Implantable surface acoustic wave devices for long term clinical monitoring

Abstract

A surface or bulk acoustic wave device can be implanted in or worn on a human or animal body to monitor various parameters thereof. The device comprises a pair of interdigitated transducers spaced apart over the surface of a piezo-electric substrate that is exposed to the parameter to be monitored. The device is interrogated by a radio-frequency signal being supplied to one of the transducers and detected after reflection by the other transducer. The parameter is measured by comparison of the supplied and received signals.

The invention claimed is: 
     
       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 surface acoustic wave device comprises an interdigitated transducer on the a surface of a piezo-electric substrate that closes a sealed chamber to form a transducer body, which wherein the piezo-electric 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, the method comprising: 
 whereinsupplying a radio-frequency signal is supplied externally of the body to the antenna, wherein the radio-frequency signal is transmitted as a surface acoustic wave over the surface of the piezo-electric substrate surface to a reflector reflectors on the surface, wherein the surface acoustic wave is reflected therefrom back to the said interdigitated transducer and;  
 transmittedreceiving a return radio-frequency signal from the antenna to an external receiver,and;  
 whereby comparison ofcomparing the supplied and receivedreturn radio-frequency signals, whereby the comparison 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 comparing the supplied and return radio-frequency signals comprises determining 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 comparing the supplied and return radio-frequency signals comprises determining a 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 as recited in claim 1, wherein a pair of the surface acoustic wave devices device which is implanted in the body, is a first of the a pair of surface acoustic wave devices being and is a pressure-sensitive acoustic wave device used in the method claimed in any one  claims 1 ,  2 ,  3  or  4 , and a second of the pair of devices being a surface acoustic wave device which is relatively insensitive to the cardiovascular or intra-cranial pressure and being is operated as a reference device thereby to cancel any effect on the cardiovascular or intra-cranial 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 first interdigitated transducer and a second interdigitated transducer, the first and second interdigitated transducers spaced apart over the a surface of a piezoelectric substrate that seals over a chamber to form a transducer body, which wherein the piezoelectric 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 first interdigitated transducers transducer, the method comprising: 
 whereinsupplying a radio-frequency signal is supplied externally of the body to the antenna, wherein the radio-frequency signal is transmitted over the surface of the piezoelectric substrate surface as an acoustic wave to the other of the transducers second interdigitated transducer, wherein the acoustic wave is reflected therefrom back to the said one of the transducers and first interdigitated transducer; 
 transmittedreceiving a return radio-frequency signal from the antenna thereof to an external receiver, and;  
 whereby comparison ofcomparing the supplied and receivedreturn radio-frequency signals, whereby the comparison provides a measurement of the cardiovascular or intra-cranial pressure. 
 
     
     
       7. A method of monitoring cardiovascular or intra-cranial pressure within a human or animal body as recited in claim 2, wherein the surface acoustic wave device which is implanted in the body is a first of a pair of surface acoustic wave devices and is a pressure-sensitive acoustic wave device and a second of the pair of devices is relatively insensitive to the cardiovascular or intra-cranial pressure and is operated as a reference device thereby to cancel any effect on the cardiovascular or intra-cranial pressure measurement of unwanted parameters. 
     
     
       8. A method of monitoring cardiovascular or intra-cranial pressure within a human or animal body as recited in claim 3, wherein the surface acoustic wave device which is implanted in the body is a first of a pair of surface acoustic wave devices and is a pressure-sensitive acoustic wave device and a second of the pair of devices is relatively insensitive to the cardiovascular or intra-cranial pressure and is operated as a reference device thereby to cancel any effect on the cardiovascular or intra-cranial pressure measurement of unwanted parameters. 
     
     
       9. A method of monitoring cardiovascular or intra-cranial pressure within a human or animal body as recited in claim 4, wherein the surface acoustic wave device which is implanted in the body is a first of a pair of surface acoustic wave devices and is a pressure-sensitive acoustic wave device and a second of the pair of devices is relatively insensitive to the cardiovascular or intra-cranial pressure and is operated as a reference device thereby to cancel any effect on the cardiovascular or intra-cranial pressure measurement of unwanted parameters.