Microbubbles are just what they sound like: tiny gas-filled bubbles. Scientists have engineered them further, to be coated with a protective outer shell and made capable of carrying drugs or genetic material to cells in the body. They are microscopic, roughly the width of spider silk, but still hundreds or thousands of times larger than nanoparticles or liposomes. This means they’re too large to leave the bloodstream. Instead, microbubbles deliver drugs by bursting on command.