It is said that the steel-string acoustic guitars in the world today was created by a German immigrant to the United States named Christian Frederick Martin (1796-1867). Sheep intestines was the material for the Guitar strings at that time and it was called “catgut strings”. To avoid discomfort playability of the material, Martin decided to use steel strings to allow banjo players to play popular style country music. Martin cleverly created a brand new inner structure design to withstand the tension of the steel strings.

Reference: "The Fun Guitar Encyclopedia" (Yamaha Music Media)