Modelli Costitutivi dei Materiali Corso di Laurea in Ingegneria Biomedica Pavia, 2013 Introduction to soft tissues Ferdinando Auricchio 1 2 3 4 1 Dipartimento di Ingegneria Civile e Architettura, Università di Pavia, Italy 2 CESNA: Center for Advanced Numerical Simulations IUSS, Pavia, ITALY 3 UME Graduate School: Understanding and Managing Extremes IUSS, Pavia Pavia, Italy 4 IMATI Istituto di Matematica Applicata e Tecnologie Informatiche, CNR, Italy Thanks to Anna Ferrara September 30, 2013 F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 1 / 20
Why is it interesting to study Soft Tissues? I Provide contributions to minimally invasive intervention optimization, e.g., balloon angioplasty or stenting Understand onset and progression of cardiovascular diseases, e.g., atherosclerosis Design of artificial (mechanical or biological) prostheses Tissue engineering F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 2 / 20
Soft Tissues I Soft tissues include skin, ligaments, tendons, cartilage, blood vessel Soft tissues contain collagen, elastin, endothelial cells, smooth muscle cells Soft tissue behavior strongly influenced by concentration and structural arrangement of its constituents F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 3 / 20
Soft Tissues: basic properties I Structural properties non-homogeneous materials due to composition (properties change through tissue thickness) incompressible materials due to higher water quantity, 60 70% anisotropic materials due to fiber preferred orientations (properties change with direction) Mechanical elastic properties large deformations (strain larger than 20% under physiological pressure) nonlinear stress-strain response (exponential stiffening at higher loads) Mechanical anelastic properties hysteresis and preconditioning stress relaxation and creep F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 4 / 20
Soft Tissues: application I Arteries Aorta F.Auricchio (UNIPV) Heart valves Carotids Intro to soft tissues Aortic valves September 30, 2013 5 / 20
Arteries I Arterial system consists of a branching network of elastic conduits and high resistance terminals Arteries subdivided into elastic and muscolar Elastic arteries: closer to heart with large diameters Aorta Pulmonary artery Common carotids Common iliacs Muscolar arteries: located at the periphery with smaller diameters Coronaries Cerebrals Renals Femorals F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 6 / 20
Arteries: histology I Artery wall is made of three concentric layers tunica adventitia (A), tunica media (M), tunica intima (I) Each layer contains in different proportions and structural arrangement elastin, collagen, endothelial cells and smooth muscle cells muscolar artery (coronaries) elastic artery (aorta) F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 7 / 20
Arteries: preferred orientations I Collagen fibers: embedded in ground matrix, symmetrically arranged with respect to the circumferential direction, forming a typically fibrous tissue F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 8 / 20
Arteries: residual stresses I Arteries not stress-free, but subject to residual stresses axial residual stresses, i.e. artery segment excised from body shortens axially circumferential residual stresses, i.e.circular ring radially cut springs open into a circular sector F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 9 / 20
Heart valves I Heart valve: composed of aortic valve (AV), pulmonary valve (PV), mitral valve (MV), and tricuspid valve (TV) Each valves show similar architecture, but different function with regard to their role in the cardiac cycle Aortic valve: located between left ventricle and ascending aorta, allowing blood flow from left ventricle to body and preventing flow in reverse direction F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 10 / 20
Aortic valve I Different views of an excised porcine aortic valve Note aortic leaflets! F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 11 / 20
Aortic leaflets: histology I Aortic leaflets. Made of three layers: ventricularis, spongiosa and fibrosa Each layer appears as a fibrous tissue containing in different concentration: elastin, collagen, proteoglycans Collagen fiber highly oriented in the circumferential direction F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 12 / 20
Mechanical properties: nonlinear response I stress I II III aorta-unstretched aorta-stretched strain?? Tendon Ligament Aorta Skin Ultimate tensile strength [MPa] 50-100 50-100 0.3-0.80 1-20 Ultimate tensile strain [%] 10-15 10-15 50-100 30-70 F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 13 / 20
Mechanical properties: stiff and anisotropy I Behavior of separate layers is different media stiffer in circumferential than axial direction adventitia and intima stiffer in axial than circumferential direction intima stiffer than adventitia F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 14 / 20
Mechanical properties: hysteresis and preconditioning I Difference in the stress-strain relation between loading and unloading Hysteresis loop area: energy dissipation as heat during load cycle Stress-strain curve changes between 1st and 2nd cycle of loading-unloading Preconditioning: loading-unloading response becomes repeatable with less energy dissipation after a proper number of subsequent cycles F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 15 / 20
Mechanical properties: creep and stress relaxation I Stress relaxation: stress response over time to a step change of strain Creep: strain response over time to a step change of stress Stress relaxation Creep F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 16 / 20
How can you investigate Soft Tissue behavior? I uniaxial Mechanical in-vitro tests biaxial Different in-vitro tests are possible uniaxial tensil test biaxial tensil test extension-inflation test torsion test extension-inflation torsion F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 17 / 20
Uniaxial in-vitro tests: loading device I Federica Lanzo Master Thesis Tensile testing machine MTS Insight 10 kn Load cell 250 N - Specimen stretching F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 18 / 20
Uniaxial in-vitro Test: human carotid arteries I? Mechanical properties of human carotid arteries with a focus on intimal failure Digital images of human carotid artery under loading Stress-strain plot. Points corresponding to intimal failures... and anisotropy? F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 19 / 20
Uniaxial in-vitro Test: human iliac arteries I? Rectangular strip samples with axial and circumferential orientations Displacement controlled strain increased until fracture F.Auricchio (UNIPV) Intro to soft tissues September 30, 2013 20 / 20