Last fall I was looking for a way to keep the stray cats warm on my front porch. I found a round disk, made for that purpose, at a local pet store. They cost about $20. I put it in the microwave for 4-1/2 min. to heat them. They last up to 12 hours. I put them under a blanket in a heavy cardboard box. The cats loved it! So I tried it for my cold feet in bed. Worked great! The only problem with using them for humans would be that they aren't flexible.

They're called Snuggle Safe. I thought I saw them in a Foster & Smith catalog, but since it's a seasonal item, they've disappeared.

Mary