(660a) Fabrication of Sharp-Tipped Hollow Metal Microneedles

Hypodermic injection is painful and requires training. Intradermal injection is even more difficult and notably unreliable. We and others have explored the use of hollow microneedles as a simple, reliable way to inject into the skin with little or no pain. However, progress in this field has been limited by the need for low-cost industrial manufacturing methods that make microneedles capable of injection into tissue. In this study, we describe a novel fabrication process and assess the performance of the resulting microneedles.

The fabrication process starts with the creation of a polymeric microneedle master structure by rotational-inclined UV lithography. A shallow 50x50 micron square cavity is ablated into one side of the microneedle using an excimer laser operating at 248 nm. From the ablated master structure, we form a reusable poly(dimethylsiloxane) (PDMS) inverse mold. The PDMS mold is then used to create a poly(lactic acid) (PLA) replica of the ablated master structure. The PLA replica is sputter-coated with a gold seed layer, and electroplated with nickel to a thickness of 30 microns. The PLA is then dissolved in chloroform to release the nickel structure. The aspect ratio of the cavity is such that the sputtering process does not coat the bottom of the cavity with gold. We use this to create the orifices in the microneedle because the nickel can only plate in areas covered with the seed layer. This technique can be used to fabricate single microneedles, arrays or microneedles, or other hollow MEMS structures without having to fabricate an original master structure repeatedly.

Using this method, we fabricated single-microneedle devices with 500 micron tall pyramidal microneedles. These devices were glued to plastic syringe adaptors to examine their fluid flow properties. We were able to deliver 200 microliters of sulforhodamine dye to in vitro muscular tissue (a chicken leg) in less than 10 seconds, and on-going studies are assessing fluid injection into pig skin.

The results show that sharp-tipped metal microneedles can be fabricated using an inexpensive electroplating and sacrificial micromolding process. Single-microneedle devices made by this method can achieve high flow rates and deliver model drugs into tissue.