Another idea was to impregnate the cotton pads with anti-microbial chemicals, let them dry, then use them in the mask. When breathing, the the humidity in air would transfer to the chemicals, making them humid and active. I considered the following:

• Methylene Blue
• Rivanol
• Betadine

Sadly, all these are slightly toxic when inhaled, so it’s definitely not an option for a face mask filter. But then Ionut sent me this link in Nature.com.

The study shows that a saline solution can act as antimicrobial agent for a similar scenario – single use masks. The filters can be soaked in a saturated saline solution, left to dry, then inserted in the #masca3d filter container. The exhaled air will reactive the hygroscopic salt crystals and enable them to kill viruses. Other salts can be used as well and may have a better yield (Sodium bicarbonate).