Cells and Nanoparticles in Flow - Namish Kokkula

Introduction

The study of the interplay between cells and nanoparticles in flow has been an ongoing interest in the field of vascular-targeted therapeutics. Lola Eniola Adefeso at the University of Michigan has used microfluidic models to simulate conditions required for drug delivery via nano-sized vascular-targeted carriers (VTCs). By using microfluidic chambers designed after human blood vessels, complete with walls coated in endothelial cells, the flow and adhesive behavior of nanoparticles can be evaluated.¹

Microvessels

Microvessels are a crucial location of the vascular system, as they are where nutrients and leukocytes cross over from blood into tissues. These vessels are also valuable in the study of human diseases, particularly in cancer due to an enhanced permeability and retention effect that is present in tumor capillaries. This brought rise to treatments via the localization of nanoparticles to tumor tissue through microvessels. The majority of the microvessels in a tumor have diameters between 20-100 microns, which can be simulated by microfluidic channels with endothelial walls.¹

Nanoparticle Margination

The effectiveness of vascular-targeted drug delivery is dependent on the margination of VTCs to the vascular walls. The endothelial cells at these walls have unique or overexpressed biomolecules caused by diseases, which can be detected by VTCs for therapy. In blood flow, red blood cells tend to congregate in the midstream of a vessel, leaving a red blood cell-free plasma layer at the vessel walls. This causes leukocytes and platelets to concentrate at this cell free layer, and this “near wall excess” allows for optimal interaction with endothelial cells regardless of vessel diameters. The margination of particles, which refers to both localization and adhesion, to the vessel walls is affected greatly by their physical properties, such as size and shape. Thus, VTCs must be designed for optimal margination by taking their physical properties into consideration.²

VTC Spheres

Figure 1: VTC sphere binding density vs sphere diameter. ¹

The use of microspheres (>1 micron diameter) and nanospheres (<1 micron diameter) are common structures in vascular-targeted drug delivery. In one study, polystyrene spheres from a range of 200 nm to 5 microns were coated with the ligand sLe^A, which is specific to E-selectin expressed by inflamed endothelial cells, to evaluate their margination in human blood flow in microfluidic channels of 28 and 43 microns. It was found that nanospheres exhibited minimal near wall excess, as they remained in the red blood cell core, filling in space between red blood cells. Therefore, microspheres were much more effective in binding to the endothelial walls than microspheres at the same concentration.¹

VTC Rods

The use of rods in vascular-targeted drug delivery is a viables alternative to spheres due to their improved resilience in blood flow. Another study used polystyrene rods of aspect ratios between 2 to 11 and diameters from 0.5 to 2 microns coated in sLe^A flowed through a channel with height of 254 microns and 1 cm width. It was found that the VTC microrods marginated to the walls much better than microspheres at the same concentration and shear rates, proving to be a competitive VTC design. However, nanorods performed worse than their nanosphere counterparts. Overall, it was found that for rods the limiting measure of margination ability was the rod’s minimum major axis length of 2.4 microns, and rods with a smaller major axis exhibited significantly lower adhesion rates.²

Effect of Blood Components on VTC Margination

The composition of blood has a significant impact on the margination of VTCs to the walls, including the local concentration of leukocytes and red blood cells as well as the flow patterns. In laminar flow, a higher hematocrit, the percentage by volume of red blood cells, leads to an increased collision rate between VTC and red blood cells, which increases the margination speed of VTCs towards the walls. However, in pulsatile flow, the same behavior is not experienced, and as the red blood cell-free layer varies significantly regardless of a constant hematocrit, leading to periods of higher and lower local concentrations of red blood cells. Furthermore, Leukocytes interfere with VTC margination in two main methods. The first is the competitive binding of surface ligands on endothelial cells, which prevent VTCs from adhering to the walls. The second is the collision-driven dissociation of adhered VTCs, which is especially present in pulsatile flow.³

Channel Design for VTCs

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Recent Advances in Combination VTCs

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References

[1] Namdee, K.; Thompson, A. J.; Charoenphol, P.; Eniola-Adefeso, O. Margination Propensity of Vascular-Targeted Spheres from Blood Flow in a Microfluidic Model of Human Microvessels. Langmuir 2013, 29 (8), 2530–2535. https://doi.org/10.1021/la304746p.

[2] Thompson, A. J.; Mastria, E. M.; Eniola-Adefeso, O. The Margination Propensity of Ellipsoidal Micro/Nanoparticles to the Endothelium in Human Blood Flow. Biomaterials 2013, 34 (23), 5863–5871. https://doi.org/10.1016/j.biomaterials.2013.04.011.

[3] Charoenphol, P.; Onyskiw, P. J.; Carrasco-Teja, M.; Eniola-Adefeso, O. Particle-Cell Dynamics in Human Blood Flow: Implications for Vascular-Targeted Drug Delivery. J. Biomech. 2012, 45 (16), 2822–2828. https://doi.org/10.1016/j.jbiomech.2012.08.035.

[4] Charoenphol, P.; Huang, R. B.; Eniola-Adefeso, O. Potential Role of Size and Hemodynamics in the Efficacy of Vascular-Targeted Spherical Drug Carriers. Biomaterials 2010, 31 (6), 1392–1402. https://doi.org/10.1016/j.biomaterials.2009.11.007.

[5] Fish, M. B.; Banka, A. L.; Braunreuther, M.; Fromen, C. A.; Kelley, W. J.; Lee, J.; Adili, R.; Holinstat, M.; Eniola-Adefeso, O. Deformable Microparticles for Shuttling Nanoparticles to the Vascular Wall. Sci. Adv. 2021, 7 (17), eabe0143. https://doi.org/10.1126/sciadv.abe0143.