Swiss researcher prints concrete-like material bound by bacteria

A bath, a printer, and a handful of bacteria. That is the workshop Karen Antorveza Paez used to make architectural parts without a single kiln. The mixture looks like wet sand, and it is still soft when it comes out of the printer. Only later does it become solid, because the bacteria do the work that cement normally does.

The organism is Sporosarcina pasteurii. When it is combined with sand or other grains, it joins them into a solid material by forming calcium carbonate, the mineral that makes up limestone. The result is a mesh of microscopic, interlocking calcite crystals. Antorveza Paez describes it simply: "We're using bacteria to produce a sort of mineral glue." The process, called biocementation, had already been used to stabilise soils and in self-healing concrete. Her work applies it to printed building parts.

In the first method, the team prints a "bio-ink" made of a gel of water, naturally occurring alginate and xanthan, sand and live bacteria. The object is then immersed in a bath of urea and calcium chloride. The bacteria break down the urea with an enzyme called urease, which releases carbonate ions. Those ions combine with the calcium to form limestone between the grains. The whole process takes two to five days, depending on the strength wanted, and the bath is renewed every day. Antorveza Paez compares it to baking: "You've got the dough, which is still very malleable and delicate. Only the oven can turn it into something solid," with the bacteria playing the role of the oven.

The second method drops the bath. A robotic arm sprays bacteria, the mineral ingredients and sand onto a surface, layer after layer. The printed method suits architectural elements, while spraying is aimed more at surfaces, according to the researcher. Both have been used to make parts, including a facade element, and the material could also serve for shade structures and partition walls.

What changes for builders is still a question of time and testing. The material cannot yet bear loads, and more research is needed to make it load-bearing. The spray method also requires study of how the grains and binding medium are arranged inside the object. The researcher's own summary of the next step is: "The big question is no longer whether biocement works, but rather how well its properties can be managed." If those properties can be controlled, a binder that needs no heating could one day reduce the emissions linked to cement, but the sources do not yet say when, or for which applications first.