Several biomedical applications involve the use of the scalp, including skin grafting, skin expansion and head impact biomechanics. Similar to skin, scalp tissue exhibits a non-linear stress-strain relationship, anisotropy and viscoelastic behaviour, all of which should be considered when modelling the scalp. Here we consider the role of the scalp in head impact biomechanics in particular. Being the most external tissue of the head, scalp is the first tissue involved in head impacts and is therefore of great relevance in the field of head protection. Currently, scalp tissue is not realistically represented in either artificial headforms for helmet testing nor finite elements (FE) head models. The aim of this work is to first assess the importance of scalp tissue in impact biomechanics, to characterise its hyperelastic and viscoelastic properties and thereafter to develop a realistic scalp model for head impact simulations.
Impact tests performed with artificial headforms (commonly used for testing helmets) both with and without scalp tissue attached revealed that the presence of scalp reduces the peak linear acceleration (an important parameter for assessing the effectiveness of helmets) by 17-26%, depending on the impact location. The presence of the scalp was also found to significantly affect the kinematics of the impact by changing the boundary conditions. It was found that neither FE models nor artificial headforms represent accurately the complex sliding interaction between the skull, scalp and helmet.
Stress relaxation tests were conducted examining the influence of the orientation of the skin tension lines, storage of the sample, level of extension and strain rates. Interestingly, while the hyperelastic response at low strain rates is anisotropic, our results showed that anisotropy is not evident in the viscous response. The storage method of the sample does not affect the relaxation behaviour, but the level of extension and the strain rate play an important role. A quasi-linear hyperviscoelastic (QLHV) model has been used to fit stress-relaxation data to a different set of parameters for each level of extension and strain rate. Ultimately, a non-linear hyperviscoelastic (NLHV) model is required to accurately represent the scalp tissue. However, for certain applications, a QLHV model with two set of material model parameters; one for low levels of extension (<10%) and one for high level of extension (>15%) will suffice.
This work provides accurate data on the mechanical and sliding properties of the scalp which was previously absent in the literature. This information will help to improve artificial headforms for helmets standards and head numerical models for impact simulations, ultimately leading to more accurate testing and therefore safer helmets.
Marie Sklodowska – Curie grant agreement No. 642662