PEG-diacrylate (PEGDA) was prepared from linear PEG with a molecular weight of 4 kDa. Briefly, acrylation of PEG-OH was carried out under argon by reacting a dichloromethane (DCM) solution of PEG-OH with acryloyl chloride and triethylamine at a molar ratio of 1-OH to 1.5-acryloyl chloride to 1.5-triethylamine (0.2 g PEG/ml DCM) (Figure ). The final product was precipitated in ice-cold diethyl ether and dried under vacuum overnight. The following procedure was applied to PEGylate the protein: tris(2carboxyethyl)phosphine hydrochloride (TCEP HCl) was added to a 7 mg/ml solution of fibrinogen in 50mM PBS with 8M urea (molar ratio 68:1 TCEP to fibrinogen cysteines). The solution was stirred for 15 minutes at 25 °C to fully dissolve. After dissolution, a solution of PEG-DA (250-300 mg/ml) in 50 mM PBS and 8 M urea was added and reacted overnight in the dark at 25 °C. The molar ratio of PEG to fibrinogen is 145:1 (linear PEG-DA, MW 4 kDa).