Figure 6.

Model of pH dependent coacervate structure and adhesive mechanisms. a) The polyphosphate (black) with low charge density paired with the polyamine (dark grey) form nm-scale complexes. The complexes have a net positive charge. b) Extended high charge density polyphosphates form a network connected by more compact lower charge density polyamines and when present divalent cations (light grey symbols). The net charge on the copolymers is negative. c) Oxidation of 3,4-dihydroxyphenol (D) by O₂ or an added oxidant initiates crosslinking between the quinone (Q) and primary amine sidechains. The coacervate can adhere to the hydroxyapatite surface through electrostatic interactions, 3,4-dihydroxyphenol sidechains, and quinone-mediated covalent coupling to matrix proteins.