Rapamycin concentration detected by LC-MS/MS from flow chamber in vitro release test in HBSS (black circles, left axis), and cumulative silicon erosion detected as dissolved oxides of silicon by ICP-OES (red squares, right axis), for microparticle formulations pSi-COOH-RAP (left), pSi-C12-RAP (middle), and pSiO2-C8-RAP (right).