Wireless Interrogation of Implantable SAW Sensors

Implantable sensors provide long-term, accurate physiological measurements after a minimally invasive procedure, particularly when designed as transponders. Wireless interrogation of deeply implanted transponders with RF remains a challenge due to the high loss at the skin-air interface and large tissue RF absorption. This paper presents a system for wirelessly interrogating surface acoustic wave (SAW) sensors implanted in the main pulmonary artery (PA), where the pressure (PAP) is a very important parameter in the management of heart failure patients. The proposed PAP monitoring system consists of an implantable SAW pressure sensor integrated with an antenna and anchor in a housing, an external antenna and an electronic interrogator. The PAP is determined by measuring the frequency of the echo signal from SAW sensor accurately. An asymmetric antenna was designed and integrated with the sensor. The combination of simulation, theoretical calculation and phantom measurement indicates that the path loss to the implant location, about 6 cm below the skin, is around 25 dB. A portable interrogator was designed based on a dual conversion receiver and single echo high frequency sampling approach to assess achievable frequency estimation accuracy predicted by Cramer-Rao Lower Bound (CRLB) analysis. The system was characterized using a high quality (Q) factor SAW sensor, fabricated at wafer level, wire-connected to the interrogator via an attenuator to simulate path loss. The signal-to-noise ratio (SNR) of captured echo signals was calculated and used in CRLB analysis. The analysis indicates that without using signal post processing, the sensor sensitivity has to be at least 440 Hz/mmHg in order to achieve a target 1 mmHg accuracy. Although the current sensor sensitivity is only 200 Hz/mmHg, the in vivo measurement showed that acceptable accuracy can be obtained by signal post processing. The results from an invasive catheter tip transducer measured simultaneously with the SAW sensor showed that the differences in pulse pressure and relative mean pressure are 0.8 mmHg and 1.4 mmHg, respectively. The accuracy could be further improved by increasing the sensor Q factor and sensitivity and reducing path loss.