Modelling the rate-dependent mechanical behaviour of the brain tissue

A new modelling approach is employed in this work for application to the rate-dependent mechanical behaviour of the brain tissue, as an incompressible isotropic material. Extant datasets encompassing single- and multi-mode compression, tension and simple shear deformation(s) are considered, across a wide range of deformation rates from quasi-static to rates akin to blast loading conditions, in the order of 1000 s-1 . With a simple functional form and a reduced number of parameters, the model is shown to capture the considered rate-dependent behaviours favourably, including in both single- and multi-mode deformation fits, and over all range of deformation rates. The provided modelling results here are obtained from either first fitting the model to the quasi-static data, or/and predicting the behaviour at a different rate than those used for calibrating the model parameters. Given its simplicity, versatility, predictive capability and accuracy, the application of the utilised modelling framework in this work to the rate-dependent mechanical behaviour of the brain tissue is proposed.

Rate-dependent mechanical behaviour of semilunar valves under biaxial deformation: From quasi-static to physiological loading rates

In this study we investigate the rate-dependency of the mechanical behaviour of semilunar heart valves under biaxial deformation, from quasi-static to physiological loading rates. This work extends and complements our previous undertaking, where the rate-dependency in the mechanical behaviour of semilunar valve specimens was documented in sub-physiological rate domains (Acta Biomater. 2019; https://doi.org/10.1016/j.actbio.2019.02.008). For the first time we demonstrate herein that the stress-stretch curves obtained from specimens under physiological rates too are markedly different to those at sufficiently lower rates and at quasi-static conditions. The results importantly underline that the mechanical behaviour of semilunar heart valves is rate dependent, and the physiological mechanical behaviour of the valves may not be correctly obtained via material characterisation tests at arbitrary low deformation rates. Presented results in this work provide an inclusive dataset for material characterisation and modelling of semilunar heart valves across a 10,000 fold deformation rate, both under equi-biaxial and 1:3 ratio deformation rates. The important application of these results is to inform the development of appropriate mechanical testing protocols, as well as devising new models, for suitable determination of the rate-dependent constitutive mechanical behaviour of the semilunar valves.

A transverse isotropic constitutive model for the aortic valve tissue incorporating rate-dependency and fibre dispersion: Application to biaxial deformation

This paper presents a continuum-based transverse isotropic model incorporating rate-dependency and fibre dispersion, applied to the planar biaxial deformation of aortic valve (AV) specimens under various stretch rates. The rate dependency of the mechanical behaviour of the AV tissue under biaxial deformation, the (pseudo-) invariants of the right Cauchy-Green deformation-rate tensor Ċ associated with fibre dispersion, and a new fibre orientation density function motivated by fibre kinematics are presented for the first time. It is shown that the model captures the experimentally observed deformation of the specimens, and characterises a shear-thinning behaviour associated with the dissipative (viscous) kinematics of the matrix and the fibres. The application of the model for predicting the deformation behaviour of the AV under physiological rates is illustrated and an example of the predicted σ-λ curves is presented. While the development of the model was principally motivated by the AV biomechanics requisites, the comprehensive theoretical approach employed in the study renders the model suitable for application to other fibrous soft tissues that possess similar rate-dependent and structural attributes.

Modulation of L-type Ca²⁺ channel activity by neuronal nitric oxide synthase and myofilament Ca²⁺ sensitivity in cardiac myocytes from hypertensive rat

Neuronal nitric oxide synthase (nNOS) is important in cardiac protection in diseased heart. Recently, we have reported that nNOS is associated with myofilament Ca(2+) desensitization in cardiac myocytes from hypertensive rats. So far, the effect of myofilament Ca(2+) desensitization or nNOS on L-type Ca(2+) channel activity (I(Ca)) in cardiac myocyte is unclear. Here, we examined nNOS regulation of I(Ca) in left ventricular (LV) myocytes from sham and angiotensin II (Ang II)-induced hypertensive rats. Our results showed that basal I(Ca) was not different between sham and hypertension (from -60 to +40 mV, 0.1 Hz). S-methyl-L-thiocitrulline (SMTC), a selective nNOS inhibitor, increased peak I(Ca) similarly in both groups. However, chelation of intracellular Ca(2+) [Ca(2+)]i with BAPTA increased I(Ca) and abolished SMTC-augmentation of I(Ca) only in hypertension. Myofilament Ca(2+) desensitization with butanedione monoxime (BDM), a myosin ATPase inhibitor, decreased I(Ca) in both groups but to a greater extent in hypertension. Intracellular BAPTA or nNOS inhibition reinstated I(Ca) in the presence of BDM to the basal level, suggesting Ca(2+)-dependent inactivation of I(Ca) by nNOS and greater vulnerability in hypertension. Increasing stimulation frequencies (2, 4 and 8 Hz) attenuated myofilament Ca(2+) sensitivity in sham and reduced peak ICa in both groups. Nevertheless, SMTC or BAPTA exerted no effect on I(Ca) at high frequencies in either group. These results suggest that nNOS attenuates I(Ca) via Ca(2+)-dependent mechanism and the vulnerability is greater in hypertension subject to myofilament Ca(2+) desensitization. nNOS or [Ca(2+)]i does not affect I(Ca) at high stimulation frequencies. The results were recapitulated with computer simulation.