Polyacrylamide Network Permeation in Hyaluronic Acid and Poly- l -lysine Complex Coacervates

Abstract

For many biomolecular condensates, complex coacervation occurs in a viscoelastic medium, which can permeate the droplets and regulate phase separation. Here, we study the permeation of complex coacervate droplets composed of hyaluronic acid (HA) and poly-l-lysine (PLL) inside synthetic polyacrylamide (PAM) hydrogels (160–2200 Pa). From the confocal images of fluorescently conjugated complex coacervate phases in the background of a separately labeled gel, we measure the intensity-derived normalized gel concentration λg. λg rose from ∼0.4 in the soft gels to ∼0.6 in the stiffest gels, showing that the extent of permeation is dependent on elasticity. By varying the solvent ionic strength (10–280 mM) and loading polyelectrolyte concentration (2–40 g L–1), we find that λg is maximized near the critical ionic strength and at the lowest polyelectrolyte loading concentration. Straightforward theory is used to explain the overall physical trends, showing that coacervation in a crosslinked network is governed by a competition between network elasticity and excluded volume interactions.