Development of a Stabilized Alginate-based Hydrogel for Oral Delivery of Encapsulated Live Cultures and Allowing their Intact Passage Through the Digestive System

Alginate hydrogels are widely used for biocompatible encapsulation due to their low cost, mild gelation conditions, and scalability; however, their limited mechanical strength and poor chemical stability under physiological conditions restrict their utility for oral delivery applications. In particular, the development of robust alginate formulations capable of surviving gastrointestinal salt and pH exposures is critical for advancing encapsulated microbial therapeutics for chronic kidney disease (CKD). In this study, we investigated the incorporation of ferric iron into calcium alginate networks as a strategy to enhance gel stability while maintaining biocompatibility. Using a three-ion competition approach, we achieved controlled introduction of ferric ions into calcium alginate gels without significantly altering bulk mechanical properties relative to standard calcium alginate. Although the initial ferric-containing gels displayed comparable modulus and structure, post-treatment with chitosan under mildly acidic conditions produced a dramatic increase in gel stability in physiological salt concentrations across both acidic and neutral pH environments. Ferric-containing gels formed at pH 4.6 absorbed negligible chitosan, in contrast to iron-free alginate gels, which incorporated substantial chitosan under identical conditions. These results support the formation of a thin, dense interfacial complex between chitosan, ferric ions, and alginate at the gel surface, which reinforces the matrix and inhibits dissolution. The resulting hybrid ferric-calcium alginate formulation enabled the production of sub-millimeter beads capable of encapsulating live <i>Thauera aminoaromatica</i> while preserving anaerobic p-cresol degradation activity at 37 °C using nitrate as an electron acceptor. Collectively, these findings establish ferric-modified alginate hydrogels as a promising, scalable platform for stable oral delivery of encapsulated microbial therapeutics.