Brain injuries caused by improvised explosive devices are being called the “signature wounds” of the Iraq and Afghanistan wars.
Blasts from roadside bombs are capable of sending shockwaves through the air that penetrate armored vehicles as well as standard headgear.
The effects as the wave ripples through the brain are poorly understood, but the results have a name: mild traumatic brain injury — better known as a concussion.
Scientists are now taking a closer look at exactly what a shockwave does in the milliseconds it takes for it to pass through a helmet, skull and brain.
The University of Nebraska–Lincoln’s college of engineering has built a test bed where these waves can be examined in detail and at their simplest forms. It’s hoped that this basic research can lead to better helmets for war fighters.
“At the end of the day, the actual damage takes place at the neuron level,” Namas Chandra, associate dean of research at the college said in an interview. Neurons are the basic cells that comprise the brain. Shockwaves stretching these cells as they pass through the brain may be the cause of short- and long-term TBI, he said.
The Rand Corp. in a 2008 study estimated the number of war fighters suffering from brain injuries at 360,000. Senior Defense Department medical officials in a March 2009 press conference said that number might be closer to 180,000. However, anywhere from 45,000 to 90,000 of them were categorized as suffering from severe and lasting symptoms.
There is evidence that relatively mild cases of brain trauma may cause problems as well. A January 2008 New England Journal of Medicine report, “Mild Traumatic Brain Injury in U.S. Soldiers Returning from Iraq,” found a strong association between mild TBI and post-traumatic stress disorder. Soldiers who temporarily lost consciousness or reported being dazed and confused after a blast were later significantly more likely to report poor general health, missed workdays and medical visits.
“However, the epidemiology of combat-related mild traumatic brain injury is poorly understood,” the six researchers who co-wrote the article noted.
UNL researchers hope that by studying shockwaves in a controlled laboratory setting, they can better understand exactly how they interact with brain cells. Working on a $3.2 million Army Research Office grant, the college has designed two enclosed shock tubes where they can perform numerous controlled blasts in a laboratory setting at a much lower cost than open-air tests.
The lab isn’t going for realism, Chandra said. The effects of an IED blast in a battle zone are tremendously complex. There are a series of “echo waves” as the energy sent out by the bomb reflects off surfaces such as buildings, vehicles or both. An open-air test will have so many variables, such as the distance from the victim to the bomb, or the shape of a charge, that it is impossible to establish a baseline reading and test equipment such as helmets with consistency, Chandra said.
Fundamental laboratory work is needed to completely understand how the shockwave propagates through the helmet and skull and how it causes brain damage.
“Then we can clearly see what are the possible effects of a combination of shockwaves a soldier experiences,” he said.
Shock tubes have been used to measure blasts since at least World War II, said Aaron Holmberg, an engineering mechanics department graduate student, who helped set up the lab. Most of the previous research focused on the pulmonary effects of explosions on a soldier. The lungs were then seen as the weakest link. “Since the advancement of flak jackets, it is no longer the weakest link; it has become the head,” Holmberg said.
Other tubes place the subject being tested outside an opening. However, even during the few inches the wave travels from the tube’s opening to the subject, it changes “dramatically,” he said.
The lab constructed two enclosed tubes, one six-inch and one nine-inch. The six-inch tube allows researchers to reproduce blasts consistently and at a lower cost. Open-air tests are expensive because of all the extra safety precautions, and can only be carried out at certified blast ranges, which are not always readily available to researchers.
“The shape of the charge also has a big effect on the kind of shockwave, so if they are doing [open-air] tests with different shapes, it’s not exactly the same, and it’s not exactly repeatable for these experiments,” Holmberg said.
And if evaluators can’t repeat a test the same way consistently, they can’t truly measure the effectiveness of equipment such as helmets.
A shockwave has three main components, Holmberg explained.
The amplitude, or strength of the blast; the positive duration or the length of the wave as it rises above the atmospheric pressure; and the negative duration, how long the wave lasts as it falls below atmospheric pressure.
The tubes can be adjusted so researchers can lengthen or shorten the duration of each of these three components, and mimic bombs placed at different distances from the subject.
“We can adjust those three according to what we want,” he said.
The nine-inch tube is actually square, Holmberg said. This innovation allows researchers to place rats or other lab animals in the tube as well as human cadavers, although experiments have not yet progressed to that phase. The square shape also permits them to install a flat window so a high-speed video camera can record what exactly is happening to a helmet or a test subject.
Chandra said: “If I know what causes the injury, then I can relate that injury to the external shockwave, and then I can come up with a better design for a helmet. When I know what I’m protecting, then I can protect it.”
To that end, the lab is working on a dummy head with a cavity filled with sensors that can record the waves as they pass through.
Called, “the redhead” — a nod to the university’s “Big Red” nickname as well as an acronym for realistic, explosion-resistant dummy — it is filled with a jelly-like material that mimics brain matter. Sensors at various points inside the head are placed in the simulated brain to measure the shockwave as it passes through.
Crash test dummy heads used by the automotive industry are similar, but an imperfect fit for testing bomb blasts, Chandra said. A car crash usually involves motion caused by an impact, and researchers in that field want to test how the head moves. A shockwave may not move the head at all.
Since it’s believed that the shockwave stretches neurons as it passes through, the redhead can give researchers more insight into this process, Chandra said.
Researchers from the university’s biology department and school of medicine are assisting the engineers so they can come up with the right simulations of a skull, skin and brain.
“A lot of work has been done in this area,” Chandra said. “We want to combine them all to see if the redhead can give us a measure of what we’re looking for.”