An integrated continuous-flow microfluidic sensor for long-term monitoring of microalgae growth in a tubular photobioreactor

<i>Spirulina</i> (<i>Arthrospira platensis</i>) is a valuable cyanobacterium used for various applications, including health supplements, cosmetics, biofertilizers, carbon capture, and biofuels. Efficient monitoring of microalgae growth in photobioreactors is crucial for optimizing yields in large-scale culturing. Existing monitoring systems take samples from the bioreactor at different intervals and perform the visualization and quantification of algae growth parameters. In this work, a microfluidic platform is mounted on a tubular photobioreactor, and the system continuously monitors the growth behavior of <i>Spirulina</i> over several days, with algal development captured on demand. Furthermore, the microfluidic sensor is fabricated using a novel xurography-based approach on photopolymer sheets. It captures real-time micrographs of algae continuously for 5 days (over 120 hours) under two different conditions: open-loop and closed-loop. In the open-loop configuration, the sensor hydrostatically taps the algal medium from the bioreactor at regular intervals. In contrast, the closed-loop sensor continuously (24/7) circulates the culture medium through the microchip for visualization without the use of any driving mechanism. From the micrographs, algal cell density, cell count, and trichome length are estimated continuously, and all parameters exhibited an increasing trend over time. Importantly, the cell density obtained from the microfluidic sensor closely matches with the conventional benchmark glass slide method, with an error of less than 3.3%. The microfluidic monitoring platform is found to be low-cost, accurate, fast, and efficient compared to existing systems, and moreover, it is easily amenable to automation.