(489i) Viscoelastic Measurements of Biocompatible Polyelectrolyte Multilayers

The layer-by-layer deposition of polyelectrolytes (PEs) is a simple and versatile technique to assemble multilayers through the electrostatic attraction between cationic and anionic PEs. Polyelectrolyte multilayers (PEMs) have been used to design anti-bacterial coatings, non-cytotoxic surfaces, and to assemble three dimensional cellular architectures. The mechanical properties of PEMs have been shown to modulate cellular response. However, the precise measurement of the physical properties of PEMs, specifically under hydrated conditions is non trivial. PEM films were formed using layer-by-layer (LbL) assembly of chitosan, a cationic PE, and hyaluronic acid, an anionic PE. Both PEs are used extensively in cell and tissue engineering applications. We report the measurement of the viscoelastic properties of hydrated PEMs under conditions used in cell-culture.

A quartz crystal microbalance with dissipation monitoring (QCM-D) was used to monitor the PEM assembly and show successful alternating deposition of the polyelectrolytes on a gold coated quartz crystal surface. Using a static QCM-D module, we have shown how polyelectrolyte bilayers of chitosan and hyaluronic acid are assembled under in vitro conditions. Using the Voigt viscoelastic model for the PEM, we have modeled the hydrated film thickness, film shear modulus and film viscosity for each polyanion and polycation bilayer deposited. Ellipsometry was used to determine the dry thickness of chitosan and hyaluronic acid PEM films.

Using LbL deposition of PEs, properties of PEMs can be tailored to fit a wide range of mechanical properties. For example, different pH conditions affect dissociation of weak acidic and basic functional groups, resulting in a range of variable properties such as composition and thickness of the polymer films. PEs deposited at a pH very close to the pKa of the functional groups tend to adsorb with loops and tails resulting in a thicker layer due to decreased charge density along the polymer chain. Alternatively, at pH values greater than 2 pH units from the pKa of the polyelectrolytes, the polymer chains tend to have a higher charge density along the polymer chain, thus resulting in a thinner, more rigid adsorbed PE layer.

Within the physiological pH range (pH 6.5-7.2), we observed rigid adsorption of hyaluronic acid due to a much lower pKa of the functional groups (pH 3.4). Chitosan deposited at physiological pH, very close to the pKa of the functional groups (pH 6.4), adsorbed at a much lower charge density and resulted in a thicker PEM film.

These results provide insight into the mechanical properties of chitosan/ hyaluronic acid PEM films, such as viscosity, hydrated/ dry thickness and modulus, and their implications on modulating cellular response.