Mechanical characterisation and structural analysis of normal and remodeled cardiovascular soft tissue

Characterization of multiaxial mechanical properties of cardiovascular soft tissue is essential in order to better understand their growth and remodeling in homeostatic conditions and in response to injury or pathological conditions. Though numerous phenomenological models have been proposed to characterize such multiaxial mechanical behavior, the approach has certain drawbacks regarding experimental determination of the model coefficients. We propose a method that aims to overcome these drawbacks. The approach makes use of orthogonal polynomials to fit the biaxial test data and suggests a way to derive the strain energy function from these analytical fits by way of minimizing the deviation of the behavior from hyperelastic ideal. Using the proposed method, a strain energy function for a lymphatic vessel is derived and the method is compared with traditional ones that used non-orthogonal polynomials as independent variables in the functional form for strain energy. The unique coefficient values obtained using the proposed method, for the first time gives us an opportunity to attribute a physical characteristic of the material to the coefficient values. The method also provides a way to assess two different material behaviors by way of comparing their deviation from the hyperelastic behavior when a similar test protocol is used to collect the data, over a similar deformation range and the order of polynomial function is chosen so as to give a similar error of fit. The behavior of mesenteric lymph vessels from normal cows, cows subjected to sham surgery and those subjected to 3 days of edematous conditions by venous occlusion are compared using this method. To be able to better understand the changes in mechanical behavior, morphological analysis of the vessels was carried out and the geometric and structural changes in these vessels were studied. We found that the behavior of bovine mesenteric lymph vessels subjected to a high flow condition shows a small difference in their mechanical behavior as compared to the vessels from normal a cow and a cow subjected to sham surgery. The geometry and structure of these vessels also showed marked differences from the other two. The thickness to radius ratio increased and a rise in percentage of area occupied by smooth muscle cells and medial collagen was observed. Though not all the differences were statistically significant, we conclude that the behavior and the morphology are suggestive of the remodeling of the vessel in response to altered hemodynamic conditions and require further investigation

Development of a two dimensional optical fiber force transducer for constitutive modeling of soft tissue

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaf 101).Issued also on microfiche from Lange Micrographics.Constitutive modeling of soft tissue has provided better understanding of their function and behavior in both normal and pathophysiological conditions. The ability to predict growth and remodeling of the tissue under the influence of applied mechanical loads suggests novel treatment methods for diseases like atherosclerosis, cardiac ischema, aneurysms, congestive heart failure and many others. Material properties such as anisotropy, nonlinearity, inhomogenity, and viscoelasticity make it difficult to characterize soft tissue. Biaxial testing (first suggested by Revlin) is an important tool to analyze nonlinear materials and biaxial testing devices have been widely used by bioengineers to develop constitutive models of soft tissue. The most broadly used model is that suggested by Y. C. Fung. Although a useful instrument, conventional biaxial testing devices suffer from certain limitations such as the inability to test small sized specimens, gripping techniques and its effects on behavior of tissue under biaxial loading state. Also the device cannot be used to perform shear tests on tissue and hence cannot be effectively used to model behavior of tissues like endocardium and myocardium that undergo large shear deformations under normal physiological conditions. The two-dimensional optical fiber force transducer aims at overcoming some of these limitations. Since the transducer can measure both axial and shear forces it can be used in fabricating devices capable of shear testing. Also the small size optical fibers make the transducers suitable to be used in instruments for testing small specimens. This work presents a theoretical design of such a transducer, its fabrication and calibration procedure, and its testing for accuracy and sensitivity