Anisotropic diffusion of concentrated hard-sphere colloids near a hard wall studied by evanescent wave dynamic light scattering.

Evanescent wave dynamic light scattering and Stokesian dynamics simulations were employed to study the dynamics of hard-sphere colloidal particles near a hard wall in concentrated suspensions. The evanescent wave averaged short-time diffusion coefficients were determined from experimental correlation functions over a range of scattering wave vectors and penetration depths. Stokesian dynamics simulations performed for similar conditions allow a direct comparison of both the short-time self- and collective diffusivity. As seen earlier [V. N. Michailidou, G. Petekidis, J. W. Swan, and J. F. Brady, Phys. Rev. Lett. 102, 068302 (2009)] while the near wall dynamics in the dilute regime slow down compared to the free bulk diffusion, the reduction is negligible at higher volume fractions due to an interplay between the particle-wall and particle-particle hydrodynamic interactions. Here, we provide a comprehensive comparison between experiments and simulations and discuss the interplay of particle-wall and particle-particle hydrodynamics in the self- and cooperative dynamics determined at different scattering wave vectors and penetration depths.

A semi-automated approach towards generating three-dimensional mesh of the heart using a hybrid MRI/histology database.

Both the forward and inverse problems of electrocardiography rely on the precise modelling of the anatomic and electrical properties of the thoracic tissues. This, in turn, requires good knowledge of the electrical anisotropy as well as conductivity inhomogeneity of the heart, lungs and the rest of the thorax. Cardiac electrical anisotropy is related to its microstructure (fibre length, density and orientation). We hereby present detailed three-dimensional (3D) meshes of the thorax and heart, using image data from contiguous 2D magnetic resonance (MR) imaging slices as well as a realistic 3D cardiac fibre orientation model that derives its data from high-resolution ex vivo human heart MR images and from histology specimens of heart tissue. Using specific software, we integrated the 3D thorax and heart meshes in one that addresses the related modelling requirements for the solution of the forward and inverse problems of electrocardiography.

One heart, two bodies: a simulation study of body surface potential differences between donor and recipient of heart transplantation.

In cardiac transplantation has been recognized some "abnormalities" in recipient ECG. We investigated the influence of heart geometrical position within the chest cavity as well as somatometric parameters on body surface torso potentials. Two control patients with different Body Mass Index (BMI) were undergone a chest MRI scan. Using specific software we created two tetrahedral meshes that could be applied in our study. A post-mortem human heart was undergone a MRI scan and we also created its tetrahedral mesh. Using second software we extracted the heart mesh of control's torsos and we replaced them with the mesh of the post-mortem heart. The last program also assessed the influence of heart (re)positioning within the thorax, on the body surface potentials. The Finite Elements Method (FEM) was used to solve the forward electrocardiographic problem for both torsos, under the assumption that all the ventricular myocardium of the one post-mortem heart was excited. FEM was also applied in simulating Body Surface Potential Mapping (BSPM) on the first thorax torso for nine different heart positions. For BSPM, FEM has been applied on Poison equation. The results show higher BSPM in patient with lower BMI and significant changes in BSPM when heart was rotated round its long axis. Conversely, the heart shifts (long x- or y- axis) didn't cause significant changes on simulated BSPM.

Dynamics of concentrated hard-sphere colloids near a wall.

We investigate the Brownian motion of hard-sphere colloids near a solid wall by Evanescent Wave Dynamic Light Scattering (EWDLS). We carried out measurements for various volume fractions of sterically stabilized poly(methyl methacrylate) (PMMA) particles over a range of scattering wave vectors, q. While in the dilute regime, the near wall short-time diffusion is significantly slowed down due to particle-wall hydrodynamic interactions (HI); as volume fraction increases, the wall effect is progressively diminished at all q's. We present a new analysis for the EWDLS short-time self- and collective diffusivities applicable to all volume fractions and a simple model for the self-diffusion describing the interplay between particle-wall and particle-particle HI. Moreover, a weaker decay of the near-wall self-diffusion coefficient with volume fraction is predicted by Stokesian dynamics simulations.

Observation of slow down of polystyrene nanogels diffusivities in contact with swollen polystyrene brushes.

The diffusion of dilute colloids in contact with swollen polymer brushes has been studied by evanescent wave dynamic light scattering. Two polystyrene nanogels with 16 nm and 42 nm radius were put into contact with three polystyrene brushes with varying grafting densities. Partial penetration of the nanogels within the brushes was revealed by the evanescent wave penetration depth-dependent scattering intensities. The experimental short-time diffusion coefficients of the penetrating particles were measured and found to strongly slow down as the nanoparticles get deeper into the brushes. The slow down is much more marked for the smaller (16 nm) nanogels, suggesting a size exclusion type of mechanism and the existence of a characteristic length scale present in the outer part of the brush.

Brownian diffusion close to a polymer brush.

In an effort to control particle diffusion near surfaces, we have studied the dynamics of colloidal hard spheres and soft compliant star copolymers on surfaces coated with polymer brushes using evanescent wave dynamic light scattering. The same experiments provide information on the brush structure and confined particle motion. The penetration into dense polydisperse brushes is size- and solvent-dependent.

Synthesis and characterization of novel pH-responsive microgels based on tertiary amine methacrylates.

Emulsion polymerization of 2-(diethylamino)ethyl methacrylate (DEA) in the presence of a bifunctional cross-linker at pH 8-9 afforded novel pH-responsive microgels of 250-700 nm diameter. Both batch and semicontinuous syntheses were explored using thermal and redox initiators. Various strategies were evaluated for achieving colloidal stability, including charge stabilization, surfactant stabilization, and steric stabilization. The latter proved to be the most convenient and effective, and three types of well-defined reactive macromonomers were examined, namely, monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA), styrene-capped poly[2-(dimethylamino)ethyl methacrylate] (PDMA50-St), and partially quaternized styrene-capped poly[2-(dimethylamino)ethyl methacrylate] (10qPDMA50-St). The resulting microgels were pH-responsive, as expected. Dynamic light scattering and 1H NMR studies confirmed that reversible swelling occurred at low pH due to protonation of the tertiary amine groups on the DEA residues. The critical pH for this latex-to-microgel transition was around pH 6.5-7.0, which corresponds approximately to the known pKa of 7.0-7.3 for linear PDEA homopolymer. The microgel particles were further characterized by electron microscopy and aqueous electrophoresis studies. Their swelling and deswelling kinetics were investigated by turbidimetry. The PDEA-based microgels were compared to poly[2-(diisopropylamino)ethyl methacrylate] (PDPA) microgels prepared with identical macromonomer stabilizers. These PDPA-based microgels had a lower critical swelling pH of around pH 5.0-5.5, which correlates with the lower pKa of PDPA homopolymer. In addition, the kinetics of swelling for the PDPA microgels was somewhat slower than that observed for PDEA microgels; presumably this is related to the greater hydrophobic character of the former particles.

Ovarian response to hCG injections during the prepubertal period in three breeds of sheep with different ovulation rates and litter sizes.

A positive correlation was reported between ovulation (OR) rate in response to hCG in prepubertal ewe lambs and adult ewes. The finding suggested that the response to hCG may be used to select prepubertal ewe lambs for prolificacy. We studied plasma FSH concentrations and ovarian response to hCG injection during the prepubertal period in breeds with high (Chios), medium (Florina) and low (Karagouniki) OR and litter sizes (LS). The results showed that Chios ewe lambs presented a higher profile of plasma FSH during the 3rd to 7th wk of age when compared to the breeds with medium (Florina) and low (Karagouniki) OR and LS. The ovarian response to hCG showed that the Chios ewe lambs responded earlier, by exhibiting ovulation at 6 wks and more effectively, by demonstrating a higher OR than the other two breeds. There was no correlation among plasma FSH concentrations during prepubertal age (3rd to 7th wk), ovulation rate after hCG injection, or litter size at first and second lambing for all breeds. Therefore we concluded that the plasma FSH concentrations and/or the ovarian response to hCG injection during the prepubertal period is not a suitable criterion for early selection for litter size in the adult ewe of these breeds.

The combined action of exercise and methylprednisolone sodium succinate on the rat gastrocnemius muscle.

1. Methylprednisolone sodium succinate (SM) administration reduces the amount of contractile proteins and increases the collagen in the rat gastrocnemius muscle; it also increases the muscle's isotonic contraction capacity. 2. Exercise in combination with SM treatment reduces this capacity and causes also further reductions of the contractile proteins and increases of collagen. 3. There are no changes of either the relative muscle weight or the number of acetylcholine receptors in any experimental group as compared to controls.