A truly cylindrical inner tracker for ALICE

After the successful installation and first operation of the upgraded Inner Tracking System (ITS2), which consists of about $10\,$m$^2$ of monolithic silicon pixel sensors, ALICE is pioneering the usage of bent, wafer-scale pixel sensors for the ITS3 for LHC Run 4. Sensors larger than typical reticle sizes can be produced using the technique of stitching. At thicknesses of about $30\,\mu$m, silicon is flexible enough to be bent to radii of the order of $1\,$cm. By cooling such sensors with a forced airflow, it becomes possible to construct truly cylindrical layers which consist practically only of the silicon sensors. The reduction of the material budget and the improved pointing resolution will allow new measurements, in particular of heavy-flavor decays and electromagnetic probes. In this presentation, we will report on sensor developments, the performance of bent sensors in test beams, and the mechanical studies on truly cylindrical layers.