Experimental and Numerical Evaluation of the Effect of Shock Waves on the Brain

 

Albert King - king@rrb.eng.wayne.edu

King Yang - king.yang@wayne.edu

Feng Zhu - fengzhume@gmail.com

Pam VandeVord - pvord@wayne.edu

Cindy Bir - cbir@wayne.edu

Department of Biomedical Engineering

Wayne State University

Detroit, MI 48202, USA

 

Popular version of paper 2aBB9

Presented Tuesday morning, April 20, 2010

159th Meeting, Baltimore, MD

 

 

This study is part of our continuing study to determine the causes of brain injury due to improvised explosive devices encountered by our troops in combat environments. The injury to the brain is generally mild and the victim may not even be aware of it until he/she begins to have problems with memory, impatience and other behavioral abnormalities. Our current goal is to determine the possible causes of this injury because prevention is not possible without knowing the cause. The plan calls for an experimental study using rodents to relate observed injuries and physiological changes to the magnitude and direction of the pressure wave. We are also using the computer to mimic the blast and to study the effect of a pressure wave on a deformable object that simulates a head so that we can model a rodent head and brain exposed to a shock wave. The eventual goal is to use the computer to develop countermeasures that will protect our troops in the field.

 

We use a shock tube to simulate a blast in the laboratory, a photograph of which is shown in Figure 1. Its two sections are shown in Figure 2. The shorter high pressure chamber on the left is pumped up with air or helium which is released when the pressure exceeds the rupture strength of some plastic (Mylar) sheets placed between the two sections. As the gas travels down the longer section of the tube on the right, it forms a supersonic shock wave that goes over any object placed in it. In our case, we want to study the effect of a shock wave on an egg-shaped plastic object which contains a silicone gel inside to simulate the skull and brain. The idea was to measure the pressure in the gel when the object is exposed to the blast and to compare the computed results with the measured data so that we can validate our computer model. To develop such a model, it was first necessary to determine the characteristics of the shock wave travelling down the shock tube. Using the principles of fluid mechanics and numerical methods, we calculated the magnitude of the pressure as the wave moved down the shock tube. This was verified by comparing the computed pressures with data from pressure sensors located along the tube. The next step was to calculate the effect of the shock wave on the deformable plastic egg. A different numerical technique was used to compute this interaction and the pressure at the center of the egg was computed. It compared favorably with the pressure measured by a sensor placed inside the egg at its center. We then performed some what-if experiments on the computer by reversing the ends of the egg and by changing the orientation of the egg with respect to the direction of the gas flow. We found that the curvature of the leading edge or the shape of the egg facing the blast had a significant effect on the pressures generated inside it.

 

This presentation will also provide some preliminary data on rodent testing in the shock tube. The animal was anesthetized and placed in a holder that was inserted into the shock tube, with the rat facing the shock wave straight on. The strength of the wave was monitored by sensors along the tube and, in certain tests, a pressure sensor was placed inside the brain of the animal. Preliminary results show that there is degeneration of the grey matter cells (neurons) in the midbrain area (hippocampus) after exposure to low levels of blast. It is not clear if this was a direct injury to the cells or if injury to the supporting cells in the brain resulted in neuronal degeneration. In any case, the damage is apparently not in the white matter of the brain (axons) which is normally injured in a blunt head impact experienced in a car crash or in a sporting accident. We hope to develop a computer model of the rat head and brain and to validate it against the data being collected now. There are many advantages to the use of computer models. Once validated, they can be used to study a variety of blast conditions without having to sacrifice more animals. We can expose the simulated animal to blasts that cannot be achieved by testing with our shock tube and determine the pressures generated in the brain. However, the model cannot, as yet, identify what cells will be injured by a blast of a given level.

 

 

Figure 1. Photograph of a 12-inch diameter shock tube

 

 

Figure 2. Diagrammatic representation of the shock tube