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Scientists Have Figured Out How To 'see' Through Mice – Could Humans Be Next?

Imagine being able to see right through your skin to watch your muscles or organs in action. It sounds like science fiction, but a group of scientists at Stanford University were recently able to make the skin of live mice appear transparent – at least under certain light conditions.

This breakthrough has unquestionably opened up new possibilities in biological research and medical imaging. So how did they do it, and could it ever lead to humans becoming invisible?

When we look at objects, light reflects off them, allowing our eyes to see shapes and colours. However, living tissue such as skin behaves differently because it is comprised of things such as water, proteins and lipids (fats), which all bend light at different angles. This means that light is scattered by skin, which limits how deeply we can see into the body without invasive surgery.

To try and get around this problem, scientists have developed more sophisticated imaging techniques over the years, such as two-photon microscopy and near-infrared fluorescence. But they often require harmful chemicals or only work on dead tissue. Instead, the goal has been to find a way to achieve transparency in living organisms safely and reversibly.

In the Stanford study, the researchers turned to a surprising tool: food dye. Tartrazine (also known as E102), a common yellow food dye found in crisps and soft drinks, has a unique property. When dissolved in water and applied to skin tissues, it alters how light interacts with biological matter.

Imaging of internal mouse organs

How tartrazine made mice see-through under certain light. Science

The key to this lies in the physics of light absorption and refraction, specifically something called the "Kramers-Kronig relations", which describe how materials interact with light across different wavelengths. Tartrazine has been used in microscopy for years as a way of staining certain parts of the anatomy to make them more visible, but it has never been used on the whole tissue of living animals.

By adding tartrazine to water and applying it to the tissues of anaesthetised live mice, the researchers were able to change the refractive index of water in the tissue, meaning the extent to which it bends light. This brought its refractive index closer to that of lipids, which enabled the light to pass through the skin of the mice more easily, making them appear transparent.

Astoundingly, the researchers were able to see in unprecedented detail deep structures inside the mice such as blood vessels and even muscle fibres. In one example, they could see the movements of the intestines in real-time through the transparent abdomen. This level of visibility was achieved without any apparent harmful effects to the mice, including being able to return their skin to its normal, opaque state once the dye was washed off.

This discovery could be revolutionary. Imagine being able to monitor organ function without invasive procedures, or see precisely where a vein is to draw blood. It could also pave the way for breakthroughs in understanding how diseases affect the body at a microscopic level.

As fascinating as this all is, making humans fully invisible remains unlikely for several reasons.

Firstly, the transparency achieved in the Stanford study is clearly not total invisibility. And although the tartrazine allows light to pass through tissues, it works best with specific wavelengths of light, mainly in the red and infrared regions of the spectrum. This means that under normal lighting conditions, the mice aren't truly invisible to the naked eye. Instead, they are transparent under specific imaging equipment designed to capture this phenomenon.

Secondly, this transparency only affects the tissues where the dye has been applied, and even then, it is limited by how deeply the dye can penetrate. Human bodies are significantly more complex and skin much thicker than those of mice. Making a whole human transparent would require a different level of application and technology.

Disappear this! BigBlueStudio

For one thing, light behaves differently when passing through larger volumes of tissue. Also, even if we could scale up the technology, achieving full-body transparency would involve significant challenges, such as ensuring the dye reached all parts of the body evenly without causing harm. Tartrazine is safe to consume within daily limits, but can cause side effects, allergic reactions and, at large doses, there is conflicting data regarding it having toxic effects on cells or potentially causing genetic mutations.

In addition, the transparency effect works by modifying how light interacts with biological tissues, but it doesn't address the issue of light absorption by other components of the body, such as bones, which are denser and would likely require different methods to become transparent.

So, is human invisibility possible? Not in the way we see in movies. But we may in future see further developments that push the boundaries of what's possible with transparency in living organisms.

Human Invisibility On The Horizon? Scientists Figure Out How To 'see' Through Mice

(Credit: fran_kie/Shutterstock)

Imagine being able to see right through your skin to watch your muscles or organs in action. It sounds like science fiction, but a group of scientists at Stanford University were recently able to make the skin of live mice appear transparent – at least under certain light conditions.

This breakthrough has unquestionably opened up new possibilities in biological research and medical imaging. So how did they do it, and could it ever lead to humans becoming invisible?

When we look at objects, light reflects off them, allowing our eyes to see shapes and colors. However, living tissue such as skin behaves differently because it is comprised of things such as water, proteins, and lipids (fats), which all bend light at different angles. This means that light is scattered by skin which limits how deeply we can see into the body without invasive surgery.

To try and get around this problem, scientists have developed more sophisticated imaging techniques over the years, such as two-photon microscopy and near-infrared fluorescence. But they often require harmful chemicals or only work on dead tissue. Instead, the goal has been to find a way to achieve transparency in living organisms safely and reversibly.

In the Stanford study, the researchers turned to a surprising tool: food dye. Tartrazine (also known as E102), a common yellow food dye found in crisps and soft drinks, has a unique property. When dissolved in water and applied to skin tissues, it alters how light interacts with biological matter.

The key to this lies in the physics of light absorption and refraction, specifically something called the "Kramers-Kronig relations", which describe how materials interact with light across different wavelengths. Tartrazine has been used in microscopy for years as a way of staining certain parts of the anatomy to make them more visible, but it has never been used on the whole tissue of living animals.

By adding tartrazine to water and applying it to the tissues of anesthetized live mice, the researchers were able to change the refractive index of water in the tissue, meaning the extent to which it bends light. This brought its refractive index closer to that of lipids, which enabled the light to pass through the skin of the mice more easily, making them appear transparent.

Astoundingly, the researchers were able to see in unprecedented detail deep structures inside the mice such as blood vessels and even muscle fibers. In one example, they could see the movements of the intestines in real-time through the transparent abdomen. This level of visibility was achieved without any apparent harmful effects to the mice, including being able to return their skin to its normal, opaque state once the dye was washed off.

This discovery could be revolutionary. Imagine being able to monitor organ function without invasive procedures or see precisely where a vein is to draw blood. It could also pave the way for breakthroughs in understanding how diseases affect the body at a microscopic level.

As fascinating as this all is, making humans fully invisible remains unlikely for several reasons.

Firstly, the transparency achieved in the Stanford study is clearly not total invisibility. And although the tartrazine allows light to pass through tissues, it works best with specific wavelengths of light, mainly in the red and infrared regions of the spectrum. This means that under normal lighting conditions, the mice aren't truly invisible to the naked eye. Instead, they are transparent under specific imaging equipment designed to capture this phenomenon.

Secondly, this transparency only affects the tissues where the dye has been applied, and even then, it is limited by how deeply the dye can penetrate. Human bodies are significantly more complex and skin much thicker than those of mice. Making a whole human transparent would require a different level of application and technology.

For one thing, light behaves differently when passing through larger volumes of tissue. Also, even if we could scale up the technology, achieving full-body transparency would involve significant challenges, such as ensuring the dye reached all parts of the body evenly without causing harm. Tartrazine is safe to consume within daily limits, but can cause side effects, allergic reactions and, at large doses, there is conflicting data regarding it having toxic effects on cells or potentially causing genetic mutations.

In addition, the transparency effect works by modifying how light interacts with biological tissues, but it doesn't address the issue of light absorption by other components of the body, such as bones, which are denser and would likely require different methods to become transparent.

So, is human invisibility possible? Not in the way we see in movies. But we may in future see further developments that push the boundaries of what's possible with transparency in living organisms.

The Scent Of Truth: The Mystery Of Human Pheromones

My friend Marty was absolutely convinced that humans give off pheromones. She swore up and down that one time, on a crowded bus, she accidentally bumped into a stranger and was suddenly, inexplicably, drawn to him. "It must be the pheromones!" she declared, cocky as ever, and then laughed. "I even followed him for a few blocks."

Showing that humans use pheromones, however, is a lot harder in science. One of the first attempts came from Martha McClintock, then a student at Wellesley College. In the late 1960s, she noticed that while her dormmates had different menstrual cycles at the start of the semester, by the end of those three to four months, their periods had magically synced up. There had to be a pheromone for that effect, she inferred (1).

But where exactly is this mysterious pheromone hiding? Some guessed the armpit—nature's little chemical factory. The problem? The armpit churns out a whole cocktail of funky substances, so which one is the guilty party? Could it be 3M2H (short for (E)-3-methyl-2-hexenoic acid), the main ingredient behind that "delightful" scent we call human sweat?

I was itching to test the idea, but I never got around to it. Getting my hands on the chemical wasn't the issue—you don't have to go scraping armpits to collect it. A quick lab synthesis, a few days of work, and voilà! But testing its effects on enough women? Now, that was the real hurdle.

Lucky for me, I had Wendy Williams, a sharp psychologist, and Corinna Avelos, a pre-med powerhouse, on my side. Together, we ran a double-blind experiment with dozens of young, heterosexual women volunteers. Their nightly ritual? Dabbing their upper lips with rubbing alcohol—either plain (our control) or spiked with an unperceivable dose of 3M2H (10 ppm)—just before bed. For four months, we tracked their progress with painstaking detail. Finally, 16 of our dedicated volunteers made it to the finish line, and the moment of truth arrived. The big reveal? Drumroll... Nothing. A big fat statistical zero! So much for the grand, life-altering effects of 3M2H (2).

Did we just lose our scientific bet, or was our sample size simply too small to catch the action? It's hard to say. But one thing's clear: plenty of other studies have struggled to replicate McClintock's findings. Critics have pointed fingers at everything from statistical slip-ups to pure chance. And just like that, the groundbreaking "first evidence" of human pheromones starts to look shaky.

But in 2023, a fresh discovery reignited the flickering hope for human pheromones. In a study led by Shani Agron, researchers managed to collect 149ml (about two-thirds of a cup) of emotional tears from six women known for their "easy crying" skills. They then had male volunteers take a whiff. The results? Astonishing. The neural link between aggression and smell lit up, testosterone levels plummeted, and male aggression dropped by a staggering 43.7 percent. They even pinpointed four olfactory receptors in our noses that latch onto the magic aggression-taming protein in women's tears (3). With evidence this solid, the idea of human pheromones starts to feel more like fact than fiction.

A new research article, authored by my long-time collaborator Jianxu Zhang and his team at the Chinese Academy of Sciences, reveals a wide array of chemicals from human sweat glands alone that could potentially broadcast who we are—our individuality, gender, sex, emotion, physiological state, and reproductive status (4). Most of these chemicals haven't undergone the same rigorous testing as those linked to menstrual synchronization or aggression reduction. So, there's plenty of room for more thrilling research on human pheromones in the future. Stay tuned!

References

1. McClintock, M.K. (1971). Menstrual synchrony and suppression. Nature. 229 (5282): 244–5.

2. Sun, L., Williams, W.A., & Avalos, C. (2005). Human sweaty smell does not affect the menstrual cycle. In R. T. Mason, M. P. LeMaster, & D. Müller-Schwarze (eds), Chemical Signals in Vertebrates X. Springer, New York, pp. 308 – 312.

3. Agron, S., de March, C.A., Weissgross, R., Mishor, E., Gorodisky, L., Weiss, T., Furman-Haran, E., Matsunami, H. And Sobel, N. (2023). A chemical signal in human female tears lowers aggression in males. PLoS biology, 21(12), p. E3002442.

4. Zhang, Y.H., Du, Y.F. And Zhang, J.X. (2024). Main compounds in sweat, such as squalene and fatty acids, should be promising pheromone components for humans. BioRxiv, pp. 2024-08.

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