Here is one of those things you make every effort not to think about: paper money and coins go hand to hand, passing from person to person and as a result are really, really dirty stuff. 

The use of paper currency has lost much of of its proeminence due to plastic currency and other digital payment methods, but it is still required on certain situations.

The idea here is to devise a currency that, somehow, remains germ-free after use. 

On the same line of this idea, we could build a desinfecting machine that could help to keep money safe, instead of of as disease vector. What technology to employ to sterilize money, that would no damage the bills? 

I know that are people around the world working to make these machines real.