Two types of polyelectrolyte multilayer films have been reported in the literature. These are (i) films whose mass and thickness increase linearly as the number of deposited bilayers increases and (ii) films that grow exponentially. We present a model for the buildup of exponentially growing films that allows a discussion of the behavior of them in a unified manner. This model is based on the diffusion both in and out the whole film of part of the chains of at least one of the polyelectrolytes constituting the multilayer. In short, the film is brought into contact with the solution of polyelectrolytes that are able to diffuse into the film. Inside of the film, chains of this polyelectrolyte constitute the “free” chains. At the subsequent rinsing step, some of them diffuse outward from the film. The remaining chains leave the film as it is brought into contact further with the polyelectrolyte solution of opposite charge. As the “free” chains reach the film/solution interface, they are complexed by the polyelectrolytes of opposite charge. These complexes, which are composed of both types of polyelectrolytes, contribute to the formation of the additional mass of the multilayer. The model relies on the evaluation of the electrostatic potential in the film within the framework of the Debye−Hückel approximation and takes into consideration the Donnan effect, which is due to noncompensated fixed charges in the film. It also includes the situation where none of the polyelectrolytes diffuse within the multilayer, in which case the film grows linearly. The model predicts the existence of a free-energy barrier that prevents total diffusion of any “free” polyelectrolyte outward from the film during a rinsing step, following contact with a polyelectrolyte solution. It also predicts that usually only one of the two polyelectrolytes that comprise the film diffuses readily into it. Both polyelectrolytes that comprise the film can diffuse “into” and “out of” the multilayer only when the concentration of noncompensated fixed charges within the film is very small. Several predictions of the model are discussed in the light of experimental results that have already been published or are new.

There exist two types of polyelectrolyte multilayers: those whose thickness increases linearly with the number of deposition steps, which are nicely structured, and those whose thickness increases exponentially, which resembles hydrogels. This simple picture has recently slightly evolved with the finding that some exponentially growing films enter into a linear growth phase after a certain number of deposition steps. In this study, we investigate the buildup process of hyaluronic acid/poly(L-lysine) (HA/PLL) multilayers that constitute one of the best known exponentially growing systems. The films are built by using two deposition methods: the well-known dipping method and the more recent spraying method where the polyelectrolyte solutions are sprayed alternately onto a vertical substrate. The goal of this study is twofold. First, we investigate the influence of the main parameters (i.e., spraying rate and spraying time) of the spraying method on the film growth process. We find that, as for the dipping method, the film thickness first evolves exponentially with the number of deposition steps, and after a given number of deposition steps, it follows a linear evolution. We find that similar behavior is observed with the dipping method. Second, because the spraying method allows the very fine variation of the different parameters of the buildup, we use this method to investigate the exponential-to-linear transition. We find that this transition always takes place after about 12 deposition steps whatever the values of the parameters controlling the deposition process. We discuss our results in light of a model proposed by Hübsch et al. (Hübsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P. Langmuir 2004, 20, 1980-1985) and later by Salomäki et al. (Salomäki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232-11240) in which it is assumed that the exponential-to-linear transition is due to a film restructuring that progressively forbids the diffusion of one of the polyelectrolytes constituting the film over part of the film. This "forbidden" zone then grows with the number of deposition steps so that the outer zone of the film that is still concerned with diffusion keeps a constant thickness and moves upward as the total film thickness increases.

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.

Alternated deposition of polyanions and polycations on a charged solid substrate leads to the buildup of polyelectrolyte multilayer (PEM) films. Two types of PEM films were reported in the literature: films whose thickness increases linearly and films whose thickness increases exponentially with the number of deposition steps. However, it was recently found that, for exponentially growing films, the exponential increase of the film thickness takes place only during the initially deposited pairs of layers and is then followed by a linear increase. In this study, we investigate the growth process of hyaluronic acid/poly(L-lysine) (HA/PLL) and poly(L-glutamic acid)/poly(allylamine) (PGA/PAH) films, two films whose growth is initially exponential, when the growth process enters the linear regime. We focus, in particular, on the influence of the molecular weight (Mw) of the polyelectrolytes. For both systems, we find that the film thickness increment per polyanion/polycation deposition step in the linear growth regime is fairly independent of the molecular weights of the polyelectrolytes. We also find that when the (HA/PLL)n films are constructed with low molecular weight PLL, these chains can diffuse into the entire film during each buildup cycle, even for very thick films, whereas the PLL diffusion of high molecular weight chains is restricted to the upper part of the film. Our results lead to refinement of the buildup mechanism model, introduced previously for the exponentially growing films, which is based on the existence of three zones over the entire film thickness. The mechanism no longer needs all the "in" and "out" diffusing polyanions or polycations to be involved in the buildup process to explain the linear growth regime but merely relies on the interaction between the polyelectrolytes with an upper zone of the film. This zone is constituted of polyanion/polycation complexes which are "loosely bound" and rich in the polyelectrolyte deposited during the former deposition step.

The short-term interactions of chondrosarcoma cells with polyelectrolyte multilayer films built up by the alternate adsorption of poly(L-lysine) (PLL) and poly(L-glutamic acid) (PGA) was studied in the presence and in the absence of serum. The films and their interaction with serum proteins were first characterized by means of optical waveguide lightmode spectroscopy, quartz crystal microbalance, and zeta potential measurements. In a serum-containing medium, the detachment forces measured by the micropipet technique were about eight times smaller on PGA-ending than on PLL-ending films. For these latter ones, the adhesion force decreased when the film thickness increased. In a serum-free medium, the differences between the negative- and positive-ending films were enhanced: adhesion forces on PLL-ending films were 40-100% higher, whereas no cellular adherence was found on PGA-terminating films. PGA-ending films were found to prevent the adsorption of serum proteins, whereas important protein adsorption was always observed on PLL-ending films. These results show how cell interactions with polyelectrolyte films can be tuned by the type of the outermost layer, the presence of proteins, and the number of layers in the film.

The interactions between polystyrenesulfonate (PSS)/polyallylamine (PAH) multilayers with human serum albumin (HSA) were investigated by means of scanning angle reflectometry (SAR). We find that albumin adsorbs both on multilayers terminating with PSS (negatively charged) or PAH (positively charged) polyelectrolytes. On films terminating with PSS only, an albumin equivalent monolayer is found whereas when PAH constitutes the outer layer, albumin interacts with the multilayer in such a way as to form a protein film that extends over thicknesses that can be as high as four times the largest dimension of the native albumin molecule. Once the protein film is formed, it is found that when the albumin solution is replaced by a pure buffer solution of same ionic strength as the adsorption solution almost no desorption takes place. On the other hand, when a buffer solution of higher ionic strength is brought in contact with the albumin film, a significant amount of adsorbed proteins is released. One also observes that, for albumin solutions of a given protein concentration, the adsorbed protein amount depends on the ionic strength of the adsorption solution. On surfaces terminating with PAH, the adsorbed protein amount first increases rapidly but passes through a maximum and decreases with the ionic strength. The ionic strength corresponding to the maximum of the adsorbed albumin amount itself depends on the albumin concentration. On the other hand, on films terminating with PSS the adsorbed amount increases with the salt concentration before leveling-off. These results show that the underlying complexity of concentration and pH dependent adsorption/desorption equilibria often simply termed "protein adsorption" is the result of antagonist competing interactions that are mainly of electrostatic origin. We also propose two microscopic models, that are compatible with our experimental observations.

When multilayer films are built up with polycations and polyanions, their thickness can grow either linearly or exponentially with the number of deposited layer pairs, depending for example on the nature of the polyelectrolytes used. We investigate in the present work the construction of a film using a binary mixture of polyanions as polyanion solution. The two anionic components are chosen such that one of them causes the film to grow linearly while the other causes the film to grow exponentially. It is observed that a mixture of both components leads to a hybrid growth law, depending on its composition. At the beginning of the construction, the thickness of the film increases exponentially with the number of deposited bilayers. Once a given thickness is reached, one observes the crossover to a linear growth regime. This finding is discussed on the basis of the diffusion coefficients of the polyanions that are assumed to diffuse "in" and "out" of the film.

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