C-CS was placed in Tris-base solution (pH = 8.5) and completely dissolved in an ice bath while stirring. Then Cys solution in Tris-base was added slowly to the C-CS solution and the solution was kept stirring for about 2 h to oxidize the catechol groups. The mixture solution was stirred vigorously for 5 min at controlled temperature (37 °C) for gelation to prepare the C-CS-Cys hydrogel. The C-CS hydrogel was prepared in the same way without the addition of Cys. C-CS with different grafting ratios were used to prepare modifications of the hydrogel. This work used different feed ratios to gain the catechol-chitosan with different grafting rates (as in Table S2 ). The different preparation conditions, explored for the formation of the C-CS and C-CS-Cys hydrogels, are in Table S3 .
After freeze-drying, the C-CS and C-CS-Cys hydrogels were characterized by FT-IR and XPS using a Nicolet-5700 Fourier Infrared Spectrometer and a PHI-5400 Perkin Elmer X-ray photoelectron spectrometer, respectively. C-CS-Tris sample was re-dried after dissolution in Tris-base solution.
After freeze-drying, the C-CS and C-CS-Cys hydrogels were characterized by FT-IR and XPS using a Nicolet-5700 Fourier Infrared Spectrometer and a PHI-5400 Perkin Elmer X-ray photoelectron spectrometer, respectively. C-CS-Tris sample was re-dried after dissolution in Tris-base solution.