Ellipsometry. Brush thicknesses were characterised by spectroscopic ellipsometry with an α-SE instrument (J. A. Woollam) at an incidence angle of 70°, at multiple wavelength (380-900 nm spectrum). The dry thickness of PSPMA brushes was first measured in air and then substrates were transferred to a liquid cell (fitted with quartz windows normal to the light beam path) and left to equilibrate for 15 min, to study the swelling in deionised water and PBS. All measurements were carried out in triplicate, at room temperature. Psi and delta spectroscopic traces were extracted and fitted against a simple native oxide/Cauchy model in CompleteEASE (J.A. Woollam). Swelling factors were calculated as the ratio of swollen/dry thickness.