A method of preparation of a cross-linked hydrogel by graft copolymerisation, said method comprises the steps of preparing an aqueous solution comprising one or more hydrophilic polymers, a cross-linking agent and a photoinitiator comprising a water-soluble peroxydisulphate, subjecting said solution to irradation and obtaining the cross-linked hydrogel, wherein the hydrophilic polymers are saturated and the cross-linking agent acts as a co-catalyst of cross-linking. The hydrogel is fast to produce and has low toxicity. The hydrogel may be suitable for use in medical devices such as dressings and the like.