Now showing 1 - 10 of 22
  • Publication
    Linear Viscoelastic Properties of Cerbral Cortex at Thresholds for Axonal Damage
    Traumatic brain injury (TBI) is caused by rapid deformation of the brain that leads to shearing of axons. While deformation below the limits of ultimate failure can activate pathophysiological cascades that cause neurodegeneration [1], bleeding does not always occur even after tearing of axons. Traditional imaging studies such as CT and MRI are designed to detect areas of bleeding but these can fail to detect the presence of multiple, widespread, microscopic axonal injuries that can result in devastating neurological deficits. A large knowledge gap still exists defining the relationship between axonal injury at a microscopic level (morphological injury) and the material properties of the corpus callosum, hippocampus and cerebral cortex on the macroscopic level, but at identical strain levels. This research investigates the linear viscoelastic properties of the cerebral cortex at known thresholds of axonal injury (0.14 - 0.34 strains [2]). During quasi static loading of tissue in creep tests, instantaneous strains were generated corresponding to axonal thresholds. A linear viscoelastic constitutive model was used to determine six Prony parameters suitable for finite element simulation in ABAQUS and ANSYS. Use of such properties at the levels of axonal damage will help to accurately predict injuries during numerical simulations, to design safety helmets and air bags, and also to refine existing injury criteria and to improve the precision in surgical procedures.
  • Publication
    Experimental Characterisation of Neural Tissue at Collision Speeds
    (International Research Council on the Biomechanics of Injury, 2012) ; ;
    Mechanical characterization of brain tissue at high loading velocities is particularly important for modelling Traumatic Brain Injury (TBI). During severe impact conditions, brain tissue experiences a mixture of compression, tension and shear. Diffuse axonal injury (DAI) occurs in animals and humans when both the strains and strain rates exceed 10% and 10/s, respectively. Knowing the mechanical properties of brain tissue at these strains and strain rates is of particular importance, as they can be used in finite element simulations to predict the occurrence of brain injuries under different impact conditions. In this research, we describe the design and operation of a High Rate Tension Device (HRTD) that has been used for tensile tests on freshly harvested specimens of porcine neural tissue at speeds corresponding to a maximum strain rate of 90/s. We investigate the effects of inhomogeneous deformation of the tissue during tension by quasi‐static tests (strain rate 0.01/s) and dynamic tests (strain rate 90/s) using different thickness specimens (4.0, 7.0, 10.0 and 13.0 mm) of the same diameter (15.0 mm). Based on a combined experimental and computational analysis, brain specimens of aspect ratio (diameter/thickness) S = 10/10 or lower (10/12, 10/13) are considered suitable for minimizing the effects of inhomogeneous deformation during tension tests. The Ogden material parameters were derived from the experimental data both at quasi‐static conditions (µ = 440 Pa and α = ‐4.8 at 0.01/s strain rate) and dynamic conditions (µ = 4238 Pa and α = 2.8 at 90/s strain rate) by performing an inverse finite element analysis to model all experimental data. These material parameters will prove useful for the nonlinear hyperelastic analysis of brain tissue.
  • Publication
    Slight compressibility and sensitivity to changes in Poisson's ratio
    (Wiley Blackwell (John Wiley & Sons), 2011-12-12) ; ; ;
    Finite element simulations of rubbers and biological soft tissue usually assume that the material being deformed is slightly compressible. It is shown here that, in shearing deformations, the corresponding normal stress distribution can exhibit extreme sensitivity to changes in Poisson's ratio. These changes can even lead to a reversal of the usual Poynting effect. Therefore, the usual practice of arbitrarily choosing a value of Poisson's ratio when numerically modelling rubbers and soft tissue will, almost certainly, lead to a significant difference between the simulated and actual normal stresses in a sheared block because of the difference between the assumed and actual value of Poisson's ratio. The worrying conclusion is that simulations based on arbitrarily specifying Poisson's ratio close to 1∕2 cannot accurately predict the normal stress distribution even for the simplest of shearing deformations. It is shown analytically that this sensitivity is caused by the small volume changes, which inevitably acy all deformations of rubber-like materials. To minimise these effects, great care should be exercised to accurately determine Poisson's ratio before simulations begin.
      218Scopus© Citations 24
  • Publication
    A high rate tension device for characterizing brain tissue
    (Sage Publications, 2012-03-08) ; ;
    The mechanical characterization of brain tissue at high loading velocities is vital for understanding and modeling traumatic brain injury. The most severe form of traumatic brain injury is diffuse axonal injury, which involves damage to individual nerve cells (neurons). Diffuse axonal injury in animals and humans occurs at strains >10% and strain rates >10 s−1. The mechanical properties of brain tissues at these strains and strain rates are of particular significance, as they can be used in finite element human head models to accurately predict brain injuries under different impact conditions. Existing conventional tensile testing machines can only achieve maximum loading velocities of 500 mm/min, whereas the Kolsky bar apparatus is more suitable for strain rates >100 s−1. In this study, a custom-designed high rate tension device is developed and calibrated to estimate the mechanical properties of brain tissue in tension at strain rates ≤ 90 s−1, while maintaining a uniform velocity. The range of strain can o be extended to 100% depending on the thickness of a sample. The same apparatus can be used to characterize the dynamic behavior of skin and other soft biological tissues by using appropriately sized load cells with a capacity of 10 N and above.
      290Scopus© Citations 9
  • Publication
    Automated Estimation of Collagen Fibre Dispersion in the Dermis and its Contribution to the Anisotropic Behaviour of Skin
    Collagen fibres play an important role in the mechanical behaviour of many soft tissues. Modelling of such tissues now often incorporates a collagen fibre distribution. However, the availability of accurate structural data has so far lagged behind the progress of anisotropic constitutive modelling. Here, an automated process is developed to identify the orientation of collagen fibres using inexpensive and relatively simple techniques. The method uses established histological techniques and an algorithm implemented in the MATLAB image processing toolbox. It takes an average of 15 s to evaluate one image, compared to several hours if assessed visually. The technique was applied to histological sections of human skin with different Langer line orientations and a definite correlation between the orientation of Langer lines and the preferred orientation of collagen fibres in the dermis (p<0.001,R 2 =0.95) was observed. The structural parameters of the Gasser–Ogden–Holzapfel (GOH) model were all successfully evaluated. The mean dispersion factor for the dermis was κ=0.1404±0.0028. The constitutive parameters μ, k 1 and k 2 were evaluated through physically-based, least squares curve-fitting of experimental test data. The values found for μ, k 1 and k 2 were 0.2014 MPa, 243.6 and 0.1327, respectively. Finally, the above model was implemented in ABAQUS/Standard and a finite element (FE) computation was performed of uniaxial extension tests on human skin. It is expected that the results of this study will assist those wishing to model skin, and that the algorithm described will be of benefit to those who wish to evaluate the collagen dispersion of other soft tissues.
      553Scopus© Citations 131
  • Publication
    Determination of friction coefficient in unconfined compression of brain tissue
    Unconfined compression tests are more convenient to perform on cylindrical samples of brain tissue than tensile tests in order to estimate mechanical properties of the brain tissue because they allow homogeneous deformations. The reliability of these tests depends significantly on the amount of friction generated at the specimen/platen interface. Thus, there is a crucial need to find an approximate value of the friction coefficient in order to predict a possible overestimation of stresses during unconfined compression tests. In this study, a combined experimental–computational approach was adopted to estimate the dynamic friction coefficient μ of porcine brain matter against metal platens in compressive tests. Cylindrical samples of porcine brain tissue were tested up to 30% strain at variable strain rates, both under bonded and lubricated conditions in the same controlled environment. It was established that μ was equal to 0.09±0.03, 0.18±0.04, 0.18±0.04 and 0.20±0.02 at strain rates of 1, 30, 60 and 90/s, respectively. Additional tests were also performed to analyze brain tissue under lubricated and bonded conditions, with and without initial contact of the top platen with the brain tissue, with different specimen aspect ratios and with different lubricants (Phosphate Buffer Saline (PBS), Polytetrafluoroethylene (PTFE) and Silicone). The test conditions (lubricant used, biological tissue, loading velocity) adopted in this study were similar to the studies conducted by other research groups. This study will help to understand the amount of friction generated during unconfined compression of brain tissue for strain rates of up to 90/s.
      290Scopus© Citations 25
  • Publication
    Extreme softness of brain matter in simple shear
    We show that porcine brain matter can be modelled accurately as a very soft rubber-like material using the Mooney–Rivlin strain energy function, up to strains as high as 60%. This result followed from simple shear experiments performed on small rectangular fresh samples (2.5 cm3 and 1.1 cm3) at quasi-static strain rates. They revealed a linear shear stress–shear strain relationship (R2>0.97), characteristic of Mooney–Rivlin materials at large strains. We found that porcine brain matter is about 30 times less resistant to shear forces than a silicone gel. We also verified experimentally that brain matter exhibits the positive Poynting effect of non-linear elasticity, and numerically that the stress and strain fields remain mostly homogeneous throughout the thickness of the samples in simple shear.
      442Scopus© Citations 55
  • Publication
    Third- and fourth-order constants of incompressible soft solids and the acousto-elastic effect
    (Acoustical Society of America, 2010) ; ;
    Acousto-elasticity is concerned with the propagation of small-amplitude waves in deformed solids. Results previously established for the incremental elastodynamics of exact non-linear elasticity are useful for the determination of third- and fourth-order elastic constants, especially in the case of incompressible isotropic soft solids, where the expressions are particularly simple. Specifically, it is simply a matter of expanding the expression for ρv2, where ρ is the mass density and v the wave speed, in terms of the elongation e of a block subject to a uniaxial tension. The analysis shows that in the resulting expression: ρv2=a+be+ce2, say, a depends linearly on μ; b on μ and A; and c on μ, A, and D, the respective second-, third, and fourth-order constants of incompressible elasticity, for bulk shear waves and for surface waves.
      260Scopus© Citations 49
  • Publication
    Temperature effects on brain tissue in compression
    Extensive research has been carried out for at least 50 years to understand the mechanical properties of brain tissue in order to understand the mechanisms of traumatic brain injury (TBI). The observed large variability in experimental results may be due to the inhomogeneous nature of brain tissue and to the broad range of test conditions. However, test temperature is also considered as one of the factors influencing the properties of brain tissue. In this research, the mechanical properties of porcine brain have been investigated at 22 °C (room temperature), and at 37 °C (body temperature) while maintaining a constant preservation temperature of approximately 4–5 °C. Unconfined compression tests were performed at dynamic strain rates of 30 and 50 s−1 using a custom made test apparatus. There was no significant difference (p=0.8559–0.9290) between the average engineering stresses of the brain tissue at the two different temperature conditions. The results of this study should help to understand the behavior of brain tissue at different temperature conditions, particularly in unconfined compression tests.
      448Scopus© Citations 26