A Highly Miniaturized Low-Power CMOS-Based pH Monitoring Platform

This paper presents the design and fabrication of a wireless, highly miniaturized, low-power electrochemical pH sensing system employing complementary metal-oxide- semiconductor (CMOS) electronics. Since plasma pH readings directly correlate to carbon dioxide levels present in the human body, this paper holds great promise for continuous monitoring of carbon dioxide in totally implantable device applications. In this paper, we have integrated a CMOS voltage controlled oscillator, which consumes only 120 μW of power and occupies an area of 0.045 mm², together with a miniature electrochemical pH sensor which detects real-time changes in pH levels. The fabricated sensor employs an electropolymerized poly(o-phenylenediamine) layer atop a platinum working electrode which yields linear operation well above and below the physiological pH range of 7.38-7.42, with sensitivities as high as 56 mV/pH. In turn, the fabricated CMOS electronics convert the voltage generated by the sensor to output frequency pulses in a linear fashion. Furthermore, a wireless transmission link was designed which broadcasts the resulting sensor data to a computer which displays real-time continuous pH readings. The miniature footprint of both the sensor and electronics, together with its low power consumption, renders this a versatile platform for facile carbon dioxide monitoring and other metabolic sensing systems.