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2 Acta of Bioengineering and Biomechanics

2 2022

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Jankowski Krzysztof, Pawlikowski Marek, Barcz Katarzyna

Acta of Bioengineering and Biomechanics

2022

Vol. 24, no. 3

169--177

The aim of this research was to verify to what extent the shape of an indenter tip influences the final form of the constitutive equation for the trabecular bone. Methods: Trabecular bone was formulated as a non-linear viscoelastic material with Mooney–Rivlin hyperelastic model to describe the purely elastic response of the bone tissue. Tests of the mechanical properties of the trabecular bone, resected from the femoral head of a 56-year-old patient, were carried out with two types of indenter: the spherical tip of a diameter of 200 μm and pyramid Vickers tip with 136° between plane faces. Tests with both indenters included loading and unloading phases with no hold at peak force and with hold time t = 20 s and were conducted with a maximum load Pmax = 500 mN and loading/unloading rate V = 500 mN/min. Results: The formulated constitutive model describes the trabecula behaviour very well. The model curves match the experimental results in the loading phase, holding period and most of the unloading ramp. The purely viscoelastic material constants are very close in value for both considered tips, but purely elastic constants differ. Conclusions: The results indicate that the constitutive model based on the indentation with the Vickers tip does not cover the plastic residual deformation. When a viscoelastic response of bone is expected, a model with constants calibrated for the spherical tip should be used, and the other set of parameters values (Vickers tip) when trabeculae may undergo plastic deformation.

Comparative analyses of blood flow through mechanical trileaflet and bileaflet aortic valves

Pawlikowski Marek, Nieroda Anna

Acta of Bioengineering and Biomechanics

2022

Vol. 24, no. 2

141--152

The primary aim of the present study was to compare the bileaflet and trileaflet aortic valves’ performance during uniform blood flow model and boundary conditions. The secondary aim of the study was to determine the effect of Newtonian/non-Newtonian fluid flow assumption on blood flow directly behind the trileaflet valve. Methods: The geometrical model of the whole system consist of the left ventricle, fragment of the aorta and mechanical valves. A representation of pulsatile flow was obtained by measuring blood flow velocity (Doppler ultrasound examination). We have assumed turbulent blood flow. We considered two blood models, Newtonian and non-Newtonian (Carreau model). The valves’ performance was assessed using the reduced stress in the valves, the shear stress in the aortic wall, flow velocity field and the effective orifice area. Results: The maximum von Mises stress for the bileaflet valve leaflets was 0.3 MPa and for the trileaflet valve – 0.06 MPa. The maximum flow velocity for the bileaflet valve was 4.52 m/s for 40° and for the trileaflet valve – 5.74 m/s. Higher shear stress was present in the bileaflet (151.5 Pa) than for the trileaflet valve (49.64 Pa). Conclusions: The results indicate that central blood jet for the trileaflet valve contributes to more physiological blood flow and decreases the risk of haemolysis. The central flow minimises the risk of leaflet dislocation. In addition, lower stresses extend the durability of the valve. However, the trileaflet valve geometry has also disadvantages, for instance, small peripheral streams or relatively low effective orifice area.