Abstract
Electrowetting-on-dielectric is a decent technique to manipulate discrete volumes of liquid in form of droplets. In the last decade, electrowetting-on-dielectric systems, also called digital microfluidic systems, became more frequently used for a variety of applications because of their high flexibility and reconfigurability. Thus, one design can be adapted to different assays by only reprogramming. However, this flexibility can only be useful if the entire system is portable and easy to use. This paper presents the development of a portable, stand-alone digital microfluidic system based on a Linux-based operating system running on a Raspberry Pi, which is unique. We present “PortaDrop” exhibiting the following key features: (1) an “all-in-one box” approach, (2) a user-friendly, self-explaining graphical user interface and easy handling, (3) the ability of integrated electrochemical measurements, (4) the ease to implement additional lab equipment via Universal Serial Bus and the General Purpose Interface Bus as well as (5) a standardized experiment documentation. We propose that PortaDrop can be used to carry out experiments in different applications, where small sample volumes in the nanoliter to picoliter range need to be handled an analyzed automatically. As a first application, we present a protocol, where a droplet is consequently exchanged by droplets of another medium using passive dispensing. The exchange is monitored by electrical impedance spectroscopy. It is the first time, the media exchange caused by passive dispensing is characterized by electrochemical impedance spectroscopy. Summarizing, PortaDrop allows easy combination of fluid handling by means of electrowetting and additional sensing.
Highlights
In digital microfluidics (DMF), discrete nL-droplets of reagents are controlled by applying a series of electrical potentials to an array of electrodes coated with a hydrophobic insulator
The boost converter runs at 9 kHz frequency of the pulse width modulation (PWM) signal and the duty cycle is controlled by the voltage regulator microcontroller ATtiny45
Droplet transport using the EWOD effect has been demonstrated with a maximum droplet velocity of 32.9 mm/s for a track of 24 path electrodes
Summary
In digital microfluidics (DMF), discrete nL-droplets of reagents are controlled by applying a series of electrical potentials to an array of electrodes coated with a hydrophobic insulator. A commonly used DMF device, schematically shown, consists of two parallel-adjusted glass slides with a slit for droplet transport, called closed configuration. The ITO and Teflon layer can be patterned to form locally hydrophilic spots This allows the integration of sensor elements, where the droplet tears-off while being moved across and building a column of media underneath the sensor element, which is termed virtual microwell. The underlying mechanism is called passive dispensing [14] Spacer structures separate both slides and guarantee parallel plates. Only the electrodes at the bottom plate are addressed for initiating droplet movements across the surface [3]
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