Uniform mixing in paper-based microfluidic systems using surface acoustic waves

Abstract

Paper-based microfluidics has recently received considerable interest due to their ease and low cost, making them extremely attractive as point-of-care diagnostic devices. The incorporation of basic fluid actuation and manipulation schemes on paper substrates, however, afford the possibility to extend the functionality of this simple technology to a much wider range of typical lab-on-a-chip operations, given its considerable advantages in terms of cost, size and integrability over conventional microfluidic substrates. We present a convective actuation mechanism in a simple paper-based microfluidic device using surface acoustic waves to drive mixing. Employing a Y-channel structure patterned onto paper, the mixing induced by the 30 MHz acoustic waves is shown to be consistent and rapid, overcoming several limitations associated with its capillary-driven passive mixing counterpart wherein irreproducibilities and nonuniformities are often encountered in the mixing along the channel--capillary-driven passive mixing offers only poor control, is strongly dependent on the paper's texture and fibre alignment, and permits backflow, all due to the scale of the fibres being significant in comparison to the length scales of the features in a microfluidic system. Using a novel hue-based colourimetric technique, the mixing speed and efficiency is compared between the two methods, and used to assess the effects of changing the input power, channel tortuousity and fibre/flow alignment for the acoustically-driven mixing. The hue-based technique offers several advantages over grayscale pixel intensity analysis techniques in facilitating quantification without limitations on the colour contrast of the samples, and can be used, for example, for quantification in on-chip immunochromatographic assays.

Precipitation of hydroxyapatite at 37 °C in a meso oscillatory flow reactor operated in batch at constant power density

“…24 In the present work, mixing was monitored by "color" measurements. 25 The technique uses the hue instead of the intensity (or brightness), enabling the use of multiple colors in a manner not possible with traditional methods that use grayscale or a specific color channel, for example, the red component of the RGB color space. 24 Concerning pH monitoring, conventional glass-type electrodes have been widely used.…”

“…This method has been developed by Rezk et al 25 to evaluate the mixing performance of paper-based microfluidic systems. They proposed hue instead of intensity (or brightness) as an independent means to quantify mixing.…”

On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves

“…To date, many acoustic-based particle manipulation functions (e.g., focusing, separating, sorting, mixing, and patterning) have been realized (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43). None of these approaches, however, have achieved the dexterity of optical tweezers; in other words, none of the previous acoustic-based methods are capable of precisely manipulating single microparticles or cells along an arbitrary path in two dimensions.…”

Laser micromachined hybrid open/paper microfluidic chips

“…25 We hypothesize that these irregularities would be small for our device because the length-scale of the fibers (pore size: 11 lm) was small in comparison to the microfluidic structures (channel diameter: w p ¼ 1 mm). Furthermore, the orientation of the fibers in Whatman Grade 1 paper is isotropic in the plane of the paper, eliminating irregularities that arise from the orientation of the channels relative to the paper fibers.…”