Those dreams of having computers in your clothing might be more realistic than you think. Japanese researchers have developed a printable conductive ink that maintains a circuit even when you stretch fabric to three times its usual length -- you could have athletic gear with hidden activity trackers, sensors and other computing devices. The key is a careful mix of fluorine, an organic solvent and silver flakes which, when combined, keeps transmitting electricity even under heavy abuse.
The current prototype for the ink, a wristband that tracks muscle movement, is pretty crude. You'd need much smaller circuitry before your apparel replaces your step counter or smartwatch. However, it only takes one step to print the ink. As such, it'd be relatively easy to produce on the large scales you need for shirts and wristbands. Smart fitness clothing already exists, but this invention would make it both more commercially viable and a heck of a lot more comfortable.