stealth nanoparticle

A stealth nanoparticle is a carrier engineered to slip past the body's defences, much like a stealth aircraft designed to avoid radar. Normally, the moment a foreign particle enters the blood it is coated with proteins and swallowed up by immune cells of the liver and spleen within minutes. A stealth particle is given a surface that resists this recognition, so it stays in circulation far longer.

The usual way to achieve stealth is to coat the surface with a dense layer of a water-loving polymer, most often polyethylene glycol, which holds a shield of water that proteins cannot easily stick to. The longer a carrier circulates, the more chances it has to slowly accumulate in target tissue such as a leaky tumour, the basis of much passive tumour targeting.

Stealth is partial, not absolute. The body still clears these particles eventually, the protective coat can hinder uptake into the very cells you want to reach, and in some patients pre-existing or induced anti-PEG antibodies undermine the effect after repeated dosing. Stealth coating buys time and improved distribution, but it does not make a carrier invisible forever.