Dear this could be as a result of coagulant property of the solution.

It could depend on the presence of calcium in the fibrinogen solution that would lead to the production of calcium alginate and the formation of a molecular cocktail with the fibrinogen.

I assume there is a formation of polyelectrolite complex(PEC) between oppositely charged groups of fibrinogen and alginate. I also agree that there may be not enough Ca ions concentration as Alginate tend to bind it. We studied formation of PEC for systems chitosan-gelatin Article Oxidized Dextran as Crosslinker for Chitosan Cryogel Scaffol...

I am aware that calcium ions crosslinks alginate, but this is not the case. I meant fibrinogen flocculates in alginate and gelatin solution (free of calcium).

Thanks Dmitry for your answer.

Our lab has also experienced the precipitation of fibrinogen out of solution in the presence of alginate (we don't use gelatin). We haven't completely solved the issue thus far, but we have discovered the following:

• The higher the concentrations of alginate and fibrinogen, the worse the precipitation is.
• Some alginates seem to intensify this process. For instance, when we tested an alginate from Sigma (A1112) side by side with one from Kimica (IL-1), we found that the precipitation was much worse for the Sigma alginate. We requested information about the two alginates (M/G ratio, molecular weight, etc.) to see if we could find a reason for this difference, but Sigma could only give us estimates for these values and Kimica never responded to our request.
• It also becomes worse, depending on what we dissolve the solutions in e.g. DI water, HEPES, PBS, or 0.9% NaCl. But this is specific to the type of alginate used.

As Dmitriy mentioned, we suspect that this has something to do with the charges of the fibrinogen and alginate. This is discussed in several papers which describe the interactions between polysaccharides and proteins:

Article Protein–polysaccharide interactions

I am happy to discuss this with you further if you think that would be helpful.

If you can manage to prepare alginate in 1-1.5M NaCl and fibrinogen preparedin 1 - 1.5M NaCl an then mix both solutions, it should not precipitate, but I am not sure about preservation of the native structure of fibrinogen.

Thanks a lot for sharing your experience, Marita!