Effect of shape on cell internalization of polymeric hydrogel nanoparticles

Recent progress in drug discovery has enabled us to target specific intracellular molecules to achieve therapeutic effects. These next generation therapeutics are often biologics which cannot enter cells by mere diffusion. Therefore it is imperative that drug carriers are efficiently internalized by cells before releasing their cargo. Nanoscale polymeric carriers are particularly suitable for such intra-cellular delivery. Although size and surface-charge has been the most studied parameters for nanocarriers, it is now well appreciated that particle shape also plays a critical role in their transport across physiological barriers. Hence there is increasing interest in fabricating shape-specific polymeric nano and microparticles for efficient delivery of drugs and imaging agents. Nanoimprint lithography methods, such as Jet-and-flash imprint lithography (J-FIL), provide versatile top-down processes to fabricate shape-specific, biocompatible nanoscale hydrogels that can deliver therapeutic and diagnostic molecules in response to disease-specific cues. However, the key challenges in top-down fabrication of such nanocarriers are scalable imprinting with biological and biocompatible materials, ease of particle-surface modification using both aqueous and organic chemistry as well as simple yet biocompatible harvesting. Here we report that a biopolymer-based sacrificial release layer in combination with improved nanocarrier-material formulation can address these challenges. The sacrificial layer improves scalability and ease of imprint-surface modification due to its switchable solubility through simple ion exchange between monovalent and divalent cations. This process enables large-scale bio-nanoimprinting and efficient, one-step harvesting of hydrogel nanoparticles in both water- and organic-based imprint solutions. We also show that when shape is decoupled from volume, charge and composition, mammalian cells preferentially internalize disc-shaped nanohydrogels of higher aspect ratios over nanorods. Interestingly, unlike nanospheres, larger-sized hydrogel nanodiscs and nanorods are internalized more efficiently. Uptake kinetics, efficiency and internalization mechanisms are all shape-dependent and cell-type specific. Although macropinocytosis is used by all cells, epithelial cells uniquely internalize nanodiscs using caveolae pathway. On the other hand, endothelial cells use clathrin-mediated uptake along with macropinocytosis for all shapes and show significantly higher uptake efficiency compared to epithelial cells. We also study the effect of shape and surface properties for their tissue uptake and penetration using spheroids as a 3D tumor model and show that hydrophobic particles show no difference in penetration inside such models even after 125 fold reduction in volume. These results provide a fundamental understanding of how cell and tissue behavior is influenced by nanoscale shape and surface properties and are critical for designing improved nanocarriers and predicting nanomaterial toxicity.