New 2-in-1 Polyelectrolyte Step-by-Step Film Buildup without Solution Alternation: From PEDOT-PSS to Polyelectrolyte Complexes

Although never emphasized and increasingly used in organic electronics, PEDOT-PSS (poly­(3,4-ethylenedioxythiophene)-poly­(styrene sulfonate)) layer-by-layer (lbl) film construction violates the alternation of polyanion and polycation rule stated as a prerequisit for a step-by-step film buildup. To demonstrate that this alternation is not always necessary, we studied the step-by-step construction of films using a single solution containing polycation/polyanion complexes. We investigated four different systems: PEDOT-PSS, bPEI-PSS (branched poly­(ethylene imine)-poly­(sodium 4-styrene sulfonate)), PDADMA-PSS (poly­(diallyl dimethyl ammonium)-PSS), and PAH-PSS (poly­(allylamine hydrochloride)-PSS). The film buildup obtained by spin-coating or dipping-and-drying process was monitored by ellipsometry, UV–vis-NIR spectrophotometry, and quartz-crystal microbalance. The surface morphology of the films was characterized by atomic force microscopy in tapping mode. After an initial transient regime, the different films have a linear buildup with the number of deposition steps. It appears that, when the particles composed of polyanion-polycation complex and complex aggregates in solution are more or less liquid (case of PEDOT-PSS and bPEI-PSS), our method leads to smooth films (roughness on the order of 1–2 nm). On the other hand, when these complexes are more or less solid particles (case of PDADMA-PSS and PAH-PSS), the resulting films are much rougher (typically 10 nm). Polycation/polyanion molar ratios in monomer unit of the liquid, rinsing, and drying steps are key parameters governing the film buildup process with an optimal polycation/polyanion molar ratio leading to the fastest film growth. This new and general lbl method, designated as <i>2-in-1 method</i>, allows obtaining regular and controlled film buildup with a single liquid containing polyelectrolyte complexes and opens a new route for surface functionalization with polyelectrolytes

Boric Acid as an Efficient Agent for the Control of Polydopamine Self-Assembly and Surface Properties

The deposition of polydopamine (PDA) films on surfaces, a versatile deposition method with respect to the nature of the used substrate, is unfortunately accompanied by deposition of insoluble precipitates in solution after a prolonged oxidation time of dopamine solutions. Therefore, there is evident interest to find methods able to stop the deposition of PDA on surfaces and to simultaneously control the self-assembly of PDA in solution to get stable colloidal aggregates. In addition to proposed methods relying on the use of polymers like poly­(vinyl alcohol) and proteins like human serum albumin, we show herein that boric acid is an efficient adjuvant that is simultaneously able to stop the self-assembly of PDA in solution as well as on surfaces and to change the adhesive properties of the resulting PDA coatings

Layer-by-Layer Enzymatic Platform for Stretched-Induced Reactive Release

An original “all-in-one” platform combining polymers, enzymes, and enzymatic substrates in a unique film is designed. A polymeric barrier stratum prevents any contact between enzymes adsorbed on top of the film and substrates loaded in an underlying reservoir. Upon stretching of the film, a continuous diffusion of substrates through the barrier is triggered, followed by a catalytic reaction. This leads to the formation of products that are released from the film. This new platform acts as a stretch-induced reactive release system and emerges as an innovative concept in mechano-responsive materials

Efficient Gas and Water Vapor Barrier Properties of Thin Poly(lactic acid) Packaging Films: Functionalization with Moisture Resistant Nafion and Clay Multilayers

Poly­(lactic acid) (PLA) represents one of the most promising and attractive biobased polymer for the industrial development of environmentally sustainable packaging. However, oxygen and water barrier properties of PLA based films cannot compete with those of commercially available composite multilayers. To fill this gap, we used the layer-by-layer deposition technique on commercially used PLA thin films (30 μm thick) in order to increase their barrier properties to oxygen and water vapor. Nanometric films were grown by alternating branched poly­(ethylene imine) (BPEI), hydrophobic fluorinated polymer (Nafion), and montmorillonite clay (MMT) layers with the aim of obtaining low gas permeability in both dry and moist conditions as well as low water vapor permeability. Two different kinds of architectures were designed and successfully prepared, based on a 4 layer repeating unit (BPEI/MMT/BPEI/Nafion), represented here as quadlayer (QL), and on a 6 layer repeating-unit ((BPEI/Nafion)₂/BPEI/MMT), hexalayer (HL). Reduction in oxygen and water permeabilities is observed for films based on both types of repeat units. The reduction of the permeabilities increases with the number of quad and hexalayers achieving reductions in terms of oxygen permeability in both dry and humid conditions up to 98% and 97% respectively for 10 HL and QL. Furthermore, a reduction of 78% of water vapor transmission rate through the functionalized film was obtained for these films. As far as oxygen permeability is concerned, HL films are more efficient than QL films for smaller numbers of deposition units. These properties are shown to result from the complementarity between the presence of BPEI/Nafion and MMT layers

Nanosized Films Based on Multicharged Small Molecules and Oppositely Charged Polyelectrolytes Obtained by Simultaneous Spray Coating of Interacting Species

Simultaneous spraying of polyelectrolytes and small multicharged molecules of opposite charges onto a vertical substrate leads to continuous buildups of organic films. Here, we investigate the rules governing the buildup of two such systems: poly­(allylamine hydrochloride)/sodium citrate (PAH/citrate) and PAH/sulfated α-cyclodextrin (PAH/CD-S). Special attention is paid to the film growth rate as a function of the spraying rate ratio of the two constituents. This parameter was varied by increasing the spraying rate of one of the constituents while maintaining constant that of the other. For PAH/CD-S systems, whatever the constituent (PAH or CD-S) whose spraying rate was kept fixed, the film growth rate first increases and passes through a maximum before decreasing when the spraying rate of the other constituent is increased. For PAH/citrate, the film growth rate reaches a plateau value when the spraying rate of citrate is increased while that of PAH is maintained constant, whereas when the spraying rate of citrate is maintained constant and that of PAH is increased, a behavior similar to that of PAH/CD-S is observed. The composition of PAH/CD-S sprayed films determined by X-ray photoelectron spectroscopy is independent of the spraying rate ratio of the two constituents and corresponds to one allylamine for one sulfate group. For PAH/citrate, by increasing the PAH/citrate spraying rate ratio, the carboxylic/nitrogen ratio in the film increases and tends to 1. There is thus always a deficit of carboxylic groups (COO^– + COOH) with respect to amines (NH₂ + NH₃⁺). Yet, the ratio (COO^–/NH₃⁺) is always close to 1, ensuring exact charge compensation. The film morphology determined by atomic force microscopy is granular for PAH/CD-S and is smooth and liquid-like for PAH/citrate. A model based on strong (respectively weak) interactions between PAH and CD-S (respectively citrate) is proposed to explain these features

Collagen-Based Fibrillar Multilayer Films Cross-Linked by a Natural Agent

Surface functionalization plays an important role in the design of biomedical implants, especially when layer forming cells, such as endothelial or epithelial cells, are needed. In this study, we define a novel nanoscale surface coating composed of collagen/alginate polyelectrolyte multilayers and cross-linked for stability with genipin. This buildup follows an exponential growth regime versus the number of deposition cycles with a distinct nanofibrillar structure that is not damaged by the cross-linking step. Stability and cell compatibility of the cross-linked coatings were studied with human umbilical vein endothelial cells. The surface coating can be covered by a monolayer of vascular endothelial cells within 5 days. Genipin cross-linking renders the surface more suitable for cell attachment and proliferation compared to glutaraldehyde (more conventional cross-linker) cross-linked surfaces, where cell clumps in dispersed areas were observed. In summary, it is possible with the defined system to build fibrillar structures with a nanoscale control of film thickness, which would be useful for in vivo applications such as inner lining of lumens for vascular and tracheal implants

Biomimetic Cryptic Site Surfaces for Reversible Chemo- and Cyto-Mechanoresponsive Substrates

Chemo-mechanotransduction, the way by which mechanical forces are transformed into chemical signals, plays a fundamental role in many biological processes. The first step of mechanotransduction often relies on exposure, under stretching, of cryptic sites buried in adhesion proteins. Likewise, here we report the first example of synthetic surfaces allowing for specific and fully reversible adhesion of proteins or cells promoted by mechanical action. Silicone sheets are first plasma treated and then functionalized by grafting sequentially under stretching poly(ethylene glycol) (PEG) chains and biotin or arginine-glycine-aspartic acid (RGD) peptides. At unstretched position, these ligands are not accessible for their receptors. Under a mechanical deformation, the surface becomes specifically interactive to streptavidin, biotin antibodies, or adherent for cells, the interactions both for proteins and cells being fully reversible by stretching/unstretching, revealing a reversible exposure process of the ligands. By varying the degree of stretching, the amount of interacting proteins can be varied continuously