Might as well use this thread to remind people that thermal print paper is covered in BPA which is a toxic chemical. Don't touch receipts with your bare hands.
https://www.consumerreports.org/cro/news/2014/03/the-health-risk-of-bpa-in-receipts/index.htm
>Can't we print by blackening the paper using powerful IR or UV LEDs? Or maybe a very fine arc of high voltage. There has to be something
That's called carbonization and there's a company called Inkless that already has developed this technology. Don't expect it to come to market anytime soon though.
Maybe in a few years a typical desktop laser engravers will be able to burn the paper precisely enough to only char the surface so you can build you own. We'll then finally have a free hardware design printer.