The method could be used to restore damaged artworks © Eva Baur.
The method could be used to restore damaged artworks © Eva Baur.

When organic-inorganic composites are constructed in nature, two plus two can add up to more than four where the mechanical properties of the composite exceed those of the component parts. The secret to nature’s success lies in the ability to control the structure and local composition using compartmentalization of reagents. Inspired by nature, researchers from École Polytechnique Fédérale de Lausanne and the University of Cambridge have devised an energy efficient process that uses bacteria to produce bone-like porous CaCO3-based composites [Hirsch et al., Materials Today (2023), ].

“We took advantage of the rheological properties of soft hydrogel-based microparticles, so-called microgels, [to 3D print] lightweight porous minerals,” explains Esther Amstad, who led the research.

When microgels are densely packed, they possess ideal properties for direct ink writing 3D printing or injecting. However, the resulting 3D structures are difficult to mineralize fully after printing, typically resulting in a hard shell around a soft core. To solve this problem, Amstad and her team added a benign urease-producing bacteria, Sporosarcina pasteurii, which is omnipresent in soils, to the microgels. When exposed to a urea-containing solution, the bacteria produce the enzyme urease, which catalyzes the hydrolysis of urea into ammonia and carbonic acid. Placing the bacteria-infused microgel in media containing Ca2+ and urea triggers the initiation of the mineralization of the scaffolds. Since bacteria are distributed throughout the scaffold, the entire soft structure becomes mineralized. The bacteria partially consume the scaffold, resulting in an exceptionally high mineral content.

“The bacteria efficiently mineralize the scaffold and only the scaffold,” says Amstad. “We obtain mineral-based materials whose 3D structure is identical to the polymeric scaffold that we initially printed yet, because the structure contains up to 93 wt% CaCO3, are much harder and stiffer.”

The porosity of the composite resembles that of human trabecular bone with mechanical properties that can be matched to load-bearing applications. The easy and versatile process enables 3D printing of structures on the centimeter scale. The ink, which the researchers dub ‘BactoInk’, could be useful in the restoration of ceramic artefacts or serve as an artificial support for the regeneration of corals. With additional refinement, the biocomposites have the potential to help in the repair of damaged or degraded bones.

“We envisage replacing the bacteria with other carbonate-containing or producing entities, which do not generate ammonia as a side product, to render the ink biocompatible,” says Amstad.