Donnan Contribution and Specific Ion Effects in Swelling of Cationic Hydrogels are Additive: Combined High-Resolution Experiments and Finite Element Modeling.

Finite element modeling applied to analyze experimentally determined hydrogel swelling data provides quantitative description of the hydrogel in the aqueous solutions with well-defined ionic content and environmental parameters. In the present study, we expand this strategy to analysis of swelling of hydrogels over an extended concentration of salt where the Donnan contribution and specific ion effects are dominating at different regimes. Dynamics and equilibrium swelling were determined for acrylamide and cationic acrylamide-based hydrogels by high-resolution interferometry technique for step-wise increase in NaCl and NaBr concentration up to 2 M. Although increased hydrogel swelling volume with increasing salt concentration was the dominant trend for the uncharged hydrogel, the weakly charged cationic hydrogel was observed to shrink for increasing salt concentration up to 0.1 M, followed by swelling at higher salt concentrations. The initial shrinking is due to the ionic equilibration accounted for by a Donnan term. Comparison of the swelling responses at high NaCl and NaBr concentrations between the uncharged and the cationic hydrogel showed similar specific ion effects. This indicates that the ion non-specific Donnan contribution and specific ion effects are additive in the case where they are occurring in well separated ranges of salt concentration. We develop a novel finite element model including both these mechanisms to account for the observed swelling in aqueous salt solution. In particular, a salt-specific, concentration-dependent Flory-Huggins parameter was introduced for the specific ion effects. This is the first report on finite element modeling of hydrogels including specific ionic effects and underpins improvement of the mechanistic insight of hydrogel swelling that can be used to predict its response to environmental change.

Data on residual stresses of mooring chains measured by neutron diffraction and hole drilling techniques.

Residual stresses in large offshore mooring chains have been measured for the first time and presented in this article. Two chain links with the same size and material, one only subjected to proof load and no cyclic service loads and the other exposed to service loads as well as the proof load, were selected for the experiment. Residual stresses just below the surface were measured using the hole-drilling technique and the neutron diffraction technique was employed for deeper measurements. The data can be used to investigate residual stress redistribution in the chain links because of material removal due to corrosion and cyclic service loads that the chains are exposed to during their service time. Moreover, the data can be used to validate numerical models for predicting residual stresses. A more detailed interpretation of the data presented in this article is provided in "Experimental and numerical study of mooring chain residual stresses and implications for fatigue life" [1].

MicroCT-based finite element models as a tool for virtual testing of cortical bone.

The aim of this study was to assess a virtual biomechanics testing approach purely based on microcomputed tomography (microCT or µCT) data, providing non-invasive methods for determining the stiffness and strength of cortical bone. Mouse femurs were µCT scanned prior to three-point-bend tests. Then microCT-based finite element models were generated with spatial variation in bone elastoplastic properties and subject-specific femur geometries. Empirical relationships of density versus Young's moduli and yield stress were used in assigning elastoplastic properties to each voxel. The microCT-based finite element modeling (µFEM) results were employed to investigate the model's accuracy through comparison with experimental tests. The correspondence of elastic stiffness and strength from the µFE analyses and tests was good. The interpretation of the derived data showed a 6.1%, 1.4%, 1.5%, and 1.6% difference between the experimental test result and µFEM output on global stiffness, nominal Young's modulus, nominal yield stress, and yield force, respectively. We conclude that virtual testing outputs could be used to predict global elastic-plastic properties and may reduce the cost, time, and number of test specimens in performing physical experiments.

Skeletal effects of a gastrin receptor antagonist in H+/K+ATPase beta subunit KO mice.

Epidemiological studies suggest an increased fracture risk in patients taking proton pump inhibitors (PPIs) for long term. The underlying mechanism, however, has been disputed. By binding to the gastric proton pump, PPIs inhibit gastric acid secretion. We have previously shown that proton pump (H(+)/K(+)ATPase beta subunit) KO mice exhibit reduced bone mineral density (BMD) and inferior bone strength compared with WT mice. Patients using PPIs as well as these KO mice exhibit gastric hypoacidity, and subsequently increased serum concentrations of the hormone gastrin. In this study, we wanted to examine whether inhibition of the gastrin/CCK2 receptor influences bone quality in these mice. KO and WT mice were given either the gastrin/CCK2 receptor antagonist netazepide dissolved in polyethylene glycol (PEG) or only PEG for 1year. We found significantly lower bone mineral content and BMD, as well as inferior bone microarchitecture in KO mice compared with WT. Biomechanical properties by three-point bending test also proved inferior in KO mice. KO mice receiving netazepide exhibited significantly higher cortical thickness, cortical area fraction, trabecular thickness and trabecular BMD by micro-CT compared with the control group. Three-point bending test also showed higher Young's modulus of elasticity in the netazepide KO group compared with control mice. In conclusion, we observed that the gastrin receptor antagonist netazepide slightly improved bone quality in this mouse model, suggesting that hypergastrinemia may contribute to deteriorated bone quality during acid inhibition.

Effects of the Histamine 1 Receptor Antagonist Cetirizine on the Osteoporotic Phenotype in H(+) /K(+) ATPase Beta Subunit KO Mice.

Epidemiological studies suggest increased fracture risk in patients using proton pump inhibitors (PPIs). We have previously shown that the H(+) /K(+) ATPase beta subunit knockout (KO) mouse, which is a model of PPI-use, have lower bone mineral density (BMD) and impaired bone quality compared to wild type (WT) mice. Like PPI users, these KO mice display elevated gastric pH and hypergastrinemia, which in turn stimulates gastric histamine release. Previous studies have suggested a negative effect of histamine on bone, thus, we wanted to study whether a histamine 1 receptor (H1R) antagonist could improve bone quality in KO mice. Female KO and WT mice aged 8 weeks received either an H1R antagonist (cetirizine) or polyethylene glycol (PEG) for 6 months. At the end of the study, KO mice displayed elevated plasma histamine levels compared to WT. As demonstrated previously, the KO mice also exhibited lower whole body BMD, reduced mechanical bone strength, and impaired bone quality assessed by µCT. No significant differences, however, were found between the KO groups receiving cetirizine or PEG for any of the measured bone parameters. In vitro gene expression analyses of histamine receptors revealed the presence of H1R and H2R both in osteoblasts and osteoclasts, and H3R in late stage osteoblasts. In conclusion, administration of the H1R antagonist cetirizine in a concentration of 3 mg/kg did not rescue the osteoporotic phenotype in H(+) /K(+) ATPase beta subunit KO mice. It can, however, not be ruled out that histamine may influence bone via other receptors. J. Cell. Biochem. 117: 2089-2096, 2016. © 2016 Wiley Periodicals, Inc.

Mechanical instability of monocrystalline and polycrystalline methane hydrates.

Despite observations of massive methane release and geohazards associated with gas hydrate instability in nature, as well as ductile flow accompanying hydrate dissociation in artificial polycrystalline methane hydrates in the laboratory, the destabilising mechanisms of gas hydrates under deformation and their grain-boundary structures have not yet been elucidated at the molecular level. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected crossover from strengthening to weakening in polycrystals. Upon uniaxial depressurisation, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilises the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insight not only into the metastable structures of grain boundaries, but also into unusual ductile flow with hydrate dissociation as observed during macroscopic compression experiments.