Pop, Focus, Destroy

A new technique developed at Caltech that uses gas-filled microbubbles for focusing light inside tissue could one day provide doctors with a minimally invasive way of destroying tumors with lasers, and lead to improved diagnostic medical imaging.

The primary challenge with focusing light inside the body is that biological tissue is optically opaque. Unlike transparent glass, the cells and proteins that make up tissue scatter and absorb light. “Our tissues behave very much like dense fog as far as light is concerned,” says Changhuei Yang, professor of electrical engineering, bioengineering, and medical engineering. “Just like we cannot focus a car’s headlight through fog, scientists have always had difficulty focusing light through tissues.”

To get around this problem, Yang and his team turned to microbubbles, commonly used in medicine to enhance contrast in ultrasound imaging. First, gas-filled microbubbles encapsulated by thin protein shells and injected into tissue are ruptured with ultrasound waves. By measuring the difference in light transmission before and after such an event, the Caltech researchers can modify the wavefront of a laser beam so that it focuses on the original locations of the microbubbles. The result, Yang explains, “is as if you’re searching for someone in a dark field, and suddenly the person lets off a flare. For a brief moment, the person is illuminated and you can home in on their location.”

If the technique is shown to work effectively inside living tissue—without, for example, any negative effects from the bursting microbubbles—it could enable a range of research and medical applications. For example, by combining the microbubbles with an antibody probe engineered to seek out biomarkers associated with cancer, doctors could target and then destroy tumors deep inside the body or detect malignant growths much sooner.

“Ultrasound and X-ray techniques can only detect cancer after it forms a mass,” Yang says. “But with optical focusing, you could catch cancerous cells while they are undergoing biochemical changes but before they undergo morphological changes.”

Photo: Courtesy of Haowen Ruan Mooseok Jang & Changhuei Yang

Tracking down the “missing” carbon from the martian atmosphere

Mars is blanketed by a mostly carbon dioxide atmosphere—one that is far too thin to prevent large amounts of water on the surface of the planet from subliming or evaporating. But many researchers have suggested that the planet was once shrouded in an atmosphere many times thicker than Earth’s. For decades that left the question, “Where did all the carbon go?”

Now scientists from Caltech and JPL think they have a possible answer. The team suggests that 3.8 billion years ago, Mars might have had only a moderately dense atmosphere. The researchers have identified a photochemical process that could have helped such an early atmosphere evolve into the current thin one without creating the problem of “missing” carbon.

“With this new mechanism, everything that we know about the martian atmosphere can now be pieced together into a consistent picture of its evolution,”says Renyu Hu, a postdoctoral scholar at JPL, a visitor in planetary science at Caltech, and lead author on the paper that appeared in Nature Communications.

When considering how the early atmosphere might have transitioned to its current state, there are two possible mechanisms for the removal of excess carbon dioxide (CO2). Either the CO2 was incorporated into minerals in rocks called carbonates or it was lost to space.

A separate study coauthored by Bethany Ehlmann, assistant professor of planetary science at Caltech, used data from several Mars-orbiting satellites to inventory carbonate rocks, showing that there are not enough carbonates in the upper crust to contain the missing carbon from a very thick early atmosphere.

To study the escape-to-space scenario, scientists examined the ratio of carbon-12 and carbon-13, two stable isotopes of the element carbon that have the same number of protons in their nuclei but different numbers of neutrons, and thus different masses. Comparing measurements from martian meteorites to those recently collected by NASA’s Curiosity rover, they found that the atmosphere is unusually enriched in carbon-13. To explain that, they describe a mechanism involving a photochemical cascade that produces carbon atoms that have enough energy to escape the atmosphere, and they show that carbon-12 is far more likely to escape than carbon-13.

“With this mechanism, we can describe an evolutionary scenario for Mars that makes sense of the apparent carbon budget, with no missing processes or reservoirs,” says Ehlmann, who is also a coauthor on the Hu study.

Photo credit: NASA/JPL-Caltech/Univ. of Arizona

They Came From Outer Space

While more than 4,900 minerals have been identified on Earth, at present only about 65 such substances have been identified that are believed to have first existed in the cloud, called the solar nebula, from which the planets of our solar system were formed.

“As our solar system started to develop about 4.6 billion years ago, high-temperature minerals arose when condensation processes turned some gasses into solids,” explains Ma. “These refractory minerals—meaning they are the first solar solids and formed at high temperatures—along with presolar grains mark the very beginning of mineral evolution at the very beginning of our solar system, and also mark my proudest contribution to science.”

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Photo by HC Van Urfalian