Sessile droplets for chemical and biological assays

Sessile droplets are non-movable droplets spanning volumes in the nL-to-μL range. The sessile-droplet-based platform provides a paradigm shift from the conventional, flow-based lab-on-a-chip philosophy, yet offering similar benefits: low reagent/sample consumption, high throughput, automation, and most importantly flexibility and versatility. Moreover, the platform relies less heavily on sophisticated fabrication techniques, often sufficient with a hydrophobic substrate, and no pump is required for operation. In addition, exploiting the physical phenomena that naturally arise when a droplet evaporates, such as the coffee-ring effect or Marangoni flow, can lead to fascinating applications. In this review, we introduce the physics of droplets, and then focus on the different types of chemical and biological assays that have been implemented in sessile droplets, including analyte concentration, particle separation and sorting, cell-based assays, and nucleic acid amplification. Finally, we provide our perspectives on this unique micro-scale platform.