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?