Thermochromic Ink Circuit with Soft Pushbutton
Conductive thread stitched to make the circuit and heat the thermochromic ink which disappears when it reaches the right temperature. The voltage needs to be about 9v to cause the thread to heat, but be careful not to leave it on for too long as the thread can burn. You can make heat controlling circuits with a microcontroller to switch the the circuit on and off at intervals so it doesn’t overheat but. For simplicity I’ve used a soft pushbutton made with conductive tape and wadding with a .
I used thermochromic pigment from proto-pic mixed with acrylic base. Maplin also sell it online in readymixed form as well as SFXC who have a wide range some of which change colour rather than going transparent.
The thread is from cool components - it is worth testing out different threads, they need to have a certain amount to resistance in order to heat up.