  The high integration density of MAPS, with silicon sensor and readout
electronics implemented in the same device, allows very thin structures with a
greatly reduced material budget. Thicknesses of $\mathcal{O}$(50~$\mu$m),
values at which silicon chips become flexible, are readily used in many
applications. In addition, MAPS can be produced in sensors of wafer size by a
process known as stitching. This in turn allows to build detector elements that
are large enough to cover full tracker half-layers with single bent sensors.
The ALICE ITS~3 project is planning to build a new vertex tracker based on
truly cylindrical wafer-scale sensors, with <0.05\% X/X$_{0}$ per layer and
located as close as 18 mm to the interaction point. R\&D on all project aspects
(including mechanics for bent wafer-scale devices, test beams of bent MAPS,
design of stitched sensors) is rapidly progressing with the aim for
installation during LHC long shutdown 3 (2025--2027). This contribution
summarises the project motivation, its R\&D schedule, and will show selected
highlights of recently accomplished project milestones, including full-scale
engineering prototypes with dummy chips and small-scale, fully functional
assemblies of functional, bent MAPS.
