Most piezoelectric gravimetric biosensors are based on quartz crystal microbalances or surface acoustic wave devices. In this paper we describe a polymer film system, made of piezoelectric polyvinylidene difluoride (PVDF) and the Immobilon-P membrane (porous PVDF), for biosensing applications. In operation a film is accommodated in a flow cell and connected to an electronic circuit, constituting an oscillatory resonant device; the output signal is its resonance frequency. This device successfully detected the binding between bovine IgG and bovine anti-IgG. Work on the modelling of this film system is presented. Two different approaches are being considered: the finite element method (FEM) and the ABCD matrices. Results comparing theoretical and experimental data are presented.