title: Direct-referencing Two-dimensional-array Digital Microfluidics Using Multi-layer Printed Circuit Board.
abstract: Digital (i.e. droplet-based) microfluidics, by the electrowetting-on-dielectric (EWOD) mechanism, has shown great potential for a wide range of applications, such as lab-on-a-chip. While most reported EWOD chips use a series of electrode pads essentially in one-dimensional line pattern designed for specific tasks, the desired universal chips allowing user-reconfigurable paths would require the electrode pads in two-dimensional pattern. However, to electrically access the electrode pads independently, conductive lines need to be fabricated underneath the pads in multiple layers, raising a cost issue especially for disposable chip applications. In this article, we report the building of digital microfluidic plates based on a printed-circuit-board (PCB), in which multilayer electrical access lines were created inexpensively using mature PCB technology. However, due to its surface topography and roughness and resulting high resistance against droplet movement, as-fabricated PCB surfaces require unacceptably high (~500 V) voltages unless coated with or immersed in oil. Our goal is EWOD operations of aqueous droplets not only on oil-covered but also on dry surfaces. To meet varying levels of performances, three types of gradually complex post-PCB microfabrication processes are developed and evaluated. By introducing land-grid-array (LGA) sockets in the packaging, a scalable digital microfluidics system with reconfigurable and low-cost chip is also demonstrated.
authors.affiliation: J. Gong was with the Department of Mechanical and Aerospace Engineering, University of California Los Angeles, Los Angeles, CA 90095 USA. He is now with the EHD technology group, Duarte CA, 91010 USA (phone: 626-357-7350; fax: 626-357-2692; e-mail:  jgong@ehdtg.com ).
authors.forename: Jian
authors.initials: J
authors.lastname: Gong
authors.forename: Chang-Jin Cj
authors.initials: CJ
authors.lastname: Kim