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The aim of this study was to analyse the influence of the template/functional monomer proportion on the achievement of molecularly imprinted hydrogels with cavities with a high enough affinity for the drug to sustain drug release. Imprinted hydrogels were prepared from N,N-dimethylacrylamide and tris(trimethylsiloxy)sililpropyl methacrylate (DMAA and TRIS; main components), methacrylic acid (MAA; functional monomer), ethylene glycol dimethacrylate (EGDMA; cross-linker), and timolol (template drug). Photo-polymerization of the monomer solutions was carried out in poly(propylene) molds (0.3 mm thickness) to obtain contact lens-like devices. Non-imprinted control hydrogels were also prepared in the same way but without the addition of timolol. The imprinted hydrogels showed a higher affinity for timolol and a slower release rate than the non-imprinted hydrogels. The release rate decreased by increasing the MAA/timolol ratio in the gel recipe. Hydrogels prepared with 400 x 10(-3) M MAA, 600 x 10(-3) M EGDMA, and a timolol/MAA mole ratio of 1:16-1:32 had drug diffusion coefficients two orders of magnitude below those of non-imprinted hydrogels. The results obtained clearly indicate that the timolol release rate is critically affected by the conditions under which the hydrogels were synthesized. These effects are discussed on the basis of the influence of drug proportion on the conformation of the imprinted cavities.

Imprinted gels incorporating two different breakable cross-linkers, a PbMAA2 complex and a disulfide (S−S) bond, were prepared by radical polymerization. After the lead ions were removed by washing, these gels showed a high affinity for calcium ions. Breakage and subsequent reconnection of the S−S bonds in the absence of Ca2+ decreases the Ca2+ binding affinity of the gel. This indicates that random reconnection of the S−S bonds produces a frustration in the adsorption of Ca2+ by the carboxyl groups. However, if the S−S bonds were reconnected in the presence of Ca2+ and the Ca2+ was subsequently removed (the post-imprinting technique), the resulting gels showed a higher binding affinity for Ca2+. This indicates that the post-imprinting technique creates a more favorable conformation for Ca2+ binding in the polymer network. We interpret our data to mean that “memory” of target-binding sites was encoded effectively into the polymer network by the initial imprinting technique and then enhanced by the post-imprinting technique.