Trouble With Bubbles

Water jets produced by the collapse of micro bubbles in the liquid areas of the brain may be responsible for some of the damage from blast-related TBIs

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By Melinda Mahaffey Icden

 
 

This article was adapted from a story first published by UT Arlington’s Inquiry Magazine.


According to the Centers for Disease Control and Prevention, traumatic brain injury (TBI) leads to more than 2 million visits to the emergency room each year. Most patients will experience mild TBI, which the CDC characterizes as “a brief change in mental status or consciousness.” While a return to normal cognitive functioning can be expected within three to six months in a majority of cases, the Department of Veterans Affairs notes that some studies indicate soldiers may still be experiencing problems two years after the initial injury.

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Although blast-related brain trauma has been observed in military personnel since World War I, mild TBI has become the “signature wound” of the Iraq and Afghanistan campaigns, with an estimated 20 percent or more of these veterans returning home with a wide array of symptoms, from headaches and memory problems to impulsivity and depression. There are other wartime causes of mild TBI, including motor-vehicle accidents, but most of the injuries have been attributed to blast shockwaves generated by the increased use of improvised explosive devices on the battlefield.

However, despite the prevalence of these injuries and the potential for progressive neurodegenerative effects like Alzheimer’s disease and chronic traumatic encephalopathy, blast-related mild TBI is not well understood, as there are no physically observable wounds and the condition cannot generally be detected by standard imaging techniques. More fundamentally, how exactly these injuries occur in the brain and their precise physiological impacts remain unknown. But two UTA researchers, supported by grants from the Office of Naval Research (ONR), are investigating a promising lead: microcavitation.

It’s not yet clear what takes place in the brain as a shockwave generated by a high-order explosion passes through. One theory gaining traction is that, during the resulting pressure drop, micro bubbles form in the liquid areas of the brain. When these bubbles collapse—within five to 10 seconds in a lab setting—they produce a water jet that can impact surrounding tissues. It may not sound like much, but consider this: Cavitation is the phenomenon that causes the small pits observed in metal boat propellers.

Microcavitation is a phenomenon that requires liquid, so one of the challenges in investigating it is that the human brain is mostly made of water.

“Inside the brain, there are segments that are mostly fluidic—such as locations near the neurons or the cerebrospinal fluid under our skull—and there are also some small fluidic regions within the gray matter and white matter that include water, plus some other floating ions and biomolecules,” explains Ashfaq Adnan, associate professor of mechanical and aerospace engineering. “Hypothetically, cavitation can take place in any location, as long as the conditions for creating it are fulfilled.”

Through the development of a high-fidelity computation model, Dr. Adnan is investigating the impact of mechanical force on the neurons and their surrounding perineuronal nets—extracellular matrix structures that are more than 80 percent liquid. His work is funded by a two-and-a-half-year, $401,786 grant from ONR and UTA.

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In July, Adnan and his postdoctoral associate, Yuan Ting Wu, published a study in Nature’s Scientific Reports that detailed their findings investigating whether the water jet released when a cavitation bubble pops can break the hyaluronan, a chief structural component of the perineuronal nets that, when damaged, is known to have some links with neurodegenerative diseases.

Using reactive molecular dynamics simulation, they modeled 12 events, using four bubble sizes (no bubble, 5 nanometers, 8 nanometers, and 10 nanometers) and three different shockwave velocities (3.6 km/s, 5.35 km/s, and 7.2 km/s). They then rebuilt the models two more times to statistically validate it, which resulted in 36 independent sets of simulations.

The team observed that the shockwave itself was not enough to break the hyaluronan; when no bubble is present, the hyaluronan is largely unaffected, regardless of the shockwave speed. However, as the front side of the bubble compresses from shockwave pressure, the bubble collapses at an even faster speed, creating stronger water jets. At the 3.6 km/s and 5.35 km/s speeds, a shockwave acting on the 5-nanometer bubble bends the hyaluronan. In the remaining sets, as the bubble size grows and/or the speed increases, the hyaluronan breaks, at times in multiple pieces.

Based on these findings, Adnan and Dr. Wu concluded that the larger the bubble, the stronger the jet—and the more damage done to the perineuronal net.

“The cavitation bubble, if it forms, will always collapse, and if we have shock-induced jet formation, this may or may not lead to damage of the extracellular matrix,” Adnan says.

He notes that he wouldn’t have been able to pursue this research without the top-notch computational power provided by the Texas Advanced Computing Center at UT Austin.

But even with that support, available computational tools limited the study to small-scale, small-duration events in the neuronal substructure. Moving forward, Adnan will use the evidence he’s gathered at the nanoscale and employ multiscale methods to develop a model that permits the team to examine what happens at an increased length of scale and duration.

The ultimate goal of the project is to build a complete model of an individual neuron.

“Then, we’ll have a better picture of what is going on and what could happen to the neuron and its surroundings.”

 

THE TROUBLE WITH BUBBLES

For Michael Cho’s microcavitation study, the first task was to establish whether the phenomenon could even occur in the brain. In spring 2015, the Alfred R. and Janet H. Potvin Endowed Chair in Bioengineering and his collaborators constructed phantom models of the brain—plastic molds filled with Jell-O. When shockwaves were passed through them in the lab, tiny bubbles did indeed form.

“The conclusion is that it is likely that these micro bubbles are being formed in the brain,” says Dr. Cho, who is a fellow of the American Institute for Medical and Biological Engineering.

That finding prompted a slew of questions about what was actually occurring. With the support of a three-year, $1.24 million grant from ONR, Cho and his team set out to record, monitor, and characterize the micro bubbles, following their trajectory in real-time from formation to collapse and assessing the damage done to surrounding brain cells.

To do this, they are engineering biomimetic brain tissue—which can mimic the major functions of real brain tissue—from mouse astrocytes and neurons. Using a controlled electrical discharge system (analogous to what happens with lightning and thunder), Cho generates shockwaves in the lab and then views through a microscope the cellular and subcellular responses stemming from the formation of micro bubbles. He can watch in real-time as the bubbles form and rise to the top, observing the impact they have as they hit the cells and pop.

The experiments have found that the force of the bubble collapse knocks some cells off the substrate, presumably killing them. Others become partially detached and start undergoing apoptosis, or programmed cell death.

In addition to examining the general effects of the micro bubbles on biomimetic tissue, the collaborative team—which includes researchers from Old Dominion University, Purdue University, and the UTA Research Institute (UTARI)—is focusing on how blood vessels could be impacted by microcavitation and how this might affect the blood-brain barrier.

In the brain, capillaries are lined with tightly packed endothelial cells, which form a semi-permeable wall and function as a sort of security system, preventing “foreign” substances—including toxic compounds and many medicinal drugs—from passing from the blood into the brain.

“If some of those micro bubbles are formed inside the vessel, that creates even more lasting problems,” Cho says. “If they collapse with that kind of force, it’s going to make your vessel leaky, allowing substances that are not supposed to enter the brain to diffuse out.”

The first challenge in tackling this region is the microfabrication of a biomimetic blood vessel approximately the width of a strand of hair from appropriate mouse cells. One of Cho’s doctoral students has spent the last year at UTARI creating, validating, and characterizing an initial model, then testing it for leakiness with alcohol, which is one of the few substances that can pass through the blood-brain barrier. The next step is to test the impact of micro bubbles to determine if and how such mechanical force could damage or destroy the cells, thus compromising the barrier.

Considering that the mechanisms behind mild TBI are still being explored and current imaging techniques cannot generally detect the damage, it perhaps comes as no surprise that there are no treatments for the injury itself.

“There are a limited number of FDA-approved drugs that are used to address symptoms, like [PTSD-related] nightmares, but the damage is done,” Cho says. “That’s very frustrating to me.”

But he and his fellow researchers see therapeutic promise in a polymeric compound known as poloxamer 188, which is FDA-approved for use as a blood thinner. While Cho believes the detached cells cannot be rescued, the thought is that the poloxamer could function as a molecule-sized Band-Aid, patching up the leaky spots while the partially detached cells work to repair themselves.

“I don’t want to stop at knowing what the problems are,” Cho says. “We’re in the business of figuring out what could be done. That’s why I’m so excited by bioengineering programs like ours that are asking the question, ‘What can we do to impact society?’ For us, it’s about improving human health. That’s the goal driving what we do.”