Controlled release systems of natural phenolic antioxidants encapsulated inside biocompatible hydrogels

Encapsulated systems of antioxidants control the bioavailability in foods, enhancing and conserving their quality. Biocompatible hydrogels based on poly-(N-isopropylacrylamide) (PNIPAM) and copolymers, crosslinked with N,N-methylenebisacrylamide (BIS), are proposed as scaffold materials of antioxidants. Hydrogels are synthesized by copolymerization of NIPAM and N-hydroxyethyl acrylamide (HEAA) or N-acryloyl-tris-(hydroxymethyl) aminomethane (NAT) co-monomers. In order to compare the behaviors of delivery systems, the same proportions of co-monomers are added. The studied phenolic compounds (PHc) are: 2-(2-hydroxyethyl) phenol (2-OH) and its isomer 4-(2-hydroxyethyl) phenol (4-OH) or tyrosol, and ferulic acid (FAc). By UV-visible spectroscopy, the partition coefficients of antioxidants in matrix/aqueous medium are determined. To describe the behavior of each controlled release systems, new kinetic parameters are defined: initial concentration of loaded drug (ICLD), the maximum molar concentration of released drug from hydrogel (Ceq) and the elapsed time for releasing the 50% and 100% of loaded substrate, among others. Determined Pc values indicate that the intramolecular interactions between PHc and hydrogel increase with OH group amount inside matrix. The release kinetics of PHc from hydrogels are different depending on substrate-matrix intermolecular interactions. A long-time release could be favored by a more hydrophilic matrix such as PNIPAM-co-8%NAT, and quick release is expected by less hydrophilic matrixes as PNIPAM or PNIPAM-co-8%HEAA. Free and encapsulated PHc show antioxidant activity against photogenerated reactive oxygen species in aqueous solution.