The Invisible Man? In a Not Too Distant Future It Might Not Be Science Fiction Anymore. A team of scientists promises to open a window on the human body, or rather on the internal organs, making the skin like a ‘transparent glass’invisible indeed. The technique, defined as “revolutionary”, developed to succeed in the enterprise is illustrated in a study to be published in ‘Science’. The researchers from Stanford University who developed it used a counterintuitive process, the topical (reversible) application of a food coloring, and succeeded in tests conducted on animals, in finding a way to see the organs inside a body by making the overlying tissues transparent to visible light. A sort of live anatomy lesson.
This strategy could potentially be applied to a wide range of medical diagnoses, from localizing lesions to monitoring digestive disorders to identifying tumors. For example, “looking to the future, the technology could make veins more visible for blood sampling, or make it easier to remove tattoos with lasers, or even help with the early diagnosis and treatment of cancer,” says Guosong Hong, an associate professor of materials science and engineering at Stanford University and a recipient of the National Science Foundation Career Grant who helped lead the work. Other possible applications? “For example, some therapies use lasers to kill cancer and precancerous cells, but they are limited to areas close to the surface of the skin. This technique could improve the penetration of light,” he continues.
How did the team make skin ‘invisible’? Researchers have developed a way to predict how light interacts with colored biological tissue. Scattering is the reason we can’t see through our bodies: fats, fluids in cells, proteins and other materials each have a different refractive index, a property that determines how much an incoming light wave is bent. The scientists involved in the study realized that if they wanted to make biological material transparent, they had to find a way to match the different refractive indices, so that light could pass through it unimpeded. One dye they predicted would be particularly effective was the food coloring tartrazine: when dissolved in water and absorbed into tissue, its molecules are perfectly structured to match the refractive indices and prevent light from scattering, resulting in transparency. The researchers first ‘tested’ it on “thin slices of chicken breast.” The result? As the concentrations of tartrazine increased, the slice became transparent.
Next, the researchers gently rubbed a temporary tartrazine solution on the mice. First, they applied the solution to their heads, making the skin transparent to reveal blood vessels crisscrossing the brain. Then, they applied the solution to their abdomens: it ‘vanished’ within minutes, to reveal contractions of the intestines and movements caused by heartbeats and breathing. The technique even improved microscopic observations, the authors continue to explain. The process is reversible, they finally showed: when the dye was rinsed off, the tissues quickly returned to normal opacity. The tartrazine did not appear to have any long-term effects, and any excess was excreted within 48 hours. The experts hypothesize that injecting the dye could lead to even deeper insights into organisms, “with implications for both biology and medicine”.
Researcher Nick Rommelfanger, working on an NSF Graduate Research Fellowship, was one of the first to realize that the same modifications that make materials transparent to microwaves could be tailored to affect the visible spectrum, with potential applications in medicine. Moving from theory to experiment, his colleague Zihao Ou, the study’s lead author, ordered a series of powerful dyes and began the process of meticulously evaluating each one for ideal optical properties.
Eventually, the team grew to include 21 students, collaborators, and consultants, involving several analytical systems. What proved key to the group’s discoveries was a decades-old instrument called an ellipsometer. In a possible medical first, the researchers realized it was perfect for predicting the optical properties of their target dyes. The researchers hope their approach could spark a new field of study that matches dyes to biological tissues based on optical properties, potentially opening the door to a wide range of medical applications. Experts caution, of course, that the technique they describe has not yet been tested in humans, and that the dyes can be harmful and should not be misused.
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