@article{ma_zhang_chang_velev_wiltberger_kshirsagar_2019, title={Real-time monitoring and control of CHO cell apoptosis by in situ multifrequency scanning dielectric spectroscopy}, volume={80}, ISSN={["1873-3298"]}, DOI={10.1016/j.procbio.2019.02.017}, abstractNote={Process control strategies based on the physiological status of cells have recently been used to enhance mammalian cell culture productivity and robustness. In this study, we investigated the feasibility of using full-spectrum dielectric spectroscopy for detecting shifts in cell physiology and as a feedback tool to increase process efficiency. Multi-frequency permittivity spectra were collected from cell culture processes in which apoptosis was induced by glucose depletion, nutrient depletion, or chemical treatment. Meanwhile, key parameters of critical frequency (fc) and Cole-Cole alpha (α) were calculated in real time from the β-dispersion curve and correlated to data from off-line viability measurements. Results show that physiological changes in apoptotic cells were reflected in the on-line parameters earlier than from off-line methods. Using information from the on-line parameters, we achieved partial or full recovery from early apoptosis by replenishing the depleted feed. We also demonstrate that by using trends in fc, we could detect a deviation in media preparation in a manufacturing process which could not be achieved using conventional measurements. The results demonstrate that full-spectrum dielectric spectroscopy can be used as a facile tool for early detection of physiological changes and process adjustment in real-time to enhance bioreactor process productivity and robustness.}, journal={PROCESS BIOCHEMISTRY}, author={Ma, Fuduo and Zhang, An and Chang, David and Velev, Orlin D. and Wiltberger, Kelly and Kshirsagar, Rashmi}, year={2019}, month={May}, pages={138–145} }
 @article{chang_fox_hicks_ferguson_chang_osborne_hu_velev_2017, title={Investigation of interfacial properties of pure and mixed poloxamers for surfactant-mediated shear protection of mammalian cells}, volume={156}, ISSN={["1873-4367"]}, DOI={10.1016/j.colsurfb.2017.05.040}, abstractNote={The Poloxamer family of surfactants are commonly used in the biopharmaceutical industry as cell culture media additives to protect cells from the turbulent environment of sparged bioreactors. Despite the widespread use of poloxamers in cell culture, their performance as cell protectants varies depending on their physical structure, molecular weight, and batch-to-batch composition. In this study, the interfacial properties of Poloxamer 188 (P188), Poloxamer 407 (P407), and a mixture of P188 and P407 were characterized to investigate the mechanism of surfactant-mediated shear protection of mammalian cells. The foam stability and equilibrium surface tension of these surfactant systems correlated with their ability to mitigate physical damage to cells in a turbulent environment. We demonstrate that while P188 can function as highly effective shear protectant, the presence of a surface-active contaminant can greatly hinder its protective characteristics. P407 was found to function as such an interfacially active “impurity,” disrupting shear protection when mixed with P188 by preferentially adsorbing to the gas-liquid and membrane-liquid interface. Addition of surface-active impurities altered the interfacial properties of the surfactant system and could be detected using an equilibrium surface tension assay. The mechanism of disruption by P407 was determined to be independent of cell-to-bubble attachment, suggesting that poloxamer adsorption to and subsequent reinforcement of the cell membrane may play a key role in protecting cells in high shear environments. This investigation contributes to our understanding of the mechanism of surfactant-mediated shear protection of cells and demonstrates that a surface tension assay can be utilized as a screening tool to ensure that poloxamer lots are free of surface active impurities.}, journal={COLLOIDS AND SURFACES B-BIOINTERFACES}, author={Chang, David and Fox, Ryan and Hicks, Ethan and Ferguson, Rachel and Chang, Kevin and Osborne, Douglas and Hu, Weiwei and Velev, Orlin D.}, year={2017}, month={Aug}, pages={358–365} }