In light of the high-luminosity upgrade of the LHC at the end of this decade, the ATLAS experiment will replace its current inner detector by an all-silicon tracker, the ITk. The ITk will outperform the current tracker not only in terms of granularity and radiation hardness, it will also significantly reduce the material budget. To demonstrate that large scale detectors can be built following the envisioned ITk design, smaller scale prototypes are constructed for each of the ITk sub-systems. Three prototypes are built from silicon pixel modules, mimicking the structures in the innermost ITk layers. They provide the unique opportunity to evaluate aspects like the mechanical integration, the serial powering and cooling of modules, as well as the data acquisition in a realistic environment.