  Fully exploiting the properties of 2D crystals requires a mass production
method able to produce heterostructures of arbitrary complexity on any
substrate, including plastic. Solution processing of graphene allows simple and
low-cost techniques such as inkjet printing to be used for device fabrication.
However, available inkjet printable formulations are still far from ideal as
they are either based on toxic solvents, have low concentration, or require
time-consuming and expensive formulation processing. In addition, none of those
formulations are suitable for thin-film heterostructure fabrication due to the
re-mixing of different 2D crystals, giving rise to uncontrolled interfaces,
which results in poor device performance and lack of reproducibility. In this
work we show a general formulation engineering approach to achieve highly
concentrated, and inkjet printable water-based 2D crystal formulations, which
also provides optimal film formation for multi-stack fabrication. We show
examples of all-inkjet printed heterostructures, such as large area arrays of
photosensors on plastic and paper and programmable logic memory devices, fully
exploiting the design flexibility of inkjet printing. Finally, dose-escalation
cytotoxicity assays in vitro also confirm the inks biocompatible character,
revealing the possibility of extending use of such 2D crystal formulations to
drug delivery and biomedical applications.
