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Postprint
This is the accepted version of a paper presented at Acoustofluidics 2016, Kongens Lyngby, Denmark, September 22-23 2016.
Citation for the original published paper:
Fornell, A., Nilsson, J., Jonsson, L., Periyannan Rajeswari, P., Joensson, H. et al. (2016) Particle enrichment in two-phase microfluidic systems using acoustophoresis.
In:
N.B. When citing this work, cite the original published paper.
Permanent link to this version:
http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-309851
Particle enrichment in two-phase microfluidic systems using acoustophoresis
Anna Fornell
1, Johan Nilsson
1, Linus Jonsson
1, Prem Kumar Periyannan Rajeswari
2, Haakan N. Joensson
2and Maria Tenje
1,31Dept. Biomedical Engineering, Lund University, Lund, Sweden E-mail: anna.fornell@bme.lth.se
2Div. Proteomics and Nanobiotechnology, Science for Life Laboratory, KTH Royal Institute of Technology, Stockholm, Sweden
3Engineering Sciences, Science for Life Laboratory, Uppsala University, Uppsala, Sweden Introduction
Droplet microfluidics provides a tool to create confined and discrete reaction chambers for biological and chemical experiments at high throughput [1]. The method has attracted special interest for single-cell studies since it is possible to encapsulate single cells inside droplets. This makes it possible to create more sensitive analysis of cells since the results arise from a particular cell and is not the average response of thousands of cells [2]. To miniaturize biological assays using droplets the same processes and steps performed in standard protocols have to be integrated on microfluidic chips. Here, we present a method to concentrate particles in droplets by combing acoustic particle focusing inside droplets with a trident shaped droplet split, and we report ten times higher concentration of particles in the center daughter droplets compared with the side daughter droplets when ultrasound is applied [3].
Experimental
An illustration of the developed concept for particle enrichment inside droplets and a photograph of the fabricated acoustofluidic device are shown in figure 1. The microfluidic channels were wet-etched on a silicon wafer and sealed by anodic bonding of a 1.1 mm thick glass lid. Aqueous droplets with polystyrene microparticles (5 µm) were generated in a continuous organic phase (olive oil). All liquid flows were controlled by syringe pumps. A piezoelectric transducer was glued on the silicon side of the chip. To achieve particle focusing inside the droplets the transducer was actuated with a frequency matched to create λ/2- resonance in the main channel. After the particle focusing step each droplet was split into three daughter droplets in a trifurcation. The performance of the system was evaluated by measuring the concentration of particles in the center and side daughter droplets using a Coulter Counter. To demonstrate the possibility to manipulate cells inside droplets and show the suitability of the system for biological applications, red blood cells were encapsulated and manipulated inside the droplets.
Figure 1: Left: Illustration of the concept for acoustic particle enrichment inside droplets. Aqueous droplets containing particles are generated. The applied acoustic field moves the particles to the pressure node in the center of the droplets, and finally each droplet is split into three daughter droplets at a trifurcation. Right: Photograph of the microfluidic chip for particle enrichment in droplets. Channel width x channel height 435 µm x 165 µm.
Results
At λ/2-resonance (1.8 MHz, 25 Vpk-pk) the particles inside the droplets were moved to the center of the droplets. The droplets were transported downstream to a trifurcation where each droplet was split into three daughter droplets with approximately the same size (25-28 nl). When ultrasound was applied the majority of the particles were enriched in the center daughter droplets as seen in figure 2.
Ultrasonic transducer Water
Oil
Oil Flow
1 2 3 4
1 Droplet generation
2 Particle encapsulation
3 Acoustic focusing
4 Droplet splitting
2 cm
Piezo
Droplet split Droplet
generation
Figure 2: Particle enrichment in droplets. Without ultrasound (left photograph) the particles are positioned in the entire main droplet resulting in particles in all daughter droplets after the droplet split. When ultrasound is applied (right photograph) the particles are directed to the center daughter droplet during the splitting step resulting in particle enrichment.
The concentration of particles in the daughter droplets after the acoustic focusing step and the droplet split is presented in figure 3. When ultrasound was applied the concentration of particles in the center daughter droplets was more than ten times higher than in the side daughter droplets. In the control experiment without ultrasound only a small concentration difference was observed between the center and side daughter droplets.
Figure 3: Concentration of
particles with and without ultrasound during the droplet split. Experiments were performed three times (n=3) with data acquisition in triplicates.
Error bars represent ± standard deviation.
To investigate the possibility to use the described method for droplet based cell assays, red blood were encapsulated inside the droplets. When ultrasound was applied to the system the cells were found to be immediately moved to the center of the droplets, see figure 4.
Figure 4: Acoustic focusing of
red blood cells in droplets. At application of ultrasound the cells were positioned to the center of the droplet (right photo). It was also observed that the droplet interface was slightly deformed by the ultrasound.
Conclusion
We have developed a method to enrich particles inside droplets by combining acoustic focusing with a trident shaped droplet split. 89% of the particles were collected in the center daughter droplets when 2/3 of the original droplet volume was removed. It was also shown that the method can be used to manipulate red blood cells inside droplets. These results opens up for novel droplet based assays that are not possible to perform today.
References
[1] R. Seemann, M. Brinkmann, T. Pfohl and S. Herminghaus, Rep Prog Phys, 75, 16601 (2012).
[2] D. Di Carlo and L. P. Lee, Anal. Chem., 78, 7918–7925 (2006).
[3] A. Fornell, J. Nilsson, L. Jonsson, P. K. Periyannan Rajeswari, H. N. Joensson and M. Tenje, Anal. Chem., 87, 10521–10526 (2015).
Ultrasound off Ultrasound on
Focused particles Randomly distributed particles
Ultrasound off Ultrasound on
Randomly distributed cells Focused cells With ultrasound
0 3 6 9 12 15
Concentration x106 particles/ml Original SideCenter Original Center Side
Without ultrasound ENRICHMENT
