Enlarge / Zihao Ou, who helped develop this solution, holds a tube of it.

One key challenge in medical imaging is to look past skin and other tissue that are opaque to see internal organs and structures. This is the reason we need things like ultrasonography, magnetic resonance, or X-rays. There are chemical clearing agents that can make tissue transparent, like acrylamide or tetrahydrofuran, but they are almost never used in living organisms because they’re either highly toxic or can dissolve away essential biomolecules.

But now, a team of Stanford University scientists has finally found an agent that can reversibly make skin transparent without damaging it. This agent was tartrazine, a popular yellow-orange food dye called FD&C Yellow 5 that is notably used for coloring Doritos.

Playing with light

We can’t see through the skin because it is a complex tissue comprising aqueous-based components such as cell interiors and other fluids, as well as protein and lipids. The refractive index is a value that indicates how much light slows down (on average, of course) while going through a material compared to going through a vacuum. The refractive index of those aqueous components is low, while the refractive index of the proteins and lipids is high. As a result, light traveling through skin constantly bends as it endlessly crosses the boundary between high and low refractive index materials.

Enlarge / Light painting reveals a cloud of particulates on Airport Road, Addis Ababa, Ethiopia (PM2.5 10-20 micrograms per cubic meter). (credit: Robin Price)

Light painting is a technique used in both art and science that involves taking long-exposure photographs while moving some kind of light source—a small flashlight, perhaps, or candles or glowsticks—to essentially trace an image with light. A UK collaboration of scientists and artists has combined light painting with low-cost air pollution sensors to visualize concentrations of particulate matter (PM) in select locations in India, Ethiopia, and Wales. The objective is to creatively highlight the health risks posed by air pollution, according to a new paper published in the journal Nature Communications.

“Air pollution is the leading global environmental risk factor," said co-author Francis Pope, an environmental scientist at the University of Birmingham in the UK who spearheaded the Air of the Anthropocene project with artist Robin Price. "[The project] creates spaces and places for discussions about air pollution, using art as a proxy to communicate and create dialogues about the issues associated with air pollution. By painting with light to create impactful images, we provide people with an easy-to-understand way of comparing air pollution in different contexts—making something that was largely invisible visible."

Light painting has been around since 1889, when Étienne-Jules Marey and Georges Demeny, who were investigating the use of photography as a scientific tool to study biological motion, created the first known light painting called Pathological Walk From in Front. In 1914, Frank and Lillian Mollier Gilbreth tracked the motion of manufacturing and clerical workers using light painting techniques, and in 1935, Man Ray "signed" his Space Writing series with a penlight—a private joke that wasn't discovered until 74 years later by photographer/historian Ellen Carey in 2009.