Associate Professor Old Dominion University, United States
Introduction: The ability to separate cells and microparticles in a sessile drop enables new capabilities in biomedical engineering applications such as developing a point of care circulating tumor cell separation in blood samples and detection of bloodborne pathogens. Recently, we demonstrated the utility of AC electric fields to produce a fluid flow in the sessile drop and subsequently dielectrophoretic (DEP) force was used to concentrate the particles of interested on the electrodes. Broadly, electric fields applied on a sessile droplet induces electrohydrodynamic effects in the sample fluid and suspended particles such as electro osmosis flow and DEP forces, which depend on factors like the electric field strength, frequency, and properties of the liquid. Additionally, to effectively separate the particles and concentration of particles on the electrodes, additives are added to the sample. For example, Tween, specifically Tween 20 or Tween 80, is a nonionic surfactant that can significantly alter the interfacial properties of droplets. Tween lowers the surface tension of the liquid, which affects the droplet shape and stability. In this work, we studied the effects of surfactant and AC electric fields in the enrichment of target microparticles.
Materials and
Methods: Binary mixtures of commercially available 5 µm (non-target; intended to get rid of) and 10 µm (target; intend to isolate from non-target) diameter polystyrene beads samples (Phosphorex LLC, Hopkinton, USA) were prepared with 0.4%, 4% ,40% ratios of 10 µm:5 µm beads in Tris EDTA (TE) buffer (Thermo Fisher Scientific, Ward Hill, USA) with 20 µs/cm conductivity. The commercially available tween 20 (Fisher bioreagents, NJ, USA) was added by volume ratios of 0%, 0.0002%, 0.002%, 0.02%, 0.1%. Interdigitated T-electrodes were used in experiments which were designed manufactured and used biomedical applications (e.g., biosensing) by our group. 50 µl of the sample was pipetted over the T-electrode array and a sequence of time dependent electric potentials (125kHz-1V-5s, 20MHz-9V-20s, 1MHz-9V-5s & 1kHz-5V-1s; Tektronix AFG-1062, Beaverton, USA) was applied and repeated the sequence by using lab view program with NI LabVIEW (2023 Q3). When the above signal sequence is applied, 2 clusters of beads started to form on either side of the droplet (Figure 1 (a)) and 2.5 µl of each cluster was extracted at the 15th cycle using a pipette, mixed and diluted to 150 µl with deionized water and, analyzed using the Flow Cytometer (MACSQuant10, Bergisch Gladbach, Germany). The total amount of beads was kept approximately constant (~ 53000 beads per droplet) in all the experiments.
Results, Conclusions, and Discussions: As discussed earlier, bead clusters were formed in 15th cycle (Figure 1 (a)). Figure 1(b) shows the experimental results. The enrichment is decreased with increasing the tween amount (figure 1(b)). In addition, the enrichment is also reduced when no tween in the sample. When the percentage of target beads decreased below 1% while keeping the total beads count of the droplet constant, the number of target beads got too low in the clusters and making it difficult to count. In conclusion, the movement of microparticles was affected by the hydrodynamic effects such as DEP force, electroosmosis flow and sample composition. The effect of tween on microparticle enrichment in a sessile droplet under an electric field involves complex interdependencies between surfactant dynamics, electrohydrodynamic forces, and particle behavior. Since hydrodynamic properties are dependent on particle properties such as size and dielectric constant, our experimental results can be extended for enrichment of rare cells (e.g., tumor cells) in the blood samples.
Acknowledgements (Optional): DN acknowledged the funding National Science Foundation (NSF): grant numbers: 2300064 and 2310106.
