Compressive deformation of a single microcapsule

Viscoelastic Properties of Bioprinted Alginate Microbeads Compared to Their Bulk Hydrogel Analogs

Hydrogel microbeads are engineered spherical microgels widely used for biomedical applications in cell cultures, tissue engineering, and drug delivery. Their mechanical and physical properties (i.e., modulus, porosity, diffusion) heavily influence their utility by affecting encapsulated cellular behavior, biopayload elution kinetics, and stability for longer term cultures. There is a need to quantify these properties to guide microbead design for effective application. However, there are few techniques with the μN-level resolution required to evaluate these relatively small, compliant constructs. To circumvent mechanically testing individual microbeads, researchers often approximate microbead properties by characterizing larger bulk gel analogs of the same material formulation. This approach provides some insight into the hydrogel properties. However, bulk gels possess key structural and mechanical differences compared to their microbead equivalents, which may limit their accuracy and utility as analogs for estimating microbead properties. Herein, we explore how microbead properties are influenced by hydrogel formulation (i.e., alginate concentration, divalent cation crosslinker, and crosslinker concentration), and whether these trends are accurately reflected in bulk gel analogs. To accomplish this, we utilize laser direct-write bioprinting to create 12 × 12 arrays of alginate microbeads and characterize all 144 microbeads in parallel using a commercially available microcompression system. In this way, the compressive load is distributed across a large number of beads, thus amplifying sample signal. Comparing microbead properties to those of their bulk gel analogs, we found that their trends in modulus, porosity, and diffusion with hydrogel formulation are consistent, yet bulk gels exhibit significant discrepancies in their measured values.

Performance of Capsules in Self-Healing Cementitious Material

Encapsulation is a very promising technique that is being explored to enhance the autonomous self-healing of cementitious materials. However, its success requires the survival of self-healing capsules during mixing and placing conditions, while still trigger the release of a healing agent upon concrete cracking. A review of the literature revealed discontinuities and inconsistencies in the design and performance evaluation of self-healing cementitious material. A finite element model was developed to study the compatibility requirements for the capsule and the cementing material properties while the cement undergoes volume change due to hydration and/or drying. The FE results have provided insights into the observed inconsistencies and the importance of having capsules' mechanical and geometrical properties compatible with the cementitious matrix.

Study of the Effect of Hydrolysis Time on the Mechanical Properties of Polysiloxane Microcapsules

It is known that the microstructure and thereby the mechanical properties of membranes constituting microcapsules are sensitive to parameters such as precursor concentration and pH. In the case of polysiloxane microcapsules, the oligomers, which are already formed in the continuous oil phase, because of the inherent moisture content in the oil phase, deposit on the membrane surface, resulting in the formation of a microstructure with a hairy layer. An electrodeformation investigation shows that the deposition of these oligomers is predominant in the smaller microcapsules compared to the larger ones and results in strain hardening and plasticity in the microcapsule membrane at high deformation. However, if the hydrolysis time during the synthesis of microcapsules is controlled, a smooth morphology (with a diminished hairy layer) can be realized for smaller capsules, as well. This work, using the electrodeformation method, demonstrates significant viscoelasticity and plasticity in the response of the capsules to applied electric stress and establishes an equivalence between simple spring and dashpot element-based phenomenological models with respect to the membrane properties using a linearized viscoelastic elasto-electrohydrodynamic model. The model can capture plasticity and strain hardening that are otherwise missed in simplified elasticity-based models.

Long-Term Autonomic Thermoregulating Fabrics Based on Microencapsulated Phase Change Materials

Microcapsules loaded with n-docosane as phase change material (mPCMs) for thermal energy storage with a phase change transition temperature in the range of 36-45 °C have been employed to impregnate cotton fabrics. Fabrics impregnated with 8 wt % of mPCMs provided 11 °C of temperature buffering effect during heating. On the cooling step, impregnated fabrics demonstrated 6 °C temperature increase for over 100 cycles of switching on/off of the heating source. Similar thermoregulating performance was observed for impregnated fabrics stored for 4 years (1500 days) at room temperature. Temperature buffering effect increased to 14 °C during heating cycle and temperature increase effect reached 9 °C during cooling cycle in the aged fabric composites. Both effects remained stable in aged fabrics for more than 100 heating/cooling cycles. Our study demonstrates high potential use of the microencapsulated n-docosane for thermal management applications, including high-technical textiles, footwear materials, and building thermoregulating covers and paints with high potential for commercial applications.

Simulated and Experimental Investigation of the Mechanical Properties and Solubility of 3D-Printed Capsules for Self-Healing Cement Composites

In the concrete industry, various R&D efforts have been devoted to self-healing technology, which can maintain the long-term performance of concrete structures, which is important in terms of sustainable development. Cracks in cement composites occur and propagate because of various internal and external factors, reducing the composite's stability. Interest in "self-healing" materials that can repair cracks has led researchers to embed self-healing capsules in cement composites. Overcoming the limitations of polymer capsules produced by chemical manufacturing methods, three-dimensional (3D) printing can produce capsules quickly and accurately and offers advantages such as high material strength, low cost, and the ability to fabricate capsules with complex geometries. We performed structural analysis simulations, experimentally evaluated the mechanical properties and solubility of poly(lactic acid) (PLA) capsules, and examined the effect of the capsule wall thickness and printing direction on cement composites embedded with these capsules. Thicker capsules withstood larger bursting loads, and the capsule rupture characteristics varied with the printing angle. Thus, the capsule design parameters must be optimized for different environments. Although the embedded capsules slightly reduced the compressive strength of the cement composites, the benefit of the encapsulated self-healing agent is expected to overcome this disadvantage.

Simple production of cellulose nanofibril microcapsules and the rheology of their suspensions

Microcapsules are commonly used in applications ranging from therapeutics to personal care products due to their ability to deliver encapsulated species through their porous shells. Here, we demonstrate a simple and scalable approach to fabricate microcapsules with porous shells by interfacial complexation of cellulose nanofibrils and oleylamine, and investigate the rheological properties of suspensions of the resulting microcapsules. The suspensions of neat capsules are viscous liquids whose viscosity increases with volume fraction according to a modified Kreiger-Dougherty relation with a maximum packing fraction of 0.74 and an intrinsic viscosity of 4.1. When polyacrylic acid (PAA) is added to the internal phase of the microcapsules, however, the suspensions become elastic and display yield stresses with power-law dependencies on capsule volume fraction and PAA concentration. The elasticity appears to originate from associative microcapsule interactions induced by PAA that is contained within and incorporated into the microcapsule shell. These results demonstrate that it is possible to tune the rheological properties of microcapsule suspensions by changing only the composition of the internal phase, thereby providing a novel method to tailor complex fluid rheology.

Influence of microcapsule parameters and initiator concentration on the self-healing capacity of resin-based dental composites

OBJECTIVE

Fracture is one of the main causes for failure of resin-based composite restorations. To overcome this drawback, self-healing resin-based composites have been designed by incorporation of microcapsules. However, the relationship between their self-healing capacity and microcapsule and resin parameters is still poorly understood. Therefore, the objective of this study was to systematically investigate the effect of initiator concentration (in the resin) and microcapsule size and concentration on the self-healing performance of commercially available flowable resin-based composites.

METHODS

Poly(urea-formaldehyde) (PUF) microcapsules containing acrylic healing liquid were synthesized in small (33±8μm), medium (68±21μm) and large sizes (198±43μm) and characterized. Subsequently, these microcapsules were incorporated into a conventional flowable resin-based composite (Majesty Flow ES2, Kuraray) at different contents (5-15wt%) and benzoyl peroxide (BPO) initiator concentrations (0.5-2.0wt%). Fracture toughness (K_IC) of test specimens was tested using a single edge V-notched beam method. Immediately after complete fracture (K_IC-initial), the two fractured parts were held together for 72h to allow for healing. Subsequently, fracture toughness of the healed resin-based composites (K_IC-healed) was tested as well.

RESULTS

The fracture toughness of healed dental composites significantly increased with increasing microcapsule size and concentration (2wt% BPO, p<0.05). The highest self-healing efficiencies (up to 76%) were obtained with microcapsules sized 198±43 um.

SIGNIFICANCE

commercially available resin-based composites can be rendered self-healing most efficiently by incorporation of large microcapsules (198±43μm). However, long-term tests on fatigue and wear behavior are needed to confirm the clinical efficacy.

Separating the contributions of zona pellucida and cytoplasm in the viscoelastic response of human oocytes

The successful characterization of the mechanical properties of human oocytes and young embryos is of crucial relevance to reduce the risk of pregnancy arrest in in-vitro fertilization processes. Unfortunately, current study has been hindered by the lack of accuracy in describing the mechanical contributions of each structure (zona pellucida, cytoplasm) due to its high heterogeneity. In this work, we present a novel approach to model the oocyte response taking into account the effect of both zona and cytoplasm, as well as different loading conditions. The model is then applied to develop an experimental protocol capable of accurately separating the viscoelastic contribution of zona and cytoplasm by simply varying the loading condition. This new protocol has the potential to open the door to improving our understanding the mechanical properties of oocytes at different stages, and provide a quantitative predictive ability to the evaluation of oocyte quality. STATEMENT OF SIGNIFICANCE: Assisted reproductive technologies, such as in vitro fertilization, often rely on identifying high quality oocytes or female egg cells. The viscoelastic properties of these cells, such as stiffness and stress relaxation time, have been identified as potential objective indicators of cell quality. However, their characterization has proven difficult due to the structural heterogeneity of the cell and inconsistent loading conditions. This paper presents a new model that, although simple, addresses the above difficulties to provide accurate estimations of the cell's mechanical properties. Learning from this model, we then propose a novel non-invasive testing protocol to allow oocyte characterization with increased accuracy. We believe this effort would improve consistency in measurements and enhance our knowledge on the mechanics of oocytes.

Ultrasonically synthesized organic liquid-filled chitosan microcapsules: part 2: characterization using AFM (atomic force microscopy) and combined AFM-confocal laser scanning fluorescence microscopy

Atomic Force Microscopy (AFM) is used to measure the stiffness and Young's modulus of individual microcapsules that have a chitosan cross-linked shell encapsulating tetradecane. The oil filled microcapsules were prepared using a one pot synthesis via ultrasonic emulsification of tetradecane and crosslinking of the chitosan shell in aqueous solutions of acetic acid. The concentration of acetic acid in aqueous solutions of chitosan was varied from 0.2% to 25% v/v. The effect of acetic acid concentration and size of the individual microcapsules on the strength was probed. The deformations and forces required to rupture the microcapsules were also measured. Three dimensional deformations of microcapsules under large applied loads were obtained by the combination of Laser Scanning Confocal Microscopy (LSCM) with Atomic Force Microscopy (AFM). The stiffness, and hence the modulus, of the microcapsules was found to decrease with an increase in size with the average stiffness ranging from 82 to 111 mN m-1 and average Young's modulus ranging from 0.4 to 6.5 MPa. The forces required to rupture the microcapsules varied from 150 to 250 nN with deformations of the microcapsules up to 62 to 110% relative to their radius, respectively. Three dimensional images obtained using laser scanning confocal microscopy showed that the microcapsules retained their structure and shape after being subjected to large deformations and subsequent removal of the loads. Based on the above observations, the oil filled chitosan crosslinked microcapsules are an ideal choice for use in the food and pharmaceutical industries as they would be able to withstand the process conditions encountered.

Parallel Compression Is a Fast Low-Cost Assay for the High-Throughput Screening of Mechanosensory Cytoskeletal Proteins in Cells

Cellular mechanosensing is critical for many biological processes, including cell differentiation, proliferation, migration, and tissue morphogenesis. The actin cytoskeletal proteins play important roles in cellular mechanosensing. Many techniques have been used to investigate the mechanosensory behaviors of these proteins. However, a fast, low-cost assay for the quantitative characterization of these proteins is still lacking. Here, we demonstrate that compression assay using agarose overlay is suitable for the high throughput screening of mechanosensory proteins in live cells while requiring minimal experimental setup. We used several well-studied myosin II mutants to assess the compression assay. On the basis of elasticity theories, we simulated the mechanosensory accumulation of myosin II's and quantitatively reproduced the experimentally observed protein dynamics. Combining the compression assay with confocal microscopy, we monitored the polarization of myosin II oligomers at the subcellular level. The polarization was dependent on the ratio of the two principal strains of the cellular deformations. Finally, we demonstrated that this technique could be used on the investigation of other mechanosensory proteins.

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.

Magneto-responsive alginate capsules

Upon incorporation of magnetic nanoparticles (mNPs) into gels, composite materials called ferrogels are obtained. These magneto-responsive systems have a wide range of potential applications including switches and sensors as well as drug delivery systems. In this article, we focus on the properties of calcium alginate capsules, which are widely used as carrier systems in medicine and technology. We studied the incorporation of different kinds of mNPs in matrix capsules and in the core and the shell of hollow particles. We found out that not all particle-alginate or particle-CaCl2 solution combinations were suitable for a successful capsule preparation on grounds of a destabilization of the nanoparticles or the polymer. For those systems allowing the preparation of switchable beads or capsules, we systematically studied the size and microscopic structure of the capsules, their magnetic behavior and mechanical resistance.

Monitoring reactive microencapsulation dynamics using microfluidics

We use microfluidic polydimethylsiloxane (PDMS) devices to measure the kinetics of reactive encapsulations occurring at the interface of emulsion droplets. The formation of the polymeric shell is inferred from the droplet deformability measured in a series of expansion-constriction chambers along the microfluidic chip. With this tool we quantify the kinetic processes governing the encapsulation at the very early stage of shell formation with a time resolution of the order of the millisecond for overall reactions occurring in less than 0.5 s. We perform a comparison of monomer reactivities used for the encapsulation. We study the formation of polyurea microcapsules (PUMCs); the shell formation proceeds at the water-oil interface by an immediate reaction of amines dissolved in the aqueous phase and isocyanates dissolved in the oil phase. We observe that both monomers contribute differently to the encapsulation kinetics. The kinetics of the shell formation process at the oil-in-water (O/W) experiments significantly differs from the water-in-oil (W/O) systems; the component dissolved in the continuous phase has the largest impact on the kinetics. In addition, we quantified the retarding effect on the encapsulation kinetics by the interface stabilizing agent (surfactant). Our approach is valuable for quantifying in situ reactive encapsulation processes and provides guidelines to generate microcapsules with soft interfaces of tailored and controllable interfacial properties.

Microcapsule mechanics: from stability to function

Microcapsules are reviewed with special emphasis on the relevance of controlled mechanical properties for functional aspects. At first, assembly strategies are presented that allow control over the decisive geometrical parameters, diameter and wall thickness, which both influence the capsule's mechanical performance. As one of the most powerful approaches the layer-by-layer technique is identified. Subsequently, ensemble and, in particular, single-capsule deformation techniques are discussed. The latter generally provide more in-depth information and cover the complete range of applicable forces from smaller than pN to N. In a theory chapter, we illustrate the physics of capsule deformation. The main focus is on thin shell theory, which provides a useful approximation for many deformation scenarios. Finally, we give an overview of applications and future perspectives where the specific design of mechanical properties turns microcapsules into (multi-)functional devices, enriching especially life sciences and material sciences.

A mechanosensory system governs myosin II accumulation in dividing cells

The mitotic spindle is generally considered the initiator of furrow ingression. However, recent studies suggest that furrows can form without spindles, particularly during asymmetric cell division. In Dictyostelium, the mechanoenzyme myosin II and the actin cross-linker cortexillin I form a mechanosensor that responds to mechanical stress, which could account for spindle-independent contractile protein recruitment. Here we show that the regulatory and contractility network composed of myosin II, cortexillin I, IQGAP2, kinesin-6 (kif12), and inner centromeric protein (INCENP) is a mechanical stress-responsive system. Myosin II and cortexillin I form the core mechanosensor, and mechanotransduction is mediated by IQGAP2 to kif12 and INCENP. In addition, IQGAP2 is antagonized by IQGAP1 to modulate the mechanoresponsiveness of the system, suggesting a possible mechanism for discriminating between mechanical and biochemical inputs. Furthermore, IQGAP2 is important for maintaining spindle morphology and kif12 and myosin II cleavage furrow recruitment. Cortexillin II is not directly involved in myosin II mechanosensitive accumulation, but without cortexillin I, cortexillin II's role in membrane-cortex attachment is revealed. Finally, the mitotic spindle is dispensable for the system. Overall, this mechanosensory system is structured like a control system characterized by mechanochemical feedback loops that regulate myosin II localization at sites of mechanical stress and the cleavage furrow.

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.

Robust, double-walled microcapsules for self-healing polymeric materials

Double-walled polyurethane/poly(urea-formaldehyde) microcapsules (PU/UF) are prepared for use in self-healing materials. This modified encapsulation procedure combines two chemistries to form more robust capsule shell walls in a single operation. Robust capsules are formed by this procedure as long as the aromatic polyisocyanate prepolymer is soluble in the core liquid and the core liquid is compatible with isocyanates. Compared to a standard UF encapsulation, the modified procedure results in capsules with an increase in shell wall thickness from 200 to 675 nm as a function of the amount of PU added to the core liquid. Thermal stability of PU/UF microcapsules prepared with varying amounts of PU is compared to UF microcapsules. Mechanical properties of the PU/UF microcapsules are assessed from single-capsule compression testing.

Mechanical Stability of Ionotropic Alginate Beads

In a series of experiments we measured the mechanical properties of single alginate beads by means of squeezing experiments between two parallel plates. We used multivalent counter-ions as cross-linking molecules for the formation of three dimensional alginate gels. In this article we examined pure Fe(II) (ferric), Fe(III) (ferrous) and Ca(II) (calcium) ions as cross-linking agents and different mixtures between these charged compounds. The results of squeezing experiments showed that capsules formed with pure ferrous ions were less stable than particles which were cross-linked with calcium or ferric ions. It turned out that at equal molar concentrations calcium and ferrous ions formed stronger gels than ferric ions. In addition to squeezing capsule experiments we also investigated different particles by optical microscopy and scanning electron microscopy. Experiments of Energy-dispersive X-ray spectroscopy (EDAX) show different compositions of these beads.

Investigation of the morphology, viability and mechanical properties of yeast cells in environmental SEM

The mechanical properties of biological cells at nanoscale may be characterized using an environmental scanning electron microscopy (ESEM) combined with a force measurement device. However, the electron beam radiation in an ESEM may damage a specimen. So far, little is known about the radiation damage to biological cells. In this work, single yeast cells were imaged using an ESEM under both high and low vacuum modes. The changes in their morphology and viability were monitored as a function of radiation time for a given beam current of 538 pA corresponding to 10 kV accelerating voltage and spot size 4. Under the two modes, the radiation damage to the morphology of yeast cells became evident after an exposure time of 3 min, but under the low vacuum mode, the damage to their morphology was more severe. However, all cells lost their viability after 5 min under the high vacuum mode with the electron beam off from an initial viability of 95+/-1%. In contrast, the viability of cells under the low vacuum mode was found to be approximately 20% after 20 min. In addition, a newly developed ESEM-based nanomanipulation technique was applied to measure the force imposed on single yeast cells and their deformation, including contact diameter and central lateral diameter for the compression of single yeast cells to a given displacement within a time frame of 1 min, and the data obtained may be used to validate mathematical modeling of the stress-strain relationship for the compression of cells in order to determine their intrinsic mechanical property parameters.

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.

Finite-element analysis of geometrical factors in micro-indentation of pollen tubes

Micro-indentation is a new experimental approach to assess physical cellular properties. Here we attempt to quantify the contribution of geometrical parameters to a cylindrical plant cell's resistance to lateral deformation. This information is important to correctly interpret data obtained from experiments using the device, such as the local cellular stiffness in pollen tubes. We built a simple finite-element model of the micro-indentation interacting partners - micro-indenter, cell (pollen tube), and underlying substratum, that allowed us to manipulate geometric variables, such as geometry of the cell, cell radius, thickness of the cell wall and radius of the indenting stylus. Performing indentation experiments on this theoretical model demonstrates that all four parameters influence stiffness measurement and can therefore not be neglected in the interpretation of micro-indentation data.

Coupling bending and shear effects on liposome deformation

Cell membrane deformation induced by external mechanical stimuli has been studied extensively over the past three decades. The present study focuses on the coupling of in-plane shear H and out-of-plane bending B of liposome membrane and its influences on the deformation of a single vesicle subjected to (i) external compressive load via two parallel platens and (ii) contact forces caused by a rigid substrate. Our results show that the increase of membrane resultant stress in both loading configurations causes the liposome to become more rigid and the degree of vesicle deformation decreases when the in-plane shearing effect is dominant. A theoretical approach is developed to facilitate cell membrane characterization under different biomechanical stimuli.

Nanoscale compression of polymer microspheres by atomic force microscopy

Atomic force microscopy (AFM) was employed to probe the mechanical properties of surface-charged polystyrene microspheres with 1-12 mol% of vinylbenzyl(trimethyl)ammonium chloride (VBTA) units. On the basis of Hertz's theory of contact mechanics, compressive moduli between 1 and 2 GPa were measured by the analysis of force-displacement curves captured on the particles via the force-volume technique. The deformation of the top of the polystyrene particles by the AFM tip was used to calculate the surface modulus. The compressive moduli are slightly less than the moduli of polystyrene bulk materials. The modulus of the polystyrene microspheres increases with an increase of the VBTA content.

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.

Contact mechanics of a thin-walled capsule adhered onto a rigid planar substrate

A thin-walled capsule, modelled as an incompressible liquid droplet contained in a thin flexible membrane, was allowed to adhere onto a rigid substrate. The contact mechanics were formulated, based on linear elasticity, to portray quantitatively the relationships between osmotic inflation, contact area and angle, membrane stretching and adhesion strength. The predicted results shed light on fundamental adhesive contact mechanics in a cell-substrate system.