Preparation of hollow hafnium oxide microneedles for transdermal drug delivery

Transdermal drug delivery is an important approach due to its advantages of controlled release, easy wiping off, high safety and low degree of side effects. However, the efficiency and success of the drug delivery method is seriously hampered by the incapacity of many drugs to cross the skin at therapeutically useful rates. A microneedle approach can dramatically promote transdermal delivery, especially for macromolecules. In this paper, a fabrication process for hollow hafnium oxide microneedles on silicon wafer using only one mask has been developed. Photoresist is spun on a 4″ silicon (100) wafer and patterned by a mask. Deep reactive ion etching is used to form deep molds inside the wafer. A hafnium oxide film is deposited by atomic layer deposition to cover the top side of the wafer and the inside walls of the molds. Then the back side of silicon wafer is etched by a 30 wt.% KOH solution until exposed hollow hafnium oxide microneedles are obtained. Compared to the processes that have been developed for silicon, silicon oxide, or polymer microneedles, the process presented here is simpler, and the strength and flexibility of hollow hafnium oxide microneedles can be easily adjusted by modifying the depositing time of hafnium oxide films. Hafnium oxide microneedles have potential applications in transdermal drug delivery.