Gravimetric biosensors use the basic principle of a response to a change in mass.  Most gravimetric biosensors use thin piezoelectric quartz crystals, either as resonating crystals (QCM), or as bulk/surface acoustic wave (SAW) devices. In the majority of these the mass response is inversely proportional to the crystal thickness. Thin polymer films are also used in which biomolecules can be added to the surface with known surface mass.  Acoustic waves can be projected to the thin film to produce an oscillatory device, which then follows an equation that is nearly identical to the Sauerbrey equation used in the QCM method.   Biomolecules, such as proteins or antibodies can bind and its change in mass gives a measureable signal proportional to the presence of the target analyte in the sample.