Microfluidic-Based Oxygen (O 2 ) Sensors for On-Chip Monitoring of Cell, Tissue and Organ Metabolism.

Oxygen (O 2 ) quantification is essential for assessing cell metabolism, and its consumption in cell culture is an important indicator of cell viability. Recent advances in microfluidics have made O 2 sensing a crucial feature for organ-on-chip (OOC) devices for various biomedical applications. OOC O 2 sensors can be categorized, based on their transducer type, into two main groups, optical and electrochemical. In this review, we provide an overview of on-chip O 2 sensors integrated with the OOC devices and evaluate their advantages and disadvantages. Recent innovations in optical O 2 sensors integrated with OOCs are discussed in four main categories: (i) basic luminescence-based sensors; (ii) microparticle-based sensors; (iii) nano-enabled sensors; and (iv) commercial probes and portable devices. Furthermore, we discuss recent advancements in electrochemical sensors in five main categories: (i) novel configurations in Clark-type sensors; (ii) novel materials (e.g., polymers, O 2 scavenging and passivation materials); (iii) nano-enabled electrochemical sensors; (iv) novel designs and fabrication techniques; and (v) commercial and portable electrochemical readouts. Together, this review provides a comprehensive overview of the current advances in the design, fabrication and application of optical and electrochemical O 2 sensors.