Ytdyklly

The humanoids I'm talking about are designed with all the characteristics pertaining to robots developed with the help of soft robotics technologies. Although the skeleton is made of titanium, their "muscles" are produced from soft multi-material actuators. There's a network of nanosensors forming a substitute to human sensory receptors. The only thing I haven't yet come up with are heating elements, which will give warmth of a normal human body and at the same time won't melt delicate synthetic materials. Can anyone give a clue what sort of material can be used here? Can it be an alternative of human vascular system (as my humanoids can work on biobatteries, so this energy can be distributed by "vessels")?
The warmth is a key feature for humanoids in my world, because they work with humans and sometimes for humans providing elder care services or medical care.

The material of your heaters is irrelevant. Running the right amount of current through any resistor will heat up the material. Heaters can be made thin, flexible, all it needs to have is slightly higher resistance than the leads going to the heater.
What matters is that anything else can stand 37° Celsius, which is not that exotic. Any standard plastic will do, there are very few plastics being commonly used that can't stand 37°, since you can reach that on a hot summer day.

They'd probably produce enough heat and be heat conductive enough to not actually have a problem but if we assume we need to fake it...

I'd have a dense mesh of infrared LEDs and sensors under the skin.

This would essentially (with some processing) give the robots the ability to see all around them with reasonable accuracy. This would allow them to accurately avoid embarrassing accidents. They'd know if somebody is standing behind them or if something is the path of their arm or if something is where their foot is about to land. They'd even "see" what is in the box they pushed their hand into without having to look.

It would also give them a finely controllable infrared source, so they could give a precisely tuned experience of warmth directly to your skin without having to waste energy to warm themselves.

Also, and this is actually an answer in itself without complex and expensive IR systems, simply make their skin have low heat capacity and conductivity. A surface like that will actually have the exact same temperature as your own skin when you touch it. If you are human and the aim is to make it feel the same as human temperature, this is exactly what you want.

As mentioned this is actually enough to make them feel right. The IR system is only needed if you also want to mimic the way you can sometimes sense nearby human body heat.

You will need the skin to be warm, for the very reason human skin is warm. And it will have nothing to do with 'feeling like a human', and certainly no need for heating elements.

No matter how your muscles work, they will produce heat. Same for all of the electronics in the AI and the nervous system. This heat will have to be dissipated, for the same reason computers need to have cooling fans. I could see the mouth and nose working as cooling vents for fans, but I doubt that would be enough.

Your best bet would be something like the liquid cooling and ventilation garments readily available for human use. They would circulate the coolant (water? robot blood?) through the muscle fibers and electronics, to collect the heat, and then circulate it to the cooling tubes on the surface (or just below the surface) of your robot skin.

There would, of course, be no need for seams or openings. It would probably be an all one piece suit.