Novel Technique Makes Transparent Skin Possible for Deeper Imaging

Using a common food dye, researchers have developed a pioneering method that can make living tissues transparent.  Published in Science, this technique enables deeper and clearer imaging in live animals, opening new possibilities for medical diagnostics and research.

Traditional optical imaging has its limitations

Optical imaging is incredibly important in the field of biology and medicine. It is one of the most used molecular and cellular imaging technologies in preclinical applications and offers a simple, non-invasive technique to provide morphological and functional data. There are several types of optical imaging, such as two-photon microscopy, near-infrared-II fluorescence imaging and optical tissue clearing, which use light in the ultraviolet, visible and infrared regions of the electromagnetic spectrum to obtain detailed images. The technique relies on absorption, scattering, emission and the fluorescence properties of cells, molecules or tissues to form a picture. However, this method comes with limitations.

Traditional optical imaging technologies often lack the ability to penetrate the thick layers of human tissue or are unsuitable for living animals, due to unwanted scattering and absorption of light.

“Biological tissue is a complex mixture of water (low refractive index) with other biomolecules including lipid and protein (high refractive index). Similar to the physics behind an oil-water mixture, light will be scattered in all different directions when propagating through the tissue, leading to optical opacity,” said lead author Dr. Zihao Ou, an assistant professor in the biomedical science program and the department of physics at the University of Texas at Dallas.

Existing methods have aimed to minimize RI differences by using high-RI chemicals, such as tetrahydrofuran and acrylamide, or by removing lipids to create an all-aqueous environment. However, these approaches are rarely suitable for live tissues because they often involve toxic substances and the depletion of essential molecules like water and lipids, which are critical for maintaining cell structure, hydration, and metabolic function.

Absorbing molecules make skin and muscle transparent in mice

Ou and his team recognized that to make biological material transparent, they needed to match different RIs, allowing light to pass through unimpeded. Drawing on principles from optics, they realized that light-absorbing dyes could direct light across various RIs.

After evaluating several strong dyes, the team identified tartrazine, a food dye also known as FD & C Yellow 5, for its potential optical properties. The tartrazine molecules become structured to match different RIs when dissolved in water and absorbed into tissues, resulting in transparency.

“By introducing strongly absorbing molecules into an aqueous medium, the RI of the solution at certain wavelengths increases and the RI differences in biological tissues are strongly reduced. This leads to reduced light scattering of biological tissue and the tissue becomes transparent visually,” said Ou.

Their novel method was originally tested on thin slices of chicken breast. Once their hypothesis was confirmed they moved to testing the method on live mice. A topical temporary tartrazine solution was applied to the scalp, resulting in transparency and the revealing of blood vessels crisscrossing the brain. The dye was also applied to the abdomen of mice, which allowed the researchers to observe contractions of the intestine and movements caused by heartbeats and breathing.

“For those who understand the fundamental physics behind this, it makes sense. But if you aren’t familiar with it, it looks like a magic trick,” said Ou.

“It takes a few minutes for the transparency to appear. It’s similar to the way a facial cream or mask works: The time needed depends on how fast the molecules diffuse into the skin,” he added.

The process was also reversible. Once the remaining dye was washed off and the dye that had diffused into the skin was metabolized, the tissue returned to normal opacity.

“Based on our observations, the dyes will be metabolized and excreted out of the animal body within a few days after application. For the yellow food dye tartrazine, we haven’t observed any long-term adverse effects in living animals,” said Ou.

The researchers also believe that injecting the dye could allow for even deeper views within organisms.

Applying the technology to human skin 

Although this process has not yet been tested on humans, whose skin is around 10 times thicker than a mouse’s, Ou and his team do not see this as a barrier.

“Indeed, human skin is much thicker than mouse skin. However, the fundamental physics behind optical opacity is the same and we envision there is no fundamental limit in applying our invention to the human,” Ou said.

The team plans to start investigating what dosage of the dye molecule will work best in human tissue, as well as experimenting with other molecules that may work more efficiently than tartrazine.

Advancing medical imaging

The novel technique will have a massive impact on biological research and the medical industry.

“Our method can improve existing optical imaging modalities to probe deep features that could not be visualized before in live animals. Due to its unique non-invasiveness, it will allow chronic investigations of biological questions across a relatively long time, such as cancer metastasis and Alzheimer’s diseases. Beyond that, we envision the potential clinical translation of this invention for early disease diagnosis and health monitoring in humans,” said Ou.

″Looking forward, this technology could make veins more visible for the drawing of blood, make laser-based tattoo removal more straightforward or assist in the early detection and treatment of cancers,″ said corresponding author Dr. Guosong Hong, assistant professor of materials science and engineering at Stanford University.

″For example, certain therapies use lasers to eliminate cancerous and precancerous cells but are limited to areas near the skin’s surface. This technique may be able to improve that light penetration,″ said Hong.

“Now that we can make tissue transparent, it will allow us to look at more detailed dynamics. It will completely revolutionize existing optical research in biology,” Ou added.

Reference: Ou Z, Duh YS, Rommelfanger NJ, et al. Achieving optical transparency in live animals with absorbing molecules. Science. 2024. doi: 10.1126/science.adm6869

About the interviewee:

Dr. Zihao Ou is an assistant professor in the biomedical science program and the department of physics at the University of Texas at Dallas. He earned his BS in physics from the University of Science and Technology of China in 2015 and completed his PhD in materials science and engineering at the University of Illinois, Urbana-Champaign in 2020. Dr. Ou’s research focus lies at the intersection of fundamental physical principles and cutting-edge genetic and molecular advancements. His goal is to create advanced imaging platforms that allow real-time monitoring of materials and biological processes in their native environments.