Research Finds That Blood Vessels Help IEDs Inflict More Damage
By Annette M. Boyle
LINCOLN, NE — With blast-induced traumatic brain injuries (TBIs) the signature injury of the conflicts in Iraq and Afghanistan, research and intense military focus on improvised explosive devices (IEDs) have continued even as the military engagements have wound down.
The Joint Improvised Explosive Device Defeat Organization (JIEDDO), established in 2006 as a wartime activity to combat IEDs, has been transitioned into a defense agency under the undersecretary of defense for acquisition, technology and logistics. Now called the Joint Improvised-Threat Defeat Agency(JIDA), the organization continue efforts to defeat improvised threats globally, and to help the U.S. military better counter the threats.
Research also continues on how to better protect troops against the devastating effects of the improvised devices. Recently, engineers designing a more protective helmet discovered that the shock from a roadside blast explains only part of the damage to the brain. A previously unrecognized factor comes from a structure within the brain itself — its network of blood vessels.
At least 300,000 servicemembers have suffered TBIs since 2000, and the number may be as high as 460,000, if subconcussive injuries are included, according to DoD statistics.1
To examine how blood vessels affect TBI, a team of researchers at the University of Nebraska-Lincoln’s Trauma Mechanics Research Initiative created a “shock tube” that sent 900 mile per hour blasts at two simplified models of a human head, one with blood vessels and one without.
“Originally, we hypothesized that the cerebral vasculature would strengthen the brain by sharing the external loading. The blood vessels are much stiffer than the brain,” said Linxia Gu, PhD, associate professor of mechanical and materials engineering at the University of Nebraska-Lincoln. The engineers expected the vessels would act like the mesh of steel wires in reinforced concrete to protect the surrounding tissue in the brain.
While that assumption may hold true for lower-frequency blast loading, in high-frequency situations such as improvised explosive device blasts, the blood vessels appear to worsen the strain, Gu said.
In a study published in Computational and Mathematical Models in Medicine, the model with blood vessels suffered three times as much principal strain and six times the shear strain in the brain stem and twice as much principal strain and 250% the shear strain in the corpus callosum as the model without blood vessels. Principal strain measures the maximum compression at a specific point, while shear strain captures the angular shift in the shape of an object.2
“Blood vessels could be 100 times stiffer than the brain tissue. This material mismatch induced stress concentrations at the interface,” explained Gu, who was the lead author of the study.
The denser the vasculature, the greater the brain injury in the Nebraska model. “If the vasculature density is doubled, the peak maximum principal strain in the corpus callosum and brainstem could be increased by 98.19% and 176.38%, respectively,” Gu told U.S. Medicine. Maximum principal strain indicates the extent of diffuse axonal injuries and mechanical injuries to the blood-brain barrier, she noted.
The authors noted that the vessel diameter also affected the maximum principal and shear strain measures, with both measures increasing with blood vessel size. Still, the researchers expected more variety in blood vessel size to refine but support their findings. “The fine vascular network with varied diameters might alter the magnitude of brain responses, but the observations on the role of blood vessel networks in brain dynamics will be the same,” they wrote.
The higher strains in the regions of denser vasculature shown in the study are consistent with the “clinical observation that the axonal bulbs were located near the blood vessels for the patients who suffered blast-induced TBI,” noted by other researchers, wrote the authors.
With the new information about the role the network of blood vessels play in shock-related brain damage, the engineering team can design a more protective helmet. Further research that uses a human head model with brain components, layers of brain tissue and a more realistic distribution of blood vessels networks would likely change the peak stress and strain experienced in the brain following a blast.
Gu and her colleagues are looking at such a model now, using a human head model that was developed for traumatic brain research from MRI and microCT images.
But the published research already has given Gu some ideas. “No helmet can protect equally against every type of force. Current helmets protect well against bullets and shrapnel, but they don’t protect as well against blasts,” she noted. To build a helmet that provides better blast protection, Gu suggested that a foam pad filling between the helmet and the head or a carbon fiber “sandwich” sheet built into the helmet.
Gu’s previous research found that gaps between the head and helmet increase overpressure and impulse on the back of the head, with the intensity increasing as gap size narrows. Tight foam pads between the head and helmet can eliminate these and prevent any pressurization within the head-helmet gap.3
Another study had found that the carbon fiber sandwich panels were especially beneficial in low intensity blasts and that great core thicknesses and larger facesheets provided more blast resistance, although at the cost of greater weight and size.4
Research on IEDs remains a high priority for the U.S. military, which has worked diligently to combat the effects of the damaging devices. In July, Defense Secretary Ash Carter presented the Joint Meritorious Unit Award to JIDA at the Pentagon.
“You are founders and pioneers in the things that you’ve done for the department,” Carter said. “You made rapid acquisition what it is today. We recognize your contributions and applaud the great things you’ve done to make a difference against IEDs.”
Flanked by members of the unit, Carter presented the award to JIDA’s director, Lt. Gen. John D. Johnson. The award recognizes JIEDDO for its efforts in mitigating the IED-threat during 2006-2012 in support of Operations Enduring Freedom, Iraqi Freedom and New Dawn. The citation said the efforts of the organization have directly assisted U.S. and coalition forces.
11 Taber KH, Hurley RA, Haswell CC, Rowland JA, Hurt SD, Lamar CD, Morey RA. White Matter Compromise in Veterans Exposed to Primary Blast Forces. J Head Trauma Rehabil. 2014 Mar 4. [Epub ahead of print]
2 Hua Y, Lin S, Gu LX. Relevance of Blood Vessel Networks in Blast-Induced Traumatic Brain Injury. Computational and Mathematical Methods in Medicine. April 2015.
3 Ganpule S, Gu, L., et al. Role of Helmet in the Mechanics of Shock Wave Propagation under Blast Loading Conditions. Computer Methods in Biomechanics and Biomedical Engineering. 2012;15(11):1233-1244.
4 Hua Y, Akula PK, and Gu, LX. 2014. Experimental and numerical investigation of carbon fiber sandwich panels subjected to blast loading, Composites Part B-Engineering. 2014;56:456-463.