Solid carriers from polymers and lipids
A polymeric nanoparticle is a tiny solid bead of polymer — often a biodegradable one such as PLGA — with drug dissolved or trapped throughout. Because the polymer is solid, the drug cannot simply leak out; it leaves slowly as the drug diffuses through the matrix or as the polymer itself erodes. That gives you a built-in diffusion-controlled or erosion-controlled release, the same logic as a sustained-release tablet, shrunk to the nanoscale.
A solid lipid nanoparticle takes the same idea but uses a fat that is solid at body temperature instead of a polymer. It marries the gentleness and biocompatibility of a lipid with the slow, steady release of a solid matrix — handy when you want a liposome's friendly chemistry but a tablet's release control.
Self-assembling micelles
A polymeric micelle is built from block copolymers — chains with a water-loving end and a water-hating end. Above a threshold concentration they snap together into a sphere with a greasy core that dissolves poorly soluble drugs, ringed by a water-friendly shell. That threshold is the critical micelle concentration: dilute the formulation below it — for instance after injection into a large volume of blood — and the micelles can fall apart and dump their cargo.
Dendrimers: trees with countable arms
A dendrimer is not an aggregate at all but a single, precisely branched molecule, grown layer by layer like a tree. Each new generation doubles the branch ends, so a dendrimer has an exact, repeatable size and a known number of surface groups — unusual in a world of fuzzy size distributions. Drugs can sit in its internal pockets or be tethered to its surface arms, and those countable arms make it a favourite scaffold for attaching targeting ligands.
One more cousin deserves a mention: the nanocrystal, which is not a carrier at all but the drug itself ground down to nanometre crystals. With no carrier to load, its whole purpose is to expose enormous surface area and so dissolve faster — a neat reminder that not every nanomedicine needs a shell.