The final PEGylated protein product was precipitated in a 5 fold excess of acetone at room temperature for 20 minutes while stirring. The precipitated protein solution was centrifuged for 20 minutes at 5000 rpm, and the obtained pellet was redissolved to a 20 mg/ml protein concentration in PBS containing 8 M urea. The PEGylated protein solution was then dialyzed against PBS containing 0.1% (v/v) glacial acetic acid at 4 °C for 2 days with twice-daily changes of PBS (Spectrum, 12-14 kDa MW cutoff). The lyophilized PEGylated fibrinogen was used right after lyophilization for creating the hybrids.