Microfluidic techno­logies have seen great advances over the past few decades in addressing appli­cations such as biochemical analysis, pharma­ceutical development, and point-of-care diagnostics. Miniaturi­zation of biochemical operations performed on lab-on-a-chip micro­fluidic platforms benefit from reduced sample, reagent, and waste volumes, as well as increased paralleli­zation and automation. This allows for more cost-effective opera­tions along with higher throughput and sensitivity for faster and more efficient sample analysis and detection. Optoelectro­wetting (OEW) is a digital opto­fluidic technology that is based on the principles of light-controlled electro­wetting and enables the actuation and mani­pulation of discrete droplets.

OEW devices have many advantages, such as the ability for large-scale, real-time, and recon­figurable control of picoliter- to micro­liter-sized droplets by adjusting the number and size of low-intensity optical light patterns incident on the device. With each indi­vidual droplet on the OEW device acting as its own bioreaction chamber, the OEW device also has the ability to support multiplex capa­bilities. This can prove to be beneficial in applications such as single-cell analysis and genomics or combinatorial libraries. Previous tradi­tional OEW devices provide a flexible platform to perform chemical and biological assays such as real-time isothermal polymerase chain reaction with basic droplet mani­pulation techniques. However, in these OEW devices, droplets are sandwiched between a bottom active OEW substrate and a top layer ground electrode substrate, forcing any input/output fluidic configura­tions to be integrated from the side openings. Although feasible, this can prove to be limiting for system inte­gration.

Researchers from the University of California, Berkeley, created a single-sided, co-planar OEW device that allows for indivi­dualized and parallel droplet actuation and benefits from easier droplet accessi­bility from above for more input/output confi­guration schemes. This was achieved by eliminating the need for a top cover electrode found in tradi­tional OEW devices by fabricating a metal mesh grid integrated on the OEW device. Droplets can still move freely around the two-dimensional device surface and are now accessible from above due to the open-top design. In their research, they have also derived a theoretical model of the co-planar OEW device to better understand how the inte­grated metal mesh grid affects device and droplet performance. Analysis gathered from the co-planar OEW model was used to optimize the co-planar device structure and operation. They have demons­trated the ability for basic droplet mani­pulation, such as indi­vidual droplet operations in parallel, merging of multiple droplets, and the ability to handle and move droplets with varying volumes simul­taneously.

The co-planar device improves on the traditional OEW device's droplet actuation performance with speeds more than two times faster, up to 4.5 cm/s. Higher droplet speeds on the co-planar OEW device achieved despite a marginal reduction in effective force compared to the tradi­tional OEW device can be partly attri­buted to the reduction in friction due to elimi­nation of the top cover. In addition, the ability to operate co-planar OEW devices with 95% reduced light intensity was demons­trated.

To showcase the benefit of having exposed droplets to accommo­date a wider range of input/output confi­gurations, a droplet-on-demand dispensing system from above was integrated with the co-planar OEW device to inject, collect, and position indi­vidual droplets and form large-scale droplet arrays of up to 20 by 20, covering the whole device surface area. Creating larger OEW devices should allow for even more droplets to be accommo­dated on chip. With this research, the team has developed an OEW platform for reliable droplet mani­pulation that can accomplish most basic biological and chemical benchtop techniques. The co-planar OEW device expands the flexi­bility and range of possi­bilities for opto­fluidic techno­logies to realize greater system integration capa­bilities and biological and chemical appli­cations. (Source: SPIE)

Reference: J. Loo et al.: Co-planar light-actuated optoelectrowetting microfluidic device for droplet manipulation, J. Opt. Microsys. 1, 034001 (2021); DOI: 10.1117/1.JOM.1.3.034001

Link: Integrated Photonics Laboratory, University of California Berkeley, Berkeley, USA