1. A method performed by data processing apparatus, the method comprising: obtaining a three-dimensional model of a first object to be fabricated by a 3D printer; receiving an input identifying an additional component to be embedded in the first object when the first object is fabricated, wherein the additional component uniquely identifies the first object; obtaining a spatial pattern of conductive circles that uniquely identifies the first object, wherein each conductive circle is connected to a pathway of conductive material that leads through a body of the first object to an outer surface of the first object; generating a digital representation of a machine-readable tag that includes the spatial pattern of conductive circles; integrating the digital representation of the machine-readable tag with the three-dimensional model; providing the three-dimensional model with the integrated digital representation to the 3D printer for fabrication; and storing data associating the first object with the spatial pattern of conductive circles that uniquely identifies the first object.