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Due to its ability to manipulate droplets flexibly, electrowetting-on-dielectric (EWOD) based digital microfluidics have been widely researched. This paper focuses on the design principles of improved open EWOD based digital microfluidics actuators. EWOD devices of different electrode formation were fabricated and tested. Based on the analysis and experiments, evaluation methods of open EWOD devices have been presented, which gives guidance to the improvement of EWOD actuators. © 2016, Editorial Office of Nanotechnology and Precision Engineering. All right reserved.
Although digital microfluidics has shown great potential in a wide range of applications, a lab-on-a-chip with integrated digital droplet actuators and powerful biochemical sensors is still lacking. To address the demand, a fully integrated chip with electrowetting-on-dielectric (EWOD) and a film bulk acoustic resonator (FBAR) sensor is introduced, where an EWOD actuator manipulates digital droplets and the FBAR sensor detects the presence of substances in the droplets, respectively. The piezoelectric layer of the FBAR sensor and the dielectric layer of the EWOD share the same aluminum nitride (AlN) thin film, which is a key factor to achieve the full integration of the two completely different devices. The liquid droplets are reliably managed by the EWOD actuator to sit on or move off the FBAR sensor precisely. Sessile drop experiments and limit of detection (LOD) experiments are carried out to characterize the EWOD actuator and the FBAR sensor, respectively. Taking advantage of the digital droplet operation, a ‘dry sensing mode’ of the FBAR sensor in the lab-on-a-chip microsystem is proposed, which has a much higher signal to noise ratio than the conventional ‘wet sensing mode’. Hg2+ droplets with various concentrations are transported and sensed to demonstrate the capability of the integrated system. The EWOD–FBAR chip is expected to play an important role in many complex lab-on-a-chip applications.
Although digital microfluidic system has shown great potential in a wide range of applications, a single-chip platform integrating biochemical sensors with digital microfluidic devices is still lacking. In this paper, film bulk acoustic resonator (FBAR) sensors and electrowetting-on-dielectric (EWOD) actuator are integrated on a single silicon chip, where the EWOD actuator manipulates digital droplets and the FBAR sensor detects analyte in the droplets. Thin aluminum nitride (AlN) film is a critical material as both the FBAR piezoelectric layer and the EWOD dielectric layer, which facilitates the compact integration of the two devices on a chip. A preliminary demonstration of Hg2+ droplet transportation and detection is conducted on the platform. The integrated platform is expected to play an important role in many complex lab-on-a-chip applications in chemistry, biology and medicine.
Micro/nano scale biosensors integrated with the local adsorption mask have been demonstrated to have a better limit of detection (LOD) and less sample consumptions. However, the molecular diffusions and binding kinetics in such confined droplet have been less studied which limited further development and application of the local adsorption method and imposed restrictions on discovery of new signal amplification strategies. In this work, we studied the kinetic issues via experimental investigations and theoretical analysis on microfabricated biosensors. Mass sensitive film bulk acoustic resonator (FBAR) sensors with hydrophobic Teflon film covering the non-sensing area as the mask were introduced. The fabricated masking sensors were characterized with physical adsorption of bovine serum albumin (BSA) and specific binding of antibody and antigen. Over an order of magnitude improvement on LOD was experimentally monitored. An analytical model was introduced to discuss the target molecule diffusion and binding kinetics in droplet environment, especially the crucial effects of incubation time, which has been less covered in previous local adsorption related literatures. An incubation time accumulated signal amplification effect was theoretically predicted, experimentally monitored and carefully explained. In addition, device optimization was explored based on the analytical model to fully utilize the merits of local adsorption. The discussions on the kinetic issues are believed to have wide implications for other types of micro/nano fabricated biosensors with potentially improved LOD. Copyright © 2015 Elsevier B.V. All rights reserved.