Implantable blood pressure sensor

Materials World magazine
,
1 Mar 2009

UK researchers are exploiting surface acoustic wave (SAW) technology to
create a wireless and implantable blood pressure sensor. Scientists at Imperial College London, UK, believe this is the first implantable SAW device, and say it could transform blood pressure monitoring and control.

In SAW technology, an electrical impulse is applied to surface electrodes on a piezoelectric material – commonly quartz, lithium niobate or lithium tantalate – generating mechanical waves that travel on the surface.

When they hit the secondary electrodes, the waves can be reflected to set up standing waves (a resonator), or the piezoelectric effect converts the energy into electrical signals on the electrodes. This is exploited in filters, oscilators and transformers.

Such devices become sensors (a relatively new market) when the external environment interacts with the surface to alter the mechanical waves. By design, temperature, pressure, torque, and chemically and biologically reactive compounds can do this.


The team at Imperial has created a proof-of-concept implantable SAW device from quartz for monitoring blood pressure.

The product could fulfil a clinical need for patients suffering from high blood pressure who have to travel to a clinic for tests, or where pressure cannot be measured non-invasively.

‘Surface acoustic wave sensors have features which commend them,’ says Professor Chris McLeod at the University’s Institute of Biomedical Engineering. ‘By attaching aerials to the electrodes, an incoming radio wave can excite the SAW and the signal detected by the secondary electrodes can be
transmitted. This offers a wireless and battery-less device, and therefore an unlimited life.’

Furthermore, the instrument, which is smaller than a five pence piece, uses highly stable quartz, ensuring calibration over time and temperature change.

A diaphragm is used to effect the alterations in the electrodes due to pressure. McLeod explains, ‘The SAW device replaces conventional strain gauges in other pressure sensors on the unexposed surface of the diaphragm.

Making waves

 

Professor Roger Whatmore, who has a background in SAW devices and is based at the Tyndall National Institute in Cork, Ireland, can see potential.

He says, ‘Competing technologies would be based on microelectromechanical systems. This requires in-built circuitry, [which] is more complex. It would be harder, although not impossible, to make this a remotely read device which needs no in-built power (see box below entitled ‘In the artery’). The advantage of SAW devices is that they can be remotely read and therefore are likely to be more reliable’.

In vitro tests at Imperial have revealed that the SAW sensor operates in wired mode with a targetted accuracy of one millimetre of mercury (mmHg) over the range of 0-250mmHg. A single measurement is made in microseconds – 100 measurements per second are adequate ‘to see the detail of every heartbeat’, notes McLeod.

The aim now is to improve wireless accuracy and continue with in vivo measurements to ensure safety and efficacy. All exposed surfaces must be biocompatible and stable in the human body, and the reference pressure chamber hermetically sealed.

Whatmore adds, ‘SAW devices will not work in liquids so the device has to be packaged in a way that the ambient pressure of fluids in the body stress the device to change the SAW velocity, without [contacting] the surface on which the SAW is propagating’.

The team at Imperial is also examining biological reactions on bare quartz and quartz that has been coated with biocompatible films.