Researchers at Microsoft have developed an answer: a new method of marking objects without leaving a visual trace. The method involves creating objects with various internal gaps, or bubbles, within its body that form predetermined patterns. These patterns can then be observed using a Terahertz scanner, a device that has been used in airport security since 2007.
Terahertz radiation is a type of electromagnetic light that is not visible to the human eye and also doesn't harm organic matter like nuclear radiation does. It can also pass through most plastics, fabric, wood and organic material, making it ideal for imaging the insides of objects. By analyzing the rate at which the Terahertz radiation beams pass through the object, the scanner can locate the gaps that make up the pattern and interpret the gaps' meaning.
Microsoft calls this type of tag an InfraStruct. It's similar to a barcode or a QR code, but the mark is structural, not visual, and therefore doesn't have to be on the outside of an object. Unlike radio-frequency identification tags, or RFIDs, tags such as InfraStructs don't require any kind of electricity to exist; they're just a part of the object's architectural makeup.
This method is particularly easy to implement with 3D-printed objects, as the printers work by creating an object layer by layer, so adding the pattern is a relatively simple modification.
3D printers capable of printing in multiple materials, such as different types of polymers or even metal, could easily create a different type of InfraStruct. If you were to make one of those layers a different material than the rest, a Terahertz scanner would detect it because the radiation would pass through that layer at a different rate than the rest.