Can the brain be illuminated with a photobiomodulation device?

During photobiomodulation therapy, it is sometimes necessary for light to penetrate deep into the body in order to stimulate, heal or regenerate cells in a deep organ. This is especially the case during transcranial photobiomodulation sessions, where we want to reach the brain. But can we really illuminate our brain by placing a photobiomodulation device on our skull? Can we do it in a non-invasive and painless way?

One might be skeptical when imagining this... So in this article, we will take stock of what has been scientifically demonstrated. Why does red and near-infrared light manage to penetrate our body? How far can it go? And finally, I will offer you a fun little experiment to reproduce at home.

A photobiomodulation device emits red or near-infrared light

To begin with, if you are not familiar with photobiomodulation , I invite you to discover this therapy by reading this page .

This very specific light is directly “absorbed” by our cells

During the therapy, we use a photobiomodulation device that emits a very specific red or near-infrared light. This light, which is a form of energy, is absorbed by an enzyme, called cytochrome c oxidase, present in the mitochondria of our cells. https://www.sciencedirect.com/science/article/pii/B9780128153055000038 .

So, this enzyme uses the energy of light to trigger a series of biochemical reactions, which are beneficial and energizing for our cells, organs and body.

The brain: an organ inaccessible to light?

In photobiomodulation therapy, one of the many challenges is to be able to reach a deep organ with this very specific light. It is very easy to illuminate the skin in order to stimulate and accelerate, for example, healing. https://link.springer.com/article/10.1007/s10103-017-2387-3 . But conversely, it is more complex to reach a deep organ... like the brain!

If we want neurons, as well as all the other cells in our brain, to benefit from the positive reactions associated with photobiomodulation, then we need to make sure that light can reach them. So before that, it will have to cross several natural barriers. Among them: hair, scalp, skull bones and blood.

What does science say about these different tissues and bodily fluids that protect our brain?

Hair: enemies of photobiomodulation devices

Our hair, a true vestige of our evolution, is a natural barrier to UV rays https://onlinelibrary.wiley.com/doi/abs/10.1111/php.12433 . Thanks to it, our skull stays cool, even on sunny days. But it is also a real barrier against red and near infrared light!

By the way, I already wrote an article about this. The conclusion was clear! To be able to illuminate the brain, there is no secret: go around the hair, or use bald areas (like the forehead)!

Scalp, blood, bones...

Once the hair is bypassed, the light produced by our photobiomodulation device will have to pass through other natural barriers before reaching the brain: the scalp, some fluids and the bones.
These biological tissues contain molecules that have the ability to absorb light. These are called chromophores . The main chromophores encountered by our light will be: water, body fat, enzymes, melanin, hemoglobin and collagen https://discovery.ucl.ac.uk/id/eprint/10104457 .

Fortunately for us, these chromophores have an optical window in which their light absorption capacity is lower!

The penetrating power of red and near infrared light

The absorption power of chromophores has been studied for several decades. Thus, we know that red and near infrared light will be less absorbed by different tissues, compared to other lights. If the light is less absorbed by molecules, then it can continue its path through the body to finally reach a deep organ.

If you look at the graph above, you will notice that the optical window (gray area) corresponds to red and near-infrared light. Using this information you can, for example, understand that green light will be much less penetrating (because it is more absorbed) than red light. You can also see that it is near-infrared light, with a wavelength of 810 nm , which is the most penetrating. So much for the theory... but what about the practice?

Studies conducted on humans

There are many studies that have already measured and validated the penetration of light emitted by photobiomodulation devices. Whether on animals, but also on humans. However, to verify the penetration on a human brain, the solutions are a little more limited... impossible to do it in vivo (on a living human).

Indeed, the brain is enclosed in its skull, its access is impossible. Nevertheless, researchers have been able to confirm this by using other techniques, such as medical computer simulation and post-mortem studies (on human cadavers). Below, I present some examples.

The so-called "Monte Carlo" simulations

In 2017, a Chinese team https://www.worldscientific.com/doi/abs/10.1142/S1793545817430027 created a model of a human head on a precision medical simulator (called Monte Carlo ). Each tissue of the brain was modeled, and its biological characteristics were entered. The team checked the penetration of light with different wavelengths. They concluded that red and near-infrared light could indeed penetrate the brain non-invasively.

"Our study showed that 810 nm was the ideal choice for transcranial photobiomodulation, for the broadest and deepest penetration (…) This allowed the light to pass through both the gray matter and the white matter (editor's note: of the brain)"

Ting Li - Institute of Biomedical Engineering Chinese Academy of Medical Science and Peking Union Medical College

In 2019, Doctor Paolo Cassano https://pubmed.ncbi.nlm.nih.gov/30796882/ , who I already told you about in a previous article , also carried out a medical simulation. With the help of his team, he confirmed that light could indeed reach the brain.

Also in the same study, he confirmed that light could reach the frontal lobe, using an intranasal photobiomodulation device.

Post mortem studies

It is difficult, if not impossible, to do this kind of study on human subjects in vivo . But researchers have been able to analyze the effectiveness of transcranial photobiomodulation devices on human cadavers. Here are some examples.

In 2012, an American team https://pubmed.ncbi.nlm.nih.gov/23077622/ was able to use an intact human head, with the various tissues still present on it. They confirmed that near-infrared radiation was indeed detected at a depth of 1 cm. In addition, according to their results, near-infrared light (830 nm) had better penetration than red light (633 nm).

It is worth noting that in this study, an LED photobiomodulation device was used. Despite its low power, compared to laser photobiomodulation devices, the light still managed to reach the brain. A patient using an LED device will not feel anything, just a little heat on the scalp.

In 2015, a second American team https://pubmed.ncbi.nlm.nih.gov/25772014/ also validated the penetration of light into the head of a human cadaver. Here, a very powerful laser photobiomodulation device (5 Watt) was used, and the penetration distance is enormous: between 4 and 5 cm deep!

Normally, you don't need that much power to benefit from the beneficial effects of photobiomodulation. I selected this example just to show you that great depths are achievable with near infrared light.

Our results clearly demonstrate the penetration of measurable 808 nm wavelength light through the scalp, skull and meninges to a brain depth of 40 to 50 mm" (…) 808 nm wavelength light demonstrated superior penetration of central nervous system tissues

Dr. Clark E. Tedford

A little experiment to see the penetration of red light

Now, here's a little experiment to illustrate the penetrating power of red light. Turn on your smartphone's flash and place your finger on it:

See how the tip of your finger turns red. What is causing this? Your first thought is probably that the white light from your flash is "illuminating" the blood. Well, no!

The white light from your flash is produced by a white LED. If we break this white light down through a prism, we can see that it is composed of a set of colors

So if your finger is all red, it is because its tissues manage to absorb all the lights, except the red one which has a greater penetrating power. The white light from the flash penetrates your finger, only the red light comes out and ends up in the retina of your eye.

Yes, a photobiomodulation device can illuminate the brain!

Yes, science has clearly demonstrated that red and near-infrared light can reach the brain. In this article, I have only offered you a few examples of studies. Be aware that the penetration measurements vary depending on the studies, ranging from a few millimeters to a few centimeters. But be careful, transcranial photobiomodulation is simple to understand but difficult to set up. Indeed, not all photobiomodulation devices can illuminate the brain. For example, a photobiomodulation panel, which can be easily found in stores, is ineffective for illuminating deep organs.

Here is a summary of this article in a few points:

Finally, I would like to point out that to succeed in brain photobiomodulation therapy, it is not all about penetration . It is also necessary to use a compatible device (whether LED or laser), the right power and calculate the exposure time in order to receive the right dosage of light.