In their demonstration, the MIT researchers have 3D printed a multicolored tree-shaped tattoo made of live bacteria cells. As a test the engineers printed a thin tree-like structure made up of cells in branches, each of which reacts to a different chemical. Using a 3-D printer, several layers of ink are printed onto a transparent patch to form a living, three-dimensional, interactive device-in this case in the shape of a tree. The tree's three different branch colors indicate a different cell type, and each branch has been engineered to react to a different chemical of molecular compound. Applying such compounds to areas of the skin causes the "tree" to light up in response.
"We found this new ink formula works very well and can print at a high resolution of about 30 micrometres per feature", said Xuanhe Zhao, Professor in MIT's Department of Mechanical Engineering. "That means each line we print contains only a few cells. We can also print relatively large-scale structures, measuring several centimetres", Zhao added.
The design of 3D-printed living materials is guided by quantitative models that account for the responses of programed cells in printed microstructures of hydrogels.
In the future, this 3D-printing technique could be used to develop an entire living computer composed of cells that communicate with one another, much like the components inside a microchip. They fit into the same technological trend, researchers say, as wearable sensors and interactive displays. "You need the ink to flow out of a nozzle like toothpaste, and it can maintain its shape after it's printed".
The team also designed certain bacterial strains to work only in tandem with other elements. 3D printing has been intensively explored to fabricate customized structures of responsive materials including hydrogels, liquid-crystal elastomers, shape-memory polymers, and aqueous droplets. The output filaments only lit up when they overlapped with the input layer and received a signal from them. Using this technique, scientists might be able to build a "living computer", where layers of cells talk to each other like transistors do in electronics today. That means that sometime in the future, we could all be walking around with living, responsive tattoos that tell us when it's not safe to go outside because air pollution levels are risky, or even just act as a temperature gauge right on your body. They're also looking into creating drug capsules and surgical implants containing cells engineered to produce compounds such as glucose, which could be released over time. They can be engineered to produce drugs within a 3-D scaffold, and applications should not be confined to epidermal devices. The paper's co-authors are graduate students Xinyue Liu, Hyunwoo Yuk, Shaoting Lin, German Alberto Parada, Tzu-Chieh Tang, Eléonore Tham, and postdoc Cesar de la Fuente-Nunez. And, "as long as the fabrication method and approach are viable" applications such as implants and ingestibles aren't off the table either, the authors conclude.
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