Abstract: The invention relates to the field of antimicrobial materials, in particular to implantable and other medical devices exhibiting antimicrobial activity. Provided is a method for providing a substrate with an antimicrobial coating by immobilizing a quaternary ammonium compound onto the surface of said substrate, comprising the polycondensation of AB2 monomers comprising a secondary amine as A-group and blocked isocyanates as B-groups to obtain a low number average molecular weight polyurea of at least 2500 Da, contacting said polyurea with a surface grafted with coupling agent to covalently anchor the polyurea, and continuing polycondensation to obtain a hyperbranched polyurea coating; followed by immobilizing onto said coating a hydrophobic N-alkylated polyethylenimine (PEI) having antimicrobial properties.