“…PU NPs were synthesized according to the procedures described previously. 25 Briefly, prepolymerization was carried out by adding oligodiols, isophorone diisocyanate (IPDI, Evonik Degussa GmbH, Essen, Germany), and the catalyst tin(II)2-ethylhexanoate (T-9, Alfa Aesar) at 75°C for 3 h under nitrogen environment with mechanical stirring at 180 rpm. After this, 2,2-bis(hydroxymethyl)propionic acid (Sigma-Aldrich) and methyl ethyl ketone (MEK) (Avantor Performance Materials, Pennsylvania, PA, USA) were added and made to react for 1 h. The temperature was cooled down to 45°C and triethylamine (Avantor Performance Materials) was added.…”