The body clears strangers fast
The moment a bare nanoparticle enters the blood, plasma proteins stick to its surface — an adsorption event that forms a so-called protein corona. To the immune system this coating is a flag reading “foreign,” and scavenger cells of the liver and spleen swallow the particle within minutes. All the careful work on loading and size is wasted if the carrier never survives long enough to do its job.
A brush that hides the particle
PEGylation is the standard fix: coat the surface with strands of polyethylene glycol, a flexible, water-loving polymer. The strands wave about in the water like a dense brush, and that watery cushion physically keeps plasma proteins from reaching the surface. With nothing to stick to, the “foreign” flag never goes up, and the particle slips past the scavengers. We call such a long-circulating particle a stealth nanoparticle.
Charge: stability versus stickiness
The carrier's surface charge, read as zeta potential, plays a double game. A strongly charged surface, positive or negative, makes particles repel one another and so resist clumping — good for shelf stability and against aggregation. But a strongly positive surface also grabs the negatively charged membranes of every cell it meets, getting cleared fast. Most long-circulating designs therefore aim for a near-neutral or mildly negative surface: stable enough not to clump, bland enough not to be grabbed.
Long circulation becomes targeting
Stealth coating is not only defensive — it is the engine of passive targeting. A particle that circulates for hours keeps passing through the leaky, disorganised blood vessels of a tumour, and because tumours drain poorly it gradually accumulates there. This is the enhanced permeability and retention (EPR) effect: no homing device, just a long-lived particle and a leaky target. It is the basis of drug targeting we develop in the final guide.