Effect of crosslinking on the elasticity of polyelectrolyte multilayer films measured by colloidal probe AFM

A homemade colloidal probe atomic force microscope was used to perform nanoindentation with a spherical probe of 5 microm in diameter, at different approach velocities in order to extract the Young's modulus, E0, of poly(L-lysine)/hyaluronan (PLL/HA) films. This parameter is of prime importance to control cellular adhesion. The films were either kept in their native form or cross-linked with a mixture of 1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide (EDC) and N-hydrosulfosuccinimide (sulfo-NHS), where the EDC concentration was varied from 1 up to 100 mg mL(-1) (approximately from 5 to 500 mM). A model based on Hertz mechanics was used to account for the interactions between film and probe. It is shown that the Young's modulus varies with the approach velocity for the native (PLL/HA) films, whereas for cross-linked ones, E0 is independent from the velocity over the whole range investigated. It is found that for native films, E0 takes a value of 3 kPa at low approach velocities, a velocity domain that should be relevant in cellular adhesion processes. The Young's modulus increases with the EDC concentration used to cross-link the films and levels off at a value of about 400 kPa for EDC concentrations exceeding 40 mg mL(-1). Thus, it is possible by crosslinking PLL/HA films to control their elastic properties with the aim to alter their behavior as to the cellular adhesion.