Analysis of In Vivo Skin Anisotropy Using Elastic Wave Measurements and Bayesian Modelling.

In vivo skin exhibits viscoelastic, hyper-elastic and non-linear characteristics. It is under a constant state of non-equibiaxial tension in its natural configuration and is reinforced with oriented collagen fibers, which gives rise to anisotropic behaviour. Understanding the complex mechanical behaviour of skin has relevance across many sectors including pharmaceuticals, cosmetics and surgery. However, there is a dearth of quality data characterizing the anisotropy of human skin in vivo. The data available in the literature is usually confined to limited population groups and/or limited angular resolution. Here, we used the speed of elastic waves travelling through the skin to obtain measurements from 78 volunteers ranging in age from 3 to 93 years old. Using a Bayesian framework allowed us to analyse the effect that age, gender and level of skin tension have on the skin anisotropy and stiffness. First, we propose a new measurement of anisotropy based on the eccentricity of angular data and conclude that it is a more robust measurement when compared to the classic "anisotropic ratio". Our analysis then concluded that in vivo skin anisotropy increases logarithmically with age, while the skin stiffness increases linearly along the direction of Langer Lines. We also concluded that the gender does not significantly affect the level of skin anisotropy, but it does affect the overall stiffness, with males having stiffer skin on average. Finally, we found that the level of skin tension significantly affects both the anisotropy and stiffness measurements employed here. This indicates that elastic wave measurements may have promising applications in the determination of in vivo skin tension. In contrast to earlier studies, these results represent a comprehensive assessment of the variation of skin anisotropy with age and gender using a sizeable dataset and robust modern statistical analysis. This data has implications for the planning of surgical procedures and questions the adoption of universal cosmetic surgery practices for very young or elderly patients.

Ranking and Rating Bicycle Helmet Safety Performance in Oblique Impacts Using Eight Different Brain Injury Models.

Bicycle helmets are shown to offer protection against head injuries. Rating methods and test standards are used to evaluate different helmet designs and safety performance. Both strain-based injury criteria obtained from finite element brain injury models and metrics derived from global kinematic responses can be used to evaluate helmet safety performance. Little is known about how different injury models or injury metrics would rank and rate different helmets. The objective of this study was to determine how eight brain models and eight metrics based on global kinematics rank and rate a large number of bicycle helmets (n=17) subjected to oblique impacts. The results showed that the ranking and rating are influenced by the choice of model and metric. Kendall's tau varied between 0.50 and 0.95 when the ranking was based on maximum principal strain from brain models. One specific helmet was rated as 2-star when using one brain model but as 4-star by another model. This could cause confusion for consumers rather than inform them of the relative safety performance of a helmet. Therefore, we suggest that the biomechanics community should create a norm or recommendation for future ranking and rating methods.

The effect of the scalp on the effectiveness of bicycle helmets' anti-rotational acceleration technologies.

OBJECTIVE: Medical data has lead to the common understanding that bicycle helmets need to be improved to better protect against brain injuries resulting from rotational acceleration. Although many different technologies exist for reducing rotational acceleration during impacts, the lack of an official testing standard means that their evaluation is based on customized set-ups that may differ and not represent real accident conditions. Previously, the authors have shown that scalp tissue plays an important role during helmet testing by absorbing energy and creating a low friction interface between head and helmet, thus reducing rotational accelerations and velocities. However, no published study has yet examined the effectiveness of anti-rotational helmet technologies in the presence of a biofidelic scalp layer. The objective of this study is to address this gap. METHODS: Three different commercially available helmet models, each one equipped with a different technology, were tested in the presence of scalp tissue, in two different scenarios; with and without the technology present. The effectiveness of each of these technologies is already documented in other studies, but only in the absence of a biofidelic scalp layer. Tests were carried out using HIII headform with porcine scalp attached to the outmost layer. Motion tracking was used to compare the impact kinematics of each helmet model in both scenarios. RESULTS: Results showed that when a biofidelic scalp layer is present, there is no statistical difference between helmet models with and without the anti-rotational technology in terms of rotational acceleration, velocity, relative rotation, impact duration and injury risk. CONCLUSIONS: Results suggest that the presence of the scalp can obscure the functionality of anti-rotational acceleration technologies. This could indicate that the effectiveness of technologies tested in previous studies, which have not tested anti-rotational acceleration technologies in the presence of a realistic scalp layer, may exaggerate the contribution of such technologies if compared with a more biofidelic set-up. The study supports the fact that headforms should be better designed by incorporating artificial skin layers that can better imitate scalp's behavior and, in addition, provides insights for the design of technologies against rotational acceleration.

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.

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 of µ = 900 ± 312 Pa and for the second Mooney-Rivlin parameter of c2 = 297 ± 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.

Head-to-nerve analysis of electromechanical impairments of diffuse axonal injury.

The aim was to investigate mechanical and functional failure of diffuse axonal injury (DAI) in nerve bundles following frontal head impacts, by finite element simulations. Anatomical changes following traumatic brain injury are simulated at the macroscale by using a 3D head model. Frontal head impacts at speeds of 2.5-7.5 m/s induce mild-to-moderate DAI in the white matter of the brain. Investigation of the changes in induced electromechanical responses at the cellular level is carried out in two scaled nerve bundle models, one with myelinated nerve fibres, the other with unmyelinated nerve fibres. DAI occurrence is simulated by using a real-time fully coupled electromechanical framework, which combines a modulated threshold for spiking activation and independent alteration of the electrical properties for each three-layer fibre in the nerve bundle models. The magnitudes of simulated strains in the white matter of the brain model are used to determine the displacement boundary conditions in elongation simulations using the 3D nerve bundle models. At high impact speed, mechanical failure occurs at lower strain values in large unmyelinated bundles than in myelinated bundles or small unmyelinated bundles; signal propagation continues in large myelinated bundles during and after loading, although there is a large shift in baseline voltage during loading; a linear relationship is observed between the generated plastic strain in the nerve bundle models and the impact speed and nominal strains of the head model. The myelin layer protects the fibre from mechanical damage, preserving its functionalities.

Evaluation of the head-helmet sliding properties in an impact test.

The scalp plays a crucial role in head impact biomechanics, being the first tissue involved in the impact and providing a sliding interface between the impactor and/or helmet and the skull. It is important to understand both the scalp-skull and the scalp-helmet sliding in order to determine the head response due to an impact. However, experimental data on the sliding properties of the scalp is lacking. The aim of this work was to identify the sliding properties of the scalp using cadaver heads, in terms of scalp-skull and scalp-liner (internal liner of the helmet) friction and to compare these values with that of widely used artificial headforms (HIII and magnesium EN960). The effect of the hair, the direction of sliding, the speed of the test and the normal load were considered. The experiments revealed that the sliding behaviour of the scalp under impact loading is characterised by three main phases: (1) the low friction sliding of the scalp over the skull (scalp-skull friction), (2) the tensioning effect of the scalp and (3) the sliding of the liner fabric over the scalp (scalp-liner friction). Results showed that the scalp-skull coefficient of friction (COF) is very low (0.06 ±â€¯0.048), whereas the scalp-liner COF is 0.29 ±â€¯0.07. The scalp-liner COF is statistically different from the value of the HIII-liner (0.75 ±â€¯0.06) and the magnesium EN960-liner (0.16 ±â€¯0.026). These data will lead to the improvement of current headforms for head impact standard tests, ultimately leading to more realistic head impact simulations and the optimization of helmet designs.

The Importance of the Scalp in Head Impact Kinematics.

The best way to reduce the risk of head injury (up to 69% reduction) is to wear a helmet. In recent years, the improvement of helmet standard tests focused on reproducing realistic impact conditions and including the effect of rotational acceleration. However, less importance has been given to the development of a realistic headform. The goal of this work was to evaluate the role of scalp tissue in head impact kinematics; both with respect to its mechanical properties and with respect to its sliding properties. An EN960 and HIII headform were subjected to linear and oblique impacts, respectively, both with and without porcine scalp attached. Different speeds, impact locations and impact surfaces were tested. Standard linear drop tests (EN960) showed that the scalp reduced the impact energy by up to 68.7% (rear impact). Oblique head impact tests showed how the headform-anvil friction coefficient changes when the HIII is covered with scalp, affecting linear and rotational accelerations. Therefore, the scalp plays an important role in head impacts and it should be realistically represented in headforms used for impact tests and in numerical models of the human head.

A Computational Approach for the Prediction of Fatigue Behaviour in Peripheral Stents: Application to a Clinical Case.

Nickel-Titanium (NiTi) peripheral stents are commonly used for the treatment of diseased femoropopliteal arteries (FPA). However, cyclic deformations of the vessel, induced by limb movements affect device performance and fatigue failure may occur. Stent strut fracture has been described in the literature, and is implicated as a potential causative factor in vessel re-occlusion. In this paper, a numerical approach is proposed to predict the fatigue behaviour of peripheral NiTi stents within patient-specific arterial geometries, as additional information to aid clinician intervention planning. The procedure needs some patient-specific vessel features derived from routine clinical images but, when this information is not available, reference data from the literature may be used, obviously increasing the uncertainties of the results. In addition, specific stent material data are required and can be obtained from experimental tests. Several 3D finite element models resembling stented vessel segments are built and used for fatigue analyses. For each model, axial cyclic boundary conditions are obtained from a patient-specific lumped parameter model representing the entire artery as a series of suitable springs. This allows the simplification of stiffness changes along the vessel due to plaque and stent that affect local axial deformations. Imposed local cyclic bending values depend on the stent location along the FPA. The procedure is exemplified by its application to an actual clinical case that showed two strut fractures at 18 months follow-up. Interestingly, despite the lack of some of patient-specific information and the use of data from the literature to inform the model, the numerical approach was able to interpret the in vivo fractures.