2.3 Synthesis of GelMA, PDA and PDA@E2 particles
GelMA synthesis was similar to previous studies[19]. Briefly, 10 g gelatin was dissolved in 100 ml phosphate buffered saline (PBS, 0.01M, pH 7.4) at 50 ℃. 0.8 ml MA was added dropwise into the gelatin solution. After reaction for 1 h, 500 ml PBS was added to stop the reaction and the obtained solution was transferred into dialysis tubes to dialyze against deionized (DI) water for 7 days at 40 ℃. The DI water was refreshed daily. GelMA was obtained after lyophilization. To quantify the degree of substitution of GelMA,1H-NMR spectroscopy (Bruker Avance III 400, USA) analysis was conducted.
The synthesis of PDA and PDA@E2 particles followed previously reported procedures [20] but with some small modifications. Because E2 is poorly water-soluble, a water/ethanol mixture was used to synthesize PDA@E2 particles. Different amounts of E2 (0 or 0.75 g) were dissolved in absolute ethanol under constant magnetic stirring for at least 2 h. 10 ml ammonia and 290 ml DI water were added into ethanol solution under stirring for 30 min. Subsequently, 50 ml 5 w/v% dopamine hydrocholoride solution was poured into the ethanol/water mixture under constant stirring for 24 h at room temperature in open air. PDA and PDA@E2 particles were obtained after centrifugation at 12,000 rpm/min for 10 min. The morphology and microstructure of PDA and PDA@E2 particles were characterized using SEM and a transmission electron microscope (TEM, Tecnai G2 20, USA). The diameter distribution of particles was analyzed using Image J software.