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The collagen diffraction patterns of human arteries under uniaxial tensile test conditions have been investigated by time resolved synchrotron small angle X-ray diffraction [1]. Different types of arteries were chosen according to their clinical interest and have been studied after dissection into their major layers (intima, media, adventitia). Using a recently designed tensile testing device [2], the orientation and d-spacing of the collagen fibers in the layers have been measured in situ under physiological conditions, together with the macroscopic force and sample deformation. This allows reconstruction of true stresses and strains and the fitting of this data to a non linear mechanical model [3]. The results show a relation between the orientation/extension of the collagen fibers on the nanoscopic level and the macroscopic stress and strain. This is attributed first to a straightening, second to a reorientation of the collagen fibers, and third to an uptake of the increasing loads by the collagen fibers.

Preconditioned prepared strips (n ϭ 78) of segments from the midregion of the left anterior descending coronary artery from the individual layers in axial and circumferential directions were subjected to cyclic quasi-static uniaxial tension tests, and ultimate tensile stresses and stretches were documented. The ratio of outer diameter to total wall thickness was 0.189 Ϯ 0.014; ratios of adventitia, media, and intima thickness to total wall thickness were 0.4 Ϯ 0.03, 0.36 Ϯ 0.03, and 0.27 Ϯ 0.02, respectively; axial in situ stretch of 1.044 Ϯ 0.06 decreased with age. Stress-stretch responses for the individual tissues showed pronounced mechanical heterogeneity. The intima is the stiffest layer over the whole deformation domain, whereas the media in the longitudinal direction is the softest. All specimens exhibited small hysteresis and anisotropic and strong nonlinear behavior in both loading directions. The media and intima showed similar ultimate tensile stresses, which are on average three times smaller than ultimate tensile stresses in the adventitia (1,430 Ϯ 604 kPa circumferential and 1,300 Ϯ 692 kPa longitudinal). The ultimate tensile stretches are similar for all tissue layers. A recently proposed constitutive model was extended and used to represent the deformation behavior for each tissue type over the entire loading range. The study showed the need to model nonstenotic human coronary arteries with nonatherosclerotic intimal thickening as a composite structure composed of three solid mechanically relevant layers with different mechanical properties. The intima showed significant thickness, load-bearing capacity, and mechanical strength compared with the media and adventitia.human left anterior descending coronary artery; elasticity; material model; mechanical properties; ultimate tensile strength ONE CENTRAL AIM IN CARDIOVASCULAR solid mechanics is the investigation of the mechanobiological behavior of arteries in health and disease, which may better explain their function on the basis of their structure and mechanics, i.e., vital information for clinical treatments of artery diseases, for designs of vascular implants such as stents and grafts, and for tissue engineering. For example, 657,000 percutaneous transluminal coronary angioplasty (PTCA) procedures were performed in the United States in 2002 (1). To improve our understanding of the mechanisms involved in PTCA procedures and stent designs, it is fundamental to better explore the mechanical properties and role of the separate arterial layers of coronary arterial walls and to develop efficient computational models.Most of the studies have focused on the mechanical properties of animal coronary arteries (3,11,22,25,26,33,42). In addition to a collagenous adventitia and a medial layer (consisting of smooth muscle, collagen, and some elastin), a coronary arterial wall may consist of a complex intimal layer that develops rapidly in early years and continues to grow gradually throughout life (41). This process is not observed in animals. The intima is a ...

In this paper, we first of all review the morphology and structure of the myocardium and discuss the main features of the mechanical response of passive myocardium tissue, which is an orthotropic material. Locally within the architecture of the myocardium three mutually orthogonal directions can be identified, forming planes with distinct material responses. We treat the left ventricular myocardium as a non-homogeneous, thick-walled, nonlinearly elastic and incompressible material and develop a general theoretical framework based on invariants associated with the three directions. Within this framework we review existing constitutive models and then develop a structurally based model that accounts for the muscle fibre direction and the myocyte sheet structure. The model is applied to simple shear and biaxial deformations and a specific form fitted to the existing (and somewhat limited) experimental data, emphasizing the orthotropy and the limitations of biaxial tests. The need for additional data is highlighted. A brief discussion of issues of convexity of the model and related matters concludes the paper.

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This review article is concerned with the mathematical modelling of the mechanical properties of the soft biological tissues that constitute the walls of arteries. Many important aspects of the mechanical behaviour of arterial tissue can be treated on the basis of elasticity theory, and the focus of the article is therefore on the constitutive modelling of the anisotropic and highly nonlinear elastic properties of the artery wall. The discussion focuses primarily on developments over the last decade based on the theory of deformation invariants, in particular invariants that in part capture structural aspects of the tissue, specifically the orientation of collagen fibres, the dispersion in the orientation, and the associated anisotropy of the material properties. The main features of the relevant theory are summarized briefly and particular forms of the elastic strain-energy function are discussed and then applied to an artery considered as a thickwalled circular cylindrical tube in order to illustrate its extension-inflation behaviour. The wide range of applications of the constitutive modelling framework to artery walls in both health and disease and to the other fibrous soft tissues is discussed in detail. Since the main modelling effort in the literature has been on the passive response of arteries, this is also the concern of the major part of this article. A section is nevertheless devoted to reviewing the limited literature within the continuum mechanics framework on the active response of artery walls, i.e. the mechanical behaviour associated with the activation of smooth muscle, a very important but also very challenging topic that requires substantial further development. A final section provides a brief summary of the current state of arterial wall mechanical modelling and points to key areas that need further modelling effort in order to improve understanding of the biomechanics and mechanobiology of arteries and other soft tissues, from the molecular, to the cellular, tissue and organ levels.

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