Structural and rheological properties of collagen hydrogels containing tannic acid and chlorhexidine digluconate intended for topical applications

The increasing microbial resistance to conventional antibiotics turned the attention to phytochemicals used traditionally to treat infected wounds. Some can modulate or modify bacterial resistance and potentiate their activity. Collagen hydrogels containing all the combinations between 5, 10 and 15% of tannic acid (TA) and 1.82, 4.55 and 9.09% chlorhexidine digluconate (CHDG), pH ≈ 4 were prepared. FT–IR spectra showed that collagen interacts stronger with TA than with CHDG, TA interacts with CHDG at high concentrations and collagen conformation is not affected, so hydrogels can be used as valuable biomaterials. The preservation of collagen's triple-helical conformation was demonstrated also by the ultraviolet-circular dichroism (UV–CD) spectroscopy. The hydrogels revealed an overall pseudoplastic rheological behaviour, being easily destroyed under the action of shear rates higher than about 20 s−1 during rotational viscometry tests. However, all the hydrogels showed a viscoelastic behaviour with an elastic part that prevails over the viscous contribution, as shown from oscillatory rheometry results. The high viscosity at rest assures a good adherence of the hydrogels on wound, while their elasticity prevents the breakdown of the applied layer.