Smart Alginate–Chitosan Oligosaccharide Microspheres for Controlled Protein Release in the Gastrointestinal Tract with Superior Protein Protection

The design of biopolymer-based carriers that protect proteins from gastrointestinal degradation while enabling controlled release is essential for advancing oral delivery systems. In this study, smart alginate–chitosan oligosaccharide (ALG–COS) microspheres were fabricated via an initial rapid divalent cation-mediated cross-linking followed by carbodiimide-mediated coupling and systematically characterized. The microspheres displayed excellent cytocompatibility, as well as favorable hematological and histopathological safety profiles. ALG–COS microspheres exhibited pH-responsive behavior, compacting under gastric conditions and swelling in intestinal fluid, which enabled limited protein release in the stomach, followed by rapid and sustained release in the intestine while preserving the protein’s secondary structure during gastrointestinal transit. Furthermore, in vivo imaging confirmed markedly enhanced intestinal retention of encapsulated protein compared with free protein, consistent with the mucoadhesive properties of chitosan oligosaccharide. Collectively, these results establish ALG–COS microspheres as a promising platform for protecting proteins during gastrointestinal transit and enhancing intestinal residence and delivery.