Bigels based on polyelectrolyte complexes as vaginal drug delivery systems

[Display omitted] •Bigels constitute a system that enables the controlled release of hydrophilic drugs.•Cationic and anionic polymers form a polyelectrolyte complex in the aqueous phase.•Electrochemical impedance is employed to evaluate the rehydration process of bigels.•Bigel mechanical properties improve with increasing chitosan content.•Mucoadhesion is favored by the inclusion of a higher ratio of an anionic polymer. Bigels in which the cationic polymer —chitosan— is combined with an anionic polymer —karaya gum, pectin or xanthan gum— were prepared. These polymers, thanks to the attraction of their charged surface groups, are liable to form a polyelectrolyte complex that would modify the characteristics of the bigel. The obtained bigels were subsequently freeze-dried and tested to study their behavior against different conditions occurring in the vaginal environment. Swelling and drug release profiles have been evaluated in a medium that simulates the ordinary vaginal conditions and simulating the conditions after ejaculation. Thanks to the formation of polyelectrolyte complexes, the bigel is able to provide a pH-independent sustained drug release. However, when one of the polymers predominates, the release of the active ingredient turns out to be pH-dependent —which can also be beneficial in some therapeutic applications—. It has been proved that the inclusion of a higher percentage of chitosan in the bigel improves its mechanical properties, while the mucoadhesivity of the system can be improved by increasing the anionic polymer. The versatility of these systems in modulating their physicochemical properties, provides a substantial advantage over the formulations currently available on the market for vaginal drug delivery.