Modes of adsorption of polyelectrolytes to model substrates of hydroxyapatite

Stoichiometric hydroxyapatite is a particularly important type of bioceramic sharing crystallographic similarities to the dominant mineral phases found in hard tissues such as bone and enamel, and routinely applied in the field of bone engineering. Adsorption of polymer coatings, often polyelectrolytes, is typically used to alter hydroxyapatite biointerfaces and alter their bioactivity, for example to promote cell adhesion or prevent biofilm formation. The morphology of the resulting coatings is typically expected to play an important role in determining its bioactivity, yet a full picture of such structures at the surface of hydroxyapatite remains incomplete. In this study, a combination of three model substrates was characterized via in situ ellipsometry, in combination with X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscopy (AFM), to study the in situ conformation of hyaluronic acid films adsorbed at the surface of hydroxyapatite. The modes of adsorption of such polyelectrolytes are found to strongly depend on the ability of the bioceramic to readily undergo ion exchange, resulting in the remodeling and partial dissolution of the mineral phase, prior to complexation and re-precipitation at the surface, therefore forming a relatively compact, poorly solvated coating. This is in contrast to the very open and swollen polymer film that is formed on more chemically stable interfaces. Finally, the chemical structure of polyelectrolytes, their molecular weight and crosslinking are observed to impact the morphology of the films generated.