Development of an Anatomical Head Model, using Simulant Materials, to measure Traumatic Brain Injury

Abstract

Head injury biomechanics is a complex area involving a combination of physical and biological principles. Blunt force head injury mechanisms remain poorly understood, especially repetitive short duration high impacts which may occur in both concussion and subconcussion. To understand forces and injury patterns involved with head injury and subsequent brain injury, head impacts must be simulated in such a way to accurately represent associated biodynamic responses of the scalp, skull and brain in vivo.

The phenomena of concussion and subconcussion, as well as the structural and mechanical properties of various head layers are outlined in the literature review of this thesis. It highlights that although various materials had previously been investigated, there is a lack of information regarding the mechanical properties of a suitable scalp, skull and brain simulant for blunt force impact experimental head models. To date, less than a handful of studies had reported the use of an anatomical model, which had limitations in the use of simulant materials. Therefore, this thesis attempted to find more suitable simulant materials for various head layers, as well as develop a more anatomically correct head model.

The study on skin/scalp simulant materials showed that dental silicones are a good alternative for studies that require a skin simulant. In this instance, the medium bodied polyvinyl siloxane presented with adequate tear strength and hardness, thus chosen as the scalp simulant for the skin/skull/brain model. For the skull simulant, it was found that epoxy resin had an elastic modulus and flexural strength closest to that of the mean human skull values reported in the literature, with impact forces not exceeding the fracture stress. Consequently, it was used for the skin/skull/brain model. The search for a suitable brain simulant identified agar/glycerol/water conditioned to 22°C to be the most suitable material for high-speed imaging while measuring its elastic behaviour at ballistic strain rates. Although it had slightly higher apparent elastic moduli in the lower strain rate range it was chosen as the brain simulant material in the skin/skull/brain model.

To validate the use of the simulant materials (except for the brain) and an electronic data capture system (piezoelectric sensors) a basic spherical head model was developed. While the simulant materials performed in a satisfactory manner and were deemed suitable materials to be incorporated in an anatomical skin/skull/brain model, the use of piezoelectric sensors failed to accurately measure force transfer through the various head layers and therefore were not suitable.

In order to quantify the forces involved, in particular the degradation of these forces upon impact through the various head layers, accelerometers were embedded in the various simulant layers and used in the development of an anatomical skin/skull/brain model. This study identified that short duration, high intensity forces to the top of the head transfer through the various layers and undergo varying amounts of displacement. This suggests that the brain is subject to potentially traumatic forces upon repetitive short duration blunt force impact.

The novel research described in this thesis identified the suitability of various simulant materials for the use in an anatomically correct experimental skin/skull/brain model, as well as suitable electronic data capture system to measure force degradation and displacement highlighting the forces the brain is subjected to upon repetitive blunt force impacts.