A method for the preparation of a cross-linked chitosan comprising the following steps: a) providing a chitosan and leaving the said chitosan to swell in a solvent; b) acylating the amino groups of said chitosan with an acrylic compound of Formula (I): wherein R 1 is an a halogen or any other leaving group that upon removal, ensures acylation of an amino group such as 3-hydroxybenzotriazole ester, anhydride (including mixed anhydrides), N-hydroxysuccinimide, pentachlorophenol, 2-nitro-4-sulfophenol esters and other similar leaving groups; R 2 , R 3 and R 4 are independently selected from H, optionally substituted alkyl (e.g. C 1 -C 6 alkyl), optionally substituted alkenyl (e.g. C 2 -C 6 alkenyl), optionally substituted alkynyl (e.g. C 3 -C 6 alkynyl), optionally substituted cycloalkyl (e.g. C 3 -C 8 -cycloalkyl), optionally substituted cycloalkenyl (e.g. C 4 -C 8 cycloalkenyl), optionally substituted cycloalkynyl (e.g. C 5 -C 8 cycloalkynyl), optionally substituted heterocycloalkyl; optionally substituted aryl (e.g. optionally substituted phenyl), optionally substituted heteroaryl and optionally substituted aryl C 1 -C 6 alkyl, in particular benzyl; wherein the term "substituted refers to groups substituted with from 1 to 5 substituents selected from the group consisting of halogen, -COOR', - NR'R", =O, -OR', -COR', -CONR'R", -SR', -SO 3 R', -SO 2 NR'R ", -SOR', -SO 2 R', - NO 2 , or -CN; or R 1 and R 2 or R 1 and R 3 , or R 1 and R 4 together form 4-24 membered aryl, heteroaryl, cyccloalkyl or heterocycloalkyl (e.g. a 6-24 membered aryl, heteroaryl, cycloalkyl or heterocycloalkyl such as 8-24 membered aryl, heteroaryl, cycloalkyl or heterocycloalkyl); c) reacting the acylation product of step b) in presence of a base (Aza-Michael reaction); d) purifying the cross-linked chitosan obtained from step c) (e.g. from salt impurities and/or aprotic solvent). a) providing a chitosan and leaving the said chitosan to swell in a solvent; b) acylating the amino groups of said chitosan with an acrylic compound of Formula (I): wherein R 1 is an a halogen or any other leaving group that upon removal, ensures acylation of an amino group such as 3-hydroxybenzotriazole ester, anhydride (including mixed anhydrides), N-hydroxysuccinimide, pentachlorophenol, 2-nitro-4-sulfophenol esters and other similar leaving groups; R 2 , R 3 and R 4 are independently selected from H, optionally substituted alkyl (e.g. C 1 -C 6 alkyl), optionally substituted alkenyl (e.g. C 2 -C 6 alkenyl), optionally substituted alkynyl (e.g. C 3 -C 6 alkynyl), optionally substituted cycloalkyl (e.g. C 3 -C 8 -cycloalkyl), optionally substituted cycloalkenyl (e.g. C 4 -C 8 cycloalkenyl), optionally substituted cycloalkynyl (e.g. C 5 -C 8 cycloalkynyl), optionally substituted heterocycloalkyl; optionally substituted aryl (e.g. optionally substituted phenyl), optionally substituted heteroaryl and optionally substituted aryl C 1 -C 6 alkyl, in particular benzyl; wherein the term "substituted refers to groups substituted with from 1 to 5 substituents selected from the group consisting of halogen, -COOR', - NR'R", =O, -OR', -COR', -CONR'R", -SR', -SO 3 R', -SO 2 NR'R ", -SOR', -SO 2 R', - NO 2 , or -CN; or R 1 and R 2 or R 1 and R 3 , or R 1 and R 4 together form 4-24 membered aryl, heteroaryl, cyccloalkyl or heterocycloalkyl (e.g. a 6-24 membered aryl, heteroaryl, cycloalkyl or heterocycloalkyl such as 8-24 membered aryl, heteroaryl, cycloalkyl or heterocycloalkyl); c) reacting the acylation product of step b) in presence of a base (Aza-Michael reaction); d) purifying the cross-linked chitosan obtained from step c) (e.g. from salt impurities and/or aprotic solvent). <EOS>