Science of Laser Hair Removal in SLOW MOTION

Uh, we are driving to a very strange location somewhere I have never been, and it's because of the woman in the back seat. This is Dianna, 'Physics Girl.' Yeah, uh, we are gonna go get a consultation for something very special right now. It is... Look, I'm not gonna keep in suspense. Laser hair removal. We wanna talk about the science that makes laser hair removal work.

I told him about this idea last, like, probably a year ago, and then I was gonna do it in July or August. I've been waiting for this moment. Now I'm scared.

"Now you're scared?"

"Here we go."

"Oh, yes!"

"-Aw, it stinks, man."

"-It went directly into your nose."

"Yeah, it did. It's called a 'laser plume.'"

"-Laser plume? Is that the smoke that comes out?"

"-It's actually a word for it. That's the smoke, yeah. How was that shot?"

The laser we are using has a wavelength of 1064 nanometers, meaning it is infrared radiation, invisible to the naked eye. But without an infrared filter, the camera's sensor picks up the laser pulses, which in my case were six at a time, each one lasting about one and a half milliseconds. We dialed it up a little.

"We dialed it up just a tap though."

"-Nice."

"Ready? The laser will now deliver 25 joules of energy per square centimeter. That was beautiful!"

"Ohhhhhh, wow! Now we're talking!"

"Wow, oh my God."

"-That is insane."

"-There's five bursts, did you see that?"

"Yeah, it looks like, like, a shootout at the O.K. Corral. That's what that looks like. All the hairs are just getting, like, slaughtered. That is amazing! That is an awesome shot. That's really slick."

"I had no idea that it was a shootout at the O.K. Corral! That makes me really, kinda, reignites my excitement about my job. That's awesome! I'm gonna get even closer on this next one. That'll be cool."

So, what's actually going on here? Well, dark eyes, skin, and hair contain a molecule called melanin, and melanin absorbs a wide range of wavelengths of light, especially in the ultraviolet part of the spectrum. This makes it an ideal shield against the sun's harmful rays, which is why you get a tan when you go out in the sun. That's your body producing and releasing more melanin in the skin. But the idea with laser hair removal is to get the melanin in dark hairs to absorb the laser light without the skin around it absorbing much of that energy.

So, it actually works best if you have really dark hairs and light skin. Infrared lasers are commonly used because melanin absorbs in this part of the spectrum more than other common molecules in the skin like water and oxyhemoglobin. Red lasers, at 755nm, are also often used for the same reason.

As the melanin absorbs the laser energy, the hair heats up to well over 100 degrees Celsius, causing it to burn and vaporizing the water it contains, and this puffs the hair up a little bit like a Cheeto. It's that expanding water vapor. Looking closely at this shot, you can actually see the bubbles on the hair from the vaporization of water. But the objective is not to destroy the hair; it's to actually use the hair to destroy the germ cells in the follicle that produce the hair in the first place. So, it's kind of like using the hair to kill the hair.

This image shows a cross-section of three hair follicles following a laser hair removal treatment. The one in the middle contained little melanin and therefore shows no damage. The other two show the hair shaft has been destroyed, rupturing the inner root sheath and damaging the outer root sheath. Cells suffer damage when their temperature rises above 60 degrees Celsius. This is because at that temperature, the bonds in proteins and collagen molecules start breaking apart, and that's a process known as denaturation.

So, the hotter it is and the longer that temperature is maintained, the more denaturation occurs and the higher the likelihood that that cell will die. Now, if you cause enough damage to the cells in the follicle that make the hair, the hair will never grow back, and that is the goal of laser hair removal.

This sequence of ultrashort pulses is used to ensure that the heat doesn't spread too far into the region surrounding the hair. So, the idea is the hair heats up, damages the germ cells right next to it, but then the laser light turns off before that heat can spread too much further, and then you hit the hair with another very short pulse again.

If you were instead just to keep heating the hair, it would heat not only the germ cells around it, but also your skin that you don't want to damage, and you would end up with burns. So, that's the reason for the series of pulses that we saw.

But if you think about it, this is kind of a weird irony. I mean, melanin is a molecule meant to protect us, meant to protect our skin from harsh rays of sunlight, and that's why it absorbs that light before it can penetrate further into your body and do damage. But now with laser hair removal, we're using melanin - its unique absorption powers - against it.

By overpowering it with lots of laser energy, we get it to heat up the hair to such high temperatures that the hair kills the neighboring cells, the germ cells in the follicle that produce the hair. So, we are using melanin's powers against it. Normally, before a laser hair removal procedure, the hair is shaved, and that serves a few purposes. For one thing, not too much energy goes into burning the external hair, which does you no good in the first place, and second, it prevents the hot hair from landing on the skin and causing surface burns.

Well, we found that shaved patches were less interesting to watch because you can't really see what's happening under the skin. After the creation of the first working laser, in 1960, its inventor, Theodore Maiman, called it "a solution looking for a problem." An unwanted hair may just be that problem because laser hair removal is the most requested cosmetic procedure.

Hey, I have a lot of people to thank, who made this video possible, like Diana, "Physics Girl," whose idea this was. Thanks to her, I no longer have that strange patch of hair on my shoulders. So, you should really go check out her channel, Physics Girl. And I also wanna thank Darren. Without him at BeyondSlowMotion, we would never have been able to do this. He was simply amazing, so you should also check out BeyondSlowMotion.

And, of course, I want to thank Audible, the leading audiobook provider with hundreds of thousands of titles in all areas of literature, including fiction, nonfiction, and periodicals. Whenever I'm out, I'm on the go, and I can't really watch videos, like when I'm driving, then I listen to Audible. I also listen to Audible when I'm on the plane, and I'm about to get on a plane right now and fly to Svalbard, Norway, which is nearly the North Pole, and that is gonna be a very long flight, so I will definitely be listening to, uh, my audiobooks.

In fact, the audiobook I'm listening to at the moment is called Creativity, Inc. And I am learning a lot from this book about how to be a creative person, how to be productive, and it's a great book to listen to because it's a story told by Ed Catmull, and you kind of feel like he's there with you, telling you this story of how Pixar was founded and how they made creativity work. So, I'm loving that book.

If you wanna check it out, you can actually download it for free, or you can get a free 30-day trial by going to audible.com/Veritasium. You know, one of the reasons Audible keeps supporting Veritasium is because they know that the people who watch this channel like learning things, and another way to learn things, besides YouTube, is to listen to audiobooks.

So I highly recommend you give 'em a try for a 30-day free trial. If you wanna do that, go to audible.com/Veritasium. Thank you so much for watching.