A group of German engineers at the University of Stuttgart led by Dr. Timo Gissibl have developed a process for creating a functional micro-camera lens. The triple lens optical head is only about 100 micrometers wide and can be can be injected into the human body with a standard syringe needle allowing for the internal examination of microscopic structures.
Source: http://www.abc.net.au/news/2016-06-28/3d-printed-injectable-micro-camera/7548966