We report on initial experiments and theory describing the operation of gel-coated Lamb wave sensors. The lowest-order flexural Lamb wave can propagate in gel-coated plates with low attenuation. This allows some new sensing approaches to be investigated, such as: (1) Using gels as filters for larger particles and molecules, while allowing smaller analytes to diffuse through to the sensor surface, e.g., for in vitro biosensing; (2) Realizing sensors based on gel density and viscoelastic changes. We develop two theories to predict the response of the sensor to the viscoelastic properties of the gel, and find excellent agreement between predictions from these theories and those from a numerical simulation program for waves in multi-layered media. One uses a Rayleigh-type solution with modified boundary conditions to derive the mechanical radiation impedance presented to the plate surface by the gel, and from this impedance calculates the phase velocity. The other uses transmission line theory originally developed by Oliner et al.

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