https://www.youtube.com/watch?v=5VNZ4ol0wqs
Allowing white-rot fungi to partially decay wood for several weeks softens the cellular structure so that repeated compression — like footsteps — produces a measurable piezoelectric voltage.
Lab cubes of treated wood have yielded on the order of 0.85 V per ~15 mm square, enough in principle to trickle-charge low-power sensors embedded in floors without batteries.