Size tunable nanoparticle formation employing droplet fusion by acoustic streaming applied to polyplexes

Automated mixing of fluids with control over mixing parameters is of highest importance for reproducible production and chemical synthesis processes. We here introduce surface acoustic waves (SAW) induced mixing of µl droplets for tailorable nanoparticle (NP) formation. Nucleic acid therapeutics represent extremely potent and innovative approaches to a variety of medical challenges, such as the treatment of cancer and genetic diseases. In this study, we apply this method to produce nucleic acid polymer complexes. Fusing two droplets containing either pDNA or cationic polymers leads to the formation of well-defined polyplexes. We show that droplet size and incubation time do not influence the desired particle characteristics significantly. However, the resulting nanoparticle diameter strongly depends on the SAW power level and educt concentrations, which indicates a kinetically controlled assembly process, while the particle shape is largely unaffected. Applying our novel technique to the formation of three-component-NP, we find that the choice of the mixing order can be used to decrease NP size even further. To address the kinetic interplay between mixing and particle growth, we apply our technique to homogenous mixing at high salt concentrations followed by a subsequent dilution step. Finally, by comparing various hand and SAW mixed polyplexes, we demonstrate significant differences in size while the cytotoxicity and in vitro efficacy remain roughly the same.