Passive Impedance Sensing Using a SAW Resonator-Coupled Biosensor for Zero-Power Wearable Applications

A bio-sensing scheme, which acquires impedance information of a capacitive biosensor by using the reflected RF signal from a surface acoustic wave (SAW) resonator connected to the biosensor, is proposed. This technique requires no power to be supplied to the biosensor node and hence is highly applicable to wearable applications. Theoretical analysis has demonstrated that the sensitivity of the SAW resonator-coupled biosensor is higher than that of traditional impedance loaded SAW sensors and therefore it is more suitable for measuring the very small impedance changes in biosensors. The passive detection of the change in the impedance of a capacitive biosensor, as a result of biological binding events associated with the capture of a target analyte, has been demonstrated by preliminary experimentation. Dry tests of the SAW coupled capacitive biosensor using a cable connected network analyzer showed the aF level capacitance measurement resolution, which was only achieved in transistor level circuits previously, could be attained. When liquid samples with concentrations of C-Reactive Protein (CRP) in the range of 0.1 to 2 mu g/ml were applied to the biosensor, a corresponding change in the resonant frequency of the SAW resonator-coupled biosensor (in the order of sub-hundred kHz) was observed. This has demonstrated the potential for applying this technique in applications where a zero-power requirement at the biosensor node could be a distinct advantage, when the cable link between the network analyzer and the biosensor node is replaced by the RF transmission.

Automated summary

This article proposes a bio-sensing scheme that uses a surface acoustic wave resonator connected to a capacitive biosensor to acquire impedance information. The scheme is suitable for wearable applications, requiring no power supply to the biosensor node. Experimental results demonstrate that the scheme can passively detect changes in the impedance of the biosensor, and the resonant frequency of the SAW resonator-coupled biosensor changes when liquid samples with concentrations of C-Reactive Protein (CRP) are applied. The technique shows potential for applications where zero-power requirement and RF transmission are advantageous.