Performance of tapered column packed-bed bioreactor for ethanol production

A tapered column type of bioreactor system packed with immobilized Saccharomyces cerevisiae was used to study the bioreactor performance as a function of design and operating variables. The performance of tapered column bioreactor system was found to be better than that of the conventional cylindrical column reactor system for the ethanol fermentation. The new bioreactor design alleviated problems associated with carbon dioxide evolution and provided a significantly better flow pattern for both liquid and gas phases in the bioreactor without local channelling. A mathematical simulation model, which takes into account of the axial convection and dispersion, interphase mass transfer, and apparent kinetic design parameters, was developed. The effect of radial concentration gradients on the bioreactor performance was found to be insignificant. For the reactor system studied, the maximum ethanol productivity obtained was 60 g ethanol/L gel/h, and the maximum glucose assimilation rate was 140 g glucose/L gel/h. One of the most important findings from this study was that the apparent kinetic parameters change at the glucose concentration of 2 g/L This change was found to be due to the changes in yeast physiology and metabolism. The values of V(m) (') and V(m) (') decreased from 0.8 to 0.39 g ethanol/g cell/h and from 97mM to 11mM, respectively. The substrate inhibition constant was estimated as 0.76M and the product inhibition constant was determined as 113 g ethanol/L The degree of product inhibition showed practically a linear relationship with an increasing ethanol concentration. Based on the hydro-dynamic analysis of the bioreactor system, it was found that the Peclet number, N(Pe) was not a strong function of the flow velocity at low flow rates through the bioreactor system, but its value decreased somewhat at an interstitial velocity greater than 0.03 cm/s. The tapered column bioreactor system showed a much better flow pattern of gas and liquid phases within the reactor, thereby providing a more homogeneous distribution of gas-liquid-solid phases in the reactor without any phase separation.     

Models of the pulsatile hydrodynamics of cerebrospinal fluid flow in the normal and abnormal intracranial system

Images obtained from magnetic resonance imaging have helped to ascertain that both the cerebrospinal fluid (CSF) and brain move in a pulsatile manner within the cranium. However, these images are not able to reveal any quantitative information on the physiological forces that are associated with pulsatile motion. Understanding both the pressure and velocity flow field of CSF in the ventricles is important to help understand the mechanics of hydrocephalus. Four separate fluid structure interaction models of the ventricular system in the sagittal plane were created for this purpose. The first model was of a normal brain. The second and third models were pathological brain models with aqueductal stenosis at various locations along the fluid pathway. The fourth model was of a hydrocephalic brain. Results revealed the hydrodynamics of CSF pulsatile flow in the ventricles of these models. Most importantly, it has also revealed the different changes in CSF pulsatile hydrodynamics caused by the various locations of fluid flow obstructions.     

Pressure-flow relationships in a model for the arterial system

A mathematical model is developed in state variable form suitable for the study of the control of blood pressure and flow in the mammalian cardiovascular system. The applicability of the model to steady state, both mean and pulsatile, and transient phenomena is demonstrated by the agreement of the results with experimental data. This model was developed to study the neural and renal-endocrine-electrolyte control of cardiovascular functions. Presented at the Society for Mathematical Biology Meeting, University of Pennsylvania, Philadelphia, August 19–21, 1976.     

Design of a sterile organ culture system for the ex vivo study of aortic heart valves

The biological response of valves to mechanical forces is not well understood. The aim of this study was to design a pulsatile system to enable the ex vivo study of aortic valves when subjected to various hemodynamic conditions. A bioreactor was designed to subject porcine aortic valves to physiological and pathophysiological pressure and flow conditions, while maintaining viability and sterility. Pressure and flow rate could be independently controlled to produce clinically relevant mechanical conditions. The oxygen transfer rate was characterized and sterile operation was achieved over 96 hours. The oxygenation capabilities ensure sufficient oxygen transport to valves, allowing operation for extended periods.     

Cells in fluidic environments are sensitive to flow frequency

Virtually all cells accommodate to their mechanical environment. In particular, cells subject to flow respond to rapid changes in fluid shear stress (SS), cyclic stretch (CS), and pressure. Recent studies have focused on the effect of pulsatility on cellular behavior. Since cells of many different tissue beds are constantly exposed to fluid flows over a narrow range of frequencies, we hypothesized that an intrinsic flow frequency that is optimal for determining cell phenotype exists. We report here that cells from various tissue beds (bovine aortic endothelial cells (BAEC), rat small intestine epithelial cells (RSIEC), and rat lung epithelial cells (RLEC)) proliferate maximally when cultured in a perfusion bioreactor under pulsatile conditions at a specific frequency, independent of the applied SS. Vascular endothelial and pulmonary epithelial cell proliferation peaked under 1 Hz pulsatile flow. In contrast, proliferation of gastrointestinal cells, which in their physiological context are subject to no flow or higher wavelength signal, was maximum at 0.125 Hz or under no flow. Moreover, exposure of BAEC to pulsatile flow of varying frequency influenced their nitric oxide synthase activity and prostacyclin production, which reached maximum values at 1 Hz. Notably, the "optimal" frequencies for the cell types examined correspond to the physiologic operating range of the organs from where they were initially derived. These findings suggest that frequency, independent of shear, is an essential determinant of cell response in pulsatile environments.     

Design of a large-scale surface-aerated bioreactor for biomass production using a VOC substrate

The design of a large-scale bioreactor for the production of bacterial biomass adapted to the biodegradation of volatile organic compounds was carried out. The bioreactor model used integrated the microbial kinetics and fluid dynamics described by the compartment model approach. The process conditions and kinetic parameters were adopted from the laboratory experimental study of (León, E., Seignez, C., Adler, N., Péringer, P., 1999. Growth inhibition of biomass adapted to the degradation of toluene and xylenes in mixture in a batch reactor with substrates supplied by pulses. Biodegradation 10, 245-250). The performance of the pulsed-batch stirred bioreactor under surface aeration conditions was simulated for different mixing configurations and conditions such as the impeller diameter, number of impellers, stirring speed, and oxygen pressure. The simulations were used for the cost analysis which resulted in the optimal design of the bioreactor.     

Modeling and design of optimal flow perfusion bioreactors for tissue engineering applications

Perfusion bioreactors have been used in different tissue engineering applications because of their consistent distribution of nutrients and flow-induced shear stress within the tissue-engineering scaffold. A widely used configuration uses a scaffold with a circular cross-section enclosed within a cylindrical chamber and inlet and outlet pipes which are connected to the chamber on either side through which media is continuously circulated. However, fluid-flow experiments and simulations have shown that the majority of the flow perfuses through the center. This pattern creates stagnant zones in the peripheral regions as well as in those of high flow rate near the inlet and outlet. This non-uniformity of flow and shear stress, owing to a circular design, results in limited cell proliferation and differentiation in these areas. The focus of this communication is to design an optimized perfusion system using computational fluid dynamics as a mathematical tool to overcome the time-consuming trial and error experimental method. We compared the flow within a circular and a rectangular bioreactor system. Flow simulations within the rectangular bioreactor are shown to overcome the limitations in the circular design. This communication challenges the circular cross-section bioreactor configuration paradigm and provides proof of the advantages of the new design over the existing one.     

Pulsatile blood flow in the entire coronary arterial tree: theory and experiment

The pulsatility of coronary circulation can be accurately simulated on the basis of the measured branching pattern, vascular geometry, and material properties of the coronary vasculature. A Womersley-type mathematical model is developed to analyze pulsatile blood flow in diastole in the absence of vessel tone in the entire coronary arterial tree on the basis of previously measured morphometric data. The model incorporates a constitutive equation of pressure and cross-section area relation based on our previous experimental data. The formulation enables the prediction of the impedance, the pressure distribution, and the pulsatile flow distribution throughout the entire coronary arterial tree. The model is validated by experimental measurements in six diastolic arrested, vasodilated porcine hearts. The agreement between theory and experiment is excellent. Furthermore, the present pulse wave results at low frequency agree very well with previously published steady-state model. Finally, the phase angle of flow is seen to decrease along the trunk of the major coronary artery and primary branches toward the capillary vessels. This study represents the first, most extensive validated analysis of Womersley-type pulse wave transmission in the entire coronary arterial tree down to the first segment of capillaries. The present model will serve to quantitatively test various hypotheses in the coronary circulation under pulsatile flow conditions.     

Evaluation of dynamic response and biomechanical properties of isolated blood vessels

In this study we present the experimental and mathematical model for a precise assessment of isolated blood vessels dynamic response under a sudden change of blood pressure. Only the end points within the time interval of the considered dynamic response of the blood vessel, or so-called “alternate steady states” of the processes, were usually considered in various studies. These studies do not provide an insight how the process variables change between these alternate steady states. Isolated blood vessels (rat abdominal aorta) were used to determine how the process dynamics can be described in detailed quantitative terms by mathematical parameters. The experimental model and mathematical procedures presented in this study describe precisely (at a high sensitivity level) the time history of the pressure and the diameter change in between alternate steady states, when an abrupt change of blood pressure occurs at the vessel outlet. Also, the experimental model and mathematical procedures were used to determine changes in the stress–strain law, caused by the action of L-arginine. The presented experimental design and mathematical model can be used for assessment of isolated blood vessel dynamic responses under different stimuli, such as drug effects, electrostimulation etc.     

Model of the mass transport to the surface of animal cells cultured in a rotating bioreactor operated in micro gravity

A mathematical model is used to investigate the transport of dissolved oxygen from the bulk fluid to the surface of aggregates of animal cells cultured in a rotating bioreactor. These aggregates move through different regions of the bioreactor with a local flow field and concentration distribution that vary with time. The time variation of the Sherwood number and the surface concentration for a range of parameters typical of a cell science experiment executed in the Rotating Wall Perfused Vessel (RWPV) bioreactor in space are investigated. The Reynolds numbers experienced by the aggregate are generally low (Re < 1) and the Peclet numbers range from O(1) to O(100). Comparison of the results from the numerical solution of the mathematical model with those from a quasi-steady model, using a steady-state correlation for mass transport on a sphere, indicate that the quasi-steady assumption is not a good model to compute the instantaneous Sherwood number. This indicates a significant history effect in the Sherwood number response to the variations of acceleration of the aggregates in the bioreactor. A high resistance to the mass transport from the bulk fluid to the surface of the aggregate exists for the bioreactor operated in micro gravity. The difference between the surface concentration and the free stream concentration was as high as 30% for aggregates larger than 3 mm. Diffusion reduces the variations of the free stream concentration resulting in a nearly constant value for the concentration at the surface of the aggregates.     

Gas transfer characteristics of a novel membrane bioreactor

We show the design features of a membrane bioreactor based on pulsatile flow across dimpled membranes. Results show an enhanced mass transfer of air of at least five-fold magnitude as compared with flat membranes. An increased working volume form 20 mL to 120 mL reduced the k(L)A at a given Reynolds number because of axial mixing of fluid from the deoxygenated end chamber. The bioreactor was used to supply air to a hybridoma mammalian cell line, and the calculated oxygen uptake showed that high-density cultures could be maintained in a 20mL, single-dimpled cultures could be maintained in a 20 mL, single-dimpled membrane system. Indirect aeration of a 2 L continuous stirred tank reactor, by a double-membrane system, showed that air could be supplied to mammalian cells at cell densities of approximately 4 x 10(6) /mL.     

Computational fluid dynamic analysis of bioprinted self-supporting perfused tissue models

Natural tissues are incorporated with vasculature, which is further integrated with a cardiovascular system responsible for driving perfusion of nutrient-rich oxygenated blood through the vasculature to support cell metabolism within most cell-dense tissues. Since scaffold-free biofabricated tissues being developed into clinical implants, research models, and pharmaceutical testing platforms should similarly exhibit perfused tissue-like structures, we generated a generalizable biofabrication method resulting in self-supporting perfused (SSuPer) tissue constructs incorporated with perfusible microchannels and integrated with the modular FABRICA perfusion bioreactor. As proof of concept, we perfused an MLO-A5 osteoblast-based SSuPer tissue in the FABRICA. Although our resulting SSuPer tissue replicated vascularization and perfusion observed in situ, supported its own weight, and stained positively for mineral using Von Kossa staining, our in vitro results indicated that computational fluid dynamics (CFD) should be used to drive future construct design and flow application before further tissue biofabrication and perfusion. We built a CFD model of the SSuPer tissue integrated in the FABRICA and analyzed flow characteristics (net force, pressure distribution, shear stress, and oxygen distribution) through five SSuPer tissue microchannel patterns in two flow directions and at increasing flow rates. Important flow parameters include flow direction, fully developed flow, and tissue microchannel diameters matched and aligned with bioreactor flow channels. We observed that the SSuPer tissue platform is capable of providing direct perfusion to tissue constructs and proper culture conditions (oxygenation, with controllable shear and flow rates), indicating that our approach can be used to biofabricate tissue representing primary tissues and that we can model the system in silico.     

Flow in the well: computational fluid dynamics is essential in flow chamber construction

A perfusion system was developed to generate well defined flow conditions within a well of a standard multidish. Human vein endothelial cells were cultured under flow conditions and cell response was analyzed by microscopy. Endothelial cells became elongated and spindle shaped. As demonstrated by computational fluid dynamics (CFD), cells were cultured under well defined but time varying shear stress conditions. A damper system was introduced which reduced pulsatile flow when using volumetric pumps. The flow and the wall shear stress distribution were analyzed by CFD for the steady and unsteady flow field. Usage of the volumetric pump caused variations of the wall shear stresses despite the controlled fluid environment and introduction of a damper system. Therefore the use of CFD analysis and experimental validation is critical in developing flow chambers and studying cell response to shear stress. The system presented gives an effortless flow chamber setup within a 6-well standard multidish.     

A global mathematical model of the cerebral circulation in man

A mathematical model of the cerebral circulation has been formulated. It was based on non-linear equations of pulsatile fluid flow in distensible conduits and applied to a network simulating the entire cerebral vasculature, from the carotid and vertebral arteries to the sinuses and the jugular veins. The quasilinear hyperbolic system of equations was numerically solved using the two-step Lax-Wendroff scheme. The model's results were in good agreement with pressure and flow data recorded in humans during rest. The model was also applied to the study of autoregulation during arterial hypotension. A close relationship between cerebral blood flow (CBF) and capillary pressure was obtained. At arterial pressure of 80 mmHg, the vasodilation of the pial arteries was unable to maintain CBF at its control value. At the lower limit of autoregulation (60 mm Hg), CBF was maintained with a 25% increase of zero transmural pressure diameter of nearly the whole arterial network.     

The fluid dynamic and shear environment in the NASA/JSC rotating-wall perfused-vessel bioreactor

The rotating-wall perfused-vessel (RWPV) bioreactor, used for both microgravity and Earth-based cell science experiments, is characterized in terms of the fluid dynamic and fluid shear stress environment. A numerical model of the flow field is developed and verified with laser Doppler velocimeter measurements. The effects of changes in operating conditions, including rotation rates and fluid perfusion rates, are investigated with the numerical model. The operating conditions typically used for ground-based experiments (equal rotation of the inner and outer cylinders) leads to flow patterns with relatively poor mass distribution characteristics. Approximately 50% of the inlet-perfused fluid bypasses the bulk of the fluid volume and flows to the perfusion exit. For operating conditions typical in microgravity, small differential rotation rates between the inner and outer cylinders lead to greatly improved flow distribution patterns and very low fluid shear stress levels over a large percentage of the fluid volume. Differences in flow patterns for the different operating conditions are explored. Large differences in the hydrodynamic environments for operating conditions typical of true microgravity and ground-based "microgravity simulations" are demonstrated.     

A perturbation model for the oscillatory flow of a Bingham plastic in rigid and periodically displaced tubes.

An approximate analytical model for the pulsatile flow of an ideal Bingham plastic fluid in both a rigid and a periodically displaced tube has been developed using regular perturbation methods. Relationships are derived for the velocity field and dimensionless flow rate. The solution compares adequately with available experimentally measured oscillatory non-Newtonian fluid flow data. These solutions provide useful analytical models supporting experimental and computation studies of arterial blood flow.     

On bone adaptation due to venous stasis

This paper addresses the question of whether or not interstitial fluid flow due to the blood circulation accounts for the observed periosteal bone formation associated with comprised venous return (venous stasis). Increased interstitial fluid flow induced by increased intramedullary pressure has been proposed to account for the periosteal response in venous stasis. To investigate the shear stresses acting on bone cell processes due to the blood circulation-driven interstitial fluid flow, a poroelastic model is extended to the situation in which the interstitial fluid flow in an osteon is driven by the pulsatile extravascular pressure in the osteonal canal as well as by the applied cyclic mechanical loading. Our results show that under normal conditions, the pulsatile extravascular pressure in the osteonal canal due to cardiac contraction (10mm Hg at 2Hz) and skeletal muscle contraction (30mm Hg at 1Hz) induce peak shear stresses on the osteocyte cell processes that are two orders of magnitude lower than those induced by physiological mechanical loading (100 microstrain at 1Hz). In venous stasis the induced peak shear stress is reduced further compared to the normal conditions because, although the mean intramedullary pressure is increased, the amplitude of its pulsatile component is decreased. These results suggest that the interstitial fluid flow is unlikely to cause the periosteal bone formation in venous stasis. However, the mean interstitial fluid pressure is found to increase in venous stasis, which may pressurize the periosteum and thus play a role in periosteal bone formation.     

Computational fluid modeling and performance analysis of a bidirectional rotating perfusion culture system

A myriad of bioreactor configurations have been investigated as extracorporeal medical support systems for temporary replacement of vital organ functions. In recent years, studies have demonstrated that the rotating bioreactors have the potential to be utilized as bioartificial liver assist devices (BLADs) owing to their advantage of ease of scalability of cell‐culture volume. However, the fluid movement in the rotating chamber will expose the suspended cells to unwanted flow structures with abnormally high shear conditions that may result in poor cell stability and in turn lower the efficacy of the bioreactor system. In this study, we compared the hydrodynamic performance of our modified rotating bioreactor design with that of an existing rotating bioreactor design. Computational fluid dynamic analysis coupled with experimental results were employed in the optimization process for the development of the modified bioreactor design. Our simulation results showed that the modified bioreactor had lower fluid induced shear stresses and more uniform flow conditions within its rotating chamber than the conventional design. Experimental results revealed that the cells within the modified bioreactor also exhibited better cell‐carrier attachment, higher metabolic activity, and cell viability compared to those in the conventional design. In conclusion, this study was able to provide important insights into the flow physics within the rotating bioreactors, and help enhanced the hydrodynamic performance of an existing rotating bioreactor for BLAD applications. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1002–1012, 2013     

A comparison of two fouling‐resistant heat exchangers

Fouling cannot always be prevented; it is important to consider the design of fouling‐resistant heat exchangers. To examine these exchangers, a test fluid whose fouling behaviour is understood should be used. Experiments have been conducted to examine the response of two model systems, a pulsatile flow and a fluid bed heat exchanger, to fouling from whey protein concentrates. Both systems are effective in certain cases, although the enhanced mass transfer possible in the pulsatile flow exchanger can increase fouling when mass transfer controls deposition. This demonstrates the possible danger in installing “antifouling”; systems. The possible mechanisms by which antifouling exchangers operate is discussed; they may work both by slowing the kinetics of fouling or enhancing the heat transfer coefficient. A simple model to demonstrate the design of antifouling exchangers is presented.