Chia-Chi Ho

Effect of membrane morphology on the initial rate of protein fouling during microfiltration

Protein fouling remains a major problem in the use of microfiltration for many bioprocessing applications. Experiments were performed to evaluate the effect of membrane morphology and pore structure on protein fouling using different track-etched, isotropic, and asymmetric microfiltration membranes. Fouling of membranes with straight-through pores occurred by pore blockage caused by deposition of large protein aggregates on the membrane surface. However, the rate of blockage was a function of the membrane porosity due to the possibility of multiple pore blockage by a single protein aggregate on high porosity membranes. Membranes with interconnected pores fouled more slowly since the fluid could flow around the blocked pores through the interconnected pore structure. This behavior was quantified using model membrane systems with well-defined pore morphology constructed from track-etch and isotropic membranes in a layered series combination. These results provide important insights into the effects of membrane pore structure and morphology on protein fouling.

Transmembrane pressure profiles during constant flux microfiltration of bovine serum albumin

Many microfiltration systems are now run at constant filtrate flux to achieve improved performance; however, large increases in transmembrane pressure are often required to maintain the flux at a constant value due to membrane fouling. We have developed a new mathematical model to describe the change in transmembrane pressure during constant flux microfiltration. Fouling is assumed to occur first by pore blockage, with a cake then forming over the blocked areas of the membrane. This combined pore blockage-cake filtration model is in good agreement with experimental data obtained during the constant flux filtration of bovine serum albumin through track-etched microfiltration membranes. The total volume of the feed solution that can be filtered through the membrane before the transmembrane pressure exceeds some critical value increases with decreasing flux due to the reduction in the rate of cake growth at low flux. Model simulations were used to provide important insights into the design and operation of constant flux microfiltration processes.

Fouling with protein mixtures in microfiltration: BSA-lysozyme and BSA-pepsin

Abstract Protein fouling during microfiltration has been investigated for mixtures of bovine serum albumin (BSA) and lysozyme and of BSA and pepsin. Flux decay curves were analyzed using a recently developed model that accounts for simultaneous pore blockage and cake formation. The model is in good agreement with the data and can be used to evaluate the effect of mixture composition on the concentration of protein aggregates and the properties of the protein deposit. For pepsin–BSA mixtures, the initial fouling appears to be dominated by the BSA, whereas the rate of cake growth occurs primarily by the pepsin. This behavior is consistent with the large concentration of pepsin aggregates and the electrostatic repulsive interactions between the negatively-charged BSA and pepsin. The behavior is more complex for mixtures of BSA and lysozyme. In this case, the fouling is dominated by the lysozyme, although mixtures with small amounts of added BSA foul more slowly than observed with either of the pure proteins.

Micropatterning of proteins and mammalian cells on biomaterials

Controlling the spatial organization of cells is vital in engineering tissues that require precisely defined cellular architectures. For example, functional nerves or blood vessels form only when groups of cells are organized and aligned in very specific geometries. Yet, scaffold designs incorporating spatially defined physical cues such as microscale surface topographies or spatial patterns of extracellular matrix to guide the spatial organization and behavior of cells cultured in vitro remain largely unexplored. Here we demonstrate a new approach for controlling the spatial organization, spreading, and orientation of cells on two micropatterned biomaterials: chitosan and gelatin. Biomaterials with grooves of defined width and depth were fabricated using a two‐step soft lithography process. Selective attachment and spreading of cells within the grooves was ensured by covalently modifying the plateau regions with commercially available protein resistant triblock copolymers. Precise spatial control over cell spreading and orientation has been observed when human microvascular endothelial cells are cultured on these patterned biomaterials, suggesting the potential of this technique in creating tissue culture scaffolds with defined chemical and topographical features.

Overview of Fouling Phenomena and Modeling Approaches for Membrane Bioreactors

Abstract Membrane bioreactors are used extensively for the treatment of municipal and industrial wastewaters. Membrane fouling remains a critical issue in the design and operation of these systems. The complex nature of the feed, the high solids concentration, and the limited options available for pretreatment and cleaning all serve to exacerbate the problem of membrane fouling. This manuscript provides an overview of the key fouling phenomena involved in membrane bioreactors, with a particular emphasis on the nature of the fouling components and the different modeling approaches used to describe the flux decline and to identify the underlying fouling mechanisms. Submitted to Separation Science and Technology, Special Issue on Membrane Fouling and Fouling Control in Membrane Bioreactor and Related Biological Process.

Measurement of membrane pore interconnectivity

Abstract Several studies have suggested that the rate and extent of membrane fouling can be strongly affected by the interconnectivity of the membrane pore structure. For example, membranes with highly interconnected pores should allow fluid to flow around and under any pore blockage on the membrane surface, significantly reducing the effect of this blockage on the filtrate flux. It has not, however, been possible to quantify these effects due to the absence of any experimental technique for measuring the pore connectivity. We have developed a new technique for evaluating the pore connectivity from data for the hydraulic permeability and/or solute diffusion coefficient in the directions normal to and parallel to the membrane surface. Experiments were performed by blocking different regions of the upper and lower surfaces of the membrane to change the relative contributions of the normal and transverse flows. Data were analyzed using a theoretical model for two-dimensional flow or transport in the porous membrane. Studies performed with polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membranes showed distinct differences in the extent of pore connectivity, consistent with the different formation methods and underlying pore morphologies for these membranes.

Scale-up of microfiltration systems: fouling phenomena and Vmax analysis

Abstract Proper sizing and scale-up of normal flow filtration devices requires an understanding of the effects of membrane fouling on system capacity. The V max test is often used to accelerate testing and reduce the required process volume, but the underlying assumption that fouling occurs by uniform constriction of cylindrical membrane pores is rarely met in practice. We have examined the validity of the V max model and have compared the results with predictions of a new model that accounts for fouling due to both pore blockage and cake formation. The V max analysis significantly over-estimates the system capacity for proteins like bovine serum albumin that foul primarily by pore blockage, but it under-estimates the capacity for lysozyme which fouls primarily by cake formation. In contrast, the pore blockage—cake filtration model provides a much better description of membrane fouling, leading to more accurate sizing and scale-up of normal flow filtration devices.

Steering cell migration using microarray amplification of natural directional persistence.

Cell locomotion plays a key role in embryonic morphogenesis, wound healing, and cancer metastasis. Here we show that intermittent control of cell shape using microarrays can be used to amplify the natural directional persistence of cells and guide their continuous migration along preset paths and directions. The key to this geometry-based, gradient-free approach for directing cell migration is the finding that cell polarization, induced by the asymmetric shape of individual microarray islands, is retained as cells traverse between islands. Altering the intracellular signals involved in lamellipodia extension (Rac1), contractility (RhoA), and cell polarity (Cdc42) alters the speed of fibroblast migration on these micropatterns but does not affect their directional bias significantly. These results provide insights into the role of cell morphology in directional movement and the design of micropatterned materials for steering cellular traffic.

Theoretical Analysis of the Effect of Membrane Morphology on Fouling during Microfiltration

Previous studies of membrane fouling have often employed one of the classical blocking laws to describe the variation of filtrate flux with time. However, these models implicitly assume that the membrane has straight-through noninterconnected pores, even though most commercial microfiltration and ultrafiltration membranes have a highly interconnected pore structure. We have developed a theoretical model for the effects of pore blockage on the fluid velocity and pressure profiles within membranes having different interconnected pore structures assuming Darcy flow in the porous membrane. Model calculations are in good agreement with filtrate flux data obtained during protein microfiltration using membranes with very different pore morphologies. The results clearly demonstrate that the membrane pore connectivity has a significant influence on the flux decline due to the possibility for fluid to flow around the pore blockage, an effect which has been ignored in previous studies.

Directing cell migration in continuous microchannels by topographical amplification of natural directional persistence

Discrete micropatterns on biomaterial surfaces can be used to guide the direction of mammalian cell movement by orienting cell morphology. However, guiding cell assembly in three-dimensional scaffolds remains a challenge. Here we demonstrate that the random motions of motile cells can be rectified within continuous microchannels without chemotactic gradients or fluid flow. Our results show that uniform width microchannels with an overhanging zigzag design can induce polarization of NIH3T3 fibroblasts and human umbilical vein endothelial cells by expanding the cell front at each turn. These continuous zigzag microchannels can guide the direction of cell movement even for cells with altered intracellular signals that promote random movement. This approach for directing cell migration within microchannels has important potential implications in the design of scaffolds for tissue engineering.