See https://en.wikipedia.org/wiki/Energy_harvesting

For a while YOShInOn was showing me a lot of papers in MDPI journals where somebody made wearable device that had peizeoelectric crystals that harvest energy from the wearer's motion or remote sensor stations that are powered by raindrops

https://www.eurekalert.org/news-releases/996074

or sensor dust that captures power from WiFi emissions

https://pmc.ncbi.nlm.nih.gov/articles/PMC9460457/