Poynting effect of brain matter in torsion

We investigate experimentally and model theoretically the mechanical behaviour of brain matter in torsion. Using a strain-controlled rheometer we perform torsion tests on fresh porcine brain samples. We quantify the torque and the normal force required to twist a cylindrical sample at constant twist rate. Data fitting gives a mean value for the shear modulus {\mu} = 900 $\pm$ 312 Pa and for the second Mooney-Rivlin parameter $c_2$ = 297 $\pm$ 189 Pa, indicative of extreme softness. Our results show that brain always displays a positive Poynting effect; in other words, it expands in the direction perpendicular to the plane of twisting. We validate the experiments with Finite Element simulations and show that when a human head experiences a twisting motion in the horizontal plane, the brain can experience large forces in the axial direction.Comment: 6 pages, 6 figures, 2 table

Tension lines of the skin

Skin tension lines are natural lines of tension that occur within the skin as a result of growth and remodeling mechanisms. Researchers have been aware of their existence and their surgical implications for over 150 years. Research in the twentieth century showed clearly, through destructive mechanical testing, that the orientation of skin tension lines greatly affects the mechanical response of skin in situ. More recent work has determined that this anisotropic response is, at least in part, due to the structural arrangement of collagen fibres within the dermis. This observation can be incorporated into mathematical and mechanical models using the popular Gasser-Ogden-Holzapfel constitutive equation. Advances in non-invasive measurement techniques for the skin, such as those based on elastic wave propagation, have enabled patient-specific identification of skin tension lines in an accurate and rapid manner. Using this technique on humans, we show that there is considerable variation in the level of anisotropy as the skin ages. Furthermore, we identify that both the structural arrangement of fibres and the in vivo levels of pre-strain play a significant role in the anisotropic behavior of skin.Peer reviewed2021-05-2

Mechanical characterisation of human and porcine scalp tissue at dynamic strain rates

Several biomedical applications require knowledge of the behaviour of the scalp, including skin grafting, skin expansion and head impact biomechanics. Scalp tissue exhibits a non-linear stress-strain relationship, anisotropy and its mechanical properties depend on strain rate. When modelling the behaviour of the scalp, all these factors should be considered in order to perform realistic simulations. Here, tensile tests at strain rates between 0.005 and 100 s−1 have been conducted on porcine and human scalp in order to investigate the non-linearity, anisotropy, and strain rate dependence of the scalp mechanical properties. The effect of the orientation of the sample with respect to the Skin Tension Lines (STLs) was considered during the test. The results showed that anisotropy is evident in the hyperelastic response at low strain rates (0.005 s−1) but not at higher strain rates (15-100 s−1). The mechanical properties of porcine scalp differ from human scalp. In particular, the elastic modulus and the Ultimate Tensile Strength (UTS) of the porcine scalp were found to be almost twice the values of the human scalp, whereas the stretch at failure was not found to be significantly different. An anisotropic hyperelastic model (Gasser-Ogden-Holzapfel) was used to model the quasi-static behaviour of the tissue, whereas three different isotropic hyperelastic models (Fung, Gent and Ogden) were used to model the behaviour of scalp tissue at higher strain rates. The experimental results outlined here have important implications for those wishing to model the mechanical behaviour of scalp tissue both under quasi-static and dynamic loading conditions.European Commission Horizon 202

Mechanical characterisation of human and porcine scalp tissue at dynamic strain rates

Several biomedical applications require knowledge of the behaviour of the scalp, including skin grafting, skin expansion and head impact biomechanics. Scalp tissue exhibits a non-linear stress-strain relationship, anisotropy and its mechanical properties depend on strain rate. When modelling the behaviour of the scalp, all these factors should be considered in order to perform realistic simulations. Here, tensile tests at strain rates between 0.005 and 100 s−1 have been conducted on porcine and human scalp in order to investigate the non-linearity, anisotropy, and strain rate dependence of the scalp mechanical properties. The effect of the orientation of the sample with respect to the Skin Tension Lines (STLs) was considered during the test. The results showed that anisotropy is evident in the hyperelastic response at low strain rates (0.005 s−1) but not at higher strain rates (15-100 s−1). The mechanical properties of porcine scalp differ from human scalp. In particular, the elastic modulus and the Ultimate Tensile Strength (UTS) of the porcine scalp were found to be almost twice the values of the human scalp, whereas the stretch at failure was not found to be significantly different. An anisotropic hyperelastic model (Gasser-Ogden-Holzapfel) was used to model the quasi-static behaviour of the tissue, whereas three different isotropic hyperelastic models (Fung, Gent and Ogden) were used to model the behaviour of scalp tissue at higher strain rates. The experimental results outlined here have important implications for those wishing to model the mechanical behaviour of scalp tissue both under quasi-static and dynamic loading conditions.European Commission Horizon 202

Manufacturing custom-contoured wheelchair seating: A state-of-the-art review

Background: Custom-contoured wheelchair seating lowers risk of pressure injury and postural deterioration while custom-contoured wheelchair seating lowers risk of pressure injury and postural deterioration while increasing the stability and functional activity of the wheelchair occupant. Producing custom-contoured seating systems has historically been a labour-intensive process custom-contoured seating systems is historically labour-intensive. Objectives: Evaluate the strengths and limitations of current manufacturing processes for custom-contoured wheelchair seating to suggest potential future manufacturing processes. Study Design: Literature review of the state of the art. Methods: Research conducted through a literature review focused on the performance of different types of custom-contoured wheelchair seating products and processes over the last 40 years. Recent literature in orthotics and prosthetics was also consulted to assess future trends in seating. Results: There are five main manufacturing processes currently used to produce custom-contoured wheelchair seating systems. No single process is yet suitable for all wheelchair users, but many manufacturers are transitioning to computer numerical controlled (CNC) milling to reduce manual labour. Adjustable micro-modular seating and moulded seat insert manufacturing are also prevalent and offer alternative seating to soft foam carving. Conclusion: There is a need in the custom wheelchair seating sector for processes that are fast, cost-effective, produce little to no material waste, and that can effectively maintain a comfortable seating micro-climate. Additive manufacturing may meet these criteria, but further evaluation is required. Clinical relevance: This review suggests that the custom-contoured wheelchair seating manufacturers are moving away from labour-intensive processes towards digital techniques, like CNC foam milling. Additive manufacturing is a potential new process that may reduce overall costs, the lead time in preparing seats and has the potential to better manage the seating micro-climate.Irish Research CouncilEnable Irelan

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 remains no accepted method for determining 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 geometric changes to the resulting wounds and excised samples were used to determine the orientation of STLs.Results: A marked anisotropy in the speed of the elastic wave travelling radially was observed. The orientation of the fastest wave was found to correlate with the orientation of the elongated wound (Ppeer-reviewed2018-11-2

Characterising the anisotropic mechanical properties of excised human skin

The mechanical properties of skin are important for a number of applications including surgery, dermatology, impact biomechanics and forensic science. In this study, we have investigated the influence of location and orientation on the deformation characteristics of 56 samples of excised human skin. Uniaxial tensile tests were carried out at a strain rate of 0.012 s(-1) on skin from the back. Digital Image Correlation was used for 2D strain measurement and a histological examination of the dermis was also performed. The mean ultimate tensile strength (UTS) was 21.6 +/- 8.4 MPa, the mean failure strain 54% +/- 17%, the mean initial slope 1.18 +/- 0.88 MPa, the mean elastic modulus 83.3 +/- 34.9 MPa and the mean strain energy was 3.6 +/- 1.6 MJ/m(3). A multivariate analysis of variance has shown that these mechanical properties of skin are dependent upon the orientation of the Langer lines (P<0.0001- P=0.046). The location of specimens on the back was also found to have a significant effect on the UTS (P =0.0002), the elastic modulus (P=0.001) and the strain energy (P=0.0052). The histological investigation concluded that there is a definite correlation between the orientation of Langer Lines and the preferred orientation of collagen fibres in the dermis (P<0.001). The data obtained in this study will provide essential information for those wishing to model the skin using a structural constitutive model.Marie Curie Fellowshi

A combined experimental and numerical study of stab-penetration forces

The magnitude of force used in a stabbing incident can be difficult to quantify, although the estimate given by forensic pathologists is often seen as 'critical' evidence in medico-legal situations. The main objective of this study is to develop a quantitative measure of the force associated with a knife stabbing biological tissue, using a combined experimental and numerical technique. A series of stab-penetration tests were performed to quantify the force required for a blade to penetrate skin at various speeds and using different 'sharp' instruments. A computational model of blade penetration was developed using ABAQUS/EXPLICIT, a non-linear finite element analysis (FEA) commercial package. This model, which incorporated element deletion along with a suitable failure criterion, is capable of systematically quantifying the effect of the many variables affecting a stab event. This quantitative data could, in time, lead to the development of a predictive model that could help indicate the level of force used in a particular stabbing incident. (C) 2013 Elsevier Ireland Ltd. All rights reserved.peer-reviewe

Deficiencies in numerical models of anisotropic nonlinearly elastic materials

Incompressible nonlinearly hyperelastic materials are rarely simulated in finite element numerical experiments as being perfectly incompressible because of the numerical difficulties associated with globally satisfying this constraint. Most commercial finite element packages therefore assume that the material is slightly compressible. It is then further assumed that the corresponding strain-energy function can be decomposed additively into volumetric and deviatoric parts. We show that this decomposition is not physically realistic, especially for anisotropic materials, which are of particular interest for simulating the mechanical response of biological soft tissue. The most striking illustration of the shortcoming is that with this decomposition, an anisotropic cube under hydrostatic tension deforms into another cube instead of a hexahedron with non-parallel faces. Furthermore, commercial numerical codes require the specification of a \u27compressibility parameter\u27 (or \u27penalty factor\u27), which arises naturally from the flawed additive decomposition of the strain-energy function. This parameter is often linked to a \u27bulk modulus\u27, although this notion makes no sense for anisotropic solids; we show that it is essentially an arbitrary parameter and that infinitesimal changes to it result in significant changes in the predicted stress response. This is illustrated with numerical simulations for biaxial tension experiments of arteries, where the magnitude of the stress response is found to change by several orders of magnitude when infinitesimal changes in \u27Poisson\u27s ratio\u27 close to the perfect incompressibility limit of 1/2 are made

A combined experimental and numerical study of stab-penetration forces

The magnitude of force used in a stabbing incident can be difficult to quantify, although the estimate given by forensic pathologists is often seen as 'critical' evidence in medico-legal situations. The main objective of this study is to develop a quantitative measure of the force associated with a knife stabbing biological tissue, using a combined experimental and numerical technique. A series of stab-penetration tests were performed to quantify the force required for a blade to penetrate skin at various speeds and using different 'sharp' instruments. A computational model of blade penetration was developed using ABAQUS/EXPLICIT, a non-linear finite element analysis (FEA) commercial package. This model, which incorporated element deletion along with a suitable failure criterion, is capable of systematically quantifying the effect of the many variables affecting a stab event. This quantitative data could, in time, lead to the development of a predictive model that could help indicate the level of force used in a particular stabbing incident.Other RMSIDd: 383140886Deposited by bulk impor