Sponge That Keeps Vegetables Fresh?
I have seen two sorts: a polymer felt-type pad and a natural clay version sandwiched between synthetic fabric. Both are meant to regulate humidity. Soak up excess moisture and expire it back as needed. 

Both claim to be anti-microbial in design. 
The clay, I suspect, is near in structure to montmorillonite which is micro-crystalline; actually bursts bacteria! The synthetic would be engineered to discourage growth of mold such as some sport socks do.

Neither need to touch veggies directly to function. Potatoes, I would place directly as I peel them anyway. Bamboo washcloth can be dampened and used as a cover over delicate greens similarly -bamboo is naturally anti-microbial.