Droplet-Based Sensing: Optical Microresonator Sensors Embedded in Digital Electrowetting Microfluidics Systems

Electrowetting-on-dielectric (EWD) microfluidics is an emerging platform for practical applications such as water quality testing and medical diagnostics. Low power consumption, low sample and reagent volumes, small size, and rapid fluid transport are features of electrowetting microfluidic platforms that will enable the development of cost-effective, rapid time-to-result, and portable point-of-care diagnostic devices. Microresonator sensors are an excellent sensor technology for integration into these microfluidic systems, because they perform high sensitivity detection of proteins, DNA, and other biologically relevant molecules while tolerating a droplet oil encapsulation layer. This paper reports on a SU-8 polymer microresonator embedded in the top plate of the EWD system, which enables addressing of the sensor with a single droplet of in volume and enables droplets to be moved onto and off of the sensor. This system is the first to demonstrate actuation of droplets onto and off of an integrated microresonator sensor. Both photolithographically patterned and electron beam lithographically patterned microresonator sensors were tested, and the effect of a conventional filler medium, silicone oil, on the sensor sensitivity was investigated.