Effects of microneedle tip geometry on injection and extraction in the skin

Needles of microns dimensions can be fabricated by MEMS techniques and used for painless injections or extraction. However, the effects of microneedle tip geometry, injection/extraction pressure and time on the injected/extracted volume have not been quantified. Thus, to provide microneedles with easily controlled tip geometry, we fabricated hollow beveled glass microneedles with a range of tip diameters (20 - 80 μm) and bevel angles (30 - 90 degrees) and punctured them into human cadaver or in vivo rat skin. For infusion, cadaver skin specimens were immediately frozen in liquid nitrogen after injecting dye solutions, sectioned using a microtome and digitally imaged, and then analyzed to determine the volume of fluid injected. Over the range of pressures (10 50 psi) and durations (1 - 20 min) studied, the infusion volume was in the range of 1 - 100 nl and the loading depth ranged from 200 to 500 μm. For extraction, interstitial fluid (ISF) was collected by beveled microneedles under pressure of -0.02 MPa for 5 min from hairless rat skin. The ISF volume measured in the needle lumen was in the range of 2 - 30 nl. We found that the injected volume increased with both pressure and injection time, but extracted volume was less when suction pressure was too low or too high. We believe this study provides the first quantitative measurements of fluid injection and extraction through microneedles into skin.