It's simple method to get your hydrogel out of solution. First you have to freeze solution of hydrogel and then you have to thawed it. You should do it many times (depends of kind of hydrogel, and many others factors) to get hydrogel structure. Personaly, I haven't done it yet, but I've read about this method many times and I think that it will looks like I write you here. I hope I'm not wrong.