Characterization of microchannels created by metal microneedles: formation and closure

Transdermal delivery of therapeutic agents for cosmetic therapy is limited to small and lipophilic molecules by the stratum corneum barrier. Microneedle technology overcomes this barrier and offers a minimally invasive and painless route of administration. DermaRoller(®), a commercially available handheld device, has metal microneedles embedded on its surface which offers a means of microporation. We have characterized the microneedles and the microchannels created by these microneedles in a hairless rat model, using models with 370 and 770 μm long microneedles. Scanning electron microscopy was employed to study the geometry and dimensions of the metal microneedles. Dye binding studies, histological sectioning, and confocal microscopy were performed to characterize the created microchannels. Recovery of skin barrier function after poration was studied via transepidermal water loss (TEWL) measurements, and direct observation of the pore closure process was investigated via calcein imaging. Characterization studies indicate that 770 μm long metal microneedles with an average base width of 140 μm and a sharp tip with a radius of 4 μm effectively created microchannels in the skin with an average depth of 152.5 ± 9.6 μm and a surface diameter of 70.7 ± 9.9 μm. TEWL measurements indicated that skin regains it barrier function around 4 to 5 h after poration, for both 370 and 770 μm microneedles. However, direct observation of pore closure, by calcein imaging, indicated that pores closed by 12 h for 370 μm microneedles and by 18 h for 770 μm microneedles. Pore closure can be further delayed significantly under occluded conditions.