Controlled Lateral Positioning of Microparticles Inside Droplets Using Acoustophoresis

Abstract

In this paper, we utilise bulk acoustic waves to control the position of microparticles inside droplets in two-phase microfluidic systems and demonstrate a method to enrich the microparticles. In droplet microfluidics different unit operations are combined and integrated on-chip to miniaturise complex biochemical assays. We present a droplet unit operation capable of controlling the position of microparticles during a trident shaped droplet split. An acoustic standing wave field is generated in the microchannel, and the acoustic forces direct the encapsulated microparticles to the centre of the droplets. The method is generic and requires no labelling of the microparticles, and is operated in a non-contact fashion. It was possible to achieve 2+fold enrichment of polystyrene beads (5 µm in diameter) in the centre daughter droplet with an average recovery of 89% of the beads. Red blood cells were also successfully manipulated inside droplets. These results show the possibility to use acoustophoresis in two-phase systems to enrich microparticles, and opens up for new dropletbased assays that are not possible to perform today.

Intra-droplet acoustic particle focusing: simulations and experimental observations

“…Of these, acoustophoresis has the advantages of being label free, gentle, and operated in non-contact mode. However, in our previous work, there have been experimental indications that the acoustic particle focusing inside droplets is impaired by a difference in acoustic properties between the droplet and the continuous phase (Fornell et al 2015). Typically, water-based solutions are used as the dispersed phase as living cells are often encapsulated inside the droplets, and fluorinated oils such as Novec HFE-7500 or FC-40 with the addition of fluorosurfactants are used as the continuous phase (also known as the carrier oil).…”

“…Recently, acoustic particle manipulation has also been implemented in droplet-based microfluidic systems for manipulation of whole droplets (Schmid et al 2014;Leibacher et al 2015;Sesen et al 2015) and focusing of particles and cells inside droplets (Fornell et al 2015Park et al 2017).…”

“…To control the position of particles inside droplets both active and passive methods have been implemented including magnetophoresis (Lombardi and Dittrich 2011;Lee et al 2014;Brouzes et al 2015), dielectrophoresis (Han et al 2017), acoustophoresis (Fornell et al 2015Park et al 2017) and hydrodynamic methods (Kurup and Basu 2012;Sun et al 2012;Hein et al 2015). Of these, acoustophoresis has the advantages of being label free, gentle, and operated in non-contact mode.…”

Acoustic impedance-based manipulation of elastic microspheres using travelling surface acoustic waves

“…3,4 The ARF depends on the size, shape, density, and compressibility of the suspended micro-objects, and variations in any of these parameters can be used to distinguish, separate, sort, trap and manipulate microobjects on-demand. 5,6 Most acoustouidics devices, which use bulk acoustic waves (BAWs), [7][8][9][10] Lamb waves (LWs), 11,12 travelling surface acoustic waves (TSAWs) [13][14][15][16][17][18] or standing surface acoustic waves (SSAWs) [19][20][21][22] for the manipulation of microparticles, cells, bacteria, or droplets, depend upon variations in the sizes of the micro-objects. A few studies have reported the use of variations in the intrinsic properties such as the density or compressibility of the micro-objects for their manipulation instead of their sizes.…”

Bead mediated separation of microparticles in droplets

“…All of the above-described systems require fabrication or use of an electric or magnetic system on-chip. Some other works have demonstrated separation or enrichment using acoustic waves [27,28] or flow fields [29,30]. …”