Fabrication of a Master Mold for Microneedles with a Micron-sized Air-vent Hole

Microneedles (MNs) are a novel drug-delivery system that can penetrate the skin barrier in a minimally invasive way to deliver drugs. Because they are less painful than injections and easy to use, they are seeing expanding applications in the cosmetic and pharmaceutical fields. Conventional MN fabrication involves creating a master mold and then producing MNs in that mold. The primary methods for fabricating the master mold include micro-electromechanical system processes, metal machining, and 3D printing. However, the major weakness of master molds produced by these methods is that air can become trapped inside the mold while casting the very viscous polymer used to make MNs. As a result, the MN tips may not form completely. In MN array fabrication, this can reduce the reliability of the MN structures. This study proposes a solution to air entrapment that involves forming a microscale air-vent hole (AVH) at the tip of the master mold. The AVH allows air to escape, but prevents the viscous fluid from leaking out during the curing process. Because conventional metal machining or 3D printing methods have resolution limitations, silicon micromachining was used to create the AVH-equipped master mold. The side length of the fabricated AVH ranged from 1.4 to 7.0 µm. Using this mold, we successfully fabricated 375 µm-high gelatin MNs with sharp tips. Furthermore, by controlling the volume of gelatin applied, thin-walled hollow pyramidal MNs with 23 µm thick walls and an internal cavity volume of 0.027 µL were also fabricated. Since the AVH-equipped master mold allows fabrication of both water-soluble and water-insoluble MNs, it could serve as a next-generation high-efficiency drug delivery system for applications in cosmeceutical and biomedical fields.