In a process of forming a hydrogel from a mixture comprising hydrogen peroxide (H2O2), horseradish peroxidase (HRP), and a polymer comprising a crosslinkable phenol group, the gelation rate in the solution and the crosslinking density in the hydrogel can be independently adjusted or controlled by selection of the molarity of H2O2 and concentration of HRP in the solution when the molarity of H2O2 is limited to be within a range and the concentration of HRP is limited to be above a threshold. A method for determining the range and threshold is disclosed. The hydrogel may be used to grow cells, in which case, the molarity of H2O2 may be selected to affect the differentiation or growth rate of the cells in the hydrogel. Also, the hydrogel system may be used for sustained delivery of a therapeutic protein, for example in the treatment of liver cancer, fibrosis or hepatitis.