The swelling of PSPMA brushes generated via ARGET and ATRP was next examined via ellipsometry (Figure ). Upon immersion in deionised water, PSPMA brushes swell considerably, from 30 to 88 nm and from 20 to 42 nm for ARGET-initiated and ATRP-initiated brushes, respectively. Upon exposure to PBS, swelling increased slightly in the case of ARGET-initiated brushes and decrease slightly in the case of ATRP-initiated brushes. In addition, swelling was more pronounced in the case of thin (HD-10) and sparse (LD-10) brushes, in particular in ARGETinitiated systems. These swellings, weaker than those reported for cationic polyelectrolyte brushes , are consistent with those reported for the literature for ATRP-initiated PSPMA brushes . The enhanced swelling of PSPMA brushes generated via ARGET, in particular those with lower thicknesses and sparser densities, imply a reduced initiation density and/or increased polydispersity, as predicted by self-consistent field theoretical studies . Although this could not be tested in our study, owing to the limited material that can be recovered from planar substrates and the difficulty of characterising the molecular weight of strong polyanionic materials by size exclusion chromatography, these results imply that brushes grown via an ARGET mechanism display longer chains (enhanced rates of polymerisation) but also higher polydispersity and lower surface density compared to those generated via an ATRP mechanism. This may be resulting from the precise regulation of the ratios of Cu(I)/Cu(II) species in ARGET, compared to ATRP, and the impact that this has on initiation rates, the persistence of radicals and recombination events. Having explored some of the physico-chemical properties of PSPMA brushes generated via ARGET and ATRP mechanisms, their ability to bind and sequester BMPs was investigated next, first via SPR (Figure and Supplementary Figures ). BMP2 rapidly adsorbed at the surface of dense and thick PSPMA brushes (HD-30), giving rise to stable anchorage of proteins at surface densities (G) of 756 ± 64 ng/cm 2 (upon injection at 10 µg/mL), 5 min after injection. This density corresponds to 0.1 molecules per nm 2 , indicating the formation of relatively dense monolayers.