Synthesis, Structural and Micromechanical Properties of 3D Hyaluronic Acid-Based Cryogel Scaffolds.

In this study, macroporous, elastic, three-dimensional scaffolds formed of hyaluronic acid mixed with ethylene glycol diglycidyl ether as a chemical cross-linker have been prepared by cryogelation for application in tissue engineering. These cryogels are characterized by large interconnected pores of size ∼50-300 µm and pore wall thickness of ∼5-30 µm as determined from confocal microscopy images. Variation of pH, freezing temperature, and polymerization time allows for control of pore size and shape as well as matrix thickness. These structural properties then determine mechanical strength as well as swelling capacity. Furthermore, increasing hyaluronic acid concentration decreases cryogel pore size, reduces swelling properties, and reinforces mechanical properties. On the other hand, decreasing cross-linker concentration, at a constant hyaluronic acid concentration, increases pore size and swelling capacity but provides less rigidity. Additionally, for the first time, local elastic properties of the polymer matrix and viscous properties of the pores have been characterized using multiple particle tracking microrheology. Local matrix elasticity, relaxation time of hyaluronic acid chains, and the degree of heterogeneity are discussed in detail. These latter properties are crucial for the development of new tissue engineering constructs and will help to understand how local matrix viscoelasticity affects cell cultivation. Finally, elastic moduli obtained in bulk rheology are much higher than corresponding values deduced from microrheology. This discrepancy might be explained by the formation of very highly cross-linked cores in the network where no tracer particle can penetrate.

Chain flexibility and dynamics of polysaccharide hyaluronan in entangled solutions: a high frequency rheology and diffusing wave spectroscopy study.

We have investigated the linear viscoelastic properties of high molecular weight hyaluronan in aqueous solution using an experimental approach combining mechanical rheometry and optical microrheology. The complex shear modulus has been measured over a broad frequency range from 10(-1) to 10(7) rad/s. Chain flexibility is characterized by the persistence length lpand this parameter has been determined for the first time in the entangled regime here from high frequency modulus data. At NaHA concentrations above the entanglement concentration ce, lp is essentially independent of polymer concentration, temperature, and ionic strength. The latter is consistent with the Odijk-Skolnick-Fixman theory. The scaling exponent describing the concentration dependence of the plateau modulus G0 agrees well with predictions for polymers in good solvents. The scaling exponents for the specific viscosity ηsp and relaxation time TR are slightly higher than theoretically predicted for polyelectrolytes in the high salt limit, indicating, that molecular aggregation occurs at higher polymer concentrations.

Effect of counterion binding efficiency on structure and dynamics of wormlike micelles.

We have studied the effect of counterion binding efficiency on the linear viscoelastic properties of wormlike micelles formed from hexadecyltrimethylammonium bromide (CTAB) in the presence of different nonpenetrating inorganic salts: potassium bromide (KBr), sodium nitrate (NaNO(3)), and sodium chlorate (NaClO(3)). We have varied the salt/surfactant ratio R at fixed surfactant concentration of 350 mM. Results are compared to data for the system cetylpyridinium chloride (CPyCl) and the penetrating counterion sodium salicylate (NaSal) (Oelschlaeger, C.; Schopferer, M.; Scheffold, F.; Willenbacher, N. Langmuir 2009, 25, 716-723). Mechanical high-frequency rheology and diffusing wave spectroscopy (DWS) based tracer microrheology are used to determine the shear moduli G' and G'' in the frequency range from 0.1 Hz up to 1 MHz (Willenbacher, N.; Oelschlaeger, C.; Schopferer, M.; Fischer, P.; Cardinaux, F.; Scheffold, F. Phys. Rev. Lett. 2007, 99, 068302, 1-4). This enables us to determine the plateau modulus G(0), which is related to the cross-link density or mesh size of the entanglement network, the bending stiffness kappa (also expressed as persistence length l(p) = kappa/k(B)T) corresponding to the semiflexible nature of the micelles, and the scission energy E(sciss), which is related to their contour length. The viscosity maximum shifts to higher R values, and the variation of viscosity with R is less pronounced as the binding strength decreases. The plateau modulus increases with R at low ionic strength and is constant around the viscosity maximum; the increase in G(0) at high R, which is presumably due to branching, is weak compared to the system with penetrating counterion. The scission energy E(sciss) approximately = 20 k(B)T is independent of counterion binding efficiency irrespective of R and is slightly higher compared to the system CPyCl/NaSal, indicating that branching may be significant already at the viscosity maximum in this latter case. The micellar flexibility increases with increasing binding efficiency of counterions according to the Hofmeister series. The persistence length values for systems CTAB/KBr, CTAB/NaNO(3), and CTAB/NaClO(3) are 40, 34, and 29 nm, respectively, independent of R, and are significantly higher than in the case of CPyCl/NaSal.

[Severe aseptic leucoencephalopathy. Manifested as immune reconstitution inflammatory syndrome in Caucasian and African patients]. / Schwere Leukenzephalopathie. Manifestationsform des aseptischen Immune-reconstitution-inflammatory-Syndroms bei Kaukasiern und Afrikanern.

BACKGROUND: We hypothesize that CNS immune reconstitution inflammatory syndrome (IRIS) after highly active antiretroviral therapy (HAART) in HIV-1-positive patients may become manifest without any opportunistic infection as an aseptic leucoencephalopathy. This opens a window of opportunity for successful treatment with corticosteroids. DESIGN: We describe a case series of immunocompromised HIV-1-positive patients who were started on HAART. All of them had clinical laboratory follow-up tests and cerebral MRI in order to investigate the course and the underlying pathophysiology of this aseptic form of IRIS. One African patient died and we performed a neuropathological examination. RESULTS: No infectious agent was detected before and during HAART. Three of four immunocompromised patients were successfully treated with corticosteroids while HAART was never interrupted and have survived up to now. One African patient died within 2 days despite intensive care due to cerebral oedema. CONCLUSIONS: Starting HAART, HIV-1-positive patients may develop an aseptic type of IRIS of the CNS without any detectable opportunistic infection, a finding that has not yet been published. This makes them susceptible for successful treatment with corticosteroids. Perhaps IRIS has a higher incidence in African patients and the patients have a poorer outcome than Caucasians.

Linear-to-branched micelles transition: a rheometry and diffusing wave spectroscopy (DWS) study.

The frequency-dependent shear modulus of aqueous wormlike micellar solutions of cetylpyridinium chloride (CPyCl) and sodium salicylate (NaSal) has been measured over a broad frequency range from 10(-2) to 10(6) rad/s using diffusing wave spectroscopy (DWS) based tracer microrheology as well as mechanical techniques including rotational rheometry and oscillatory squeeze flow. Good agreement between mechanical and optical techniques is found in the frequency range from 10(-1) to 10(5) rad/s (Willenbacher, N.; Oelschlaeger, C.; Schopferer, M.; Fischer, P.; Cardinaux, F.; Scheffold, F. Phys. Rev. Lett. 2007, 99 (6), 068302). At intermediate frequencies between 10 and 10(4) rad/s, squeeze flow provides most accurate data and is used to determine the plateau modulus G(0), which is related to the cross-link density or mesh size of the entanglement network, as well as the scission energy E(sciss), which is deduced from the temperature dependence of the shear moduli in the plateau zone. In the frequency range above 10(4) rad/s, DWS including a new inertia correction is most reliable and is used to determine the persistence length l(p). The system CPyCl/NaSal is known to exhibit two maxima in zero-shear viscosity and terminal relaxation time as the salt/surfactant ratio R is varied (Rehage, H.; Hoffman, H. J. Phys. Chem. 1988, 92 (16), 4712-4719). The first maximum is attributed to a transition from linear to branched micelles (Lequeux, F. Europhys. Lett. 1992, 19 (8), 675-681), and the second one is accompanied by a charge reversal due to strongly binding counterions. Here, we discuss the variation of G(0), E(sciss), and l(p) with salt/surfactant ratio R at constant surfactant concentration of 100 mM CPyCl. G(0) increases at the linear-to-branched micelles transition, and this is attributed to the additional contribution of branching points to the cross-link density. E(sciss) exhibits two maxima analogous to the zero-shear viscosity, which can be understood in terms of the variation of micellar length and variation of the amount of branched micelles and contour length between branching points consistent with the results of a comprehensive cryo-transmission electron microscopy (TEM) study (Abezgauz, L.; Ramon, O.; Danino, D. Department of Biotechnology and Food Engineering, Technion, Haifa, Israel. European Colloid and Interface Society, Geneva, 2007). The persistence length decreases with increasing R. This decrease is stronger than expected from the decrease of Debye length according to the Odijk-Skolnick-Fixman (OSF) theory and is attributed to the penetration of salicylate ions into the micelles; the linear-to-branched transition obviously does not have an effect on l(p).

Broad bandwidth optical and mechanical rheometry of wormlike micelle solutions.

We characterize the linear viscoelastic shear properties of an aqueous wormlike micellar solution using diffusing wave spectroscopy (DWS) based tracer microrheology as well as various mechanical techniques such as rotational rheometry, oscillatory squeeze flow, and torsional resonance oscillation covering the frequency range from 10(-1) to 10(6) rad/s. Since DWS as well as mechanical oscillatory squeeze flow and torsional resonance oscillation cover a sufficiently high frequency range, the persistence length of wormlike micelles could be determined directly from rheological measurements for the first time.

Structural and dynamical properties of ribbonlike self-assemblies of a fluorinated cationic surfactant.

The structural and dynamic properties of low ionic strength micellar solutions of the cationic surfactant perfluorooctylbutane trimethylammonium bromide have been investigated by cryo-TEM, small-angle neutron scattering, small-angle X-ray scattering, T-jump and rheological experiments. The surfactant molecules self-assemble into narrow ribbons with average dimensions on the order of 4 nm x 3 nm, either under salt-free conditions or in the presence of up to 30 mM KBr or NaF. Cryo-TEM also reveals in the salt-free systems the presence of networks of multiconnected micelles. Rheological experiments showed that these surfactant systems exhibit a strong shear-thickening effect even in the presence of up to 30 mM KBr. The T-jump response of the micellar solutions was found to be multiexponential. This observation rules out the presence of only linear micelles with an exponential length distribution and suggests more complex topologies of the micellar aggregates. The relaxation time associated with the predominant process in the T-jump relaxation is strongly correlated to the critical shear rate beyond which shear thickening occurs, thus indicating that this critical shear rate is controlled by the micellar kinetics.

Synergistic effects in mixed wormlike micelles of dimeric and single-chain cationic surfactants at high ionic strength.

The mixed micellization between the cationic gemini surfactant [C12H25(CH3)2N+(C2H4)N+(CH3)2C12H25*2Br-] and the cationic cetyltrimethylammonium bromide (CTAB) in 150 mM KBr solutions has been investigated. The variation of the cmc of the mixtures, measured by surface tension experiments, with composition revealed synergism in micelle formation. T-Jump and light scattering experiments performed in the vicinity of the crossover volume fraction showed the existence of two micellar populations, possibly linear and toroidal micelles. Rheological and dynamic light scattering experiments allowed to fully characterize the linear viscoelasticity of the mixtures. These measurements revealed synergistic gains in viscoelastic properties with a maximum of the stress-relaxation time around the equimolar composition. These effects are ascribed to a progressive intermicellar crosslinking resulting from a continuous increase of the end-cap energy with the 12-2-12 content in the mixture.