Over the last 10 years, thousands of troops have returned from Iraq and Afghanistan with traumatic brain injuries triggered by blasts from improvised explosive devices. Growing evidence suggests that the shockwaves produced by these explosions lead to injuries that are different from concussions suffered in car accidents and football games—and that even seemingly minor blasts, from which a soldier might walk away apparently unharmed, could damage the brain, especially with repeated exposure.
A new device being developed by researchers at the University of Pennsylvania School of Medicine could provide a simple way to measure the magnitude of explosions to which a soldier is exposed over time. It could also help scientists better understand the threshold for brain injury.
“Soldiers [with mild traumatic brain injury] can often appear normal, so it’s critically important to have some kind of objective measure to denote which soldiers have been exposed to a blast that is powerful enough to cause brain injury,” says Kacy Cullen, assistant professor of neurosurgery at Penn and leader of the study. “These devices wouldn’t diagnose brain injury, but they would indicate who needs a more thorough workup, and could influence decisions about when a soldier can return to action.”
The powerful blasts triggered by improvised explosive devices generate a supersonic wave followed by another shock wave called an overpressure wave. These forces are often strong enough to throw someone in the air, triggering the kind of blunt impact one might experience in a car accident. But many scientists believe that the waves themselves, in addition to the impact, can damage the brain.
The military has amped up efforts to measure the specific properties of explosions using helmet-mounted accelerometers and pressure sensors, but these devices have drawbacks. “They can be expensive, cumbersome, and require power to operate,” says Cullen. “Ours is a materials-based indicator, so you don’t need an internal power supply; the power from the blast induces the color change.”
As with a butterfly’s wing, the color of the material used in the detector is determined by its structure rather than chemical composition or pigment. It contains photonic crystals made up of layers of pores separated by columns a few hundred nanometers in width—the size of the pores and columns and how they are arranged within the structure determines the color of the sensor. When exposed to a shockwave, the columns collapse, either changing the color of the material or making it lose color altogether.
The three-dimensional crystal is made with multiple laser beams that carve precise shapes into a photosensitive plastic sheet, using holographic lithography technology developed by Shu Yang, a professor in the department of materials science and engineering at Penn. The result is a material that is mechanically strong but lightweight. By varying the chemistry and composition of the materials, the three-dimensional photonic crystals can be made very resistant to extreme heat or cold and wet or dry conditions. “You can hit it with a hammer, and it won’t change color,” says Smith. “It will only break at the type of very-high-frequency shockwave you might get in a blast.”
One of the benefits of the color-changing approach is that it would immediately alert soldiers and field medics when an individual has been exposed to a blast level that could cause injury.
Researchers can change the nanoscale structure or components of the material in order to alter the specific frequency and magnitude of the supersonic blast wave required to make the badge change color, allowing researchers to tune the badge to respond to different kinds of shock. “We can also make material so it will fail in response to repeated exposure,” says Douglas Smith, director of the Center for Brain Injury and Repair at Penn, who was senior author on the study. “We might create a device with multiple components that can detect both a single exposure and cumulative exposure like with a radioactivity badge.”
Part of the next phase of the work will be determining what the threshold should be. Researchers are studying animals exposed to explosions, searching for the minimal levels capable of inducing brain damage. Smith aims to begin field testing of the devices within the next two years. The researchers say that because the manufacturing process is similar to that used to make electronics, it should be easy to scale up.
Smith notes that the brain may respond to blasts similar to the way the nanocrystals do. “Skeletal structures within cells, particularly axons [the thin fibers than connect neurons], may be very vulnerable to the high frequency rate of the blast wave,” he says. Much like silly putty can stretch to great lengths if pulled slowly, but can snap in half if pulled apart very quickly, axons become stiffer when deformed rapidly. “With blast exposure, the rapid vibration might rattle apart the structures that make up your cells, so they go from behaving like Jell-O to almost like glass,” he says. “Even a small amount of stretching could break parts of them.”
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