User blog comment:Xilinoc/Ch. 593 - Marching Out the ZOMBIES 4/@comment-29449108-20140820150921/@comment-5398287-20140820215259

Wouldn't surprise me if he helped heal Hitsugaya after FKT (when he was missing limbs and stuff) and took the time to put some drugs in his body that Mayuri could activate should he ever turn against the Seireitei.

That would be outlandishly Batman-y for any other character, but for Mayuri, it's just another day in the life of a madman.