prof. Ing. Jiří Burša, Ph.D.
E-mail:   bursa@fme.vutbr.cz  WWW:   http://www.old.umt.fme.vutbr.cz/~jbursa/ Dept.:   Institute of Solid Mechanics, Mechatronics and Biomechanics Dept. of Biomechanics Position:   Head of Department Room:   A2/702 Dept.:   Institute of Solid Mechanics, Mechatronics and Biomechanics Dept. of Biomechanics Position:   Professor Room:   A2/702

Supervised courses:

• Biomechanics III - Cardiovascular (RBM)
• Computational Models of Non-linear Material Behaviour (9VMM)
• Constitutive Equations for BIO (RKB-A)
• Constitutive Equations for IME (RKI-A)
• Diploma Project (M-BIO) (RDT)
• Mechanics of Biological Tissues (9MBT)

Publications:

• POLZER, S.; GASSER, T.; FORSELL, C.; DRUCKMÜLLEROVÁ, H.; TICHÝ, M.; STAFFA, R.; VLACHOVSKÝ, R.; BURŠA, J.:
  Automatic Identification and Validation of Planar Collagen Organization in the Aorta Wall with Application to Abdominal Aortic Aneurysm,
  MICROSCOPY AND MICROANALYSIS, Vol.19, (2013), No.6, pp.1395-1404, ISSN 1431-9276, Cambridge Journals
  journal article - other
• HOENICKA, M.; SIEGFRIED, S.; BURŠA, J.; HUBER, G.; HOLGER, B.; BIRNBAUM, D.; SCHMID, C.:
  Development of endothelium-denuded human umbilical veins as living scaffolds for tissue-engineered small-calibre vascular grafts, Wiley-Blackwell
  journal article in Web of Science
• POLZER, S.; GASSER, T.; BURŠA, J.; STAFFA, R.; VLACHOVSKÝ, J.; MAN, V.; SKÁCEL, P.:
  Importance of material model in wall stress prediction in abdominal aortic aneurysms,
  MEDICAL ENGINEERING & PHYSICS, Vol.35, (2013), No.4, pp.1282-1289, ISSN 1350-4533, Elsevier
  journal article - other
• POLZER, S.; GASSER, T.; MARKERT, B.; BURŠA, J.; SKÁCEL, P.:
  Impact of poroelasticity of intraluminal thrombus on wall stress of abdominal aortic aneurysms,
  BIOMED ENG ONLINE, Vol.11, (2012), No.62, pp.1-13, ISSN 1475-925X, Biomed Central
  journal article - other
• BURŠA, J.; HOLATA, J.; LEBIŠ, R.:
  Tensegrity finite element models of mechanical tests of individual cells,
  Technology and Health Care, Int. Journal of Health Care Engineering, Vol.20, (2012), No.2, pp.135-150, ISSN 0928-7329, Elsevier Science B.V.
  journal article - other
• POLZER, S.; BURŠA, J.; SWEDENBORG, J.; GASSER, T.:
  The impact of Intra-luminal Thrombus failure on the mechanical stress in the wall of Abdominal Aortic Aneurysms,
  EUROPEAN JOURNAL OF VASCULAR AND ENDOVASCULAR SURGERY, Vol.41, (2011), No.4, pp.467-473, ISSN 1078-5884, Elsevier
  journal article - other
• SKÁCEL, P.; BURŠA, J.:
  Material parameter identification of arterial wall layers from homogenized stress-strain data,
  COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING, Vol.14, (2011), No.01, pp.33-41, ISSN 1025-5842, Taylor & Francis Group
  journal article - other

List of publications at Portal BUT

Abstracts of most important papers:

• SKÁCEL, P.; BURŠA, J.:
  Numerical implementation of constitutive model for arterial layers with distributed collagen fibre orientations, Taylor & Francis Group
  journal article in Web of Science
  Several constitutive models have been proposed for the description of mechanical behaviour of soft tissues containing collagen fibres. Some of the commonly used approaches accounting for the dispersion of fibre orientations are based on the summation of (mechanical) contributions of differently oriented fibre families. This leads to the need of numerical integration on the sphere surface, and the related numerical consumption is the main disadvantage of this category of constitutive models. The paper is focused on the comparison of various numerical integration methods applied to specific constitutive model applicable for arterial walls. Robustness and efficiency of several integration rules were tested with respect to application in finite element (FE) codes. Among all the analysed numerical integration rules, the best results were reached by Lebedev quadrature; the related parameters for the specific constitutive model are presented in the paper. The results were implemented into the commercial FE code ANSYS via user subroutines, and their applicability was demonstrated by an example of FE simulation with non-homogenous stress field.
• POLZER, S.; BURŠA, J.; GASSER, T.; STAFFA, R.; VLACHOVSKÝ, R.:
  A numerical implementation to predict residual strains from the homogeneous stress hypothesis with application to abdominal aortic aneurysms,
  ANNALS OF BIOMEDICAL ENGINEERING, Vol.41, (2013), No.7, pp.1516-1527, ISSN 0090-6964, Springer
  journal article - other
  Wall stress analysis of abdominal aortic aneurysm (AAA) is a promising method of identifying AAAs at high risk of rupture. However, neglecting residual strains (RS) in the load-free configuration of patient-specific finite element analysis models is a sever limitation that strongly affects the computed wall stresses. Although several methods for including RS have been proposed, they cannot be directly applied to patient-specific AAA simulations. RS in the AAA wall are predicted through volumetric tissue growth that aims at satisfying the homogeneous stress hypothesis at mean arterial pressure load. Tissue growth is interpolated linearly across the wall thickness and aneurysm tissues are described by isotropic constitutive formulations. The total deformation is multiplicatively split into elastic and growth contributions, and a staggered schema is used to solve the field variables. The algorithm is validated qualitatively at a cylindrical artery model and then applied to patient-specific AAAs (n = 5). The induced RS state is fully three-dimensional and in qualitative agreement with experimental observations, i.e., wall strips that were excised from the load-free wall showed stress-releasing-deformations that are typically seen in laboratory experiments. Compared to RS-free simulations, the proposed algorithm reduced the von Mises stress gradient across the wall by a tenfold. Accounting for RS leads to homogenized wall stresses, which apart from reducing the peak wall stress (PWS) also shifted its location in some cases. The present study demonstrated that the homogeneous stress hypothesis can be effectively used to predict RS in the load-free configuration of the vascular wall. The proposed algorithm leads to a fast and robust prediction of RS, which is fully capable for a patient-specific AAA rupture risk assessment. Neglecting RS leads to non-realistic wall stress values that severely overestimate the PWS.
• POLZER, S.; GASSER, T.; BURŠA, J.; STAFFA, R.; VLACHOVSKÝ, J.; MAN, V.; SKÁCEL, P.:
  Importance of material model in wall stress prediction in abdominal aortic aneurysms,
  MEDICAL ENGINEERING & PHYSICS, Vol.35, (2013), No.4, pp.1282-1289, ISSN 1350-4533, Elsevier
  journal article - other
  Background. Results of biomechanical simulation of the abdominal aortic aneurysm (AAA) depend on the constitutive description of the wall. Based on in-vitro and in-vivo experimental data several constitutive models for the AAA wall have been proposed in literature. Those models differ strongly from each other and their impact on the computed stress in biomechanical simulation is not clearly understood. Methods. Finite Element (FE) models of AAAs from 7 patients who underwent elective surgical repair were used to compute wall stresses. AAA geometry was reconstructed from CT angiography (CTA) data and patient-specific constitutive descriptions of the wall were derived from planar biaxial testing of anterior wall tissue samples. In total 28 FE models were used, where the wall was described either by patient-specific or previously reported study-average properties. This data was derived from either uniaxial or biaxial in-vitro testing. Computed wall stress fields were compared on node-by-node basis. Results. Different constitutive models for the AAA wall cause significantly different predictions of wall stress. While study-average data from biaxial testing gives globally the same stress field as the patient-specific wall properties, the material model based on uniaxial test data overestimates the wall stress on average by 30kPa or about 67% of the mean stress. A quasi-linear description based on the in-vivo measured distensibility of the AAA wall leads to a completely altered stress field and overestimates the wall stress by about 75kPa or about 167% of the mean stress. Conclusion. The present study demonstrated that the constitutive description of the wall is crucial for AAA wall stress prediction. Consequently, results obtained using different models should not be mutually compared unless different stress gradients across the wall are not taken into account. Highly nonlinear material models should be preferred when the response of AAA to increased blood pressure is investigated, while the quasi-linear model with high initial stiffness produces negligible stress gradients across the wall and thus, it is more appropriate when response to mean blood pressure is calculated.
• BURŠA, J.; HOLATA, J.; LEBIŠ, R.:
  Tensegrity finite element models of mechanical tests of individual cells,
  Technology and Health Care, Int. Journal of Health Care Engineering, Vol.20, (2012), No.2, pp.135-150, ISSN 0928-7329, Elsevier Science B.V.
  journal article - other
  A three-dimensional finite element model of a vascular smooth muscle cell is based on models published recently; it comprehends elements representing cell membrane, cytoplasm and nucleus, and a complex tensegrity structure representing the cytoskeleton. In contrast to previous models of eucaryotic cells, this tensegrity structure consists of several parts. Its external and internal parts number 30 struts, 60 cables each, and their nodes are interconnected by 30 radial members; these parts represent cortical, nuclear and deep cytoskeletons, respectively. This arrangement enables us to simulate load transmission from the extracellular space to the nucleus or centrosome via membrane receptors (focal adhesions); the ability of the model was tested by simulation of some mechanical tests with isolated vascular smooth muscle cells. Although material properties of components defined on the basis of the mechanical tests are ambiguous, modelling of different types of tests has shown the ability of the model to simulate substantial global features of cell behaviour, e.g. action at a distance effect or the global load-deformation response of the cell under various types of loading. Based on computational simulations, the authors offer a hypothesis explaining the scatter of experimental results of indentation tests.
• SKÁCEL, P.; BURŠA, J.:
  Material parameter identification of arterial wall layers from homogenized stress-strain data,
  COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING, Vol.14, (2011), No.01, pp.33-41, ISSN 1025-5842, Taylor & Francis Group
  journal article - other
  Multi-layer structure of the artery can have significant effects on the resulting mechanical behaviour of the artery wall. Separation of the artery into individual layers is sometimes performed to identify the layer-specific parameters of constitutive model proposed by Holzapfel (2000). Inspired by this single-layer model, a double-layer model was formulated and used for identification of material parameters from homogenized stress-strain data (of non-separated artery wall). The paper demonstrates that the layer-specific parameters of the double-layer constitutive model can be indentified without the need of artery separation. The resulting double-layer model can credibly describe the homogenized stress-strain behaviour of the real artery wall including large-strain stiffening effects attributed to multi-layer nature of the artery.