Brain bubbles: Researchers describe the dynamics of cavitation in soft porous material



Hector Gomez, professor of mechanical engineering and lead researcher, explained that when a bubble bursts inside a liquid, it creates pressure shock waves. We refer to cavitation as the formation and collapse of a vapor cavity.

According to Pavlos Vlachos, the Regenstrief Center for Healthcare Engineering's director and the St. Vincent Health Professor of Healthcare Engineering, "Cavitation has been investigated since the 1800s." "The non-equilibrium thermodynamics, continuum mechanics, and numerous other factors on a scale of micrometers and microseconds make it a tremendously complex subject of research. We have been studying these occurrences for hundreds of years, yet we are only now beginning to grasp them."                                                                     
There is even less information available on bubbles that burst in soft porous materials like the brain or other human tissues. That's important because knowing how those bubbles operate could help us understand concussions and possibly help us administer specific treatments to certain parts of the body.                 
Gomez, Vlachos, and colleagues described the creation of a mathematical model to characterize the dynamics of these cavitation bubbles in a deformable porous medium in new research published in the Proceedings of the National Academies (PNAS) Nexus.

Cavitation happens all over the body; for instance, the sound of your knuckles cracking is the sound of bubbles popping in the synovial fluid of your joints. The fluid around the brain may develop bubbles when internal body fluids are subjected to pressure waves, such as when football players experience head collisions. Additionally, bubbles bursting close to the brain could harm its soft tissue in a manner similar to how they harm boat propellers.

The human brain, according to Vlachos, "is like a water-filled spongy sponge; it has the consistency of gelatin." "Because it is made of porous, heterogeneous, and anisotropic material, the situation is much more complicated. When such occurrences take place inside the body, our present understanding of cavitation does not directly apply."

A theoretical and computational model created by Gomez and colleagues demonstrates how the deformability of a porous material inhibits the collapse and expansion of cavitation bubbles. The traditional scaling relationship between bubble size and time is broken down by this.

According to Yu Leng, a postdoctoral research associate who collaborated with Gomez and is the paper's first author, "Our model embeds the bubbles within deformable porous materials." The study of cavitation bubbles in pure liquid can then be expanded to soft tissues like the human brain.

This model, however intricate, can also be simplified to a simple ordinary differential equation. "Lord Rayleigh created the equation that captures the dynamics of a bubble in a fluid 100 years ago," Gomez added. "To characterize when the medium is poroelastic, we were able to add to that equation. It's quite astounding how these intricate physics can result in such a straightforward and beautiful equation."

Gomez and Vlachos are already organizing tests to verify their findings physically, but they are also considering the big picture. Targeted medicine distribution is one potential application, according to Gomez. "Say you wish to administer a medication directly to a tumor. You don't want that medicine to spread to other areas. Encapsulations that keep the medicine segregated until it reaches its target have been seen. The use of bubbles can dislodge the encapsulation. Our study offers a clearer picture of how these bubbles burst within the body, which may improve drug delivery."

Traumatic brain injury is "another example of future possibilities," Leng said. The study of the effects of uncontrolled cavitation collapse on brain tissue when military personnel and civilians are subjected to blast shock waves can be expanded upon in this research.

Gomez and Vlachos declare that they are overjoyed to lay fresh scientific groundwork for comprehending bubble dynamics in soft porous materials. Gomez stated, "We look forward to how we and others will use this knowledge in the future. This offers up all sorts of opportunities for future research."

The National Science Foundation (Award 1805817), the U.S. Department of Energy, and the U.S. Department of Defense contributed to the funding of this study through the DEPSCoR program (Award FA9550-20-1-0165) (Grant DE-SC0018357).

Purdue University

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