The LNPs used in the Pfizer and Moderna formulations are a composite of four different lipids (fats): cationic lipids, polyethylene glycol, phospholipids and cholesterol. The latter two are for structural integrity, PEG is for optimizing distribution and stealth qualities in vivo, and the cationic lipids (+) are to hold the mRNA (-) ‘in place’.
… The LNPs used in the COVID-19 injectable products owe their existence to liposomes. Liposomes are artificial bi-layer structures with hydrophobic membranes that can be used as delivery vehicles for drugs. The LNPs are unique, however, in a number of ways, but most relevantly in the realm of toxicity to humans due to the incorporation of cationic lipids. The phospholipids and cholesterol are no big deal.
… LNPs get taken up by cells and stuffed into vacuoles called endosomes (well, this is the proposed mechanism of action) where, due to the acidic pH inside the endosome, the magical cationic lipids get positively charged due to their ionizability, which is thought to change the shape of the nanoparticle itself to allow it to escape the endosome to release its fancy payload.