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.

Flexural plate waves can propagate non-radiatively along liquid-immersed plates provided the plate wave velocity is lower than the speed of sound in the liquid. This low-loss condition has been exploited to measure biochemical processes in liquids. Proteins or other biomolecules that adsorb to the plate surface lower the plate wave velocity, while the viscosity of the liquid causes attenuation of the plate wave.Thin plate-wave structures have been formed with microfabrication techniques. The response of these structures has been experimentally calibrated and found to agree well with theory. The plate-wave delay-line oscillator tested has a mass sensitivity which permits the detection of less than ten nanograms of mass adsorbed over the 0.24 cm2 membrane area. Fluid viscosities were measured in the range of zero to 10.6 poise.The plate-wave delay-line oscillator frequency was monitored to study the adsorption of protein onto the Sensor surface in real time. Plate wave attenuation was seen to rise sharply over time as a drop of blood on the sensor coagulated.

Recently, a new microsensor employing low-velocity ultrasonic Lamb waves was developed and demonstrated to be capable of measuring the viscosity of solutions in small volumes. The microsensor, when attached to a temperature-controlled stage, can measure viscosity as a function of temperature. In this investigation, the ultrasonic Lamb-wave oscillator is employed to experimentally measure the viscosity of dimethylsulfoxide (Me2SO) solutions as a function of temperature. The microsensor and the experimental procedure are described and results for 1M, 3M, and 5M Me2SO aqueous solutions are presented. Dimethylsulfoxide is a compound commonly employed as a cryoprotectant in cryopreservation, the low-temperature preservation of biological materials. The temperature dependence of viscosity obtained through this study can be used in determining the probability for ice nucleation in biological materials, a parameter of importance during cryopreservation.