P. Schaaf

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (143)488.47 Total impact

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    Advanced Functional Materials 04/2013; · 9.77 Impact Factor
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  • Adv. Funct. Mater. 01/2013; 23(38):4785-4792.
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    ABSTRACT: 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.
    Langmuir 05/2012; 28(22):8470-8. · 4.19 Impact Factor
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    ABSTRACT: Human gingival fibroblasts (HGFs) cell sheets have a potential use for in vivo wound healing due to the ability of HGFs to adopt a contractile phenotype which is typically expressed during extracellular matrix tissue remodeling. For this purpose, we developed a chemically detachable platform based on poly(allylamine hydrochloride)/poly(styrene sulfonate) multilayer film built on a sacrificial precursor film which served as a substrate for HGF cell layer formation. The sacrificial precursor film, based on disulfide-containing polycation and polyanion, is degradable under mild conditions compatible for cell sheet detachment. Cellular viability and cell phenotype analysis of HGF show that the designed platform promotes cell phenotype switch into contractile phenotype, maintained after cell sheet lift-off. This contractile phenotype is acquired by fibroblasts during in vivo wound healing and tissue remodeling. HGFs cell sheet fragments, obtained by this detachment process, could be cultured later on showing a good retention of the typical spindle-shape of differentiated cells after 10 days of culture. HGFs cell sheets have great potential applications as autologous substrates for tissue repair and cellular synthetic platforms for research on connective tissue diseases or evaluation of novel therapeutic agents.
    Chemistry of Materials. 01/2012; 24(5):930-937.
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    ABSTRACT: 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.
    Langmuir 03/2011; 27(8):4653-60. · 4.19 Impact Factor
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    ABSTRACT: We report on a novel approach for the design of stimuli-responsive surfaces based on the immobilization of charged ABC triblock terpolymer micelles. The terpolymer consists of a hydrophobic polybutadiene (B) block, a weak anionic poly(methacrylic acid) (MAA) middle block, and a strong cationic end block of quaternized poly(2-(dimethylamino) ethyl methacrylate) (Dq) (BMAADq). In alkaline solutions, this polymer self-assembles into core-shell-corona micelles with a hydrophobic B core, a pH-sensitive MAA/Dq intramicellar interpolyelectrolyte complex (im-IPEC) shell, and a cationic corona of excess Dq. These micelles were directly adsorbed onto charged silica as a monolayer creating laterally structured surfaces. The adsorption kinetics was found to follow the theoretical model of random sequential adsorption (RSA). Exposure of the adsorbed micelles to external stimuli (at pH < pK(a,apparent) of PMAA) induces im-IPEC dissolution due to protonation of the MAA block resulting in a changed composition of the shell and both the length and charge density of the corona. Two types of conformational response to pH trigger and therefore, two types of dynamics coupled to short and long relaxation times are involved in the system. The response to pH cycling was shown to be reversible on the short-term scale while the long-term exposure to acidic media causes irreversible changes in the morphology of the micelles as a consequence of the micelles' core mobility and slow rearrangement. In particular, we find that exposure to low pH causes a shape transition to a "doughnut"-like morphology, exposing the core parts of the micelles. In contrast, adsorbed micelles with covalently crosslinked B cores show higher stability to irreversible morphology changes while maintaining the reversible response to pH cycling.
    Soft Matter. 01/2011; 23:11144-11153.
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    ABSTRACT: It is known that changes of the ionic strength of the solution in contact with a polyelectrolyte multilayer film leads to the swelling or the deswelling of this film. Recently, we found that increasing the ionic strength of the solution in contact with an exponentially growing polyelectrolyte multilayer changes vigorously its internal structure with the formation of holes before the film finally dissolves (Mjahed et al., Soft Matter 2009, 5, 2269). Here we generalize these observations and show that any change of ionic strength of the solution (increase or decrease) in contact with the film induces a film restructuring with the formation of holes. Two types of holes are found: spherical holes formed by a direct increase or decrease of the ionic strength and non-spherical holes, resembling cracks which appear after an increase followed by a decrease sequence of the ionic strength. Later on, these holes heal. The film restructuring thus depends upon the path of ionic changes imposed to the film and takes place through complex processes which are far from being fully clear.
    Journal of Materials Chemistry 01/2011; 21(23):8416-8421. · 5.97 Impact Factor
  • Angew. Chem. Int. Ed. 01/2011; 123:4466–4469.
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    ABSTRACT: In this paper, we report on the mechanical characterization of polyelectrolytes multilayer (PEM) films prepared from poly(glutamic acid)-poly(styrenesulfonate) (PGA-PSS) blends, deposited in alternated spray deposition with poly(allylamine hydrochloride) (PAR). The polyanion composition of the blended film was first investigated by Fourier transformed infrared spectroscopy in the attenuated total reflection mode. The monomer molar fraction of PGA, in the film increases almost linearly with x, i.e., the monomer molar fraction of PGA in the sprayed polyanion solution. The mechanical properties of the blended (PAH/PGA(x)-PSS(1-x))(n) film were measured using two methods: (i) the wrinkling metrology method and (ii) by colloidal probe atomic force microscopy. We demonstrate that Young's modulus of the PAH/PGA(x)-PSS(1-x) multilayer films can be systematically controlled by the chemical composition of these films, depending on x. Measurements indicate that increasing the monomer molar fraction of PGA in the blended film results in a decrease in film modulus up to 2 orders of magnitude as compared to the PAH/PSS system. At a monomer molar fraction of PGA in the film around x = 0.7, this system undergoes such a transition. We also show that for a given x the elastic properties of these films are significantly affected by the humidity conditions. For a (PAH/PGA(0.88)-PSS(0.12)) film, Young's modulus of the film varies from several hundred to few MPa by solely altering the relative humidity between 12.5% and 80%.
    Macromolecules 01/2011; 44(22):8954-8961. · 5.52 Impact Factor
  • Abstracts of Papers of the American Chemical Society. 01/2011; 242.
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    ABSTRACT: Development of blood vessels remains a big challenge for vascular tissue engineering. We recently demonstrated that poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/PSS) films are excellent substrates for vascular progenitor cell differentiation. However, no intact cell sheets could be harvested. Using a spraying procedure, we develop here a user-friendly technology for the development of small blood vessels based on alginate membranes. The key point of our approach is based on a multilayered PAH/PSS film built on micro-textured alginate gels, about 140 mu m thick and which can be peeled off. The alginate, calcium chloride and polyelectrolyte solutions are sprayed on a vertical glass substrate. Due to the drainage of the calcium chloride solution on the adsorbed alginate layer, an oriented micro-textured gel is obtained. The PAH/PSS film, built on top of the gel, induces a good proliferation without phenotype alteration of smooth muscle cells. Parallel micro-textures on the top of the gel allow the orientation of cells. After peeling off the substrate, the cellularized membrane is rolled around a mandrel. Confocal microscopy observations show the formation of concentric layers with a presence of cells. This work represents the initial stages of a new, original blood vessel reconstruction strategy via tissue engineering.
    Soft Matter 01/2011; 7(7):3621-3626. · 3.91 Impact Factor
  • Macromolecules 01/2010; 43:3994–3997. · 5.52 Impact Factor
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    ABSTRACT: The electrochemically tailored degradation of polyelectrolyte assemblies holds great promises for the design of inexpensive, easily prepared and precise controlled release systems. However, the conception of such electrochemically responsive platforms for drug or gene delivery requires a detailed understanding of the degradation process of the polyelectrolyte multilayer in which the active species to release are incorporated. To this end, we assess here the influence of an applied electric potential on different polyelectrolyte systems, combining global and local investigation techniques. In situ atomic force microscopy allows us to evidence morphological changes at the nano- and micro-scale, while the investigation at a larger scale by optical waveguide lightmode spectroscopy brings complementary information relating not only to material release into the bulk solution, but also to ion migration and swelling. Weak and highly hydrated poly(L-lysine)/hyaluronic acid assemblies with thicknesses up to several hundreds of nanometre continuously dissolve upon electrochemical trigger. However, stronger and more compact films made of poly(allylamine hydrochloride) and poly(styrene sulfonate) dissolve only if their thickness is of a few tens of nanometres, while thicker films delaminate from the electrode. Additional results obtained with composite films combining both polyelectrolyte systems allow us to present mechanisms based on the continuous formation of protons at the electrode surface due to water electrolysis which fully describe the dissolution and the delamination processes. In addition, the study also reveals a novel approach for the release of free-standing polyelectrolyte membranes, which is of great interest for the development of mechanical sensors, separation membranes, or micromechanical devices.
    Soft Matter 01/2010; 6(17):4246-4254. · 3.91 Impact Factor
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    ABSTRACT: Polyelectrolyte multilayer (PEM) films consist of polyanion/polycation super-structures that are sensitive to various stresses like ionic strength changes. We investigate the swelling process of the exponentially growing poly(L-lysine)/hyaluronic acid (PLL/HA) films induced by changes of the ionic strength of the contact solution. We show that above a first critical ionic strength the swelling is accompanied by a release of both polyelectrolytes constituting the film, leading to its subsequent dissolution. At a second critical ionic strength, the swelling of the multilayer is so important that, in addition to this polyelectrolyte release, formation of spherical holes is observed inside the film. The presence of dissolved PLL and HA chains in these holes leads to an increase of the concentration of their counterions inside of them, and thus induces an extra osmotic pressure. This in turn favors the size increase of the holes before they coalesce. The release of both polyelectrolytes from the film into the supernatant ultimately allows a decrease of the osmotic pressure inside the PLL/HA film, which finally leads to the disappearance of the holes and concomitantly to a complete dissolution of the film. When the release of polyelectrolytes into the solution is prevented by a poly(diallyldimethyl ammonium chloride)/poly(styrene sulfonate) (PDADMAC/PSS) capping film, the holes appear at a smaller critical ionic strength compared to uncapped films. Here too the formation of the holes is attributed to an increase of the osmotic pressure inside the film. As soon as the capping barrier ruptures because of the swelling of the film, both PLL and HA chains can diffuse out of the film and the holes decrease in size and disappear, as does the film.
    Soft Matter 01/2009; 5(11):2269-2276. · 3.91 Impact Factor
  • P. Schaaf, J.-C. Voegel
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    ABSTRACT: The films known as polyelectrolyte multilayers are made by alternating deposition of polyanions (negatively charged polymers) and polycations (positively charged polymers). The development of these films, invented in the 1990s [1,3], has seen a considerable burst of interest, in particular due to their many applications. Indeed, these films are used to make electroluminescent diodes [4], anti-reflecting surfaces [5], water filtering substrates [6], and substrates for the separation of chiral molecules [7]. The alternating deposition of positive and 12 negative species can also be used to make films with a mechanical strength close to that of steel [8]. Applications to biosensors and especially biomaterials are currently under investigation [9]. This is the last example discussed in the present chapter. Polyelectrolytes are charged polymers, usually soluble in an aqueous solution. When a surface, supposed negatively charged, is set in contact with a solution of polycations (positively charged polyelectrolytes), the chains will immediately interact with the surface via electrostatic interaction and adsorb onto it. Like any other polymer, polyelectrolytes do not adsorb lengthwise against the surface, but form loops and tails. This adsorption is generally irreversible, and replacing the polycation solution by the solvent (water) alone will only lead to very slight desorption. This irreversibility of adsorption results from the formation of many anchoring points with the surfaces along the long polymer chains. Even if the interaction energy between a monomer, the basic building block of the polymer, and a surface is small, the fact that a number of contact points are set up makes the overall interaction between a polymer and a surface rather strong. Furthermore, in order for a chain to desorb, all the anchor points on the surface must be broken simultaneously, and such an event is highly improbable.
    Nanoscience: Nanobiotechnology and Nanobiology. 01/2009;
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    ABSTRACT: A biosensor is an analytical device which physically associates a biological receptor and a transducer [1]. The transducer transforms the interaction between the receptor and its target into an interpretable signal. No other step, such as the separation of free and complexed molecules or the addition of a further reagent, is required. Biosensors have been classified in terms of the transduction mode or the nature of the biological receptor [2-4]. The modes of transduction applicable to all molecular interactions, without labelling the molecules and whatever their nature, detect either a change in mass on a surface (surface plasmon resonance, acoustic biosensors), or a heat transfer (calorimetry) [6]. Among the devices using these transduction modes, the most widely used in biology at the present time are based on surface plasmon resonance (SPR). Since the commonest SPR biosensors are manufactured by Biacore AB, a company based in Uppsala, Sweden [5] (Biacore is a registered trademark), we begin by discussing the interpretation of SPR data obtained in this configuration.
    Nanoscience: Nanobiotechnology and Nanobiology. 01/2009;
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    ABSTRACT: The optical response of a dielectric interfacial film can be completely described, to second order in the film thickness, by three parameters called optical invariants. These allow the extraction of a maximum of information about the film while avoiding arbitrary models of that film. The information which can be obtained from experimental data on an isotropic film is, in order of decreasing precision: (i) the surface concentration of the adsorbed material, (ii) the average thickness of the layer through the first moment of the mass distribution in the layer, and (iii) a more complicated moment that gives an indication of the uniformity of the layer. This method of analysis is applied to scanning angle reflectometry data on two kinds of films at a water/silica interface: (i) Protein films, and in particular, antibody/antigen complexes forming at the interface. This provides a model of a film that is essentially homogenous in the direction parallel to the surface, but which may have concentration gradients perpendicular to the surface. (ii) Films of polystyrene latex particles. Here the information gained is the size and number of particles adsorbed at the interface, with an indication of the uniformity of the particle distribution.
    02/2008: pages 296-299;
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    ABSTRACT: Poly(dimethylsiloxane) (PDMS) substrates are used in many applications where the substrates need to be elongated and various treatments are used to regulate their surface properties. In this article, we compare the effect of three of such treatments, namely, UV irradiation, water plasma, and plasma polymerization, both from a molecular and from a macroscopic point of view. We focus our attention in particular on the behavior of the treated surfaces under mechanical stretching. UV irradiation induces the substitution of methyl groups by hydroxyl and acid groups, water plasma leads to a silicate-like layer, and plasma polymerization causes the formation of an organic thin film with a major content of anhydride and acid groups. Stretching induces cracks on the surface both for silicate-like layers and for plasma polymer thin coatings. This is not the case for the UV irradiated PDMS substrates. We then analyzed the chemical composition of these cracks. In the case of water plasma, the cracks reveal native PDMS. In the case of plasma polymerization, the cracks reveal modified PDMS. The contact angles of plasma polymer and UV treated surfaces vary only very slightly under stretching, whereas large variations are observed for water plasma treatments. The small variation in the contact angle values observed on the plasma polymer thin film under stretching even when cracks appear on the surface are explained by the specific chemistry of the PDMS in the cracks. We find that it is very different from native PDMS and that its structure is somewhere between Si(O2) and Si(O3). This is, to our knowledge, the first study where different surface treatments of PDMS are compared for films under stretching.
    Langmuir 01/2008; 23(26):13136-45. · 4.19 Impact Factor
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    ABSTRACT: A micro-stratified 3D scaffold was designed by successive stacking of alginate gel layers (AGLs) and poly(L-lysine)-hyaluronic acid (PLL- HA) multilayer films. AGLs are obtained by complexation of alginate by Ca2+ ions. Alginate solutions are first sprayed onto a solid substrate inclined such that the excess of solution be removed by natural drainage. A CaCl2 solution is then either sprayed onto the substrate or the alginate covered substrate is dipped into a CaCl2 solution. The spraying of the CaCl2 solution leads to micro-porous AGLs, whereas the dipping in a CaCl2 aqueous solution leads to a more homogeneous gel layer without porosity. The second process also allows the formation of AGLs with a controlled thickness. With the goal of stacking different AGLs and PLL- HA films, the influence of a PLL- HA precursor film on the formation of AGLs is firstly investigated. It is found that when an alginate solution is sprayed on a PLL- HA multilayer built in the presence of CaCl2, the multilayer plays the role of reservoir of Ca2+ ions and of PLL chains, which both diffuse out of the multilayer film and complex alginate chains. This leads to the formation of a "pre-alginate gel''. When this film is further dipped in the CaCl2 solution, an additional AGL forms, which is, however, free of PLL chains. Finally after the build-up of a PLL-HA film on the top of AGL, we succeeded in designing micro-stratified 3D scaffolds constituted by alternating strata of AGLs and PLL-HA films. This micro-stratified gel provides a new scaffold design with a perfectly controlled build-up: AGL aims to be a 3D scaffold for cell culture, and the PLL-HA multilayers should act as reservoirs for biologically active molecules.
    Soft Matter 01/2008; 4(7):1422-1429. · 3.91 Impact Factor

Publication Stats

2k Citations
488.47 Total Impact Points

Institutions

  • 1989–2012
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
    • University of California, Los Angeles
      • Department of Chemistry and Biochemistry
      Los Angeles, CA, United States
  • 2008
    • Université de Haute-Alsace
      Mulhousen, Alsace, France
  • 1991–2007
    • French Institute of Health and Medical Research
      Lutetia Parisorum, Île-de-France, France
    • Purdue University
      • School of Chemical Engineering
      West Lafayette, Indiana, United States
  • 2005
    • Unité Inserm U1077
      Caen, Lower Normandy, France