A micromanipulation technique with a theoretical cell model for determining mechanical properties of single mammalian cells

Fabrication of gradient hydrogels using a thermophoretic approach in microfluidics

The extracellular matrix presents spatially varying physical cues that can influence cell behavior in many processes. Physical gradients within hydrogels that mimic the heterogenous mechanical microenvironment are useful to study the impact of these cues on cellular responses. Therefore, simple and reliable techniques to create such gradient hydrogels are highly desirable. This work demonstrates the fabrication of stiffness gradient Gellan gum (GG) hydrogels by applying a temperature gradient across a microchannel containing hydrogel precursor solution. Thermophoretic migration of components within the precursor solution generates a concentration gradient that mirrors the temperature gradient profile, which translates into mechanical gradients upon crosslinking. Using this technique, GG hydrogels with stiffness gradients ranging from 20 to 90 kPa over 600µm are created, covering the elastic moduli typical of moderately hard to hard tissues. MC3T3 osteoblast cells are then cultured on these gradient substrates, which exhibit preferential migration and enhanced osteogenic potential toward the stiffest region on the gradient. Overall, the thermophoretic approach provides a non-toxic and effective method to create hydrogels with defined mechanical gradients at the micron scale suitable forin vitrobiological studies and potentially tissue engineering applications.

Understanding the mechanical performance of raw and cooked potato cells

The micromechanics of individual potato cells comprising of cell wall and embedded native or gelatinised starch were explored. Micromanipulation can be used to compare cells of distinct strengths and study (bio)mechanical issues related to industrial processing (e.g. heat treatment). Two commercial types of potato, 'baking' and 'Maris Piper' were selected to conduct microcompression experiments. Cells isolated from 'Maris Piper' raw tubers appeared to be more resistant to deformation than the respective ones from the 'baking' cultivar. Cooked cells suffered a decrease in their turgidity which resulted in clusters of observed behaviours, with force-deformation curves showing a single or multiple bursting events. This study provides fundamental work and an insight on the behaviour of potato cells via an exploratory investigation of how different elements of the potato tissue can be measured. The results obtained can be used to relate cellular biology to mechanical properties and could also pave the way to understanding other starch-containing cells (e.g. pea, lentils, wheat).

Discrete element modelling of tomato tissue deformation and failure at the cellular scale

Bruise damage in fruit results from cell wall failure and inter-cellular separation. Despite the importance of the micro-mechanics of plant tissue with respect to its integrity, it remains largely unquantified and poorly understood, due to many difficulties during experimental characterization. In this article, a 3D micro-mechanical plant tissue model that is able to model cell rupture and inter-cellular debonding and thus provide more insight into the micro-mechanics was developed. The model is based on the discrete element method (DEM) and represents the tissue as a mass-spring system. Each plant cell is represented as a deformable visco-elastoplastic triangulated mesh under turgor pressure. To model cell wall rupture, it is assumed that a spring connection in the wall breaks at a certain critical stretch ratio and that a ruptured cell is turgorless. The inter-cellular contact model assumes brittle fracture between a cell's node and an adjacent cell's triangle when their bond distance exceeds a critical value. A high-speed tomato fruit cell compression test was simulated and the modelled force-strain curve compares well with the experimental data, including for strains above the elastic limit. By varying the shape of the cell in the compression simulation it was shown that the force-strain curve is highly dependent on the cell shape and thus parameter fitting procedures based on a spherical cell model will be inaccurate. Furthermore, the wall stiffness and thickness showed a positive linear relationship with the force at cell bursting. Besides simulating compression tests of single cells, we also simulated tensile and compression tests on small tissue specimens. Realistic tissue structures of tomato mesocarp tissue were generated by a novel method using DEM simulations of deformable cells in a shrinking cylinder. The cell area, volume and anisotropy distributions of the virtual tissue compared well with micro-CT images of real tomato mesocarp tissue (normalized root mean square error values smaller than 3%). The tissue compression and tensile test simulations demonstrated an important influence of the inter-cellular bonding energy and tissue porosity on the tissue failure characteristics and elastic modulus.

Cell mechanics in biomedical cavitation

Studies on the deformation behaviours of cellular entities, such as coated microbubbles and liposomes subject to a cavitation flow, become increasingly important for the advancement of ultrasonic imaging and drug delivery. Numerical simulations for bubble dynamics of ultrasound contrast agents based on the boundary integral method are presented in this work. The effects of the encapsulating shell are estimated by adapting Hoff's model used for thin-shell contrast agents. The viscosity effects are estimated by including the normal viscous stress in the boundary condition. In parallel, mechanical models of cell membranes and liposomes as well as state-of-the-art techniques for quantitative measurement of viscoelasticity for a single cell or coated microbubbles are reviewed. The future developments regarding modelling and measurement of the material properties of the cellular entities for cutting-edge biomedical applications are also discussed.

Mechanical disruption of mammalian cells in a microfluidic system and its numerical analysis based on computational fluid dynamics

The lysis of mammalian cells is an essential part of different lab-on-a-chip sample preparation methods, which aim at the release, separation, and subsequent analysis of DNA, proteins, or metabolites. Particularly for the analysis of compartmented in vivo metabolism of mammalian cells, such a method must be very fast compared to the metabolic turnover-rates, it should not affect the native metabolite concentrations, and should ideally leave cell organelles undamaged. So far, no such a method is available. We have developed a microfluidic system for the effective rapid mechanical cell disruption and established a mathematical model to describe the efficiency of the system. Chinese hamster ovary (CHO) cells were disrupted with high efficiency by passing through two consecutive micronozzle arrays. Simultaneous cell compression and shearing led to a disruption rate of ≥90% at a sample flow rate of Q = 120 μL min(-1) per nozzle passage, which corresponds to a mean fluid velocity of 13.3 m s(-1) and a mean Reynolds number of 22.6 in the nozzle gap. We discussed the problem of channel clogging by cellular debris and the resulting flow instability at the micronozzle arrays. The experimental results were compared to predictions from Computational Fluid Dynamics (CFD) simulations and the critical energy dissipation rate for the disruption of the CHO cell population with known size distribution was determined to be 4.7 × 10(8) W m(-3). Our model for the calculation of cell disruption on the basis of CFD-data could be applied to other microgeometries to predict intended disruption or undesired cell damage.

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.

Determining the mechanical properties of yeast cell walls

The intrinsic cell wall mechanical properties of Baker's yeast (Saccharomyces cerevisiae) cells were determined. Force-deformation data from compression of individual cells up to failure were recorded, and these data were fitted by an analytical model to extract the elastic modulus of the cell wall and the initial stretch ratio of the cell. The cell wall was assumed to be homogeneous, isotropic, and incompressible. A linear elastic constitutive equation was assumed based on Hencky strains to accommodate the large stretches of the cell wall. Because of the high compression speed, water loss during compression could be assumed to be negligible. It was then possible to treat the initial stretch ratio and elastic modulus as adjustable parameters within the analytical model. As the experimental data fitted numerical simulations well up to the point of cell rupture, it was also possible to extract cell wall failure criteria. The mean cell wall properties for resuspended dried Baker's yeast were as follows: elastic modulus 185 ± 15 MPa, initial stretch ratio 1.039 ± 0.006, circumferential stress at failure 115 ± 5 MPa, circumferential strain at failure 0.46 ± 0.03, and strain energy per unit volume at failure 30 ± 3 MPa. Data on yeast cells obtained by this method and model should be useful in the design and optimization of cell disruption equipment for yeast cell processing.

Estimation of disruption of animal cells by laminar shear stress

Using mechanical cell properties measured by micromanipulation, and a model of cell distortion in laminar flow fields, a method has been developed for predicting disruption of animal cells by laminar shear stresses. Predictions of the model were compared with measured losses of cell number and viability of TB/C3 murine hybridomas sheared in a cone and plate viscometer at shear rates up to 3950 s(-1), and shear stresses up to 600 Nm(-2), achieved by enhancement of viscosity with dextran. In all cases, the experimental, results and predictions were within 30%. Such excellent agreement suggests it might be possible to use micromanipulation measurements of animal cell mechanical properties to predict cell damage in more complex flow fields, such as those in bioreactors.

Measuring the mechanical properties of single microbial cells

Many cells are considered to be susceptible to mechanical forces or "shear" in bioprocessing, leading to undesirable cell breakage or adverse metabolic effects. However, cell breakage is the aim of some processing operations, in particular high-pressure homogenisation and other cell disruption methods. In either case, the exact mechanisms of damage or disruption are obscure. One reason for this is that the mechanical properties of the cells are generally unknown, which makes investigation or prediction of the damage difficult. There are several methods for measuring the mechanical properties of single microbial cells, and these are reviewed briefly. In the context of bioprocessing research, a powerful method of characterising the mechanical properties of single cells is compression testing using micromanipulation, supplemented by mathematical modelling of the cell behaviour in compression. The method and associated modelling are described, with results mainly from studies on yeast cells. Continuing difficulties in making a priori predictions of cell breakage in processing are identified. In future, compression testing by micromanipulation might also be used in conjunction with other single cell analytical techniques to study mechanisms controlling form, growth and division of cells and their consequential mechanical behaviour. It ought to be possible to relate cell wall mechanics to cell wall composition and structure, and eventually to underlying gene expression, allowing much greater understanding and control of the cell mechanical properties.

Induction of mammalian cell death by simple shear and extensional flows

In this work we investigated whether the type of shear flow, to which cells are exposed, influences the initiation of cell death. It is shown that mammalian cells, indeed, distinguish between discrete types of flow and respond differently. Two flow devices were employed to impose accurate hydrodynamic flow fields: uniform steady simple shear flow and oscillating extensional flow. To distinguish between necrotic and apoptotic cell death, fluorescence activated cell sorting and the release of DNA in the culture supernatant was used. Results show that Chinese Hamster Ovaries and Human Embryonic Kidney cells will enter the apoptotic pathway when subjected to low levels of hydrodynamic stress (around 2.0 Pa) in oscillating, extensional flow. In contrast, necrotic death prevails when the cells are exposed to hydrodynamic stresses around 1.0 Pa in simple shear flow or around 500 Pa in extensional flow. These threshold values at which cells enter the respective death pathway should be avoided when culturing cells for recombinant protein production to enhance culture longevity and productivity.

Protein release from yeast cells as an evaluation method of physical effects in ultrasonic field

The release rate of intercellular protein from yeast cells by the ultrasonic action is proposed as a method for evaluating the physical (mechanical) effects of the ultrasonic field. The protein concentration was quantitatively determined using UV absorbance of proteins by spectrophotometry. The detail of the procedures, such as the effects of the origin of yeast cells, pretreatment of the cells, and the wavelengths for spectrophotometric determination of protein content, are examined. The effectiveness of the proposed evaluation method was experimentally demonstrated by changing the irradiation conditions of ultrasound, such as the concentration of yeast cells, temperature, ultrasound power, types of sonicator, and the superposition with the mechanical mixing. The results validate the usefulness of the proposed evaluation method for the quantification of the physical effects of ultrasound fields. Also, the range of cavitational effects of ultrasound sensed by the evaluation procedures were discussed.

Characterization of the localized hydrodynamic shear forces and dissolved oxygen distribution in sparged bioreactors

Detailed, high-resolution numerical simulations of the bubbly flows, used for oxygen delivery and mixing in mammalian cell suspensions, have been performed. The hydrodynamics, shear and normal forces, mass transfer and mass transport from and around individual bubbles and bubble clusters were resolved for different operating conditions, that is, Weber, Morton, and Schmidt numbers. Suspended animal (e.g., mammalian, insect) cells are known to be susceptible to damage potentially leading to cell death, caused by hydrodynamic stresses and oxygen deprivation. Better knowledge of the magnitude of the shear forces and the extent of mixing of the dissolved oxygen in sparged bioreactors can have a significant impact on their future design and optimization. Therefore, the computed liquid-phase velocity fields were used to calculate and compare the local shear in different types of single bubble wakes and in bubble clusters. Oxygen mass transfer and dissolved oxygen transport were resolved to examine oxygen supply to the cells in the different types of flows.

Reactor engineering in large scale animal cell culture

This article mainly addresses the issues associated with the engineering of large-scale free suspension culture in agitated bioreactors >10,000 L because they have become the system of choice industrially. It is particularly concerned with problems that become increasingly important as the scale increases. However, very few papers have been written that are actually based on such large-scale studies and the few that do rarely address any of the issues quantitatively. Hence, it is necessary very often to extrapolate from small-scale work and this review tries to pull the two types of study together. It is shown that 'shear sensitivity' due to agitation and bursting bubbles is no longer considered a major problem. Homogeneity becomes increasingly important with respect to pH and nutrients at the largest scale and sub-surface feeding is recommended despite 'cleaning in place' concerns. There are still major questions with cell retention/recycle systems at these scales, either because of fouling, of capacity or of potential and different 'shear sensitivity' questions. Fed-batch operation gives rise to cell densities that have led to the use of oxygen and enriched air to meet oxygen demands. This strategy, in turn, gives rise to a CO(2) evolution rate that impacts on pH control, pCO(2) and osmolality. These interactions are difficult to resolve but if higher sparge and agitation intensities could be used to achieve the necessary oxygen transfer, the problem would largely disappear. Thus, the perception of 'shear sensitivity' is still impacting on the development of animal cell culture at the commercial scale. Microcarrier culture is also briefly addressed. Finally, some recommendations for bioreactor configuration and operating strategy are given.

Mechanical and failure properties of single attached cells under compression

Eukaryotic cells are continuously subjected to mechanical forces under normal physiological conditions. These forces and associated cellular deformations induce a variety of biological processes. The degree of deformation depends on the mechanical properties of the cell. As most cells are anchorage dependent for normal functioning, it is important to study the mechanical properties of cells in their attached configuration. The goal of the present study was to obtain the mechanical and failure properties of attached cells. Individual, attached C2C12 mouse myoblasts were subjected to unconfined compression experiments using a recently developed loading device. The device allows global compression of the cell until cell rupture and simultaneously measures the associated forces. Cell bursting was characterized by a typical reduction in the force, referred to as the bursting force. Mean bursting forces were calculated as 8.7+/-2.5 microN at an axial strain of 72+/-4%. Visualization of the cell using confocal microscopy revealed that cell bursting was preceded by the formation of bulges at the cell membrane, which eventually led to rupturing of the cell membrane. Finite element calculations were performed to simulate the obtained force-deformation curves. A finite element mesh was built for each cell to account for its specific geometrical features. Using an axisymmetric approximation of the cell geometry, and a Neo-Hookean constitutive model, excellent agreement between predicted and measured force-deformation curves was obtained, yielding an average Young's modulus of 1.14+/-0.32 kPa.

Mechanical properties of desiccated ragweed pollen grains determined by micromanipulation and theoretical modelling

The mechanical properties of desiccated ragweed pollen grains were determined using a micromanipulation technique and a theoretical model. Single pollen grains with a diameter of approximately 20 microm were compressed and held, compressed and released, and compressed to rupture at different speeds between two parallel surfaces. Simultaneously, the force being imposed on the pollen grains was measured. It has been found that the rupture force of pollen grains increased linearly with their displacement at rupture on average, but was independent of their diameter. The mean rupture force was 1.20 +/- 0.03 mN, and mean deformation (the ratio between the displacement and diameter) at rupture was 22 +/- 0.6%. Single pollen grains were modeled as a capsule with a core full of air and a non permeable wall. A constitutive equation based on Hookean law was used to determine the mechanical property parameters Eh (product of the Young's modulus and wall thickness), and the mean value of Eh of desiccated pollen gains was estimated to be 1653 +/- 36 N/m.

Mathematical modelling of the cellular mechanics of plants

The complex mechanical behaviour of plant tissues reflects the complexity of their structure and material properties. Modelling has been widely used in studies of how cell walls, single cells and tissue respond to loading, both externally applied loading and loads on the cell wall resulting from changes in the pressure within fluid-filled cells. This paper reviews what approaches have been taken to modelling and simulation of cell wall, cell and tissue mechanics, and to what extent models have been successful in predicting mechanical behaviour. Advances in understanding of cell wall ultrastructure and the control of cell growth present opportunities for modelling to clarify how growth-related mechanical properties arise from wall polymeric structure and biochemistry.

Hydrodynamic damage to animal cells

Animal cells are affected by hydrodynamic forces that occur in culture vessel, transfer piping, and recovery operations such as microfiltration. Depending on the type, intensity, and duration of the force, and the specifics of the cell, the force may induce various kinds of responses in the subject cells. Both biochemical and physiological responses are observed, including apoptosis and purely mechanical destruction of the cell. This review examines the kinds of hydrodynamic forces encountered in bioprocessing equipment and the impact of those forces on cells. Methods are given for quantifying the magnitude of the specific forces, and the response thresholds are noted for the common types of cells cultured in free suspension, supported on microcarriers, and anchored to stationary surfaces.

Pore sizes in hydrated dextran microspheres

The average pore size of hydrated dextran microspheres is derived from rheological and protein release data. The microspheres were prepared by cross-linking an aqueous solution of methacrylated dextran emulsified in a continuous poly(ethylene glycol) phase. The rheological data were obtained using a novel micromanipulation technique, which enables the compression of a single microsphere. The so obtained pseudoelasticity moduli of the microspheres were derived from these compression data and corresponded well with the elasticity moduli of macroscopic hydrogels of the same composition, as determined with dynamic mechanical analysis. The modulus increased with decreasing water contents of the microspheres and with increasing degrees of methacrylate substitution of the dextran used. Furthermore, the average pore sizes calculated from the pseudoelasticity moduli were in good agreement with the pore sizes derived from protein release data. In conclusion, this study shows that micromanipulation provides insight into the average pore sizes of dextran microspheres, which is an important characteristic that will modulate the release of encapsulated proteins.

Observations on the shear damage to different animal cells in a concentric cylinder viscometer

A clear distinction is made between damage to the population of cells and damage to individual cells on exposure to shear stress. Work on mechanical damage to animal cells in suspension is reported for six different cell lines. Precisely controlled shears of 1 Pa, 10 Pa, and 100 Pa were generated in a viscometer and distortions in morphology of the cells-for instance, the formation of transient pores, cytoplasmic extrusions, and ghost-cell membranes-are presented with photographic evidence. Low shears are shown to be just as damaging as the higher shears, although the type of damage is different. It follows that bioreactors should be operated at intermediate shear levels for optimal yield. A mechanism to account for the unexpected stability of animal cells at intermediate levels of shear is presented.

Modelling the effect of osmolality on the bursting strength of yeast cells

When yeast cells are resuspended in buffer prior to homogenisation, the diluent osmotic pressure can have a significant effect on cell mechanical strength. In this paper a model is proposed which describes the relationship between the cell bursting force and the osmotic pressure of the diluent, using chemical potential and force balance analyses. Yeast cells were exposed for 1 h to diluents with osmolalities varying from almost 0 to 700 mmol kg-1 before their bursting strengths were measured by micromanipulation. The experimental data were compared with the predictions made from the model and in general they were in good agreement. It is expected that this model might be used to understand cell disruption behaviour in downstream processing equipment such as homogenisers.

The use of flow cytometry to study the impact of fluid mechanical stress on Escherichia coli W3110 during continuous cultivation in an agitated bioreactor

Continuous culture fermentations of Escherichia coli W3110 have been carried out at controlled dissolved oxygen levels of 40% and 10% of saturation. Satisfactory and reproducible results were obtained. Agitation speeds of 400 and 1200 rpm at an aeration rate of 1 vvm have been used as well as an aeration rate of 3 vvm at 400 rpm. The upper levels of these variables represent much higher agitation and aeration intensities than those normally used in practical fermentations. The fermentations were monitored by mass spectrometry and optical density, and cell samples were studied by flow cytometry, SEM, and TEM. Protocols were developed so the state of both cell membranes and cell size could be measured by flow cytometry. Under all the conditions of agitation and aeration, flow cytometric analysis indicated that both cell membranes were intact and that a cytoplasmic membrane potential existed; also the cell size did not change, results confirmed by SEM and TEM. There were no detectable changes in off-gas analysis or optical density during the continuous fermentation nor in the cell structure as revealed by SEM or TEM, except at the highest agitation intensity. Under the latter conditions, after 7 h, the outer polysaccharide layer on the cell was stripped away. It is concluded that any changes in biological performance of this E. coli cell line due to variations in agitation or aeration intensity or scale of operation cannot be attributed to fluid dynamic stresses associated with the turbulence generated by impellers or with bursting bubbles.

Process-scale disruption of microorganisms

Common hosts for the large-scale manufacture of biological products, such as Escherichia coli and Saccharomyces cerevisiae, do not excrete products to the medium. Effective techniques for cell disruption are therefore required. These include physical, chemical, enzymatic and mechanical methods. Mechanical methods such as bead milling, high-pressure homogenization, and microfluidization are preferred. However, gentler, specific methods are receiving increasing attention particularly when used in combination to synergistically exploit their different specificities. Benefits can also be derived by integrating product release and recovery. In all cases it is essential to consider the interaction of the disruption operation with downstream units and to clearly demonstrate the cost benefits of alternative strategies.

Prediction of mechanical damage to animal cells in turbulence

In previous work a model was proposed for estimation of disruption of animal cells in turbulent capillary flows using information about the hydrodynamics, and cell mechanical properties determined by micromanipulation. The model assumed that the capillary flow consists of a laminar sublayer and a homogeneous turbulent region, and within the latter eddies of sizes similar to or smaller than the cells interact with those cells, causing local surface deformations. The proposed mechanism of cell damage was that such deformations result in an increase in membrane tension and surface energy, and that a cell disrupts when its bursting membrane tension and bursting surface energy are exceeded. The surface energy of the cells was estimated from the kinetic energy of appropriate sized eddies. To test the model, cells were disrupted in turbulent flows in capillaries at mean energy dissipation rates ranging from 800 to 2 x 10(4) Wkg-1. The model assumed that the specific lysis rate is almost independent of the number of passes, which was verified by the experimental data. The implication was that despite the damage the cell mechanical properties did not change markedly during multiple recirculations through the capillaries. On average the model underestimated the cell disruption by about 15%. Although the model gave reasonably good predictions, it lacks proper explanation of the independence of the specific lysis rate on the number of passes. In this paper it is shown that this problem can be resolved in principle by consideration of the localisation of the energy dissipation in turbulent capillary flows. The necessity of further modelling of cell-turbulence interactions is demonstrated.

Effect of Pluronic F-68 on the mechanical properties of mammalian cells

The mechanical properties of TB/C3 hybridoma cells taken from a continuous culture were measured by micromanipulation. The culture conditions were constant except for the presence or absence of Pluronic F-68 in the medium. It was found that the mean bursting membrane tension and the mean elastic area compressibility modulus of the cells were significantly greater (60% and 120%, respectively) in a medium with 0.05% (w/v) Pluronic F-68 compared to that without Pluronic. Pluronic F-68 therefore affected the strength of the membranes when the cells were exposed to it for a long period of time, i.e., in culture. The short-term effect of Pluronic F-68 on cell strength was also tested by its addition at various levels up to 0.2% (w/v) immediately before the mechanical property measurements. The resulting cell strength depended on the Pluronic concentration, but a significant short-term effect could only be detected above a threshold of 0.1% (w/v). Previous reports on the effect of Pluronic F-68 on animal cell culture are evaluated in the light of these observations.