Chitin is the second most abundant biopolymer in the world. Only cellulose (produced by plants) exceeds its occurrence. Crustaceans, arthropods and fungi produce approximately
100.000.000.000 metric tons of the material annually in their biosynthesis. In Nature chitin appears in a wide range of different ways and forms composites with a varity of materials. For example crab's shell becomes a hard, thick, porous layer because the chitin is mixed with calcium, while mixing it with the proteins of a silkworm creates a smooth, soft and flexible material.
Science still seems to be on the lookout for a lot of answers on how nature's chitinous materials are being created, even though there have been a bunch of discoveries already and chitosan, a deacetylised derivate of chitin, is already used in medical procedures.
However, chitin is still relatively unexplored and rarely used.