Biomacromolecular phase separation and assembly in elastic networks

Gels are key materials in biological systems such as tissues and may control biocondensate formation and structure. To further understand the effects of elastic environments on biomacromolecular assembly, we have investigated phase behavior and radii of coacervate droplets and collagen assemblies in polyacrylamide (PAM) networks as a function of the gel modulus. Poly-L-lysine (PLL) and sodium hyaluronate (HA) coacervate phases are prepared in PAM gels with moduli varying from 0.035 – 9.0 kPa. The size of the coacervate droplets is reported from brightfield microscopy and confocal fluorescent microscopy. Overall, the coacervate droplet volume decreases inversely with the modulus. Fluorescence microscopy is used to determine the phase behavior and concentration of fluorescently tagged HA in the coacervate phases as a function of ionic strength (100 - 250 mM). We find the critical ionic strength and coacervate stability is nonmonotonic as a function of the network modulus and that the local gel concentration can be used to control phase behavior and coacervate droplet size scale. On the other hand, the structure of collagen type II is not controlled by the gel modulus, and instead, we observe less bundling of the fibers. By studying the diffusion of collagen through the network, collagen is able to assemble by "hopping" in larger pores defined by lower density of the network as a path of least resistance. By understanding how elastic environments influence simple electrostatic assembly, we can further understand more complicated biomacromolecular assemblies.