Temperature-Dependent Complex Coacervation of Engineered Elastin-like Polypeptide and Hyaluronic Acid Polyelectrolytes

Coacervates have enormous potential due to their diverse functional properties supporting a wide number of applications in personal care products, pharmaceuticals, and food processing. Normally, separation of coacervate phases is induced by changes in pH, ionic strength, and/or polyelectrolyte concentration. This study investigates the microphase separation and coacervate complex formation of two natural polyelectrolytes, elastin-like polypeptides (ELPs) and hyaluronic acid (HA), as simple models for biological coacervates. These complex coacervates are formed over a wide range of stoichiometric molar charge ratios without the presence of salt or changes in pH and are primarily induced by changes in temperature. Unlike pure ELP solutions, the ELP/HA coacervates result in well-formed spherical particles after the temperature-induced phase transition. We also note that the formation of these complex coacervates is reversible with low hysteresis. We have demonstrated via fluorescent imaging and dynamic light scattering that high positive/negative charge ratios at elevated temperatures produced 400-600 nm particles with relatively low polydispersity indices (PDIs) of ∼0.1. Furthermore, dynamic light scattering, fluorescence microscopy, and optical microscopy revealed that the ratio of the two polyions strongly influenced the size and structure of these ELP/HA complex coacervates. Finally, we showed that the ELP/HA coacervates were able to sequester the hydrophobic fluorescent molecule pyrene, highlighting their potential for use as delivery vehicles for hydrophobic payloads.