SAW Microsensor Brain Implant for Prediction and Monitoring of Seizures

An implantable surface acoustic wave (SAW) microsensor has been developed for early detection and monitoring of seizures based on local temperature changes in the brain´s epileptogenic zones that occur prior to and during an epileptic event. Three SAW sensors were designed and fabricated: a 172 MHz filter, a 434 MHz filter, and a 434 MHz delay line. Their temperature sensitivities were tested by measuring the phase change between the input and output waveforms as a function of temperature. We achieved a phase sensitivity of 144 phase degrees per degC and a minimum detectable temperature of 5 mK for the 434-MHz, 10.2-mus delay line. Based on the sensitivity tests, a prototype 434 MHz SAW sensor was fabricated to a size of 11times1times1.1 mm, which is commensurate with existing brain implantable probes. Because of possible damping of the surface waves by the surrounding tissue or fluid, a glass housing with dry air was built on the top of the SAW substrate. Test and reference sensors were used in the prototype system to minimize the effect of source instabilities and to amplify the temperature effect. The phase change between the output waveforms of the sensors was measured with phase detector electronics after they were converted to lower (10.7 MHz) frequencies by standard mixers. The complete prototype sensor was tested in a saline water bath and found to detect as low as 3 mK changes of temperature caused by the addition of hot water.