Adhesion of polyether-modified poly(acrylic acid) to mucin

Pluronic-PAA, a thermogelling copolymer composed of side chains of poly(acrylic acid) (PAA) grafted onto a backbone of Pluronic copolymer, is of interest as a vehicle for the controlled release of compounds. An important feature of such a vehicle is its bioadhesive/mucoadhesive properties, which in the case of Pluronic-PAA are significant due to the presence of the PAA side chains. An atomic force microscopy (AFM) method has been developed and utilized to investigate the interactions between a Pluronic-PAA-modified microsphere and mucous substrates. The bioadhesive force was successfully measured, and trends were observed under conditions of varying pH and ionic strength. Pluronic-PAA exhibits significant mucoadhesion over a range of pH values, with mucoadhesion being optimal at pH 4-5 (adhesive force approximately 80 mN/cm(2)) and dropping sharply at higher pH, to a value of approximately 20 mN/cm(2) at pH 8. The mucoadhesive force decreased with increasing ionic strength, from a value of approximately 80 mN/cm(2) in 0.025 M NaCl to approximately 25 mN/cm(2) in 1.0 M NaCl. These results have been interpreted in terms of the effect of changing pH and ionic strength on electrostatic interactions and swelling of the polymer and mucin layers. Tensiometric force measurements indicated that hydrophobic interactions, as well as hydrogen bonding and electrostatic interactions, were significant in the mucoadhesion of Pluronic-PAA copolymers. Experiments with a range of Pluronic-PAA copolymers with varying PPO contents in the Pluronic segments showed that increasing the overall PPO content increased the hydrophobicity of the polymer solutions. This was reflected in the increases in the advancing contact angles with the mucin layer, indicating that hydrophobic interactions play a role in the adhesion of Pluronic-PAA to mucin.