Nichrome wire is a standard for heating wire. I think the problem you will run into is getting any kind of run-time from a battery. Electric heating is very efficient, nearly all the power consumed in turned into heat, but it's also very "hungry", it takes a lot of power to warm up something the size of a shirt.

Most heating pads use between 50-100 watts, if you have a 12V battery pack, that's going to require about 4-8 amps to run. An 8 Ah 12V SLA battery is going to weigh around 5 pounds, and if you are using 100 watts, it's only going to last an hour or so before needing a charge.

These numbers are the most likely reason you don't see more "electric" or "heated" clothing, the batteries you would have to carry around for a full day of heating would start to approach the size and weight of a car battery!

There are more energy-dense battery types available, but they are also trickier to deal with. Lithium-Ion batteries are the battery of choice for laptops, but they have stricter charging profiles, and have a nasty habit of catching fire if mistreated!

Good luck with your endeavor, and make sure you don't burn your test subjects!

Tom D.

You might purchase a set of electric heated socks and enjoy an afternoon of destructive investigation.

It's been done...

http://gerbing.com/

http://thefutureofthings.com/pod/195/electric-clothing-will-warm-your-body.html

http://www.nextag.com/heated-clothing/stores-html

This website may help. It is for making homemade heated clothing.

http://www.aumih.info/Tips/heated.html

You may download a helpful little program from wiretron company.Are differents things released heat (easy to calculate) and surface resistor temp. there are tables for so many alloys, much more than in wiretron's help.Temp. is given in round wires resistors for some alloys starting at 205ºc.Never are accurated values, just enough for most applications.-

A human being generates about 150 watts average. Clothing prevents losing that heat too fast in cold weather. A heated shirt next to the body, and then covered by insulating clothing to minimize heat loss from the combined body and electric heat would not need to be nearly as strong as the body heat itself, maybe a tenth as strong. So now we're down to fifteen watts, or about one amp at twelve volts. So a fifteen amp hour lithium battery http://www.batterystuff.com/powersports-batteries/LFX14L2-BS12.html might prevent hands and other extremities from cooling too much. 1.6 lbs.

Heated jacket

Your first task is to determine how much power you need to dissipate to heat your clothing to the required temperature. The power is normally described in watts. Your next task is to determine how many watt.hours are available from batteries that you might consider using. If you can obtain the required watt hours from an acceptable battery, the rest is fairly simple engineering. Others have posted that nichrome can be used. Although this is true, nichrome is a relatively expensive material that can be used at temperature far in excess of what you require. You might consider printing resistive organic ink on a piece of the clothing. There are various approaches, but the objective is to dissipate the required power for the specified length of time and to dissipate that power in a relatively uniform manner over the specified surface area.

What about minimizing the surface area required. Consider "patches" that are held snug against the inner arms along the brachial arteries or a single one right over the heart to pump the heat throughout the body? Maybe even "hook and loop" (velcro) them in for removal to launder the shirt.

Not exactly what you asked for but may change your calculations by minimizing your load.