Good questions, I have no idea but would love to know also.
I do know that you can use the same sponge for 4-6 cycles I think, so for people that are trying to decide between tampons and sponges, at least there is less waste, etc with the sponges and you definately get quite a bit of use out of them.
I had to stop using them for a while because I had horrible cramps (not from the sponges), but now that my cramps are better, I was able to use it last cycle. I didn't have any problem with leaks. I do want to get a Diva Cup to try though.