Distribution of SiO₂ nanoparticles in 3D liver microtissues

Nanoparticles (NPs) are used in numerous products in technical fields and biomedicine; their potential adverse effects have to be considered in order to achieve safe applications. Besides their distribution in tissues, organs, and cellular localization, their impact and penetration during the process of tissue formation occurring in vivo during liver regeneration are critical steps for establishment of safe nanomaterials.

In this study, 3D cell culture of human hepatocarcinoma cells (HepG2) was used to generate cellular spheroids, serving as in vitro liver microtissues. In order to determine their differential distribution and penetration depth in HepG2 spheroids, SiO₂ NPs were applied either during or after spheroid formation. The NP penetration was comprehensively studied using confocal laser scanning microscopy and scanning electron microscopy.

Spheroids were exposed to 100 µg mL^-1 SiO₂ NPs either at the beginning of spheroid formation, or during or after formation of spheroids. Microscopy analyses revealed that NP penetration into the spheroid is limited. During and after spheroid formation, SiO₂ NPs penetrated about 20 µm into the spheroids, corresponding to about three cell layers. In contrast, because of the addition of SiO₂ NPs simultaneously to cell seeding, NP agglomerates were located also in the spheroid center. Application of SiO₂ NPs during the process of spheroid formation had no impact on final spheroid size.

Understanding the distribution of NPs in tissues is essential for biomedical applications. The obtained results indicate that NPs show only limited penetration into already formed tissue, which is probably caused by the alteration of the tissue structure and cell packing density during the process of spheroid formation.