH of the surrounding environment and could therefore cause a change in a smart hydrogel that would initiate the release of insulin.