 These challenges arise chiefly from the fact that all substrates are different and thus require different monomer and cross-linker combinations to adequately form imprinted polymers for that substrate.The first, and lesser, challenge arises from choosing those monomers which will yield adequate binding sites complementary to the functional groups of the substrate molecule.For example, it would be unwise to choose completely hydrophobic monomers to be imprinted with a highly hydrophilic substrate.