EPOXY-BASED AMPHIPHILIC HYDROGELS: SWELLING BEHAVIOR AND MECHANICAL PROPERTIES

Introduction: Polymeric hydrogels are soft materials with a network structure that retain large amounts of water and have found widespread application in different technological areas. Amphiphilic hydrogels contain both hydrophilic and hydrophobic segments and are presently investigated for instance as devices for controlled release of pharmacological substances or biomaterials in tissue engineering. The development of amphiphilic hydrogels for a specific application requires the attainment of the most appropriate balance between hydrophobic and hydrophilic moieties, network cross-link density, and swelling degree. In the present work the versatility of epoxy resins chemistry was exploited to tune such parameters. Materials and methods: Several epoxy networks were synthesized by changing the hydrophilicity/hydrophobicity of the monomers (epoxy resins and amines) and/or their ratio in the reaction mixture. For all hydrogels the room temperature equilibrium swelling ratio in 0.1 M NaCl was determined. For some materials swelling tests were carried out also at various temperatures to assess their thermosensitivity. The room temperature mechanical behavior of hydrogels equilibrated in 0.1 M NaCl was investigated by uniaxial tensile tests. Results: The room temperature water content of the hydrogels investigated was in the 65– 95 wt.% range. The water content decreased by about 45-55 wt.% as the temperature was increased from 6°C to 70°C. A more pronounced thermosensitivity was displayed by less cross-linked hydrogels. The tensile behavior of the hydrogels was dramatically influenced by the purification procedure: if the hydrogels were purified simply by soaking in 0.1 M NaCl, their tensile modulus and elongation at break were definitely lower than if the same systems were treated with organic solvents before equilibration in 0.1 M NaCl. In spite of these considerable mechanical property differences the room temperature water content of the two sets of materials was similar. Discussion: The results obtained show that epoxy-based hydrogels with both chemical and physical cross-links can be successfully prepared. The extent of physical crosslinking, which is triggered by the association of hydrophobic groups, can be easily adjusted by a proper selection of the length and concentration of the latter. By changing the hydrophilic-hydrophobic balance and the chemical cross-link density it is possible to tune the swelling degree and the thermosensitivity (i.e. volume change as a function of temperature) of the hydrogels. The chemical cross-link density being the same, the mechanical behavior is considerably influenced by the extent of hydrophobic association, which in turn can be altered by absorption/desorption of molecules able to promote or inhibit hydrophobic interactions between the network segments.