  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.
