3D fiber-deposited scaffolds for tissue engineering: Influence of pores geometry and architecture on dynamic mechanical properties

Are in vivo and in situ brain tissues mechanically similar

▪ AbstractAlmost all vessels carrying fluids within the body are flexible, and interactions between an internal flow and wall deformation often underlie a vessel's biological function or dysfunction. Such interactions can involve a rich range of fluid-mechanical phenomena, including nonlinear pressure-drop/flow-rate relations, self-excited oscillations of single-phase flow at high Reynolds number and capillary-elastic instabilities of two-phase flow at low Reynolds number. We review recent advances in understanding the fundamental mechanics of flexible-tube flows, and discuss physiological applications spanning the cardiovascular system (involving wave propagation and flow-induced instabilities of blood vessels), the respiratory system (involving phonation, the closure and reopening of liquid-lined airways, and Marangoni flows on flexible surfaces), and elsewhere in the body (involving active peristaltic transport driven by fluid-structure/muscle interactions).

Biofluid mechanics in flexible tubes

http://www.fizjoterapeutom.pl/files/5/Biomechanics%20Of%20Knee%20Ligaments.pdf

Biomechanics of knee ligaments: injury, healing, and repair

On the biomechanics of heart valve function.

We investigated how preconditioning history and specimen recovery time affect the accuracy of measurements of mechanical properties obtained from sequential, repeated materials testing of porcine aortic valve (PAV) cusps. Strain history protocols were modeled by quasilinear viscoelastic theory and the results compared with the experimental data. Assuming that the model was predictive, the accuracy of predicting experimental data was related to the suitability of the materials testing protocol. We found that the preconditioned state of the PAV material was not unique but was a function of the deformation history that had occurred before the preconditioning cycles. Preconditioning without an adequate rest period between tests increased predictive errors, whereas allowing the material to rest without preconditioning reduced errors. Modeling more of the strain history reduced errors for specimens briefly rested between tests but had no impact on specimens with long rest periods. The smallest predictive errors were obtained for a loading protocol with a 24 h specimen recovery period followed by material preconditioning. We recommend the use of this protocol for estimating material properties of PAV tissues. © 2000 Biomedical Engineering Society.

Role of preconditioning and recovery time in repeated testing of aortic valve tissues: validation through quasilinear viscoelastic theory.

Knowledge of strain-rate sensitivity of soft tissue viscoelastic and nonlinear elastic properties is important for accurate predictions of biomechanical behavior and for quantitative assessment of the effects of disease or surgical/pharmaceutical intervention. Soft tissues are known to exhibit mild rate sensitivity, but experimental artifacts related to testing system control can confound estimation of these effects. “Perfect” ramp-and-hold stress-relaxation tests become difficult at high strain rates because of problems related to undershoot/overshoot error and vibrations. These errors can introduce unwanted bias into parameter estimation methods that rely on idealizations of the applied ramp-and-hold displacement. To address these problems, we describe a new method for estimating quasilinear viscoelastic (QLV) parameters that directly fits the QLV constitutive model to the actual point-wise stress–time history of the test, using an adaptive grid refinement (AGR) global optimization algorithm. This new method significantly improves the accuracy and predictivity of QLV parameter estimates for heart valve tissues, compared to traditional methods that use idealized displacement data. We estimated QLV parameters for aortic valve tissue over a range of physiologic displacement rates, finding that the viscoelastic content parameter (C) increased slightly with increasing strain rate, but the fast (τ1) and slow (τ2) time constants were strain rate insensitive.

The effect of strain rate on the viscoelastic response of aortic valve tissue: a direct-fit approach.

BACKGROUND Quasilinear viscoelasticity (QLV) theory has been widely and successfully used to describe the time-dependent response of connective tissues. Difficulties remain, however, particularly in material parameter estimation and sensitivities. In this study, we introduce a new alternative: the fractional order viscoelasticity (FOV) theory, which uses a fractional order integral to describe the relaxation response. FOV implies a fractal-like tissue structure, reflecting the hierarchical arrangement of collagenous tissues. METHOD OF APPROACH A one-dimensional (I-D) FOV reduced relaxation function was developed, replacing the QLV "box-spectrum" function with a fractional relaxation function. A direct-fit, global optimization method was used to estimate material parameters from stress relaxation tests on aortic valve tissue. RESULTS We found that for the aortic heart valve, FOV had similar accuracy and better parameter sensitivity than QLV, particularly for the long time constant (tau2). The mean (n = 5) fractional order was 0.29, indicating that the viscoelastic response of the tissue was strongly fractal-like. RESULTS SUMMARY: mean QLV parameters were C = 0.079, tau1 = 0.004, tau2 = 76, and mean FOV parameters were beta = 0.29, tau = 0.076, and rho = 1.84. CONCLUSIONS FOV can provide valuable new insights into tissue viscoelastic behavior Determining the fractional order can provide a new and sensitive quantitative measure for tissue comparison.

Fractional order viscoelasticity of the aortic valve cusp: an alternative to quasilinear viscoelasticity.

The elements of Quasi-Linear Viscoelastic (QLV) theory have been applied to model the internal shear mechanics of fresh and glutaraldehyde-fixed porcine aortic valve leaflets A novel function estimation method was used to extract the material functions from experimental shear data obtained at one strain rate, and the model was used to predict the material response at different strain rates In general, experiments and predictions were in good agreement, the larger discrepancies being in the prediction of peak stresses and hysteresis in cyclic shear In shear, fixed tissues are stiffer (mean initial shear modulus, 13 kPa versus 427 Pa), take longer to relax to steady state (mean τ2 4,736 s versus 1,764 s) with a slower initial relaxation rate (mean magnitude of G(0), 1 s−1 versus 5 s−1 ), and relax to a lesser extent than fresh tissues (mean percentage stress remaining after relaxation, 60 versus 45 percent) All differences were significant at p = 004 or less, except for the initial relaxation slope We conclude that shear experiments can complement traditional tensile and biaxial experiments toward providing a complete mechanical description of soft biomaterials, particularly when evaluating alternative chemical fixation techniques

Quasi-Linear Viscoelastic theory applied to internal shearing of porcine aortic valve leaflets.

A model for the coupled problem of wall deformation and fluid flow, based on thin-shell and lubrication theories, and driven by a propagating wave of smooth muscle activation, is proposed for peristaltic pumping in the ureter. The model makes use of the available experimental data on the mechanical properties of smooth muscle and accounts for the soft material between the muscle layer and the vessel lumen. The main input is the activation wave of muscular contraction. Equations for the time-dependent problem in tubes of arbitrary length are derived and applied to the particular case of periodic activation waves in an infinite tube. Mathematical (small amplitude) and numerical analyses of this case are presented. Predictions on phase-lag in wall constriction with respect to peak activation wave, lumen occlusion due to thickening lumen material with contracting smooth muscle, and the general bolus shape are in qualitative agreement with observation. Some modifications to the mechanical, elastic, and hydrodynamic properties of the ureter that will make peristalsis less efficient, due for example to disease, are identified. In particular, the flow rate-pressure rise relationship in linear for weak to moderate activation waves, but as the lumen is squeezed shut, it is seen to be nonlinear in a way that increases pumping efficiency. In every case a ureter whose lumen can theoretically be squeezed shut is the one for which pumping is most efficient.

Evelyn O. Carew

Papers

Role of preconditioning and recovery time in repeated testing of aortic valve tissues: validation through quasilinear viscoelastic theory.

The effect of strain rate on the viscoelastic response of aortic valve tissue: a direct-fit approach.

Fractional order viscoelasticity of the aortic valve cusp: an alternative to quasilinear viscoelasticity.

Quasi-Linear Viscoelastic theory applied to internal shearing of porcine aortic valve leaflets.

An Active Membrane Model for Peristaltic Pumping: Part I—Periodic Activation Waves in an Infinite Tube