Printing cells with a splash. For printing biological cells, their careful yet fast transport is necessary, i.e. typically they are transported from a liquid-filled open container onto a hydrogel-covered substrate. Here, short pulse and focused lasers are great tools to manipulate the liquid, partly because, at already very low pulse energy, they can induce a phase transition. The impressive study of Zhang et al. adds a new twist to the quest of how to print cells using such lasers. While they showed two years ago that the cells are not harmed by the method, they demonstrate here that the precise positioning of the laser focus makes all the difference. By adjusting the distance between the laser focus and the free surface of the liquid, their technique covers smoothly the range from single cell precision printing to depositing many cells yet with a larger spread. This is due to the fluid mechanics: the laser focus creates a small vapor bubble below the surface that explosively expands and ejects the cells from the container with a thin jet. As long as the flow is mostly laminar, and the jet breaks up due to surface tension, precise printing is possible. Shortening the distance too much results in a more splash-like flow that voids the precision.

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