[sylvan]

I always use a heating pad and never have a problem. By keeping it under half of the whelping box, mom and the pups can wiggle to adjust their position to get close or further from the warm end. The cord is not accessible for chewing since it is under the box, not in it under the bedding. I would be concerned that the microwaveable thing would fluctuate in temp....or get cool faster than expected.