Agitation induced cell injury in microcarrier cultures. Protective effect of viscosity is agitation intensity dependent: Experiments and modeling

Perfusion of MC3T3E1 Preosteoblast Spheroids within Polysaccharide-Based Hydrogel Scaffolds: An Experimental and Numerical Study at the Bioreactor Scale

The traditional 3D culture systems in vitro lack the biological and mechanical spatiotemporal stimuli characteristic to native tissue development. In our study, we combined porous polysaccharide-based hydrogel scaffolds with a bioreactor-type perfusion device that generates favorable mechanical stresses while enhancing nutrient transfers. MC3T3E1 mouse osteoblasts were seeded in the scaffolds and cultivated for 3 weeks under dynamic conditions at a perfusion rate of 10 mL min^-1. The spatial distribution of the cells labeled with superparamagnetic iron oxide nanoparticles was visualized by MRI. Confocal microscopy was used to assess cell numbers, their distribution inside the scaffolds, cell viability, and proliferation. The oxygen diffusion coefficient in the hydrogel was measured experimentally. Numerical simulations of the flow and oxygen transport within the bioreactor were performed using a lattice Boltzmann method with a two-relaxation time scheme. Last, the influence of cell density and spheroid size on cell oxygenation was investigated. The cells spontaneously organized into spheroids with a diameter of 30-100 μm. Cell viability remained unchanged under dynamic conditions but decreased under static culture. The cell proliferation (Ki67 expression) in spheroids was not observed. The flow simulation showed that the local fluid velocity reached 27 mm s^-1 at the height where the cross-sectional area of the flow was the smallest. The shear stress exerted by the fluid on the scaffolds may locally rise to 100 mPa, compared with the average value of 25 mPa. The oxygen diffusion coefficient in the hydrogel was 1.6×10-9 m² s^-1. The simulation of oxygen transport and consumption confirmed that the cells in spheroids did not suffer from hypoxia when the bioreactor was perfused at 10 mL min^-1, and suggested the existence of optimal spheroid size and spacing for appropriate oxygenation. Collectively, these findings enabled us to define the optimal conditions inside the bioreactor for an efficient in vitro cell organization and survival in spheroids, which are paramount to future applications with organoids.

Fluid flow-induced cell stimulation in bone tissue engineering changes due to interstitial tissue formation in vitro

In tissue engineering experiments in vitro, bioreactors have been used for applying wall shear stress (WSS) on cells to regulate cellular activities. To determine the loading conditions within bioreactors and to design tissue engineering products, in silico models are used. Previous in silico studies in bone tissue engineering (BTE) focused on quantifying the WSS on cells and the influence on appositional tissue growth. However, many BTE experiments also show interstitial tissue formation (i.e., tissue infiltrated in the pores rather than growing on the struts - appositional growth), which has not been considered in previous in silico studies. We hereby used a multiscale fluid-solid interaction model to quantify the WSS and mechanical strain on cells with interstitial tissue formation, taken from a reported BTE experiment. The WSS showed a high variation among different interstitial tissue morphologies. This is different to the situation under appositional tissue growth. It is found that a 35% filling of the pores results (by mineralised bone tissue) when the average WSS increases from 1.530 (day 0) to 5.735 mPa (day 28). Furthermore, the mechanical strain on cells caused by the fluid flow was extremely low (at the level of 10^-14 -10^-15 ), comparing to the threshold in a previous mechanobiological theory of osteogenesis (eg, 10^-2 ). The output from this study offers a significant insight of the WSS changes during interstitial tissue growth under a constant perfusion flow rate in a BTE experiment. It has paved the way for optimising the local micro-fluidic environment for interstitial tissue mineralisation.

Plant and Biomass Extraction and Valorisation under Hydrodynamic Cavitation

Hydrodynamic cavitation (HC) is a green technology that has been successfully used to intensify a number of process. The cavitation phenomenon is responsible for many effects, including improvements in mass transfer rates and effective cell-wall rupture, leading to matrix disintegration. HC is a promising strategy for extraction processes and provides the fast and efficient recovery of valuable compounds from plants and biomass with high quality. It is a simple method with high energy efficiency that shows great potential for large-scale operations. This review presents a general discussion of the mechanisms of HC, its advantages, different reactor configurations, its applications in the extraction of bioactive compounds from plants, lipids from algal biomass and delignification of lignocellulosic biomass, and a case study in which the HC extraction of basil leftovers is compared with that of other extraction methods.

Numerical accuracy comparison of two boundary conditions commonly used to approximate shear stress distributions in tissue engineering scaffolds cultured under flow perfusion

INTRODUCTION

Flow-induced shear stresses have been found to be a stimulatory factor in pre-osteoblastic cells seeded in 3D porous scaffolds and cultured under continuous flow perfusion. However, due to the complex internal structure of the scaffolds, whole scaffold calculations of the local shear forces are computationally intensive. Instead, representative volume elements (RVEs), which are obtained by extracting smaller portions of the scaffold, are commonly used in literature without a numerical accuracy standard.

OBJECTIVE

Hence, the goal of this study is to examine how closely the whole scaffold simulations are approximated by the two types of boundary conditions used to enable the RVEs: "wall boundary condition" (WBC) and "periodic boundary condition" (PBC).

METHOD

To that end, lattice Boltzmann method fluid dynamics simulations were used to model the surface shear stresses in 3D scaffold reconstructions, obtained from high-resolution microcomputed tomography images.

RESULTS

It was found that despite the RVEs being sufficiently larger than 6 times the scaffold pore size (which is the only accuracy guideline found in literature), the stresses were still significantly under-predicted by both types of boundary conditions: between 20% and 80% average error, depending on the scaffold's porosity. Moreover, it was found that the error grew with higher porosity. This is likely due to the small pores dominating the flow field, and thereby negating the effects of the unrealistic boundary conditions, when the scaffold porosity is small. Finally, it was found that the PBC was always more accurate and computationally efficient than the WBC. Therefore, it is the recommended type of RVE.

Dynamic cell culture on calcium phosphate microcarriers for bone tissue engineering applications

Developing appropriate cell culturing techniques to populate scaffolds has become a great challenge in tissue engineering. This work describes the use of spinner flask dynamic cell cultures to populate hydroxyapatite microcarriers for bone tissue engineering. The microcarriers were obtained through the emulsion of a self-setting aqueous α-tricalcium phosphate slurry in oil. After setting, hydroxyapatite microcarriers were obtained. The incorporation of gelatin in the liquid phase of the α-tricalcium phosphate slurry allowed obtaining hybrid gelatin/hydroxyapatite-microcarriers. Initial cell attachment on the microcarriers was strongly influenced by the speed of the dynamic culture, achieving higher attachment at low speed (40 r/min) as compared to high speed (80 r/min). Under moderate culture speeds (40 r/min), the number of cells present in the culture as well as the number of microcarrier-containing cells considerably increased after 3 days, particularly in the gelatin-containing microcarriers. At longer culture times in dynamic culture, hydroxyapatite-containing microcarriers formed aggregates containing viable and extracellular matrix proteins, with a significantly higher number of cells compared to static cultures.

Death rate in a small air-lift loop reactor of vero cells grown on solid microcarriers and in macroporous microcarriers

The death rate of Vero cells grown on Cytodex-3 microcarrierswas studied as a function of the gas flow rate in a smallair-lift loop reactor. The death rate may be described byfirst-order death-rate kinetics. The first-order death-rateconstant as calculated from the decrease in viable cells, theincrease in dead cells and the increase in LDH activity islinear proportional to the gas flow rate, with a specifichypothetical killing volume in which all cells are killed ofabout 2.10(-3)m(3) liquid per m(3) of air bubbles.In addition, an experiment was conducted in the sameair-lift reactor with Vero cells grown inside porous Asahimicrocarriers. The specific hypothetical killing volumecalculated from this experiment has a value of 3.10(-4)m(3) liquid per m(3) of air bubbles, which shows thatthe porous microcarriers were at least in part able to protectthe cells against the detrimental hydrodynamic forcesgenerated by the bubbles.

Scalable stirred-suspension bioreactor culture of human pluripotent stem cells

Advances in stem cell biology have afforded promising results for the generation of various cell types for therapies against devastating diseases. However, a prerequisite for realizing the therapeutic potential of stem cells is the development of bioprocesses for the production of stem cell progeny in quantities that satisfy clinical demands. Recent reports on the expansion and directed differentiation of human embryonic stem cells (hESCs) in scalable stirred-suspension bioreactors (SSBs) demonstrated that large-scale production of therapeutically useful hESC progeny is feasible with current state-of-the-art culture technologies. Stem cells have been cultured in SSBs as aggregates, in microcarrier suspension and after encapsulation. The various modes in which SSBs can be employed for the cultivation of hESCs and human induced pluripotent stem cells (hiPSCs) are described. To that end, this is the first account of hiPSC cultivation in a microcarrier stirred-suspension system. Given that cultured stem cells and their differentiated progeny are the actual products used in tissue engineering and cell therapies, the impact of bioreactor's operating conditions on stem cell self-renewal and commitment should be considered. The effects of variables specific to SSB operation on stem cell physiology are discussed. Finally, major challenges are presented which remain to be addressed before the mainstream use of SSBs for the large-scale culture of hESCs and hiPSCs.

The extended serial subculture of human diploid fibroblasts on microcarriers using a new medium supplement formulation

Human diploid fibroblasts serially passaged on microcarriers exhibit a decrease in their proliferative capacity with each transfer from microcarrier-to-microcarrier. This phenomenon, which does not occur in the same time scale with cells cultured in T-flasks, has been a serious barrier to the systematic utilization of microcarriers in the scale-up of anchorage-dependent human diploid cell cultures. This decreases in cell growth with each passage is shown to be related to the serum content of the medium, with high serum concentrations resulting in a more rapid decrease in cell growth with each serial transfer. As a result, methods for reducing the serum requirement of the cells were investigated. A new medium supplement mixture, PPRF92, has been developed, which allows the serial passaging of MRC5 cells on Cytodex 1 microcarriers through as many as 13 microcarrier-to-microcarrier transfers, and at a serum levels as low as 1%, with no decrease in the proliferative capacity of the cells until they approach their reported population doubling limit. This new supplement mixture is a significant improvement to microcarrier technology in that it enables the use of microcarriers in the early stages of inoculum build-up for the production purposes.

Computational modeling of flow-induced shear stresses within 3D salt-leached porous scaffolds imaged via micro-CT

Flow-induced shear stresses have been found to be a stimulatory factor in pre-osteoblastic cells seeded in 3D porous scaffolds and cultured under continuous flow perfusion. However, due to the complex internal structure of porous scaffolds, analytical estimation of the local shear forces is impractical. The primary goal of this work is to investigate the shear stress distributions within Poly(l-lactic acid) scaffolds via computation. Scaffolds used in this study are prepared via salt leeching with various geometric characteristics (80-95% porosity and 215-402.5microm average pore size). High resolution micro-computed tomography is used to obtain their 3D structure. Flow of osteogenic media through the scaffolds is modeled via lattice Boltzmann method. It is found that the surface stress distributions within the scaffolds are characterized by long tails to the right (a positive skewness). Their shape is not strongly dependent on the scaffold manufacturing parameters, but the magnitudes of the stresses are. Correlations are prepared for the estimation of the average surface shear stress experienced by the cells within the scaffolds and of the probability density function of the surface stresses. Though the manufacturing technique does not appear to affect the shape of the shear stress distributions, presence of manufacturing defects is found to be significant: defects create areas of high flow and high stress along their periphery. The results of this study are applicable to other polymer systems provided that they are manufactured by a similar salt leeching technique, while the imaging/modeling approach is applicable to all scaffolds relevant to tissue engineering.

Pluronic enhances the robustness and reduces the cell attachment of mammalian cells

The addition of the non-ionic surfactant, Pluronic F-68, to serum-free CHO cultures causes multi-functional effects that enhance cell yield in agitated cultures and reduce cell adhesion in stationary cultures. Three independent CHO cell lines were subjected to high liquid shear in assay systems that either included or excluded a liquid-gas interface. In the absence of Pluronic, there was a loss in cell viability in either assay system, although there was an intrinsic variability in sensitivity of the cell lines to shear damage. Supplementation with Pluronic prevented loss of cell viability, indicating protection in either a gas sparged or bubble-free environment. However, we found no evidence of long-term protection of cells once Pluronic was removed. Pluronic was capable of repairing trypsin-damaged cells as evidenced by enhanced growth, reduced membrane porosity, and improved robustness under liquid shear. The proportion of adherent cells was reduced to a minimal level by the presence of Pluronic although its effect was rapidly reversible with a high proportion (70%) of adherent cells observed within a few culture passages of its removal. The observed effects of Pluronic on these cultures are compatible with a mechanism in which the polymer forms a protective layer on the cell membrane, which has a significantly lower hydrophobicity.

The use of dinoflagellate bioluminescence to characterize cell stimulation in bioreactors

Bioluminescent dinoflagellates are flow-sensitive marine organisms that produce light emission almost instantaneously upon stimulation by fluid shear in a shear stress dose-dependent manner. In the present study we tested the hypothesis that monitoring bioluminescence by suspended dinoflagellates can be used as a tool to characterize cellular response to hydrodynamic forces in agitated bioreactors. Specific studies were performed to determine: (1) impeller configurations with minimum cell activation, (2) correlations of cellular response and an integrated shear factor, and (3) the effect of rapid acceleration in agitation. Results indicated that (1) at a volumetric mass transfer coefficient of 3 x 10(-4) s(-1), marine impeller configurations were less stimulatory than Rushton configurations, (2) bioluminescence response and a modified volumetric integrated shear factor had an excellent correlation, and (3) rapid acceleration in agitation was highly stimulatory, suggesting a profound effect of temporal gradients in shear in increasing cell stimulation. By using bioluminescence stimulation as an indicator of agitation-induced cell stimulation and/or damage in microcarrier cultures, the present study allows for the verification of hypotheses and development of novel mechanisms of cell damage in bioreactors.

Expansion of mammalian neural stem cells in bioreactors: effect of power input and medium viscosity

Multipotent neural precursors can be cultured in suspension bioreactors as aggregates of stem cells and progenitor cells. However, it is important to limit the size of the aggregates, as necrotic centers may develop at very large diameters. Previously, we have shown that the hydrodynamics within a suspension bioreactor can be used to control the diameter of NSC aggregates (D(MAVG)<150 microm) below sizes where necrosis would be expected to occur. In the present study, power law correlations were developed for our bioreactors showing the dependence of the maximum mean aggregate diameter on both the kinematic viscosity of the medium and the power input per unit mass of medium. The power input was manipulated by changing the agitation rate (60-100 rpm), and the viscosity was manipulated through the addition of non-toxic levels of carboxymethylcellulose. The study also confirmed that the maximum liquid shear generated at the surface of the aggregates was sufficient to dislodge single cells, thus limiting the maximum diameter of the aggregates, without causing cell damage (tau(max)=9.76 dyn/cm(2)). This is a first step in the development of a reproducible, scaled-up process for the production of neural stem cells for therapeutic applications including the treatment of neurodegenerative disorders and acute central nervous system injuries.

Estimation of hyphal tensile strength in production-scale Aspergillus oryzae fungal fermentations

Fragmentation of filamentous fungal hyphae depends on two phenomena: hydrodynamic stresses, which lead to hyphal breakage, and hyphal tensile strength, which resists breakage. The goal of this study was to use turbulent hydrodynamic theory to develop a correlation that allows experimental data of morphology and hydrodynamics to be used to estimate relative (pseudo) tensile strength (sigma(pseudo)) of filamentous fungi. Fed-batch fermentations were conducted with a recombinant strain of Aspergillus oryzae in 80 m(3) fermentors, and measurements were made of both morphological (equivalent hyphal length, L) and hydrodynamic variables (specific power input, epsilon; kinematic viscosity, v). We found that v increased over 100-fold during these fermentations and, hence, Kolmogorov microscale (lambda) also changed significantly with time. In the impeller discharge zone, where hyphal fragmentation is thought to actually take place, lambda was calculated to be 700-3500 microm, which is large compared to the size of typical fungal hyphae (100-300 microm). This result implies that eddies in the viscous subrange are responsible for fragmentation. Applying turbulent theory for this subrange, it was possible to calculate sigma(pseudo)from morphological and hydrodynamic measurements. Pseudo tensile strength was not constant but increased to a maximum during the first half and then decreased during the second half of each fermentation, presumably due to differences in physiological state. When a literature correlation for hyphal fragmentation rate (k(frag)) was modified by adding a term to account for viscosity and tensile strength, the result was better qualitative agreement with morphological data. Taken together, these results imply hyphal tensile strength can change significantly over the course of large-scale, fed-batch fungal fermentations and that existing fragmentation and morphology models may be improved if they accounted for variations in hyphal tensile strength with time.

Higher production of rabies virus in serum-free medium cell cultures on microcarriers

Rabies virus suspensions were obtained from VERO cells cultivated on solid microcarriers in a bioreactor after infection with the Pasteur rabies virus strain (PV). Virus production-serum free medium (VP-SFM) or Leibovitz 15 (L15) medium supplemented or not with fetal calf serum (FCS) were used to cultivate the VERO cells, before and after virus infection. The cell growth was shown to reach higher densities (1.6 x 10(6) cellsmol(-l)), when VP-SFM supplemented with 1% of FCS was used during the cell growth phase of culture, and then replaced by VP-SFM alone for the virus multiplication phase. In the cultures performed from the beginning with VP-SFM, lower densities accompanied by an altered cell morphology and detachment from the microcarriers were always observed. In rabies virus infected cultures, kinetic studies showed that higher virus yields (10(4.7) FFD(50) per 0.05 ml) were always obtained in cultures performed initially on VP-SFM supplemented with 1% FCS and after infection on VP-SFM alone. In agreement with that, rabies virus production, as measured by the average of virus titers in harvests obtained at different times after infection were shown to be 5.5 times higher in the cell cultures using initially VP-SFM+1%FCS and, following infection, VP-SFM alone. Besides the advantages of using media with a well-controlled composition, these data indicate the usefulness of serum free media also in terms of virus productivity.

Shear sensitivity of animal cells from a culture-medium perspective

Recently, several groups have published data on the shear sensitivity of suspended animal cells and the protective effect of certain polymers. These findings did not, at the time, seem to have great practical application because shear sensitivity did not cause great problems for large-scale applications in sparged and stirred-tank reactors using the then-current culture media and fermentation procedures. However, two recent developments might require renewed attention in sparged animal-cell cultures--protein-free media and new fermentation techniques.

Shear sensitivity in animal cell culture

Over the past year, considerable progress has been made in understanding shear sensitivity in animal cell culture as a result of extensive theoretical and experimental work. Here we review this progress, paying special attention to the physical and biological mechanisms by which mechanical forces act upon cells, and the effects of such forces.

Animal cells in turbulent fluids: Details of the physical stimulus and the biological response

Animal cells in large scale bioreactors are subjected to a variety of fluid forces for which they are not adapted by evolution. In severe cases the result is cell death, but under more modest agitation conditions an increasing number of nonlethal responses affecting growth rate, metabolism, and product formation have been reported. The forces causing these responses have not been characterized because particle-turbulence interactions are extremely complex. The current understanding of the microscopic structure of turbulence in an infinite liquid and in boundary layers shows that an average shear stress alone is not likely to be adequate to describe the bioreactor environment. Combining knowledge of the physical stimuli and the biological responses will lead to better ways of limiting cell damage and possibly to using physical stresses as a means of specifically modifying cell behavior.