What startups are working on hard, technically challenging problems?

Niron magnetics[1] is a startup trying to scale production of Fe16N2 magnets up to commercial scale. If successful, they'll end up with magnets that compete favorably with Neodymium magnets at elevated temperatures, without the need for rare earths. I hope they succeed in their quest.

Genode Labs[2] has been slowly pushing capability based security towards the masses, with their Genode project. Currently their focus seems to be on smartphones. I'm hoping one day to run Genode as my daily driver.

IVO Limited[3] might have a new type of thruster for outer space... they've got a payload due to get a ride share to orbit in a few days, and should know if it works before the end of the year. I remain hopeful that it works, but as it's based on a theory of science that isn't mainstream (Quantized Inertia)[4], I await evidence. I understand a bit of the math, enough to get a hint of where it's coming from, but I'm not a physicist, and I don't play one on TV.

[1] https://www.nironmagnetics.com/

[2] https://www.genode-labs.com/

[3] https://ivolimited.us/

[4] https://quantizedinertia.com/about/

One that comes top of mind is Swedish "startup" H2 Green Steel (https://www.h2greensteel.com/). They're building a steel plant powered by a giga-scale electrolyser to produce hydrogen (rather than using coal).

Comma.ai is a retrofit driving assist system that supports many models of cars. Computer vision and open source.