Martin's redesigned flexible core strings have tin-plated core wire for extra corrosion resistance, "because what you don’t see matters!" 
Flexible thin core wire is wound with highest quality phosphor bronze wrap wire.  Extra light gauge 010/010 014/014 025w/010 032w/012 042w/018 054w/027w. 

Thinner core wire allows control with sharp attack, increased flexibility, and improved harmonic response.

** This set replaces MFX700 **