Classical and all-floating FETI methods for the simulation of arterial tissues

High-resolution and anatomically realistic computer models of biological soft tissues play a significant role in the understanding of the function of cardiovascular components in health and disease. However, the computational effort to handle fine grids to resolve the geometries as well as sophisticated tissue models is very challenging. One possibility to derive a strongly scalable parallel solution algorithm is to consider finite element tearing and interconnecting (FETI) methods. In this study we propose and investigate the application of FETI methods to simulate the elastic behavior of biological soft tissues. As one particular example we choose the artery which is - as most other biological tissues - characterized by anisotropic and nonlinear material properties. We compare two specific approaches of FETI methods, classical and all-floating, and investigate the numerical behavior of different preconditioning techniques. In comparison to classical FETI, the all-floating approach has not only advantages concerning the implementation but in many cases also concerning the convergence of the global iterative solution method. This behavior is illustrated with numerical examples. We present results of linear elastic simulations to show convergence rates, as expected from the theory, and results from the more sophisticated nonlinear case where we apply a well-known anisotropic model to the realistic geometry of an artery. Although the FETI methods have a great applicability on artery simulations we will also discuss some limitations concerning the dependence on material parameters.Comment: 29 page

A comparative study of hyperelastic constitutive models for colonic tissue fitted to multiaxial experimental testing

For colonic stents design, the interaction with colonic tissue is essential in order to characterize the appropriate radial stiffness which provides a minimum lumen for intestinal transit to be maintained. It is therefore important to develop suitable constitutive models allowing the mechanical behavior of the colon tissue to be characterized. The present work investigates the biomechanical behavior of colonic tissue by means of biaxial tests carried out on different parts of the colonic tract taken from several porcine specimens. Samples from the colonic tract were quasi-statically tensioned using a load-controlled protocol with different tension ratios between the circumferential and the axial directions. Fitting techniques were then used to adjust specific hyperelastic models accounting for the multilayered conformation of the colonic wall and the fiber-reinforced configuration of the corresponding tissues. It was found that the porcine colon changed from a more isotropic to a more anisotropic tissue and became progressively more flexible and compliant in circumferential direction depending on the position along the duct as it approaches the rectum. The best predictive capability of mechanical behavior corresponds to the Four Fiber Family model showing mean values of coefficient of determination R2 ¼ 0:97, and a normalized root mean square error of eNRMS ¼ 0:0814 for proximal spiral samples, and R2 ¼ 0:89 ; eNRMS ¼ 0:1600 and R2 ¼ 0:94 ; eNRMS ¼ 0:1227 for distal spiral and descending colon samples, respectively. The other analyzed models provide good results for proximal spiral colon specimens, which have a lower degree of anisotropy. The analyzed models with the fitted elastic parameters can be used for more realistic and reliable FE simulations, providing the appropriate framework for the design of optimal devices for the treatment of colonic diseases

Over length quantification of the multiaxial mechanical properties of the ascending, descending and abdominal aorta using Digital Image Correlation

In this paper, we hypothesize that the biaxial mechanical properties of the aorta may be dependent on arterial location. To demonstrate any possible position-related difference, our study analyzed and compared the biaxial mechanical properties of the ascending thoracic aorta, descending thoracic aorta and infrarenal abdominal aorta stemming from the same porcine subjects, and reported values of constitutive parameters for well-known strain energy functions, showing how these mechanical properties are affected by location along the aorta. When comparing ascending thoracic aorta, descending thoracic aorta and infrarenal abdominal aorta, abdominal tissues were found to be stiffer and highly anisotropic. We found that the aorta changed from a more isotropic to a more anisotropic tissue and became progressively less compliant and stiffer with the distance to the heart. We observed substantial differences in the anisotropy parameter between aortic samples where abdominal samples were more anisotropic and nonlinear than the thoracic samples. The phenomenological model was not able to capture the passive biaxial properties of each specific porcine aorta over a wide range of biaxial deformations, showing the best prediction root mean square error e=0.2621 for ascending thoracic samples and, especially, the worst for the infrarenal abdominal samples e=0.3780. The micro-structured model with Bingham orientation density function was able to better predict biaxial deformations (e=0.1372 for ascending thoracic aorta samples). The root mean square error of the micro-structural model and the micro-structured model with von Mises orientation density function were similar for all positions

Supramolecular assembly and mechanical properties of dermis

The present work is a part of a wider research project which aims at the in vitro tissues and biohybrid generation. The process of generating biological tissues requires benchmarks in order to define the optimal set of design and performance parameters for the tissue of interest. As a consequence of that, my efforts have been devoted to the study of natural tissue. In particular I have focused my attention to their composition, microstructure and macroscopic properties. The first part of the thesis reviews recent studies concerning the assembly and spatial arrangement of some biological macromolecules of interest, which compose the extracellular matrix. The extracellular matrix is indeed largely responsible for the macroscopic physical properties of connective tissues. Skin has been chosen as model of connective tissue to study. This choice is motivated by the fact that skin is a more general model rather then tendons, which are mainly subjected to uniaxal tension, and the osmosis-supported cartilage. An experimental campaign has been designed in order to gather information on dermal composition and structure, and how these characteristics can affect the macroscopic behaviour of the tissue. The results of this experimental campaign are shown in the second part of the work. At last two constitutive equations are presented. Both of them are developed within the framework of continuum mechanics. The first one is a full three dimensional model able to capture the elastic behaviour of dermis at large deformations. The second model is able to predict the viscoelastic behaviour. Both model accounts for the anisotropy of the native tissue and are structural model, since they contain parameters on the underlying histology. The development of these models provide noteworthy information on the performance of tissue-engineered constructs whose properties have been designed ab initio. In particular, since the mechanical properties of biohybrids can be on-line monitored during culturing in bioreactors. Thus constitutive models can provide cues on the evolution of the mechanical properties, giving the chance to investigate on the complex relationship between mechanical stimulus and tissue remodelling

Mechanical characterization of animal derived starting materials for tissue engineering

Animal derived starting materials are well established in the production of Tissue Engineered Medical Devices. Porcine specifically can be found in products ranging in application from hernia repair to breast reconstruction. Although this material has been largely accepted in the Tissue Engineering industry, little is known of its material properties and mechanical characteristics. A review of the scientific literature describes limited mechanical measures only on uncontrolled research grade material. The objective of this work is to mechanically characterize porcine starting material used in the medical device industry. Porcine skin is provided by Midwest Research Swine, LLC (MRS) an established supplier to Medical Device companies. The experiments are established to evaluate if the skin’s mechanical characteristics vary by location and direction. The porcine skin samples are marked for their location (Back and Neck) as well as Orientation (Head and Spine). A custom die is used to prepare uniaxial tensile samples parallel, perpendicular, and at 45 degrees from the Spine landmark. An MTS load frame and Digital Image Correlation (DIC) measurement system is used to acquire the stress-stretch relationship. Mechanical indices from the stress-stretch relationship is analyzed by first separating it into a toe and linear region through a bilinear curve fitting method, apply the Ogden hypereslastic material model to the Toe, and linear model to the linear region. The Ogden fit in the toe region reveals anisotropic behavior that varied by location on the porcine skin, where the Back region behaves anisotropically and the Neck isotopically. The Gasser-Ogden-Holzepfel (GOH) structural model is explored to unify the tissue’s directional properties. The GOH model requires that the microstructural element’s (e.g. collagen fiber) arrangement within the macroscopic tissue is known beforehand. In the literature this is accomplished through histological measurements on the tissue itself. This requirement limits the ability of the GOH model to be used in the real time analysis of experimental work, which is needed in both academia and the tissue engineering industry. A method is developed to determine the microstructural arrangement (angle and dispersion) by utilizing the mechanical response at two orientations. This developed Microstructural Arrangement Determination Method (MADM) is verified by reproducing a GOH ABAQUS model from the literature. MADM is then applied to porcine skin experimental data, revealing a potential limitation of the GOH model in its treatment of transverse strain

Mekanik malzeme modeli ve ilgili parametrelerin kestirilmesi için yumuşak biyolojik dokular üstünde yapılan yerinde indentör deneyleri.

Soft biological tissues, being live and due to their physiological structures, display considerably complex mechanical behaviors. For a better understanding and use in various applications, first study to be carried out is the tests made particularly as in vivo. An indenter test device developed for this purpose in the METU, Department of Mechanical Engineering, Biomechanics Laboratory is operational. In this study, in order to carry out precise and dependable tests, initially, various tests and improvements were conducted on the device and the software controlling the device. At the end of this study, displacement and load measurement accuracies and precisions were improved. Better algorithms for filtering the noisy data were prepared. Some test protocols within the software were improved and new protocols were annexed. To be able to conduct more dependable tests a new connection system was attached to the device. In order to study the anisotropic behavior of soft tissues ellipsoid tips were designed and produced. In the second phase of the study, tests on medial forearm were carried out. In these tests, hysteresis, relaxation and creep behaviors displaying the viscoelastic v properties of the soft biological tissues were observed. In addition to viscoelastic behaviors, preconditioning (Mullin’s) effect and anisotropic response were examined. By using the results of the relaxation and creep tests, parameters of the Prony series capable of modelling these data were determined. With this study, some important conclusions regarding the soft biological tissues were drawn and thus the behaviors of the soft biological tissues were better understood. Besides, the difficulties inherent to in-vivo tests were recognized and actions to reduce these difficulties were explained. Finally, clean experimental data, to be used in the computer simulations, were obtained.M.S. - Master of Scienc

Biomechanical properties of the spinal cord: implications for tissue engineering and clinical translation

Spinal cord injury is a severely debilitating condition which can leave individuals paralyzed and suffering from autonomic dysfunction. Regenerative medicine may offer a promising solution to this problem. Previous research has focused primarily on exploring the cellular and biological aspects of the spinal cord, yet relatively little remains known about the biomechanical properties of spinal cord tissue. Given that a number of regenerative strategies aim to deliver cells and materials in the form of tissue-engineered therapies, understanding the biomechanical properties of host spinal cord tissue is important. We review the relevant biomechanical properties of spinal cord tissue and provide the baseline knowledge required to apply these important physical concepts to spinal cord tissue engineering

Effect of Nicotine Exposure on Fatigue Mechanics of Murine Arteries

Nicotine is an addictive substance found in electronic cigarettes (e-cigarettes) and cigarettes, and the use of such products has growing concern because of the prevalent use by young adults. Nicotine exposure degrades arterial tissue and results in increased arterial stiffness, which is linked to cardiovascular events, such as stroke and myocardial infarction. The hypothesis of this research is that fatigue loading reveals changes to the mechanical behavior of nicotine-treated arteries that is more informatively discriminating than stiffness alone. Ten murine arteries, five untreated and five nicotine-treated, were subjected to cyclic fatigue loading in an open-circumferential configuration by a custom tensometer with a motorized actuator and inline load cell. The specimens were subjected to an alternating strain of ±50%, superimposed on a mean strain of 150%. A power law was fit to the experimental data to extract parameters indicative of peak stress, loss of tension, degradation slope, and oscillation band. Compared to untreated specimens, nicotine-treated specimens exhibited a 108% higher peak stress, 118% greater loss of tension, and 107% larger width of the oscillation band. The tension loss and oscillation band width provided additional discriminating evidence of different mechanical behavior, supporting the hypothesis that fatigue testing can reveal distinctions in mechanical behavior that are not evident in static testing alone

Mechanical Behaviour of Tendinopathic Tendon: An Engineering Perspective

Tendinopathy is a debilitating disease affecting millions of people worldwide. The aetiology of this disease is not well understood, and treatment remains difficult due to a lack of evidence-based management. This dissertation sought to quantify the mechanical behaviour of tendon in order to understand the difference between healthy and tendinopathic tendons. The findings in this dissertation offer insights that may contribute toward the development of better clinical management of tendinopathy