Engineers have devised a 3-D printing technique that uses a new kind of ink made from genetically programmed living cells. The cells are engineered to light up in response to a variety of stimuli. When mixed with a slurry of hydrogel and nutrients, the cells can be printed, layer by layer, to form 3-D, interactive structures and devices. The team has then demonstrated its technique by printing a "living tattoo" -- a thin, transparent patch patterned with live bacteria cells in the shape of a tree. Each branch of the tree is lined with cells sensitive to a different chemical or molecular compound. When the patch is adhered to skin that has been exposed to the same compounds, corresponding regions of the tree light up in response. The technique can be used to fabricate "active" materials for wearable sensors and interactive displays. Such materials can be patterned with live cells engineered to sense environmental chemicals and pollutants as well as changes in pH and temperature.
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