From the paper:

> The ink used for the proof of extrusion demonstration is a ready-to-use, polyethylene oxide–based training bioink purchased and used directly from the vendor (Cellink Start, Cellink)

> The ink used for the honeycomb demonstration and the maple leaf demonstration is a sacrificial, temperature-sensitive, 40% (w/v) Pluronic F-127 in deionized water bioink purchased and used directly from the vendor (Pluronic F-127, Allevi).

> The ink used for the first cell-laden grid demonstration is Pluronic F-127 bioink with B16 cancer cells suspended in solution.

> The ink used for the second cell-laden grid demonstration is Pluronic F-127 bioink embedded with RBCs.

> The ink used for the cell viability experiments is Pluronic F-127 bioink with B16 cancer cells suspended in solution.

Aha, thanks! That makes a lot of sense.