High-frequency ultrasound scattering from microspheres and single cells.

Assessing the proportion of biological cells in a volume of interest undergoing structural changes, such as cell death, using high-frequency ultrasound (20-100 MHz), requires the development of a theoretical model of scattering by any arbitrary cell ensemble. A prerequisite to building such a model is to know the scattering by a single cell in different states. In this paper, a simple model for the high-frequency acoustic scattering by one cell is proposed. A method for deducing the backscatter transfer function from a single, subresolution scatterer is also devised. Using this method, experimental measurements of backscatter from homogeneous, subresolution polystyrene microspheres and single, viable eukaryotic cells, acquired across a broad, continuous range of frequencies were compared with elastic scattering theory and the proposed cell scattering model, respectively. The resonant features observed in the backscatter transfer function of microspheres were found to correspond accurately to theoretical predictions. Using the spacing of the major spectral peaks in the transfer functions obtained experimentally, it is possible to predict microsphere diameters with less than 4% error. Such good agreement was not seen between the cell model and the measured backscatter from cells. Possible reasons for this discrepancy are discussed.

"Active" and "passive" learning of three-dimensional object structure within an immersive virtual reality environment.

We used a fully immersive virtual reality environment to study whether actively interacting with objects would effect subsequent recognition, when compared with passively observing the same objects. We found that when participants learned object structure by actively rotating the objects, the objects were recognized faster during a subsequent recognition task than when object structure was learned through passive observation. We also found that participants focused their study time during active exploration on a limited number of object views, while ignoring other views. Overall, our results suggest that allowing active exploration of an object during initial learning can facilitate recognition of that object, perhaps owing to the control that the participant has over the object views upon which they can focus. The virtual reality environment is ideal for studying such processes, allowing realistic interaction with objects while maintaining experimenter control.

Enhancing penicillin fermentations by increased oxygen solubility through the addition of n-hexadecane.

n-Hexadecane was added to fermentation media to increase the medium oxygen solubilities, thus enhancing oxygen transfer rates in penicillin fermentations. For shake flask fermentations, cells were found to grow faster in the flasks with n-hexadecane than those without. The addition of n-hexadecane to penicillin fermentations was shown to significantly increase cell growth and penicillin production and reduce formation of mycelial pellets. The result was attributed to the enhancement of oxygen transfer in mycelial fermentations due to the higher oxygen solubilities of fermentation media achieved by adding n-hexadecane.

Simultaneous measurements of oxygen diffusion coefficients and solubilities in fermentation media with polarographic oxygen electrodes.

A membrane-covered oxygen electrode was used to measure oxygen diffusion coefficients and solubilities in aqueous glucose solutions and various fermentation media following a newly developed methodology. The fermentation media studied were tryptic soy broth and those for fermentations of Penicillium chrysogenum, Saccharomyces cerevisiae, and Micrococcus glutamicus. The experimental results of oxygen diffusion coefficients and solubilities in glucose solutions were in good accord with the literature data. As for the fermentation media, both oxygen diffusion coefficients and solubilities were found to decrease with an increased fractional composition of these media, and log-additive behaviors of the oxygen diffusion coefficients and solubilities in fermentation media were observed.

The anomaly of oxygen diffusion in aqueous xanthan solutions.

A membrane-covered polarographic oxygen electrode was used to measure oxygen diffusion coefficients in aqueous polyelectrolyte solutions of xanthan gum, sodium alginate, and sodium carboxymethylcellulose (CMC). In sodium alginate solutions, dilute xanthan solutions, and solutions containing more than 0.3 wt % CMC, oxygen diffusion coefficients decrease with increasing polymer concentrations. Interestingly, in dilute CMC solutions and concentrate xanthan solutions containing more than 0.5 wt % xanthan gum, oxygen diffusion coefficients increase with increasing polymer concentrations, and values exceeding that in pure water are generally observed.

Effective diffusivity of oxygen in microbial pellets.

In a typical submerged aerobic fermentation with microbial pellets, the effective diffusivity of oxygen in the pellets is probably the most important, yet most difficult transport property to characterize experimentally. Its values directly indicate the efficiency or deficiency of oxygen to individual cells, and thus the biological activity of the microorganisms. In the past, it was not possible to assess reliably the effective diffusivity of oxygen in pellets due to several reasons. Firstly, most oxygen electrodes available were coarse, and hence not suitable for in situ measurements. Secondly, there was a lack of methods rigorous enough to characterize the structure of the microbial pellets. A state-of-the-art review of the literature relating to the feature subject is presented. Emphasis is laid upon development and evolution of the means for quantitative characterization of the effective diffusivity of oxygen in microbial pellets.