Couple of questions come to mind:

1. Are we certain this is graphene oxide and not some other compound? Graphene oxide is carbon, hydrogen, and oxygen. There are no metallic elements in that compound and yet this is clearly showing magnetic properties. Ink can have iron mixed in the complex mixture that makes up ink. We need to be specific about what we're dealing with.

2. These are large pieces, visible to the naked eye. Typically, when I read about toxicities of GO, it involves nanoparticles. So, then I need to know have they prepared the ink in such a way to generate nanoparticles (this takes considerable effort, as I understand it)? And does the body absorb them through the lining of the GI tract? We eat a lot of things that don't get absorbed and just pass through the system into the feces. It's not a trivial question.

I don't know what precisely I'm looking at here, so I'm not going to start jumping up and down until I have a better idea of what this is and whether it is in fact harmful.