The first ever Thermochroic (heat-sensitive) designs. They look black on the sheet but on contact with skin, begin to change through red, amber, green, turquoise and finally a brilliant Azure blue. The material changes colours within temperatures of 25-37 degrees Celsius, (77-98.6 degrees Fahrenheit) the Heartstrings are made to stand away from the skin so that you get the full spectrum of colours from cool to warm. We package them with extra adhesive dots for repeated use. The little heart outlines are UV Red, which glow under black lights.
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