QCM is a well-suited technique for monitoring mass attached to coating equipment in a vacuum. It can be used for the investigation of the adsorption and surface reaction in the monolayer range via changes in the resonant frequency . The substrate of QCM is standard quartz crystal, and this is used to detect minute changes on the surface at the nano-gram level. Since no hydroxyapatite substrate was used, the results of QCM could only indirectly indicate the binding between polydopamine and PEG based on changes in the surface density. A FTIR study is necessary to determine whether PEG reacted with polydopamine. In addition, the results of the contact angle assessment demonstrated that the polydopamine-induced-PEG modified the hydroxyapatite surface by making it more hydrophilic.