Shaping nanoparticle diffusion through biological barriers to drug delivery

Nanoparticle drug delivery systems encounter many biological barriers, such as the extracellular matrix and mucus gels, that they must bypass to gain access to target cells. A design parameter that has recently gained attention is nanoparticle shape, as it has been shown elongated rod–shaped nanoparticles achieve higher diffusion rates through biological gels. However, the optimal dimensions of rod-shaped nanoparticles to enhance this effect has yet to be established. To systematically approach this, rod-shaped nanoparticles were synthesized by mechanically stretching 100 nm, 200 nm, and 500 nm spherical nanoparticles. Transmission electron microscopy confirmed this procedure yields a significant fraction of elongated rods and remaining spheres could be removed by centrifugation. Fluorescent microscopy and multiple particle tracking analysis was then used to characterize rod-shaped and spherical nanoparticle diffusion in MaxGel®, a model extracellular matrix hydrogel. When dispersed in MaxGel, we found rod-shaped nanoparticles exhibited the greatest enhancement in diffusion rate when their length far exceeds the average hydrogel network size. These results further establish the importance of shape as a design criterion to improve nanoparticle-based drug delivery systems.