I was wondering if anyone knows about making a small 2-4oz heated water pouch? It would be like a very small camel back (hiking water reserve) with a wrapped heating element around the outside and sandwiched between some isolation. I read that electric blankets use self regulating heating elements. I'm not sure what type of conducting wire is used for that. Anyway...if anyone has some ideas, let me know.
Most of these (well, as of 40 years ago, when my aunt used to work for a company that manufactured them) used a nichrome wire in a high temperature insulation. I remember that she brought me some samples of the scrapped wire. I'm not sure what the brand name was for that stuff (and, it probably wouldn't help much anyway, since that was 40+ years ago).
The old version used discrete thermostats embedded in the device to approximate the temperature, and control the heating. The more modern approach is to measure the resistance of the wire, realizing that it changes with temperature, and calculate the temperature based on the wire resistance. That's not necessarily an easy thing to do, but it is certainly possible, given adequate sensors and adequate computing power.
The trickiest part of your project may be to obtain a suitable power source for your device. Heating water requires a LOT of power, due to the high heat capacity of water.
I'm assuming that your design will use a container for the water with a high thermal conductivity (e.g., an Aluminum can), with the heating wire wrapped around the outside of the container, and with that surrounded by thermal insulation? The trickiest part may be ensuring a good thermal conductivity from the heating element to the water container (e.g., between the resistive wire and the Aluminum can). I wonder if embedding the heating wire in some sort of heat conductive paste may enhanced the heat conductivity to the water in the container?