[Sayantan Pal] did this for the humble RGB LED matrix, creating an extra-thin version by recessing WS2812b NeoPixel LEDs ... used PCB thickness. Using EasyEDA, [Sayantan] designed a 8×8 matrix ...

Look around for heart rate sensors that interface easily to microcontrollers, and you’ll come up with a few projects that use LEDs and other ... a pipe with a few piezoelectric sensors.