Chemical cross-linking of gelatin is achieved using a thermosensitive reactive copolymer based on N-isopropylacrylamide (NIPAM). The copolymer bears 5 mol % acrylic acid units which form amide bonds with the amino groups of gelatin in the presence of a water-soluble carbodiimide. The cross-linking reaction occurs only below the LCST congruent to 34 degrees C (lower critical solution temperature), i.e., when the copolymer is in the coil conformation. Above the LCST the copolymer adopts a globule conformation and its ability to react with gelatin is drastically reduced. By setting the temperature above or below the LCST it is possible to switch off or on the reactivity of the system and control the gelation process. The switch temperature can be set at the desired value by adjusting the composition of the thermosensitive copolymer.