a method and apparatus for removing and / or modifying specific target substances in the blood is disclosed . the invention includes a column or chamber in which selected bioreactant molecules are suitably retained for reaction within a porous hydrogel polymer matrix by reversibly quenching the reactant propensity with a displaceable surrogate and polymerizing an insoluble cross - linked hydrogel in such manner that the hydrogel polymer replicates the shape of the bioreactant molecule . this replicated phantom shape in the polymer then exhibits profound dispersion - force affinity for the bioreactant . the invention provides for retention of a wide variety of therapeutic reactants and reaction products while confining the reaction within the column .