Swollen brush Swollen brush/PNA complex Layer Thickness (Å) SLD × 10 -6 (Å -2 ) Solvent (%) Roughness (Å) Thickness (Å) SLD × 10 Following the successful characterisation and fitting of swollen brushes, nucleic acid materials were injected into the cells, and the neutron reflectometry profiles were again measured for all three contrasts and fitted using the data obtained in the absence of RNA (bare brushes) as a starting point. Throughout this step, the silicon blocks were not in any way and alignment was maintained throughout. Nucleic acid materials were injected at two different concentrations. The first, 3 µg/mL, corresponds to the partial loading of the brush with nucleic acid materials, typically achieved during complexation steps, for which N/P ratios of 5-10 are typically used for transfections. The second concentration, 10 µg/mL, corresponded to concentrations that should allow saturation of the brush with nucleic acid materials, according to surface plasmon resonance data and the volume of solution injected (7 mL for each injection).