Fig. 3.

Schematics and SEM images of Si geometries during the isotropic etching (the 2nd DRIE) process to sharpen microneedles. a Fabricated microwells after the 1st DRIE step. b Early stage of the 2nd DRIE: Initial microwells are widened due to isotropic etching. c, d Intermediate stage: Adjacent widened microwells connected to each other, forming blunt microneedles and bridged valleys. e, f Final stage: Sharp microneedles are produced and the bridged valleys are gradually disappeared. SEM images in top view and bird’s-eye view are shown at the bottom of schematics for early (bi, bii), intermediated (di, dii), and final (fi, fii) stages