Fabrication and controlled release behavior of metal-polyphenol-decorated alginate hydrogel beads

Hydrogel-based controlled release systems are widely used in pharmaceutical and agricultural fields. To achieve controlled release of hydrophilic drugs, which otherwise release rapidly through hydrated polymer networks, we develop a facile ferric ion co-crosslinking strategy to fabricate composite hydrogel beads. The highly porous composite hydrogel beads were prepared by blending recycled household formaldehyde adsorbent (RFA) with sodium alginate (Alg) at a 1:1 (w/w) ratio, followed by Fe³⁺-induced crosslinking and subsequent surface functionalization with tannic acid (TA). Through altering the Alg hydrogel structure, RFA enabled the hydrogel to synergistically restrict drug molecules, and coupled with the blocking function of metal-polyphenol complexes (MPC), this composite structure provided effective regulation of water-soluble drug molecule release. The carrier demonstrated pronounced swelling and disassembly of the metal-polyphenol microfilm, resulting in rapid gentian violet (GV) release under alkaline pH conditions. In acidic environments, the conversion of ternary metal-polyphenol complexes into double or single complexes weakened the "on-off" effect, thereby accelerating GV release compared with neutral conditions. The release of GV was governed by a combination of diffusion-controlled processes and progressive hydrogel bead relaxation. Additionally, the carrier exhibited no detectable toxicity in murine fibroblast cells and during rice seed germination, indicating its favorable biosafety. These findings demonstrated that the controlled-release system had the potential to be further explored for environmental protection and hazardous substances management.