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.
Author
Timothy Hearn
Senior Lecturer in Bioinformatics, Anglia Ruskin University
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
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.
Next stop, invisibility?
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.
Timothy Hearn does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
