This invention relates to a method of making a composition comprising a colloid comprising particles having a colloidal silica core and a shell, said method comprising the steps of: simultaneously introducing into a vessel a first liquid stream containing silica core particles, a second liquid stream containing a polymer having amine functionalities; and a third liquid stream having a pH adjusting substance such that said vessel is maintained at a pH below 6.0, and followed by readjusting the pH of the colloid to a pH between about 7-10; the addition of a cross-linking reagent, and adjusting the pH of the colloid to approximately physiological conditions, wherein the core-shell colloid contains between 10 and 30 μmol amine-monomer/m2 core surface area.