Researchers at Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have developed a method to 3D print mineralized constructs with a little helping hand from bacteria. The technique consists of printing a polymer bioink that contains bacteria that will produce calcium carbonate when exposed to a urea solution. The two-step process also involves printing a construct and then exposing it to urea for several days, over which time the bacteria naturally mineralize the structure. The approach allows researchers to accommodate some of the limitations of 3D printing, such as flow requirements for bioinks, but still create a rigid mineralized structure that could pave the way for bone implants.
Bioprinting is going from strength to strength, with enormous potential in creating custom tissue implants to treat a variety of conditions. The printing process allows researchers to print live cells other biocompatible matrix materials to create a living construct. While the technique has enormous promise, some tissues are more challenging than others to print.
First off, bioinks must adhere to several basic characteristics in order to be printed effectively. “3D printing is gaining increasing importance in general, but the number of materials that can be 3D printed is limited for the simple reason that inks must fulfill certain flow conditions,” said Esther Amstad, a researcher involved in the study. “For example, they must behave like a solid when at rest, but still be extrudable through a 3D printing nozzle — sort of like ketchup.”
This flow requirement makes it more difficult for the final printed product to be rigid, which is a limitation when printing bone. In the past, researchers have tried to include small mineral particles within the ink, but the resulting printed constructs were often too soft or suffered other mechanical issues, including shrinking and cracking.
“So, we came up with a simple trick: instead of printing minerals, we printed a polymeric scaffold using our BactoInk, which is then mineralized in a second, separate step,” said Amstad. “After about four days, the mineralization process triggered by the bacteria in the scaffold leads to a final product with a mineral content of over 90%.”
The bacteria within the ink are called Sporosarcina pasteurii and will produce and secrete calcium carbonate when the researchers expose the printed construct to a urea-containing solution. However, the robust constructs can be decontaminated after this mineralization process by soaking them in ethanol, ensuring that the bacteria do not make it into the patient.
Study in journal Materials Today: 3D printing of living structural biocomposites