Programmable Relaxation Dynamics in Bioinspired Coacervates for 3D Printing Cell Scaffolds

Inspired by the tunable physical properties and biological roles of intracellular biomolecular condensates, this study developed chitosan and hyaluronic acid coacervates via liquid-liquid phase separation for 3D-printed cell scaffolds. Multiscale analysis revealed that relaxation dynamics governed the structural organization and extrusion processability of coacervates. Using time-salt-molecular weight-pH superposition principles, relaxation times were continuously and predictably tuned across seven orders of magnitude (10^-3 to 10⁴ s) by modulating pH, ionic strength, and chain length, enabling programmable solid-gel-liquid transitions through a dynamic balance of electrostatic, hydrophobic, and hydration interactions. Furthermore, quantitative extrusion force measurements and subjective assessment revealed that salt enhanced extrudability but compromised stability, while higher molecular weight and pH restored structural integrity via strengthened hydrophobic interactions. Notably, relaxation time strongly correlated with filament deformation, with a threshold exceeding 0.5 s ensuring structural integrity during extrusion. Scaffolds printed within the relaxation time range (0.5-500 s) exhibited outstanding mechanical properties and reliable reprocessability while effectively supporting the adhesion, spreading, and proliferation of L929 mouse fibroblasts and adipose-derived stem cells, confirming their biocompatibility and functionality as cell-supporting matrices. Collectively, this multi-parameter framework, guided by relaxation dynamics, achieved simultaneous extrudability and stability in natural polyelectrolyte coacervates, advancing their application in 3D bioprinting and tissue engineering.