design an automatic hand washing system

I want to make a default sanitary hygiene system.
In particular, I want to develop a hydrogel dispenser that can be attached to a doorknob, so that when the knob is operated it is “necessary” to press the hydrogel and wash your hands.
I am open to design options, although I would prefer it to be a dispenser external to the knob itself, so it can be easily put on or taken off.

Ideally would love for you to manufacture as well