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
    Influence of preservation temperature on the measured mechanical properties of brain tissue
    The large variability in experimentally measured mechanical properties of brain tissue is due to many factors including heterogeneity, anisotropy, age dependence and post-mortem time. Moreover, differences in test protocols also influence these measured properties. This paper shows that the temperature at which porcine brain tissue is stored or preserved prior to testing has a significant effect on the mechanical properties of brain tissue, even when tests are conducted at the same temperatures. Three groups of brain tissue were stored separately for at least 1 h at three different preservation temperatures, i.e., ice cold, room temperature (22 °C) and body temperature (37 °C), prior to them all being tested at room temperature (∼22 °C). Significant differences in the corresponding initial elastic shear modulus μ (Pa) (at various amounts of shear, 0≤K≤1.0) were observed. The initial elastic moduli were 1043±271 Pa, 714±210 Pa and 497±156 Pa (mean±SD) at preservation temperatures of ice cold, 22 °C and 37 °C, respectively. Based on this investigation, it is strongly recommended that brain tissue samples must be preserved at an ice-cold temperature prior to testing in order to minimize the difference between the measured in vitro test results and the in vivo properties. A by-product of the study is that simple shear tests allow for large, almost perfectly homogeneous deformation of brain matter.
      640Scopus© Citations 29
  • Publication
    Third- and fourth-order elasticities of biological soft tissues
    (Acoustical Society of America, 2010) ; ;
    In the theory of weakly nonlinear elasticity,Hamilton et al. [J. Acoust. Soc. Am.116, 41–44 (2004)] identified W=μI2+(A/3)I3+DI22 as the fourth-order expansion of the strain-energy density for incompressible isotropic solids. Subsequently, much effort focused on theoretical and experimental developments linked to this expression in order to inform the modeling of gels and soft biological tissues. However, while many soft tissues can be treated as incompressible, they are not in general isotropic, and their anisotropy is associated with the presence of oriented collagen fiber bundles. Here the expansion of W is carried up to fourth order in the case where there exists one family of parallel fibers in the tissue. The results are then applied to acoustoelasticity, with a view to determining the second- and third-order nonlinear constants by employing small-amplitude transverse waves propagating in a deformed soft tissue.
      230Scopus© Citations 43
  • 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.
      594Scopus© Citations 134
  • 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.
      267Scopus© Citations 50
  • 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.
      453Scopus© Citations 26
  • Publication
    Non-invasive evaluation of skin tension lines with elastic waves
    Background: Since their discovery by Karl Langer in the 19th Century, Skin Tension Lines (STLs) have been used by surgeons to decide the location and orientation of an incision. Although these lines are patient-specific, most surgeons rely on generic maps to determine their orientation. Beyond the imprecise pinch test, there still exists no accepted method for determining the STLs in vivo. Methods: (i) The speed of an elastic motion travelling radially on the skin of canine cadavers was measured with a commercial device called the Reviscometer R . (ii) Similar to the original experiments conducted by Karl Langer, circular excisions were made on the skin and the subsequent geometric changes to the resulting wounds and excised samples were used to determine the orientation of STLs. Results A marked anisotropy in the speed in the elastic wave travelling radially was observed. The orientation of the fastest wave was found to correlate with the orientation of the elongated wound (P < 0.001, R2 = 74%). Similarly, the orientation of fastest wave was the same for both in vivo and excised isolated samples, indicating that the STLs have a structural basis. Resulting wounds expanded by an average area of 9% (+16% along STL and −10% across) while excised skin shrunk by an average of 33% (23% along STL and 10% across). Conclusion: Elastic surface wave propagation has been validated experimentally as a robust method for determining the orientation of STLs nondestructively and non-invasively. This study has implications for the identification of STLs and for the prediction of skin tension levels, both important factors in reconstructive surgeries for both medicine and veterinary medicine.
      432Scopus© Citations 15
  • Publication
    Inhomogeneous deformation of brain tissue during tension tests
    Mechanical characterization of brain tissue has been investigated extensively by various research groups over the past fifty years. These properties are particularly important for modelling Traumatic Brain Injury (TBI) by using finite element human head models to simulate brain injuries under different impact conditions. They are also increasingly important for computer assisted neurosurgery. During severe impact conditions, brain tissue experiences compression, tension and shear; however only limited tests have been performed in tension. Typically, cylindrical specimen are prepared and glued to platens to perform tensile tests which produce an inhomogeneous deformation field near the boundaries, thus contributing to higher magnitudes of stresses. In this research, we present the design and calibration of a High Rate Tension Device (HRTD) capable of performing tests up to a maximum strain rate of 90/s. We use experimental and numerical methods to investigate the effects of inhomogeneous deformation of porcine brain tissue during tension at different specimen thicknesses (4.0 – 14.0 mm), by performing tension tests at a strain rate of 30/s. One-term Ogden material parameters ( = 4395.0 Pa, a = - 2.8) were derived by performing an inverse finite element analysis to model all experimental data. A similar procedure was adopted to determine the Young’s modulus ( E = 11200 Pa) of the linear elastic regime. Based on this 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.
      409Scopus© Citations 35