Implantable intravascular pressure sensing apparatus and method of operating it

1. An implantable intravascular pressure sensor comprising:

a first transducer arranged to provide a pressure dependent signal in response to alternating electrical signals of a first frequency band;

a second transducer arranged to provide a reference signal in response to alternating electrical signals of a second frequency band different from the first frequency band; and

an antenna coupling for sending and receiving said signals, wherein the pressure dependence of the response provided by the first transducer is associated with deflection of a deflectable member by changes in intravascular pressure; and

the reference response is associated with a reference member arranged to be deflected less than the deflectable member by those same changes in intravascular pressure;

wherein the first transducer comprises a first interdigitated transducer, (IDT), on a surface of a piezoelectric crystalline substrate having a first crystal plane orientation, and

the second transducer comprises a second IDT on the surface of the piezoelectric crystalline substrate having a second crystal plane orientation which is the same as the first crystal plane orientation.

2. The apparatus of claim 1 , wherein the signals sent and received by the antenna coupling comprise radio frequency (RF) signals.

3. The apparatus of claim 2 wherein the antenna coupling sends and receives signals in a bandwidth which encompasses the first frequency band and second frequency band.

4. The apparatus of claim 1 wherein the different deflectability of the reference member and the deflectable member is associated with at least one of:

(a) different thicknesses of the substrate; and

(b) arrangement of support of the substrate.

5. The apparatus of claim 1 wherein the deflectable member is coupled to an enclosed cavity that provides a reference pressure in response to changes in intravascular pressure.

6. The apparatus of claim 1 wherein the deflectable member at least partially encloses said cavity, wherein the deflectable member is provided by a membrane, wherein the membrane has a thickness of less than 200 μm, or less than 60 μm, or more than 5 μm, or more than 10 μm, or more than 20 μm, or less than 100 μm.

7. The apparatus of claim 1 wherein the two transducers each comprise an assembly of conductive elements and the frequency band in which the transducers respond is selected by the spatial arrangement of said conductive elements.

8. The apparatus of claim 7 wherein the conductive elements are arranged to provide an LCR circuit.

9. The apparatus of claim 1 , wherein the reference member and the deflectable member are provided by different regions of the same substrate.

10. The apparatus of claim 1 , wherein the first IDT and the second IDT are surface acoustic wave (SAW) IDTs.