Tunable Mucin–Chitosan Complex Coacervates as Model Biological Interfaces

Complex coacervates—liquid–liquid phase-separated mixtures formed by oppositely charged macromolecules—offer versatile soft-matter platforms to mimic biological fluids. We developed a mucin–chitosan coacervate in which ~90% of the droplet interior is mucin 5B (MUC5B) and ~10% is chitosan, yielding spherical droplets that remain liquid-like yet structurally coherent. The droplets were stabilized with genipin crosslinking, preventing fusion for over 24h while maintaining internal mobility. Using zeta-potential analysis, confocal microscopy, and optical imaging, we characterized the charge balance and morphology, and constructed a comprehensive phase diagram across pH and ionic strength identifying conditions for stable liquid–liquid phase separation near physiological pH with low-millimolar divalent ions. Rheological measurements revealed that by varying polymer concentration, the coacervate viscosity can be tuned continuously—from low-viscosity, liquid-like droplets to soft and then gels. Together, these findings establish a quantitative framework for mucin-based complex coacervates as model biological interfaces and adaptable soft materials.