Configuration, Anion-Specific Effects, Diffusion, and Impact on Counterions for Adsorption of Salt Anions at the Interfaces of Clay Minerals

Interfacial interactions of clay minerals with salt solutions are ubiquitous and play a crucial role in a wide range of fields, where salt cations are the focus while anions are generally regarded as spectators. Here, molecular dynamics simulations show that the various anions are strongly adsorbed on the surfaces of clay minerals, and the resulting anion-specific effects are pronounced. Although constructing only H-bonds, anions form stable inner- and outer-sphere complexes with clay minerals, and F^– and OH^– can result in even more stable complexes than metal ions. The underlying anion-specific effects abide by the sequence OH^– > F^– > Cl^– > I^– and show apparent enhancements with increase of salt concentrations. OH^– is particular at relatively high concentrations, forming clusters and capturing metal ions at octahedral AlO₆ surfaces and approaching tetrahedral SiO₄ surfaces with help of metal ions in addition to the monodispersive inner- and outer-sphere species at octahedral AlO₆ surfaces that are similar for all anions. Diffusion coefficients of anions are the same order of magnitude as those of metal ions and are affected by counterions, concentrations, and distances to the surfaces of clay minerals. Diffusion coefficients of both inner- and outer-sphere anions decrease as I^– > Cl^– > F^– > OH^–. Adsorption of anions is affected by counterions (metal ions) and vice versa. Impact of anions on the adsorption of counterions also shows ion-specific effects that follow the sequence OH^– > F^– > Cl^– > I^–, and OH^– can even alter the adsorption structure and distribution of counterions

3D Differentiation of Neural Stem Cells in Macroporous Photopolymerizable Hydrogel Scaffolds

<div><p>Neural stem/progenitor cells (NSPCs) are the stem cell of the adult central nervous system (CNS). These cells are able to differentiate into the major cell types found in the CNS (neurons, oligodendrocytes, astrocytes), thus NSPCs are the mechanism by which the adult CNS could potentially regenerate after injury or disorder. Microenviromental factors are critical for guiding NSPC differentiation and are thus important for neural tissue engineering. In this study, D-mannitol crystals were mixed with photocrosslinkable methacrylamide chitosan (MAC) as a porogen to enhance pore size during hydrogel formation. D-mannitol was admixed to MAC at 5, 10 and 20 wt% D-mannitol per total initial hydrogel weight. D-mannitol crystals were observed to dissolve and leave the scaffold within 1 hr. Quantification of resulting average pore sizes showed that D-mannitol addition resulted in larger average pore size (5 wt%, 4060±160 µm², 10 wt%, 6330±1160 µm², 20 wt%, 7600±1550 µm²) compared with controls (0 wt%, 3150±220 µm²). Oxygen diffusion studies demonstrated that larger average pore area resulted in enhanced oxygen diffusion through scaffolds. Finally, the differentiation responses of NSPCs to phenotypic differentiation conditions were studied for neurons, astrocytes and oligodendrocytes in hydrogels of varied porosity over 14 d. Quantification of total cell numbers at day 7 and 14, showed that cell numbers decreased with increased porosity and over the length of the culture. At day 14 immunohistochemistry quantification for primary cell types demonstrated significant differentiation to the desired cells types, and that total percentages of each cell type was greatest when scaffolds were more porous. These results suggest that larger pore sizes in MAC hydrogels effectively promote NSPC 3D differentiation.</p> </div

Pore size analysis of MAC scaffolds.

<p>(A) Microscope images of acellular MAC scaffolds with varying mass percentages of D-mannitol. (B) Pore sizes of MAC scaffolds with varied D-mannitol percentages. Letters denote significance by single factor ANOVA with Tukey’s <i>post hoc</i> analysis (p<0.001). (C) SEM images of MAC scaffolds with varying mass percentages of D-mannitol. Freeze-dried scaffolds collapse during the process, so the pore sizes are not directly comparable to those shown in A and B. Mean ± SD with n = 3.</p

Total cell number at day 7 and 14 for porous scaffolds cultured in control (+EGF+FGF, -GF) and differentiation (INF-γ, PDGF-AA, BMP-2) media.

<p>NSPCs were initially seeded at 200 × 10³ cells/scaffold. *** denotes significance by two-factor ANOVA (p<0.0001). Mean ± SD with n = 3. All scaffolds were cultured for 1 d in expansion media (+EGF+FGF) then switched to conditions labeled in the caption.</p

Rheology and swelling results

<p>Letters denote significance by single factor ANOVA with Tukey’s <i>post hoc</i> analysis (p<0.01).</p

Figure 1

<p>(A) Methodology for creating 3D porous MAC scaffolds and procedure for NSPC culture and differentiation in 3D environments. (B) Images of a 20% D-mannitol scaffold captured immediately after crosslinking and after PBS dissolution for 1 hr at 37°C.</p

Fluorescence staining results for NSPC differentiation.

<p>Quantification of IHC at day 14 shows that more porous scaffolds (up to 20 wt% D-mannitol initially) in (A) neuron specific media (IFN-γ) favor neurons. (B) Oligodendrocyte specific media (PDGF-AA) favor oligodendrocytes and (C) in astrocyte specific media (BMP-2) favor astrocytes. (D) Control media with no growth factors (-GF) as well as with proliferation growth factors (+EGF+FGF) maintain nestin expression (note: error bars are included but too small to see). Letters denote significance by single factor ANOVA (p<0.001). Mean ± SD with n = 3.</p

Multiphoton confocal images of fluorescence staining for neurons, oligodendrocytes and astrocytes in 10 wt% scaffolds obtained at the center region of whole scaffolds.

<p>Corresponding zoomed regions (white rectangle) for each image are provided below. Nuclei appear blue by Hoechst 33342, cell staining for each differentiation marker appear red by Alexa-Fluor 546.</p

Oxygen diffusion data through 0, 5, 10, 20 wt% MAC scaffolds over 8 hrs.

<p>Mean ± SD with n = 3.</p

Oxygen diffusion in macroporous MAC scaffolds.

<p>Oxygen diffusion device allowing us to measure depletion of oxygen in the top chamber as diffusion occurs through the gel into the oxygen free bottom chamber.</p