Influence of Silica Nanoparticle Density and Flow Conditions on Sedimentation, Cell Uptake, and Cytotoxicity

Careful evaluation of the toxicological response of engineered nanomaterials (ENMs) as a function of physicochemical properties can aid in the design of safe platforms for biomedical applications including drug delivery. Typically, in vitro ENM cytotoxicity assessments are performed under conventional static cell culture conditions. However, such conditions do not take into account the sedimentation rate of ENMs. Herein, we synthesized four types of similar size silica nanoparticles (SNPs) with modified surface roughness, charge, and density and characterized their cytotoxicity under static and dynamic conditions. Influence of particle density on sedimentation and diffusion velocities were studied by comparing solid dense silica nanoparticles of approximately 350 nm in diameter with hollow rattle shape particles of similar size. Surface roughness and charge had negligible impact on sedimentation and diffusion velocities. Lower cellular uptake and toxicity was observed by rattle particles and under dynamic conditions. Dosimetry of ENMs are primarily reported by particle concentration, assuming homogeneous distribution of nanoparticles in cell culture media. However, under static conditions, nanoparticles tend to sediment at a higher rate due to gravitational forces and hence increase effective doses of nanoparticles exposed to cells. By introducing shear flow to SNP suspensions, we reduced sedimentation and nonhomogeneous particle distribution. These results have implications for design of in vitro cytotoxicity assessment of ENMs and suggest that among other factors, sedimentation of nanoparticles in toxicity assessment should be carefully considered.