The delivery of polynucleotides such as messenger RNA is now industrialized via vectorization in lipid nanoparticles. These nano-objects are nanocomposites in which the mRNA is condensed by complexation with a cationic lipid and coated with a shell of phospholipids and cholesterol. Their synthesis involves several rapid supramolecular assembly processes, including nanoprecipitation of hydrophobic lipids by solvent switching, electrostatic complexation of mRNA with cationic lipids to form a coacervate, and self-assembly of lipids. If the industrial production of these objects is now recorded, several challenges remain to be met to ensure their full development and generalization as a vaccine and therapeutic tool. Thus, their structure is complex, highly dependent on the formulations and the process, and there is no consensus. The efficiency of encapsulation and delivery remains very empirical and improvable. The process conditions, in particular the mixing of the species and the purification by filtration, are not optimized because the elementary mechanisms and their kinetics are not described. Storage conditions remain complex and the efficiency of encapsulation and delivery must also be improved. Finally, costs must be lowered. The Prosalide project targets to deliver a detailed understanding of the complete manufacturing chain thanks to a multidisciplinary team combining skills in physical-chemistry of assemblies, micro and millifluidics, filtration, hydrodynamics, microbiology, structural characterization. We will study in detail the assembly phenomena via the intensified establishment of phase diagrams. We will deploy a micro/millifluidic tandem allowing rapid mixing of solutions, in operando characterization and purification by tangential filtration. This approach will make it possible both to exploit the potential of microfluidics for the screening and characterization of mechanisms and that of millifluidics, which is the industrial production scale for these objects. We will propose new synthetic routes based on the use of composition sequences. Finally, we will quantify in detail the internal structure of the objects, their ability to encapsulate and deliver the polynucleotide, and their metastability. The PROSALIDE project will thus provide generic and free knowledge to understand the link between formulation, synthesis path, mesostructure, metastability, encapsulation and delivery.