[1][2][3] It was first used in the 1970s in novelty toys like mood rings, but has found some practical uses in things such as thermometers, product packaging, and pens.
At lower temperatures, the crystals are mostly solid and hardly reflect any light, causing it to appear black.
When pressed with a finger, body heat turns the black circle in the center of the stamp into an image of the full moon.
The stamp image is a photo of a total solar eclipse seen in Jalu, Libya, on March 29, 2006.
In medical training, thermochromic ink can be used to imitate human blood because it shares its color changing property.
It's important to accurately identify this change in order to safely and correctly operate the ECMO machines.
This has led to simulation-based trainings (SBT) which allows medical students to run simulations that mimic real ECMO machines before using them in serious situations.
By using thermochromic ink in these simulations, the color changing effect can be realistically copied and observed without using real human blood or other costly methods.