source: They are rivaled as biological materials in toughness only by chitin
target: They are compared to biological materials in toughness because of chitin.
references: They are compared to biological materials in toughness because of chitin. They are rivaled as biological materials only compared to chitin. Only chitin rivals them as biological materials. They are rivaled as biological materials in toughness. Their durability are comparable to chitin. Chitin is the only natural material that is stronger. They are rivaled as biological materials in toughness only by chitin