Sub 100 nm-sized lipid nanoparticles (LNPs) have been widely used in drug delivery systems (DDSs). The size of the LNPs is an important parameter for the DDS performance, such as biodistribution and gene silencing using siRNAs. However, the LNPs prepared by the conventional preparation method show a wide size distribution. To improve the LNP size distribution, we developed a microfluidic device, named the iLiNP™ device, in a previous study. This device could produce LNPs in the size range of 20 to 150 nm, but the size distribution of the large-sized LNPs needs to be further improved. From the viewpoint of the LNP formation process, a homogeneous and slow rate dilution of ethanol plays an important role in improving the large-size LNP size distribution. In this study, we developed a three-dimensional, symmetrically assembled microfluidic device named the 3D-iLiNP device with the aim of precise size control of large-sized LNPs. We designed the 3D-iLiNP device using a computational fluid dynamics simulation and demonstrated that the 3D-iLiNP device can improve the LNP size distribution. The gene silencing activity of four kinds of siRNA-loaded LNPs was investigated via in vitro and in vivo experiments to elucidate the effect of the LNP size distribution. The results revealed that the LNPs with a size between 90 and 120 nm showed higher gene silencing activity than those with other sizes. The 3D-iLiNP device is expected to improve DDS performance by precisely controlling the size of LNPs.
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