Abrasion in pyroclastic flows

The relative size of glass rims coating crystals in the matrix ash provides a semi-quantitative measure of abrasion of ash grains in pyroclastic flows. Median abrasion indices (= areacrystal / areaglass rim) are 8.4 to 18.5 in Laacher See pyroclastic flow units but only 4 to 6.3 in assocciated fallout, showing stronger abrasion of ash particles in the pyroclastic flows. All pyroclasts undergo strong attrition in the vent but clasts in pyroclastic flows undergo a second major phase of abrasion during high-energy near-vent flow. Abrasion of ash particles is weaker during lower-energy higher-strength motion further downstream, suggesting that high contents of fine ash in distal deposits are due to diminishing elutriation rather than high rate of attrition

Preparation of small unilamellar vesicles (SUV) and biophysical characterization of their complexes with poly-L-lysine-condensed plasmid DNA

Liposomes have numerous applications in the (bio)pharmaceutical industries as agents in the synthesis of new biomaterials for use in areas including gene delivery. There is currently a need to establish efficient scaleable methods for the manufacture of liposomes, and in the present paper we describe the operation of a new high-velocity jet homogenizer for downsizing of multilamellar large vesicles to produce small unilamellar vesicles (SUV). Measurements of size distribution of SUVs are presented and compared with mathematical simulations based on the solution of a population balance equation combined with computational-fluid-dynamics analysis of flow in the homogenizer. Anionic SUVs are produced by the new method and incubated with poly-L-lysine (PLL)-condensed plasmid DNA (pDNA) to generate complexes under different physico-chemical conditions. The colloidal properties of the resulting complexes, including their size and charge, are measured using a Zetasizer and the encapsulation efficiency is obtained experimentally using a Pico Green assay. The results show that between 85 and 95% of the PLL-pDNA condensed plasmids were encapsulated by the liposomes, the smaller liposomes being more effective in encapsulating the complexes.Maguire, LA; Zhang, H and Shamlou, PA

Design of a prototype miniature bioreactor for high throughput automated bioprocessing

A new miniature bioreactor with a diameter equal to that of a single well of a 24-well plate is described and its engineering performance as a fermenter assessed. Mixing in the miniature bioreactor is provided by a set of three impellers mechanically driven via a microfabricated electric motor and aeration is achieved with a single tube sparger. Parameter sensitive fluorophors are used with fibre optic probes for continuous monitoring of dissolved oxygen tension and an optical based method is employed to monitor cell biomass concentration during fermentation. Experimental measurements are provided on volumetric mass transfer coefficient for air–water and bacterial fermentation data are presented for Escherichia coli. The local and average power input, energy dissipation rate and bubble size are derived from an analysis of the multiphase flow in the miniature bioreactor using computational fluid dynamics (CFD). Volumetric mass transfer coefficients are predicted using Higbie's penetration model with the contact time obtained from the CFD simulations of the turbulent flow in the bioreactor. Comparative data are provided from parallel experiments carried out in a 20 l (15 l working volume) conventional fermenter. Predicted and measured volumetric mass transfer coefficients in the miniature bioreactor are in the range 100–400 h−1, typical of those reported for large-scale fermentation.S. R. Lamping, H. Zhang, B. Allen and P. Ayazi Shamlouhttp://www.elsevier.com/wps/find/journaldescription.cws_home/215/description#descriptio

Computational-fluid-dynamics (CFD) analysis of mixing and gas liquid mass transfer in shake flasks

CFD (computational fluid dynamics) techniques were used to predict mixing and gas-liquid mass transfer in a 250 ml shake flask operating over a range of shaking frequencies between 100 and 300 rev./min, shaking diameters between 20 and 60 mm, and fill volumes between 25 and 100 ml. Interfacial area, a, volumetric mass-transfer coeffcient, kLa, and the power input per unit volume, epsilonv, of the liquid were predicted to be 300<a<800 m2 . m(-3), 10<kLa<100 h(-1) and 40<epsilonv<600 W . m(-3) respectively. These values are significantly different from the reported range for laboratory and pilot-scale bioreactors used in the fermentation of bacterial and fungal micro-organisms (100<a<300 m2 . m(-3), 100<kLa<400 h(-1) and 1000<epsilonv<3000 W . m(-3)). Our analysis showed that, at the highest shaking frequency and amplitude of operation, the specific power input in the shake flask was much lower than in laboratory bioreactors. Bacterial and fungal micro-organisms require dissolved oxygen concentrations typically in the range 50-250 mmol of O2 . h(-1) . litre(-1), corresponding to volumetric mass-transfer coefficients, kLa, in the range of 250-400 h(-1). Poor mixing and dissolved-oxygen limitation in shake flasks may limit their use in process design and media optimization in fermentation. In contrast, mammalian cells have relatively low demand for oxygen and consequently require a lower specific power input, this being typically between 1 and 10 W . m(-3), allowing efficient operation in shake flasks. Experimental data presented as part of the present study showed that mammalian cell growth in shake flasks was essentially independent of the specific power input, the maximum specific cell growth rate being 0.056 h(-1). The corresponding maximum oxygen-uptake rate was 0.74 mmol of O2 . h(-1) . litre(-1) for a viable cell count of 1.3 x 10(6) cells . ml(-1). These values are comparable with reported values for laboratory and pilotscale bioreactors. This analysis suggests that growth of mammalian cells in shake flasks (and hence in laboratory bioreactors) is not limited by the gas-liquid mass-transfer rate. In mammalian cell cultures, the requirement for good mixing is driven by other considerations, including the need for good cell suspension and reduction in heterogeneity, for example, in pH, temperature, nutrient concentration, osmolality and lactate/glucose ratio.Zhang, H; Williams-Dalson, W; Keshavarz-Moore, E. and Shamlou, PA.http://www.ncbi.nlm.nih.gov/pubmed/1531028

Engineering characterisation of a single well from 24-well and 96-well microtitre plates

The detailed engineering characterisation of shaken microtitre-plate bioreactors will enhance our understanding of microbial and mammalian cell culture in these geometries and will provide guidance on the scale-up of microwell results to laboratory and pilot scale stirred bioreactors. In this work computational fluid dynamics (CFD) is employed to provide a detailed characterisation of fluid mixing, energy dissipation rate and mass transfer in single well bioreactors from deep square 24-well and 96-well microtitre plates. The numerical predictions are generally found to be in good agreement with experimental observation of the fluid motion and measured values of the key engineering parameters. The CFD simulations have shown that liquid mixing is more intensive in 96-well than in 24-well bioreactors due to a significant axial component to the fluid velocity. Liquid motion is strongly dependent on the orbital shaking amplitude which generally has a greater impact than the shaking frequency. Average power consumptions of 70–100 W m−3 and 500–1000 W m−3, and overall mass transfer coefficient, kLa, values of 0.005–0.028 s−1 and 0.056–0.10 s−1 were obtained for 24-well and 96-well bioreactors respectively at an orbital shaking amplitude of 3 mm and shaking frequencies ranging from 500 rpm to 1500 rpm. The distribution of energy dissipation rates within each bioreactor showed these to be greatest at the walls of the well for both geometries. Batch culture kinetics of E. coli DH5α showed similar maximum specific growth rates and final biomass yields in shaken 24-well and shake flask bioreactors and in stirred miniature and 20 L bioreactors at matched kLa values. The CFD simulations thus give new insights into the local and overall engineering properties of microwell bioreactor geometries and further support their use as high throughput tools for the study and optimisation of microbial and mammalian cell culture kinetics at this scale.Hu Zhang, Sally R. Lamping, Samuel C.R. Pickering, Gary J. Lye and Parviz Ayazi Shamlo