Blast badge: This photonic crystal changes color when exposed to an explosion that generates a shockwave with a certain magnitude and frequency.
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.”