* Scientific: "Keratin is a fibrous protein that forms the main structural component of hair, skin, and nails."
* Beauty: "The keratin treatment helped to smooth my hair and reduce frizz."
* Animal: "The rhinoceros's horn is made of keratin, just like human fingernails."
* Comparative: "Keratin is stronger than collagen, making it ideal for protective structures."
* Figurative: "His skin was as tough as keratin, impervious to any insult."
The best way to use "keratin" depends on the context of your writing or conversation.