In agreement with SPR data, BMP4 binding to PSPMA brushes was found to be significantly weaker than BMP2 (Figure ). Upon BMP4 binding, changes in dissipation and frequency shift were less pronounced than those detected for BMP2, on all three brush types, although trends (e.g. comparing HD-30, HD-10 and LD-10) were qualitatively similar to those described above for BMP2. However, one major difference clearly observed was that, upon washing, dissipation and frequency shifts did not stabilize, as in the case of BMP2, but reduced. This implies a weaker binding of BMP4, with significant desorption during washing steps. QCM-D also enabled probing the impact of the chemistry of the underlying substrate on BMP binding (Supplementary Figure ). In contrast to trends observed for silica substrates, it was observed that BMP4 and BMP2 (in particular) bound more strongly to sparse brushes (LD-10) generated from gold substrates, compared to generated from silica substrates. Frequency shifts were more pronounced for these brushes, with reduced dissipation components, indicating an increased binding and suggesting the formation of a more rigid protein-brush complex.