Effects of the Properties of Dielectric Materials on the Fabrication and Operation of Digital Microfluidic Systems

Abstract

Digital microfluidic systems (DMS), where liquid droplets are manipulated on a planar surface, are the recent generation of microfluidic systems. Specifically, the systems where electrowetting force is employed for the actuation of droplets on a 2-D array of electrodes are capable of performing many of the basic fluidic operations. In these systems, the actuation force on the liquid droplet is a function of the applied voltage and its frequency, dielectric properties of the liquid and the insulating material, and finally the instantaneous position of the droplet between two adjacent electrodes. At lower frequencies the applied voltage is dropped almost entirely in the insulating layer for droplets of aqueous liquids. The unit area capacitance of the insulating layer depends linearly on the dielectric constant and inversely on the thickness. Therefore, for a given voltage and frequency, stronger actuation force can be generated if a thinner layer of material with high dielectric strength is used. However, the electric-field strength of the material poses the limitation on reducing the thickness of insulating layer. Typical insulators, with high electric field strength, used in microelectromechanical systems require quite sophisticated equipment for their deposition on the substrates. On the other hand, spin-coating of polymer insulators have been adopted as an economic and faster alternative technique for the fabrication of DMS. Polymer insulators possess reasonable dielectric strength and lower thickness can be achieved from higher spin speeds. However, low electric-field strengths of these materials prohibit working with thinner layers. As a result, actuation of droplets in DMS, fabricated using these materials as insulators, requires higher voltages. Higher applied voltages are not desirable for portable systems dealing with sensitive liquid samples. This paper presents an investigation on different insulating materials in terms of the ease of fabrication and the minimum voltage requirements for operation. The results can be useful in choosing the insulating material that will result in the expected level of performance while meeting the resource constraints. As a result, this paper specifies the material, based on the comparative study, that can be used to avoid the need for a sophisticated deposition system and the fabricated DMS can be operated with much lower voltages.