Polyelectrolyte hydrogels with complexing ability towards metal ions

Stimuli-responsive hydrogels have been synthesized, that exhibit dramatic changes in their swelling behaviour, network structure and permeability in response to a number of external stimuli, including pH, ionic strength of the surrounding fluid, temperature and presence of specific solutes. Because of their nature, these materials can be used in a wide variety of applications, including separation, biosensors, drug delivery devices and tissue engineering. The produced hydrogels are based on L-phenylalanine and L-histidine residues, the first being poly(anionic), while the second poly(ampholyte). Figure 1 shows the equilibrium degree of swelling (EDS) in relation to the temperature, at different pHs, for the hydrogel CP2, i.e., a gel containing the thermosensitive N-isopropylacrylamide (NIPAAm) and the pH-responsive N-acryloyl-L-phenylalanine (Phe) 1-2. Low pH values improve the protonation of the carboxyl group, thus reducing the Lower Critical Solution Temperature (LCST). Figure 1: Effect of the temperature on the Equilibrium Degree of Swelling for the hydrogel CP2 at different pHs. On the other hand, the ampholyte hydrogels 3-4 containing N-acryloyl-L-histidine (H5 and H10) show a greater swellability before and after their isoelectric point (Figure 2). The different cross-linking density (5 and 10 mol%) affects the swellability only at high pHs. The different basicity constants (logK) of the two basic groups in the monomer unit is in line with this behaviour. Both the hydrogel systems are able to form complex species with Cu(II) and Ru(II) ions, showing a decreasing pattern of the EDS as a consequence of the charge neutralization and of the stoichiometric arrangement of the gel basic groups around the metal ion. The comparison with the free polymer analogues better elucidates the protonation and complex-formation mechanisms. Figure 2: Effect of the pH on the Equilibrium Degree of Swelling (EDS) for the hydrogel H5 and H10 at 25°C. All the synthetic hydrogels of the present study are not toxic, as shown by the leukemic monocyte (RAW264, from a mouse) cells. This result, in conjunction with the kinetic reversibility of their swelling, makes these kinds of material suitable candidates for biomedical and pharmaceutical applications. REFERENCES (1) A.Fini, M. Casolaro, Temperature and pH sensitive hydrogels based on N-acryloyl-L-histidine and N-acryloyl-L-phenylalanine. European Conference on Drug delivery and Pharmaceutical Technology, Sevilla (Spain) May 10-12, 2004 (2) M.Casolaro, E.Paccagnini, R.Mendichi, Y.Ito. Macromolecules 2005, 38, 2460-2468 (3) M.Casolaro, S.Bottari, A.Cappelli, R:Mendichi, Y.Ito. Vinyl polymers based on L-histidine residues. Part 1. The thermodynamics of poly(ampholyte)s in the free and in the cross-linked gel form. Biomacromolecules 2004, 5, 1325-133 (4) M. Casolaro, I. Carbonaro, A.Fini. Vinyl Polymers Based on L-Histidine Residues: The Thermodynamics of Soluble and Cross-linked Poly(ampholyte) Hydrogels. ISMEC 03, Capri 6-11 giugno 2003