A New Paradigm in Microfluidic Systems: Laser-Induced Graphene Microfluidic Integrated Sensors (LIGMIS) for On-Site Biomedical and Environmental Monitoring

The integration of electrochemical sensors with pumpless paper microfluidic systems enables the in-field quantification of analyte concentrations that conventional lateral flow assays, such as urine pregnancy tests and home COVID-19 nasal swab tests, or multiplexed colorimetric paper microfluidics like µPADs, cannot quantify. However, the reliance on wax printing or photolithography to create hydrophobic channels hinders the scalable manufacturing of electrochemical paper microfluidic systems. Additionally, the non-conductive nature of paper necessitates interfacing metal or carbon electrodes with the further reducing scalability and increasing costs.

Herein, we introduce a novel category of microfluidic sensors, termed Laser Induced Graphene Microfluidic Integrated Sensors (LIGMIS), which seamlessly integrate both the electrical and microfluidic components of electrochemical systems that circumvents the mentioned challenges. With LIGMIS, both the microfluidics and connecting electrochemical electrodes are made of the same material, laser-induced graphene (LIG) fabricated simultaneously from the same standard carbon dioxide laser in ambient conditions on a single polyimide substrate. We demonstrate how this one-step, mask-free process uses different laser settings to create open microfluidic hydrophilic LIG tracks outlined with highly hydrophobic LIG walls that are capable of splitting and transferring fluid samples to micro/nanostructured electrically conductive LIG-based electrodes. We highlight recent key innovations including Y-shaped reservoirs for consistent fluid flow, hydrophilic polyethyleneimine coatings to maintain wettability, and separable microfluidic and electrochemical components enabling isolated electrode nanoparticle metallization and biofunctionalization. We demonstrate how LIGMIS can be used to selectively monitor pesticides and plant nutrient ions for soil and water quality and uric acid and calcium in saliva for oral health monitoring.