3D-printed 'living ink' is full of microbes and can release drugs

An ink made using engineered bacterial cells can be 3D-printed into structures that release anti-cancer drugs or capture toxins from the environment.

The microbial ink is the first printable gel to be made entirely from proteins produced by E.coli cells, without the addition of other polymers.

“This is the first of its kind… a living ink that can respond to the environment. We have repurposed the matrix that these bacteria normally utilise as a shielding material to form a bio-ink,” says Avinash Manjula-Basavanna at the Massachusetts Institute of Technology in Boston.

By embedding another kind of genetically modified E.coli within the gel, Manjula-Basavanna and his colleagues built living structures that either released the anti-cancer drug azurin or captured the toxin bisphenol A (BPA) from the environment. BPA is commonly used to make plastics and has been linked to infertility and cancer.

The researchers made the ink from protein polymer molecules called curli nanofibres. First, they genetically engineered E. coli cells to produce subunits of curli nanofibres that had one of two oppositely charged modules, known as either a “knob” or a “hole”, attached to them. By growing a mixture of the two types of cells, they produced curli fibres that crosslinked with each other when the knobs from one fibre locked into the oppositely charged holes from another fibre.

The team then filtered the bacteria through a nylon membrane to concentrate the crosslinked fibres, before removing the cells from the mixture. This produced a gel that had a suitable viscosity and elasticity for printing.

The gel could be piped through a nozzle to produce threads around half a millimetre wide. Despite the narrow width of the fibres, they were strong enough to hold together without breaking when stretched between two pillars 16 millimetres apart.

“I remember that moment when it bridged this gap and I was screaming and jumping,” he says.

By genetically modifying additional E.coli to produce azurin in the presence of a chemical called IPTG, then seeding these cells into the gel, the researchers found that they could turn the gel into a living structure that releases azurin on demand.

They continued their experiments by engineering another population of E. coli to produce curli subunits that could bind to BPA. These cells were then embedded in the gel, which allowed it to capture nearly 30 per cent of the toxin from the liquid around it within 24 hours.

The lifetime of the gel has yet to be specifically tested, but there are living structures in the lab that have remained stable for more than a couple of years, says Manjula-Basavanna.

“The beauty of the work lies in the ability to genetically programme the functional response of the printed living material,” says André Studart at ETH Zürich in Switzerland.