Ion Pairing and Hydration in Polyelectrolyte Multilayer Films Containing Polysaccharides

Engineering nanoassemblies of polysaccharides

Polysaccharides offer a wealth of biochemical and biomechanical functionality that can be used to develop new biomaterials. In mammalian tissues, polysaccharides often exhibit a hierarchy of structure, which includes assembly at the nanometer length scale. Furthermore, their biochemical function is determined by their nanoscale organization. These biological nanostructures provide the inspiration for developing techniques to tune the assembly of polysaccharides at the nanoscale. These new polysaccharide nanostructures are being used for the stabilization and delivery of drugs, proteins, and genes, the engineering of cells and tissues, and as new platforms on which to study biochemistry. In biological systems polysaccharide nanostructures are assembled via bottom-up processes. Many biologically derived polysaccharides behave as polyelectrolytes, and their polyelectrolyte nature can be used to tune their bottom-up assembly. New techniques designed to tune the structure and composition of polysaccharides at the nanoscale are enabling researchers to study in detail the emergent biological properties that arise from the nanoassembly of these important biological macromolecules.

Glycosaminoglycans as polyelectrolytes

One of the barriers to understanding structure-property relations for glycosaminoglycans has been the lack of constructive interplay between the principles and methodologies of the life sciences (molecular biology, biochemistry and cell biology) and the physical sciences, particularly in the field of polyelectrolytes. To address this, we first review the similarities and differences between the physicochemical properties of GAGs and other statistical chain polyelectrolytes of both natural and abioitic origin. Since the biofunctionality and regulation of the structures of GAGs is intimately connected with interactions with their cognate proteins, we particularly compare and contrast aspects of protein binding, i.e. effects of both GAGs and other polyelectrolytes on protein stability, protein aggregation and phase behavior. The protein binding affinities and their dependences on pH and ionic strength for the two groups are discussed not only in terms of observable differences, but also with regard to contrasting descriptions of the bound state and the role of electrostatics. We conclude that early studies of the heparin-Antithromin system, proceeding to a large extent through the methods and models of protein chemistry and drug discovery, established not only many enabling precedents but also constraining paradigms. Current studies on heparan sulfate and chondroitin sulfate seem to reflect a more ecumenical view likely to be more compatible with concepts from physical and polymer chemistry.

Polysaccharide-blend multilayers containing hyaluronan and heparin as a delivery system for rhBMP-2

It is shown that blend multilayers of hyaluronan (HA) and heparin (HEP) as polyanions and poly(L-lysine) (PLL) as a polycation can be used to prepare films with different thicknesses and chemical compositions. The amounts of recombinant human BMP-2 (rhBMP-2) loaded and the fraction initially released from the films depend on the film's chemical composition. The amounts of rhBMP-2 loaded in the films are much higher for HA mass fractions of more than 0.4. The bioactivity of the rhBMP-2-loaded films is investigated on C2C12 myoblasts, which differentiates into osteoblasts in contact with the films. The alkaline phosphatase expression for cells grown on nanoblend films of various compositions falls over a unique curve. This suggests that the cells "sensing" the rhBMP-2 are not influenced by the film's chemistry. The rhBMP-2 can sustain at least three successive culture sequences while remaining bioactive, thus confirming the important and protective effect of rhBMP-2. Altogether, these results indicate that crosslinked PLL/HA films have superior properties for the incorporation of rhBMP-2 and on its long-lasting bioactivity.

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).