Formation of back-spattered bloodstain patterns: Air flow visualisation and form of the damage in a brain tissue simulant

Back-spattered bloodstain patterns are often important in investigations of cranial gunshot fatalities, particularly where there is doubt whether the death is suicide or homicide. Back-spatter is the projection of blood and tissue back toward the firearm. Three mechanisms are known to cause back-spatter: the interaction of blood with muzzle gases; a momentum effect known as tail-splash, and the collapse of a temporary cavity which forms around the wound channel [1]. The simulant most widely used at present for human soft tissue is gelatine gel, with 10 and 20 % w/w concentrations of gelatine [2-4]. Recent work has questioned the validity of the use of this material as a brain simulant [5]. Two series of experiments were performed to understand the effect the elastic response of different materials has on the form of the permanent damage and the ejection of air from the temporary cavity. The results indicate that by reducing the elasticity of the material the cross-sectional area of the permanent wound track increases. None of the simulants used in this experiment experienced exactly the same form of damage as bovine brain. Air ejection observed in the gelatine gel was caused by air being drawn into the temporary cavity at the moment of formation with an average velocity of 64.8 m/s, 75.6 m/s and 81 m/s for 10%, 5% and 3% gelatine respectively. The average velocity of the air ejected from the entry hole was 72 m/s for 10% gelatine, 43 m/s for 5 % gelatine. By reducing the elasticity of the material, the velocity of the ejected air reduced. Air ejection was not observed from bovine brain, nor from simulant material M1 [5]. Overall, this study suggested that the ejection of the blood from the head is strongly dependent on the pressure inside the skull. It is hypothesised that the elasticity of the brain is not enough to cause the ejection of the blood backward. However as the bullet penetrates the head, pressure inside the head increases and this pressure has the potential to eject the brain and blood backward towards the firearm.