A 3D model of muscle reveals the causes of nonuniform strains in the biceps brachii

Traumatic brain injury (TBI) occurs when local mechanical load exceeds certain tolerance levels for brain tissue. Extensive research has been done previously for brain matter experiencing compression at quasistatic loading; however, limited data is available to model TBI under dynamic impact conditions. In this research, an experimental setup was developed to perform unconfined compression tests and stress relaxation tests at strain rates≤90/s. The brain tissue showed a stiffer response with increasing strain rates, showing that hyperelastic models are not adequate. Specifically, the compressive nominal stress at 30% strain was 8.83±1.94, 12.8±3.10 and 16.0±1.41 kPa (mean±SD) at strain rates of 30, 60 and 90/s, respectively. Relaxation tests were also conducted at 10%- 50% strain with the average rise time of 10 ms, which can be used to derive time dependent parameters. Numerical simulations were performed using one-term Ogden model with initial shear modulusµ o =6.06±1.44, 9.44±2.427 and 12.64±1.227 kPa (mean±SD) at strain rates of 30, 60 and 90/s, respectively. A separate set of bonded and lubricated tests were also performed under the same test conditions to estimate the friction coefficient µ, by adopting combined experimental-computational approach. The values ofµwere 0.1±0.03 and 0.15±0.07 (mean±SD) at 30 and 90/s strain rates, respectively, indicating that pure slip conditions cannot be achieved in unconfined compression tests even under fully lubricated test conditions. The material parameters obtained in this study will help to develop biofidelic human brainfinite element models, which can subsequently be used to predict brain injuries under impact conditions. c

http://www.maths.nuigalway.ie/~destrade/Publis/destrade_90.pdf

Mechanical characterization of brain tissue in compression at dynamic strain rates.

The viability of chronic neural microelectrodes for electrophysiological recording and stimulation depends on several factors, including the encapsulation of the implant by a reactive tissue response. We postulate that mechanical strains induced around the implant site may be one of the leading factors responsible for the sustained tissue response in chronic implants. The objectives of this study were to develop a finite-element model of the probe-brain tissue interface and analyze the effects of tethering forces, probe-tissue adhesion and stiffness of the probe substrate on the interfacial strains induced around the implant site. A 3D finite-element model of the probe-brain tissue microenvironment was developed and used to simulate interfacial strains created by 'micromotion' of chronically implanted microelectrodes. Three candidate substrates were considered: (a) silicon, (b) polyimide and (c) a hypothetical 'soft' material. Simulated tethering forces resulted in elevated strains both at the tip and at the sharp edges of the probe track in the tissue. The strain fields induced by a simulated silicon probe were similar to those induced by a simulated polyimide probe, albeit at higher absolute values for radial tethering forces. Simulations of poor probe-tissue adhesion resulted in elevated strains at the tip and delamination of the tissue from the probe. A tangential tethering force results in 94% reduction in the strain value at the tip of the polyimide probe track in the tissue, whereas the simulated 'soft' probe induced two orders of magnitude smaller values of strain compared to a simulated silicon probe. The model results indicate that softer substrates reduce the strain at the probe-tissue interface and thus may also reduce tissue response in chronic implants.

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A finite-element model of the mechanical effects of implantable microelectrodes in the cerebral cortex.

Almost all computer models of the musculoskeletal system represent muscle geometry using a series of line segments. This simplification (i) limits the ability of models to accurately represent the paths of muscles with complex geometry and (ii) assumes that moment arms are equivalent for all fibers within a muscle (or muscle compartment). The goal of this work was to develop and evaluate a new method for creating three-dimensional (3D) finite-element models that represent complex muscle geometry and the variation in moment arms across fibers within a muscle. We created 3D models of the psoas, iliacus, gluteus maximus, and gluteus medius muscles from magnetic resonance (MR) images. Peak fiber moment arms varied substantially among fibers within each muscle (e.g., for the psoas the peak fiber hip flexion moment arms varied from 2 to 3 cm, and for the gluteus maximus the peak fiber hip extension moment arms varied from 1 to 7 cm). Moment arms from the literature were generally within the range of fiber moment arms predicted by the 3D models. The models accurately predicted changes in muscle surface geometry over a 55 degrees range of hip flexion, as compared to changes in shape predicted from MR images (average errors between the model and measured surfaces were between 1.7 and 5.2 mm). This new framework for representing muscle will enhance the accuracy of computer models of the musculoskeletal system.

/pdf/three-dimensional-representation-of-complex-muscle-1k1jr9zqpx.pdf

Three-dimensional representation of complex muscle architectures and geometries.

The influence of the non-Newtonian properties of blood on the flow in large arteries: unsteady flow in a 90° curved tube

http://www.mate.tue.nl/mate/pdfs/3124.pdf

Design and numerical implementation of a 3-D non-linear viscoelastic constitutive model for brain tissue during impact

Laser-Doppler velocity measurements were performed on the entry flow in a 90° bend of circular cross-section with a curvature ratio a/R = 1/6. The steady entry velocity profile was parabolic, having a Reynolds number Re = 700, with a corresponding Dean number ? = 286. Both axial and secondary velocities were measured, enabling a detailed description of the complete flow field. The secondary flow at the entrance of the bend was measured to be directed completely towards the inner bend. Significant disturbance of the axial velocity field was not measured until a downstream distance (aR)½. Maximum secondary velocities were measured at 1.7 (aR)½ downstream from the inlet. The development of the axial flow field can be quite well explained from the secondary velocity field.

Steady entry flow in a curved pipe

The large strain dynamic behaviour of brain tissue and silicone gel, a brain substitute material used in mechanical head models, was compared. The non-linear shear strain behaviour was characterised using stress relaxation experiments. Brain tissue showed significant shear softening for strains above 1% (approximately 30% softening for shear strains up to 20%) while the time relaxation behaviour was nearly strain independent. Silicone gel behaved as a linear viscoelastic solid for all strains tested (up to 50%) and frequencies up to 461 Hz. As a result, the large strain time dependent behaviour of both materials could be derived for frequencies up to 1000 Hz from small strain oscillatory experiments and application of Time Temperature Superpositioning. It was concluded that silicone gel material parameters are in the same range as those of brain tissue. Nevertheless the brain tissue response will not be captured exactly due to increased viscous damping at high frequencies and the absence of shear softening in the silicone gel. For trend studies and benchmarking of numerical models the gel can be a good model material.

The large shear strain dynamic behaviour of in-vitro porcine brain tissue and a silicone gel model material.

http://www.mate.tue.nl/mate/pdfs/4148.pdf

Evaluation of a fictitious domain method for predicting dynamic response of mechanical heart valves

The degree of restoration of pump function by ventricular pacing depends on the pacing site and timing of pacing. Numerical models of cardiac electromechanics could be used to investigate the relation between the ventricular pacing site and timing on the one side, and pump function on the other. In patient-specific models, these numerical models could be used to optimize location and timing for best pump function. The aim of this study was to demonstrate the potential for modeling patient-specific electromechanic during ventricular pacing by means of the extension of an existing three-dimensional finite-element model of LV electromechanics with the right ventricle. A parametrized geometry of the LV and RV was made from canine (non-invasively obtained) cine-MR short axis images. Depolarization was modeled using the eikonal-diffusion equation. Mechanics was computed from balance of momentum, with nonlinear anisotropic passive and time-, strain-, and strainrate-dependent uniaxial active behavior. Simulations of complete cardiac cycles were performed for a normal heart beat with synchronous activation and ventricular pacing at the right ventricular apex and left ventricular free wall. We focused on timing of LV and RV hemodynamics, asynchrony in depolarization and myofiber shortening, regional stroke work, and systolic septal motion. In the simulation of sinus rhytm, ventricular ejection was found to start earlier for the right side than for the left side, which is in agreement with experimental data. In simulations with ventricular pacing, results agreed with experimental findings in the following aspects: 1) depolarization sequence; 2) the spatial distributions of sarcomere length and stroke work density depended mainly on timing of depolarization; 3) maximum pressure and maximum increase of pressure were lower than during sinus rhythm; 4) the earliest activated ventricle had the earliest start of ejection, and 5) the septum moved towards the last activated ventricle at the onset of systole. As a first step, the potential of patient-specific modeling in simulating conduction disturbances has been demonstrated by inserting a ventricular geometry, obtained from non-invasively measured short axis MR images. Later steps would include the implementation of adaptation models to estimate patient myofiber orientation and to assess the effects of pacing in the long term.

Phm Peter Bovendeerd

Papers

Design and numerical implementation of a 3-D non-linear viscoelastic constitutive model for brain tissue during impact

Steady entry flow in a curved pipe

The large shear strain dynamic behaviour of in-vitro porcine brain tissue and a silicone gel model material.

Evaluation of a fictitious domain method for predicting dynamic response of mechanical heart valves

Intra- and interventricular asynchrony of electromechanics in the ventricularly paced heart three-dimensional numerical modeling