Intergrated CMOS Micorsystems for Electrochemical Sensing
Abstract This work presents a novel integrated CMOS microsystem for electrochemical sensing and biological sensing applications. The system provides a platform to enable concurrent monitoring of chemical and biological signals in vitro. The microsystem consists of microelectrode arrays (MEAs), a potentiostat, a bio-potential amplifier and an analog-to-digital converter, featuring portability, sample volume reduction, high spatial resolution, low power consumption and compatibility with mass manufacturing. Microelectrode arrays offer numerous benefits over macroelectrodes due to their smaller sample size requirement, small form factor, low power consumption, and higher sensitivity due to increased rates of mass transport. These features make MEAs well-suited for lab-on-a-chip applications. MEAs are implemented with an individually addressable 32 × 32 array of 7 um square microelectrodes with 37 um center to center spacing on a CMOS chip with a built-in VLSI potentiostat for electrochemical analysis. The potentiostat features a low input impedance, high sensitivity, high gain and low power trans-impedance amplifier front end, with programmable control of electrode potential, thus facilitating continuous bi-directional measurement of reduction-oxidation (redox) current in a potentiostat for cyclic voltammetry (CV) experiment. The integrated CMOS MEAs is post-processed at the die level to coat the exposed Al layers with Au. To verify microelectrode array behavior with individual addressability, cyclic voltammetry was performed using a potassium ferricyanide (K3Fe(CN)6)) solution. For biological signal recoding applications, we present a low noise, low power, bandwidth tunable amplifier for bio-potential signal recording applications. By employing depletion-mode pMOS transistor in diode configuration as a tunable sub pA current source to adjust the resistivity of a MOS-Bipolar pseudo-resistor, the bandwidth is adjusted without any need for a separate band-pass filter stage. For high CMRR, PSRR and dynamic range, a fully differential structure is used in the design of the amplifier. In order to save power, asynchronous analog-to-digital converter (ADC), and digital signal processing (DSP) are employed. Data compression is achieved by the inherent signal dependent sampling rate of the asynchronous architecture. This makes the system attractive for compact and portable biological signal monitoring applications. The amplifier and ADC were fabricated in a 0.18µm CMOS technology and consume a total of only 79µW. A three electrode electrocardiogram (ECG) measurement is performed using the proposed amplifier and ADC to show its feasibility for low power biological monitoring applications. With both electrochemical and biological sensing techniques, we have implemented both sensing system on a single CMOS chip with dynamic gain delta-sigma analog-to-digital converter for chemical and biological application. The CMOS microsystem is designed in a 0.5µm CMOS technology and consumes a total of 20µW in front-end stages.