Viscosity is a vital but underrated property of everyday substances. Toothpaste wouldn’t work if it dripped through the bristles; it needs to form a pleasing, stable dollop. Custard should be moderately thick but not gloopy; and glacé icing just fluid enough to creep, lava-like, down the summit of a cupcake before solidifying.


Viscosity, defined as the measure of a fluid’s resistance to flow, was central to a striking development announced last week. Scientists in Switzerland have made a 3D printer capable of using “living ink”. It is a blend of several ingredients: a sugar-containing hydrogel base, which forms the structure of the ink; bacteria, which is the living component; and a culture medium for keeping the bacteria alive. The researchers named their creation “flink”, shorthand for functional living ink. By using different bacteria, the flink printouts can be tailored for specific use. One bacterium, Acetobacter xylinum, secretes a pure form of nanocellulose, a stable material that retains moisture and can be used to treat burns.


The new printing platform was created by a team led by professor Andre Studart, from the Laboratory of Complex Materials at the Swiss Federal Institute of Technology in Zurich. His team also developed a flink containing the bacterium Pseudomonas putida, which can break down phenol, a common industrial pollutant. The phrase “strong and stable” could have been invented for flink: it is fluid enough to squirt through a nozzle yet solid enough to support the layers grafted on top of it. The winning formula, according to its inventors, needed to be “as viscous as toothpaste and have the consistency of Nivea hand cream”.