Characterization of surface micromachined metallic microneedles

The purpose of the paper is to provide quantitative characterization of metallic microneedles. Mechanical and fluid flow experiments were performed to evaluate the buckling force, the penetration force, and the pressure versus flow rate characteristics of the microneedles. The microneedle design variations characterized included varying the shaft lengths, varying the tip taper angles/geometries, and the inclusion of micromechanical barbs. The penetration force was found to range from 7.8 gF for a microneedle of shaft length 500 μm, to 9.4 gF for a length of 1500 μm, both with a tip taper angle of 30°. Microneedles with a linear tip taper angle of 30° penetrated 95 +% of the time without failure. The microneedles with a 15° and 20° linear tip taper penetrated 10% and 25% of the time, respectively. The buckling force was found to be 98.4 gF for a 500 μm long microneedle shaft, 72.3 gF for a needle of shaft length 1000 μm, and 51.6 gF for a 1500 μm long shaft. The results demonstrate that the penetration force was 7.9% of the buckling force for 500 μm long shafts, 11.6% for a 1000 μm long shaft, and 18.2% for a 1500 μm long microneedle shafts. The microneedle fluid flow characteristics were studied. An inlet pressure of 49.0 Pa was required for a flow rate of 1000 μL/h and 243.0 Pa for a flow rate of 4000 μL/h using air as the fluid medium. For water, an average pressure of 30.0 kPa was required for a flow rate of 1000 μL/h and 106.0 kPa for a flow rate of 4000 μL/h.