The structure of poly(L-lysine) (PLL)͞hyaluronan (HA) polyelectrolyte multilayers formed by electrostatic self-assembly is studied by using confocal laser scanning microscopy, quartz crystal microbalance, and optical waveguide lightmode spectroscopy. These films exhibit an exponential growth regime where the thickness increases exponentially with the number of deposited layers, leading to micrometer thick films. Previously such a growth regime was suggested to result from an ''in'' and ''out'' diffusion of the PLL chains through the film during buildup, but direct evidence was lacking. The use of dye-conjugated polyelectrolytes now allows a direct three-dimensional visualization of the film construction by introducing fluorescent polyelectrolytes at different steps during the film buildup. We find that, as postulated, PLL diffuses throughout the film down into the substrate after each new PLL injection and out of the film after each PLL rinsing and further after each HA injection. As PLL reaches the outer layer of the film it interacts with the incoming HA, forming the new HA͞PLL layer. The thickness of this new layer is thus proportional to the amount of PLL that diffuses out of the film during the buildup step, which explains the exponential growth regime. HA layers are also visualized but no diffusion is observed, leading to a stratified film structure. We believe that such a diffusion-based buildup mechanism explains most of the exponential-like growth processes of polyelectrolyte multilayers reported in the literature.hyaluronan ͉ poly(L-lysine) ͉ confocal laser scanning microscopy ͉ diffusion ͉ film structure

The formation of a new kind of biocompatible film based on poly(L-lysine) and hyaluronic acid (PLL/HA) by alternate deposition of PLL and HA was investigated. Optical waveguide lightmode spectroscopy, streaming potential measurements, atomic force microscopy, and quartz crystal microbalance (QCM) were used to analyze the different aspects of the buildup process such as the deposited mass after each new polyelectrolyte adsorption, the overall surface charge of the film, and its morphology. As for "conventional" polyelectrolyte multilayer systems, the driving force of the buildup process is the alternate overcompensation of the surface charge after each PLL and HA deposition. The construction of (PLL/HA)n films takes place over two buildup regimes. The first one is characterized by the formation of isolated islands that grow both by addition of new polyelectrolytes on their top and by mutual coalescence of the islands. The second regime sets in once a continuous film is formed after the eighth layer pair deposition in our working conditions and is characterized by an exponential increase of the mass. QCM measurements at different frequencies evidenced a viscoelastic behavior of the films with a shear viscosity on the order of 0.1 Pa‚s. This new kind of biocompatible film incorporating a natural polymer of the cartilage and a widely used polypeptide is of potential use for cell-targeted action.

The buildup of the first layers of a polystyrenesulfonate (PSS)/polyallylamine (PAH) multilayer is studied in situ by means of streaming potential measurements (SPM) and by scanning angle reflectometry (SAR). The results are discussed in the framework of a schematic representation of the multilayer in three zones:  a precursor zone (I), a core zone (II), and an outer zone (III). This view seems to be supported by our experimental findings. The ζ potential of the multilayer determined by the SPM shows a symmetrical and constant charge inversion during the multilayer buildup. This seems to indicate an exact charge compensation in zone II and an excess charge that is entirely located in the outer zone III. It is also shown by SAR that a regular buildup regime, in which the thickness increment per layer is constant, is reached after the deposition of the first six polyelectrolyte layers, which gives an indication of the extension of zone I. The influence of the salt concentration C NaCl present in the polyelectrolyte solutions during multilayer buildup is also investigated. It is found that an increase of the salt concentration in the polyelectrolyte solutions leads to larger amounts of deposited polyelectrolytes and to thicker multilayers. The amount deposited per polyelectrolyte layer δ Q (PSS or PAH) is correctly predicted by the law δQ = a· + b where α lies between 0.05 and 0.15. In addition, when a multilayer built up in salty solutions is brought in contact with pure water, it expands, indicating that the rinsing step mainly affects zone III of the multilayer, which appears thus to behave like a polyion layer. The structural changes of the multilayer consecutive to the replacement of the salt solution by pure water occur with characteristic times ranging from a few tens of minutes to several hours depending on the initial salt concentration. Finally, it is also found that the structural modifications of the film are fully reversible so that the initial multilayer structure is recovered when water is replaced again by the initial salt solution.