To find out, I asked Wiendelt Steenbergen, Professor of Biomedical Optics at the University of Twente. He knows all about how light interacts with human tissue.
What does the laser do to tattoo ink?
A laser doesn’t literally remove the ink from your skin. Tattoo ink consists of tiny pigment particles. During treatment, these particles are hit by extremely short, powerful pulses of light. “The aim is to give the pigment particles such a shock that they break apart,” Wiendelt explains. So, the laser breaks the ink into smaller and smaller pieces.
Then your own immune system gets to work. Immune cells called macrophages act a bit like your body’s rubbish trucks. They clear up the ink particles by effectively ‘eating’ them. Some of this material can then be carried away through your lymphatic system.
So, the laser doesn’t make your tattoo disappear straight away. It breaks the ink particles into smaller pieces, after which your body gradually tries to clear them away. This takes time and helps explain why you usually need several treatments.
Why do you need several treatments?
Not every tattoo is the same. Ink particles can vary in size and colour, and the ink can sit at different depths in your skin. This means there isn’t a single laser treatment that works exactly the same way for every tattoo.
For a start, the size of the ink particles matters. The laser uses very short pulses of light. Some last only a few nanoseconds, or billionths of a second. Other pigment particles may even require picosecond pulses. These are another thousand times shorter. Which pulse works best therefore depends partly on the size of the ink particles.
The colour of your tattoo matters too. Different colours of ink absorb different colours of light. Red pigment, for example, mainly absorbs blue and green light, while green pigment mainly absorbs red light. Different ink colours may therefore require different colours of laser light.
Got a tattoo with several colours? Then things quickly become more complicated. In theory, you can choose the most suitable laser light for each colour. In practice, however, practitioners only have a limited number of laser types available. Your own skin also contains pigment that can absorb light. Ideally, you want to target the tattoo ink while leaving your own skin as unaffected as possible.
Finally, it matters how the tattoo was applied. According to Wiendelt, deep, professionally applied tattoos usually require more treatments than tattoos where the ink sits less deeply in the skin. And even after several treatments, a tattoo may not disappear completely. “All in all, laser treatment is a very simple approach to quite a complicated problem,” says Wiendelt. So, success isn’t guaranteed.
Why doesn’t the laser burn your skin?
If a laser is powerful enough to break apart ink particles, why doesn’t it damage everything around them too? There’s a clever principle behind this. The laser is designed to target the ink as much as possible while sparing the surrounding skin. The colour of the laser light helps, because some pigments absorb certain colours of light better than others. But just as important is how short the light pulse is.
A pulse can last only a billionth or even a trillionth of a second. In that extremely short time, the ink particle absorbs a large amount of energy. This gives the heat very little time to spread to the surrounding tissue.
This principle is called selective photothermolysis. It sounds complicated, but the name actually describes exactly what happens. Selective means that you mainly want to target the pigment, photo refers to light, thermo to heat and lysis to breaking down structures.
According to Wiendelt, with such extremely short light pulses, forces also play a role alongside light and heat. So, behind those few quick flashes from a laser machine lies a surprising amount of physics!
So how do you remove a tattoo with a laser?
A laser doesn’t simply erase your tattoo or remove the ink directly from your skin. Instead, extremely short pulses of light break the pigment particles into smaller pieces. Your immune system then helps to clear those pieces away.
How well this works varies from tattoo to tattoo. The size and colour of the ink particles matter, as do the depth of the tattoo and the pigment in your own skin. That’s why several treatments are usually needed, and there’s no guarantee that a tattoo will disappear completely.
Pretty remarkable, really. To get a tattoo, you mainly need a needle and some ink. To get rid of one, you need pulses of light lasting billionths or even trillionths of a second, quite a bit of physics and the rubbish trucks of your own body.
And perhaps in the future, you won’t even need a needle. Watch Kees visit UT researchers to find out whether it’s possible to get a tattoo without a needle.
