For years, scientists have experimented with using responsive materials in 3D-printing inks. This has allowed for the creation of complex objects which can "shapeshift" in response to heat, light or other stimuli. Never before, however, have researchers successfully printed genetically engineered cells and kept them alive.
Previous attempts to do so have suffered due to the mammalian cells used in the ink rupturing and dying, often when under the pressure of being pushed through the nozzle of the 3D printer.
Engineers at MIT, led by Professor Timothy Lu and Professor Xuanhe Zhao, turned their attentions elsewhere: to bacterial cells, which have considerably stronger cell walls capable of withstanding printing.
These cells could be turned into an ink by combining them in a mixture of nutrients and hydrogel, an absorbent polymer gel containing mostly water. This allows the cells to be printed into living structures and devices, while being sustained throughout the process.
"We found this new ink formula works very well and can print at a high resolution of about 30 micrometres per feature," said Professor Xuanhe Zhao
"That means each line we print contains only a few cells. We can also print relatively large-scale structures, measuring several centimetres."
The team printed a "living tattoo" of an abstract tree design to demonstrate the new technique. They began by printing the bacterial cells on an elastomer layer, which was then cured with ultraviolet light and adhered to the skin.
Branches of the tree containing different cells lit up in response to the presence of their corresponding chemical and molecular stimuli.
The researchers were also able to engineer the bacterial cells to communicate with each other, fluorescing when they detect a signal sent from another cell in the structure. This could, the researchers suggest, allow for the possibility of 3D-printed "living computers", which pass signals between cells, rather than transistors.
According to the researchers, this technique could be used to create living, wearable devices such as health-monitoring devices, sensors for pollutants in the wearer's surroundings, or surgical implants that release compounds over time.
