The pads that come with your AED should be replaced every 4-5 years or as directed by the owners manual of your device. AED pads are manufactured with tin and lubricated with a special conductive gel that forms a bond with the skin on contact. It is extremely important to make sure that the pads are in prime shape or the device will be useless. Over time the gel will deteriorate and essentially dry out (especially if opened). The pads of your AED should have an expiration date on them to let you know when to change them out. Most suppliers that sell AEDs will also sell their accompanying pads.