One of the unique features of our robot design is that it's warm and has a squishy tummy. This is a surprisingly hard effect to achieve once you factor in safety, long-term reliability, and manufacturability. I'd originally looked at electric hot water bottles, because then some other company would be taking care of all those problems, and I'd just have to make it easy to plug in. Not so much. The short version is that every one I tested or researched had one or more terrible problems, from scalding people to short lifespans.

The longer version is that I was pretty enamored with one of the heated bottles. The Amazon description was filled with confidence-inspiring words and phrases like "made in USA" and "reliable". Upon arrival, however, the sticker on it actually says "made in China." 

The design itself is pretty smart, with several layers of waterproof bladder to make it tough.

The more recent reviews mention that it starts out heating up rapidly and to a (maybe too high) temperature, and then gradually tapering off until it doesn't heat at all. Inside the bladder you can see why:

These are some uuugly heating elements, clearly corroding already, and the liquid inside was tapwater rather than some oil or inert gel. They must dissolve or cake over until they no longer work.

Add to this my general discomfort with putting a 120V AC device inside our otherwise low voltage DC product, and this solution is dead.

So we're building our own bladders and heater and controller! We've already been experimenting with this silicone heating pad, which is much safer with its 12V needs (from which we can charge the electronics, too) and completely encapsulated heating elements. This fancy fellow even has a temperature sensor built in! Heats our test bladder up to toasty in just 10 minutes, too.