Open cell foam acts more like a sponge, it will absorb water. The blackstars act like that, you can squeeze them and water is released. If you try that with an ez-clonner collar, you will find that they don't absorb or release water when squeezed. They have like a "skin" on the top and bottom and if you look closer you'll see it in the middle. This skin is closed cell and acts as a barrier, plus if you try to grow with them they leave a real dent in the stem that looks to me as a choke point.

I'll look again for the stuff I used and where I got it (tho I took a quick look before when I answered the other day...I knew you would ask). This goes back at least 11 years since I made collars, so my memory is a little weak.