Qraft

The Science of QR Code Tattoos - Why They Become Unreadable Within a Few Years

Last updated: 4 min read

The Trend and the Reality of QR Code Tattoos

Tattoos that put a QR code on the body are said to have appeared in various parts of the world from the late 2010s. As a "living link" that displays your social media profile, portfolio site, or a message to your partner when scanned, they gained popularity mainly among a younger generation with a strong affinity for technology.

However, it is said that few QR code tattoos actually remain scannable 2 to 3 years after they are done. Among tattoo artists, the understanding has spread that "a QR code tattoo is something to enjoy for its visual impact, and you should not expect it to function." Why do QR code tattoos become unreadable? The reason lies in the characteristics of human skin as a "printing medium."

Skin Stretching and Module Distortion

For a QR code to be read correctly, the modules (the smallest black and white units) need to be arranged in an accurate grid. The reader estimates the position of the grid based on the finder patterns in three corners and judges whether each module is black or white.

The rate at which human skin stretches and contracts differs greatly depending on the part of the body. Skin near the joints (elbows, knees, wrists) stretches and contracts greatly every day, and the tautness of the skin also changes as body weight goes up and down. This stretching makes the spacing between the tattoo's modules uneven, and the reader can no longer estimate the position of the grid correctly.

The stiffness of the skin (elastic modulus) differs by body part, and a 1980 biometric study (Agache et al.) reported that skin elasticity declines after the age of 30 and that Young's modulus doubles with age. Because of the physical properties of skin, it is unavoidable that modules which formed a perfect grid when the tattoo was done become irregularly distorted a few years later.

Ink Spreading - The Physics of "Blurring"

Tattoo ink is injected into the dermis layer of the skin (below the epidermis, at a depth of about 1 to 2 mm). Right after injection, the ink particles are densely packed, but over time the particles spread into the surrounding tissue and the lines blur. This phenomenon is called "tattoo blowout," and it appears especially noticeably in designs made up of thin lines.

A QR code is exactly "a collection of thin lines." The smaller the module size, the more susceptible it is to the effects of ink spreading. When the ink of adjacent black modules spreads and encroaches on the area of the white modules, the reader can no longer distinguish the boundary between black and white.

The speed at which tattoo ink spreads depends on the ink particle size, the injection depth, the water content of the skin, the amount of exposure to ultraviolet light, and so on. The particle size of tattoo ink differs by color, and a 2011 survey of particle sizes (Høgsberg et al.) reported that the majority of black pigment consisted of nanoparticles smaller than 100 nanometers. The smaller the particles, the more easily they tend to spread. To extend the life of a QR code tattoo, using ink with a large particle size, making the module size large, and avoiding ultraviolet light are effective in theory, but even so, permanent readability cannot be guaranteed.

How Much Can Error Correction Save

According to the published specifications of the developer Denso Wave, QR code error correction level H can restore damage of up to about 30% of the codewords. In the case of a tattoo, this 30% margin has to deal with both "distortion from skin stretching" and "blurring from ink spreading."

The problem is that this deterioration occurs uniformly across the whole QR code. Ordinary dirt and damage are local (only part of the code is damaged), but skin stretching and ink spreading affect the whole code. Reed-Solomon codes have the characteristic of being strong against local damage but weak against overall distortion. A state in which all the modules are slightly shifted is more troublesome for the reader than 30% local damage.

Furthermore, if the finder patterns (the position detection patterns in three corners) are distorted, the reader cannot recognize it as a QR code in the first place. The finder patterns are not covered by error correction, and if they break down, there is no room for restoration.

If You Still Want a QR Code Tattoo

There are best practices for when you understand the scientific constraints and still want a QR code tattoo.

  1. Choosing the body part: Choose a part with little stretching (the outside of the upper arm, between the shoulder blades, the outside of the calf). Avoid areas near the joints and the abdomen
  2. Make it large: Secure at least 8 cm square. The larger the modules, the higher the resistance to ink spreading and skin stretching
  3. Error correction level H: Choose the highest level of error correction to maximize the margin for deterioration
  4. Dynamic QR code: With a dynamic QR code whose link destination can be changed, you can redirect to another URL even if the service at the link destination ends
  5. Short URL: The less data stored, the fewer the modules, and the larger each module can be made. Make use of a URL shortener

However, even with these measures, it is difficult to guarantee readability for 5 years or more. It is realistic to think of a QR code tattoo not as "a link that works forever" but as "interactive art that can be enjoyed for a certain period."