Resistive heating vs. Hot pack

Dear All,

I am comparing two methods for keeping premature, and ill babies warm.

On the one hand I have resistive heating, and on the other 'hot packs'

Goal is to find a reusable, and reliable method. It has to maintain a constant temperature for at least 2 hours, and preferable has to be cheap.

I know a resistive heating element suits all these purposes, but I am wondering if there are any 'hot packs' that do so...?

Just to be clear: By 'hot pack' I mean the elements which contain a liquid that crystallizes when you break a coin inside the element.

Does anybody know more about these things, and has a source maybe?

Thanks,
Pieter

Take a look at the below websites for a start.

[U][COLOR=#810081][URL]http://www.thomasnet.com/products/hand-warmers-92140607-1.htm[/URL][/COLOR][/U]