What's the meaning behind S.A.D. and Smiles? From the Punk Hazard arc, we have learned that Vegapunk had created a unique substance called S.A.D., which is one of the few things needed to create Smiles, artificial Zoan Devil Fruits.

So basically we have (a joker) using sadness to create smiles. I doubt Oda named both objects that way accidentally. So does anybody know if Oda has ever mentioned what the symbolism is behind S.A.D and Smiles?
The substance is just "SAD" not "S.A.D". Considering that Caesar Clown is the only person in the world who knows how to manufacture it, and he uses it to create artificial fruits that he calls "smiles", it was just an appropriate name to fit into the theme. Clown, smiles, sad, etc.