Imperial College is developing an implantable passive blood pressure monitor based around a SAW (surface acoustic wave) oscillator.

"In essence, it is an oscillator that you can ping from the outside with an RF pulse, then listen to as some of the RF energy is re-transmitted," researcher Professor Chris McLeod told ElectronicsWeekly. "We designed this device so that it is sensitive to pressure and the change in pressure alters its resonant frequency."

In use, the sensor would be attached directly in a heart chamber or blood vessel - monitoring blood pressure deep inside the patient as they live life normally.

"The case for using it in the heart or pulmonary circulation is that the alternative is to insert a catheter each time you want to make a measurement," said McLeod. "The case for using it in patients with high blood pressure is based on the relative risk between not knowing blood pressure and the small risk associated with implanting."

The prototype has a 10x8mm SAW tile connected to a 10mm³ coil antenna.

Now the team has £760,000 from the Wellcome Trust over three years to shrink and improve the sensor and receiver and gain approval for a medical device.

"We have a design that gets down to 8x2 or 8x3mm, and ideally we want to integrate the antenna," said McLeod. For the receiver "you need a very well tuned aerial to detect the signal and a very low noise front end on the system. The plan is to end up with a pocket sized box to put into your top pocket".

More sensitivity is desirable. Right now the sensor can detect pressure changes equal to 3mm of Hg when wired directly to a receiver, but noise means it is not as sensitive operating wirelessly. "The target is to achieve 3mm over radio, and in the longer term we want to get to 1mm. We know what we have got to do."

At the end of three years McLeod intends to have a system ready for human medical trials.

Imperial is aiming to spin out a company to exploit the technology.