Acoustic cell filter for high density perfusion culture of hybridoma cells

Application of therapeutic ultrasonic waves across the dialyzer membrane: A pilot study on the impact on dialyzer clearance and safety

INTRODUCTION

Progressive clogging of the dialyzer membrane during hemodialysis can compromise solute removal efficiency. Existing solutions fall short in addressing intradialytic reduction of dialyzer clearance. This pilot study aims to assess the impact and safety of applying therapeutic ultrasonic waves to dialyzers for mitigating intradialytic clogging.

METHODS

In this pilot study, 15 stable maintenance hemodialysis patients (12 males and 3 females) were enrolled. Each patient served as their own control. They underwent one session of hemodialysis with the application of therapeutic ultrasonic waves (Ultrasonic session) and were crossed-over to a second session without the use of ultrasonic waves (Control session). All the study sessions operated at a fixed dialysate flow rate of 500 mL/min and a blood flow rate of 250 or 300 mL/min. The adequacy of dialysis achieved during each session was monitored using Online Clearance Monitoring of the dialysis machines, and clearance K values, varying between 135 and 209 mL/min, were recorded, and plotted. A direct comparison between Control and Ultrasonic sessions was performed to assess the impact and safety of using ultrasonic waves during hemodialysis.

FINDINGS

The mean percentage decline in dialyzer clearance values was 4.41% for Ultrasonic sessions (SD: 5.3) and 12.69% for Control sessions (SD: 6.35) (p-value <0.001). This indicates that the application of ultrasonic waves reduced the decline in clearance values. The mean differences of the blood component parameters were comparable between both Ultrasonic sessions and Control sessions, suggesting the safety of utilizing ultrasonic waves during dialysis. Microscopic membrane analysis corroborated the safety.

DISCUSSION

Intradialytic clogging of dialyzer membranes is a significant problem that can cause dialysis inadequacy. Our study tackles this issue by introducing therapeutic ultrasonic waves to improve dialyzer clearance during hemodialysis sessions in patients.

Towards manufactured red blood cells for the treatment of inherited anemia

Patients with inherited anemia and hemoglobinopathies (such as sickle cell disease and β-thalassemia) are treated with red blood cell (RBC) transfusions to alleviate their symptoms. Some of these patients may have rare blood group types or go on to develop alloimmune reactions, which can make it difficult to source compatible blood in the donor population. Laboratory-grown RBC represent a particularly attractive alternative which could satisfy an unmet clinical need. The challenge, however, is to produce - from a limited number of stem cells - the 2x1012 RBC required for a standard adult therapeutic dose. Encouraging progress has been made in RBC production from adult stem cells under good manufacturing practice. In 2011, the Douay group conducted a successful proof-of-principle mini-transfusion of autologous manufactured RBC in a single volunteer. In the UK, a trial is planned to assess whether manufactured RBC are equivalent to RBC produced naturally in donors, by testing an allogeneic mini-dose of laboratory-grown manufactured RBC in multiple volunteers. This review discusses recent progress in the erythroid culture field as well as opportunities for further scaling up of manufactured RBC production for transfusion practice.

Miniature auto-perfusion bioreactor system with spiral microfluidic cell retention device

Medium perfusion is critical in maintaining high cell concentration in cultures. The conventional membrane filtration method for medium exchange has been challenged by the fouling and clogging of the membrane filters in long-term cultures. In this study, we present a miniature auto-perfusion system that can be operated inside a common-size laboratory incubator. The system is equipped with a spiral microfluidic chip for cell retention to replace conventional membrane filters, which fundamentally overcomes the clogging and fouling problem. We showed that the system supported continuous perfusion culture of Chinese hamster ovary (CHO) cells in suspension up to 14 days without cell retention chip replacement. Compared to daily manual medium change, 25% higher CHO cell concentration can be maintained at an average auto-perfusion rate of 196 ml/day in spinner flask at 70 ml working volume (2.8 VVD). The auto-perfusion system also resulted in better cell quality at high concentrations, in terms of higher viability, more uniform and regular morphology, and fewer aggregates. We also demonstrated the potential application of the system for culturing mesenchymal stem cells on microcarriers. This miniature auto-perfusion system provides an excellent solution to maintain cell-favorable conditions and high cell concentration in small-scale cultures for research and clinical uses.

Acoustofluidic separation of cells and particles

Acoustofluidics, the integration of acoustics and microfluidics, is a rapidly growing research field that is addressing challenges in biology, medicine, chemistry, engineering, and physics. In particular, acoustofluidic separation of biological targets from complex fluids has proven to be a powerful tool due to the label-free, biocompatible, and contact-free nature of the technology. By carefully designing and tuning the applied acoustic field, cells and other bioparticles can be isolated with high yield, purity, and biocompatibility. Recent advances in acoustofluidics, such as the development of automated, point-of-care devices for isolating sub-micron bioparticles, address many of the limitations of conventional separation tools. More importantly, advances in the research lab are quickly being adopted to solve clinical problems. In this review article, we discuss working principles of acoustofluidic separation, compare different approaches of acoustofluidic separation, and provide a synopsis of how it is being applied in both traditional applications, such as blood component separation, cell washing, and fluorescence activated cell sorting, as well as emerging applications, including circulating tumor cell and exosome isolation.

Selective Particle Filtering in a Large Acoustophoretic Serpentine Channel

The objective of this study is to investigate the performance of a serpentine channel for acoustically driven selective particle filtering. The channel consists of sharp corners and straight sections, and the acoustic field is affecting the particles throughout the channel. A prototype of the separator channel is manufactured using 3D printing. Acoustic waves are generated by a piezoelectric transducer operating near 2 MHz. Computer simulations are carried out to explore and visualize the flow field and acoustic field in the separator. Selective particle trapping is aimed to be achieved in the hairpin sections, which is confirmed by experiments. Spherical polyethylene particles of 34 µm, 70 µm and 100 µm diameter are used to demonstrate selective trapping by adjusting the flow rate in the channel or voltage input to the transducer. In addition, wheat beer containing yeast up to 20 µm size is selectively filtered by adjusting the flow rate to the channel. Experiments demonstrate that selective particle filtering is possible in the serpentine channel as both methods yield clear separation thresholds.

Continuous Manufacturing of Recombinant Therapeutic Proteins: Upstream and Downstream Technologies

Continuous biomanufacturing of recombinant therapeutic proteins offers several potential advantages over conventional batch processing, including reduced cost of goods, more flexible and responsive manufacturing facilities, and improved and consistent product quality. Although continuous approaches to various upstream and downstream unit operations have been considered and studied for decades, in recent years interest and application have accelerated. Researchers have achieved increasingly higher levels of process intensification, and have also begun to integrate different continuous unit operations into larger, holistically continuous processes. This review first discusses approaches for continuous cell culture, with a focus on perfusion-enabling cell separation technologies including gravitational, centrifugal, and acoustic settling, as well as filtration-based techniques. We follow with a review of various continuous downstream unit operations, covering categories such as clarification, chromatography, formulation, and viral inactivation and filtration. The review ends by summarizing case studies of integrated and continuous processing as reported in the literature.

Applications of Acoustofluidics in Bioanalytical Chemistry

Acoustics has a broad spectrum of applications, ranging from noise cancelation to ultrasonic imaging. In the past decade, there has been increasing interest in developing acoustic-based methods for biological and biomedical applications. This Perspective summarizes the recent progress in applying acoustofluidic methods (i.e., the fusion of acoustics and microfluidics) to bioanalytical chemistry. We describe the concepts of acoustofluidics and how it can be tailored to different types of bioanalytical applications, including sample concentration, fluorescence-activated cell sorting, label-free cell/particle separation, and fluid manipulation. Examples of each application are given, and the benefits and limitations of these methods are discussed. Finally, our perspectives on the directions that developing solutions should take to address the bottlenecks in the acoustofluidic applications in bioanalytical chemistry are presented.

Advances in high frequency ultrasound separation of particulates from biomass

In recent years the use of high frequency ultrasound standing waves (megasonics) for droplet or cell separation from biomass has emerged beyond the microfluidics scale into the litre to industrial scale applications. The principle for this separation technology relies on the differential positioning of individual droplets or particles across an ultrasonic standing wave field within the reactor and subsequent biomass material predisposition for separation via rapid droplet agglomeration or coalescence into larger entities. Large scale transducers have been characterised with sonochemiluminescence and hydrophones to enable better reactor designs. High frequency enhanced separation technology has been demonstrated at industrial scale for oil recovery in the palm oil industry and at litre scale to assist olive oil, coconut oil and milk fat separation. Other applications include algal cell dewatering and milk fat globule fractionation. Frequency selection depends on the material properties and structure in the biomass mixture. Higher frequencies (1 and 2MHz) have proven preferable for better separation of materials with smaller sized droplets such as milk fat globules. For palm oil and olive oil, separation has been demonstrated within the 400-600kHz region, which has high radical production, without detectable impact on product quality.

Membrane-less microfiltration using inertial microfluidics

Microfiltration is a ubiquitous and often crucial part of many industrial processes, including biopharmaceutical manufacturing. Yet, all existing filtration systems suffer from the issue of membrane clogging, which fundamentally limits the efficiency and reliability of the filtration process. Herein, we report the development of a membrane-less microfiltration system by massively parallelizing inertial microfluidics to achieve a macroscopic volume processing rates (~ 500 mL/min). We demonstrated the systems engineered for CHO (10-20 μm) and yeast (3-5 μm) cells filtration, which are two main cell types used for large-scale bioreactors. Our proposed system can replace existing filtration membrane and provide passive (no external force fields), continuous filtration, thus eliminating the need for membrane replacement. This platform has the desirable combinations of high throughput, low-cost, and scalability, making it compatible for a myriad of microfiltration applications and industrial purposes.

Ultrasound-enhanced attenuated total reflection mid-infrared spectroscopy in-line probe: acquisition of cell spectra in a bioreactor

This article presents a novel method for selective acquisition of Fourier transform infrared (FT-IR) spectra of microorganisms in-line during fermentation, using Saccharomyces cerevisiae as an example. The position of the cells relative to the sensitive region of the attenuated total reflection (ATR) FT-IR probe was controlled by combing a commercially available ATR in-line probe with contact-free, gentle particle manipulation by ultrasonic standing waves. A prototype probe was successfully constructed, assembled, and tested in-line during fed-batch fermentations of S. cerevisiae. Control over the position of the cells was achieved by tuning the ultrasound frequency: 2.41 MHz was used for acquisition of spectra of the cells (pushing frequency f(p)) and 1.87 MHz, for retracting the cells from the ATR element, therefore allowing spectra of the medium to be acquired. Accumulation of storage carbohydrates (trehalose and glycogen) inside the cells was induced by a lack of a nitrogen source in the feed medium. These changes in biochemical composition were visible in the spectra of the cells recorded in-line during the application of f(p) and could be verified by reference spectra of dried cell samples recorded off-line with a FT-IR microscope. Comparison of the cell spectra with spectra of trehalose, glycogen, glucose, and mannan, i.e., the major carbohydrates present in S. cerevisiae, and principal components analysis revealed that the changes observed in the cell spectra correlated well with the bands specific for trehalose and glycogen. This proves the applicability and capability of ultrasound-enhanced in-line ATR mid-IR spectroscopy as a real-time PAT method for the in situ monitoring of cellular biochemistry during fermentation.

An ultrasonically enhanced inclined settler for microalgae harvesting

Microalgae have vast potential as a sustainable and scalable source of biofuels and bioproducts. However, algae dewatering is a critical challenge that must be addressed. Ultrasonic settling has already been exploited for concentrating various biological cells at relatively small batch volumes and/or low throughput. Typically, these designs are operated in batch or semicontinuous mode, wherein the flow is interrupted and the cells are subsequently harvested. These batch techniques are not well suited for scaleup to the throughput levels required for harvesting microalgae from the large-scale cultivation operations necessary for a viable algal biofuel industry. This article introduces a novel device for the acoustic harvesting of microalgae. The design is based on the coupling of the acoustophoretic force, acoustic transparent materials, and inclined settling. A filtration efficiency of 70 ± 5% and a concentration factor of 11.6 ± 2.2 were achieved at a flow rate of 25 mL·min(-1) and an energy consumption of 3.6 ± 0.9 kWh·m(-3) . The effects of the applied power, flow rate, inlet cell concentration, and inclination were explored. It was found that the filtration efficiency of the device is proportional to the power applied. However, the filtration efficiency experienced a plateau at 100 W L(-1) of power density applied. The filtration efficiency also increased with increasing inlet cell concentration and was inversely proportional to the flow rate. It was also found that the optimum settling angle for maximum concentration factor occurred at an angle of 50 ± 5°. At these optimum conditions, the device had higher filtration efficiency in comparison to other similar devices reported in the previous literature.

Acoustofluidics 20: applications in acoustic trapping

This part of the Acoustofluidics tutorial series reviews applications in acoustic trapping of micron-sized particles and cells in microfluidic systems. Acoustic trapping enables non-invasive and non-contact immobilisation of cells and particles in microfluidic systems. Acoustic trapping has been used for reducing the time needed to create 3D cell clusters, enhance particle-based bioassays and facilitated interaction studies of both cells and particles. An area that is increasingly interesting is the use of acoustic trapping for enriching low concentration samples and the washing or fractioning of cell populations prior to sensitive detection methods (MALDI-MS, PCR etc.) The main focus of the review is systems where particles can be retained against a flow while applications in which particles are positioned in a stationary fluid will be addressed in part 21 of the Acoustofluidics tutorial series (M. Wiklund, S. Radel and J. J. Hawkes, Lab Chip, 2012, 12, ).

Acoustofluidics 5: Building microfluidic acoustic resonators

Acoustophoresis is getting more attention as an effective and gentle non-contact method of manipulating cells and particles in microfluidic systems. A key to a successful assembly of an acoustophoresis system is a proper design of the acoustic resonator where aspects of fabrication techniques, material choice, thickness matching of involved components, as well as strategies of actuation, all have to be considered. This tutorial covers some of the basics in designing and building microfluidic acoustic resonators and will hopefully be a comprehensive and advisory document to assist the interested reader in creating a successful acoustophoretic device.

Computer programs for modeling mammalian cell batch and fed-batch cultures using logistic equations

A MATLAB(®) toolbox was developed for applying the logistic modeling approach to mammalian cell batch and fed-batch cultures. The programs in the toolbox encompass sensitivity analyses and simulations of the logistic equations in addition to cell specific rate estimation. The toolbox was first used to generate time courses of the sensitivity equations for characterizing the relationship between the logistic variable and the model parameters. Subsequently, the toolbox was used to describe CHO cell data from batch and fed-batch mammalian cell cultures. Cell density, product, glucose, lactate, glutamine, and ammonia data were analyzed for the batch culture while fed-batch analysis included cell density and product concentration. In all instances, experimental data were well described by the logistic equations and the resulting specific rate profiles were representative of the underlying cell physiology. The 6-variable batch culture data set was also used to compare the logistic specific rates with those from polynomial fitting and discrete derivative methods. The polynomial specific rates grossly misrepresented cell behavior in the initial and final stages of culture while those based on discrete derivatives had high variability due to computational artifacts. The utility of logistic specific rates to guide process development activities was demonstrated using specific protein productivity versus growth rate trajectories for the 3 cultures examined in this study. Overall, the computer programs developed in this study enable rapid and robust analysis of data from mammalian cell batch and fed-batch cultures which can help process development and metabolic flux estimation.

Emerging clinical applications of microchip-based acoustophoresis

Acoustophoresis is currently in a state of transition from the academic laboratories, moving into the biomedical laboratories and industries. Clear areas of interest are seen in clinical diagnostics and therapeutics, where new approaches to cell handling and purification are emphasized as highly potent areas. This article outlines some of the basic unit operations of acoustophoresis, where applications as cell washing, binary separation, free-flow acoustophoresis, and affinity acoustophoresis are highlighted. The most recent steps to move acoustophoresis into clinical and preclinical applications are also presented.

Evaluation of a Serum-free Medium for the Production of rAAV-2 using HeLa Derived Producer Cells

During the last decade, recombinant AAVs have become of increasing interest for gene therapy. Clinical trials have been conducted following promising in vivo evaluations, thus leading laboratories to adapt their production systems for larger and higher quality demands. Classical transfection protocols seem difficult and cumbersome to adapt to a bioreactor scale. The use of stable producer cells appears as an attractive alternative, as this system requires only a single infection step to induce rAAV production. Furthermore, the switch to a serum-free medium is an interesting strategy to increase the biosafety level to satisfy clinical grade requirements for gene therapy products. Here, we have combined both approaches and evaluated different rAAV producer clones in a serum-free medium. We first evaluated the cell growth in a serum-free medium and then did a partial optimisation of the medium composition to obtain vector yields as close as possible to the yields obtained in a classical serum containing medium. Different helper viruses, multiplicity of infection, times of infection and harvest have been compared in small scale cultures in order to determine the optimal settings which were then transferred and evaluated in suspension cultures in spinner flasks. The yields obtained in this system were similar to or at most 2 times lower than those obtained in a serum-containing medium. The scale-up of such a production system as well as the use of high cell density perfusion culture systems will probably lead to considerably higher yields than those obtained in a classical process.

Constructive improvement of the ultrasonic separation device ADI 1015

The use of the ultrasonic separation deviceis a very important step in the direction forimproving animal cell bioreactor cultures. However,the normal construction of the ultrasonic separationdevice ADI 1015 has an inherent disadvantage inpumping the cell suspension continuously through thedevice by using a peristaltic pump. The cells aretaken out of the reactor and are transported to theside inlet located below the separation chamber of thedevice. This cycling leads to cell death and aconsiderable reduction of the viable cell density. Themodification of the configuration of the device (nocirculation of the cell suspension through theretention device; during approximately 9 minutescell-free supernatant is extracted; every 9 minute forabout one minute, the volume which is equivalent tothe interior volume of the chamber and the tubingconnecting the device to the reactor, is flushed backin order to return the retained cells back to thereactor) allows cell densities from 10(6) to2.7 x 10(6) c/ml with a viability of at least90% (tested for the shear sensitive insect cell lineHigh Five), whereas the maximal cell densitiesobtained were 0.76 x 10(6) c/ml for the periodof continuous culture and 10(5) c/ml at the end ofthe use of the device in the classical mode.

Dielectrophoretic forces can be safely used to retain viable cells in perfusion cultures of animal cells

Dielectrophoresis is a well established and effective means for the manipulation of viable cells. However, its effectiveness greatly depends upon the utilization of very low electrical conductivity media. High conductivity media, as in the case of cell culture media, result only in the induction of weaker repulsive forces (negative dielectrophoresis) and excessive medium heating. A dielectrophoresis-based cell separation device (DEP-filter) has been recently developed for perfusion cultures that successfully overcomes these obstacles and provides a very high degree of viable cell separation while most of the nonviable cells are removed from the bioreactor by the effluent stream. The latter results in high viabilities throughout the culture period and minimization of lysed cell proteases in the bioreactor. However, an important question that remains to be answered is whether we have any adverse effects by exposing the cultured cells to high frequency electric fields for extended periods of time. A special chamber was constructed to quantitate the effect under several operational conditions. Cell growth, glucose uptake, lactate and monoclonal antibody production data suggest that there is no appreciable effect and hence, operation over long periods of time of the DEP-filter should not have any adverse effect on the cultured cells.

Production of a self-activating CBM-factor X fusion protein in a stable transformed Sf9 insect cell line using high cell density perfusion culture

Factor Xa is a serine protease, whose high selectivity can be used to cleave protein tags from recombinant proteins. A fusion protein comprised of a self-activating form of factor X linked to a cellulose-binding module, saCBMFX, was produced in a stable transformed Sf9 insect cell line. The activity of the insect cell produced saCBMFX was higher than the equivalent mammalian cell produced material. A 1.5 l batch fermentation reached a maximum cell concentration of 1.6 x 10(7) cells ml(-1) and a final saCBMFX concentration of 4 mg l(-1). The production of saCBMFX by this cell line was also analyzed in a 1.5 l perfusion system using an ultrasonic filter as a cell-retention device for flow rates up to 3.5 l day(-1). The cell-retention efficiency of an air backflush mode of acoustic filter operation was greater than 95% and eliminated the need to pump the relatively shear sensitive insect cells. In the perfusion system over 4 x 10(7) Sf9 cells ml(-1) were obtained with a viability greater than 80%. With a doubling of viable cell concentration from 1.5 to 3 x 10(7) cells ml(-1) the saCBMFX production rate was doubled to 6 mg l(-1) day(-1). The saCBMFX volumetric productivity of the perfusion system was higher than the batch fermentations (0.6 mg l(-1) day(-1)) by an order of magnitude.

Mammalian cell retention devices for stirred perfusion bioreactors

Within the spectrum of current applications for cell culture technologies, efficient large-scale mammalian cell production processes are typically carried out in stirred fed-batch or perfusion bioreactors. The specific aspects of each individual process that can be considered when determining the method of choice are presented. A major challenge for perfusion reactor design and operation is the reliability of the cell retention device. Current retention systems include cross-flow membrane filters, spin-filters, inclined settlers, continuous centrifuges and ultrasonic separators. The relative merits and limitations of these technologies for cell retention and their suitability for large-scale perfusion are discussed.

Supermacroporous polymer-based cryogel bioreactor for monoclonal antibody production in continuous culture using hybridoma cells

Cryogel matrices composed of different polymeric blends were synthesized, yielding a unique combination of hydrophilicity and hydrophobicity with the presence or absence of charged surface. Four such cryogel matrices composed of polyacrylamide-chitosan (PAAC), poly(N-isopropylacrylamide)-chitosan, polyacrylonitrile (PAN), and poly(N-isopropylacrylamide) were tested for growth of different hybridoma cell lines and production of antibody in static culture. All the matrices were capable for the adherence of hybridoma cell lines 6A4D7, B7B10, and H9E10 to the polymeric surfaces as well as for the efficient monoclonal antibody (mAb) production. PAAC proved to be relatively better in terms of both mAb production and cell growth. Further, PAAC cryogel was designed into three different formats, monolith, disks, and beads, and used as packing material for packed-bed bioreactor. Long-term cultivation of 6A4D7 cell line on PAAC cryogel scaffold in all the three formats could be successfully done for a period of 6 weeks under static conditions. Continuous packed-bed bioreactor was setup using 6A4D7 hybridoma cell line in the three reactor formats. The reactors ran continuously for a period of 60 days during which mAb production and metabolism of cells in the bioreactors were monitored periodically. The monolith bioreactor performed most efficiently over a period of 60 days and produced a total of 57.5 mg of antibody in the first 30 days (in 500 mL) with a highest concentration of 115 μg mL(-1) , which is fourfold higher than t-flask culture. The results demonstrate that appropriate chemistry and geometry of the bioreactor matrix for cell growth and immobilization can enhance the reactor productivity.

Induction of apoptosis in nutrient-deprived cultures of hybridoma and myeloma cells

In the present study, cell death was investigated in cultures of NS/0 myelomas and SP2/0-derived D5 hybridomas through morphological examination of cells stained with acridine orange and ethidium bromide. The relative contribution of elevated levels of lactic acid and ammonia, as well as deprivation of glutamine, cystine, and glucose on the induction of necrosis or apoptosis, was investigated. In batch culture of D5 hybridoma cells, induction of apoptotic cell death correlated with the exhaustion of glutamine, while in the case of NS/0 myelomas, it coincided with exhaustion of cystine. To determine whether limiting nutrients were the actual triggering factors for apoptosis in batch culture, exponentially growing cells were resuspended in glutamine or cystine-free media. Within 30 to 40 h, viability decreased to 50% and the nonviable cell population displayed typical apoptotic morphology, with crescents of condensed chromatin around the periphery of the nucleus, or with the entire nucleus present as one or a group of featureless, brightly staining spherical beads. Similarly, D5 hybridomas and NS/0 myelomas cultivated in glucose-free medium died mainly from apoptosis. Cells were also cultivated in fresh medium supplemented with elevated concentrations of ammonia (3.0 mM) and/or lactate (35 mM, 50 mM). This resulted in decreased viabilities and necrotic death in both cell lines. From these results, we conclude that D5 hybridomas and NS/0 myelomas deprived of essential nutrients die by apoptosis, whereas incubation in the presence of elevated levels of metabolic byproducts such as ammonia and lactate will induce necrotic cell death in these cells.

Effect of Feed and Bleed Rate on Hybridoma Cells in an Acoustic Perfusion Bioreactor: Metabolic Analysis

For the development of optimal perfusion processes, insight into the effect of feed and bleed rate on cell growth, productivity, and metabolism is essential. In the here presented study the effect of the feed and bleed rate on cell metabolism was investigated using metabolic flux analysis. Under all tested feed and bleed rates the biomass concentration as calculated from the nitrogen balance (biomass-nitrogen) increased linearly with an increase in feed rate, as would be expected. However, depending on the size of the feed and bleed rate, this increase was attained in two different ways. At low feed and bleed rates (Region I) the increase was obtained through an increase in viable-cell concentration, while the cellular-nitrogen content remained constant. At high feed and bleed rates (Region II) the increase was attained through an increase in cellular-nitrogen content, while the cell concentration remained constant. Per gram biomass-nitrogen, the specific consumption and production rates of the majority of the nutrients and products were identical in both regions, as were most of the fluxes. The major difference between the two regions was an increased flux from pyruvate to lactate and a decreased flux of pyruvate toward citrate in region II. The decreased in-flux at the level of citrate can either be balanced by a decreased out-flux toward lipid biosynthesis leading to a lower fraction of lipids in the cell, by a decreased out-flux toward the citric acid cycle resulting in a decreased energy generation, or by a combination of these. Finally, the specific productivity increases less than the nitrogen content per cell in region II, which implies that for obtaining maximum production rates it is important to increase the cell density and not only the biomass density.

Ultrasound, cavitation bubbles and their interaction with cells

This article reviews the basic physics of ultrasound generation, acoustic field, and both inertial and non-inertial acoustic cavitation in the context of localized gene and drug delivery as well as non-linear oscillation of an encapsulated microbubble and its associated microstreaming and radiation force generated by ultrasound. The ultrasound thermal and mechanical bioeffects and relevant safety issues for in vivo applications are also discussed.

Chip integrated strategies for acoustic separation and manipulation of cells and particles

Acoustic standing wave technology combined with microtechnology opens up new areas for the development of advanced particle and cell separating microfluidic systems. This tutorial review outlines the fundamental work performed on continuous flow acoustic standing wave separation of particles in macro scale systems. The transition to the microchip format is further surveyed, where both fabrication and design issues are discussed. The acoustic technology offers attractive features, such as reasonable throughput and ability to separate particles in a size domain of about tenths of micrometers to tens of micrometers. Examples of different particle separation modes enabled in microfluidic chips, utilizing standing wave technology, are described along a discussion of several potential applications in life science research and in the medical clinic. Chip integrated acoustic standing wave separation technology is still in its infancy and it can be anticipated that new laboratory standards very well may emerge from the current research.

Retroviral vector production using suspension-adapted 293GPG cells in a 3L acoustic filter-based perfusion bioreactor

Recombinant retroviruses are now an established tool for gene delivery. Presently they are mainly produced using adherent cells. However, due to the restrictive nature of adherent cell culture, this mode of production is hampered by low cell-specific productivity and small production units. The large-scale production of retroviral vectors could benefit from the adaptation of retrovirus packaging cell lines to suspension culture. Here, we describe the ability of a 293 packaging cell line to produce retroviral vectors in suspension culture at high titer. Adherent 293GPG cells, producing a Moloney Murine Leukemia Virus (MoMLV) retrovirus vector pseudotyped with the vesicular stomatitis virus G (VSVG) envelope protein and expressing a TK-GFP fusion protein, were adapted to suspension culture in calcium-free DMEM. At a cell density similar to adherent cell culture, the suspension culture produced retroviral vector consistently in the range of 1 x 10(7) infectious viral particles/mL (IVP/mL), with a specific productivity threefold higher than adherent culture. Furthermore, at the same medium replacement frequency, the suspension producer cells could be cultured at higher density than their adherent counterparts, which resulted in virus titer of 3-4 x 10(7) IVP/mL at 11.0 x 10(6) cells/mL. This corresponds to a 10-fold increase in viral concentration compared to adherent cells. The capacity to up scale the retroviral vector production was also demonstrated by performing a 2 VVD perfusion culture for 9 days in a 3L Chemap bioreactor. The combination of suspension and perfusion led to a 20-fold increase in maximum virus productivity compared to the adherent culture.

Towards industrial application of quasi real-time metabolic flux analysis for mammalian cell culture

Cellular physiology and metabolism were monitored using a quasi real-time combination of on-line and off-line data to estimate metabolic fluxes in an established bioreaction network. The utility of this approach towards optimizing bioreactor operation was demonstrated for CHO cells cultivated in 15 L perfusion reactors at 20 x 10(6) cells/mL. Medium composition and dilution rates were changed to obtain several steady states with varying glucose and glutamine concentrations. When cells were restored to initial culture medium and perfusion rate conditions after being exposed to lower glucose and glutamine concentrations, the pyruvate flux into the TCA cycle was increased 30% while the pyruvate flux through lactate was decreased 30%, suggesting steady-state multiplicity. By appropriately altering cellular metabolism, perfusion bioreactors can operate at lower perfusion rates without significant accumulation of inhibitory metabolites such as lactate. Changes in glucose, lactate and glutamine uptake/production rates had significant effects on the calculation of other fluxes in the network. Sensitivity analysis of these key metabolic fluxes highlighted the need for accurate and reliable real-time sensors. Overall, rapid observation of metabolic fluxes can be a valuable tool for bioprocess development, monitoring and control. The framework presented in this study offers a convenient means for quasi real-time estimation of metabolic fluxes and represents a step towards realizing the potential of metabolic flux analysis for accelerated bioprocess optimization.

Stable hybridoma cultivation in a pilot-scale acoustic perfusion system: long-term process performance and effect of recirculation rate

Perfusion systems have the possibility to be operated continuously for several months. It is important that the performance of the cell retention device does not limit the operation time of a perfusion process used in the production of active pharmaceutical ingredients. Therefore, the aim of this study was to investigate the reliability and long-term stability of an acoustic perfusion process using the 200 L/d BioSep. As the BioSep is an external device, it is possible that dependent on the recirculation rate nutrient gradients occur in the external loop, which could affect the cell metabolism. Therefore, the effect of possible nutrient gradients on cell metabolism, viability and productivity was studied by varying the recirculation rate. In this study, it is shown that a perfusion process using a pilot-scale acoustic cell-retention device (200 L/d) is reliable and simple to operate, resulting in a stable 75-day cultivation of a hybridoma cell line producing a monoclonal antibody. The recirculation rate had a significant effect on the oxygen concentration in the external loop, with oxygen being depleted within the cell-retention device at recirculation rates below 6 m3/m(reactor)3.d (=600 L/d). The oxygen depletion at low circulation rates correlated with a slightly increased lactate production rate. For all other parameters no effect of the recirculation rate was observed, including cell death measured through the release of lactate dehydrogenase and specific productivity. A maximum specific productivity of 12 pg/cell.d was reached.

Selective retension of active cells employing low centrifugal force at the medium change during suspension culture of Chinese hamster ovary cells producing tPA

The effect of centrifugal force applied for cell separation at the medium change on the growth, metabolism and tissue plasminogen activator (tPA) productivity of Chinese hamster ovary (CHO) cells suspension culture was investigated. The viability of the precipitated cells increased exponentially as the centrifugal force decreased. However, the cell recovery was lower than 91% when centrifugal forces applied for 5 min was less than 67 x g. In cultures incubated for 474 h with 7 medium changes employing centrifugal forces ranging from 67 to 364 x g, a centrifugal force lower than 119 x g resulted in higher specific rates of growth, glucose consumption, and lactate and tPA production during the whole culture period. On the other hand, daily centrifugation at 67 to 537 x g without discarding the supernatant had no effect on the specific rates. The cultures inoculated with cells precipitated at a centrifugal force of 67 x g showed apparently higher specific rates of metabolism compared to those inoculated with cells in the supernatant. The cells in the supernatant and the precipitate obtained following centrifugation at 67 x g have average diameters of 15.5 and 17.4 microm, respectively. The intracellular contents of amino acids, especially nonessential amino acids, of the precipitated cells were markedly higher than those of the cells in the supernatant. These results indicate that large cells with high amino acid content and metabolic activity were selectively retained in the culture by means of centrifugation at low forces such as 67 x g. Consequently, application of a low centrifugal force is recommended for medium change in order to maintain higher specific productivity of suspended mammalian cells in perfusion culture.

Characterization and optimization of acoustic filter performance by experimental design methodology

Acoustic cell filters operate at high separation efficiencies with minimal fouling and have provided a practical alternative for up to 200 L/d perfusion cultures. However, the operation of cell retention systems depends on several settings that should be adjusted depending on the cell concentration and perfusion rate. The impact of operating variables on the separation efficiency performance of a 10-L acoustic separator was characterized using a factorial design of experiments. For the recirculation mode of separator operation, bioreactor cell concentration, perfusion rate, power input, stop time and recirculation ratio were studied using a fractional factorial 2(5-1) design, augmented with axial and center point runs. One complete replicate of the experiment was carried out, consisting of 32 more runs, at 8 runs per day. Separation efficiency was the primary response and it was fitted by a second-order model using restricted maximum likelihood estimation. By backward elimination, the model equation for both experiments was reduced to 14 significant terms. The response surface model for the separation efficiency was tested using additional independent data to check the accuracy of its predictions, to explore robust operation ranges and to optimize separator performance. A recirculation ratio of 1.5 and a stop time of 2 s improved the separator performance over a wide range of separator operation. At power input of 5 W the broad range of robust high SE performance (95% or higher) was raised to over 8 L/d. The reproducible model testing results over a total period of 3 months illustrate both the stable separator performance and the applicability of the model developed to long-term perfusion cultures.

A new immobilisation method to arrange particles in a gel matrix by ultrasound standing waves

Ultrasonic forces may be used to manipulate particles in suspension. For example, a standing wave ultrasound (US) field applied to a suspension moves the particles toward areas of minimal acoustic pressure, where they are orderly retained creating a predictable heterogeneous distribution. This principle of ultrasonic retention of particles or cells has been applied in numerous biotechnological applications, such as mammalian cell filtering and red blood cell sedimentation. Here, a new US-based cell immobilisation technique is described that allows manipulation and positioning of cells/particles within various nontoxic gel matrices before polymerisation. Specifically, gel immobilisation was used to directly demonstrate that the viability of yeast cells arranged by an US standing wave is maintained up to 4 days after treatment. The versatility of this immobilisation method was validated using a wide range of acoustic devices. Finally, the potential biotechnological advantages of this US-controlled particle positioning method combined with gel immobilisation/encapsulation technology are discussed.

Real time in situ microscopy for animal cell-concentration monitoring during high density culture in bioreactor

An in situ microscope (ISM) device is utilised in this study to monitor hybridoma cells concentration in a stirred bioreactor. It generates images by using pulsed illumination of the liquid broth synchronised with the camera frame generation to avoid blur from the cell's motion. An appropriate image processing isolates the sharp objects from the blurred ones that are far from the focal plane. As image processing involves several parameters, this paper focuses on the robustness of the results of the cells counting. This stage determines the applicability of the measuring device and has seldom been tackled in the presentations of ISM devices. Calibration is secondly performed for assessing the cell-concentration from the cell automated numeration provided by the ISM. Flow cytometry and hemacytometer chamber were used as reference analytical methods. These measures and the output of the image processing allow estimating a single calibration parameter: the reference volume per image equal to 1.08 x 10(-6) mL. In these conditions, the correlation coefficient between both reference and ISM data sets becomes equal to 0.99. A saturation of this system during an ultrasonic wave perfusion phase that deeply changes the culture conditions is observed and discussed. Principal component analysis (PCA) is used to undergo the robustness study and the ISM calibration step.

Acoustic cell filter: a proven cell retention technology for perfusion of animal cell cultures

This article is a review highlighting the application of the acoustic filter as a reliable cell retention device during the long-term perfusion of animal cell cultures. Critical operating parameters such as duty cycle, perfusion and re-circulation flow rates, acoustic power and backflush frequency are discussed with regard to influence on the separation efficiency and optimal operating ranges have been identified. Perfusion data gathered from the literature have been complemented with original data from a series of perfusion experiments carried out in the context of industrial projects for industrially relevant cell lines including NS0, HEK-293, SP2-derived hybridoma and insect cells in different serum-supplemented and serum-free media at different perfusion rates and acoustic chamber volumes. Finally, scale-up potential of the acoustic filter for large-scale industrial applications is discussed.

Effect of feed and bleed rate on hybridoma cells in an acoustic perfusion bioreactor: Part I. Cell density, viability, and cell-cycle distribution

For the development of optimal perfusion processes the effect of the feed and bleed rate on cell growth in a perfusion bioreactor was studied. The viable-cell density, viability, growth, death, and lysis rate and cell-cycle distribution of a hybridoma cell line producing an IgG1 were studied over a range of specific feed and bleed rates. It was found that the feed and bleed rates applied in the different cultures could be divided into two regions based on the viable-cell density and cell-cycle distribution. The cultures in the first region, low feed rates (0.5 and 1.0 d(-1)) combined with low bleed rates (0.05 and 0.10 d(-1)), were nutrient-limited, as an increase in the feed rate resulted in an increase in the viable-cell density. The cultures in the second region, high feed and bleed rates, were nonnutrient-limited. In this region the viable-cell density decreased more or less linearly with an increase in the bleed rate and was independent of the feed rate. This suggests that the cells were limited by a cell-related factor. Comparison of Trypan-blue dye-exclusion measurements and lactate-dehydrogenase activity measurements revealed that cell lysis was not negligible in this bioreactor set-up. Therefore, lactate-dehydrogenase activity measurements were essential to measure the death rate accurately. The specific growth rate was nearly constant for all tested conditions. The viability increased with an increase of the bleed rate and was independent of the feed rate. Furthermore, the specific productivity of monoclonal antibody was constant under all tested conditions. For the optimal design of a perfusion process it should first be established whether viability is an important parameter. If not, a bleed rate as low as possible should be chosen. If low viabilities are to be avoided, the bleed rate chosen should be higher, with the value depending on the desired viability. Next, the feed rate should be set at such a rate that the cells are just in the nonnutrient-limited region.

Optimization of reovirus production from mouse L-929 cells in suspension culture

Reovirus serotype 3 Dearing (T3D) has shown potential as a novel cancer therapy. To support the increasing demand for reovirus, a two-stage perfusion mode scheme is proposed for cell growth and reovirus production. Mouse L-929 cells were used as the host for reovirus infection due to their ability to grow well in suspension culture. Several L-929 cell growth and reovirus infection characteristics were investigated and optimized in spinner flask batch cultures. For the growth of L-929 cells, a balanced nutrient-fortification of SMEM medium increased the maximum cell density by 30%, compared to normal SMEM; however, ammonia and lactate accumulations were found to inhibit further cell growth. For the production of reovirus, approximately 90% increase in viral yield resulted when the infection temperature was reduced from 37 to 33 degrees C. Infectious reovirus particles were shown to be stable in conditioned medium at 37 and 33 degrees C. The final virus titer was dependent on the multiplicity of infection (MOI) and the host cell density at the time of infection. A combination of an MOI of 0.1 pfu/cell and an initial host cell density of 1.0 x 10(6) cells/mL in fortified medium resulted in a maximum virus titer of (4.59 +/- 0.16) x 10(9) pfu/mL and a specific yield of (2.34 +/- 0.08) x 10(3) pfu/cell. At an optimal harvest time of the infection process, 99% of the virus was associated with the cellular debris. Finally, the presence of 5.0 mM ammonia in the culture medium was shown to seriously inhibit the reovirus yield, whereas lactate concentrations up to 20 mM had no effect.

Insights into adenoviral vector production kinetics in acoustic filter-based perfusion cultures

One of the major limitations in the production of adenoviral vectors is the reduction in cell-specific productivity observed for increasing cell density at infection in batch cultures. This observation strongly suggests some nutrient depletion and/or metabolite inhibition in the media. These limitations have been partially overcome through other feeding strategies, such as fed-batch and sequential batch operations. To improve these results, we evaluated perfusion as a strategy to increase the volumetric productivity of HEK-293 cell cultures, by allowing productive infection at higher cell densities. An acoustic cell separator was employed in consideration of the increased shear sensitivity of the cells during the infection phase. The effects of perfusion rate and cell density at infection on the production of a recombinant adenovirus expressing the GFP were investigated. The perfusion mode allowed successful infection at cell densities in the range of 2.4-3 x 10(6) cell/mL, while maintaining a similar cell specific productivity (17,900 +/- 2400 VP/cell) to that of a batch infected at a low cell density (5 x 10(5) cell/mL). The highest virus concentrations (4.1 +/- 0.6 x 10(10) VP/mL) were attained for a feed rate of 2 vol/d and constituted a fivefold increase compared to a batch with medium replacement. Rapid assessment of the infection status was achieved through the use of on-line monitoring of respiration, fluorescence, and biovolume. Analysis of the kinetics of nutrient consumption and metabolite production revealed that a reduction in specific productivity is correlated with reduced metabolic activity.

Quantification of a novel h-shaped ultrasonic resonator for separation of biomaterials under terrestrial gravity and microgravity conditions

A novel, h-shaped ultrasonic resonator was used to separate biological particulates. The effectiveness of the resonator was demonstrated using suspensions of the cyanobacterium, Spirulina platensis. The key advantages of this approach were improved acoustic field homogeneity, flow characteristics, and overall separation efficiency (sigma = 1 - ratio of concentration in cleared phase to input), monitored using a turbidity sensor. The novel separation concept was also effective under microgravity conditions; gravitational forces influenced overall efficiency. Separation of Spirulina at cleared flow rates of 14 to 58 L/day, as assessed by remote video recording, was evaluated under both microgravity (</=0.05 g) and terrestrial gravity conditions. The latter involved a comparison with 5- and 24-microm-diameter polystyrene microspheres. Influences of gravity on sigma were evaluated by varying the relative inclination angle (within a range of 120 degrees ) between the resonator and the gravitational vector. Cells of Spirulina behaved in a manner comparable to that of the 5-microm-diameter polystyrene microspheres, with a significant decrease in mean (+/-SE, n = 3) sigma from 0.97 +/- 0.03 and 0.91 +/- 0.02 at a flow rate of 14 L/day, to corresponding values of 0.53 +/- 0.05 and 0.57 +/- 0.03 (P < 0.05) at 58 L/day, respectively. During a typical microgravity period of ca. 22 s, achieved during the 29th ESA Parabolic Flight Campaign, sigma was unchanged at a flow rate of 14 L/day, compared with terrestrial gravity conditions; with increased flow rates, sigma was significantly reduced. Overall, these results demonstrate that, for optimum resonator performance under the relatively short microgravity period utilized in this study, flow rates of ca. 14 L/day were preferred. These data provide a baseline for exploiting noninvasive, compact, ultrasonic separation systems for manipulating biological particulates under microgravity conditions.

Scale-up and optimization of an acoustic filter for 200 L/day perfusion of a CHO cell culture

Acoustic cell retention devices have provided a practical alternative for up to 50 L/day perfusion cultures but further scale-up has been limited. A novel temperature-controlled and larger-scale acoustic separator was evaluated at up to 400 L/day for a 10(7) CHO cell/mL perfusion culture using a 100-L bioreactor that produced up to 34 g/day recombinant protein. The increased active volume of this scaled-up separator was divided into four parallel compartments for improved fluid dynamics. Operational settings of the acoustic separator were optimized and the limits of robust operations explored. The performance was not influenced over wide ranges of duty cycle stop and run times. The maximum performance of 96% separation efficiency at 200 L/day was obtained by setting the separator temperature to 35.1 degrees C, the recirculation rate to three times the harvest rate, and the power to 90 W. While there was no detectable effect on culture viability, viable cells were selectively retained, especially at 50 L/day, where there was a 5-fold higher nonviable washout efficiency. Overall, the new temperature-controlled and scaled-up separator design performed reliably in a way similar to smaller-scale acoustic separators. These results provide strong support for the feasibility of much greater scale-up of acoustic separations.