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  • 1.
    Fallqvist, B.
    et al.
    Royal Institute of Technology, (KTH).
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    A chemo-mechanical constitutive model for transiently cross-linked actin networks and a theoretical assessment of their viscoelastic behaviour2013In: Biomechanics and Modeling in Mechanobiology, ISSN 1617-7959, E-ISSN 1617-7940, Vol. 12, no 2, p. 373-382Article in journal (Refereed)
    Abstract [en]

    Biological materials can undergo large deformations and also show viscoelastic behaviour. One such material is the network of actin filaments found in biological cells, giving the cell much of its mechanical stiffness. A theory for predicting the relaxation behaviour of actin networks crosslinked with the cross-linker α-actinin is proposed. The constitutive model is based on a continuum approach involving a neo-Hookean material model, modified in terms of concentration of chemically activated cross-links. The chemical model builds on work done by Spiros (Doctoral thesis, University of British Columbia, Vancouver, Canada, 1998) and has been modified to respond to mechanical stress experienced by the network. The deformation is split into a viscous and elastic part, and a thermodynamically motivated rate equation is assigned for the evolution of viscous deformation. The model predictions were evaluated for stress relaxation tests at different levels of strain and found to be in good agreement with experimental results for actin networks cross-linked with α-actinin.

  • 2.
    Fallqvist, B.
    et al.
    Royal Institute of Technology, (KTH).
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    Constitutive modelling of composite biopolymer networks2016In: Journal of Theoretical Biology, ISSN 0022-5193, E-ISSN 1095-8541, Vol. 395, p. 51-61Article in journal (Refereed)
    Abstract [en]

    The mechanical behaviour of biopolymer networks is to a large extent determined at a microstructural level where the characteristics of individual filaments and the interactions between them determine the response at a macroscopic level. Phenomena such as viscoelasticity and strain-hardening followed by strain-softening are observed experimentally in these networks, often due to microstructural changes (such as filament sliding, rupture and cross-link debonding). Further, composite structures can also be formed with vastly different mechanical properties as compared to the individual networks. In this present paper, we present a constitutive model presented in a continuum framework aimed at capturing these effects. Special care is taken to formulate thermodynamically consistent evolution laws for dissipative effects. This model, incorporating possible anisotropic network properties, is based on a strain energy function, split into an isochoric and a volumetric part. Generalisation to three dimensions is performed by numerical integration over the unit sphere. Model predictions indicate that the constitutive model is well able to predict the elastic and viscoelastic response of biological networks, and to an extent also composite structures.

  • 3.
    Fallqvist, B.
    et al.
    Royal Institute of Technology, (KTH).
    Kulachenko, A.
    Royal Institute of Technology, (KTH).
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    Modelling of cross-linked actin networks: influence of network parameters and cross-link compliance2014In: Journal of Theoretical Biology, ISSN 0022-5193, E-ISSN 1095-8541, Vol. 350, p. 57-69Article in journal (Refereed)
    Abstract [en]

    A major structural component of the cell is the actin cytoskeleton, in which actin subunits are polymerised into actin filaments. These networks can be cross-linked by various types of ABPs (Actin Binding Proteins), such as Filamin A. In this paper, the passive response of cross-linked actin filament networks is evaluated, by use of a numerical and continuum network model. For the numerical model, the influence of filament length, statistical dispersion, cross-link compliance (including that representative of Filamin A) and boundary conditions on the mechanical response is evaluated and compared to experimental results. It is found that the introduction of statistical dispersion of filament lengths has a significant influence on the computed results, reducing the network stiffness by several orders of magnitude. Actin networks have previously been shown to have a characteristic transition from an initial bending-dominated to a stretching-dominated regime at larger strains, and the cross-link compliance is shown to shift this transition. The continuum network model, a modified eight-chain polymer model, is evaluated and shown to predict experimental results reasonably well, although a single set of parameters cannot be found to predict the characteristic dependence of filament length for different types of cross-links. Given the vast diversity of cross-linking proteins, the dependence of mechanical response on cross-link compliance signifies the importance of incorporating it properly in models to understand the roles of different types of actin networks and their respective tasks in the cell.

  • 4.
    Fallqvist, Björn
    et al.
    Royal Institute of Technology, (KTH).
    Fielden, Matthew L.
    Royal Institute of Technology, (KTH).
    Pettersson, Torbjörn
    Royal Institute of Technology, (KTH).
    Nordgren, Niklas
    SP Technical Research Institute of Sweden.
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    Gad, Annica K.B.
    Karolinska Institutet, KI.
    Experimental and computational asessment of F-actin influence in regulating cellular stiffness and relaxation behaviour of fibroblasts2016In: Journal of The Mechanical Behavior of Biomedical Materials, ISSN 1751-6161, E-ISSN 1878-0180, Vol. 59, p. 168-184Article in journal (Refereed)
    Abstract [en]

    In biomechanics, a complete understanding of the structures and mechanisms that regulate cellular stiffness at a molecular level remain elusive. In this paper, we have elucidated the role of filamentous actin (F-actin) in regulating elastic and viscous properties of the cytoplasm and the nucleus. Specifically, we performed colloidal-probe atomic force microscopy (AFM) on BjhTERT fibroblast cells incubated with Latrunculin B (LatB), which results in depolymerisation of F-actin, or DMSO control. We found that the treatment with LatB not only reduced cellular stiffness, but also greatly increased the relaxation rate for the cytoplasm in the peripheral region and in the vicinity of the nucleus. We thus conclude that F-actin is a major determinant in not only providing elastic stiffness to the cell, but also in regulating its viscous behaviour. To further investigate the interdependence of different cytoskeletal networks and cell shape, we provided a computational model in a finite element framework. The computational model is based on a split strain energy function of separate cellular constituents, here assumed to be cytoskeletal components, for which a composite strain energy function was defined. We found a significant influence of cell geometry on the predicted mechanical response. Importantly, the relaxation behaviour of the cell can be characterised by a material model with two time constants that have previously been found to predict mechanical behaviour of actin and intermediate filament networks. By merely tuning two effective stiffness parameters, the model predicts experimental results in cells with a partly depolymerised actin cytoskeleton as well as in untreated control. This indicates that actin and intermediate filament networks are instrumental in providing elastic stiffness in response to applied forces, as well as governing the relaxation behaviour over shorter and longer time-scales, respectively.

  • 5.
    Fallqvist, Björn
    et al.
    Royal Institute of Technology, (KTH).
    Kulachenko, Artem
    Royal Institute of Technology, (KTH).
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    Cross-link debonding in actin networks: influence on mechanical properties2015In: International Journal of Experimental and Computational Biomechanics, ISSN 1755-8735, Vol. 3, no 1, p. 16-26Article in journal (Refereed)
    Abstract [en]

    The actin cytoskeleton is essential for the continued function and survival of the cell. A peculiar mechanical characteristic of actin networks is their remodelling ability, providing them with a time-dependent response to mechanical forces. In cross-linked actin networks, this behaviour is typically tuned by the binding affinity of the cross-link. We propose that the debonding of a cross-link between filaments can be modelled using a stochastic approach, in which the activation energy for a bond is modified by a term to account for mechanical strain energy. By use of a finite element model, we perform numerical analyses in which we first compare the model behaviour to experimental results. The computed and experimental results are in good agreement for short time scales, but over longer time scales the stress is overestimated. However, it does provide a possible explanation for experimentally observed strain-rate dependence as well as strain-softening at longer time scales.

  • 6.
    Holzapfel, Gerhard A.
    et al.
    Royal Institute of Technology KTH ; Graz University of Technology, Austria.
    Kiousis, Dimitrios E.
    Graz University of Technology, Austria .
    Kroon, Martin
    Royal Institute of Technology KTH.
    On Modelling Multi-Layered Soft Collagenous Tissues2008In: Presented at 8th World Congress on Computational Mechanics, 30 June – 4 July, 2008, Venice, Italy, 2008Conference paper (Refereed)
  • 7.
    Kandušer, Maša
    et al.
    University of Ljubljana, Slovenia.
    Kokalj Imsirovic, Mojca
    University of Ljubljana, Slovenia.
    Ušaj, Marko
    University of Ljubljana, Slovenia.
    The Effect of Lipid Antioxidant α-Tocopherol on Cell Viability and Electrofusion Yield of B16-F1 Cells In Vitro2019In: The Journal of Membrane Biology, ISSN 0022-2631, Vol. 252, no 1, p. 105-114Article in journal (Refereed)
    Abstract [en]

    Induced cell fusion is a powerful method for production of hybridoma in biotechnology and cell vaccines in medical applications. Among different alternatives, physical methods have an advantage, as they do not require any additives. Among them electrofusion, an electroporation-based cell fusion method holds a great promise. Electric pulses cause cell membrane permeabilization and due to pore formation bring cell membrane into the fusogenic state. At the same time, however, they compromise cell viability. We used a train of 8 × 100 µs electric pulses, delivered at 1 Hz with strengths ranging from 400 to 1600 V/cm. We evaluated electrofusion efficiency by dual color microscopy. We determined cell viability, because during electroporation reactive oxygen species are generated affecting cell survival. The novelty of our study is evaluation of the effect of lipid antioxidant α-tocopherol on cell fusion yield and cell viability on mouse B16-F1 cells. Pretreatment with α-tocopherol slowed down dynamic of cell fusion shortly after electroporation. Twenty-four hours later, fusion yields between α-tocopherol treated and untreated cells were comparable. The viability of α-tocopherol pretreated cells was drastically improved. Pretreatment of cells with α-tocopherol improved whole electrofusion process by more than 60%. We believe that α-tocopherol holds great promise to become an important agent to improve cell electrofusion method.

  • 8.
    Kandušer, Maša
    et al.
    University of Ljubljana, Slovenia.
    Ušaj, Marko
    University of Ljubljana, Slovenia.
    Cell electrofusion: past and future perspectives for antibody production and cancer cell vaccines2014In: Expert Opinion on Drug Delivery, ISSN 1742-5247, E-ISSN 1744-7593, Vol. 11, no 12, p. 1885-1898Article in journal (Refereed)
    Abstract [en]

    Introduction: In the past few decades, new methods for drug and gene delivery have been developed, among which electroporation and electrofusion have gained noticeable attention. Lately, advances in the field of immunotherapy have enabled new cancer therapies based on immune response, including monoclonal antibodies and cell vaccines. Efficient cell fusion is needed for both hybridoma production and cell vaccine preparation, and electrofusion is a promising method to achieve this goal.Areas covered: In the present review, we cover new strategies of cancer treatment related to antibody production and cell vaccines. In more detail, cell electroporation and electrofusion are addressed. We briefly describe principles of cell electroporation and focus on electrofusion and its influential factors, with special attention on the fusogenic state of the cell membrane, contact formation, the effect of electrofusion media and cell viability. We end the review with an overview of the very promising field of microfluidic devices for electrofusion.Expert opinion: In our opinion, electrofusion can be a very efficient method for hybridoma and cell vaccine production. Advances in the development of microfluidic devices and a better understanding of the underlying (biological) mechanisms will overcome the current limitations.

  • 9.
    Kondori, Farid Abedan
    et al.
    Umeå universitet, Institutionen för tillämpad fysik och elektronik.
    Yousefi, Shahrouz
    Umeå universitet, Institutionen för tillämpad fysik och elektronik.
    Liu, Li
    Umeå universitet, Institutionen för tillämpad fysik och elektronik.
    Active human gesture capture for diagnosing and treating movement disorders2013In: Proceeding of The Swedish Symposium on Image Analysis (SSBA2013), Gothenburg, Sweden, 2013Conference paper (Other academic)
    Abstract [en]

    Movement disorders prevent many people fromenjoying their daily lives. As with other diseases, diagnosisand analysis are key issues in treating such disorders.Computer vision-based motion capture systems are helpfultools for accomplishing this task. However Classical motiontracking systems suffer from several limitations. First theyare not cost effective. Second these systems cannot detectminute motions accurately. Finally they are spatially limitedto the lab environment where the system is installed. In thisproject, we propose an innovative solution to solve the abovementionedissues. Mounting the camera on human body, webuild a convenient, low cost motion capture system that canbe used by the patient in daily-life activities. We refer tothis system as active motion capture, which is not confinedto the lab environment. Real-time experiments in our labrevealed the robustness and accuracy of the system.

  • 10.
    Kroon, Martin
    Royal Institute of Technology (KTH).
    A constitutive model for smooth muscle including active tone and passive viscoelastic behaviour2010In: Mathematical Medicine and Biology, ISSN 1477-8599, E-ISSN 1477-8602, Vol. 27, no 2, p. 129-155Article in journal (Refereed)
    Abstract [en]

    A new constitutive model for the biomechanical behaviour of smooth muscle tissue is proposed. The active muscle contraction is accomplished by the relative sliding between actin and myosin filaments, comprising contractile units in the smooth muscle cells. The model includes a chemical part, governing the cross-bridge (myosin head) cycling, that is responsible for the filament sliding. The number of activated cross-bridges govern the contractile force generated and also the contraction speed. A strain-energy function is used to describe the mechanical behaviour of the smooth muscle tissue. Besides the active contractile apparatus, the mechanical model also incorporates a passive viscoelastic part. The constitutive model was calibrated with respect to experiments on smooth muscle tissue from swine carotid artery and guinea pig taenia coli, in terms of isometric and isotonic tensile test results. The model was fully able to reproduce the experimental results.

  • 11.
    Kroon, Martin
    Royal Institute of Technology (KTH).
    A continuum mechanics framework and a constitutive model for remodelling of collagen gels and collagenous tissues2010In: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 58, no 6, p. 918-933Article in journal (Refereed)
    Abstract [en]

    Collagen is a very important protein of the human body and is responsible for the structural stability of many body components. Furthermore, collagen fibre networks are able to grow and remodel themselves, which enables them to adjust to varying physiological conditions. This remodelling is accomplished by fibre-producing cells, such as fibroblasts. The ability to adjust to new physiological conditions is very important, for example in wound healing. In the present paper, a theoretical framework for modelling collagenous tissues and collagen gels is proposed. Continuum mechanics is employed to describe the kinematics of the collagen, and affine deformations of fibres are assumed. Biological soft tissues can be approximated as being hyperelastic, and the constitutive model for the collagen fabric is therefore formulated in terms of a strain energy function. This strain energy function includes a density function that describes the distribution of the collagen fibre orientation. The density function evolves according to an evolution law, where fibres tend to reorient towards the direction of maximum Cauchy stress. The remodelling of the collagen network is also assumed to include a pre-stretching of collagen fibres, accomplished by fibroblasts. The theoretical framework is applied to experiments performed on collagen gels, where gels were exposed to remodelling under both biaxial and uniaxial constraints. The proposed model was able to predict both the resulting collagen distribution and the resulting stress–strain relationships obtained for the remodelled collagen gels. The influence of the most important model parameters is demonstrated, and it appears that there is a fairly unique set of model parameters that gives an optimal fit to the experimental data.

  • 12.
    Kroon, Martin
    Royal Institute of Technology (KTH).
    A numerical framework for material characterization of inhomogeneous hyperelastic membranes by inverse analysis2010In: Journal of Computational and Applied Mechanics, ISSN 1586-2070, Vol. 234, no 2, p. 563-578Article in journal (Refereed)
    Abstract [en]

    An inverse method for estimating the distributions of the elastic properties of hyperelastic, inhomogeneous membranes is proposed. The material description of the membrane is based on a versatile constitutive model, in which two stiffness parameters govern the nonlinear elastic behaviour of the material. The estimation procedure includes a finite element framework. The two stiffness parameters in the constitutive law are assumed to vary continuously over the inhomogeneous membrane, and in the finite element framework the distributions of the two parameters are approximated using standard linear shape functions. Experimental results are assumed to exist in terms of nodal displacements from a test with known geometry and boundary conditions. The experimental membrane is modelled in the finite element framework, and the deformation of it is predicted. An error function, quantifying the discrepancy between the experimentally obtained deformation pattern and the numerically predicted pattern, is then minimised with respect to the nodal values of the two interpolated parameters. In a number of numerical examples, the proposed procedure is assessed by attempting to reproduce the given random reference distributions of material properties. The proposed estimation method is fully able to reproduce the reference distributions of the two material parameters with excellent accuracy.

  • 13.
    Kroon, Martin
    Royal Institute of Technology (KTH).
    An efficient method for material characterization of hyperelastic anisotropic inhomogeneous membranes based on inverse finite element method and an element partition strategy2010In: Quarterly Journal of Mechanics and Applied Mathematics, ISSN 0033-5614, E-ISSN 1464-3855, Vol. 63, no 2, p. 201-225Article in journal (Refereed)
    Abstract [en]

    An inverse method for estimating the distributions of the nonlinear elastic properties of inhomogeneous and anisotropic membranes is investigated. The material description of the membrane is based on a versatile constitutive model, including four material parameters: two initial stiffness values pertaining to the principal directions of the material, the angle between these principal directions and a reference coordinate system and a parameter related to the level of nonlinearity of the material. The estimation procedure consists of the following three steps: (i) perform experiments on the membranous structure whose properties are to be determined, (ii) define a corresponding finite-element (FE) model and (iii) minimise an error function (with respect to the unknown parameters) that quantifies the deviation between the numerical predictions and the experimental data. For this finite deformation problem, an FE framework for membranous structures exposed to a pressure boundary loading is outlined: the principle of virtual work, its linearisation and the related spatial discretisation. To achieve a robust parameter estimation, an element partition method is employed. In numerical examples, the proposed procedure is assessed by attempting to reproduce given random reference distributions of material fields in a reference membrane. The deviations between the estimated material parameter distributions and the related reference fields are within a few percent in most cases. The standard deviation for the resulting maximum principal stress was consistently below 1%.

  • 14. Kroon, Martin
    An Inverse Method to Estimate the Material Parameters and Wall Stresses of a Saccular Cerebral Aneurysm2008In: Presented at 11th Euromech – Mecamat Conference, 10-14 March, 2008, Torino, Italy, 2008Conference paper (Refereed)
  • 15.
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    Cell contraction of an elastic substrate assessed by an axisymmetric model2012In: International Journal of Experimental and Computational Biomechanics, ISSN 1755-8735, Vol. 2, no 1, p. 61-73Article in journal (Refereed)
    Abstract [en]

    In the present paper, a computational model for cell contraction of an elastic substrate is proposed. Axisymmetry is assumed and the cell is represented by its contractile apparatus, which is taken to consist of radially oriented stress fibres. The constitutive behaviour of the contractile apparatus is modelled by use of a strain energy function, and contraction of stress fibres is modelled by giving them a different natural configuration compared with the underlying elastic substrate. The model was compared with experiments, in which fibroblasts were put on an elastic substrate. The contracted cell radius depends on the stiffness of the elastic substrate, and model predictions were compared with the experimental results for different values of the stiffness of the elastic substrate. The model also predicts that the contraction of the cell tends to cause a small crater below the cell, which is qualitatively in agreement with experimental observations.

  • 16.
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    Influence of dispersion in myosin filament orientation and anisotropic filament contractions in smooth muscle2011In: Journal of Theoretical Biology, ISSN 0022-5193, E-ISSN 1095-8541, Vol. 272, no 1, p. 72-82Article in journal (Refereed)
    Abstract [en]

    A new constitutive model for the biomechanical behaviour of smooth muscle tissue is proposed. The active muscle contraction is accomplished by the relative sliding between actin and myosin filaments, comprising contractile units in the smooth muscle cells. The orientation of the myosin filaments, and thereby the contractile units, are taken to exhibit a statistical dispersion around a preferred direction. The number of activated cross-bridges between the actin and myosin filaments governs the contractile force generated by the muscle and also the contraction speed. A strain-energy function is used to describe the mechanical behaviour of the smooth muscle tissue. Besides the active contractile apparatus, the mechanical model also incorporates a passive elastic part. The constitutive model was compared to histological and isometric tensile test results for smooth muscle tissue from swine carotid artery. In order to be able to predict the active stress at different muscle lengths, a filament dispersion significantly larger than the one observed experimentally was required. Furthermore, a comparison of the predicted active stress for a case of uniaxially oriented myosin filaments and a case of filaments with a dispersion based on the experimental histological data shows that the difference in generated stress is noticeable but limited. Thus, the results suggest that myosin filament dispersion alone cannot explain the increase in active muscle stress with increasing muscle stretch.

  • 17. Kroon, Martin
    Mechanical Modelling of Calcium-Activated Contraction of Smooth Muscle Cells2008In: Presented at IUTAM Symposium on Cellular, Molecular and Tissue Mechanics, 18-21 June, 2008, Woods Hole, Massachusetts, USA, 2008Conference paper (Refereed)
  • 18. Kroon, Martin
    Modelling of smooth muscle Cells2008In: Presented at 8th World Congress on Computational Mechanics, 30 June – 4 July, 2008, Venice, Italy, 2008Conference paper (Refereed)
  • 19. Kroon, Martin
    Modelling of Thin Anisotropic Collagen-Dominated Soft Biological Tissue2008In: Presented at 2nd International Conference on Heterogeneous Material Mechanics, 3-8 June, 2008, Huangshan, China, 2008Conference paper (Refereed)
  • 20.
    Kroon, Martin
    Royal Institute of Technology (KTH).
    On the correlation between continuum mechanics entities and cell activity in biological soft tissues: Assessment of three possible criteria for cell-controlled fibre reorientation in collagen gels and collagenous tissues2010In: Journal of Theoretical Biology, ISSN 0022-5193, E-ISSN 1095-8541, Vol. 264, no 1, p. 66-76Article in journal (Refereed)
    Abstract [en]

    The biomechanical behaviour of biological cells is of great importance in many physiological processes. One such process is the maintenance of fibrous networks, such as collagenous tissues. The activity of the fibre-producing cells in this type of tissue is very important, and a comprehensive material description needs to incorporate the activity of the cells. In biomechanics, continuum mechanics is often employed to describe deforming solids, and modelling can be much simplified if continuum mechanics entities, such as stress and strain, can be correlated with cell activity. To investigate this, a continuum mechanics framework is employed in which remodelling of a collagen gel is modelled. The remodelling is accomplished by fibroblasts, and the activity of the fibroblasts is linked to the continuum mechanics theory. The constitutive model for the collagen fabric is formulated in terms of a strain energy function, which includes a density function describing the distribution of the collagen fibre orientation. This density function evolves according to an evolution law, where fibroblasts reorient fibres towards the direction of increasing Cauchy stress, elastic deformation, or stiffness. The theoretical framework is applied to experimental results from collagen gels, where gels have undergone remodelling under both biaxial and uniaxial constraint. The analyses indicated that criteria 1 and 2 (Cauchy stress and elastic deformations) are able to predict the collagen fibre distribution after remodelling, whereas criterion 3 (current stiffness) is not. This conclusion is, however, tentative and pertains, strictly speaking, only to fibre remodelling processes, and may not be valid for other types of cell activities.

  • 21.
    Kroon, Martin
    Royal Institute of Technology (KTH).
    Optimal length of smooth muscle assessed by a microstructurally and statistically based constitutive model2011In: Computer Methods in Biomechanics and Biomedical Engineering, ISSN 1025-5842, E-ISSN 1476-8259, Vol. 14, no 1, p. 43-52Article in journal (Refereed)
    Abstract [en]

    Smooth muscle exhibits an optimal length at which it is able to generate a maximum amount of force. In this study, the optimal length is assessed by use of a microstructurally and statistically based constitutive model for smooth muscle. The model is based on the sliding filament theory, and a modified version of Hill's mechanical model was adopted. It was conjectured, that a variation in the overlap in the actomyosin contractile units together with a statistical dispersion in the size of the dense bodies are responsible for the optimal length characteristics. The influence of contractile unit length, dense body size and dense body compliance was investigated, and the model was fully able to predict experimental data. The results indicate that the compliance of the dense bodies does not contribute significantly to the total compliance of the contractile apparatus.

  • 22.
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    Simulation of cerebral aneurysm growth and prediction of evolving rupture risk2011In: Modelling and Simulation in Engineering, ISSN 1687-5591, E-ISSN 1687-5605, article id 289523Article in journal (Refereed)
    Abstract [en]

    Cerebral aneurysms are local expansions of blood vessel walls in the brain blood system. The rupture of an aneurysm is a very severe event associated with a high rate of mortality. When cerebral aneurysms are detected, clinicians need to decide if operation is required. The risk of aneurysm rupture is then compared to the risks associated with the medical intervention. In the present paper, a probabilistic framework for a mechanically based rupture risk assessment of cerebral aneurysms is proposed. The method is based on the assumption that the strength of aneurysmal tissues can be described by a statistical distribution. A structural analysis of the aneurysm in question is performed, and the maximum stress experienced by the aneurysm is compared to the strength distribution. The proposed model was compared with clinical results for ruptured aneurysms in terms of rupture density and accumulated rupture risk as a function of aneurysm size. The model was able to reproduce the clinical results well. The proposed framework may potentially be used under in vivo conditions to predict the risk of rupture for diagnosed aneurysms.

  • 23.
    Murtada, Sae-Il
    et al.
    Royal Institute of Technology (KTH).
    Kroon, Martin
    Royal Institute of Technology (KTH).
    Holzapfel, Gerhard A.
    Graz University of Technology, Austria ; Royal Institute of Technology (KTH).
    A calcium-driven mechanochemical model for prediction of force generation in smooth muscle.2010In: Biomechanics and Modeling in Mechanobiology, ISSN 1617-7959, E-ISSN 1617-7940, Vol. 9, no 6, p. 749-762Article in journal (Refereed)
    Abstract [en]

    A new model for the mechanochemical response of smooth muscle is presented. The focus is on the response of the actin–myosin complex and on the related generation of force (or stress). The chemical (kinetic) model describes the cross-bridge interactions with the thin filament in which the calcium-dependent myosin phosphorylation is the only regulatory mechanism. The new mechanical model is based on Hill’s three-component model and it includes one internal state variable that describes the contraction/relaxation of the contractile units. It is characterized by a strainenergy function and an evolution law incorporating only a few material parameters with clear physical meaning. The proposed model satisfies the second law of thermodynamics. The results of the combined coupled model are broadly consistent with isometric and isotonic experiments on smooth muscle tissue. The simulations suggest that the matrix in which the actin–myosin complex is embedded does have a viscous property. It is straightforward for implementation into a finite element program in order to solve more complex boundary-value problems such as the control of short-term changes in lumen diameter of arteries due to mechanochemical signals.

  • 24.
    Murtada, Sae-Il
    et al.
    Royal Institute of Technology, (KTH).
    Kroon, Martin
    Royal Institute of Technology, (KTH).
    Holzapfel, Gerhard A.
    Graz University of Technology, Austria ; Royal Institute of Technology (KTH).
    Modeling the dispersion effects of contractile fibers in smooth muscles2010In: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 58, no 12, p. 2065-2082Article in journal (Refereed)
    Abstract [en]

    Micro-structurally based models for smooth muscle contraction are crucial for a better understanding of pathological conditions such as atherosclerosis, incontinence and asthma. It is meaningful that models consider the underlying mechanical structure and the biochemical activation. Hence, a simple mechanochemical model is proposed that includes the dispersion of the orientation of smooth muscle myofilaments and that is capable to capture available experimental data on smooth muscle contraction. This allows a refined study of the effects of myofilament dispersion on the smooth muscle contraction. A classical biochemical model is used to describe the cross-bridge interactions with the thin filament in smooth muscles in which calcium-dependent myosin phosphorylation is the only regulatory mechanism. A novel mechanical model considers the dispersion of the contractile fiber orientations in smooth muscle cells by means of a strain-energy function in terms of one dispersion parameter. All model parameters have a biophysical meaning and may be estimated through comparisons with experimental data. The contraction of the middle layer of a carotid artery is studied numerically. Using a tube the relationships between the internal pressure and the stretches are investigated as functions of the dispersion parameter, which implies a strong influence of the orientation of smooth muscle myofilaments on the contraction response. It is straightforward to implement this model in a finite element code to better analyze more complex boundary-value problems.

  • 25.
    Rems, Lea
    et al.
    University of Ljubljana, Slovenia.
    Ušaj, Marko
    University of Ljubljana, Slovenia.
    Kandušer, Maša
    University of Ljubljana, Slovenia.
    Reberšek, Matej
    University of Ljubljana, Slovenia.
    Miklavčič, Damijan
    University of Ljubljana, Slovenia.
    Pucihar, Gorazd
    University of Ljubljana, Slovenia.
    Cell electrofusion using nanosecond electric pulses2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, article id 3382Article in journal (Refereed)
    Abstract [en]

    Electrofusion is an efficient method for fusing cells using short-duration high-voltage electric pulses. However, electrofusion yields are very low when fusion partner cells differ considerably in their size, since the extent of electroporation (consequently membrane fusogenic state) with conventionally used microsecond pulses depends proportionally on the cell radius. We here propose a new and innovative approach to fuse cells with shorter, nanosecond (ns) pulses. Using numerical calculations we demonstrate that ns pulses can induce selective electroporation of the contact areas between cells (i.e. the target areas), regardless of the cell size. We then confirm experimentally on B16-F1 and CHO cell lines that electrofusion of cells with either equal or different size by using ns pulses is indeed feasible. Based on our results we expect that ns pulses can improve fusion yields in electrofusion of cells with different size, such as myeloma cells and B lymphocytes in hybridoma technology.

  • 26.
    Ušaj, Marko
    et al.
    University of Ljubljana, Slovenia.
    Flisar, Karel
    University of Ljubljana, Slovenia.
    Miklavcic, Damijan
    University of Ljubljana, Slovenia.
    Kanduser, Masa
    University of Ljubljana, Slovenia.
    Electrofusion of B16-F1 and CHO cells: the comparison of the pulse first and contact first protocols2013In: Bioelectrochemistry, ISSN 1567-5394, E-ISSN 1878-562X, Vol. 89, p. 34-41Article in journal (Refereed)
    Abstract [en]

    High voltage electric pulses induce permeabilisation (i.e. electroporation) of cell membranes. Electric pulses also induce fusion of cells which are in contact. Contacts between cells can be established before electroporation, in so-called contact first or after electroporation in pulse first protocol. The lowest fusion yield was obtained by pulse first protocol (0.8%±0.3%) and it was only detected by phase contrast microscopy. Higher fusion yield detected by fluorescence microscopy was obtained by contact first protocol. The highest fusion yield (15%) was obtained by modified adherence method whereas fusion yield obtained by dielectrophoresis was lower (4%). The results are in agreement with current understanding of electrofusion process and with existing electrochemical models. Our data indicate that probability of stalk formation leading to fusion pores and cytoplasmic mixing is higher in contact first protocol where cells in contact are exposed to electric pulses. Another contribution of present study is the comparison of two detection methods. Although fusion yield can be more precisely determined with fluorescence microscopy we should note that by using this detection method single coloured fused cells cannot be detected. Therefore low fusion yields are more reliably detected by phase contrast microscopy.

  • 27.
    Ušaj, Marko
    et al.
    University of Ljubljana, Slovenia.
    Kandušer, Maša
    University of Ljubljana, Slovenia.
    Modified Adherence Method (MAM) for Electrofusion of Anchorage-Dependent Cells2015In: Cell Fusion: Overviews and Methods, New York, NY: Humana Press, 2015, p. 203-216Chapter in book (Refereed)
    Abstract [en]

    The artificially induced cell fusion is a useful experimental tool in biology, biotechnology and medicine. The electrofusion is a physical method for cell fusion that applies high-voltage electric pulses. The use of electric pulses causes cell membrane structural changes which bring the cell membrane in the so-called fusogenic state. When such fusogenic membranes are in close contact cell fusion takes place. Physical contact between fusion partners can be achieved by various methods and one of them is modified adherence method (MAM) described in detail here on B16-F1 cell line. The method is based on the fact that living cells form contacts in confluent culture. However, instead of using confluent cell culture, in modified adherence method cells are plated in suitable concentration and allowed to form contacts for only short predetermined period of time. During that time the cells are only slightly attached to the dish surface maintaining the spherical shape. Observed high fusion yields up to 50 % obtained by MAM in situ by dual-color fluorescence microscopy are among the highest in field of electrofusion. The method can be readily adapted to other anchorage-dependent cell lines.

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