Open-Top Optofluidics Architecture Adds Flexibility, Increases Droplet Speeds

Optoelectrowetting (OEW) is a digital optofluidic technology based on the principles of light-controlled electrowetting that enables the actuation and manipulation of discrete droplets of liquid. In traditional OEW devices, droplets are sandwiched between a bottom active OEW substrate and a top layer ground electrode substrate. As a result, any droplet-dispensing or -extraction techniques must be integrated from the side openings.

In research published by the University of California, Berkeley (UC Berkeley), a single-sided, coplanar OEW device that features a conductive metal mesh grid integrated with the device surface to allow droplets to be exposed from above. Droplets can still move freely around the 2D device surface, but are now accessible from above due to the open-top design. The new design allows for easier access as well as more fluidic operations and integration schemes than previous generations of OEW devices.

The coplanar OEW device has the potential to automate and scale down biological and chemical processes that currently use conventional well plates, resulting in reduced volumes of solutions, reduced manual labor, and increased processing throughput. Due to its open-top configuration, it introduces possibilities for optofluidic technologies to realize greater system integration capabilities and new biological and chemical applications.

To determine how the integrated metal mesh grid would influence device and droplet performance, the researchers developed a theoretical model of the coplanar OEW device. Based on analysis of the model, they optimized droplet actuation while maintaining reliable droplet movement.

Schematic of the coplanar light-actuated optoelectrowetting microfluidic device that features an integrated metal mesh grid. A droplet on the device surface is actuated and moved around the 2D plane under the influence of an incident optical pattern. Courtesy of Jodi Loo et al.

While performing basic droplet manipulations (such as merging and parallel actuation of droplets), the coplanar OEW device demonstrated speeds up to 4.5 cm/s — more than 2× faster than the droplet actuation performance of existing OEW devices. The coplanar device achieved higher droplet speeds in spite of a marginal reduction in effective force compared to a traditional device. The researchers attributed this result to the reduction in friction due to the elimination of the top cover.

The researchers also showed that the coplanar OEW device could operate with 95% reduced light intensity.

To demonstrate the versatility of their optofluidic platform, the researchers integrated a droplet-on-demand dispensing system from above with the coplanar OEW device. The researchers injected, collected, and positioned individual droplets and achieved large-scale droplet arrays of up to 20 × 20, covering the whole device surface area. Larger OEW devices could allow for even more droplets to be accommodated on a chip, the researchers believe.

The UC Berkeley team has developed an OEW platform for reliable droplet manipulation that can accomplish most basic biological and chemical benchtop techniques. With faster droplet speeds and more efficient operational conditions, this coplanar device could improve OEW’s versatility as a light-actuated digital microfluidics platform.

The research was published in the Journal of Optical Microsystems (www.doi.org/10.1117/1.JOM.1.3.034001).