Whale baleen is the mostly mineralized keratin-based bio-material consisting of parallel plates suspended down the mouth of the whale. Baleen's mechanical properties being strong and flexible made it a popular material for numerous applications requiring such a property (see Human uses section). The basic structure of the whale baleen was characterized to be a tubular structure with a medulla hollow core enclosed by a tubular layer with a diameter varying from 60 to 900 microns, which had approximately 2.7 times higher calcium content. The elastic module in the longitudinal direction and the transverse direction are 270MPa and 200MPa, respectively. This difference in elastic module could be attributed to packing of the sandwiched tubular structure.