Mechanical Properties of Osmotically Stressed Polyelectrolyte Complexes and Multilayers: Water as a Plasticizer

Compacted, macroporous complexes of poly­(styrenesulfonate) and poly­(diallyldimethylammonium) were dehydrated under defined osmotic stress using poly­(ethylene glycol), PEG. A strong mechanical response to dehydration was observed. At the lowest osmotic stress applied, macropores within the complex were compacted, and the material became transparent. With additional osmotic stress, the decrease in water content with increasing stress slowed considerably, but the complex became much stiffer, the equilibrium modulus reaching several hundred MPa. Concurrently, the complexes became more brittle. Multilayers of the same polyelectrolytes reached equilibrium hydration levels much faster and also increased significantly in modulus. Using an empirical fit, the plasticizing efficiency of water was shown to be exceptionally strong

Homogeneity, Modulus, and Viscoelasticity of Polyelectrolyte Multilayers by Nanoindentation: Refining the Buildup Mechanism

Atomic force microscopy, AFM, and nanoindentation of polyelectrolyte multilayers, PEMUs, made from poly­(diallyldimethylammonium), PDADMA, and poly­(styrene sulfonate), PSS, provided new insight into their surface morphology and growth mechanism. A strong odd/even alternation of surface modulus revealed greater extrinsic (counterion-balanced) charge compensation for fully hydrated multilayers ending in the polycation, PDADMA. These swings in modulus indicate a much more asymmetric layer-by-layer growth mechanism than previously proposed. Viscoelastic properties of the PEMU, which may contribute to cell response, were highlighted by variable indentation rates and minimized by extrapolating to zero indentation rate, at which point the surface and bulk equilibrium moduli were comparable. Variations in surface composition were probed at high resolution using force mapping, and the surface was found to be uniform, with no evidence of phase separation. AFM comparison of wet and dry films terminated with PSS and PDADMA revealed much greater swelling of the PDADMA-terminated PEMU by water, with collapse of surface roughness features in dry conditions. Dynamic and static contact angle measurements suggested less rearrangement for the glassy PSS surface

Cell Durotaxis on Polyelectrolyte Multilayers with Photogenerated Gradients of Modulus

Behaviors of rat aortic smooth muscle (A7r5) and human osteosarcoma (U2OS) cells on photo-cross-linked polyelectrolyte multilayers (PEMUs) with uniform, or gradients of, moduli were investigated. The PEMUs were built layer-by-layer with the polycation poly­(allylamine hydrochloride) (PAH) and a polyanion poly­(acrylic acid) (PAA) that was modified with photoreactive 4-(2-hydroxyethoxy) benzophenone (PAABp). PEMUs with different uniform and gradients of modulus were generated by varying the time of uniform ultraviolet light exposure and by exposure through optical density gradient filters. Analysis of adhesion, morphology, cytoskeletal organization, and motility of the cells on the PEMUs revealed that A7r5 cells established a polarized orientation toward increasing modulus on shallow modulus gradients (approximately 4.7 MPa mm^–1) and durotaxed toward stiffer regions on steeper gradients (approximately 55 MPa mm^–1). In contrast, U2OS cells exhibited little orientation or durotaxis on modulus gradients. These results demonstrate the utility of photo-cross-linked PEMUs to direct vascular and osteoblast cell behavior, a potential application for PEMU coatings on biomedical implants