A novel micromanipulation technique for measuring the bursting strength of single mammalian cells

Microplastic-Free Microcapsules Using Supramolecular Self-Assembly of Bis-Urea Molecules at an Emulsion Interface

Encapsulation technology is well established for entrapping active ingredients within an outer shell for their protection and controlled release. However, many solutions employed industrially use nondegradable cross-linked synthetic polymers for shell formation. To curb rising microplastic pollution, regulatory policies are forcing industries to substitute the use of such intentionally added microplastics with environmentally friendly alternatives. This work demonstrates a one-pot process to make microplastic-free microcapsules using supramolecular self-assembly of bis-ureas. Molecular bis-urea species generated in-situ spontaneously self-assemble at the interface of an oil-in-water emulsion via hydrogen bonding to form a shell held together by noncovalent bonds. In addition, Laponite nanodiscs were introduced in the formulation to restrict aggregation observed during the self-assembly and to reduce the porosity of the shell, leading to well-dispersed microcapsules (mean Sauter diameter d [3,2] ∼ 5 μm) with high encapsulation efficiency (∼99%). Accelerated release tests revealed an increase in characteristic release time of the active by more than an order of magnitude after encapsulation. The mechanical strength parameters of these capsules were comparable to some of the commercial, nondegradable melamine-formaldehyde microcapsules. With mild operating conditions in an aqueous environment, this technology has real potential to offer an industrially viable method for producing microplastic-free microcapsules.

Improving Cleaning Efficiency through the Measurement of Food Fouling Adhesive Strength

This study aims to investigate the impacts of factors, including textural properties, surface roughness, and contact angle, on the cleaning performance of food soils and develop a preliminary mathematical model to predict the cleaning score, depending on the soil-surface properties. The force required to remove soil from the surface was determined by a texture analyzer equipped with a newly designed probe. Potato puree and egg yolk soils showed high adhesive forces compared to other deposits. Margarine required the lowest force to detach from the surfaces. A soil-surface characteristic number (SSCN) was constructed from the results of contact angle, roughness, and textural analysis to predict the cleaning score depending on the soil-surface properties. The experimental work presented indicates that a higher SSCN was associated with lower cleaning scores for soil-surface combinations. Furthermore, a predictive model was developed to define the relationship between cleaning scores and SSCN. The applicability of the model was validated by measuring the cleaning performance of caramel and pudding soils on glass, porcelain, and stainless-steel household surfaces by using an automatic method. Therefore, it can be concluded that the SSCN approach can be improved in further studies to predict cleaning scores of soil-surface combinations in the experimental rig or automatic dishwasher.

Analytical Models for Measuring the Mechanical Properties of Yeast

The mechanical properties of yeast play an important role in many biological processes, such as cell division and growth, maintenance of internal pressure, and biofilm formation. In addition, the mechanical properties of cells can indicate the degree of damage caused by antifungal drugs, as the mechanical parameters of healthy and damaged cells are different. Over the past decades, atomic force microscopy (AFM) and micromanipulation have become the most widely used methods for evaluating the mechanical characteristics of microorganisms. In this case, the reliability of such an estimate depends on the choice of mathematical model. This review presents various analytical models developed in recent years for studying the mechanical properties of both cells and their individual structures. The main provisions of the applied approaches are described along with their limitations and advantages. Attention is paid to the innovative method of low-invasive nanomechanical mapping with scanning ion-conductance microscopy (SICM), which is currently starting to be successfully used in the discovery of novel drugs acting on the yeast cell wall and plasma membrane.

Viscoelastic Properties of Polymeric Microneedles Determined by Micromanipulation Measurements and Mathematical Modelling

Microneedles, including dissolvable ones made from biocompatible and biodegradable materials, have been widely studied and can potentially be used for transdermal drug delivery, disease diagnosis (sampling), skin care, etc. Characterizing their mechanical properties is essential, as being mechanically strong enough to pierce the skin barrier is one of the most fundamental and crucial requirements for them. The micromanipulation technique was based on compressing single microparticles between two flat surfaces to obtain force and displacement data simultaneously. Two mathematical models had already been developed to calculate the rupture stress and apparent Young's modulus, which can identify variations of these parameters in single microneedles within a microneedle patch. In this study, a new model has been developed to determine the viscoelasticity of single microneedles made of hyaluronic acid (HA) with a molecular weight of 300 kDa loaded with lidocaine by using the micromanipulation technique to gather experimental data. The modelling results from the micromanipulation measurements suggest that the microneedles were viscoelastic and their mechanical behaviour was strain-rate dependent, which implies that the penetration efficiency of viscoelastic microneedles can be improved by increasing their piercing speed into the skin.

Prediction of vascular injury by cavitation microbubbles in a focused ultrasound field

Many studies have shown that microbubble cavitation is one mechanism for vascular injury under ultrasonic excitation. Previous work has attributed vascular damage to vessel expansions and invaginations due to the expansion and contraction of microbubbles. However, the mechanisms of vascular damage are not fully understood. In this paper, we investigate, theoretically and experimentally, the vessel injury due to stress induced by ultrasound-induced cavitation (UIC). A bubble-fluid-vessel coupling model is constructed to investigate the interactions of the coupling system. The dynamics process of vessel damage due to UIC is theoretically simulated with a finite element method, and a focused ultrasound (FU) setup is carried out and used to assess the vessel damage. The results show that shear stress contributes to vessel injury by cell detachment while normal stress mainly causes distention injury. Similar changes in cell detachment in a vessel over time can be observed with the experimental setup. The severity of vascular injury is correlated to acoustic parameters, bubble-wall distance, and microbubble sizes, and the duration of insonation..

Micromanipulation and Automatic Data Analysis to Determine the Mechanical Strength of Microparticles

Microparticles are widely used in many industrial sectors. A micromanipulation technique has been widely used to quantify the mechanical properties of individual microparticles, which is crucial to the optimization of their functionality and performance in end-use applications. The principle of this technique is to compress single particles between two parallel surfaces, and the force versus displacement data are obtained simultaneously. Previously, analysis of the experimental data had to be done manually to calculate the rupture strength parameters of each individual particle, which is time-consuming. The aim of this study is to develop a software package that enables automatic analysis of the rupture strength parameters from the experimental data to enhance the capability of the micromanipulation technique. Three algorithms based on the combination of the “three-sigma rule”, a moving window, and the Hertz model were developed to locate the starting point where onset of compression occurs, and one algorithm based on the maximum deceleration was developed to identify the rupture point where a single particle is ruptured. Fifty microcapsules each with a liquid core and fifty porous polystyrene (PS) microspheres were tested in order to produce statistically representative results of each sample, and the experimental data were analysed using the developed software package. It is found that the results obtained from the combination of the “3σ + window” algorithm or the “3σ + window + Hertz” algorithm with the “maximum-deceleration” algorithm do not show any significant difference from the manual results. The data analysis time for each sample has been shortened from 2 to 3 h manually to within 20 min automatically.

Determination of the mechanical properties of polymeric microneedles by micromanipulation

Precise characterization of the mechanical properties of polymeric microneedles is crucial for their successful penetration into skin and delivery of the loaded active ingredients. However, most available strategies for this purpose are based on compression of the whole patch, which only provide the average rupture force of the needles and can not give information on the variations across individual microneedles in the patch. In this study, we determined the mechanical strength of individual microneedles of two types of hyaluronic acid microneedles with or without loaded model drugs using a micromanipulation technique. The applied force as a function of displacement of the microneedles was recorded, which was used to determine the rupture displacement, rupture force, and then to derive and calculate normal stress-deformation curve, rupture stress and Young's modulus of individual microneedles. The obtained data suggest that the molecular weight of the polymer and the loading of drug into the microneedles can significantly affect the rupture behavior and mechanical properties of the microneedles, which provides a foundation for preparing sufficiently strong microneedles for controlled drug delivery.

Microfluidic assessment of mechanical cell damage by extensional stress

Mammalian cells have been widely used in bioreactors to produce biological products such as pharmaceutical materials. The productivity of such bioreactors is vastly affected by flow-induced cell damage in complicated flow environments, such as agitation-driven turbulence and oxygen bubble bursting at the interface between the culturing medium and air. However, there is no systematic approach to diagnose the cell damage caused by the hydrodynamic stress. In this work, we propose a novel microfluidic method to accurately assess the mechanical cell damage under a controlled extensional stress field, generated in a microfluidic cross-slot geometry. The cell damage in the extensional field is related to the oxygen bubble bursting process. We employed viscoelasticity-induced particle focusing to align the cells along the shear-free channel centerline, so that all the cells experience a similar extensional stress field, which also precludes the cell damage due to wall shear stress. We applied our novel microfluidic sensor to find the critical extensional stress to damage Chinese hamster ovary (CHO) cells; the critical stress is found to be ∼250 Pa. Our current results are relevant in the design of practical bioreactors, as our results clearly demonstrate that the control of the bubble bursting process is critical in minimizing cell damage in bioreactor applications. Further, our results will provide useful information on the biophysical cell properties under fluid flow environments.

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.

Molecular profiling of single cells in response to mechanical force: comparison of chondrocytes, chondrons and encapsulated chondrocytes

A chondrocyte and its surrounding pericellular matrix (PCM) are defined as a chondron. The PCM plays a critical role in enhancing matrix production, protecting the chondrocyte during loading and transducing mechanical signals. Tissue engineering involves the design of artificial matrices which aim to mimic PCM function for mechanical strength and signalling motifs. We compare the mechanical performance and mechanoresponsiveness of chondrocytes with and without PCM, and encapsulated by alternate adsorption of two oppositely charged polyelectrolytes; chitosan and hyaluronan. Zeta potential measurements confirmed the success of the encapsulation. Encapsulation did not influence chondrocyte viability or metabolic activity. Cells were compressed by micromanipulation with final deformations to 30%, 50% and 70%. Force-displacement data showed that the larger the deformation at the end of compression, the greater the force on the cell. Mechanoresponsiveness of cells was studied by combining single cell PCR with dynamic compression at 20% and 40%. Aggrecan and Type II collagen gene expression were significantly increased in encapsulated chondrocytes and chondrons compared to chondrocytes whereas dynamic compression had no effect on SOX9 or lubricin gene expression. Our results demonstrate that although encapsulation can mimic responses of chondrocytes to biomechanical compression the molecular profile did not reach the enhanced levels observed with chondrons.

An atomic force microscope operating at hypergravity for in situ measurement of cellular mechano-response

We present a novel atomic force microscope (AFM) system, operational in liquid at variable gravity, dedicated to image cell shape changes of cells in vitro under hypergravity conditions. The hypergravity AFM is realized by mounting a stand-alone AFM into a large-diameter centrifuge. The balance between mechanical forces, both intra- and extracellular, determines both cell shape and integrity. Gravity seems to be an insignificant force at the level of a single cell, in contrast to the effect of gravity on a complete (multicellular) organism, where for instance bones and muscles are highly unloaded under near weightless (microgravity) conditions. However, past space flights and ground based cell biological studies, under both hypogravity and hypergravity conditions have shown changes in cell behaviour (signal transduction), cell architecture (cytoskeleton) and proliferation. Thus the role of direct or indirect gravity effects at the level of cells has remained unclear. Here we aim to address the role of gravity on cell shape. We concentrate on the validation of the novel AFM for use under hypergravity conditions. We find indications that a single cell exposed to 2 to 3 x g reduces some 30-50% in average height, as monitored with AFM. Indeed, in situ measurements of the effects of changing gravitational load on cell shape are well feasible by means of AFM in liquid. The combination provides a promising technique to measure, online, the temporal characteristics of the cellular mechano-response during exposure to inertial forces.

Strain-dependent viscoelastic behaviour and rupture force of single chondrocytes and chondrons under compression

The chondron in articular cartilage includes the chondrocyte and its surrounding pericellular matrix (PCM). Single chondrocytes and chondrons were compressed between two parallel surfaces by a micromanipulation technique to investigate their biomechanical properties and to discover the mechanical significance of the PCM. The force imposed on the cells was measured directly during deformation at various compression speeds and deformations up to cell rupture. When the deformation at the end of compression was 50%, relaxation showed that the cells were viscoelastic, but this viscoelasticity was generally insignificant at 30% deformation or lower. When the deformation was 70%, the cells had deformed plastically. Chondrons ruptured at a mean deformation of 85 +/- 1%, whilst chondrocytes ruptured at a mean deformation of 78 +/- 1%. Chondrons were generally stiffer than chondrocytes and showed less viscoelastic behaviour than chondrocytes. Thus, the PCM significantly influences the mechanical properties of the cells.

Gene expression profiles of dynamically compressed single chondrocytes and chondrons

A chondrocyte produces a hydrated pericellular matrix (PCM); together they form a chondron. Previous work has shown that the presence of the PCM influences the biological response of chondrocytes to loading. The objective of this study was to determine the gene expression profiles of enzymatically isolated single chondrocytes and chondrons in response to dynamic compression. Cartilage specific extracellular matrix components and transcription factors were examined. Following dynamic compression, chondrocytes and chondrons showed variations in gene expression profiles. Aggrecan, Type II collagen and osteopontin gene expression were significantly increased in chondrons. Lubricin gene expression decreased in both chondrons and chondrocytes. Dynamic compression had no effect on SOX9 gene expression. Our results demonstrate a clear role for the PCM in interfacing the mechanical signalling in chondrocytes in response to dynamic compression. Further investigation of single chondrocytes and chondrons from different zones within articular cartilage may further our understanding of cartilage mechanobiology.

Spherical Alginate Granules Formulated for Quick-Release Active Subtilisin

Novel attrition-resistant and spherical enzyme granules encapsulating active subtilisin were formed by emulsification of 2% alginate sol loaded with active enzyme, instantaneous gelation triggered through in situ release of Ca(2+) (internal gelation), particle separation, and finally acetone extractive drying. Granular subtilisin was highly active, readily dispersible, and mechanically robust. This technique serves as a new and attractive alternative to established enzyme granulation processes, such as fluid bed coating, extrusion followed by marumerization, drum granulation, or prilling, for use in industrial enzyme applications such as detergents, textile manufacturing, and food processing. The formulation and encapsulation conditions were optimized to maximize the resistance of the granule to compression and impact forces, consistent with enzyme release and particle dispersion in detergent solutions. Well characterized alginates, with specified guluronic/mannuronic acid (G/M) content and molecular weight, were used in the formulation. The characteristics of the resulting microspheres, including their size and distribution, morphology, shrinkage, compression resistance, impact strength, solubility and encapsulation yield, were examined. Spherical dry granules were formulated with a mean diameter of 500 microm with particle sizes ranging from 300 to 800 microm. Dry alginate granules were discrete, spherical, and glossy white and exhibited impact strength, compression resistance, and solubility difference dependent on composition. Reduced starch levels, high alginate concentration, low alginate molecular weight, and use of high guluronate alginates resulted in the lowest dust level and highest compression resistance. Subtilisin mass yields were approximately 50%, and specific activity yields ranged from 60% to 100%. A formulation consisting of 3% SG150 alginate, 10% starch, 10% TiO(2), and 1% CaCO(3) provided granules appropriate for use in detergent application.

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.

Viscoelastic properties of single attached cells under compression

The viscoelastic properties of single, attached C2C12 myoblasts were measured using a recently developed cell loading device. The device allows global compression of an attached cell, while simultaneously measuring the associated forces. The viscoelastic properties were examined by performing a series of dynamic experiments over two frequency decades (0.1-10 Hz) and at a range of axial strains (approximately 10-40%). Confocal laser scanning microscopy was used to visualize the cell during these experiments. To analyze the experimentally obtained force-deformation curves, a nonlinear viscoelastic model was developed. The nonlinear viscoelastic model was able to describe the complete series of dynamic experiments using only a single set of parameters, yielding an elastic modulus of 2120 +/- 900 Pa for the elastic spring, an elastic modulus of 1960 +/- 1350 for the nonlinear spring, and a relaxation time constant of 0.3 +/- 0.12 s. To our knowledge, it is the first time that the global viscoelastic properties of attached cells have been quantified over such a wide range of strains. Furthermore, the experiments were performed under optimal environmental conditions and the results are, therefore, believed to reflect the viscoelastic mechanical behavior of cells, such as would be present in vivo.

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.

Monitoring the biomechanical response of individual cells under compression: a new compression device

Skeletal muscle cells are sensitive to sustained compression, which can lead to the development of pressure sores. Although it is known that this type of tissue breakdown depends on the magnitude and duration of the applied load, the exact relationship between cell deformation and damage remains unclear. To gain more insight into this process, a method has been developed, that incorporates the use of a new loading device and confocal microscopy. The loading device is able to compress individual cells, either statically or dynamically, while measuring the resulting forces. Experiments can be performed under ideal environmental conditions, comparable with those of a CO2 incubator. First compression experiments on C2C12 mouse myoblasts showed the shape changes that cells undergo during static compression by the loading device. Calculations using the three-dimensional confocal images showed no change in volume and an increase in the surface area of the cell as a result of compression. The device presented here provides a useful way to monitor the biomechanical response of skeletal muscle cells during long-term compression experiments. Therefore it will contribute to the knowledge about strain-induced cell damage, as seen in pressure sores and other mechanically induced clinical conditions.

Inertial shear forces and the use of centrifuges in gravity research. What is the proper control?

Centrifuges are used for 1 x g controls in space flight microgravity experiments and in ground based research. Using centrifugation as a tool to generate an Earth like acceleration introduces unwanted inertial shear forces to the sample. Depending on the centrifuge and the geometry of the experiment hardware used these shear forces contribute significantly to the total force acting on the cells or tissues. The inertial shear force artifact should be dealt with for future experiment hardware development for Shuttle and the International Space Station (ISS) as well as for the interpretation of previous space-flight and on-ground research data.

Mechanical strength of microcapsules made of different wall materials

The mechanical strength of microcapsules made of three different wall materials, including melamine-formaldehyde resin, urea-formaldehyde resin and gelatin-gum arabic coacervate, were measured by a micromanipulation technique. Single microcapsules were compressed to large deformations or rupture and the force being imposed on them were measured simultaneously. Melamine-formaldehyde and urea-formaldehyde microcapsules showed clear bursting under compression, and their bursting force, deformation at bursting and deformation at a pesudo yield point were determined. Gelatin microcapsules did not show clear bursting under compression, and their mechanical strength was characterized by the force required to cause their deformation to 50%. The mechanical strengths of these three types of microcapsules are compared in this paper.

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.

Strength of mid-logarithmic and stationary phase Saccharopolyspora erythraea hyphae during a batch fermentation in defined nitrate-limited medium

A method for measuring mechanical properties of Saccharopolyspora erythraea is reported with data from a batch fermentation. Briefly, hyphae were glued to the end of a tungsten filament mounted horizontally on a sensitive force transducer. Free ends of hyphae were trapped against a flat surface by a second probe. The force transducer and tungsten filament were then moved at a fixed rate, the hypha were strained, and the force resisting motion recorded. From these data the maximum force resisting motion is taken as the force at which breakage occurs. Hyphae from the mid-logarithmic phase of a simple batch fermentation on defined medium were found to have a breaking force of 890 +/- 160 nN (95% confidence), while stationary phase hyphae were weaker at 580 +/- 150 nN. Video recordings of the experiments allowed an approximation of breaking strain, which did not differ significantly between samples at 0.18 +/- 0.03. Electron microscopy was used to measure cell wall thickness, cell diameter, and hence cell wall cross-sectional area. The ultimate tensile strength was estimated to be 24 +/- 3 MPa with no difference between the two samples, the lower breaking force of the stationary phase hyphae being attributed to a thinner cell wall. Assuming a linear relationship between stress and strain, the elastic modulus was estimated to be 140 +/- 30 MPa. These values are comparable with other structural biological materials such as yeast cell walls and collagen.

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.

Determination of growth and lysis kinetics in plant cell suspension cultures from the measurement of esterase release

The death of Medicago sativa L. cells cultivated in a batch culture was investigated by measuring both the appearance of intact dead cells determined on the basis of the trypan blue (TB) dye exclusion, and the release of the cytoplasmic esterase activity into the culture medium upon cell death. Taking into account the strong instability of this released esterase activity, the total dead cell and lysed cell densities have been estimated. A mechanism for cell death and lysis is proposed and the specific rates of cell growth, death and lysis estimated. The specific rate of appearance of TB dead cells was low and essentially constant (0.25 day(-1)) during the first 8 days of the batch culture, and then increased above 1.5 day(-1) after 2 weeks of cultivation. Whereas no lysis occurred during the first seven days, this phenomenon occurred during the second period and accounted for about 20% of the total cell death by the end of the process. Thus, the viability determined by the trypan blue exclusion method appeared to be invalid after 7 days of culture. When lysis of viable cells is taken into consideration, the specific growth rate was significantly increased and growth was shown to continue for a further 8 days. Increased sensitivity of the cells to shear stresses and consequent cell lysis could be the result of a 35% increase in the cell size

Mechanical strength of single microcapsules determined by a novel micromanipulation technique

A micromanipulation technique has been developed to measure the bursting force of single dry microcapsules coated onto a surface, such as those normally used in carbonless copying paper. For measuring the bursting force of a given microcapsule, a single fine probe with a flat end about 10 microns in diameter was used to squeeze the microcapsule against a flat surface until it burst. The force being imposed on the microcapsule was measured by a transducer connected to the probe. The bursting force and diameter of single dry microcapsules in two samples, different in size and wall thickness, were measured by this technique. The bursting force of the microcapsules in one sample ranged from 50 to 220 microN and the diameter from 1.3 to 7.0 microns, whilst the bursting force in the other was from 20 to 175 microN and the diameter from 0.7 to 3.7 microns. This technique makes it possible to compare the mechanical strength of microcapsules made of different formulations, and to infer information about microcapsule mechanical properties.