Effective embedding of liposomes into polyelectrolyte multilayered films: the relative importance of lipid-polyelectrolyte and interpolyelectrolyte interactions

Abstract

The layer-by-layer (LbL) self-assembly of polyelectrolytes has emerged as a powerful and versatile strategy for engineering (bio)surfaces with active compounds. One possibility to engineer such films is to dope them with nanoparticles or with intact vesicles as carriers filled with the compounds of interest. In our previous studies we demonstrated that intact vesicles stabilized by poly-L-lysine (PLL) coating can be embedded in polyelectrolyte multilayer films made from the combination of either poly-L-glutamic acid (PGA) and poly(allylamine hydrochloride) (PAH) or from the combination of hyaluronic acid (HA) and PLL. These findings were empirical and not correlated to the interaction mode between PLL decorated vesicles (PLL-Lip) with the multilayer film. In order to produce robust and stable multi-layered nanocoatings containing intact vesicles, one has to understand how PLL-coated vesicles are integrated in the LbL architecture. To that aim, vesicle adsorption on three kinds of films, namely (PAH/PSS)₁₂, where PSS stands for poly-(4-styrenesulfonate), (PAH/PGA)₁₂, and (PLL/HA)₁₂ was studied. PLL-Lip adsorption strongly depends on the interactions between PLL with both the vesicles and the polyanions used to form polyelectrolyte multilayers as was shown by direct evidence with confocal and atomic force microscopy as well as by the investigation of the interaction between PLL-Lip with the three polyanions in solution, namely HA, PGA and PSS. The latest was investigated by means of isothermal titration microcalorimetry. As a result, PSS in solution is able to desorb PLL from PLL coated liposomes, and on the other hand, PGA and HA can adsorb on PLL-stabilized vesicles without desorbing or inducing only partial desorption of the pre-adsorbed PLL. As a consequence, the (PLL/HA)₁₂ film was found to be the best system among the three investigated combinations of polycations/polyanions for vesicle entrapment. The amount of entrapped vesicles is then proportional to the number of vesicle deposition steps and is increased for liposomes with higher charge. The release of a dye encapsulated in the entrapped vesicles could be induced by temperature changes which shows that for the HA-PLL combination of polyelectrolytes, the embedded vesicles retain their integrity below the main phase transition temperature. The temperature increase leads to fast leakage of liposome cargo while the film structure is not changed (up to 45 °C) suggesting that the lipidic bilayer is destabilized by ionic contacts with a polyelectrolyte network of the film. The latest is supported by the fact that within increase of vesicle negative charge, the stability of film-embedded vesicles is decreased whereas stability of these vesicles in solution is increased.