Elasticity of Native and Cross-Linked Polyelectrolyte Multilayer Films

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Abstract
Mechanical properties of polyelectrolyte multilayer films were studied by nanoindentation using the atomic force microscope (AFM). Force−distance measurements using colloidal probe tips were systematically obtained for supported films of poly(l-lysine) and hyaluronan that are suited to bio-application. Both native and covalently cross-linked films were studied as a function of increasing layer number, which increases film thickness. The effective Young's modulus perpendicular to the film, E⊥, was determined to be a function of film thickness, cross-linking, and sample age. Thick PEM films exhibited a lower E⊥ than thinner PEM, whereas the Young's modulus of cross-linked films was more than 10-fold larger than native films. Moduli range from ≈20 kPa for native films up to ≈800 kPa for cross-linked ones. Young's moduli increased slightly with sample age, plateauing after ∼4 weeks. Spreading of smooth muscle cells on these substrates with pre-attached collagen proved to be highly dependent on film rigidity with stiffer films giving greater cell spreading.

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