Hello, I use Collagen type I along with a buffer solution (consisting of PBS, NaOH, and deionized water) to make a 3D hydrogel. I then freeze dry it until it becomes a 3D scaffold, and seed cells into it. All goes well until the last part, when I seed cells with medium into the scaffold it dissolves instead of transforming back into a hydrogel.

I do a similar Process with Alginate and it works perfectly, but for my research I need to use Collagen.

Some use crosslinkers like NHS/EDC before freeze-drying to make sure it doesn't dissolve, but I know others who do it without and it works with them.