Here’s another one of my favorites; – it could prevent many infections, even epidemics. I first found it here:
Tannic acid utilized to “stain” odor-eliminating silver onto clothing
which links to: Rapid assembly of colorless antimicrobial and anti-odor coatings from polyphenols and silver J.J. Richardson et. al. Nature Scientific Reports,2022
What it does is make any water-tolerant surface permanently antimicrobial by using an invisible film of tannic acid a few nanometers thick which incorporates a bit of silver. Tannic acid has been used with black iron in iron-gall permanent ink for over a thousand years. Experiments over the past 120 or so years, especially the last 20 years, have shown that tannic acid can glue any metal (salt) ion to anything; silver and its compounds have long been known to be antimicrobial; so putting the two together was a natural idea. The materials cost is only about $1 for 100 square meters, figuring 10 mL/sq m.
Braindump:
The secret formula is: 0.1 mg/mL AgNO3, 0.4 mg / mL tannic acid in distilled water, either mixed fresh and used for immersion or applied as two separate sprays, with the silver nitrate applied first. Mixed, it should last a few hours. If it starts turning blown, it’s gone bad - no guarantees, but it may still work. The first few molecular layers of coating forms instantly on any surface, including the surfaces of spray droplets, so separate application of the solutions is preferred for spray applications. I The surface should be clean or the dirt will get coated. It works best on textiles, especially natural fibers, but may be used on any surface that can tolerate cleaning with water-based solutions. A rinse is usually not needed for thinly applied sprays, but rinsing may prevent later discoloration, particularly of residual silver nitrate not incorporated into the coating. Residues of dried cleaning agents on the surface, particularly bleach, may cause the coating to turn black or brown. It seems to be resistant to discoloration from cleaning products, but long-term behavior is unknown. The coating can be removed quickly with concentrated bleach, or more gradually with lower concentrations. On smooth surfaces, high abrasion will remove it.
On very smooth or high-wear surfaces it will not be permanent, but may be reapplied. In a single application, the film thickness is self-limiting to a few nanometers, but repeated applications will lead to a buildup, which is essentially the same as tea scum or “varnish”.
The coating seems not to act by releasing silver ions, it chelates the silver, so its activity should last indefinitely in most circumstances. This chelation makes it rather environmentally safe, but silver nitrate in larger amounts if released directly in a stream could kill desirable microorganisms.
Dry silver nitrate is hazardous, an oxidizer that will cause skin burns if touched (it is sometimes used for cauterizing in medicine). In the concentration used here, it is extremely safe, and could be ingested without problems. (Not recommended.) Silver nitrate in much higher concentrations was mandated by law in many jurisdictions in the US to be dropped in all newborn babies’ eyes.
/end braindump
The paper’s author, J.J. Richardson was (and may still be) trying to get a patent on the process, but it may have been for Japan and/or Australia only. I sent him prior art from the 1890s, search for “Tannin (gallotannic acid) in alkaline solution reduces silver nitrate” in the linked article (provided courtesy of Rex Research, see first post above). J.J. was trying to market the stuff at a literal million percent markup as a t-shirt underarm spray. 
I tried to get a business off the ground making and marketing “Permanent Silver Invisible Ink Disinfectant” (“Silver Shield” would be better, but is somebody else’s trademark, unfortunately), but wasn’t able to raise any investment. There are many potential markets; I think most of the potential is in sales to institutions. I still have enough to make a couple thousand liters on hand. If anybody is interested, please contact me.