3D-Printed 'Living Ink' Could Lead to Self-Repairing Structures

A scarcity of organ donors, from lungs to livers, is one of the driving forces behind an intense research effort into 3D printing as a future source of synthetic human tissue.

Despite advances in 3D printing, creating arbitrary shapes and patterns remains difficult. However, now researchers set out to develop "microbial ink," which is made of the entirety of genetically engineered microbial cells, programmed to perform bottom-up, hierarchical self-assembly of protein monomers into nanofibers, and further into nanofiber networks that comprise extrudable hydrogels.

A team of researchers from Harvard University and Brigham and Women's Hospital, Harvard Medical School, has developed a type of living ink that can be used to print living materials. 

The group describes how they made their ink and potential applications in a paper published in Nature Communications. 

Scientists have combined the benefits of the vexing tool and the toxic microbe to create an ink that is alive and entirely composed of microbes. The microbial ink flows like toothpaste under pressure and can be 3D printed into various tiny shapes. To test the microbial ink's ability to hold its shape, the researchers printed it in a variety of shapes and patterns, including a lattice grid, a box, a ring, and a cone that looked almost like an icicle, all of which retain their shape and glisten like Jell-O.

Researchers have created this new living ink by replacing the polymers with a protein produced by genetically modified E. coli bacteria. They genetically engineered bacteria to make two different versions of this protein known as a "knob" and a "hole," which lock together to form a robust cross-linked mesh. This stiffens the material enough to print while still allowing it to flow from the 3-D printer's nozzle.

According to Neel Joshi, a synthetic biologist at Northeastern University and an author on the new paper, a material like microbial ink has broader ambitions. Such inks are a growing area of interest in engineered living materials. Unlike concrete or plastic structures, living systems would be autonomous, adaptive to environmental cues, and capable of regeneration — at least, the aspirational goal, according to Joshi.

"Imagine creating buildings that heal themselves," Joshi said.

The material is still being developed, but the authors believe it could be a critical renewable building material, capable of self-growth and healing, and ideal for building sustainable homes on Earth and in space.

It is not the first time a living ink has been discovered. Previously, scientists created printable gels that were bacteria and polymer cocktails that helped provide structure when printed. One such ink contained hyaluronic acid, a seaweed extract, and fumed silica, all of which thickened and viscous the material.

However, the new substance does not contain additional polymers; it is made from genetically engineered E. coli bacteria. The ink, which is also made of living bacteria cells, is grown in bacterial cultures by the researchers. When the ink is extracted from the liquid culture, it hardens like gelatin and can be plugged into 3D printers to be printed into living structures that do not grow further and remain in their printed forms.

The ink still needs a lot of work. It can't withstand drying out and isn't yet stable enough to be the sole foundation of larger structures. Researchers are working on ways to make more robust printed structures.