Contact angles of surfactant solutions on heterogeneous surfaces.

Using Gibbs' adsorption equation and a literature isotherm, a new general model to predict the contact angle of surfactant solutions on (smooth or rough) chemically heterogeneous surfaces is constructed based on the Cassie equation. The model allows for adsorption at the liquid-vapor, solid-liquid, and solid-vapor interfaces. Solid-vapor adsorption is allowed in order to model the autophobic effect on hydrophilic surfaces. Using representative values for the coefficients which describe adsorption at each interface, model predictions for contact angles as a function of f parameters (area fractions) and surfactant concentration are made for heterogeneous surfaces made up of different materials. On smooth surfaces, the f parameters serve as weighting factors determining how to combine the effects of surfactant adsorption on each material to predict the behavior on the heterogeneous surface. Due to the non-linear nature of the model, the inclusion of a small amount of hydrophobic material has a greater effect on a predominantly hydrophilic material than vice versa, explaining the result seen in literature that a small amount of hydrophobic contamination (such as oil) significantly increases contact angle on a hydrophilic surface. The fact that even a small amount of heterogeneity can greatly change experimental results could lead to incorrect experimental conclusions about surfactant adsorption if a surface were wrongly assumed to be homogeneous. Model predictions rapidly become more complex as the number of differently wettable materials present on the surface increases. Also, an approximately equal weighting of different materials generally leads to more complex behaviors compared to heterogeneous surfaces composed largely of a single material. Rough heterogeneous surfaces follow previous results for surfactant wetting of rough homogeneous surfaces, leading to an amplification/attenuation of surfactant effects for penetrated/unpenetrated wetting, and further increasing the complexity of predictions. These potential complexities point to the importance of characterizing the heterogeneities of any surface under consideration. With proper characterization, the model described in this paper will allow for prediction of contact angles on all types of heterogeneous surfaces, and design of surfaces for specific interactions with surfactant solutions.

Dose-injury relationships for cryoprotective agent injury to human chondrocytes.

Vitrification of articular cartilage (AC) could enhance tissue availability but requires high concentrations of cyroprotective agents (CPAs). This study investigated relative injuries caused by commonly used CPAs. We hypothesized that the in situ chondrocyte dose-injury relationships of five commonly used CPAs are nonlinear and that relative injuries could be determined by comparing cell death after exposure at increasing concentrations. Human AC samples were used from four patients undergoing total knee arthroplasty surgery. Seventy µm slices were exposed in a stepwise protocol to increasing concentrations of 5 CPAs (max = 8 M); dimethyl sulfoxide (Me(2)SO), glycerol (Gly), propylene glycol (PG), ethylene glycol (EG), and formamide (FM). Chondrocyte viability was determined by membrane integrity stains. Statistical analysis included t-tests and nonlinear least squares estimation methods. The dose-injury to chondrocytes relationships for all CPAs were found to be nonlinear (sigmoidal best fit). For the particular loading protocol in this study, the data identified the following CPA concentrations at which chondrocyte recoveries statistically deviated significantly from the control recovery; 1 M for Gly, 4 M for FM and PG, 6 M for Me(2)SO, and 7 M for EG. Comparison of individual means demonstrated that Gly exposure resulted in the lowest recovery, followed by PG, and then Me(2)SO, FM and EG in no specific order. The information from this study provides an order of damage to human chondrocytes in situ of commonly used CPAs for vitrification of AC and identifies threshold CPA concentrations for a stepwise loading protocol at which chondrocyte recovery is significantly decreased. In general, Gly and PG were the most damaging while DMSO and EG were among the least damaging.

MR spectroscopy measurement of the diffusion of dimethyl sulfoxide in articular cartilage and comparison to theoretical predictions.

UNLABELLED: Cartilage cryopreservation requires optimal loading of protective solutes, most commonly dimethyl sulfoxide (DMSO), to maximize chondrocyte survival. Previously, diffusion models have been used to predict the distribution of solutes in tissue samples, but the accuracy of spatiotemporal predictions of these models have not been validated with empirical studies and remains unknown. OBJECTIVE: In this study, magnetic resonance spectroscopic imaging was used to measure the spatial and temporal changes in DMSO and water concentrations in porcine articular cartilage plugs, throughout 1 h of solute loading. DESIGN: A custom NMR spectroscopic imaging pulse sequence provided water and DMSO concentration images with an in-plane spatial resolution of 135 µm and a temporal resolution of 150 s, repeated for 60 min throughout DMSO loading. Delayed gadolinium-enhanced magnetic resonance of cartilage (d-GEMRIC) imaging provided fixed charge density and spin-density imaging provided water density images prior to DMSO loading. RESULTS: The measured spatial and temporal distribution of DMSO in three different samples was compared to independent predictions of Fick's law and the modified triphasic biomechanical model by Abazari et al. (2011) with the empirical data more closely agreeing with the triphasic model. CONCLUSION: Dynamic NMR spectroscopic imaging can measure spatial and temporal changes in water and cryoprotectant concentrations in articular cartilage. The modified triphasic model predictions for the interstitial distribution of DMSO were confirmed and its advantage over the predictions by Fick's law model, which is commonly used in the literature of cryobiology, was demonstrated.

Cryoprotective agent toxicity interactions in human articular chondrocytes.

BACKGROUND: Vitrification is a method of cryopreservation by which cells and tissues can be preserved at low temperatures using cryoprotective agents (CPAs) at high concentrations (typically ≥6.0 M) to limit the harmful effects of ice crystals that can form during cooling processes. However, at these concentrations CPAs are significantly cytotoxic and an understanding of their toxicity characteristics and interactions is important. Therefore, single-CPA and multiple-CPA solutions were evaluated for their direct and indirect toxicities on chondrocytes. METHODS: Chondrocytes were isolated from human articular cartilage samples and exposed to various single-CPA and multiple-CPA solutions of five common CPAs (dimethyl sulfoxide (DMSO), ethylene glycol (EG), propylene glycol (PG), glycerol (Gy) and formamide (Fm)) at both 6.0 and 8.1 M concentrations at 0 °C for 30 min. Chondrocyte survival was determined using a fluorescent cell membrane integrity assay. The data obtained was statistically analyzed and regression coefficients were used to represent the indirect toxicity effect which a specific combination of CPAs exerted on the final solution's toxicity. RESULTS: Multiple-CPA solutions were significantly less toxic than single-CPA solutions (P<0.01). The indirect toxicity effects between CPAs were quantifiable using regression analysis. Cell survival rates of approximately 40% were obtained with the four-CPA combination solution DMSO-EG-Gy-Fm. In the multiple-CPA combinations, PG demonstrated the greatest degree of toxicity and its presence within a combination solution negated any benefits of using multiple lower concentration CPAs. CONCLUSIONS: Multiple-CPA solutions are less cytotoxic than single-CPA solutions of the same total concentration. PG was the most toxic CPA when used in combinations. The highest chondrocyte survival rates were obtained with the 6.0 M DMSO-EG-Gy-Fm combination solution.

Particle trapping and banding in rapid colloidal solidification.

We derive an expression for the nonequilibrium segregation coefficient of colloidal particles near a moving solid-liquid interface. The resulting kinetic phase diagram has applications for the rapid solidification of clay soils, gels, and related colloidal systems. We use it to explain the formation of bandlike defects in rapidly solidified alumina suspensions.

Model and experimental studies for contact angles of surfactant solutions on rough and smooth hydrophobic surfaces.

Despite the practical need, no models exist to predict contact angles or wetting mode of surfactant solutions on rough hydrophobic or superhydrophobic surfaces. Using Gibbs' adsorption equation and a literature isotherm, a new model is constructed based on the Wenzel and Cassie equations. Experimental data for aqueous solutions of sodium dodecyl sulfate (SDS) contact angles on smooth Teflon surfaces are fit to estimate values for the adsorption coefficients in the model. Using these coefficients, model predictions for contact angles as a function of topological f (Cassie) and r (Wenzel) factors and SDS concentration are made for different intrinsic contact angles. The model is also used to design/tune surface responses. It is found that: (1) predictions compare favorably to data for SDS solutions on five superhydrophobic surfaces. Further, the model predictions can determine which wetting mode (Wenzel or Cassie) occurred in each experiment. The unpenetrated or partially penetrated Cassie mode was the most common, suggesting that surfactants inhibit the penetration of liquids into rough hydrophobic surfaces. (2) The Wenzel roughness factor, r, amplifies the effect of surfactant adsorption, leading to larger changes in contact angles and promoting total wetting. (3) The Cassie solid area fraction, f, attenuates the lowering of contact angles on rough surfaces. (4) The amplification/attenuation is understood to be due to increased/decreased solid-liquid contact-area.

Investigating cryoinjury using simulations and experiments. 1: TF-1 cells during two-step freezing (rapid cooling interrupted with a hold time).

There is significant interest in designing a cryopreservation protocol for hematopoietic stem cells (HSC) which does not rely on dimethyl sulfoxide (Me(2)SO) as a cryoprotectant. Computer simulations that describe cellular osmotic responses during cooling and warming can be used to optimize the viability of cryopreserved HSC; however, a better understanding of cellular osmotic parameters is required for these simulations. As a model for HSC, the erythroleukemic human cell line TF-1 was used in this study. Simulations, based on the osmotic properties of TF-1 cells and on the solution properties of the intra- and extracellular compartments, were used to interpret cryoinjury associated with a two-step cryopreservation protocol. Calculated intracellular supercooling was used as an indicator of cryoinjury related to intracellular ice formation. Simulations were applied to the two-step cooling protocol (rapid cooling interrupted with a hold time) for TF-1 cells in the absence of Me(2)SO or other cryoprotectants and optimized by minimizing the indicator of cryoinjury. A comparison of simulations and experimental measurements of membrane integrity supports the concept that, for two-step cooling, increasing intracellular supercooling is the primary contributor to potential freezing injury due to the increase in the likelihood of intracellular ice formation. By calculating intracellular supercooling for each step separately and comparing these calculations with cell recovery data, it was demonstrated that it is not optimal simply to limit overall supercooling during two-step freezing procedures. More aptly, appropriate limitations of supercooling differ from the first step to the second step. This study also demonstrates why high cell recovery after cryopreservation could be achieved in the absence of traditional cryoprotectants.

Investigating cryoinjury using simulations and experiments: 2. TF-1 cells during graded freezing (interrupted slow cooling without hold time).

Cryopreservation plays a key role in the long-term storage of native and engineered cells and tissues for research and clinical applications. The survival of cells and tissues after freezing and thawing depends on the ability of the cells to withstand a variety of stresses imposed by the cryopreservation protocol. A better understanding of the nature and kinetics of cellular responses to temperature-induced conditions is required to minimize cryoinjury. An interrupted freezing procedure that allows dissection of cryoinjury was used to investigate the progressive damage that occurs to cells during cryopreservation using slow cooling. Simulations of cellular osmotic responses were used to provide interpretation linking states of the cell with events during the freezing procedure. Simulations of graded freezing (interrupted slow cooling without hold time) were correlated with cell recovery results of TF-1 cells. Calculated intracellular supercooling and osmolality, were used as indicators of the probability of cryoinjury due to intracellular ice formation and solution effects, providing direct links of cellular conditions to events in the freezing process. Using simulations, this study demonstrated that both intracellular supercooling and osmolality are necessary to explain graded freezing results.

A multisolute osmotic virial equation for solutions of interest in biology.

The osmotic virial equation was used to predict osmolalities of solutions of interest in biology. The second osmotic virial coefficients, Bi, account for the interactions between identical solute molecules. For multisolute solutions, the second osmotic virial cross coefficient, Bij, describes the interaction between two different solutes. We propose to use as a mixing rule for the cross coefficient the arithmetic average of the second osmotic virial coefficients of the pure species, so that only binary solution measurements are required for multisolute solution predictions. Single-solute data were fit to obtain the osmotic virial coefficients of the pure species. Using those coefficients with the proposed mixing rule, predictions were made of ternary solution osmolality, without any fitting parameters. This method is shown to make reasonably accurate predictions for three very different ternary aqueous solutions: (i) glycerol + dimethyl sulfoxide + water, (ii) hemoglobin + an ideal, dilute solute + water, and (iii) bovine serum albumin + ovalbumin + water.

The effect of cell size distribution on predicted osmotic responses of cells.

An understanding of the kinetics of the osmotic response of cells is important in understanding permeability properties of cell membranes and predicting cell responses during exposure to anisotonic conditions. Traditionally, a mathematical model of cell osmotic response is obtained by applying mass transport and Boyle-vant Hoff equations using numerical methods. In the usual application of these equations, it is assumed that all cells are the same size equal to the mean or mode of the population. However, biological cells (even if they had identical membranes and hence identical permeability characteristics--which they do not) have a distribution in cell size and will therefore shrink or swell at different rates when exposed to anisotonic conditions. A population of cells may therefore exhibit a different average osmotic response than that of a single cell. In this study, a mathematical model using mass transport and Boyle-van't Hoff equations was applied to measured size distributions of cells. Chinese hamster fibroblast cells (V-79W) and Madin-Darby canine kidney cells (MDCK), were placed in hypertonic solutions and the kinetics of cell shrinkage were monitored. Consistent with the theoretical predictions, the size distributions of these cells were found to change over time, therefore the selection of the measure of central tendency for the population may affect the calculated osmotic parameters. After examining three different average volumes (mean, median, and mode) using four different theoretical cell size distributions, it was determined that, for the assumptions used in this study, the mean or median were the best measures of central tendency to describe osmotic volume changes in cell suspensions.

Dynamic surface tensions of Athabasca bitumen vacuum residue including the effect of dissolved air.

The pendant drop technique was used to measure the equilibrium and dynamic surface tensions of Athabasca bitumen vacuum residue (500 degrees C+) (AVR) between 150 and 280 degrees C. A significant (16%) slow (over hours) decrease from initial to equilibrium values was found. In addition, the effect of dissolved air on surface tension was studied at 150 degrees C by comparing dynamic surface tensions of air- or nitrogen-saturated AVR in contact with air or nitrogen. It was found that the presence of dissolved air significantly decreases the dynamic change in surface tension (from 16 to 5%). In order to perform the surface tension studies, the density of AVR was required. Archimedes method was used to measure the density of AVR from 98 to 335.8 degrees C.

Cryoprotectant equilibration in tissues.

The first step in the cryopreservation of cells or tissues is often the movement of a permeating cryoprotectant into the cells or tissues from the solution into which they have been placed. The cryoprotectant enters the cells or tissues by thermodynamic equilibration with the surroundings. In the reverse case, thermodynamic equilibration also drives the removal of permeating cryoprotectants by a dilution solution at the end of the preservation process when the cells or tissues are being readied for use. There have been reports of tissues having equilibrium cryoprotectant concentrations lower than that of the surrounding carrier solution. For various tissues, the equilibrium concentration of cryoprotectant inside the tissue is either equal to, or lower than the cryoprotectant concentration of the surrounding solution. A simple thermodynamic treatment of the solution-tissue equilibrium shows that an equilibrium concentration difference can exist between a tissue and the surrounding solution if a pressure difference can be maintained.

Investigation of the dependence of inferred interfacial tension on rotation rate in a spinning drop tensiometer.

The spinning drop tensiometer is widely used to study the interfacial properties of many systems. However, there have also been several reported limitations with the spinning drop tensiometer. In this paper, it is shown that there is a relationship between the measured interfacial tension and the rotation rate of the drop. A detailed investigation of this relationship is presented.

The effect of temperature on membrane hydraulic conductivity.

The objective of this study was to use the temperature dependence of water permeability to suggest the physical mechanisms of water transport across membranes of osmotically slowly responding cells and to demonstrate that insight into water transport mechanisms in these cells may be gained from easily performed experiments using an electronic particle counter. Osmotic responses of V-79W Chinese hamster fibroblast cells were measured in hypertonic solutions at various temperatures and the membrane hydraulic conductivity was determined. The results were fit with the general Arrhenius equation with two free parameters, and also fit with two specific membrane models each having only one free parameter. Data from the literature including that for human bone marrow stem cells, hamster pancreatic islets, and bovine articular cartilage chondrocytes were also examined. The results indicated that the membrane models could be used in conjunction with measured permeability data at different temperatures to investigate the method of water movement across various cell membranes. This approach for slower responding cells challenges the current concept that the presence of aqueous pores is always accompanied by an osmotic water permeability value, P(f)>0.01 cm/s. The possibility of water transport through aqueous pores in lower-permeability cells is proposed.