Non-spherical micro- and nanoparticles for drug delivery: Progress over 15 years

Shape of particulate drug carries has been identified as a key parameter in determining their biological outcome. In this review, we analyze the field of particle shape as it shifts from fundamental investigations to contemporary applications for disease treatment, while highlighting outstanding remaining questions. We summarize fabrication and characterization methods and discuss in depth how particle shape influences biological interactions with cells, transport in the vasculature, targeting in the body, and modulation of the immune response. As the field moves from discoveries to applications, further attention needs to be paid to factors such as characterization and quality control, selection of model organisms, and disease models. Taken together, these aspects will provide a conceptual foundation for designing future non-spherical drug carriers to overcome biological barriers and improve therapeutic efficacy.

Pharmaceutical feasibility and flow characteristics of polymeric non-spherical particles

Last decade has seen emergence of particle shape as a critical design parameter to overcome several long standing problems associated with particulate drug delivery- non-specific drug effects, RES uptake, poor bioavailability, achieving controlled release profiles, predictable degradation profiles, longer circulation time and zero order release kinetics to name a few. Non-spherical particles have been synthesized by techniques ranging from classical solvent evaporation to specialized techniques like film stretching and PRINT®. Non-spherical particles tend to show a difference in macrophage uptake, adhesion to target cells and distribution in vivo. This review also discusses these effects and its implications. Lastly, the impact of particle aspect ratio and other shape-governed parameters on flow properties, dispersion viscosities and other pharmaceutically relevant aspects have been briefly explained. Although there are no thumb rules yet, modern and classical literature on behavior of non-spherical particles has been reviewed and the observations have been trend-lined.

Impact of hydrophobic micron ellipsoids on liquid surfaces

HYPOTHESIS

Hydrophobic spheres may exhibit three impact modes after impacting the liquid surface. For the impact of common non-spherical particles in practical processes, particle shape affects the flow of surrounding fluid and forces acting on them, thus the impact behaviors of non-spherical particles may differ from those of spherical particles and remain to be revealed.

SIMULATION

The impact of hydrophobic micron ellipsoidal particles is numerically studied by solving the coupled equations of particle motion and surrounding fluid flow combining VOF method and dynamic meshing technique. The motion characteristics of the ellipsoids and fluids, and the main forces acting on the ellipsoids during impact are analyzed.

FINDINGS

The increase in the axis ratio (AR) of ellipsoid decreases the surface tension and fluid force, resulting in a smaller total force acting on the ellipsoid. Correspondingly, the ellipsoid's impact behavior changes. An impact-mode phase diagram is presented for the studied ellipsoids. Three impact modes, namely, submergence, rebound, and oscillation, exist when AR > 0.8, while only submergence and oscillation exist when AR ≤ 0.8. The critical velocities decrease with the increase in AR, which is well illustrated by the analysis of energy conversion under critical conditions.

A non-crystallization-driven strategy for the preparation of non-spherical polymeric nanoparticles

To fabricate precisely defined non-spherical nanostructures like those widely exist in the biological domain by self-assembly of synthetic polymers without employing crystallization-driven forces is still a great challenge. In this study, we report a strategy to fabricate nanoparticles with advanced hierarchical architectures using styrene and methacrylic as monomers and maleamic acid-α-methyl styrene copolymer as a macroinitiator by polymerization-induced self-assembly (PISA). The structure of the prepared particles changed from common spherical micelles to cubes with edge lengths ranging from 50 to 200 nm when the solvent was 50 wt% ethanol in water and the monomer molar ratio of styrene to 2-hydroxyethyl methacrylate was 3:1. The growth of the cubic nanoparticles exhibited an interesting self-assembly process, initially forming vesicles with irregular cubes inside them. As the polymerization progressed, the inner cubes escaped from the vesicles and finally generated well-defined cubic nanoparticles. Wide-angle X-ray scattering (WAXS) results of the cubic nanoparticles indicated that no crystalline structure existed. The formation mechanism of the cubic nanoparticles was elucidated via density functional theory (DFT) calculations. This strategy was further applied to various monomers, and its universality was confirmed by successful fabrication of different non-spherical nanoparticles such as rectangles, fusiform platelets, and triangular pyramids.

Organometallic-polypeptide Block Copolymers: Synthesis and Crystallization-driven Self-assembly in Aqueous Solutions to Rod-like and Plate-like Micelles with Polypeptide Coronas

The synthesis of polyferrocenyldimethylsilane-b-poly(L-glutamic acid) block copolymers was systematically explored. Rod-like and plate-like micelles were prepared from self-assembly of the block copolymers in aqueous solution with two different approaches. In a dissolution-dialysis approach, micelles were prepared by dissolving a block copolymer sample in excess aqueous base followed by the dialysis of the solution against water. The morphology of the resultant micelles showed composition dependence. The block copolymer sample with the lowest soluble block content formed rod-like micelles, while the other samples with higher soluble block content formed spherical micelles. In a THF-mediated approach, THF was added into a block copolymer solution, which was first prepared by the dissolution-dialysis approach, and then the THF was evaporated slowly. Plate-like micelles were obtained from all the block copolymer samples examined, and these had a broad range of soluble block content. The formation of these plate-like micelles was attributed to the crystallization of the PFDMS blocks in the presence of THF. A tendency of large platelet micelles evolving to thin filaments over time was observed. This change may have been driven by the increase in the corona-chain stretching energy as THF evaporated.