Dynamics of poly(L-lysine) in hyaluronic acid/poly(L-lysine) multilayer films studied by fluorescence recovery after pattern photobleaching

Multiple functionalities of polyelectrolyte multilayer films: new biomedical applications

The design of advanced functional materials with nanometer- and micrometer-scale control over their properties is of considerable interest for both fundamental and applied studies because of the many potential applications for these materials in the fields of biomedical materials, tissue engineering, and regenerative medicine. The layer-by-layer deposition technique introduced in the early 1990s by Decher, Moehwald, and Lvov is a versatile technique, which has attracted an increasing number of researchers in recent years due to its wide range of advantages for biomedical applications: ease of preparation under "mild" conditions compatible with physiological media, capability of incorporating bioactive molecules, extra-cellular matrix components and biopolymers in the films, tunable mechanical properties, and spatio-temporal control over film organization. The last few years have seen a significant increase in reports exploring the possibilities offered by diffusing molecules into films to control their internal structures or design "reservoirs," as well as control their mechanical properties. Such properties, associated with the chemical properties of films, are particularly important for designing biomedical devices that contain bioactive molecules. In this review, we highlight recent work on designing and controlling film properties at the nanometer and micrometer scales with a view to developing new biomaterial coatings, tissue engineered constructs that could mimic in vivo cellular microenvironments, and stem cell "niches."

Internal composition versus the mechanical properties of polyelectrolyte multilayer films: the influence of chemical cross-linking

Different types of polyelectrolyte multilayer films composed of poly(L-lysine)/hyaluronan (PLL/HA), chitosan/hyaluronan (CHI/HA) and poly(allylamine hydrochloride)/poly(L-glutamic acid) (PAH/PGA) have been investigated for their internal composition, including water content, ion pairing, and ability to be covalently cross-linked, as well as for their mechanical properties. Film buildup under physiological conditions was monitored by the quartz crystal microbalance with dissipation monitoring (QCM-D) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), which allows unambiguous quantification of the different groups present in the polyelectrolytes. (PAH/PGA) films emerged as the most dense films with the lowest hydration (29%) and the highest COO(-) molar density. In addition, PAH is greatly in excess in these films (3 PAH monomers per PGA monomer). The formation of amide bonds during film cross-linking using the water-soluble carbodiimide EDC was also investigated. All of the films could be cross-linked in a tunable manner, but PAH/PGA exhibited the highest absolute number of amide bonds created, approximately 7 times more than for (PLL/HA) and (CHI/HA) films. The Young's modulus E of the films measured by AFM nanoindentation was shown to vary over 1 to 2 orders of magnitude for the different systems. Interestingly, a linear relationship between E and the density of the covalent cross-links created was observed for (PLL/HA) and (CHI/HA) films whereas (PGA/PAH) films exhibited biphasic behavior. The mean distance between covalent cross-links was estimated to be approximately 11 nm for (PLL/HA) and (CHI/HA) films and only approximately 6 nm for (PAH/PGA) films for the maximum EDC concentration tested (100 mg/mL).

Surface-supported multilayers decorated with bio-active material aimed at light-triggered drug delivery

In this work, we report on the functionalization of layer-by-layer films with gold nanoparticles, microcapsules, and DNA molecules by spontaneous incorporation into the film. Exponentially growing films from biopolymers, namely, hyaluronic acid (HA) and poly-L-lysine (PLL), and linearly growing films from the synthetic polymers, namely, poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), were examined for the embedding. The studied (PLL/HA)(24)/PLL and (PAH/PSS)(24)/PAH films are later named HA/PLL and PSS/PAH films, respectively. The HA/PLL film has been found to be more efficient for both particle and DNA embedding than PSS/PAH because of spontaneous PLL transport from the interior of the whole HA/PLL film to the surface in order to make additional contact with embedded particles or DNA. DNA and nanoparticles can be immobilized in HA/PLL films, reaching loading capacities of 1.5 and 100 microg/cm(2), respectively. The capacities of PSS/PAH films are 5 and 12 times lower than that for films made from biopolymers. Polyelectrolyte microcapsules adsorb irreversibly on the HA/PLL film surface as single particles whereas very poor interaction was observed for PSS/PAH. This intrinsic property of the HA/PLL film is due to the high mobility of PLL within the film whereas the structure of the PSS/PAH film is "frozen in". Gold nanoparticles and DNA form micrometer-sized aggregates or patches on the HA/PLL film surface. The diffusion of nanoparticles and DNA into the HA/PLL film is restricted at room temperature, but DNA diffusion is triggered by heating to 70 degrees C, leading to homogeneous filling of the film with DNA. The film has not only a high loading capacity but also can be activated by "biofriendly" near-infrared (IR) laser light, thanks to the gold nanoparticle aggregates on the film surface. Composite HA/PLL films with embedded gold nanoparticles and DNA can be activated by light, resulting in DNA release. We assume that the mechanism of the release is dependent on the disturbance in bonding between "doping" PLL and DNA, which is induced by local thermal decomposition of the HA/PLL network in the film when the film is exposed to IR light. Remote IR-light activation of dextran-filled microcapsules modified by gold nanoparticles and integrated into the HA/PLL film is also demonstrated, revealing an alternative release pathway using immobilized light-sensitive carriers (microcapsules).

Adhesion of two physically contacting planar substrates coated with layer-by-layer assembled films

Adhesives composed of synthetic and low-cost molecules that are based on simple chemical principles are attractive because of their versatility. In this article, we report adhesion between two planar substrates coated with layer-by-layer (LbL) assembled films of cationic poly(diallyldimethylammonium chloride) (PDDA) and anionic poly(sodium styrenesulfonate) (PSS) and perform lap shear measurements of the adhered substrates. Films prepared on the substrates functioned as adhesives when one substrate coated with the PDDA-surface film contacted the other surface coated with the PSS-surface film under adequate pressure in the presence of water droplets, suggesting that two films adhered on the basis of polyion complex formation. Observations suggested that the adhesives failed at the substrate-film interface rather than at the bulk films. The adhesion was compared between film-coated substrates and noncoated ones. Confocal laser scanning microscopic observation of adhesives composed of fluorescently labeled poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) revealed that the labeled PAH assembled on one substrate was well dispersed, even in a nonlabeled film assembled on another substrate. It was therefore confirmed that after adhesion in the presence of the water component, the polyelectrolytes became intermixed between the glassy films, resulting in changes in the adhesive structure at the substrate-film interface.

Remote near-IR light activation of a hyaluronic acid/poly(l-lysine) multilayered film and film-entrapped microcapsules

Spontaneous embedding of gold nanoparticle (NP) aggregates or polyelectrolyte microcapsules modified with NPs in biocompatible hyaluronic acid/poly(l-lysine) films is reported. The NPs were adsorbed in the aggregated state to induce near-IR light absorption. The films functionalized with gold NPs become active in response to a "biologically friendly" near-IR laser at a power of about 20 mW. The activation is characterized by a localized temperature increase in the film, allowing conversion of light energy to heat into confined volumes. Microcapsules adsorbed onto the film can release its cargo under stimulation with near-IR light because of localized permeability changes in their walls. This work is aimed at layer-by-layer film-based biomedical coatings and active surfaces with light-sensitive features wherein metal NPs and microcapsules are used as active centers or carriers with remote control of functionalities.

Diffusion in supported lipid bilayers: influence of substrate and preparation technique on the internal dynamics

The diffusion law of DMPC and DPPC in Supported Lipid Bilayers (SLB), on different substrates, has been investigated in details by Fluorescence Recovery After Patterned Photobleaching (FRAPP). Over micrometer length scales, we demonstrate the validity of a purely Brownian diffusive law both in the gel and the fluid phases of the lipids. Measuring the diffusion coefficient as a function of temperature, we characterize the gel-to-liquid phase transition of DMPC and DPPC. It is shown that, depending on the type of substrate and the method used for bilayer preparation, completely different behaviours can be observed. On glass substrates, using the Langmuir-Blodgett deposition technique, both leaflets of the bilayer have the same dynamics. On mica, the dynamics of the proximal leaflet is slower than the dynamics of the distal leaflet, although the transition temperature is the same for both layers. Preparing bilayers from vesicle fusion in same conditions leads to more random behaviours and shifted transition temperatures.