A polymer hydrogel having a polymer formed by the crosslinking reaction of a polymeric unit A according to formula (I),with a crosslinking unit B according to formula (II)and water, wherein n=100-10,000, preferable 250-2500, more preferable 500-1500； m=independently 2-10, preferably 3 or 4； FG is a functional moiety that can be covalently coupled to the complementary functional moiety F1 or F2 of the crosslinking unit (B)； k=0.01-0.05； h=0, 1 or 2； the spacer is an organic moiety, having a main chain comprising at least two functional moieties F1 and F2, wherein the length of the crosslinker in the extended conformation as determined by molecular modeling (including spacer and functional groups F1 and F2) is between 2.5 and 12 nm, or wherein the length is between 20 and 80 atoms.
