Batch and semicontinuous aggregation and sedimentation of hybridoma cells by acoustic resonance fields

Process intensification for the continuous production of an antimicrobial peptide in stably-transformed Sf-9 insect cells

The antibiotic resistance crisis has prompted research into alternative candidates such as antimicrobial peptides (AMPs). However, the demand for such molecules can only be met by continuous production processes, which achieve high product yields and offer compatibility with the Quality-by-Design initiative by implementing process analytical technologies such as turbidimetry and dielectric spectroscopy. We developed batch and perfusion processes at the 2-L scale for the production of BR033, a cecropin-like AMP from Lucilia sericata, in stably-transformed polyclonal Sf-9 cells. This is the first time that BR033 has been expressed as a recombinant peptide. Process analytical technology facilitated the online monitoring and control of cell growth, viability and concentration. The perfusion process increased productivity by ~ 180% compared to the batch process and achieved a viable cell concentration of 1.1 × 10⁷ cells/mL. Acoustic separation enabled the consistent retention of 98.5–100% of the cells, viability was > 90.5%. The recombinant AMP was recovered from the culture broth by immobilized metal affinity chromatography and gel filtration and was able to inhibit the growth of Escherichia coli K12. These results demonstrate a successful, integrated approach for the development and intensification of a process from cloning to activity testing for the production of new biopharmaceutical candidates.

Hydrocyclones as cell retention devices for an N-1 perfusion bioreactor linked to a continuous-flow stirred tank production bioreactor

A continuous Chinese hamster ovary (CHO) cell culture process comprised of a highly proliferative N-1 perfusion bioreactor utilizing a hydrocyclone as a cell retention device linked to a production continuous-flow stirred tank reactor (CSTR) is presented. The overflow stream from the hydrocyclone, which is only partially depleted of cells, provides a continuous source of high viability cells from the N-1 perfusion bioreactor to the 5-20 times larger CSTR. Under steady-state conditions, this linked-bioreactor system achieved a peak volumetric productivity of 0.96 g/L/day, twofold higher than the optimized fed-batch process. The linked bioreactor system using a hydrocyclone was also shown to be 1.8-3.1 times more productive than a dual, cascading CSTR system without cell retention.

Cell Adhesion, Morphology, and Metabolism Variation via Acoustic Exposure within Microfluidic Cell Handling Systems

Acoustic fields are capable of manipulating biological samples contained within the enclosed and highly controlled environment of a microfluidic chip in a versatile manner. The use of acoustic streaming to alter fluid flows and radiation forces to control cell locations has important clinical and life science applications. While there have been significant advances in the fundamental implementation of these acoustic mechanisms, there is a considerable lack of understanding of the associated biological effects on cells. Typically a single, simple viability assay is used to demonstrate a high proportion of living cells. However, the findings of this study demonstrate that acoustic exposure can inhibit cell attachment, decrease cell spreading, and most intriguingly increase cellular metabolic activity, all without any impact upon viability rates. This has important implications by showing that mortality studies alone are inadequate for the assessment of biocompatibility, but further demonstrates that physical manipulation of cells can also be used to influence their biological activity.

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.

Acoustophoretic separation of infected erythrocytes from blood plasma in a microfluidic platform using biofunctionalized, matched-impedance layers

Acoustophoresis is rapidly gaining prominence in the field of cell manipulation. In recent years, researchers have extensively used this method for separating different types of cells from the bulk fluid. In this paper, we propose a novel acoustophoresis-based technique to capture infected or abnormal erythrocytes from blood plasma. A typical acoustic device consisting of a transducer assembly, microfluidic cavity, and a reflector is considered. Based on the concept of impedance matching, a pair of antibody-coated polystyrene layers is placed in the nodal regions of an acoustic field within the cavity. This technique allows bi-directional migration of the suspended cells to the biofunctionalized surfaces. Therefore, simultaneous capture of infected erythrocytes on both the layers is feasible. Finite element method is used to model the pressure field as well as the motion of erythrocytes under the influence of acoustic radiation, drag, and gravitational forces. A parametric analysis is done by varying the excitation frequency, driving voltage, and the thickness of the polystyrene layers. The resulting changes in the pressure amplitude and field pattern are investigated. The erythrocyte collection efficiency, rate of collection, and the cell distribution on the layer surfaces are also determined under different field conditions. The occurrence of transient cavitation in the blood plasma-filled cavity at the chosen frequency is taken into account by using its threshold pressure value as the limiting factor of pressure amplitude. The study provides an insight into the phenomenon and serves as a guideline to fabricate low-cost, multifunctional rapid diagnostic devices based on acoustophoretic separation.

Manipulation of magnetic nanorod clusters in liquid by non-uniform alternating magnetic fields

It is discovered that a non-uniform alternating magnetic field can induce a translational motion of an anisotropic magnetic particle or cluster near a surface. Unlike a permanent magnet pulling a magnetic particle, the particle moves away from the magnetic source with a periodic fluctuation in its trajectory that varies with a frequency that is twice that of the field frequency. The moving speed can be tuned by varying the magnetic field strength and gradient, its alternating frequency, and the particle size. A hydrodynamic model is developed that can qualitatively explain all of the phenomena observed. Such a simple particle manipulation method has a great potential in applications such as cell biology and microfluidics.

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.

Separation of suspensions and emulsions via ultrasonic standing waves - a review

Ultrasonic standing waves (USW) separation is an established technology for micro scale applications due to the excellent control to manipulate particles acoustically achieved when combining high frequency ultrasound with laminar flow in microchannels, allowing the development of numerous applications. Larger scale systems (pilot to industrial) are emerging; however, scaling up such processes are technologically very challenging. This paper reviews the physical principles that govern acoustic particle/droplet separation and the mathematical modeling techniques developed to understand, predict, and design acoustic separation processes. A further focus in this review is on acoustic streaming, which represents one of the major challenges in scaling up USW separation processes. The manuscript concludes by providing a brief overview of the state of the art of the technology applied in large scale systems with potential applications in the dairy and oil industries.

Acoustofluidics 23: acoustic manipulation combined with other force fields

In this, the final paper of the Acoustofluidics series of tutorial articles, we discuss applications in which acoustic radiation forces are used in conjunction with competing or complementary force-fields. This may be in order to enable manipulation operations that would not be easily performed by either force-field alone, or may be used to effect separation based on the different physical principals underlying competing fields. Examples are given of a number of different applications in which acoustic forces are combined with gravitational fields, hydrodynamic forces, electric fields (including dielectrophoresis), magnetic forces and optical forces.

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, ).

High density continuous production of murine pluripotent cells in an acoustic perfused bioreactor at different oxygen concentrations

Strategies for the production of pluripotent stem cells (PSCs) rely on serially dissociated adherent or aggregate-based culture, consequently limiting robust scale-up of cell production, on-line control and optimization of culture conditions. We recently developed a method that enables continuous (non-serially dissociated) suspension culture-mediated reprogramming to pluripotency. Herein, we use this method to demonstrate the scalable production of PSCs and early derivatives using acoustic filter technology to enable continuous oxygen-controlled perfusion culture. Cell densities of greater than 1 × 10⁷  cells/mL were achieved after 7 days of expansion at a specific growth rate (µ) of 0.61 ± 0.1 day⁻¹ with a perfusion rate (D) of 5.0 day⁻¹. A twofold increase in maximum cell density (to greater than 2.5 × 10⁷  cells/mL) was achieved when the medium dissolved oxygen was reduced (5% DO). Cell densities and viabilities >80% were maintained for extended production periods during which maintenance of pluripotency was confirmed by stable expression of pluripotency factors (SSEA-1 and Nanog), as well as the capacity to differentiate into all three germ layers. This work establishes a versatile biotechnological platform for the production of pluripotent cells and derivatives in an integrated, scalable and intensified stirred suspension culture.

Microfluidic, label-free enrichment of prostate cancer cells in blood based on acoustophoresis

Circulating tumor cells (CTC) are shed in peripheral blood at advanced metastatic stages of solid cancers. Surface-marker-based detection of CTC predicts recurrence and survival in colorectal, breast, and prostate cancer. However, scarcity and variation in size, morphology, expression profile, and antigen exposure impairs reliable detection and characterization of CTC. We have developed a noncontact, label-free microfluidic acoustophoresis method to separate prostate cancer cells from white blood cells (WBC) through forces generated by ultrasonic resonances in microfluidic channels. Implementation of cell prealignment in a temperature-stabilized (±0.5 °C) acoustophoresis microchannel dramatically enhanced the discriminatory capacity and enabled the separation of 5 μm microspheres from 7 μm microspheres with 99% purity. Next, we determined the feasibility of employing label-free microfluidic acoustophoresis to discriminate and divert tumor cells from WBCs using erythrocyte-lysed blood from healthy volunteers spiked with tumor cells from three prostate cancer cell-lines (DU145, PC3, LNCaP). For cells fixed with paraformaldehyde, cancer cell recovery ranged from 93.6% to 97.9% with purity ranging from 97.4% to 98.4%. There was no detectable loss of cell viability or cell proliferation subsequent to the exposure of viable tumor cells to acoustophoresis. For nonfixed, viable cells, tumor cell recovery ranged from 72.5% to 93.9% with purity ranging from 79.6% to 99.7%. These data contribute proof-in-principle that label-free microfluidic acoustophoresis can be used to enrich both viable and fixed cancer cells from WBCs with very high recovery and purity.

Acoustofluidics 12: Biocompatibility and cell viability in microfluidic acoustic resonators

Manipulation of biological cells by acoustic radiation forces is often motivated by its improved biocompatibility relative to alternative available methods. On the other hand, it is well known that acoustic exposure is capable of causing damage to tissue or cells, primarily due to heating or cavitation effects. Therefore, it is important to define safety guidelines for the design and operation of the utilized devices. This tutorial discusses the biocompatibility of devices designed for acoustic manipulation of mammalian cells, and different methods for quantifying the cell viability in such devices.

Acoustofluidics 9: Modelling and applications of planar resonant devices for acoustic particle manipulation

This article introduces the design, construction and applications of planar resonant devices for particle and cell manipulation. These systems rely on the pistonic action of a piezoelectric layer to generate a one dimensional axial variation in acoustic pressure through a system of acoustically tuned layers. The resulting acoustic standing wave is dominated by planar variations in pressure causing particles to migrate to planar pressure nodes (or antinodes depending on particle and fluid properties). The consequences of lateral variations in the fields are discussed, and rules for designing resonators with high energy density within the appropriate layer for a given drive voltage presented.

Acoustofluidics 8: applications of acoustophoresis in continuous flow microsystems

This acoustofluidics tutorial focuses on continuous flow-based half wavelength resonator systems operated in the transversal mode, where the direction of the primary acoustic force acts in plane with the microchip. The transversal actuation mode facilitates integration with up- and downstream microchannel networks as well as visual control of the acoustic focusing experiment. Applications of particle enrichment in an acoustic half wavelength resonator are discussed as well as clarification of the carrier fluid from undesired particles. Binary separation of particle/vesicle/cell mixtures into two subpopulations is outlined based on the different polarities of the acoustic contrast factor. Furthermore, continuous flow separation of different particle/cell types is described where both Free Flow Acoustophoresis (FFA) and binary acoustophoresis are utilized. By capitalizing on the laminar flow regime, acoustophoresis has proven especially successful in performing bead/cell translations between different buffer systems. Likewise, the ability to controllably translate particulate matter across streamlines has opened a route to valving of cells/particles without any moving parts, where event triggered cell sorting is becoming an increasing area of activity. Recent developments now also enable measurements of fundamental cell properties such as density and compressibility by means of acoustophoresis. General aspects on working with live cells in acoustophoresis systems are discussed as well as available means to quantify the outcome of cell and particle separation experiments performed by acoustophoresis.

Acoustophoretic microfluidic chip for sequential elution of surface bound molecules from beads or cells

An acoustophoresis-based microfluidic flow-chip is presented as a novel platform to facilitate analysis of proteins and peptides loosely bound to the surface of beads or cells. The chip allows for direct removal of the background surrounding the beads or cells, followed by sequential treatment and collection of a sequence of up to five different buffer conditions. During this treatment, the beads/cells are retained in a single flow by acoustic radiation force. Eluted peptides are collected from the outlets and subsequently purified by miniaturized solid-phase extraction and analyzed with matrix assisted laser desorption mass spectrometry. Fundamental parameters such as the system fluidics and dispersion are presented. The device was successfully applied for wash and sequential elution of peptides bound to the surface of microbeads and human spermatozoa, respectively.

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.

Chip-based size-selective sorting of biological cells using high frequency acoustic excitation

This work presents the size-selective sorting of single biological cells using the assembly process known as templated assembly by selective removal (TASR). We have demonstrated experimentally, for the first time, the selective placement and sorting of single SF9 cells (clonal isolate derived from Spodoptera frugiperda (Fall Armyworm) IPLB-Sf21-AE cells) into patterned hemispherical sites on rigid assembly templates using TASR. Nearly 100% of the assembly sites on the template were filled with matching cells (with assembly density as high as 900 sites per mm(2)) within short time spans of 3 minutes. 3-D reconstruction of cell profiles and volume analysis of cells trapped inside assembly sites demonstrates that only those cells that match the assembly site precisely (within 0.5 μm) in size are assembled on the template. The assembly conditions are also compatible with the extension of TASR to mammalian cells. TASR-based size-selective structuring and sorting of biological systems represents a valuable tool with potential for implementation in biological applications such as cell sorting for medical research or diagnostics, templating for artificial tissue replication, or isolation of single cells for the study of biological or mechanical behavior.

Ultrasonic trapping of small particles by a vibrating rod

In this paper, the ultrasonic trapping of small particles by a vibrating rod is proposed and analyzed. An aluminum rod, which is driven by an actuator and operates in the 0 th order vibration mode (back and forth vibration mode), can trap small particles in water. The experimental phenomena and the trapping mechanism are theoretically analyzed, and the acoustic radiation force acting on a spherical particle near the surface of the vibrating rod is estimated. The effects of operating frequency, rod radius, particle radius and other physical parameters are investigated experimentally and theoretically, and useful guidelines to optimize the trapping capability are proposed.

Acoustophoresis in wet-etched glass chips

Acoustophoresis in microfluidic structures has primarily been reported in silicon microfabricated devices. This paper demonstrates, for the first time, acoustophoresis performed in isotropically etched glass chips providing a performance that matches that of the corresponding silicon microdevices. The resonance mode characteristics of the glass chip were equal to those of the silicon chip at its fundamental resonance. At higher order resonance modes the glass chip displays resonances at lower frequencies than the silicon chip. The cross-sectional profiles of acoustically focused particle streams are also reported for the first time, displaying particles confined in a vertical band in the channel center for both glass and silicon chips. A particle extraction efficiency of 98% at flow rates up to 200 microL/min (2% particle concentration) is reported for the glass chip at the fundamental resonance. The glass and silicon chips displayed equal particle extraction performance when tested for increasing particle concentrations of 2-15%, at a flow velocity of 12.9 cm/s for the glass chip and 14.8 cm/s for the silicon chip.

An on-line method for the reduction of fouling of spin-filters for animal cell perfusion cultures

The main limitation in the use of spin-filters during perfusion cultures of animal cells was revealed to be filter fouling. This phenomenon involves cell-sieve interactions as well as cell attachment to, and growth on, the filter surface. The cell attachment effect has been analysed in the present study during long-term perfusion simulations with CHO animal cells. It was demonstrated that at low filter acceleration, below 6.2 m/s2, a high perfusion rate of 25 cm/h induced rapid filter pore clogging within 3 days, whereas increasing the filter acceleration to 25 m/s2 increased filter longevity from 3 to 25 days, for filters with a pore size of 8.5 microm. Increasing the filter pore size to 14.5 microm improved filter longevity by 84% with less viable and dead cell deposits on the filter surface. However, it was demonstrated that filter longevity was not necessarily dependent on the amount of cell deposit on the filter surface. In the second part of this study, ultrasonic technology was used to reduce filter fouling. Filter vibration, induced by a piezo actuator, improved filter longevity by 113% during CHO cells perfusion cultures.

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.

Proliferation and viability of adherent cells manipulated by standing-wave ultrasound in a microfluidic chip

Ultrasonic-standing-wave (USW) technology has potential to become a standard method for gentle and contactless cell handling in microfluidic chips. We investigate the viability of adherent cells exposed to USWs by studying the proliferation rate of recultured cells following ultrasonic trapping and aggregation of low cell numbers in a microfluidic chip. The cells form 2-D aggregates inside the chip and the aggregates are held against a continuous flow of cell culture medium perpendicular to the propagation direction of the standing wave. No deviations in the doubling time from expected values (24 to 48 h) were observed for COS-7 cells held in the trap at acoustic pressure amplitudes up to 0.85 MPa and for times ranging between 30 and 75 min. Thus, the results demonstrate the potential of ultrasonic standing waves as a tool for gentle manipulation of low cell numbers in microfluidic systems.

Ultrasonic collection of small particles by a tapered metal strip

A pi-shaped ultrasonic actuator can collect small particles by its two sharp edges. However, the collection of particles is weak in air and not very stable in water. In this paper, a refinement to the pi-shaped ultrasonic actuator is made for a more efficient collection of small particles in air and water. In the refined structure, an ultrasonic actuator with a metal strip is used to collect small particles. The metal strip is mechanically driven by one corner of a rectangular, sandwich-shaped ultrasonic transducer operating in the thickness mode vibration. The metal strip is tapered along its length and has a strong vibration at its tip. Small particles in air and water can be attracted to the radiation surface near the end of the metal strip. The dependence of the number of collected particles on driving frequency and voltage is investigated for shrimp eggs, mint seeds, and grass seeds. For a given driving voltage and particle type, the number of collected particles reaches a maximum value at some driving frequency. Increasing driving voltage increases this maximum number to some extent; but too large a driving voltage decreases it. The maximum number also depends on the weight per particle. It increases as the weight per particle decreases for the particles with close densities. Furthermore, the relationship between the number of collected particles and vibration amplitude at the end of the metal strip is investigated for shrimp eggs, mint seeds, and grass seeds. The number is approximately linearly proportional to the vibration amplitude when the vibration amplitude is not too large. In addition to the application in which the length of the metal strip is parallel to gravitation, the actuator also can be used with its length perpendicular to gravitation. However, the latter has a weaker capability of collecting small particles. It is also found that the actuator has a stronger capability to collect particles in water than in air.

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.

Retention and viability characteristics of mammalian cells in an acoustically driven polymer mesh

A processing approach for the collection and retention of mammalian cells within a high porosity polyester mesh having millimeter-sized pores has been studied. Cell retention occurs via energizing the mesh with a low intensity, resonant acoustic field. The resulting acoustic field induces the interaction of cells with elements of the mesh or with each other and effectively prevents the entrainment of cells in the effluent stream. Experiments involving aqueous suspensions of polystyrene particles were used to provide benchmark data on the performance of the acoustic retention cell. Experiments using mouse hybridoma cells showed that retention densities of over 1.5 x 10(8) cell/mL could be obtained. In addition, the acoustic field was shown to produce a negligible effect on cell viability for short-term exposure.

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.

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.

Enhanced separation of filamentous fungi by ultrasonic field: possible usage in repeated batch processes

Usage of ultrasonic field-based filters in retention of filamentous fungal cells was assessed using Rhizopus arrhizus NRRL 1526 as a model organism. Effects of operating conditions, such as power input, harvest pump flow rate, run time and stop time, on the system's separation efficiency (SE) were evaluated by modulating the variables according to a Central Composite Design (CCD). The standard pump with which the ultrasonic filter was equipped was shown to be unsuitable and was, therefore, substituted for with a prime rate reverse pump that made possible separation and recycle of the mycelial biomass. The operating conditions were optimised (run time, 300 s; stop time, 3 s; power input, 6 W; harvest pump flow rate, 4 l per day) and a repeated batch process (three batches for a total of 192 h) was performed during which the SE was maintained always higher than 88%.

Cell retention devices for suspended-cell perfusion cultures

Perfusion cultures of animal cells have several advantages over batch or fed-batch cultures. They give, for instance, higher productivities and a consistent product quality, and allow steady state operation and better cell physiology control. However, one of the main aspects limiting performance and scale-up of perfusion processes is the need for an adequate cell retention device. The devices currently in use for stirred perfusion bioreactors are continuous centrifuges, tangential flow membrane filters, dynamic filters, spin-filters, ultrasonic and dielectrophoretic separators, gravity settlers and, more recently, hydrocyclones. The advantages and disadvantages of each of these methods will be discussed.

Single half-wavelength ultrasonic particle filter: predictions of the transfer matrix multilayer resonator model and experimental filtration results

The quantitative performance of a "single half-wavelength" acoustic resonator operated at frequencies around 3 MHz as a continuous flow microparticle filter has been investigated. Standing wave acoustic radiation pressure on suspended particles (5-microm latex) drives them towards the center of the half-wavelength separation channel. Clarified suspending phase from the region closest to the filter wall is drawn away through a downstream outlet. The filtration efficiency of the device was established from continuous turbidity measurements at the filter outlet. The frequency dependence of the acoustic energy density in the aqueous particle suspension layer of the filter system was obtained by application of the transfer matrix model [H. Nowotny and E. Benes, J. Acoust. Soc. Am. 82, 513-521 (1987)]. Both the measured clearances and the calculated energy density distributions showed a maximum at the fundamental of the piezoceramic transducer and a second, significantly larger, maximum at another system's resonance not coinciding with any of the transducer or empty chamber resonances. The calculated frequency of this principal energy density maximum was in excellent agreement with the optimal clearance frequency for the four tested channel widths. The high-resolution measurements of filter performance provide, for the first time, direct verification of the matrix model predictions of the frequency dependence of acoustic energy density in the water layer.

Glucose-based optimization of CHO-cell perfusion cultures

Perfusion cultures of CHO cells producing t-PA were performed using acoustic filter cell retention. A robust off-line glucose analysis and predictive control protocol was developed to maintain the process within approximately 0.5 mM of the glucose set point, without the need for a more fallible on-line sensor. Glucose usage (the difference between the inlet and reactor glucose concentrations) provided an easily measured indicator of overall medium utilization for mapping acceptable ranges of operation, including the edge of failure. Earlier onset of perfusion with a ramping glucose set point (1.5 mM/d) resulted in improved growth and consistency during the perfusion culture start-up. At steady state, the t-PA concentration variability increased gradually with increasing glucose usage up to approximately 22 mM, then up to 24 mM the variability increased threefold. Peak t-PA concentrations of over 90 mg/L were obtained by controlling at a glucose usage of approximately 24 mM, but these t-PA levels were not sustainable for more than 3 days. A consistent t-PA concentration of 40 mg/L was obtained at a glucose usage of 21.5 mM.

Performance of small-scale CHO perfusion cultures using an acoustic cell filtration device for cell retention: characterization of separation efficiency and impact of perfusion on product quality

Several small-scale Chinese hamster ovary (CHO) suspension cultures were grown in perfusion mode using a new acoustic filtration system. The separation performance was evaluated at different cell concentrations and perfusion rates for two different CHO cell lines. It was found that the separation performance depends inversely on the cell concentration and perfusion rate. High media flow rates as well as high cell concentrations resulted in a significant drop in the separation performance, which limited the maximal cell concentration achievable. However, packed cell volumes of 10% to 16% (corresponding to 3 to 6. 10(7) cells/mL) could be reached and were maintained without additional bleeding after shifting the temperature to 33 degrees C. Perfusion, up to 50 days, did not harm the cells and did not result in a loss of performance of the acoustic filter as often seen with other perfusion systems. Volumetric productivities in perfusion mode were 2- to 12-fold higher for two cell lines producing two different glycoproteins when compared to fed-batch or batch processes using the same cell lines. Product concentrations were in the range of 20% to 80% of batch or fed-batch culture, respectively. In addition, using the protease-sensitive product rhesus thrombopoietin, we could show that cultivation in perfusion mode drastically reduced proteolysis when compared to a batch culture without addition of protease inhibitors such as leupeptin.

High-density perfusion culture of insect cells with a biosep ultrasonic filter

The baculovirus/insect cell expression system has provided a vital tool to produce a high level of active proteins for many applications. We have developed a very high-density insect cell perfusion process with an ultrasonic filter as a cell retention device. The separation efficiency of the filter was studied under various operating conditions. A cell density of over 30 million cells/mL was achieved in a controlled perfusion bioreactor and cell viability remained greater than 90%. Sf9 cells from a high-density culture and a spinner culture were infected with two recombinant baculoviruses expressing genes for the production of human chitinase and monocyte-colony inhibition factor. The protein yield on a cell basis from infecting high-density Sf9 cells was the same as or higher than that from the spinner Sf9 culture. Virus production from the high-density culture was similar to that from the spinner culture. The results show that the ultrasonic filter did not affect insect cells' ability to support protein expression and virus production following infection with baculovirus. The potential applications of the high-density perfusion culture for large-scale protein expression from Sf9 cells are also highlighted.

A comparison of intensive cell culture bioreactors operating with hybridomas modified for inhibited apoptotic response

It is demonstrated, using two different perfusion reaction systems, that hybridoma modified by inhibiting their apoptotic response can give improved process performance in terms of cell number and viability in intensive cell culture. Two cell perfusion systems, one using a spin filter and the other an ultrasonic filter, are compared using two cell lines. One cell line is transfected with the bcl-2 gene (TB/C3 bcl-2) which encodes the 'anti-apoptotic' human bcl-2 protein and the other cell line (TB/C3 pEF) with a negative transfection vector. Both reactor systems give similar retention performance for both cell lines. Bcl-2 transfected cells reach higher cell densities than the control cell line, and the percentage of apoptotic cells is clearly lower than with pEF cells. The maximum cell numbers of the bcl-2 cell line are 1.21 x 10(7) ml-1 in the ultrasonic filter culture and 1.58 x 10(7) ml-1 in the spin filter culture, respectively. Using the pEF cell line the maximum cell number reaches 6.0 x 10(6) ml-1 with ultrasonic retention and 5.9 x 10(6) ml-1 in the spin filter. The use of ultrasound in this cell retention system has no apparent influence on cell growth, productivity or viability. Selective retention of viable cells is detectable but the effect of removing non-viable cells is negligible.

Ultrasonic separations in analytical biotechnology

Cells in megahertz-frequency noncavitating ultrasonic standing waves concentrate at submillimetre distances and are, as large clumps, easily removed from suspension in flow or batch systems. An ultrasonic filter for perfusion hybridoma culture is now available. Results from small-volume prototype analytical-scale systems can inform the design of effective filter or batch-clarification systems for a wide range of cell sizes, concentrations and sample volumes. Large increases in the rates of aqueous biphasic separations and of the rates and sensitivities of analytical immunocoated particle-agglutination assays occur in standing waves. Ultrasonic manipulation is briefly compared with immunomagnetic and dielectrophoretic separations.