NEWS | May 1, 2014

The GelMan Quest: NRL Materials Science Informs Helmet and Armor Design

By Kyra Wiens

GelMan is not the latest comic book superhero—though he has been: shot at, stood near explosions, dropped from towers, and held underwater. Made up of synthetic bones and soft tissue, the GelMan surrogates help U.S. Naval Research Laboratory (NRL) Materials Science and Technology Division scientists understand how helmets and armor protect our real heroes in uniform.

Says Dr. Peter Matic, Superintendent, We have the expertise to adapt the surrogates and analysis techniques to a particular situation or problem our warfighters may face. Using resources and protocols they've been developing since 2000, NRL can take any helmet design-from a football helmet to a military prototype—and in a few weeks come back with an analysis of how that design compares to current standards at protecting the brain.

The combat helmet, says Matic, is supposed to do three things. It's supposed to shield from blunt impact, ballistic impact, and blast pressure loading. Most of NRL's helmet research has focused on minimizing blasts and blunt impacts (like those from improvised explosive devices (IEDs) or forceful hits, respectively), on behalf of the U.S. Marine Corps (USMC).

Says Dr. Amit Bagchi, a scientist in Matic's division, DoD [the Department of Defense] is providing the best possible equipment to the soldiers and warfighters, but then we keep constantly looking for a better solution.

Recently, NRL started placing live cells inside a GelMan brain to see how a blast affects cell function and survival.

It's not simply a question of which design is best, especially given the innumerable threats to which warfighters are exposed. What we're doing, says Matic, is providing some of the building blocks to answer that kind of question.

Pressure in the brain? Experiments with GelMan and the computational model

NRL makes the GelMan brains in-house, even mimicking the folds of a real brain. Using baby powder, which Bagchi refers to under the technical term of mold release agent, the scientists plop out a translucent brain surrogate with pudding-like consistency. They encase the gel brain inside a hard plastic skull, which is mounted on a neck and covered with a helmet. GelMan gets sensors on the brain, skull, and helmet.

From a safe distance, the researchers set off an explosive charge. The charge can be with or without fragments, and is made to imitate an IED a Marine might have encountered in Iraq or Afghanistan.

After the experiment, Matic and Bagchi get a spreadsheet, with time and pressure data from each sensor. This tells them how pressure from the blast dissipates or is passed through the helmet and skull. Ultimately, says Bagchi, the goal is to minimize the energy and the pressure that goes into the brain.

The whole thing is over in a fraction of a second. The most harmful part, says Bagchi, is in approximately the first 25th of a second, or about 40 milliseconds. While a home video camera might shoot 24 or 30 frames per second, for these experiments the researchers capture video at 10,000 frames per second.

NRL also uses sensors to evaluate blunt impacts. Rather than simply whacking GelMan with a hammer, however, the lab has a 23-foot tall drop tower. As Matic describes, You mount the helmet on a metal head form, which rides the rail down and hits an anvil. The helmet then bounces off the anvil, and you can measure the acceleration during that event. A smaller bounce means the helmet is better absorbing the impact and protecting the brain.

To further understand how the blast moves in and around the head, the researchers run simulations with a computational model. Varying levels of pressure are simulated moving in and around regions of the brain, represented by flowing waves of color similar to a weather map showing a regional heat wave.

Dr. David Mott and colleagues in NRL's Laboratories for Computational Physics and Fluid Dynamics have also done advanced simulations of how blasts interact with different helmet geometries. They've shown that waves of pressure enter below the helmet brim and can cause significant amplifications and focal points.

The quest for a better suspension system

With established testing procedures and data, NRL evaluates new concepts and prototypes. Says Matic, We've done studies on the different designs for pads used in the current helmets, and we've looked at different commercial designs that were given to us to evaluate. The pads, or suspension system, separate the helmet from the head and absorb force.

The researchers have tested current pad materials against several prototype concepts, including solid polymers, different types of foam, and a hybrid polymer-foam.

They've also studied what Bagchi refers to as, topology—you know, what the arrangement of the suspension inside the helmet should be. Up to a point, distributing the pressure across more, smaller pads helps; today's advanced combat helmet (ACH), and the enhanced combat helmet (ECH) being fielded in 2014, are lined with seven. Changing thickness or spacing also makes a difference, as does temperature.

Matic believes NRL is the only lab to have evaluated how the pads perform under compression loading (the force of squeezing the top and bottom of a pad toward each other), and under shear loading (the force on moving the top and bottom of a pad sideways, deforming it from a rectangle into a parallelogram): And then you start asking, how much of the helmet is pushing the pads into a shearing response versus a compression response. So it's a subtle difference that does affect how the helmet protects the brain.

But there are no easy answers. We have been able to quantify and rank differences between different suspensions, says Matic, but no design was significantly better than any other design in all cases. No one single suspension configuration performs best under pressure from a front, side, and back blast; and under blunt impact; and when shot at. (Mott's lab has shown similar variation in helmet geometries, depending on the direction of the blast.)

NRL in 2014: linking pressure impacts to brain cell function

Our hypothesis, says Bagchi, is that [mild to moderate traumatic brain injury] happens at the cellular level. Another NRL researcher, Dr. Thomas O'Shaughnessy, has started looking at how blast exposure affects brain cell function.

O'Shaughnessy has developed a novel method for culturing live brain cells (from mice). The cells are encased in packages about the size of a half graham cracker, and neatly fit into a modified GelMan brain mold. Says Bagchi, They're portable and they'll support the cells for up to two weeks, so we don't have to feed anything to them. Matic believes NRL has the most robust cell culture in use for this type of research, and is the only lab to put live cells inside a soft tissue surrogate.

NRL has already exposed the live cells to blast and blunt impacts. Whether or not GelMan is wearing a helmet has resulted in different cell survival rates that are statistically relevant.

As Bagchi says, I think the fundamental unknown is the level of cellular injury that degrades motor functions or other neurological functions. It's unknown, for example, if people with post-traumatic stress disorder have lost or experienced major changes in brain cell activity. The best possible resolution with magnetic resonance imaging (MRI) is still not enough to see what's happening at the cellular level.

But while the medical community looks for a link between cognitive function and cellular function, NRL is working to connect cellular function to blast impacts. As Matic says, Understanding a threshold for those cells, or a dose-response curve, is also very important, and that's something we've done.

Additionally, NRL has been working with the Defense Advanced Research Projects Agency (DARPA) and USMC on a sensor that attaches to the nape of a helmet. If a person were exposed to a blast on deployment, the sensor would log the blast data. With enough data, scientists might be able to see if there's a correlation between blast exposure and cognitive function.

What makes providing the best helmet possible even more complicated is that Marines have other requirements. Weight is a primary factor, as are comfort, ventilation, and durability. Marines need to maintain an auditory awareness of what's going on around them, so extra protection on the ears is an issue. They are also responsible for training and building trust with local people, possibly difficult with bulky face coverings. Then there are considerations about how the helmet integrates with other equipment, like communications or night vision.

It's like putting together a puzzle, says Matic. Our job is to provide some basic understanding of what's going on during these events and how protective equipment works. Then we can contribute to the design and selection of the best protective equipment for our warfighters.