Layer-by-layer assemblies on plastic films or fabrics: can surface engineering processes mimic additive manufacturing techniques at a nano-to-micro scale?

So far, the additive manufacturing (AM) techniques developed for plastic materials allowed fulfilling such important goals as no shape limits in manufacturing process, full customisation on the single plastic artefact, localised production and limited waste material. Hence, nowadays it is possible to design the envisaged products not following the constricting conventional manufacturing processes but just focusing on their specific function. However, the AM approach seems to be essentially focused on a macro scale level, because of the thickness of the deposited layers. On the other hand, it could be useful to build up assemblies, the thickness of which could be within the nano-to-micro scale: in this context, the Layer-by-Layer (LbL) approach could be very useful. The very first work, which described the principles of this technique, was published in 1966 by Iler. Surprisingly, the potential of the LbL remained hidden till the early 1990s, when Decher and coworkers developed a practical method for the deposition based polyanions and polycations; nowadays, this technique can be tailored to produce nanostructured films, the complex functionality of which can be related to the two following categories: i) Tailoring of surface interactions for improving physical and chemical properties: as every object interacts with the environment via its surface, all the properties that depend on this interaction are dictated by the surface functionality; ii) Fabrication of surface-based functional devices: the sequence used during the deposition defines the final multilayer architecture and thus the device properties. In its simplest description, the LbL consists in an alternate adsorption of chemical species on a chosen substrate exploiting one interaction, which takes place between the selected species, as the driving force for the multilayer build-up; up to now, most of the multilayer films have been fabricated using mainly the electrostatic attraction, although this is not a prerequisite. In fact, there are many others interactions such as donor/acceptor, hydrogen bonding, covalent bonds, stereocomplex formation or specific recognition that have been utilized for the multilayer deposition. This work is aimed at demonstrating that the LbL approach may represent a possible AM strategy for obtaining functional architectures at a nano-to-micro-scale. Furthermore, some examples of the assemblies specifically devoted to enhance the barrier properties or the fire retardancy of the LbL-treated substrates (plastics films or fabrics) will be presented.