Amphiphile Blending Controls Micelle Morphology to Enhance <i>In Vitro</i> Performance and Selective <i>In Vivo</i> mRNA Delivery to Lung and Spleen

Polymeric micelles assembled from amphiphilic block copolymers represent a versatile platform for mRNA delivery due to their modular chemical design and tunable self-assembly. Here, we report a library of ternary mixed micelles composed of amphiphiles displaying a cationic oligoamine (A7), anionic carboxyethyl acrylate (CEA), and neutral poly-(ethylene glycol) (PEG), engineered to enhance <i>in vitro</i> transfection and enable passive, ligand-free <i>in vivo</i> organ targeting. Systematic blending of these components allowed modulation of micelle surface chemistry and morphology, which are key parameters that govern mRNA complexation, stability, and biodistribution. Increasing incorporation of anionic amphiphiles resulted in a distinct evolution of morphology from spherical micelles to elongated wormlike structures, while maintaining uniformly high zeta potential and efficient mRNA complexation. Top-performing ternary formulations <i>in vitro</i> achieved over 95% transfection efficiency and cell viability across multiple cell types, representing a 2-fold performance enhancement relative to homomicelle controls. These formulations also exhibited robust mRNA internalization and efficient endosomal escape, resulting in a 7-fold improvement in GFP mean fluorescence intensity. Furthermore, by controlling micelle aggregation, we demonstrate that serum stability can be significantly improved, highlighting the tunability of this mixed micelle system. Finally, <i>in vivo</i>, increasing incorporation of anionic amphiphiles consistently shifted mRNA expression from lungs to spleen. We attribute this shift in organ tropism to morphology, where spherical micelles sequester in the lungs and wormlike ternary mixed micelles accumulate in the spleen. Transmission electron microscopy confirmed the elongated wormlike architecture that correlated with spleen localization, supporting morphology as the dominant factor driving organ selectivity. This morphology-driven organ selectivity marks a departure from traditional charge-centric targeting paradigms and establishes a tunable, composition-based framework for directing polymeric micelles to specific organs. Together, these findings introduce a simple yet powerful strategy for enhancing <i>in vitro</i> mRNA delivery and achieving organ-specific mRNA delivery using a blended micelle platform.