Heat pads from last century...

My arm is sorta varying so i have been using heatpads that i used to get at walgreens with a rebate. Yesterday i used one that expired in 2001, it still worked, I dont know that it worked as well. The one in the picture has a copyright date of 2003. I dont feel any heat on the plus side it hasnt caught fire though. It says in the instruction not to microwave it. I wouldnt have thought about doing that without them telling me not to.