In reply to cas smerdon:
I just pour boiling water in my platypus every now and then and let it run through the pipe. But then, I've never used anything other than water, usually filtered, in them - after use, empty them out, hang them up to dry and store them hanging up. I use the big zip 2, so this is easy. I've never worried about the grey bits in the tube.
Boots used to do some baby gear sterilizing tablets, each of which was intended to be disolved in 2l of water - fill up platy, drop one in, shake well, rinse. Don't know if they still do them.