Blast from the Past

In the late 1980s, a company called Generra introduced a line of clothing that used thermochromic dyes. The company called the line Hypercolor. Schools across the United States were filled with students wearing the popular shirts and hats, most of them bearing a collection of handprints where other kids left their temporary marks.

Thermochromic Fabric Displays

The word "thermochromic" looks a little intimidating at first, but the concept itself is pretty simple. Thermo comes from the Greek word "thermos," which means warm or hot. Chromic comes from "chroma," meaning color. A thermochromic substance changes color as it changes temperature. In fabrics, a special dye acts as the thermochromic agent.

Some thermochromic dyes change from colorful to clear, revealing the color of the fabric underneath. Companies can use thermochromic dyes in shirts that slowly reveal a company slogan or logo as the shirt heats up. When the shirt cools down, the logo seems to disappear.

There are two widely used elements in thermochromic dyes, and both rely on chemical reactions:

  • Liquid crystals: These thermochromic dyes rely on liquid crystals contained in tiny capsules. The liquid crystals are cholestric, also known as chiral nematics, which means that its molecules arrange themselves in a very specific helical structure. These structures reflect certain wavelengths of light. As the liquid crystals heat up, the orientation of the helices changes, which causes the helices to reflect a different wavelength of light. To our eyes, the result is a change in color. As the crystals cool down, they reorient themselves into their initial arrangements and the original color returns.
  • Micro-encapsulate thermochromic system: In this system, the thermochromic dye contains millions of tiny capsules that look a little like an organic cell. Each capsule has an outer membrane and contains an organic, hydrophobic solvent, which makes it less likely that water will dilute or wash out the chemicals in the dye. The solvent contains particles of a color developer and a dye precursor. As the capsule heats up, the solvent melts and a chemical reaction causes the color developer to donate a proton to the dye precursor. In turn, this causes the precursor to develop into the dye itself and change color. When the dye cools down, the developer and precursor separate, the solvent resolidifies and the color returns to its original state.

Like fur fabric displays, thermochromic fabrics aren't animated -- they can only conceal and reveal designs or colors based on environmental conditions. While that might be enough for some people, others want even more dynamic clothing.

In the next section, we'll look at a technology that turns normal clothes into wearable neon signs -- electroluminescent fabric displays.