(a–d) SEM images of nanofibers prepared by co-electrospinning of core CA-A solution and shell CA-D solution under core-to-shell flow ratio of (a) 1:3, (b) 1:1.5, (c) 1.5:1, (d) 3:1, respectively. (e,f) The amplified SEM images corresponding to (c,d), showing the core-shell structure.