Cancer is a disease of tissue growth gone wrong. Tumor cells proliferate uncontrollably and don’t die as they should. When a solid tumor grows large enough to need to sprout its own blood vessels, those vessels, too, grow irregularly. The tumor vessel networks are tortuous and disorganized (see Physics Today, February 2016, page 14), and the vessels’ lining, or endothelium, is riddled with gaps hundreds of microns wide between cells.
The challenge of chemotherapy is to kill off the tumor cells without doing too much harm to healthy ones. The inter-endothelial gaps promised a way to do that. Nanoparticles up to 300 nm in diameter can fit through the gaps, and they can’t permeate normal blood vessels the same way. So nanoparticles loaded with a drug should, it stands to reason, selectively enter and attack tumor tissue but leave healthy tissue alone. An inherently biomedical challenge was seemingly...
