Common food dye makes skin ‘invisible’ for medical imaging

Researchers are transforming how we visualize internal organs by rendering overlying skin tissues "invisible." Using a common food dye (with minor alterations), the experts have developed a new way to see organs by making the skin transparent to visible light.

This innovative approach holds the potential to revolutionize medical diagnostics, enabling doctors to more effectively locate injuries, monitor digestive disorders, and identify cancers with unprecedented clarity.

Dye for transparent skin imaging

The process, involving the topical application of a food-safe dye, was tested on animal subjects and showed promise in a wide range of medical applications.

"Looking forward, this technology could make veins more visible for drawing blood, simplify laser-based tattoo removal, or assist in early cancer detection and treatment," noted Dr. Guosong Hong, one of the lead researchers at Stanford University .

Insights from the field of optics

To develop this technique, the researchers investigated how light interacts with biological tissues containing dyes. They predicted how refractive indices, which dictate how light bends and scatters, could be manipulated to render tissues transparent.

The research built on fundamental insights from the field of optics, including the Kramers-Kronig relations and Lorentz oscillation - concepts that are documented in textbooks from the 1970s and 1980s.

Graduate researcher Nick Rommelfanger was among the first to connect the dots between these optical theories and their applications in visible light, working under an NSF Graduate Research Fellowship.

Rommelfanger's work was instrumental in identifying tartrazine (FD & C Yellow 5) as a key dye for the process.

"We realized that the same modifications that make materials transparent to microwaves could be tailored for the visible spectrum with potential applications in medicine," noted Rommelfanger.

Testing the optical properties of dyes

In lab tests, the researchers applied a temporary tartrazine solution to mice, rendering the skin transparent and revealing the intricate network of blood vessels in the brain , as well as the abdomen's internal movements.

Once rinsed, the transparency effect faded with no long-term consequences. The team speculates that injecting the dye might provide even deeper visibility within organisms.

Zihao Ou, the study’s lead author, directed the experimental phase of the research. "We ordered a range of strong dyes and meticulously evaluated each for its optical properties," said Ou.

One key tool in the evaluation process was an ellipsometer, a piece of equipment typically used in semiconductor manufacturing but critical in predicting the optical properties of the dyes.

Tissue transparency breakthrough

National Science Foundation Program Officer Richard Nash emphasized the importance of accessible advanced research facilities.

"While a basic workhorse such as an ellipsometer would rarely make headlines, it plays a crucial role when deployed for atypical uses like this," said Nash.

Dr. Hong highlighted the role that funding played in the success of the project, especially during the pandemic.

"NSF’s support was instrumental in enabling us to continue our work during the most challenging times," said Dr. Hong. "The flexibility and encouragement provided by the NSF grants allowed us to explore new, uncharted territories in the field."

As an optics researcher, NSF Program Officer Adam Wax expressed amazement at the research team’s novel application of fundamental physics.

"This team has used the Kramers-Kronig relationship in an exciting new way, showing how a strongly absorbing dye can make skin transparent," said Wax.

A new frontier in skin imaging

This breakthrough in skin transparency could lead to applications in biology and medicine, ranging from more efficient diagnostic imaging to improved cancer treatments .

As Dr. Hong’s lab continues to explore this exciting new field, the future of medical imaging looks remarkably clear.

However, it is important to note that this technique of transparent imaging tissues with dye has not yet been tested on humans.

The use of dyes in medical applications must always follow strict safety guidelines, and they should never be consumed or applied (to people or animals) without proper medical supervision.

Video Credit: Keyi "Onyx" Li/U.S. National Science Foundation

The study is published in the journal Science .

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