Formation of polymer microneedle arrays using soft lithography

We demonstrate the fabrication of polymer microneedle arrays using soft lithography. A photomask was designed to use Fresnel diffraction of UV light to create sharp, tapered hollows in SU-8, a negative photoresist, after development. Polymer microneedles were formed using these SU-8 structures as a mold. These polymer needles may be applicable as flexible electrodes in brain-machine interfaces because they are more likely to survive movement of the skin than conventional brittle silicon needles. Similar needles, made from medicinal substances, could be used for transdermal drug administration. For these applications, the needles must be long, sharp, and stiff enough to penetrate the stratum corneum (∼20 μm in thickness) and reach the viable epidermis (200-300 μm in thickness), but must not reach the dermis, which contains sensitive nerve endings. We successfully manufactured 20×20 microneedle arrays of polydimethylsiloxane with a needle length of 200 μm. We experimentally verified that these manufactured electrodes successfully penetrated the stratum corneum of a cultured skin.