Upon immersion in deionised water, all brushes swell considerably, as determined by in situ ellipsometry (Figure ). In line with previous observations , swelling was more pronounced for thin and sparse brushes and the swelling ratio of PMETAI and PDMAEMA brushes was comparable, indicating that PDMAEMA brushes are expected to be extensively protonated in these conditions. To explore the morphology of brushes in deionised water, two different models were used to fit the ellipsometry data: a simple homogenous Cauchy layer and a single gradient Cauchy layer (Supplementary Figure ). Both models afforded comparable fits with low MSE. Beyond the determination of the overall swelling of brushes, it was therefore not possible to further study the morphology of brushes using in situ ellipsometry. The adsorption of 20 bp siRNA oligonucleotides was next investigated. Upon incubation of PDMAEMA brushes in 10 µg/mL oligo solutions (in deionised water), swollen brush thicknesses increased slightly, on average by 8.9-13.1% (Figure ). These results suggest that the conformation of PDMAEMA brushes and corresponding RNA complexes is significantly rearranged upon complexation. Levels of adsorption of the same oligos to the same panel of PDMAEMA brushes were found to be 2.9, 1.4 and 1.0 µg/cm 2 , for dense thick, dense thin and sparse brushes, respectively . The absence of correlation between such adsorption levels and measured changes in swollen thicknesses is therefore suggesting different modes of adsorption of oligos within polymer brushes. Indeed, it may be proposed that oligos could insert edge on within brushes and infiltrate deep within the brush structure, and may infiltrate fully or partially within the brush (Figure ).