In contrast, PMETAI brushes were not associated with significant changes in swollen thickness upon RNA infiltration, despite comparable uptake to dense thick PDMAEMA brushes (2.8 µg/cm 2 ) . To investigate whether such infiltration was associated with changes in brush conformation and morphology, we extracted refractive indices from the fits of in situ ellipsometry data (Supplementary Figure ). In all cases, complexation was associated with a slight increase in refractive index, to comparable levels in dense thick PDMAEMA and PMETAI brushes.