Implantable surface acoustic wave devices for long term clinical monitoring

1. A method of monitoring cardiovascular or intra-cranial pressure within a human or animal body comprising a surface acoustic wave device implanted therein, wherein the device comprises an interdigitated transducer on the surface of a piezo-electric substrate that closes a sealed chamber to form a transducer body, which substrate is exposed such that the cardiovascular or intra-cranial pressure to be monitored acts on the exposed substrate, wherein an antenna is connected to the interdigitated transducer, wherein a radio-frequency signal is supplied externally of the body to the antenna, is transmitted as a surface acoustic wave over the substrate surface to a reflector on the surface, reflected therefrom back to the said transducer and transmitted from the antenna to an external receiver, whereby comparison of the supplied and received signals provides a measurement of pressure difference across the substrate.

2. A method according to claim 1 wherein the cardiovascular or intra-cranial pressure is monitored by determination of a delay of the surface acoustic wave.

3. A method according to claim 1 wherein the cardiovascular or intra-cranial pressure is monitored by determination of the change of resonant frequency of the surface acoustic wave.

4. A method according to claim 1 wherein a plurality of said devices is employed arranged to operate at different frequencies.

5. A method of monitoring cardiovascular or intra-cranial pressure within a human or animal body, wherein a pair of surface acoustic wave devices is implanted in the body, a first of the devices being a pressure-sensitive acoustic wave device used in the method claimed in any one claim 1 , 2 , 3 or 4 , and a second of the devices being a surface acoustic wave device which is relatively insensitive to the cardiovascular or intracranial pressure and being operated as a reference device thereby to cancel any effect on the cardiovascular or intracranial pressure measurement of unwanted parameters.

6. A method of monitoring cardiovascular or intra-cranial pressure within a human or animal body comprising an implanted surface acoustic wave device which comprises a pair of interdigitated transducers spaced apart over the surface of a piezoelectric substrate that seals over a chamber to form a transducer body, which substrate is exposed such that the cardiovascular or intra-cranial pressure to be monitored acts on the exposed substrate, wherein an antenna is connected to one of the interdigitated transducers, wherein a radio-frequency signal is supplied externally of the body to the antenna, is transmitted over the substrate surface as an acoustic wave to the other of the transducers, reflected therefrom back to the said one of the transducers and transmitted from the antenna thereof to an external receiver, whereby comparison of the supplied and received signals provides a measurement of the cardiovascular or intra-cranial pressure.

7. A cardiovascular or intra-cranial pressure monitoring system comprising:

a surface acoustic wave device,

a radio-frequency signal source for supplying a first radio-frequency signal to the surface acoustic wave device, and

a radio-frequency signal receiver for receiving a second radio-frequency signal from the surface acoustic wave device,

wherein the surface acoustic wave device comprises:

an interdigitated transducer on a surface of a piezo-electric substrate that closes a sealed chamber to form a transducer body,

a reflector on the surface of the substrate, and

an antenna connected to the interdigitated transducer,

which substrate is arranged to be exposed, in use, such that the cardiovascular or intra-cranial pressure to be monitored acts on the exposed substrate, and wherein the first radio-frequency signal transmitted by the radio-frequency signal source is arranged to be:

received by the antenna and transmitted as a surface acoustic wave over the substrate surface to the reflector,

reflected therefrom back to the said transducer, and

transmitted from the antenna to the radio-frequency signal receiver as the second radio-frequency signal, whereby comparison of the supplied and received signals provides a measurement of a pressure difference across the substrate together with at least one of map information and information downloaded by using a communication unit.

8. A cardiovascular or intra-cranial pressure monitoring system according to claim 7 wherein the cardiovascular or intra-cranial pressure is monitored by determination of a delay of the surface acoustic wave.

9. A cardiovascular or intra-cranial pressure monitoring system according to claim 7 wherein the cardiovascular or intra-cranial pressure is monitored by determination of the change of resonant frequency of the surface acoustic wave.

10. A cardiovascular or intra-cranial pressure monitoring system according to claim 7 wherein a plurality of said surface acoustic wave devices operate at different frequencies.

11. A cardiovascular or intra-cranial pressure monitoring system comprising a pair of surface acoustic wave devices, a first of the devices being a pressure-sensitive acoustic wave device as defined in claim 7 and a second of the devices being a surface acoustic wave device which is relatively insensitive to the cardiovascular or intra-cranial pressure and being operated as a reference device thereby to cancel any effect on the cardiovascular or intra-cranial pressure measurement of unwanted parameters.

12. A cardiovascular or intra-cranial pressure monitoring system comprising a pair of surface acoustic wave devices, a first of the devices being a pressure-sensitive acoustic wave device as defined in claim 8 and a second of the devices being a surface acoustic wave device which is relatively insensitive to the cardiovascular or intra-cranial pressure and being operated as a reference device thereby to cancel any effect on the cardiovascular or intra-cranial pressure measurement of unwanted parameters.