Lauren Bowker's UK-based firm The Unseen is currently working on a bunch of cool thermochromic textile and dye applications, like this hair color that responds to heat.
They are taking it around to manufacturers who could make it more widely available. Their site is heavy on the mystery and light on the details, but here are the basics:
The F I R E dye is responsive to you and your environment by changing colour to passing temperature fluctuations. Available in multiple colour ranges from bright red to subtle pastels, launching at London Fashion Week in collaboration with Storm Models. The application of the dye is currently semi-permanent lasting over a few washes.
They have some additional clips here.
• THEUNSEEN FIRE (Vimeo / T H E U N S E E N)
