Very hard plastic!
The cushion... It's a very hard piece of plastic... I guess not very comfortable to sleep on...
However it keeps a good level of warmth for long time.
warm but cover doesn't last very long
I bought this for my rats. They like to snuggle up to it (if it's not too hot). I found reducing the heating times to almost half helps finding the right temperature.
The cover was chewed through in 2 seconds though. Would prefer something with a soft outer layer and a tougher inner layer.