A standard respirator, like a disposable N95 or an elastomeric P100, uses the power of the wearer's lungs to move air through a filter. An unpowered device has a lot of advantages in simplicity and reliability, but it can also be tiring, limits filtering capacity to what the wearer can tolerate, and requires a good seal on the face to prevent air intrusion during inhalation. An alternative approach is a Powered Air-Purifying Respirator (PAPR), which typically uses a battery-powered fan to push air through a filter.
A high-quality PAPR like the ones in the 3M Versaflo series will cost you >$1k, though you can get dubious cheap ones for ~$160.
But what if we want to go even farther in the "dubious" direction? Could we us a tiny fan to pull air through a filter and pipe it through hole in a KN95? I guess!
A bunch of companies make these for about $40. This one is a Bwinmak Resnr R18, but you can also buy them from other companies you haven't heard of, like OFG or 4WDKING.
I got one, and tried it for a bit. It's documented as producing 2-4 m³/h depending on speed, and with it running on high I had trouble breathing in faster than it could fill the mask with air. This wouldn't be suitable for an environment where a PAPR is a requirement, but it's probably better than a disposable mask, especially if you have facial hair or otherwise aren't getting a good seal.
The biggest issue I had was with the hose. I wore it on the provided neck strap and if I got it adjusted correctly and stood still it worked fine. As I moved my head relative to my body, however, primarily when looking up or down, it worked less well. When I looked up the hose would go taut and I'd be supporting the weight of the filter with my mask. The filter is pretty light, but this would still tend to pull my mask down and off my face a bit. Looking down was worse: the hose would kink, and I'd get much less air. You could imagine using a longer and more robust hose, but since the hose is held up by your mask I think this wouldn't work well. Instead, something where the filter was mounted on a hat could be good, and it comes with a diagram showing how you could velcro it to a hard hat:
I learned about this from Dan Elton's picture of George Church wearing one:
I didn't find mine useful enough to keep, and gave it away in the office Yankee Swap (it comes with several masks, and you can make more by putting a 12-14mm hole in a regular mask). I usually only have trouble breathing with a mask when I'm dancing energetically, but this isn't stable enough for that. And if I needed the greater protection benefits I'd use an elastomeric (which I suspect is quite a bit better, even with my facial hair). Overall, I suspect these are most useful for people who have a hard time breathing through a high-filtration mask in non-active situations.