It has recently been demonstrated that dopamine solutions put in contact with a variety of solid substrates allow the production of thin coatings probably made of melanin (Science 2007, 318, 426). In this article, we show that the thickness of these coatings can be controlled to allow a growth regime that is proportional to the reaction time if fresh dopamine is regularly proVided. We propose that the growth is initiated by the adsorption of a radical compound. When dopamine polymerization or aggregation has reached a steady state in solution, the produced species do not adhere anymore to the substrate, emphasizing the role played by unoxidized dopamine. X-ray photoelectron spectroscopy showed that the thickness of the deposit increases linearly with the number of immersion steps, but the thickness measured in ultravacuum is about 4 times smaller than the thickness measured by ellipsometry in conditions of ambient humidity. This suggests that the drying of the deposit has a considerable influence on its properties. The Si2p signal characteristic of the silicon substrate decreases progressively when the number of deposition steps increases but does vanish even after 32 deposition steps. This observation will be discussed with respect to the formation of a continuous film. Cyclic voltammetry experiments showed that a deposit impermeable to ferrocyanide is obtained after the immersion in nine freshly prepared dopamine solutions, demonstrating the formation of a film. The atomic composition of the film determined by X-ray photoelectron spectroscopy is compatible with that of melanin. Finally, we show that the deposit can be quantitatively removed from the substrate when put in a strongly alkaline solution.

Mechanical properties of model and natural gels have recently been demonstrated to play an important role in various cellular processes such as adhesion, proliferation, and differentiation, besides events triggered by chemical ligands. Understanding the biomaterial/cell interface is particularly important in many tissue engineering applications and in implant surgery. One of the final goals would be to control cellular processes precisely at the biomaterial surface and to guide tissue regeneration. In this work, we investigate the substrate mechanical effect on cell adhesion for thin polyelectrolyte multilayer (PEM) films, which can be easily deposited on any type of material. The films were cross linked by means of a water-soluble carbodiimide (EDC), and the film elastic modulus was determined using the AFM nanoindentation technique with a colloidal probe. The Young's modulus could be varied over 2 orders of magnitude (from 3 to 400 kPa) for wet poly(L-lysine)/hyaluronan (PLL/HA) films by changing the EDC concentration. The chemical changes upon cross linking were characterized by means of Fourier transform infrared spectroscopy (FTIR). We demonstrated that the adhesion and spreading of human chondrosarcoma cells directly depend on the Young's modulus. These data indicate that, besides the chemical properties of the polyelectrolytes, the substrate mechanics of PEM films is an important parameter influencing cell adhesion and that PEM offer a new way to prepare thin films of tunable mechanical properties with large potential biomedical applications including drug release.