Stretchable hydrogels of chitosan/hyaluronic acid induced by polyelectrolyte complexation around neutral pH.

In this work, we propose the formation of stretchable hydrogels at neutral pH from the physical crosslinking of chitosan (CS) and hyaluronic acid (HA) by polyelectrolyte complexation. A mixture of CS (Mw ≈ 600 kg/mol, degree of acetylation ≈ 50 %) solution and HA (Mw ≈ 77 kg/mol) solution was prepared with an excess of salts screening the electrostatic interactions CS/HA. In a controlled manner, the polyelectrolyte complexation was induced through the progressive dialysis of the salted polymer mixture against a sodium acetate solution (AcONa, 0.01 M) for 7 days. Depending on [HA], various materials were obtained: viscous solutions at [HA] = 0.75 % (w/v); hydrogels at [HA] = 1.50-2.24 % (w/v) with Young modulus of 14 kPa and stretchable to 200 %. The small angle X-ray scattering characterization of the hydrogels revealed a multiscale organization related to the conformation of the polymers induced by the physical interactions. The dialysis process with AcONa was optimized by adding a dialysis step against a zinc acetate solution containing Zn2+. The combination of polyelectrolyte complexation between CS/HA and metal complexation between Zn2+ and the polymers led to an enhancement of the hydrogel stretchability up to 400 %.

Photo-responsive supramolecular polymer bottle-brushes: The key role of the solvent on self-assembly and responsiveness.

HYPOTHESIS: Supramolecular polymer bottlebrushes (SPBs) consist in the 1D self-assembly of building blocks composed of a self-assembling core with pendant polymer arms. Kinetic hurdles often hinder their stimuli-responsiveness in solution. Changing the nature of the solvent should alleviate these hurdles by modulating the self-association strength, leading to stimuli-responsive SPBs. EXPERIMENTS: The SPBs were formed, in various solvents, by hydrogen bond-driven self-assembly of an azobenzene-bisurea decorated with poly(ethylene oxide) polymer arms. The photo-isomerization of the azobenzene unit was studied by UV/visible spectroscopy and proton NMR spectroscopy, whereas the consequences on supramolecular self-assembly were studied by small angle neutron and X-ray scattering. FINDINGS: In water, the assembly was previously shown to be driven by both hydrogen-bonds and strong hydrophobic effects, the latter rendering the system kinetically frozen and the disassembly irreversible. Here we show that in organic solvents such as toluene or chloroform, reversible light-responsive dissociation is achieved. Solvophobic effects in these solvents are expected to be much weaker than in water, which probably allows reversibility of the light-response in the former solvents. The key role of the solvent on the reversibility of the process opens up new perspectives for the design of stimuli-responsive SPBs and their applications in various fields.

Grafted chitosan thin films of various degrees of acetylation as a reusable platform for the investigation of biological interactions.

Surface treatment by adhesive polymers is a promising solution to immobilize and study bacteria cells through microscopic assays and, for example, control their growth or determine their susceptibility to antibiotic treatment. The stability of such functional films in wet conditions is crucial, as the film degradation would compromise a persistent use of the coated devices. In this work, low roughness chitosan thin films of degrees of acetylation (DA) ranging from 0.5 % to 49 % were chemically grafted onto silicon and glass substrates and we have demonstrated how the physicochemical properties of the surfaces and the bacterial response were DA-dependent. A fully deacetylated chitosan film presented an anhydrous crystalline structure while the hydrated crystalline allomorph was the preferred structure at higher DA. Moreover, their hydrophilicity increased at higher DA, leading to higher film swelling. Low DA chitosan-grafted substrate favored bacterial growth away from the surface and could be envisioned as bacteriostatic surfaces. Contrarily, an optimum of Escherichia coli adhesion was found for substrates modified with chitosan of DA = 35 %: these surfaces are adapted for the study of bacterial growth and antibiotic testing, with the possibility of reusing the substrates without affecting the grafted film - ideal for limiting single-use devices.

Preparation of highly stable and ultrasmooth chemically grafted thin films of chitosan.

Chitosan-coated surfaces are of great interest for biomedical applications (antibacterial coatings, implants, would healing, single-cell microfluidics…). However, one major limitation of chitosan-based systems is the high solubility of the polymer under acidic aqueous conditions. Herein, we describe a simple procedure to prepare extremely smooth and stable chitosan coatings. In detail, chitosan films with a low degree of N-acetylation and of thicknesses varying from 40 nm to 10 µm were grafted onto epoxy-functionalized silicon wafers via an optimized water-temperature treatment (WTT). The formation of a grafted chitosan network insoluble in acidic aqueous media (pH 3.5) was evidenced and the films were stable for at least 2 days at pH 3.5. The film morphology and the swelling behavior were characterized by atomic force microscopy (AFM) and neutron reflectivity, which showed that the film roughness was extremely low. The physical cross-linking of the films was demonstrated using infrared spectroscopy, dynamic mechanical analysis (DMA) and wide-angle X-ray scattering (WAXS). Finally, we show that the swelling behavior of such films was largely influenced by the environmental conditions, such as the pH or ionic strength of the solution.

Innovative pressure environment combining hydrostatic pressure gradient and mechanical compression for structural investigations of nanoporous soft films.

The development of a new sample environment enabling X-ray scattering measurements at small and large angles under mechanical compression and hydraulic flow is presented. The cell, which is adapted for moderate pressures, includes beryllium windows, and allows applying simultaneously a compressive pressure up to 2.5 kbar in the perpendicular direction to the flow and either a hydrostatic pressure up to 300 bar or a pressure gradient of the same amplitude. The development of high-pressure devices for synchrotron experiments is relevant for many scientific fields in order to unveil details of a material's structure under relevant conditions of stresses. In particular, mechanical constraints coupled to hydrostatic pressure or flow, leading to complex stress tensor and mechanical response, and therefore unexpected deformations (swelling and pore deformation), are poorly addressed. Here, first the design of the environment is described, and then its performance with measurements carried out on a regenerated cellulose membrane is demonstrated.

A New Generation of Ultrasmall Nanoparticles Inducing Sensitization to Irradiation and Copper Depletion to Overcome Radioresistant and Invasive Cancers.

An emerging target to overcome cancer resistance to treatments is copper, which is upregulated in a wide variety of tumors and may be associated with cancer progression and metastases. The aim of this study was to develop a multimodal ultrasmall nanoparticle, CuPRiX, based on the clinical AGuIX nanoparticle made of the polysiloxane matrix on which gadolinium chelates are grafted. Such hybrid nanoparticles allow: (i) a localized depletion of copper in tumors to prevent tumor cell dissemination and metastasis formation and (ii) an increased sensitivity of the tumor to radiotherapy (RT) due to the presence of high Z gadolinium (Gd) atoms. CuPRiX nanoparticles are obtained by controlled acidification of AGuIX nanoparticles. They were evaluated in vitro on two cancer cell lines (lung and head and neck) using the scratch-wound assay and clonogenic cell survival assay. They were able to reduce cell migration and invasion and displayed radiosensitizing properties.

Role of initial precursors on the liquid-crystalline phase behavior of synthetic aluminogermanate imogolite nanotubes.

HYPOTHESIS: Synthetic imogolite nanotubes form stable colloidal dispersions that may also exhibit a rich liquid-crystalline phase behavior according to the nanotube length to diameter ratio. Anisometric double-walled aluminogermanate nanotubes are now readily available through hydrothermal treatment of germanium and aluminum precursors. This work aims to assess how the self-organization behavior of these nanotubes is influenced by the nature of the precursors. EXPERIMENTS: Five different samples were synthesized by changing the precursors involved in the formation of either inner or outer walls, then fully characterized. From series of aqueous dispersions prepared by osmotic stress, we evaluated the phase behavior by coupling polarized optical observations and small-angle X-ray scattering. FINDINGS: The formation of anisometric nanotubes is achieved whatever the initial conditions. Their structural properties are however affected by the nature of the aluminum salt. For nanotubes synthesized with aluminum perchlorate, the dispersions present an isotropic-to-columnar phase transition with a self-organization of the nanotubes over large distances. By contrast, nanotubes synthesized with chloride and nitrate salts form only nematic or isotropic liquids and tend to group together in bi-dimensional rafts. We suggest that the different phase behaviors are related at the first order to the presence of structural vacancies in the nanotube walls.

Controlled Synthesis of Gold Nanoparticles in Copolymers Nanomolds by X-ray Radiolysis.

We show by X-ray and neutron small-angle scattering that gold nanoparticles with controlled sizes and morphologies can be obtained by the metallic reduction of AuCl4- ions trapped in 3D organic molds by X-ray radiolysis. The molds are spherical frozen micelles of polystyrene-b-poly(dimethylaminoethyl methacrylate) (PS-b-PDMAEMA) block copolymer in acidic aqueous solution with a PS spherical core surrounded by a corona of PDMAEMA chains in good solvent. The behavior of micelles is controlled by the [AuCl4-]/[DMAEMA] ratio RAuCl4-/DMAEMA. At low gold concentration, AuCl4- ions condense on the positively charged DMAEMA moieties without changing the behavior of the PDMAEMA chains. At intermediate gold concentration, the ions induce a progressive contraction of the corona's chains and dehydration of micelles. At large gold concentration, the corona becomes a fully dry phase loaded with gold ions, which induces micelle aggregation. Radiolysis of the solution by an intense X-ray beam produces different types of gold nanoparticles with respect to RAuCl4-/DMAEMA and irradiation time. At RAuCl4-/DMAEMA = 0.033, irradiation produces in the first step gold clusters in the micelle corona which in the second step merge to form nanoparticles of a similar size to that of the micelle. Conversely, at RAuCl4-/DMAEMA = 0.33, micelles do not operate as templates but only as nucleation zones and large nanoparticles grow outside the micelles.

In situ synthesis of Fe3O4 nanoparticles coated by chito-oligosaccharides: physico-chemical characterizations and cytotoxicity evaluation for biomedical applications.

Fe3O4 nanoparticles coated with chito-oligosaccharides (COS) were prepared in situ by a simple co-precipitation method through a mixing of iron ions (Fe3+ and Fe2+) and COS aqueous solutions followed by precipitation with ammonia. The impact of COS with different degree of polymerization (DP 10, 24 and 45) and degree of N-acetylation (DA) âˆ¼ 24% and 50% (exhibiting high solubility) on the synthesis and physical properties of the coated magnetic nanoparticles was evaluated. Several advantages were found when the magnetic nanoparticles were prepared in the presence of the studied COS, such as: preparation of functionalized magnetic nanoparticles with narrower size distributions and, consequently, higher saturation magnetization (an increase of up to 22%); and an expressive increasing in the concentration of COS-coated magnetic nanoparticles (up to twice) in the cell viability test in comparison with pure Fe3O4 nanoparticles. Furthermore, among the analyzed samples, the magnetic nanoparticles coated by COS with DA âˆ¼ 50% present a higher cytocompatibility. Our results allow envisioning various biomedical applications, valorizing the use of coated-magnetic nanoparticles for magnetic-field assisted drug delivery, enzyme or cell immobilization, or as a marker for specific cell tracking, among others.

Anisotropic and heterogeneous dynamics in stretched elastomer nanocomposites.

We use X-ray photon correlation spectroscopy (XPCS) to investigate the dynamics of a stretched elastomer by means of probe particles. The particles dispersed in the elastomer were carbon black or silica aggregates classically used for elastomer reinforcement but their volume fraction is very low (φ < 10-2). We show that their dynamics is slower in the direction of the tensile strain than in the perpendicular one. For hydroxylated silica which is poorly wetted by the elastomer, there is no anisotropy. Two-time correlation functions confirm anisotropic dynamics and suggest dynamical heterogeneity already expected from the q-1 behavior of the relaxation times. The height χ* of the peak of the dynamical susceptibility, determined by the normalized variance of the instantaneous correlation function, is larger in the direction parallel to the strain than in the perpendicular one. It also appears that its q dependence changes with the morphology of the probe particle. Therefore, the heterogeneous dynamic probed by the particles is not related only to that of the strained elastomer matrix. In fact, it results from modification of the dynamics of the polymer chains near the surface of the particles and within the aggregate porosity (bound polymer). It is concluded that XPCS is a powerful method for investigating the dynamics, at a given strain, of the bound polymer-particle units which are responsible, at large volume fractions, for the reinforcement.

Highly stretchable hydrogels from complex coacervation of natural polyelectrolytes.

The controlled complex coacervation of oppositely charged hyaluronic acid (Mw ≈ 800-1000 kg mol-1) and chitosan (Mw ≈ 160 kg mol-1, degree of acetylation = 15%) led to hydrogels with controllable properties in terms of elasticity and strength. In this work, we performed desalting by dialysis of high ionic strength solutions of mixed polyelectrolytes and showed that the control of the pH during the polyelectrolyte assembly greatly impacts the mechanical properties of the hydrogel. First, for pHs from 5.5 to 7.5, a slight coacervation was observed due to low chitosan protonation and poor polyelectrolyte associations. Then, for pHs from 3.0 to 5.5, coacervation and syneresis led to free-standing and easy to handle hydrogels. Finally, for pHs from 2.0 to 3.0 (close to the pKa of the hyaluronic acid), we observed the unusual stretchability of these hydrogels that could arise from the pre-folding of hyaluronic acid chains while physical crosslinking was achieved by hyaluronic acid/chitosan polyelectrolyte complexation.

Solvent contribution to the stability of a physical gel characterized by quasi-elastic neutron scattering.

The dynamics of a physical gel, namely, low-molecular-mass organic gelator methyl-4,6-O-benzylidene-α-D-mannopyranoside (α-manno) in water and toluene, are probed by neutron scattering. Using high gelator concentrations, we were able to determine, on a time scale from a few picoseconds to 1 nanosecond, the number of solvent molecules that are immobilized by the rigid network formed by the gelators. We found that only a few toluene molecules per gelator participate in the network which is formed by hydrogen bonding between the gelators' sugar moieties. In water, however, the interactions leading to the gel formations are weaker, involving dipolar, hydrophobic, or π-π interactions, and hydrogen bonds are formed between the gelators and the surrounding water. Therefore, around 10 to 14 water molecules per gelator are immobilized by the presence of the network. This study shows that neutron scattering can give valuable information about the behavior of solvent confined in a molecular gel.

Bioresorbable and nonresorbable macroporous thermosensitive hydrogels prepared by cryopolymerization. Role of the cross-linking agent.

Macroporous poly( N-isopropylacrylamide) (pNIPA) gels (so-called cryogels), cross-linked with different bis-acrylic compounds, N,N'-methylenebisacrylamide (MBAAm) and dimethacrylate-tyrosine-lysine-tyrosine (DMTLT), were prepared through free-radical polymerization at subzero temperature in dioxane/water media. DMTLT is a hydrolytically degradable cross-linker with relatively hydrophobic character. The effects of different synthesis conditions, namely the concentration of monomers, the cross-linker, and the initiator in the reaction mixture, on the structure of the pNIPA-cryogels have been studied. The equilibrium swelling ratio of the DMTLT cross-linked pNIPA cryogels at temperatures below lower critical solution temperature (LCST) of pNIPA, was over ten times higher than that of the gels synthesized at room temperature from the same feed composition. The MBAAm cross-linked pNIPA cryogels synthesized in water exhibited the highest equilibrium swelling and the fastest response. The critical transition temperature, T c, was lower ( T c approximately 31 degrees C) for pNIPA-cryogels synthesized in dioxane/water media or cross-linked with DMTLT as compared to MBAAm cross-linked pNIPA cryogels synthesized in water (T c approximately 33 degrees C). Scanning electron microscopy (SEM) revealed different porous structure and pore surface morphology depending on the cross-linker (MBAAm or DMTLT) and the solvent (water or dioxane/water) used. Gels and cryogels were also characterized by SAXS, showing that the nanostructure of the samples is related to swelling.

ASAXS, SAXS and SANS investigations of vulcanized elastomers filled with carbon black.

Small-angle X-ray scattering (SAXS) performed down to small q values (q

Adsorption of divalent cations on DNA.

The distribution of divalent ions in semidilute solutions of high-molecular-mass DNA containing both sodium chloride and strontium chloride in near-physiological conditions is studied by small-angle x-ray scattering and by small-angle neutron scattering. Both small-angle neutron scattering and small-angle x-ray scattering reveal a continuous increase in the scattering intensity at low q with increasing divalent ion concentration, while at high q the scattering curves converge. The best fit to the data is found for a configuration in which DNA strands of cross-sectional radius 10 angstroms are surrounded by a counterion sheath of outer radius approximately 13.8 angstroms, independent of the strontium chloride concentration. When the strontium chloride is replaced by calcium chloride, similar results are obtained, but the thickness of the sheath increases when the divalent salt concentration decreases. These results correspond in both cases to partial localization of the counterions within a layer that is thinner than the effective Debye screening length.

Light scattering studies of the solution properties of chitosans of varying degrees of acetylation.

The use of two techniques, differential interferometry and quasi-elastic light scattering (QELS), allowed us to study solutions of chitosan varying in degree of acetylation (DA), degree of dissociation (alpha), and concentration (C(p)). With the first technique, we demonstrated the modification of the electric polarizability of the polymer chains, through a law of behavior of the variation of the refractive index increment dn/dC with DA and alpha. This brought us information on the various kinds of interactions (H-bonds, electrostatic, and hydrophobic) involved in the evolution of the solution properties. QELS experiments performed in dilute regime showed the presence of supramolecular structures depending on DA and alpha. The topology and the nature of these objects are discussed. The typical presence of aggregates and their evolution with concentration was also demonstrated in semidilute regime.

Scattering from dilute ferrofluid suspensions in soft polymer gels.

Small angle neutron and x-ray scattering methods are used to investigate the structure of dilute suspensions of two different ferrofluid systems dispersed in soft polyacrylamide hydrogels. It is found that the particles in the fluid are fractal aggregates composed of smaller particles of radius ca. 5 nm. The fractal dimension is strongly dependent on sample, taking the value 1.7 in the first sample and 2.9 in the second sample. In the presence of a magnetic field the aggregates orient, but are restricted in both their translational and rotational freedom. The effect of the gel elasticity is treated as a hindrance to the orientation process.