Techniques for looking inside the living body have revolutionised our understanding of how it works. Magnetic-resonance imaging (MRI) can now show us slices through the brains of people while they conduct cognitive tasks, revealing which parts of the brain are active. Ultrasound and PET (positron emission tomography) scanning can reveal tumours deep within organs and tissues; X-rays have long been used to image buried bone fractures.
Such methods have their strengths and weaknesses – in particular, they don’t tend to have very good spatial resolution, so that fine details aren’t clear. How useful it would be simply to be able to see through our flesh to what is transpiring inside, in microscopic detail: blood coursing through veins, neurons firing, muscles contracting, cells proliferating.
“Deep tissue imaging” is now a major goal in medical research. Of course, flesh isn’t transparent to visible light, but infrared light (with longer wavelengths than the red end of the spectrum) can penetrate further. However, light at or close to the visible range tends to be strongly scattered by tissues, meaning that it bounces off in all directions. It is light scattering that gives clouds their opaque milkiness: the water droplets are as transparent as water in a glass, but when they are so small they scatter light strongly.
It’s much the same with tissues. Our cells are, at this level, not much more than bags of salty water, but their components, such as the cell membrane, scatter light. It might seem there is not much we can do about that – but there is a trick that can turn them transparent.
Scattering happens because of the phenomenon called refraction, whereby the path of a light ray bends when it passes from one transparent substance to another. That’s what makes objects underwater look distorted: the light bends as it travels from air into the water. The amount of bending depends on a property of the materials called the refractive index: light passing between two substances with the same refractive index doesn’t bend at all. We see a glass rod immersed in water because glass and water have different refractive indices. But the refractive index of glass is about the same as that of baby oil, so a glass rod dipped into the oil becomes virtually invisible.
Researchers at Stanford University in California have reported in Science that they can match the refractive index of cellular contents and the surrounding tissue by adding a dye to the tissue, thus turning it transparent. In this way they could make the skin of live mice virtually see-through, allowing them to look directly at the guts, blood vessels in the brain, or leg muscles. The dye is harmless; in fact it’s nothing but tartrazine, widely used as a yellow dye in foods.
It’s not a totally new idea: “tissue clearing” techniques using additives such as sugars or glycerol to match refractive indices have been around for a while. But usually they require high concentrations of the additives, which can cause problems like tissue swelling. In contrast, the Stanford team needs only small amounts of the dye, and the transparency only lasts until the dye gets cleared from the animal’s body.
Making tissues invisible by matching their refractive index with the surroundings is in fact a much older idea than this. It was exactly how HG Wells’s “invisible man” made himself disappear in the 1897 book of that name.
Wells invoked a bit of what he called jiggery-pokery to make it work. His protagonist, Griffin, drinks a potion that first bleaches the red pigment in blood and turns him transparent, and then exposes himself to a special kind of radiation (X-rays had just been discovered) that alters the refractive index of his flesh to match that of air.
Wells, who studied at what was later to become Imperial College London under the biologist Thomas Henry Huxley, was well informed about the science of his day.
So you could regard the Stanford work as an example of the prescience of science-fiction writers. The catch is that Wells’s story had a moral, which he explicitly took from the tale of a ring of invisibility in Plato’s Republic: invisibility corrupts. As Plato argued, how could our leaders stay trustworthy if, by being unseen, they could evade responsibility for their actions?
There’s little reason to worry about the morality of temporarily transparent mice – but when we consider other technologies of invisibility, in which the military is investing heavily, Plato’s warning remains worth heeding.
