Nanoparticles are especially prone to accumulating at the site of a tumor, which makes them very good for detecting tumors in the body. Nanoparticles accumulate or start to ‘stack up’ inside of a tumor in the body because tumors often make the blood vessels around them very ‘leaky’, and blood vessels grow around a tumor in order to supply the tumor with the blood and nutrients it needs to keep growing. Once nanoparticles leak out of these ‘leaky’ blood vessels into the surrounding tissue around the tumor, it is very hard for them to get back out, allowing doctors to find the tumor by looking at where the nanoparticles go!
Once the nanoparticles have served their function inside the body, they often eventually get passed to the kidneys (for very small particles) or the liver and the spleen (for larger particles), eventually being excreted from the body. Scientists often ‘engineer’ nanoparticles to only stay inside the body for as long as they are need to detect a tumor or deliver a drug, and then to get out quickly!!