Prior to use, the crystals were immersed in an EDTA solution (10 mM, pH 7.3) for 10 min. A QCM-D sensor system (Q-Sense Explorer, Biolin Scientific, Sweden) was used to monitor the adsorption of BMP2 (10 μg/mL) and BMP4 (10 μg/mL) in PBS onto the crystal and record the frequency (from 3 rd to 13 th harmonics) and dissipation shifts. Each harmonic has a specific penetration depth, described by the following equation