P. Schaaf

Spray‐Assisted Polyelectrolyte Multilayer Buildup: from Step‐by‐Step to Single‐Step Polyelectrolyte Film Constructions

The alternate deposition of polyanions and polycations on a solid substrate leads to the formation of nanometer to micrometer films called Polyelectrolyte Multilayers. This step-by-step construction of organic films constitutes a method of choice to functionalize surfaces with applications ranging from optical to bioactive coatings. The method was originally developed by dipping the substrate in the different polyelectrolyte solutions. Recent advances show that spraying the polyelectrolyte solutions onto the substrate represents an appealing alternative to dipping because it is much faster and easier to adapt at an industrial level. Multilayer deposition by spraying is thus greatly gaining in interest. Here we review the current literature on this deposition method. After a brief history of polyelectrolyte multilayers to place the spraying method in its context, we review the fundamental issues that have been addresses so far. We then give an overview the different fields where the method has been applied.

Strategies for covalently reticulated polymer multilayers

The past decade has seen a growing need for robust organic coating technologies in various fields going from biotechnology, sensing and electronic devices to membrane design. Polyelectrolyte multilayers constitute a major and versatile tool in this respect. Yet, these films usually suffer from chemical and mechanical weakness. Improvement of the chemical and mechanical stability of multilayer films can be brought about by covalent reticulation between the constituent polymers. Three main routes were developed for this purpose: post-reticulation after film buildup, step-by-step reticulation during film buildup and recently one-pot film construction. This review summarizes these main strategies with emphasis on the mechanisms of the chemical reactions used.

Polysaccharide Films Built by Simultaneous or Alternate Spray: A Rapid Way to Engineer Biomaterial Surfaces

We investigated polysaccharide films obtained by simultaneous and alternate spraying of a chitosan (CHI) solution as polycation and hyaluronic acid (HA), alginate (ALG), and chondroitin sulfate (CS) solutions as polyanions. For simultaneous spraying, the film thickness increases linearly with the cumulative spraying time and passes through a maximum for polyanion/CHI molar charge ratios lying between 0.6 and 1.2. The size of polyanion/CHI complexes formed in solution was compared with the simultaneously sprayed film growth rate as a function of the polyanion/CHI molar charge ratio. A good correlation was found. This suggests the importance of polyanion/polycation complexation in the simultaneous spraying process. Depending on the system, the film topography is either liquid-like or granular. Film biocompatibility was evaluated using human gingival fibroblasts. A small or no difference is observed in cell viability and adhesion between the two deposition processes. The CHI/HA system appears to be the best for cell adhesion inducing the clustering of CD44, a cell surface HA receptor, at the membrane of cells. Simultaneous or alternate spraying of CHI/HA appears thus to be a convenient and fast procedure for biomaterial surface modifications.

Hyaluronic Acid and Its Derivatives in Coating and Delivery Systems: Applications in Tissue Engineering, Regenerative Medicine and Immunomodulation

As an Extracellular Matrix (ECM) component, Hyaluronic acid (HA) plays a multi-faceted role in cell migration, proliferation and differentiation at micro level and system level events such as tissue water homeostasis. Among its biological functions, it is known to interact with cytokines and contribute to their retention in ECM microenvironment. In addition to its biological functions, it has advantageous physical properties which result in the industrial endeavors in the synthesis and extraction of HA for variety of applications ranging from medical to cosmetic. Recently, HA and its derivatives have been the focus of active research for applications in biomedical device coatings, drug delivery systems and in the form of scaffolds or cell-laden hydrogels for tissue engineering. A specific reason for the increase in use of HA based structures is their immunomodulatory and regeneration inducing capacities. In this context, this article reviews recent literature on modulation of the implantable biomaterial microenvironment by systems based on HA and its derivatives, particularly hydrogels and microscale coatings that are able to deliver cytokines in order to reduce the adverse immune reactions and promote tissue healing.

Simultaneous spray coating of interacting species: general rules governing the poly(styrene sulfonate)/poly(allylamine) system.

Simultaneous spraying of two solutions of interacting species onto a substrate held vertically leads to the formation of nanometer-sized coatings. Here we investigate the simultaneous spraying of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) solutions leading to the formation of a film composed of PSS/PAH complexes. The thickness of this film increases linearly with the cumulative spraying time. For a given spraying rate of PAH (respectively PSS), the growth rate of the film depends strongly upon the PSS/PAH ratio and passes through a maximum for a PSS/PAH ratio lying between 0.55 and 0.8. For a PSS/PAH ratio that is maintained constant, the growth speed of the film increases linearly with the spraying rate of polyelectrolyte of both solutions. Using X-ray photoelectron spectroscopy, we find that the film composition is almost independent of the PSS/PAH (spayed) ratio, with composition very close to 1:1 in PSS:PAH film. The 1:1 PSS:PAH composition is explained by the fact that the simultaneous spraying experiments are carried out with salt-free solutions; thus, electroneutrality in the film requires exact matching of the charges carried by the polyanions and the polycations. Zeta potential measurements reveal that, depending on whether the PSS/PAH spraying rate ratio lies below or above the optimal spraying rate ratio, the film acquires a positive or a negative excess charge. We also find that the overall film morphology, investigated by AFM, is independent of the spraying rate ratio and appears to be composed of nanometer-sized grains which are typically in the 100 nm range.

Anti-fouling phosphorylcholine bearing polyelectrolyte multilayers: Cell adhesion resistance at rest and under stretching

We investigate the anti-fouling properties of polyelectrolyte multilayers bearing phosphorylcholine and triethylene glycol moieties and their adhesive response under stretching towards mammalian cells and fungi. More precisely we use a precursor multilayer deposited on glass and on an elastomeric silicone sheet and onto which one or two layers of polyacrylic acid modified with triethylene glycol or phosphorylcholine groups are added. In previous studies, these architectures proved to be resistant to protein adsorption (A. Reisch, J. C. Voegel, E. Gonthier, G. Decher, B. Senger, P. Schaaf and P. J. Mesini, Langmuir, 2009, 25, 3610; A. Reisch, J. Hemmerle, J. C. Voegel, E. Gonthier, G. Decher, N. Benkirane-Jessel, A. Chassepot, D. Mertz, P. Lavalle, P. Mesini and P. Schaaf, J. Mater. Chem., 2008, 18, 4242.). Here we investigate the adhesion of mammalian cells (fibroblasts) and of fungi (Candida albicans) both at rest and under uniaxial stretching of the substrate. Two layers of these polyelectrolytes yield surfaces that are practically resistant to the adhesion of fungi and mammalian cells at rest. Under stretching of the substrate, fungi adhesion remains almost totally prevented at least up to a stretching degree of 1.5, while fibroblast adhesion remains only prevented up to a stretching degree of 1.2. Fibroblast adhesion starts to take place and increases when the substrate is further stretched. The onset of fibroblast adhesion under stretching is retarded for phosphorylcholine containing films compared to those that contain triethylene glycol. These systems thus provide a first example of surfaces that present excellent anti-fouling properties at rest and become specifically adhesive under stretching.

Antibacterial Peptide-Based Gel for Prevention of Medical Implanted-Device Infection.

Implanted medical devices are prone to infection. Designing new strategies to reduce infection and implant rejection are an important challenge for modern medicine. To this end, in the last few years many hydrogels have been designed as matrices for antimicrobial molecules destined to fight frequent infection found in moist environments like the oral cavity. In this study, two types of original hydrogels containing the antimicrobial peptide Cateslytin have been designed. The first hydrogel is based on alginate modified with catechol moieties (AC gel). The choice of these catechol functional groups which derive from mussel’s catechol originates from their strong adhesion properties on various surfaces. The second type of gel we tested is a mixture of alginate catechol and thiol-terminated Pluronic (AC/PlubisSH), a polymer derived from Pluronic, a well-known biocompatible polymer. This PlubisSH polymer has been chosen for its capacity to enhance the cohesion of the composition. These two gels offer new clinical uses, as they can be injected and jellify in a few minutes. Moreover, we show these gels strongly adhere to implant surfaces and gingiva. Once gelled, they demonstrate a high level of rheological properties and stability. In particular, the dissipative energy of the (AC/PlubisSH) gel detachment reaches a high value on gingiva (10 J.m-2) and on titanium alloys (4 J.m-2), conferring a strong mechanical barrier. Moreover, the Cateslytin peptide in hydrogels exhibited potent antimicrobial activities against P. gingivalis, where a strong inhibition of bacterial metabolic activity and viability was observed, indicating reduced virulence. Gel biocompatibility tests indicate no signs of toxicity. In conclusion, these new hydrogels could be ideal candidates in the prevention and/or management of periimplant diseases.

Cell guidance into quiescent state through chromatin remodeling induced by elastic modulus of substrate.

Substrate stiffness is known to strongly influence the fate of adhering cells. Yet, little is known about the influence of the substrate stiffness on chromatin. Chromatin integrates a multitude of biochemical signals interpreted by activation or gene silencing. Here we investigate for the first time the organization of chromatin of epithelial cells on substrate with various mechanical properties. On stiff substrates (100-200 kPa), where cells preferentially adhere, chromatin is mainly found in its euchromatin form. Decreasing the Young modulus to 50 kPa is correlated with a partial shift from euchromatin to heterochromatin. On very soft substrates (≪10 kPa) this is accompanied by cell lysis. On these very soft substrates, histone deacetylase inhibition by adding a drug preserves acetylated histone and thus maintains the euchromatin form, thereby keeping intact the nuclear envelope as well as a residual intermediate filament network around the nucleus. This allows cells to survive in a non-adherent state without undergoing proliferation. When transfer on a stiff substrate these cells retain their capacity to adhere, to spread and to enter a novel mitotic cycle. A similar effect is observed on soft substrates (50 kPa) without need of histone deacetylase inhibition. These new results suggest that on soft substrates cells might enter in a quiescence state. Cell quiescence may thus be triggered by the Young modulus of a substrate, a major result for strategies focusing on the design of scaffold in tissue engineering.

Adhesion of red blood cells to solid surfaces: Experimental results and modeling

Red blood cells (RBC), previously fixed with glutaraldehyde, adhere to glass slides coated with fibrinogen. The RBC solution is injected in a diffusion cell where the RBCs settle on the horizontal glass surface, which is observed with an inverted optical microscope. This experiment is repeated for a series of RBC concentrations in the solution. The relative surface covered by the RBCs, as well as the variance of this surface coverage, is obtained for each concentration by means of image processing. In order to model the process, the RBCs are approximated by disks or spheres. A simulation code was developed on the basis of the Random Sequential Adsorption (RSA) model. Usually, no overlap is allowed, and the particles, once adsorbed, stay pennanentIy fixed in place. Here, the model is extended to account for possible overlap between the RBCs. The surface covered, estimated by simulation, as a function of the number of particles deposited on the surface, shows a clear deviation from a linear relation if overlaps are taken into account; this trend is supported by the experimental data. In the same way, the coverage fluctuation is best reproduced if overlaps are allowed. On the other hand, somewhat surprisingly, a model taking into account both the random ditlusion of the RBCs and the gravitational force they experience, but assuming no overlaps, describes also quite reasonably the experimental coverage fluctuation.

Surface‐Assisted Self‐Assembly Strategies Leading to Supramolecular Hydrogels

Localized molecular self-assembly processes leading to the growth of nanostructures exclusively from the surface of a material is one of the great challenges in surface chemistry. In the last decade, several works have been reported on the ability of modified or unmodified surfaces to manage the self-assembly of low-molecular-weight hydrogelators (LMWH) resulting in localized supramolecular hydrogel coatings mainly based on nanofiber architectures. This Minireview highlights all strategies that have emerged recently to initiate and localize LMWH supramolecular hydrogel formation, their related fundamental issues and applications.