Diffraction Lithography for 3-D Microneedle Fabrication

This paper presents a diffraction ultra-violet (UV) lithography method for fabricating various micro-cone shape microneedle structures. A direct UV exposure to a liquid state photosensitive resin through a photomask generates a unique diffraction pattern of the light, in which the exposed area of the photosensitive resin becomes the needle structures. The change of liquid to solid-state of the resin by crosslinking also works as a light waveguide to create a novel sharp tip on top of the cone. Secondary and tertiary harmonic cone shapes are formed with further UV energy exposure. The proposed method is unique and versatile as it enables the formation of various cone-shaped microstructures with a straight or curved sidewall, such as tip-integrated cone and multiple harmonic cones having different heights and base shapes. The relationship between the microneedle height formed at different exposure energy and the corresponding geometry was also discussed. Insertion test and force-displacement test were conducted to demonstrate the functionality of the fabricated microneedle, and the results showed that the tip of each tested microneedle can withstand up to 0.15 N before breakage occurs. The fabricated cone-shaped microneedle with a sharp tip has a great potential for transdermal drug delivery microneedle application.