Real-time pH Monitoring using CMOS ISFET Arrays in 3D-printed Microfluidic Flow Cells

This paper presents a complete Lab-on-Chip platform that integrates CMOS ISFET arrays with 3D-printed microfluidic flow cells for real-time $\mathbf{p H}$ monitoring of samples in a constant flow. The ISFET sensors, fabricated using the unmodified TSMC 180 nm process, are arranged in a $256 \times 256$ array with per-pixel offset compensation. A dedicated digital readout system based on FPGA is implemented to acquire data and feedback calibrated sensor offsets to on-chip DACs. The microfluidic chip features a cylindrical flow cell with a height of $400 \mu \mathrm{~m}$ and a radius of $\mathbf{1. 5 5} \mathrm{mm}$, utilising a silicone O-ring gasket in a dimension of $3 \times 2 \times 0.5 \mathrm{~mm}$ to secure the contact area. The platform demonstrates the capability to monitor pH changes in fluids and visualise the diffusion of ion dynamics above the array surface in real time, with a latency of less than 350 ms. It reports a pH sensitivity of $27 \mathrm{mV} / \mathrm{pH}$ for solutions passing through the cell at a flow rate of $12.56 \mathrm{~mL} / \mathrm{min}$.