2 aminoethyl methacrylate

Gelatin (Sigma-Aldrich, USA) was modified to a 40% degree of methacrylation as previously described [14] . Briefly, Gelatin was dissolved in PBS 10 mM at a concentration of 10% (w/v), and methacrylic anhydride (Sigma-Aldrich, USA) was carefully added to the solution drop by drop.
One hour later, the reaction was stopped by adding an excess of PBS 10 mM and dialysed against Milli Q water with 6-8 kDa MWCO membranes (Spectra/por, Spectrumlabs, USA). Water was changed every 4 h during a 4 d period. Gelatin-methacryloyl (GelMA) was lyophilized and stored at -20 ºC. Sodium carboxymethylcellulose (CMC) and sodium alginate (Alg) (Sigma-Aldrich) were methacrylated at a maximum degree of methacrylation as previously described [39] . The methacrylation reaction was performed by mixing a solution of 1% (w/v) of the polymer in 50 mM MES buffer at pH 6.5 with 20 mM EDC, 10 mM N-hydroxysuccinimide and 10 mM 2aminoethylmethacrylate (Sigma-Aldrich). The reaction was stopped after 24 h with the addition of acetone (Panreac, Spain) and filtered using a vacuum flask. The precipitate was dissolved in PBS 10 mM and dyalized against Milli Q water with 3.5 kDa MWCO membranes (Thermofisher, USA). Finally, the solutions of methacrylated polymers (CMCMA and AlgMA) were lyophilized and stored at -20 ºC.