Effect of residual stress on peak cap stress in arteries

Vulnerable plaques are a subset of atherosclerotic plaques that are prone to rupture when high stresses occur in the cap. The roles of residual stress, plaque morphology, and cap stiffness on the cap stress are not completely understood. Here, arteries are modeled within the framework of nonlinear elasticity as incompressible cylindrical structures that are residually stressed through differential growth. These structures are assumed to have a nonlinear, anisotropic, hyperelastic response to stresses in the media and adventitia layers and an isotropic response in the intima and necrotic layers. The effect of differential growth on the peak stress is explored in a simple, concentric geometry and it is shown that axial differential growth decreases the peak stress in the inner layer. Furthermore, morphological risk factors are explored. The peak stress in residually stressed cylinders is not greatly affected by changing the thickness of the intima. The thickness of the necrotic layer is shown to be the most important morphological feature that affects the peak stress in a residually stressed vessel.

Interdisciplinarity: the right people, a supportive place, and a program emerges

This paper describes the St. Olaf College experience moving to a more interdisciplinary approach to student learning. The authors place this within the context of the three “P”s of higher education—people, place, and program. The key for transformation resided in focusing on the people and the place. In so doing, an interdisciplinary program emerged.

Differential growth and residual stress in cylindrical elastic structures

Cylindrical forms are among one of Nature's fundamental building blocks. They serve many different purposes, from sustaining body weight to carrying flows. Their mechanical properties are generated through the often complex arrangements of the walls. In particular, in many structures that have elastic responses, such as stems and arteries, the walls are in a state of tension generated by differential growth. Here, the effect of differential growth and residual stress on the overall mechanical response of the cylindrical structure is studied within the framework of morpho-elasticity.

Morpho-elastodynamics: the long-time dynamics of elastic growth

As elastic tissues grow and remodel, they generate stresses that influence their mechanical states which influence the way growth proceeds. This complex feedback leads to a dynamic evolution of growth and stresses which can be modelled within the theory of exact nonlinear elasticity. Here, we first review the different theoretical results considering growth laws and then, we present a new approach to look at morphoelasticity as a continuous dynamical process. These evolution laws lead to new dynamical systems that can be studied by the classical methods of dynamical systems theory.

Nonlinear Euler buckling

The buckling of hyperelastic incompressible cylindrical tubes of arbitrary length and thickness under compressive axial load is considered within the framework of nonlinear elasticity. Analytical and numerical methods for bifurcation are developed using the exact solution of Wilkes for the linearized problem within the Stroh formalism. Using these methods, the range of validity of the Euler buckling formula and its first nonlinear corrections are obtained for third-order elasticity. The values of the geometric parameters (tube thickness and slenderness) where a transition between buckling and barrelling is observed are also identified.

Morning vs evening light treatment for winter depression. Evidence that the therapeutic effects of light are mediated by circadian phase shifts

Bright light exposure has been found to alleviate the symptoms of recurrent winter depression in many patients. The mechanism of light therapy may involve shifts in the timing (phase) of circadian rhythms. In this study, morning light exposure (which shifts rhythms earlier) was compared with evening light exposure (which shifts rhythms later) in a double-blind, crossover design. The onset of melatonin secretion in the evening was measured under dim light conditions as a marker for circadian timing (phase) before and after each treatment. Eight patients with winter depression and five control subjects were studied. Morning light was found to be significantly better than evening light in reducing depressive symptoms. At baseline, there was a trend for the onset of melatonin production to be later in the patients than in the controls. Morning light shifted the melatonin onset significantly earlier in the patients but not the controls. Our findings suggest that patients with winter depression have circadian rhythms that are abnormally delayed and that bright light therapy benefits winter depression by providing a corrective advance.