Researchers at MIT have developed a new technique of 3D printing living tattoo using a special ink made from genetically programmed living cells. The cells respond to different types of stimuli. They were then mixed with a slurry of hydrogel and nutrients to print them layer-by-layer. When embedded onto a human hand, they light up in response to chemical or molecular compounds. Tattoo-like 3D printing have been attempted in the past, but printing with living cells is one of the firsts of its kind.
The team of researchers explored different types of responsive materials as the basis for 3D printed ink. There have been other research works using mammalian cells, but they are too weak and easily rupture during the printing process. Hence the team looked for a harder cell type and found the solution in bacteria. Bacteria have tough cell walls and can withstand the forces endured during 3D printing extrusion. They are also compatible with most hydrogels, the basis for slurry medium. Hydrogels are gel-like materials composed of water and some polymer. After screening tests they identified pluronic acis to be the most compatible hydrogel.
Hence they mixed the bacteria, pluronic acid and nutrients and made the recipe for 3D printing ink. It led to a resolution of 30 micrometers per feature printing accuracy. That means that each printed line would have only few cells. They printed the ink with a custom 3D printer built by them. They printed a shape of tree on an elastomer layer. After printing they cured the layer by exposing it to ultraviolet radiation. Thus they created a “3D printing living tattoo”. This printed layer was then adhered to human skin.
To test the 3D printing living tattoo, the researchers smeared chemical compounds on a subject’s hand. Then they pressed the printed patch over the smear. After few hours the branches in the tree lit up when the bacteria responded to corresponding chemical stimuli. Each branch lit up in different color based on the bacteria present and the exposed chemical compound.
The researchers also enabled the bacteria to communicate with each other. For example, they programmed some cells to glow only after receiving signals from neighboring cells. They printed a structure with one layer of signal producing bacteria and another layer of response bacteria. They observed that only after the input bacteria underneath transmitted signal, the output bacteria above lit up. This could be potentially used to print “living computers”, structures with different types of cells passing information just like transistors or integrated circuits. It is a future advancement over the developed 3D printing living tattoo. One of the researchers Xinhue Liu can be quoted as saying
“We can use bacterial cells like workers in a 3-D factory. They can be engineered to produce drugs within a 3-D scaffold, and applications should not be confined to epidermal devices. As long as the fabrication method and approach are viable, applications such as implants and ingestibles should be possible.”