India ink is a simple black ink. Previously it has been used for writing and printing and is now commonly used for drawing in comic books and comic strips. India ink is also used in medical research to visualize blood vessels when viewed under a microscope. It is used in a diluted form as an ultra-fine polishing medium for making precise optical surfaces on metals. In ophthalmology, it is used to some extent in corneal tattooing.
Spinelli, L.; Botwicz, M.; Zolek, N.; Kacprzak, M.; Milej, D.; Sawosz, P.; Zaccanti, G. Determination of reference values for optical properties of liquid phantoms based on Intralipid and India ink. Biomed. Opt. Express 2014, 5 (7), 2037-2053.
Aquaro, G. D.; Todiere, G.; Strata, E.; Barison, A.; Bella, G.; Lombardi, M. Usefulness of india ink artifact in steady-state free precession pulse sequences for detection and quantification of intramyocardial fat. J. Magn. Reson. Imaging 2014, 40 (1), 126-132.