Modeling Shear Damage to Suspended CHO Cells during Cross-Flow Filtration

Digital Twin for Continuous Production of Virus-like Particles toward Autonomous Operation

Lentiviral vector and virus-like particle (VLP) manufacturing have been published in fed-batch upstream and batch downstream modes before. Batch downstream and continuous upstream in perfusion mode were reported as well. This study exemplifies development and validation steps for a digital twin combining a physical-chemical-based mechanistic model for all unit operations with a process analytical technology strategy in order to show the efforts and benefits of autonomous operation approaches for manufacturing scale. As the general models are available from various other biologic manufacturing studies, the main step is model calibration for the human embryo kidney cell-based VLPs with experimental quantitative validation within the Quality-by-Design (QbD) approach, including risk assessment to define design and control space. For continuous operation in perfusion mode, the main challenge is the efficient separation of large particle manifolds for VLPs and cells, including cell debris, which is of similar size. Here, innovative tangential flow filtration operations are needed to avoid fast blocking with low mechanical stress pumps. A twofold increase of productivity was achieved using simulation case studies. This increase is similar to improvements previously described for other entities like plasmid DNAs, monoclonal antibodies (mAbs), and single-chain fragments of variability (scFv) fragments. The advantages of applying a digital twin for an advanced process control strategy have proven additional productivity gains of 20% at 99.9% reliability.

The role of biomechanical stress in extracellular vesicle formation, composition and activity

Extracellular vesicles (EVs) are cornerstones of intercellular communication with exciting fundamental, clinical, and more broadly biotechnological applications. However, variability in EV composition, which results from the culture conditions used to generate the EVs, poses significant fundamental and applied challenges and a hurdle for scalable bioprocessing. Thus, an understanding of the relationship between EV production (and for clinical applications, manufacturing) and EV composition is increasingly recognized as important and necessary. While chemical stimulation and culture conditions such as cell density are known to influence EV biology, the impact of biomechanical forces on the generation, properties, and biological activity of EVs remains poorly understood. Given the omnipresence of these forces in EV preparation and in biomanufacturing, expanding the understanding of their impact on EV composition-and thus, activity-is vital. Although several publications have examined EV preparation and bioprocessing and briefly discussed biomechanical stresses as variables of interest, this review represents the first comprehensive evaluation of the impact of such stresses on EV production, composition and biological activity. We review how EV biogenesis, cargo, efficacy, and uptake are uniquely affected by various types, magnitudes, and durations of biomechanical forces, identifying trends that emerge both generically and for individual cell types. We also describe implications for scalable bioprocessing, evaluating processes inherent in common EV production and isolation methods, and propose a path forward for rigorous EV quality control.

High shear resistance of insect cells: the basis for substantial improvements in cell culture process design

Multicellular organisms cultivated in continuous stirred tank reactors (CSTRs) are more sensitive to environmental conditions in the suspension culture than microbial cells. The hypothesis, that stirring induced shear stress is the main problem, persists, although it has been shown that these cells are not so sensitive to shear. As these results are largely based on Chinese Hamster Ovary (CHO) cell experiments the question remains if similar behavior is valid for insect cells with a higher specific oxygen demand. The requirement of higher oxygen transfer rates is associated with higher shear forces in the process. Consequently, we focused on the shear resistance of insect cells, using CHO cells as reference system. We applied a microfluidic device that allowed defined variations in shear rates. Both cell lines displayed high resistance to shear rates up to 8.73 × 10⁵ s⁻¹. Based on these results we used microbial CSTRs, operated at high revolution speeds and low aeration rates and found no negative impact on cell viability. Further, this cultivation approach led to substantially reduced gas flow rates, gas bubble and foam formation, while addition of pure oxygen was no longer necessary. Therefore, this study contributes to the development of more robust insect cell culture processes.

Wide-surface pore microfiltration membrane drastically improves sieving decay in TFF-based perfusion cell culture and streamline chromatography integration for continuous bioprocessing

Although several compelling benefits for bioprocess intensification have been reported, the need for a streamlined integration of perfusion cultures with capture chromatography still remains unmet. Here, a robust solution is established by conducting tangential flow filtration‐based perfusion with a wide‐surface pore microfiltration membrane. The resulting integrated continuous bioprocess demonstrated negligible retention of antibody, DNA, and host cell proteins in the bioreactor with average sieving coefficients of 98 ± 1%, 124 ± 28%, and 109 ± 27%, respectively. Further discussion regarding the potential membrane fouling mechanisms is also provided by comparing two membranes with different surface pore structures and the same hollow fiber length, total membrane area, and chemistry. A cake‐growth profile is reported for the narrower surface pore, 0.65‐µm nominal retention perfusion membrane with final antibody sieving coefficients ≤70%. Whereas the sieving coefficient remained ≥85% during 40 culture days for the wide‐surface pore, 0.2‐µm nominal retention rating membrane. The wide‐surface pore structure, confirmed by scanning electron microscopy imaging, minimizes the formation of biomass deposits on the membrane surface and drastically improves product sieving. This study not only offers a robust alternative for integrated continuous bioprocess by eliminating additional filtration steps while overcoming sieving decay, but also provides insight into membranes' fouling mechanism.

Development of a small-scale rotary lobe-pump cell culture model for examining cell damage in large-scale N-1 seed perfusion process

Perfusion technology has been identified as a process improvement capable of eliminating some of the constraints in cell culture and allows for high cell densities and viabilities. However, when implementing this N-1 seed perfusion platform in large-scale manufacturing, unexpected cell damage was observed as early as Day 1. Given that the shear rate within recirculation hollow fibers was normalized and aligned correctly across bench, pilot, and manufacture scale, the primary mitigation was placed on the rotary lobe pump. Lowering the pump rate in manufacture scale successfully alleviated the cell damage. To understand the source of cell damage within the pump, a small-scale rotary lobe-pump robustness model was developed. Testing different pump flow rates and back pressures, it was concluded that high back pressure can cause cell damage. The back pressure within the system can cause back flow and high shear within small clearances inside the pump, which lead to the primary cell damage observed at a large scale. This shear level can be significantly higher than the shear in the hollow fiber. This pump robustness model can be utilized to aid the perfusion skid design, including pump operation efficiency and cell shear sensitivity. Methods to reduce the back pressure and cell shearing were determined to better predict manufacturing performance in the future.

Droplet Demulsification Using Ultralow Voltage-Based Electrocoalescence

Demulsification of droplets stabilized with surfactant is very challenging due to their low surface energy. We report ultralow voltage-based electrocoalescence phenomenon for the demulsification of aqueous droplets with an aqueous stream. In the absence of electric field, due to the disjoining pressure resulting from the tail-tail interaction between the surfactant molecules present on the aqueous droplets and interface, coalescence of aqueous droplets with the aqueous stream is prevented. However, above a critical electric field, the electrical stress overcomes the disjoining pressure, thus leading to the droplet coalescence. The influence of surfactant concentration, droplet diameter, and velocity on the electrocoalescence phenomena is studied. The macroscopic contact between the aqueous droplet with the aqueous stream enables droplet coalescence at much lower voltage (10 to 90 V), which is at least two orders of magnitude smaller than voltages used in prior works (1.0 to 3.0 kV). The electrocoalescence phenomena is used for the extraction of microparticles encapsulated in aqueous droplets into the aqueous stream and size-based selective demulsification. A new paradigm of droplet electrocoalescence and content extraction is presented that would find significant applications in chemistry and biology.

Shear stress increases cytotoxicity and reduces transfection efficiency of liposomal gene delivery to CHO-S cells

Animal cells in suspension experience shear stress in different situations such as in vivo due to hemodynamics, or in vitro due to agitation in large-scale bioreactors. Shear stress is known to affect cell physiology, including binding and uptake of extracellular cargo. In adherent cells the effects of exposure to shear stress on particle binding kinetics and uptake have been studied. There are however no reports on the effect of shear stress on extracellular cargo delivery to suspension cells. In this study, we have evaluated the effect of shear stress on transfection of CHO-S cells using Lipofectamine 2000 in a simple flow apparatus. Our results show decreased cell growth and transfection efficiency upon lipoplex assisted transfection of CHO-S while being subjected to shear stress. This effect is not seen to the same extent when cells are exposed to shear stress in absence of the lipoplex complex and subsequently transfected, or if the lipoplex is subjected to shear stress and subsequently used to transfect the cells. It is also not seen to the same extent when cells are exposed to shear stress in presence of liposome alone, suggesting that the observed effect is dependent on interaction of the lipoplex with cells in the presence of shear stress. These results suggest that studies involving liposomal DNA delivery in presence of shear stress such as large scale transient protein expression should account for the effect of shear during lipoplex assisted DNA delivery.

Deformation and rupture of Dunaliella salina at high shear rates without the use of thickeners

BACKGROUND

High-density cultures require operating below the critical threshold of shear stress, in order to avoid reducing the specific growth rate of the cells. When determining this threshold, direct inspection of the cells in flow provides insight into the conditions of shearing.

OBJECTIVE

Aim of this study was using a novel rheo-optical setup for the observation of cells in laminar shear flow and the determination of the critical shear stress required to damage them in their natural environment.

METHODS

Dunaliella salina cells were sheared and observed in flow for shear stresses of up to 90 Pa, at ambient temperature, without adding thickeners. The critical shear stress was determined by fitting a hydrodynamics-based criterion to the experimental data on the percentage of deformed cells after shearing.

RESULTS

Single cells, clusters and strings of cells were visible in shear flow. The strings formed at maximum shear stresses of 10 Pa or higher. Cells lost motility for maximum shear stresses higher than 15 Pa, and more than 80% of the cells were deformed at maximum shear stresses higher than 60 Pa. The estimated critical shear stress was 18 Pa.

CONCLUSIONS

Shear stresses higher than 18 Pa should be avoided when cultivating D. salina.

Use of hollow fiber tangential flow filtration for the recovery and concentration of HIV virus-like particles produced in insect cells

Attenuated viruses, inactivated viruses and virus like particles (VLPs) are known to be efficient vaccines partially due to their particulate structure. A potential HIV vaccine candidate engineered as a VLP (HIV gag-VLP) and produced in insect cells is currently under preclinical trials demanding large amounts. Due to their extreme fragility and sensitivity to shear forces the recovery and concentration of these extracellular enveloped particles of approximately 120 nm in size is challenging. The current bench scale gradient ultracentrifugation and precipitation methods have been found unsuitable for larger scale processes. In this study a two-step tangential flow filtration (TFF) process using hollow fibers was developed for the clarification and concentration of HIV gag-VLPs. The first step is microfiltration for cell removal and the second step is ultrafiltration for concentrating the HIV gag-VLPs. The chosen parameters for the microfiltration step were hollow fiber membranes of 0.45 μm cut off 5000 s(-1) shear force and a flux of 10 LMH. The chosen parameters for the ultrafiltration step were a 500 kDa cut off membrane, 6000 s(-1) shear force and a trans-membrane pressure (TMP) of 1.25 bar. The utilization of these parameters provided with concentrated HIV-gag VLPs from 2L of starting cell suspension within 6h of processing time. These downstream processing conditions are extremely valuable for the further large-scale purification process development for HIV gag-VLPs and other particulate bioproducts.

Bio-Rapid-Prototyping of Tissue Engineering Scaffolds and the Process-Induced Cell Damage

Tissue scaffolds play a vital role in tissue engineering by providing a native tissue-mimicking environment for cell proliferation and differentiation as well as tissue regeneration. Fabrication of tissue scaffolds has been drawing increasing research attention and a number of fabrication techniques have been developed. To better mimic the microenvironment of native tissues, novel techniques have emerged in recent years to encapsulate cells into the engineered scaffolds during the scaffold fabrication process. Among them, bio-Rapid-Prototyping (bioRP) techniques, by which scaffolds with encapsulated cells can be fabricated with controlled internal microstructure and external shape, shows significant promise. It is noted in the bioRP processes, cells may be continuously subjected to environmental stresses such as mechanical, electrical forces and laser exposure. If the stress is greater than a certain level, the cell membrane may be ruptured, leading to the so-called process-induced cell damage. This paper reviews various cell encapsulation techniques for tissue scaffold fabrication, with emphasis on the bioRP technologies and their technical features. To understand the process-induced cell damage in the bioRP processes, this paper also surveys the cell damage mechanisms under different stresses. The process-induced cell damage models are also examined to provide a cue to the cell viability preservation in the fabrication process. Discussions on further improvements of bioRP technologies are given and ongoing research into mechanical cell damage mechanism are also suggested in this review.

The potential of hydrodynamic damage to animal cells of industrial relevance: current understanding

Suspension animal cell culture is now routinely scaled up to bioreactors on the order of 10,000 L, and greater, to meet commercial demand. However, the concern of the 'shear sensitivity' of animal cells still remains, not only within the bioreactor, but also in the downstream processing. As the productivities continue to increase, titer of ~10 g/L are now reported with cell densities greater than 2 × 10(7) cells/mL. Such high, and potentially higher cell densities will inevitably translate to increased demand in mass transfer and mixing. In addition, achieving productivity gains in both the upstream stage and downstream processes can subject the cells to aggressive environments such as those involving hydrodynamic stresses. The perception of 'shear sensitivity' has historically put an arbitrary upper limit on agitation and aeration in bioreactor operation; however, as cell densities and productivities continue to increase, mass transfer requirements can exceed those imposed by these arbitrary low limits. Therefore, a better understanding of how animal cells, used to produce therapeutic products, respond to hydrodynamic forces in both qualitative and quantitative ways will allow an experimentally based, higher, "upper limit" to be created to guide the design and operation of future commercial, large scale bioreactors. With respect to downstream hydrodynamic conditions, situations have already been achieved in which practical limits with respect to hydrodynamic forces have been experienced. This review mainly focuses on publications from both the academy and industry regarding the effect of hydrodynamic forces on industrially relevant animal cells, and not on the actual scale-up of bioreactors. A summary of implications and remaining challenges will also be presented.

Physiological responses of CHO cells to repetitive hydrodynamic stress

A majority of the previous investigations on the hydrodynamic sensitivity of mammalian cells have focused on lethal effects as determined by cell death or lysis. In this study, we investigated the effect of hydrodynamic stress on CHO cells in a fed-batch process using a previously reported system which subjects cells to repetitive, high levels of hydrodynamic stress, quantified by energy dissipation rate (EDR). The results indicated that cell growth and monoclonal antibody production of the test cells were very resistant to the hydrodynamic stress. Compared to the control, no significant variation was observed at the highest EDR tested, 6.4 x 10(6) W/m(3). Most product quality attributes were not affected by intense hydrodynamic stress either. The only significant impact was on glycosylation. A shift of glycosylation pattern was observed at EDR levels at or higher than 6.0 x 10(4) W/m(3), which is two orders of magnitude lower than the EDR where physical cell damage, as measured by lactate dehydrogenase release, was observed. While not as extensively investigated, a second monoclonal antibody produced in a different CHO clone exhibited the same glycosylation change at an intensive EDR, 2.9 x 10(5) W/m(3). Conversely, a low EDR of 0.9 x 10(2) W/m(3) had no effect on the glycosylation pattern. As 6.0 x 10(4) W/m(3), the lowest EDR that triggers the glycosylation shift, is about one order of magnitude higher than the estimated, maximum EDR in typically operated, large-scale stirred tank bioreactors, further studies in a lower EDR range of 1 x 10(3)-6.0 x 10(4) W/m(3) are needed to assess the glycosylation shift effect under typical large-scale bioreactor operation conditions. Follow-up studies in stirred tanks are also needed to confirm the glycosylation shift effect and to validate the repetitive hydrodynamic stress model.