The ALICE collaboration is currently developing a new vertex detector ("ITS3"), which is planned to be installed in 2026 - 2028 during LHC long shutdown 3 to replace the three innermost layers (Inner Barrel) of the current Inner Tracking System as from Run 4 onwards. ITS3 is based on a truly innovative concept, comprising the use of ultra-thin (50 micrometers) silicon wafers and stitching technologies in 65nm CMOS imaging process to produce large area (10cm x 26cm) Monolithic Active Pixels Sensors sensors which can be bent to half-cylindrical shapes of 18, 24 and 30 mm bending radii for layer 0, 1 and 2 respectively. The ITS3 will thus be built of six stitched sensors kept in place using slices of carbon foam. Together with a very low power consumption of about 20mV/cm2 allowing for air cooling, no further mechanical and electrical support structures are required, resulting in an extermely low material budget of less than 0.05\% of a radiation length per layer, providing extremely good tracking and vertexing capabilities.Within the comprehensive R\&D program for the ITS3 different analogue and digital test structures with various pixel pitch sizes, matrices and readout circuits have been produced and extensively tested in various particle beams before and after irradiation with TID and NIEL to validate the ITS3 technology approachIn this contribution we present an overview on the various prototypes and selected results on the detection performancve measurements in the laboratory and in test beams using various chip settings.