Microbubbles can be loaded with drugs in a variety of methods.  Some of the engineering details to these process are explained in the Fabrication section of this site.  A simple method is to attached the drugs to the outer shell surface such as shown in (A) in the image above.  Image (B) shows drug encapsulation between the phospholipid shell.  As mentioned in the discussion of flow focusing in the Fabrication section, drugs can be incorporated into a layer of oil within an inner-shell layer.  Secondary carriers can also be employed to hold smaller particles as shown in (D).  Polymer (grey) microbubbles can be coated in a biocompatible material (yellow) where the drugs are captured in the polymer layer such as image (E).  Image (F) shows surface loading microbubbles with protein shells, which is simplified by the multiple thiol groups proteins exhibit.  Finally, one can load the entire volume of a protein-shelled microbubble as shown in image (G). [1]